Symposium Organizers
Yalin Lu University of Colorado
MarkT. Lusk University of Colorado
JohnM. Merrill Air Force Research Laboratory
Sheila Bailey NASA Glenn Research Center
Alberto Franceschetti National Renewable Energy Laboratory
Symposium Support
National Renewable Energy Laboratory
Naval Research Laboratory, Solid State Devices Branch
B1: Next Generation Photovoltaics I: Multiple Exciton Generation and the Exploitation of Quantum Confinement
Session Chairs
Tuesday PM, April 26, 2011
Room 2001 (Moscone West)
9:30 AM - B1.1
Hot Carrier and Multiple Exciton Generation in PbS Colloidal Quantum Dots Characterized with an Improved Transient Grating Technique.
Qing Shen 1 2 , Kenji Katayama 3 , Tsuguo Sawada 4 , Sojiro Hachiya 1 , Taro Toyoda 1
1 Department of Applied Physics and Chemistry, The University of Electro-Communications, Tokyo Japan, 2 PRESTO, Japan Science and Technology Agency, Saitama Japan, 3 Department of Applied Chemistry, Chuo University, Tokyo Japan, 4 , Japan Science and Technology Agency, Saitama Japan
Show AbstractIn recent years, enhanced multiple exciton generation (MEG) in semiconductor quantum dots (QDs) has received much interest, because the MEG has a potential to produce an appreciable improvement in a energy conversion efficiency of solar cells through increased photocurrent [1]. The enhanced MEG in some QDs such as PbSe, PbS, CdSe, PbTe, and Si QDs has been observed at threshold photon energies of 2-3 times the HOMO-LUMO transition energy (Eg) using transient absorption (TA) spectroscopy and time-resolved photoluminescence [1,2]. However, several recent reports have questioned the claim of enhanced MEG in QDs and even its existence [3]. Further theoretical and experimental researches to better understand the MEG are expected. In this study, we apply an improved transient grating (TG) technique [4,5] to characterize hot carrier generation and MEG in PbS colloidal QDs. The improved TG technique is one kind of pump-probe methods and the transient refractive index changes in the sample due to photoexcited carriers can be measured. Thus, photoexcited carrier dynamics in the QDs can be monitored by using the improved TG technique. We have characterized pump light intensity and photon energy dependences of the TG responses in PbS colloidal QDs. We found that besides a peak existing at 500 fs in the TG responses, a new peak appeared at about 2 ps when the photon energy of the pump light is larger than 2.7Eg. The new peak intensity decreased as the photon energy of the pump light decreased and the peak disappeared for the photon energies smaller than 2.7Eg. In addition, a fast Auger recombination decay with a decay time of about 100 ps was observed when the photon energy is larger than 2.7Eg. We think that the first peak at 500 fs resulted from photoexcited hot carriers and the second peak at 2 ps resulted from MEG in the PbS QDs. We succeeded in separate detection of hot carrier generation and MEG in semiconductor QDs for the first time. Dependence of the MEG efficiency on the QD size is being studied in detail now. Acknowledgment: Part of this research was supported by JST PRESTO program, Grant in Aid for Scientific Research (No. 21310073) from the Ministry of Education, Sports, Science and Technology of the Japanese Government. [1] A. Nozik, Chem. Phys. Lett. 457, 3 (2008).[2] R. D. Schaller and V. I. Klimov, Phys. Rev. Lett 92, 186601 (2004). [3] G. Nair, S. M. Geyer, L. –Y. Chang and M. G. Bawendi, Phys. Rev. B 78, 125325 (2008).[4] K. Katayama, M. Yamaguchi, and T. Sawada: Appl. Phys. Lett. 82, 2775 (2003). [5] Q. Shen, M. Yanai, K. Katayama, T. Sawada, and T. Toyoda, Chem. Phys. Lett. 442, 89 (2007).
9:45 AM - B1.2
General Character of Long Living Carriers Created by Efficient Multiple Exciton Generation in Silicon Nanocrystals.
Dolf Timmerman 1 , Jan Valenta 2 , Tom Gregorkiewicz 1
1 Van der Waals - Zeeman institute, University of Amsterdam, Amsterdam Netherlands, 2 Department of Chemical Physics & Optics, Charles University, Prague Czech Republic
Show AbstractMultiple exciton generation (MEG) in semiconductor nanocrystals offers a possible route to overcome the theoretical conversion limit of present day first-generation photovoltaics. Occurrence of this mechanism in these nanocrystals and the possibility of increased efficiency relative to the MEG yield in their bulk counterparts are subjects that are intensively studied, but show contradictory results. Here we demonstrate results for silicon nanocrystals in solid state matrices and colloidal ethanol suspensions. These were obtained by a technique that precludes the need for ultrafast spectroscopy applied in previously conducted investigations, and thus considerably simplifies experiments and their interpretation. It is based on measurements of the quantum yield of photoluminescence and its dependence on the excitation photon energy. Results for the two differently-fabricated materials (bottom-up and top-down) show the same qualitative dependence on the excitation photon energy, with a clear step-wise increase typical for MEG processes. This generic behavior of both systems is complemented by the low threshold energy for the MEG process and its high efficiency. In the most favorable case, the MEG onset is recorded at 2.8 eV, with the exciton generation yield doubling for 3.4 eV. This highly efficient MEG occurring in the visible spectral range represents dramatic improvement in comparison to bulk Si, which has a MEG onset of 3.5 eV and shows a yield increasing to only 1.5 at 5 eV.
10:00 AM - **B1.3
Third Generation Photovoltaics: Multiple Exciton Generation in Colloidal Quantum Dots, Singlet Fission in Molecules, Quantum Dot Arrays, Quantum Dot Solar Cells, and Effects of Solar Concentration.
Arthur Nozik 1 2 , Justin Johnson 1 , M. Hanna 1 , M. Beard 1 , J. Luther 1 , A. Midgett 1 , O. Semonin 1 , Josef Michl 2
1 , NREL, Golden, Colorado, United States, 2 Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, United States
Show AbstractOne potential, long-term approach to more efficient future generation solar cells is to utilize the unique properties of quantum dots (QDs) and unique molecular chromophores to control the relaxation pathways of excited states to produce enhanced conversion efficiency through efficient multiple electron-hole pair generation from single photons . We have observed efficient multiple exciton generation (MEG) in PbSe, PbS, PbTe, and Si QDs and efficient singlet fission (SF) in molecules that satisfy specific requirements for their excited state energy level structure.. We have studied MEG in close-packed QD arrays where the QDs are electronically coupled in the films and thus exhibit good transport. We have developed simple, all-inorganic QD solar cells that produce large short-circuit photocurrents and power conversion efficiencies in the 3-5% range via both nanocrystalline Schottky junctions and nanocrystalline p-n junctions. We have observed very efficient SF in thin films of molecular crystals of 1,3 diphenylisobenzofuran with quantum yields of 200% , reflecting the creation of two excited triplet states from the first excited singlet state. Various possible configurations for novel solar cells based on MEG in QDs and SF in molecules that could produce high conversion efficiencies will be presented, along with progress in developing such new types of solar cells. Recent analyses of the effect of MEG or SF combined with solar concentration on the conversion efficiency of solar cells will be discussed.
10:30 AM - B1.4
Enhanced Charge Collection in Confined Bulk Heterojunction Organic Solar Cells.
Jonathan Allen 1 , Kevin Yager 1 , Htay Hlaing 3 2 , Chang-Yong Nam 1 , Benjamin Ocko 2 , Charles Black 1
1 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York, United States, 3 Department of Physics and Astronomy, State University of New York, Stony Brook, Stony Brook, New York, United States, 2 Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York, United States
Show AbstractWe describe a new organic semiconductor solar cell device architecture that improves the electrical performance of the organic material by confining it within nanometer-scale volumes. Confining blended poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester organic solar cell active layers within thin nanoporous aluminum oxide templates nearly doubles the supported short-circuit current density compared to equivalent unconfined volumes of the same blend, and increases the poly(3-hexylthiophene) hole mobility in the blend by over 1000 times. Grazing incidence x-ray diffraction measurements show that the confinment disrupts polymer ordering and reduces grain size, and also changes the orientational distribution. Similar confined volumes of single-component poly(3-hexylthiophene) show an almost 400 times enhancement in hole mobility, while the conductivity of confined [6,6]-phenyl-C61-butyric acid methyl ester decreases by 50 times. Organic semiconductor solar cells may enable large-scale integration of solar energy solutions through easily scalable and low-cost manufacturing. Much of the recent exciting scientific and technological progress in this vibrant field has largely come through the introduction of new polymer semiconductor materials. In this work, we demonstrate that nanostructuring the organic semiconductor solar cell active material can dramatically increase the performance in existing, well-established materials – an approach that may provide benefits for new organic semiconductors as well.
10:45 AM - B1.5
The Implications of Ultrafast Relaxation Processes in Pristine Conjugated Polymers on the Charge Separation Mechanism in Bulk Heterojunction Polymer:Fullerene Blends.
Natalie Banerji 1 , Sarah Cowan 1 , Mario Leclerc 2 , Eric Vauthey 3 , Alan Heeger 1
1 Center for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California, United States, 2 Department of Chemistry, Université Laval, Quebec City, Quebec, Canada, 3 Department of Physical Chemistry, University of Geneva, Geneva Switzerland
Show AbstractThe femtosecond-resolved evolution of the emission spectrum of two very important conjugated polymers, P3HT and PCDTBT, is presented. The early relaxation processes (due to initial localization of the excitation, exciton formation, exciton diffusion and conformational relaxation) of the pristine materials, in film and solution, were investigated in detail with fluorescence up-conversion spectroscopy. Two excitation wavelengths and several emission wavelengths, covering the entire fluorescence spectrum, were used. The data were complemented by polarization-sensitive measurements. Both P3HT and the donor-acceptor copolymer PCDTBT have been successfully used in organic bulk heterojunction (BHJ) solar cells. P3HT:PCBM devices have power conversion efficiencies as high as 5%. PCDTBT:PC70BM photovoltaic cells yield power conversion >6% and internal quantum efficiencies approaching 100%. Ultrafast (<100 fs) charge separation between the polymer electron donor and the fullerene acceptor at their interface is in both cases the key step in the functioning of the BHJ solar cells. We therefore discuss the implications of the relaxation processes observed in the pristine polymers on the charge separation mechanism in the polymer:fullerene composites. Most of those processes occur on a much slower time scale than the <100 fs charge separation in the BHJ blend. In particular, excitons do not have time to diffuse to a polymer:fullerene interface prior to charge separation, in contrast with the commonly proposed picture. A significant distance has however to be covered by the photoexcitation to reach an interface, since there is experimental evidence that phase separation between the polymer and fullerene is necessary for good solar cell functioning. As charge separation occurs during the initial self-localization of the primary excitation, we suggest that this excitation reaches the polymer:fullerene interface for charge separation before it becomes spatially self-localized and bound within an exciton. The high initial mobility of the electron and hole directly after the π-π* interband transition and their delocalization (allowing quantum effects to assist the transport) could account for the observed ultrafast charge separation rate in BHJ blends.
11:30 AM - **B1.6
Heavily Doped Semiconductor Nanocrystal Quantum Dots.
Eran Rabani 1
1 , Tel Aviv University, Tel Aviv Israel
Show AbstractElectronic doping of semiconductors by impurity atoms enabled their widespread technological application in micro and opto-electronics. Applying these principles to colloidal semiconductor nanocrystals, an emerging family of materials where size, composition and shape-control offer widely tunable optical and electronic properties, has proven elusive. This arises both from the synthetic challenge of how to introduce single impurities and from a lack of fundamental understanding of this heavily doped limit under strong quantum confinement. Here we report a method to dope semiconductor nanocrystals with metal impurities providing control of the band gap and Fermi energy. A combination of optical measurements and scanning tunneling spectroscopy reveal the emergence of a confined impurity band and confined band-tailing. We develop two theoretical models for heavily doped nanocrystals. The first is used to explain blue shifts in the absorption spectrum observed for n-doped nanocrystals and is based on a tight-binding model where impurities are localized due to the quantum confinement effect. The other is used to describe red shifts in the absorption spectrum observed for p-doped nanocrystals and is based on the treatment of lattice disorder induced by the impurities leading to band tailing. The successful control of doping and its understanding provide n- and p-doped semiconductor nanocrystals which greatly enhance the potential application of such materials in solar cells, thin-film transistors, and optoelectronic devices.
12:00 PM - B1.7
Ab initio Investigation of Exciton Transfer between Si Quantum Dots.
Zhibin Lin 1 2 , Huashan Li 1 , Mark Lusk 1 , Alberto Franceschetti 2
1 Physics, Colorado School of Mines, Golden, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractRecent advances in the synthesis of quantum confined nanostructures offer an intriguing opportunity for improving the solar cell efficiency in photovoltaic energy conversion. In particular, it may be possible to increase the efficiency with which photo-excited excitons are transferred between quantum dots by functionalizing their surfaces. Both the theoretical setting and the influence of different types of surface ligands on the exciton transfer between nanostructures have been largely unexplored. Provided that the QDs are sufficiently well separated, exciton hopping can be considered within the framework of Förster resonance energy transfer (FRET) wherein Coulomb interactions are approximated using either dipole-dipole or higher-order multipole expansions. It is not clear that this is the appropriate approach, though, when the dots are separated only by a few nanometers because of the steric influence of passivating ligands. We consider Si quantum dots (QDs) and carry out many-body ab-initio calculations within the GW approximation. Excitonic characteristics are investigated by solving the Bethe-Salpeter equation (BSE). The exciton transfer rates are calculated using Fermi’s golden rule within first-order perturbation theory. We compare our results, for Si QDs with a range of sizes, with the predictions of Förster theory by explicitly computing the Förster radius from the emission and absorption spectra of the QDs. Several types of surface termination, including H, OH and CH3, are considered. We find that, at the same QD separation, H-terminated Si QDs exhibit a higher exciton transfer rate than QDs passivated with the two other common ligands. Exciton transfer rates in reconstructed and unreconstructed QDs, as well as effects of microscopic screening in the Coulomb interaction between initial and final states, are investigated. This work is viewed as a first step in developing a framework for the design of QD assemblies with improved exciton transfer efficiency.
12:15 PM - B1.8
Quantitative Absorption Strength of Silicon Nanocrystals for Photovoltaics.
Benjamin Lee 1 , Daniel Hiller 2 , Ingrid Anderson 3 , Bhavin Jariwala 4 , Sumit Agarwal 4 , Margit Zacharias 2 , Paul Stradins 1
1 National Center for Photovoltaics, National Renewable Energy Lab, Golden, Colorado, United States, 2 IMTEK, Faculty of Engineering, Albert-Ludwigs-University Freiburg, Freiburg Germany, 3 Department of Physics, Colorado School of Mines, Golden, Colorado, United States, 4 Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado, United States
Show AbstractWe report direct measurements of the normalized absorption strength of Si NCs with sizes down to 2 nm. Our results show that the near-gap absorption follows a Tauc relationship for indirect semiconductors and the above-gap absorption is substantially similar to that of bulk crystalline Si. This has important implications for applications of Si NCs, particularly in considering them as absorbers in solar cells [1]. Due to their relatively weak absorption, a minimum absorber layer thickness of several microns is required to absorb a significant fraction of incident sunlight.This work addresses the outstanding question of whether the bandgap in Si NCs has a direct or indirect character, which is sometimes disputed, with most measurements and evidence indicating an indirect nature for sizes greater than 4 nm [2]. It also provides a quantitative report of the absorption strength of Si NCs, which is still only sparsely investigated. Moreover, a comparison of the NC absorption strength to that of bulk Si has not been clearly presented and is valuable for applications, particularly in photovoltaics.We determine the absorption strength of two types of Si NCs: NCs grown in a SiO2 matrix [3], and NCs grown in an argon-silane plasma, passivated and dispersed in a liquid [4]. Si NCs were grown as size-selected samples, with diameters down to 2 nm and narrow size distribution. The absorbance of the samples was obtained using a UV-Vis spectrophotometer with an integrating sphere and by photothermal deflection spectroscopy. For the NCs in oxide, the amount of Si present in the NCs was determined by elastic recoil detection analysis. For the plasma-synthesized NCs, the NCs were handled in an inert environment, their mass measured, passivated to prevent oxidation, and their absorbance determined.The near-gap absorption for as-grown NCs exhibits strong absorption from defect states and follows the Urbach rule. By contrast, the absorption of well-passivated and highly-luminescent NCs from the bandedge out to ~2.5 eV is well-characterized by the Tauc relationship for indirect semiconductors. The Tauc band gap values are close to the PL gap and expected values for each NC size due to quantum confinement. This provides additional evidence that the bandgap of Si NCs has an indirect nature down to ~2 nm size. Above ~2.5 eV, the dielectric-screening corrected absorption of the NCs is markedly similar to that of bulk crystalline Si and lower than amorphous Si. In no spectral region is NC absorption distinctly higher than that of bulk Si.This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-08GO28308.[1] G Conibeer et al., Thin Solid Films 511-512, 654-662 (2006).[2] C Meier et al., J. Appl. Phys. 101, 103112 (2007).[3] M Zacharias et al., Appl. Phys. Lett. 80, 661-663 (2002).[4] L Mangolini et al., Nano Lett. 5, 655-659 (2005).
12:30 PM - B1.9
Modification of Germanium Nanowires with Semiconductor Quantum Dots: A New Photoanode Material.
Ajay Singh 2 1 , Chris Barrett 2 , Hugh Geaney 2 , Robert Gunning 2 1 , Kevin Ryan 2 1
2 Department of Chemical and Environmental Sciences, Materials and Surface Science Institute, Limerick Ireland, 1 SFI-Strategic Research Cluster in Solar Energy Conversion, Materials and Surface Science Institute , Limerick, Limerick, Ireland
Show AbstractQuantum dot sensitized one dimensional structures have found significant recent interest as emerging photoanode, light sensors and emission devices. While the majority of research has focussed on carbon nanotubes, TiO2 and ZnO, sensitization of group IV nanowires has not been investigated. Here we synthesise high purity germanium nanowires directly from an ITO substrate without the use of catalysts and further decorate/sensitize the surface with II-VI quantum dots. The hybrid structure couples the wide-band gap absorption of the quantum dot to the more effective charge carrier ability of the nanowire. The ease of formation of long nanowires (< 1 micron) in vertically arrays from the ITO ensures rapid transport of the electrons to the external electrode. Complete exciton separation at the quantum dot/nanowire interface is evidenced by photoemission studies. In addition to excellent photo-anode behaviour germanium (band gap 0.66 eV) also harvests low energy photons in the infra-red contributing to the short circuit current. The hybrid material is characterized by PL, Uv-Vis, TEM, HRTEM, HRSEM and I/V nanoprobe measurements. This work is a significant advancement in the field of photoanode material and has significant implications germanium nanowire based solar-cells. In these devices germanium acts as an effective photoanode contributing to the short circuit current even though with a band gap of 0.66 eV, germanium it is only active in the infra-red portion of the spectrum
12:45 PM - B1.10
The Role of Composition and Surface Treatment in Multiple Exciton Generation Efficiency of PbSe Quantum Dots.
Aaron Midgett 1 2 , Hugh Hillhouse 3 1 , Barbara Hughes 1 2 , Arthur Nozik 1 2 , Matthew Beard 1
1 , NREL, Golden, Colorado, United States, 2 Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, United States, 3 Chemical Engineering, University of Washington, Seatle, Washington, United States
Show AbstractEfficient multiple exciton generation (MEG) has been displayed in several different nano-scale systems and has been shown to be dependent on composition and surface treatment, with some materials showing efficient MEG, and others showing none at all. Recent reports have suggested that due to the repetitive nature of pulsed fs-laser experiments for measuring the photon-to-exciton quantum yields (QYs) that result from multiple exciton generation (MEG), an alternative relaxation pathway may produce artificially high results. We present transient-absorption (TA) data for several lead chalcogenide quantum dots (QDs) at a variety of pump photon energies. We examine the dynamics of a charge separated state for PbSe QDs and obtain photon-to-exciton QYs for all samples studied under flowed or stirred conditions. We find differences in the observed TA dynamics between flowing and static conditions that depend upon photon fluence, pump photon energy, composition, and quality of the QD surfaces. The results are modeled with a spatially resolved population balance of generation, recombination, convective replacement, and accumulation of long-lived excited QDs. By comparing the simulations and experiments, the steady-state population of the long-lived QD excited states and their kinetics are determined for different experimental conditions. We also find that by treating the surface of QDs, we can increase QYs and decrease the charging probability by a factor of 3-4. While heavy metals such as lead are unlikely to be used in commercial solar cells, this proof-of-concept system explains some of the properties governing MEG efficiency so that new materials can be designed to increase QYs and eventually make better use of the available energy in the solar spectrum.
B2: Next Generation Photovoltaics II: Multiple Exciton Generation and the Exploitation of Quantum Confinement
Session Chairs
Tuesday PM, April 26, 2011
Room 2001 (Moscone West)
2:30 PM - B2.1
Investigation of Microscopic Mechanism of Space Separated Quantum Cutting in Si Nanocrystals Embedded in SiO2-matrix.
Minh Tuan Trinh 1 , Wieteke de Boer 1 , Rens Limpens 1 , Tom Gregorkiewicz 1
1 WZI, University of Amsterdam, Amsterdam Netherlands
Show AbstractSemiconductor nanocrystals (NCs) can exhibit carrier multiplication (CM), a process in which a single absorbed photon creates multiple electron-hole pairs (excitons). This is of interest for the development of highly efficient (third-generation) solar cells. CM is investigated in Si NCs embedded in SiO2-matrix by ultrafast spectroscopy. The decay dynamics of exciton are measured by two different excitation energies: below and above CM threshold, both at low excitation fluences. The results show identical decay dynamics indicating of absence of Auger recombination. Interestingly, when normalized for the number of absorbed photons, we found a higher signal intensity for the higher excitation energy. The higher signal intensity and the absence of Auger recombination are the characteristic fingerprints for the space separated quantum cutting (SSQC), a process in which multiple excitons are formed in the neighboring NCs. Two mechanisms could be invoked to explain this finding: after absorption of a single high-energy photon (i) multiple excitons are generated in different NCs, or (ii) multiple excitons are generated in a single NC and subsequently relocated very fast (faster than the experimental resolution, < 100 fs) to the neighboring NCs. In order to distinguish between these two mechanisms, we performed another experiment in which the number of excitons is measured as a function of excitation fluence. We found that the Auger recombination occurs even before all NCs are excited. This result suggests that the excitons are not very mobile on a short time scale, and that the multiple excitons must be formed in the different NCs directly after absorption of a single high-energy photon.
2:45 PM - B2.2
Photoluminescence Properties of Core/Shell CdSe/ZnS Quantum Dots Embedded in Transparent Films for Third Generation Photovoltaics.
Bahareh Sadeghimakki 1 2 , Navid Mohammad Sadegi Jahed 1 2 , Siva Sivoththaman 1 2
1 Electrical and Computer Engineering, University of Waterloo, Waterloo, Ontario, Canada, 2 Center for Advanced Photovoltaic Devices and Systems (CAPDS), University of Waterloo, Waterloo, Ontario, Canada
Show AbstractColloidal semiconductor quantum dots (QDs) with a diameter of a few nanometers have generated remarkable technological interest as the active material either in optoelectronic devices such as light-emitting diodes, lasers and solar cells or for biological applications. Their high photoluminescence (PL) quantum yield (QY), tunable emission wavelength, photostability and multiplexing capabilities are dramatically different than the bulk material due to electron confinement in three special dimensions. The luminescence properties of QDs are sensitive to surface interactions, and the degree of surface passivation has been shown to be a crucial parameter in determining the QY. In this work we studied the effects of surface passivation on the optical properties of CdSe QDs. An effective surface passivation of mono-disperse CdSe QDs is achieved by coating them with ZnS. In addition, the QDs were incorporated in transparent matrices and their optical characteristics were investigated by using steady state PL measurements and with wide-field fluorescence imaging. The enhancement in the luminescence properties of CdSe QD layers by surface passivation was evaluated. It was observed that the photobleaching caused by the photooxidation of the QDs lead to a sharp drop of PL for the QDs without passivation. Photocorrosion creates surface defects which likely enhance the nonradiative energy transfer in excited QDs. The PL images were taken through a bandpass filter under UV excitation. Fluorescence images enabled us to see the brightness and visibility of QDs. The obtained results along with the AFM and TEM images provide information on the geometry of the QDs.PL lifetime measurements using time correlated single-photon counting (TSPC) with nanosecond flash lamp excitation and temperature dependent QY measurements were also performed on the fabricated films. Experimental data was fit to the numerical model with lifetime constants in nanoseconds range. The excitonic emission of samples was also mapped using a liquid nitrogen cryostat in the 77K - 300K range. We demonstrate that the nonradiative process that limits the QY of the QDs at room temperature is the exciton-phonon coupling which is the main reason of the scattering in a medium. Full details of fabrication, characterization, and analysis will be presented. These studies give us insight to exploit the QD layers for photon down shifting and multiple exciton generation for application in photovoltaics and have led to new device designs.
3:00 PM - **B2.3
Multiple Exciton Generation and Carrier Dynamics in PbSe QDs and QD arrays: Implications and Applications towards Solar Energy Conversion.
Matt Beard 1 , Aaron Midgett 1 , Joseph Luther 1 , Octavi Semonin 1 , Barbara Hughes 1 , Danielle Smith 1 , Jianbo Gao 1 , Arthur Nozik 1
1 Chemical and Biosciences, National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractMultiple exciton generation (MEG) in quantum dots (QDs) and impact ionization (II) in bulk semiconductors are processes that describe producing more than one electron-hole pair per absorbed photon. MEG is a hotly debated subject in the scientific literature with varying results for similar QD systems. There are two critical issues to address: (1) what are the MEG yields for colloidal quantum dot system? and (2) does quantum confinement provide a benefit for MEG? I will present an overview of recent MEG measurements at NREL and compare our results to recent measurements from LANL. We discuss the proper way to compare MEG in QDs with II in bulk semiconductors, and argue that there are important differences in the photophysics between bulk semiconductors and QDs even at high excess photon energies. We find that the MEG efficiency increases by at least 2 in PbSe QDs compared to bulk PbSe. We also analyze MEG based upon a competition between producing multiple excitons and hot-carrier cooling and find the competition between cooling and multiplication increases by a factor of 3 in PbSe QDs compared to bulk PbSe. I will present detailed balance calculations that show power conversion efficiencies in QD solar cells exhibiting MEG can exceed conversion efficiencies of their bulk counterparts, especially if the MEG threshold energy can be reduced toward twice the QD bandgap energy. For wide spread solar energy utilization, increasing the power conversion efficiency over the Shockley - Queissar limit remains an important and significant scientific challenge. To achieve the maximum benefit of MEG in solar energy conversion the MEG efficiency must be improved since we show the threshold photon energy and MEG efficiency are mathematically related. If time permits I will also review QD-solar cells based on Pb-salt quantum dots as the active element and discuss future directions and basic research challenges that need to be met in order to produce highly efficient solar energy conversion based on quantum dots.
4:00 PM - **B2.4
Assessment of Carrier Multiplication in Bulk PbS and PbSe.
Joep Pijpers 1 2 , Ronald Ulbricht 2 , Klaas Jan Tielrooij 2 , Anna Osherov 3 , Yuval Golan 3 , Guy Allan 4 , Christophe Delerue 4 , Mischa Bonn 2
1 , MIT, Cambridge, Massachusetts, United States, 2 , FOM-AMOLF, Amsterdam Netherlands, 3 , Ben-Gurion University of the Negev, Beer-Sheva Israel, 4 , ISEN-IEMN, Lille France
Show AbstractThe possibility to generate, in semiconductor materials, multiple charge carriers per photon (carrier multiplication, CM) is of fundamental interest, but also of great practical interest for future solar cells: one of the key factors limiting solar cell efficiency is that incident photons generate one electron-hole pair, irrespective of the photon energy, and excess photon energy is lost as heat. CM is known to occur in bulk semiconductors, but has been thought to be enhanced significantly in nanocrystalline materials like quantum dots, owing to their discrete energy levels and enhanced Coulomb interactions. We employ time-resolved TeraHertz spectroscopy to quantify – directly and with picosecond time resolution – the carrier yield per photon in several materials. We find that the CM factor (i.e., number of generated photons per absorbed photon) at a given photon energy is higher in bulk PbSe and PbS than in QDs of the same materials, but that the energy efficiency (the relative fraction of the photon energy that is transformed into excitons rather than heat) is higher in QDs. Measured CM factors in bulk materials and in quantum dots are reproduced quantitatively using tight binding calculations, which indicate that the reduced CM factors in quantum dots can be ascribed to the reduced density of states in these structures. We discuss the consequences of our findings for solar energy conversion: for the same ~1.2 eV band gap, CM is more efficient in PbSe QDs than in bulk silicon. Nonetheless, we demonstrate that the efficiency of solar cells based on PbSe QDs is not significantly enhanced by CM compared to a bulk silicon-based device.
4:30 PM - B2.5
CdSe Quantum Dot Sensitized Solar Cell with ~100% Internal Quantum Efficiency.
Milan Sykora 1 , Nobuhiro Fuke 2 , Laura Hoch 1 , Alexey Koposov 1 , Virginia Manner 1 , Atsui Fukui 2 , Naoki Koide 2 , Hiroyuki Katayama 2
1 Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Solar Systems Development Group, Sharp Corporation, Nara Japan
Show AbstractPhotoelectrochemical cells (PECs) based on a mesoporous nanocrystaline TiO2 film (TiO2 film) sensitized with organic or organometallic dyes have been studied intensely for the past twenty years as a potential low cost alternative to more traditional, solid state photovoltaics. Significant progress has been made in optimization of the components of the dye sensitized solar cell (DSSC) with highest reported efficiencies currently exceeding 11%. As part of search for new approaches to further improvement in efficiency over past several years, a number of research groups reported studies of PECs in which the sensitizing dyes are substituted with nanocrystals (NCs) of narrow band gap semiconductors, such as InP, CdS, CdSe, CdTe, PbS and InAs. In these studies it was demonstrated that semiconductor NCs can function as efficient sensitizers across a broad spectral range from the visible to mid-infrared, and offer advantages such as the tunability of optical properties and electronic structure by simple variation in NC size, while retaining the appeal of low-cost fabrication.In the presented work we will discuss the effect of NC surface passivation on the performance of a PEC consisting of a photoanode prepared by direct deposition of CdSe NCs onto a TiO2 film, aqueous Na2S or Li2S electrolyte and a Pt counter electrode. We show that light harvesting efficiency (LHE) of the NC/TiO2 film photoanode is significantly enhanced when the NQD surface passivation is changed from tri-n-octylphosphine oxide (TOPO) to n-butylamine (BA). In the PEC the use of NCs with a shorter passivating ligand, BA, leads to a significant enhancement in both the electron injection efficiency at the NC/TiO2 interface and charge collection efficiency at the NC/electrolyte interface, with the latter attributed mostly to a more efficient diffusion of the electrolyte through the pores of the photoanode. We show that utilizing BA capped NCs and aqueous Li2S as an electrolyte, it is possible to achieve ~100% internal quantum efficiency of photon-to-electron conversion, matching the performance of dye sensitized solar cells.
4:45 PM - B2.6
Quantum Dot Sensitization of Titanium Dioxide Crystals Resulting in Multiple Exciton Collection.
Bruce Parkinson 1 , Justin Sambur 1 , Thomas Novet 2
1 Chemistry, University of Wyoming, Laramie, Wyoming, United States, 2 , Voxtel Inc, Beaverton, Oregon, United States
Show AbstractSensitization of mesoporous nanocrystalline TiO2 solar cells with quantum confined semiconductor nanocrystals (QDs) has some advantages over organic dyes or inorganic complex sensitizers, yet the reported efficiencies of laboratory devices are not currently competitive with dye sensitized cells. Several methods previously utilized to bind CdSe QDs to the mesoporous TiO2 films were investigated using low index faces of both anatase and rutile TiO2 polytypes as model systems. The in situ ligand exchange method, where 3-mercaptopropionic acid (MPA) covered TiO2 crystal surfaces are treated with trioctylphosphine (TOP)/trioctylphosphine oxide (TOPO) TOP/TOPO-capped CdSe QDs, resulted in very irreproducible and usually low sensitized photocurrents. The ex situ ligand exchange method, whereby MPA-capped QDs are synthesized and directly adsorbed onto bare TiO2 single crystals, resulted in both reproducible sensitized photocurrents and surface coverages that are verified with atomic force microscopy (AFM). Multiple exciton generation, the creation of two electron hole pairs from one high energy photon, is well established in bulk semiconductors but the efficiency and magnitude of this effect remains controversial in quantum confined systems like semiconductor nanocrystals. With the knowledge obtained from the CdSe QDs we extended this work to a photoelectrochemical system composed of PbS nanocrystals chemically bound to TiO2 single crystals to demonstrate the collection of photocurrents with quantum yields greater than one. The electronic coupling and favorable energy level alignment of PbS nanocrystals to bulk TiO2 energy levels make this a suitable system to quickly extract multiple excitons before they recombine. Our results have implications for increasing the efficiency of photovoltaic devices by avoiding losses due to the thermalization of photogenerated carriers.
5:00 PM - B2.7
Giant Exciton-exciton Attraction in Nanocrystal Quantum Dots.
Stuart Stubbs 1 , Samantha Hardman 1 , Do-Kyeong Ko 3 , Wendy Flavell 1 , Mohammad Afzaal 2 4 , Paul O'Brien 2 , David Binks 1
1 School of Physics and Astronomy, The University of Manchester, Manchester United Kingdom, 3 Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 2 School of Chemistry, The University of Manchester, Manchester United Kingdom, 4 Center of Research Excellence in Renewable Energy of the Research Institute, King Fahd University of Petroleum and Minerals, Dhahran Saudi Arabia
Show AbstractMultiple exciton generation (MEG) is a process in nanocrystal quantum dots (NQDs) by which the energy in excess of the band gap of an absorbed photon is used to create multiple excitons rather than being wasted as heat [1]. This process has the potential to increase the efficiency of solar cells based on NQDs beyond the Shockley-Queisser limit. The extent of this increase, however, is critically dependent upon the threshold photon energy at which MEG occurs, which should be as low as possible for the greatest gain in efficiency. It has been suggested previously that giant exciton-exciton attraction could be used to reduce the threshold for MEG [2]. However later theoretical work anticipated that only giant repulsion would be possible in NQDs comprising a core and a single shell [3]. Here we report giant exciton-exciton attraction in NQDs consisting of a core and two shells. Ultrafast transient absorption spectroscopy was used to investigate a quasi-type II NQD comprising a CdSe core, a CdTe inner shell and a CdS outer shell. A large transient red shift of the absorption edge was observed corresponding to an attractive exciton-exciton interaction energy of -96±7 meV which is an order of magnitude greater than previously found in NQDs. Attractive interactions of this magnitude have the potential to both reduce the threshold for and increase the slope efficiency of MEG, thus demonstrating the possibility of engineering NQDs to enhance MEG. References1M.C. Beard and R. J. Ellingson, Laser & Photonics Review 2, 377 (2008).2R. D. Schaller, J. M. Pietryga, and V. I. Klimov, Nano Letters 7, 3469 (2007).3A. Piryatinski, S. A. Ivanov, S. Tretiak, and V. I. Klimov, Nano Letters 7, 108 (2006).
5:15 PM - B2.8
Controlling Charge Transfer from Lead-salt Nanocrystals.
Byung Ryool Hyun 1 , Adam Bartnik 1 , Jin-Kyun Lee 2 , Hiroaki Imoto 4 , Liangfeng Sun 1 , David Stachnik 1 , Joshua Choi 3 , Yoshiki Chujo 4 , Tobias Hanrath 3 , Christopher Ober 2 , Frank Wise 1
1 Applied physics, Cornell University, Ithaca, New York, United States, 2 Department of Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 4 Department of Polymer Chemistry, Kyoto University, Kyoto Japan, 3 School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York, United States
Show AbstractGuided by Marcus theory, the role of the solvent reorganization energy and the electronic coupling strength in charge transfer (CT) from PbS nanocrystals (NCs) to both molecules and titanium dioxide (TiO2) nanoparticles are investigated in different solvents and with different linker molecules. We find that the CT rate from PbS NCs to molecules increases dramatically with solvent dielectric constant. The choice of solvent allows significant control of the CT process. As an important example, we find that PbS NCs dispersed in water exhibit CT rates 1000 times higher than the same NCs in organic solvent. These trends are accounted for by a modified Marcus theory that incorporates only static dielectric effects.[1] However, in a NC-TiO2 system, the CT rate is only weakly dependent on solvent dielectric constant, in contrast to the strong dependence in NC-molecule system. This is ascribed to the larger size of the acceptor in this system, and is accounted for by the same modified Marcus theory. It was known that different molecular anchor groups lead to different electronic coupling strengths between molecules and metal oxide nanoparticles[2-3]. In the NC-TiO2 system, the electronic coupling strength is investigated by changing the length, aliphatic and aromatic structure, and anchor groups of linker molecules. A shorter linker leads to faster electron transfer, while surprisingly molecules with the same length induce similar electron transfer rates regardless of their chemical structure. In contrast, the ET rate is greatly modified by using linker molecules with different anchor groups, which is attributed to the dependence of the electronic coupling strength on the anchor groups. The effects of solvent and linker molecule on CT between nanocrystals and acceptors in the frame of Marcus model are discussed and presented. (1)Hyun, B.-R.; Bartnik, A. C.; Lee, J.-K.; Imoto, H.; Sun, L.; Choi, J. J.; Chujo, Y.; Hanrath, T.; Ober, C. K.; Wise, F. W. Nano Lett. 2010, 10, 318.(2)Ernstorfer, R.; Gundlach, L.; Felber, S.; Storck, W.; Eichberger, R.; Willig, F. J. Phys. Chem. B 2006, 110, 25383.(3)She, C.; Guo, J.; Irle, S.; Morokuma, K.; Mohler, D. L.; Zabri, H.; Odobel, F.; Youm, K.-T.; Liu, F.; Hupp, J. T.; Lian, T. J. Phys. Chem. A 2007, 111, 6832.
5:30 PM - B2.9
Optical Spectroscopy and Dynamics of Nanoscale Spatially Indirect Excitons in Colloidal Heteronanocrystals.
Celso De Mello Donega 1
1 Debye Institute for Nanomaterials Science, Utrecht University, Utrecht Netherlands
Show AbstractSemiconductor heterostructures can show different carrier localization regimes after photoexcitation, depending on the energy offsets between the valence and conduction band levels of the materials that are combined at the heterointerface. In type-I heterostructures both carriers are primarily localized in the same material, whereas in type-II heterostructures electrons and holes are spatially separated, creating a spatially indirect exciton. Heteronanostructures offer additional degrees of control over the indirect exciton properties. The relative energy offsets in heteronanocrystals (HNCs) can be tailored by manipulating the composition, size and shape of each component. This offers the possibility of directly controlling the electron-hole overlap, and gives rise to an intermediate localization regime, in which one carrier is confined in one of the components, while the other is delocalized over the whole HNC. This flexibility in tailoring the optoelectronic properties of colloidal HNCs has important consequences for a number of technologies. Understanding the size-, shape- and composition-dependence of the optical properties of spatially indirect excitons in HNCs is therefore of great interest. Despite the advances made in recent years, a comprehensive fundamental understanding of nanoscale spatially indirect excitons has yet to emerge. In our group, we have systematically investigated the evolution of the optical properties of nanoscale spatially indirect excitons as a function of the size, shape, and composition of the HNC, using highly efficient CdTe/CdSe colloidal HNCs as a model system. Emphasis is given to quantitative aspects, such as the absorption cross section of the lowest energy exciton transition, the Stokes shift, transition linewidths, and the exciton radiative lifetime. Other HNCs, such as CdSe/CdS, CdSe/(Cd,Zn)S/ZnS, ZnTe/CdSe and PbSe/CdSe, are also studied. The results of our investigations provide novel fundamental insights into nanoscale spatially indirect excitons. This knowledge will impact on the design of colloidal HNCs for optoelectronic applications. Solar cells in particular may benefit from Type-II HNCs in several ways. First, the longer indirect exciton lifetimes and the (partial) charge carrier separation facilitate carrier extraction and should enhance the efficiency of photovoltaic devices. Second, Auger recombination rates and hot carrier relaxation rates should decrease and biexciton interactions become repulsive, making it possible to increase the conversion efficiency. Finally, Type-II HNCs are promising light harvesters and converters in solar concentrators due to their large absorption cross-sections in the UV-Visible range associated with small emission reabsorption cross-sections.
5:45 PM - B2.10
A Novel Approach to Ligand Exchange on Lead Sulfide Nanocrystal Quantum Dot and Studies of Inter-quantum Dots Energy/Charge Transfer.
Zachary Lingley 1 , Siyuan Lu 2 , Anupam Madhukar 1 2
1 Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California, United States, 2 Physics and Astronomy, University of Southern California, Los Angeles, California, United States
Show AbstractWe present a new route to ligand exchange in solution on lead sulfide nanocrystal quantum dots (PbS QDs) and investigation of energy/charge transfer between quantum dots made possible by this approach to controlling the ligand length and inter quantum dot separation without decreasing PbS QD quantum efficiency. The conventional approach to ligand exchange is to heat nanocrystals quantum dots in an excess of new ligand, but it generally results in a reduction in the quantum efficiency of core-only quantum dots. In the new approach, Pb2+ cations are bound to the new ligands to form cation-ligand exchange units, and PbS QDs are exposed to an appropriate solution of such exchange units at elevated temperature to promote reactions that replace lead cations on the surface of a PbS QDs and initial ligands with the new exchange units. Infrared spectroscopy is used to verify that our cation-ligand exchange procedure results in essentially complete removal of the initial ligands. Photoluminescence measurements show that our approach results in negligible reduction of quantum efficiency in solution as compared to that of the initial PbS QDs. Using such methods, we create a series of quantum dots with high quantum efficiency and varying ligand length and thereby varying inter quantum dot spacing when drop cast into densely packed assemblies. The success of our new cation-ligand exchange method in preserving quantum efficiency enables a systematic investigation of inter quantum dot energy and charge transfer. Time resolved photoluminescence measurement based findings of transfer rates as function of quantum dot separation are presented here. Tuning of inter quantum dot interactions by this new ligand exchange approach is of significance to the synthesis of PbS QDs suitable for use in quantum dot based solar cells. This work is supported by AFOSR grant number FA9550-08-1-0146.
Symposium Organizers
Yalin Lu University of Colorado
MarkT. Lusk University of Colorado
JohnM. Merrill Air Force Research Laboratory
Sheila Bailey NASA Glenn Research Center
Alberto Franceschetti National Renewable Energy Laboratory
Symposium Support
National Renewable Energy Laboratory
Naval Research Laboratory, Solid State Devices Branch
B5: Poster Session: Next Generation Photovoltaics V
Session Chairs
Wednesday PM, April 27, 2011
Salons 7-9 (Marriott)
B3: Next Generation Photovoltaics III: Emerging Strategies I
Session Chairs
Wednesday PM, April 27, 2011
Room 2001 (Moscone West)
9:30 AM - B3.1
2.8 % Efficient Hybrid Solar Cells Based on Fully Polymer Infiltrated ZnO Nanorods.
Linny Baeten 1 , Bert Conings 2 , Hans-Gerd Boyen 2 , Jan D'Haen 2 3 , An Hardy 1 3 , Jean Manca 2 3 , Marlies Van Bael 1 3
1 Institute for Materials Research, Inorganic and Physical Chemistry, Hasselt University, Diepenbeek Belgium, 2 Institute for Materials Research, Materials Physics, Hasselt University, Diepenbeek Belgium, 3 IMEC vzw, IMOMEC, Diepenbeek Belgium
Show AbstractPhotovoltaic cells have attracted great scientific interest as an alternative and clean energy source. Promising candidates for third generation photovoltaics are hybrid solar cells consisting of metal oxides with controlled nanostructure, combined with hole conducting polymers. Due to the short exciton diffusion length in hole conducting conjugated polymers (~10 nm), a nanoscale intermixing with the electron conducting material is crucial.In this work, hybrid photovoltaic solar cells consisting of hydrothermally grown ZnO nanorods which are infiltrated with poly-(3-hexylthiophene-2,5-diyl) (P3HT) have been prepared and analyzed by a combination of XPS depth profiling and electrical characterization.The intimate contact ensures adequate charge transfer, while additionally, the charge transport is improved by the one-dimensional structure of the grown nanorods thus improving device performance. Until now, hybrid solar cells based on ZnO with a controlled nanostructured morphology that were post-infiltrated with P3HT have reached maximum efficiencies up to 0.55 %.[1]Results of the current study show that device efficiency can be improved by post-annealing at the melting temperature of the polymer, resulting in an enhanced polymer crystallinity. However, prolonged annealing times diminish device performance most likely due to unfavourable ordering of the polymer chains onto the nanorods.Here, a record efficiency of 2.8%, which is five times higher compared to current literature, is achieved [2] by tuning morphological parameters and optimizing the ZnO/polymer interfacial area in combination with a control of leakage currents by means of appropriate blocking layers for electrons and holes.[1] Y. Lee, M. Lloyd, D.C. Olson, R.K. Grubbs, P. Lu, R.J. Davis, J.A. Voigt, J. Hsu, J. Phys. Chem. C 2009, 113, 15778–15782.[2] L. Baeten, B. Conings, H.-G. Boyen, J. D’Haen, A. Hardy, J.V. Manca, M.K. Van Bael, “2.8 % efficient hybrid solar cells based on fully polymer infiltrated ZnO nanorods”, submitted
9:45 AM - **B3.2
Next Generation Photovoltaics.
Ryne Raffaelle 1
1 National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractThe solar cell industry has grown at an astonishingly high rate over the past decade. This growth has been both in what one could consider the “traditional” areas such as flat panel crystalline silicon arrays, as well as in “new” technologies such as thin film CdTe arrays on glass. A review will be presented of the past developments in the photovoltaic landscape the role of our National Center for Photovoltaics (NCPV) at NREL has played. The NCPV was created to enhance communication, catalyze strategic partnerships, and serve the PV industry as the place to come to access the wealth of knowledge and facilities within the U.S. Department of Energy system. Recent highlights of the NCPV’s scientific and technical accomplishments and its success in fulfilling its original mission will be given. Also, an overview of the current PV research and development programs at NREL, as well as a vision for the future PV and what is needed to accelerate PV as a viable energy option in the U.S. will be provided.
10:15 AM - B3.3
Three-Dimensional Photovoltaics: Technology, Materials, and Devices.
Marco Bernardi 1 , Jin Wan 2 , Jeffrey Grossman 1
1 Dept. of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Dept. of Mathematics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThe concept of three-dimensional photovoltaic (3DPV) structures is introduced, motivated by the potential to absorb more light and generate more power than a flat panel of the same area footprint, to lower the installation costs and to favor the adoption of ultra-thin, cheap and flexible semiconductors. We used a genetic algorithm to optimize the energy production in a day for arbitrarily shaped 3D solar cells confined to a given area footprint and total volume[1]. Our simulations demonstrate that the performance of 3DPV structures scales linearly with height, leading to volumetric energy conversion, and provides power fairly evenly throughout the day. We show that optimal 3D structures are not simple box-like shapes, and that design attributes such as reflectivity could be optimized using three-dimensionality. In addition, we show recent calculations on the variability of optimal shapes at different places of the globe and of combination of materials with different optical properties, including combinations of mirrors and absorbing parts within the same structure, and of semiconductors of different energy gaps and reflectivity. Examples of possible integration of 3DPV technology in buildings and residential installations will also be shown. [1] Myers, B.; Bernardi, M.; Grossman, J. C. Three-Dimensional Photovoltaics. Appl. Phys. Lett. 2010, 96, 071902.
10:30 AM - B3.4
Engineered Optical Absorption of Nano/Micro-pillar Arrays for Efficient Photovoltaics.
Siu-fung Leung 1 , Yunze Long 1 , Qingfeng Lin 1 , Zhiyong Fan 1
1 Electronic and Computer Engineering, Hong Kong University of Science and Technology, Hong Kong Hong Kong
Show AbstractMaterials made of nano/micro-structures have unique optical properties as compared to their thin film and bulk counterparts, such as tunable reflectance, transmission and absorption. These structurally engineered properties will lead to novel photon managing device with potentially significant improvement for photovoltiacs. Specifically, engineered three-dimensional (3-D) arrays of nano/micro-pillars have been fabricated in our work with materials including Ge, Si and CdS, with pillar diameter close to or smaller than optical wavelength. Systematic optical property investigations on the 3-D structures have been performed experimentally assisted with optical simulations. It is found that for pillars with uniform diameter, increasing material filling ratio leads to enhancement of the reflectance while simultaneously decreasing the transmittance, with the absorbance efficiency showing a strong diameter dependency. To enhance the broadband optical absorption efficiency, a novel dual-diameter nanopillar structure is realized with a small diameter tip for minimal reflectance and a large diameter base for maximal absorption of the penetrating photons. As the result, the fabricated dual-diameter nanopillar arrays with finite thickness exhibits an impressive absorbance close to unity. These results enable a viable and convenient route toward 3-D structured high performance photonic devices.
10:45 AM - B3.5
Hole and Electron Transport Layers in PbS QD Hetero-junction Solar Cells.
Jianbo Gao 1 2 , Joseph Luther 1 , Randy Ellingson 1 2 , Arthur Nozik 1 3 , Matthew Beard 1
1 , NREL, Golden, Colorado, United States, 2 Physics , University of Toledo, Toledo, Ohio, United States, 3 Physics , University of Colorado,Boulder, Boulder, Colorado, United States
Show AbstractThe power conversion efficiency of quantum dot solar cell(QDSC) can be affected by the following factors: a) the photogeneration efficiency of electron-hole pairs, b) the hole and electron collection efficiency by electrodes. Electron-hole pair photogeneration efficiency can be more than 100% if absorbed photon energy is larger than QD band gap due to multiple exciton generation (MEG) effect. However, the realization of MEG effect in QDSC needs efficient collection of holes and electrons to electrodes. We recently reported an NREL certified ~3% efficient device with structure of ITO/ZnO nanocrystals(NC)/PbS QD/Au. The device is remarkably stable in air without encapsulation for more than 1000 hours. Therefore, in this study we focus on devices with structure of ITO/ZnO NC/PbS QD/metal fabricated in air. By introducing an electron transport layer at interface between ITO and ZnO NC thin films, and a hole transport layer at interface between PbS QD and top metal contacts, we are able to achieve efficient devices with enhanced open-circuit voltage and more than 400 hours lifetime under simulated solar illumination. Furthermore, this study may provide a pathway towards highly efficient devices using MEG effect.
11:30 AM - B3.6
Resonance Shifting: A Simple, All-optical Method for Circumventing the Reabsorption Problem in Luminescent Concentrators.
Noel Giebink 1 , Gary Wiederrecht 1 , Michael Wasielewski 1 2
1 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States, 2 Dept. of Chemistry, Northwestern University, Evanston, Illinois, United States
Show AbstractLuminescent concentrators (LSCs) were developed over three decades ago as a simple route to obtain high concentration ratio for photovoltaic cells without tracking the sun. These devices traditionally consist of a transparent slab embedded with a chromophore that absorbs sunlight and re-emits it back into the slab, where it is trapped by total internal reflection, transported through the slab, and subsequently absorbed by photovoltaic cells attached to the edges. In principle, high concentration ratios >100 are possible for commonly used chromophores. In practice, however, there is typically an overlap between the chromophore absorption and emission spectra that, although small, ultimately leads to unacceptable reabsorption losses, limiting the concentration ratio to ~10 and hence the utility of LSCs to date.We introduce a simple, all-optical means of avoiding reabsorption loss by ‘resonance shifting’ from a bilayer cavity that consists of an absorber/emitter waveguide lying upon a low refractive index layer supported by a transparent substrate. Emission into discrete modes of the slab waveguide formed by the absorber/emitter layer is evanescently coupled into the transparent substrate at sharply defined angles. Upon reflection from the substrate bottom, this light returns to a new, laterally displaced position where the cavity thickness has been changed slightly, leading to a non-resonant and near-unity reflectivity (i.e. no reabsorption). By appropriately varying the cavity thickness across the lateral extent of the device, the original absorption resonance is continually avoided at each bounce, allowing for extremely low propagation loss to the substrate edges and hence an increase in the optical concentration ratio. We validate this concept for absorber/emitter layers composed of both a typical luminescent polymer and inorganic semiconductor nanocrystals, demonstrating near-lossless propagation in each case.
11:45 AM - B3.7
Light Absorption Enhancement in Ultra-thin Film Solar Cells Using Whispering Gallery Modes in Dielectric Nanosphere Arrays.
Jonathan Grandidier 1 , Dennis Callahan 1 , Augustin Mihi 2 , Jeremy Munday 1 , Michael Deceglie 1 , Paul Braun 2 , Harry Atwater 1
1 Applied physics, California Institute of Technology, Pasadena, California, United States, 2 Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana Champaign, Illinois, United States
Show AbstractLight trapping is a critical requirement in thin film photovoltaics, and dielectric texturing is a viable method to induce light trapping, but thin film device quality often suffers upon direct texturing of the semiconductor active material. Thus it is desirable to develop a design method in which textured dielectric layers provide for light trapping on smooth planar thin film cells. We propose here an approach for coupling light into smooth untextured thin film solar cells of uniform thickness using periodic arrangements of resonant dielectric nanospheres deposited as a continuous film on top of a thin cell. Freely propagating sunlight can be diffractively coupled and transformed into several guided modes within the array of wavelength scale dielectric spheres. Incident optical power is then transferred to the thin film cell by leaky mode coupling into the cell thin absorber layer. It is shown that guided whispering gallery modes in the spheres can be coupled into particular modes of the solar cell and significantly enhance its efficiency by increasing the fraction of incident light absorbed. We numerically demonstrate this enhancement using full field finite difference time domain (FDTD) simulations of a 600nm SiO2 nanosphere array above a 100nm thin film amorphous Si (a-Si) solar cell structure featuring back reflector and transparent conducting oxide layers. The incoupling element in this design has advantages over other schemes as it is a lossless dielectric material and its spherical symmetry naturally accepts a wide angle of incidence range. Moreover, analytical models show that for SiO2 nanospheres of a given dielectric material, a large number of resonant modes can be supported which can give rise to a 15% absorption enhancement in the a-Si absorber layer at several wavelengths between 300nm and 840nm. Also, the SiO2 nanosphere array can be fabricated using simple, well developed self assembly methods and is easily scalable without the need for lithography or patterning. We will report results of full angle dependent transmission and reflection measurements done using an integrating sphere as well as photoconductivity measurements with a solar simulator. This concept can be easily extended to other thin film solar cell materials, such as gallium arsenide or copper indium gallium diselenide (CIGS) which are also considered.
12:00 PM - **B3.8
Multiple Growths of III-V Solar Cells on a Single Substrate Using Eitaxial Liftoff.
Stephen Forrest 1 , Kyusang Lee 1 , Jeramy Zimmerman 1 , Kuen Ting Shiu 1
1 Physics, Electrical and Computer Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractGroup III-V photovoltaic cells generally exhibit high power conversion efficiencies (ηp); however their applications are limited by wafer cost. One possible solution to reduce the cost of III-V thin-film solar cells is to non-destructively re-use the original substrate by removing the active region of a device structure from the parent substrate via epitaxial lift-off (ELO). Wafer reuse has been limited by the increased surface roughness caused by wet-chemical etching during the ELO process. In this work, we employ epitaxial protection layers that surround the AlAs sacrificial layer. These protection layers leave both the substrate and the lifted-off thin film with high quality regrowth interfaces[1]. Atomic force microscope (AFM) images show that the root-mean-square (RMS) roughness of the III-V (i.e. InP and GaAs) surface after protection layer removal following both the first and the second growths can be less than that of the original epi-ready substrate. Furthermore, by combining ELO with cold welding to transfer the active region to a thin, flexible plastic substrate, we demonstrate re-use of the original substrate by fabricating efficient InP and GaAs thin-film solar cells without loss of performance. We consider the cost of solar energy conversion based on multiple reuse of the seed wafer, and find that GaAs cells can be economically competitive with thin film and Si solar cell technologies if the wafer can be reused a sufficient number of times to convert the substrate from a materials to a capital cost.[1]K. Lee, K. T. Shiu, J. Zimmerman, and S. R. Forrest, "Multiple growths of epitaxial lift-off solar cells from a single InP substrate," Appl. Phys. Lett., vol. 97, p. 101107, 2010.
12:30 PM - B3.9
Fundamental Limits of Core-shell Nanowire Photovoltaics and Their Assembly Towards Efficient Ultra-thin Solar Cells.
Thomas Kempa 1 , James Cahoon 1 , Sun-Kyung Kim 1 , Robert Day 1 , Charles Lieber 1 2
1 Department of Chemistry, Harvard University, Cambridge, Massachusetts, United States, 2 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Show AbstractCore-shell nanowires are a versatile platform on which to realize single standalone photovoltaic devices whose tunable electronic and optical properties can be exploited for emerging next-generation solar cell concepts. We report in situ synthesis of silicon core-shell nanowires with highly crystalline shells and well defined diode geometries. Optimized devices yield open-circuit voltages as high as 0.50 V and fill-factors greater than 72%, even for nanowires with diameters as small as 200 nm. Notably, single nanowire devices exhibit current densities double the expectation for equivalent bulk films and 1-sun power conversion efficiencies of up to 6%. Furthermore, wavelength-dependent single nanowire photocurrent spectra reveal tunable optical resonances within the nanowires, and quantitative analyses show that absolute external quantum efficiencies approach values equal to or greater than unity. Simulations and measurements further suggest a unique approach for enhancing efficiency through assembly of designed nanowire elements. We demonstrate directed assembly and parallel interconnection of nanowires in planar geometries and vertical stacks, and show that this can yield cells with efficiencies in excess of 8%. Simulations and experiments further suggest that sub-micron thick assemblies of single nanowire elements can achieve efficiencies in excess of those obtained in thin-film solar cells. Together, these results suggest a new paradigm for development of next-generation, ultra-thin solar cells.
12:45 PM - B3.10
A Facile and Reliable Method to Build Highly Efficient Dye Sensitized Solar Cells (DSSC) Using Nanowire/Nanoparticle Composite Electrodes.
Dominik Koll 1 , Yong-Gun Lee 2 , Alexander Birkel 1 , Xavier Van Meerbek 1 , Stefan Frank 1 , Kookheon Char 2 , Wolfgang Tremel 1
1 , Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg - University of Mainz, Mainz, Rhineland Palatinate, Germany, 2 , School of Chemical and Biological Engineering, Seoul National University , Seoul Korea (the Republic of)
Show AbstractTiO2 – nanowires are commonly synthesized via hydrothermal treatment of TiO2-P25 in alkaline media [1] resulting in a sodium titanate as the reaction product. The latter is transformed into a corresponding hydrogen titanate by an acidic ion exchange, which can be followed by a thermal dehydration to obtain TiO2 nanowires. The morphology of the nanowires does not change during these processing steps. Depending on the calcination conditions, the resulting crystallographic phase of the TiO2 nanowires can be selectively tuned. The hydrothermal synthesis of the nanowires was performed using a microwave reactor, which represents a faster, less energy intense method, as compared to conventional hydrothermal procedures. In a second step a facile and reliable way to prepare a so called “semiconducting paste” was developed. The preparation of the semiconducting paste is a very important step in the device fabrication of DSSCs. Compared to the established procedure [2], the pastes used herein require less processing steps and only a small amount of starting materials. As described above, three kinds of nanowires, i.e. sodium titanate-, hydrogen titanate- and TiO2-nanowires, were used in this study. In order to obtain composite electrodes, the nanowires were mixed with TiO2-P25 nanoparticles, before paste preparation. The amount of nanowires in the pastes was varied to find the ideal ratio of nanoparticles to nanowires. When used as a composite electrode material in combination with TiO2 nanoparticles, anisotropic nanostructures of the different polymorphs of TiO2, like rods or in particular wires, are known to increase the performance of DSSC’s [3-4]. Commonly, TiO2 nanowires are used as material for these composite electrodes. In order to avoid the calcination step, hydrogen titanate nanowires were directly incorporated into the electrode material. While the pastes are sintered to remove the organic material in the paste, these nanowires are transformed from hydrogen titanate into TiO2. Additionally, sodium titanate nanowires were tested as an additive to electrodes for DSSCs, as they represent a possible substitute for TiO2 nanowires. References[1] D.V. Bavykin, J.M. Friedrich, F.C. Walsh, Adv. Mater., 2006, 18, 2807-2824[2] S. Ito, T. Kitamura, Y. Wada, S. Yanagida, Solar Energy Materials and Solar Cells, 2003, 76, 3-136[3] J. H. Yoon, S.R. Jang, R. Vittal, J. Lee, K.J. Kim, Journal of Photochemistry and Photobiology A: Chemistry, 2006, 180, 184-188[4] B. Tan, Y. Wu, J. Phys. Chem. B, 2006, 110, 15932-15938
B4: Next Generation Photovoltaics IV: Emerging Strategies II
Session Chairs
Wednesday PM, April 27, 2011
Room 2001 (Moscone West)
2:30 PM - B4.1
Exploitating Interface States in Photovoltaics: First-principles Design of GaAs-based Superlattices.
Alexie Kolpak 1 , Jeffrey Grossman 1
1 Department of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractWe use first-principles density functional theory to design a novel photovoltaic system that exploits a common yet usually undesirable property of heterointerfaces: interface states. We show that one can create parallel electron and hole conducting channels, separated by a nanoscale photo-active material, via atomic-layer thick modifications at the interfaces of GaAs-based superlattices. The nanoscale periodicity also potentially allows one to take advantage of the polar field in the GaAs to drive electron-hole separation and reduce recombination. Our work suggests a means by which to decrease the cost of GaAs photovoltaics by eliminating the need for high purity GaAs films.
2:45 PM - **B4.2
Hot Carrier Solar Cells: An Overview of Recent Progress.
Martin Green 1
1 , University of New South Wales, Sydney, New South Wales, Australia
Show Abstract Hot carrier solar cells offer one of the most promising options for high performance “third generation” photovoltaic devices. For successful operation, these need to be thin, strongly absorbing, radiatively efficient devices in a simple 2-terminal configuration. Nonetheless, they offer potential performance close to the maximum possible for solar conversion, equivalent to a multi-cell stack of six or more tandem cells possibly without some of the limitations, such as spectral sensitivity. However, hot carrier cells offer some quite fundamental challenges in implementation. The two key challenges in implementing relate to the two features that are not required in conventional cells: (1) energy selective contacts to interface between the “hot” carrier concentrations in the absorber and the “cold” carriers in the outside world; (2) an absorber designed to allow carrier collection before the photogenerated carriers relax back to the band edges.Resonant tunnelling through quantum dots is one preferred solution for the implementation of selective energy contacts. A development of this idea is to additionally incorporate thin semiconductors into the contact, the top one transparent (TCO). This has the advantage of blocking transport at energies corresponding to the semiconductor bandgap.Generation of “hot” optical phonons by creating a phonon relaxation bottleneck is the approach showing most promising for slowing photogenerated carrier energy loss. By suppression of the key Klemens mechanism involving the disassociation of high energy optical phonons into two acoustic phonons, a hot optical phonon population is created which slows carrier energy loss to this population. III-V compound semiconductors with a large difference in the III and V elemental masses have a large gap between the optical and phonon branches that suppresses this relaxation. Our modelling work suggests quantum dot superlattices can be designed to posses similar properties. Experimental measurements of energy relaxation rates in both III-V and engineered materials will be reported.Our modelling work suggests that natural materials such as some bulk III-V materials already have sufficiently suppressed optical phonon relaxation rates to allow phonon bottlenecks to be created under concentrated sunlight operation. This suggests hot carrier effects can be demonstrated under concentration in the near term. The paper will discuss the challenges in hot carrier design and operation, progress in developing of appropriate designs and in synthesizing new materials and experimental results to date.
3:15 PM - B4.3
Wide Band Gap InAlAs Solar Cells for a New Multijunction Design.
Marina Leite 1 , Robyn Woo 2 , William Hong 2 , Daniel Law 2 , Harry Atwater 1
1 , CALTECH, Pasadena, California, United States, 2 , Spectrolab, Inc., Sylmar, California, United States
Show AbstractHigh efficiency photovoltaics for both space and terrestrial applications has recently attracted considerable interest to III-V semiconductors multijunction solar cells, which are usually based on the GaInP/GaAs/Ge system due to the well known properties of these lattice-matched materials. Nevertheless, the use lattice-matched layers grown on GaAs substrates leads to poor current matching between subcells, compromising series-connected multijunction cell efficiency. To enable excellent current matching between subcells and also to work with lattice-matched epitaxial layers, we propose here a novel InP-based triple junction solar cell design, formed using a combination of lattice-matched InAlAs/InGaAsP/InGaAs alloys. Device simulations indicate that the proposed multijunction design can achieve over 46% efficiency at 100-suns illumination. The overall cost for this type of multijunction cell can be reduced by reuse of the InP substrates, via direct wafer bonding or epitaxial lift-off techniques. A key building block for an InP-based three junction high efficiency solar cell is a high quality InAlAs top subcell. To that end, we investigated InxAl1-xAs alloy layer synthesis by metalorganic vapor phase epitaxy to fabricate a single junction InxAl1-xAs top subcell, in this case lattice-matched to InP. A high band gap In0.35Al0.65As top window was used to reduce surface recombination and increase light absorption within the In0.52Al0.48As p-n absorber layer (Eg = 1.47 eV), improving cell efficiency. The window layer lattice spacing of 5.80 Å, yields a 1.94 % tensile strain with respect to the In0.52Al0.48As layer and InP substrate. Despite strain, X-ray diffraction and transmission electron microscopy confirmed that the In0.35Al0.65As top window is coherently-strained and dislocation-free with respect to the absorber layer. Photoluminescence of In0.52Al0.48As layers showed good material quality and lifetime of over 200 picoseconds. Hall mobility was measured to be 667 cm2/Vs for Nd = 2.68 x 1018 cm-3 carrier density; therefore we estimate a lower bond on the minority carrier diffusion length of approximately 1 μm. In order to improve the top contact of the cell a thin InGaAs cap layer was used. We have fabricated In0.52Al0.48As solar cells lattice-matched to InP with efficiency higher than 14 %, open circuit voltage of Voc = 1V, Jsc = 19.3 mA/cm2 and maximum external quantum efficiency of 81 % under AM1.5 global illumination. The InGaAsP and InGaAs subcells were also fabricated and efficiencies of 8.0 and 9.2 % were achieved, respectively. The behavior of these three types of subcells in a two-terminal, series-connected configuration will be discussed. The successful fabrication of all independent subcells indicates the potential for a high efficiency InAlAs/InGaAsP/InGaAs triple junction cell.
3:30 PM - B4.4
Modeling Optical Confinement Geometry Photovoltaics.
Yuan Li 1 , Eric Peterson 1 , Wanyi Nie 1 , Jie Liu 1 , Wenxiao Huang 1 , Coffin Robert 1 , MacNeill Christopher 2 , David Carroll 1
1 Center for Nanotechnology and Molecular Materials, Department of Physics, Wake Forest University, Winston-Salem, North Carolina, United States, 2 Chemistry, Wake Forest University, Winston-Salem, North Carolina, United States
Show AbstractIn this work, we combine four basic models to describe photo-electronic performance in fiber-based solar cells (Optical Confinement Geometry Photovoltaics (OCGPV)): a ray tracing model for optical path in the fiber, a model for optical field distribution in thin films round the fiber surface, a model for polymer photo-conversion physics, and finally a model for an equivalent circuit for heterogeneous optical distribution within the fiber. First, we use a ray-tracing method to describe optical paths in leaky fiber waveguides with large diameters. From this approach, it can be predicted that light absorption is influenced by the coupling angle into the fiber, the fiber diameter and length, and the layers around the fiber. Secondly a transfer matrix formalism has been developed to analyze the distribution of optical energy in the thin films of fiber solar cell. For this approach we can show that the mode structure associated with a fiber-device leads to an overall shift and broadening of the external quantum efficiency (EQE) for any given polymer absorber. Thirdly, we examine the photo-physics of the absorption layer to reveal modifications in excitonic dynamics which impact overall device function. Specifically, fiber-based solar cells not only increase the optical path length, but they also modify thin film morphology to influence the carrier mobility and exciton recommendation rate. Finally, by regarding fiber-based cells as an integrated semiconductor device, the characteristics of its equivalent circuit, using a modification of the Shockley equation, can be understood. By combining these approaches for the fiber-based PV, output voltage, output current, inner resistances in homogeneous and heterogeneous optical distribution can be fully predicted.
3:45 PM - B4.5
Efficiency Enhancement of Bulk-heterojunction Hybrid Solar Cells towards 3% and Beyond.
Michael Krueger 1 2 , Yunfei Zhou 1 2 , Michael Eck 1 2
1 Freiburg Materials Research Centre (FMf), University of Freiburg, Freiburg Germany, 2 Institute for Microsystems Technology , University of Freiburg, Freiburg Germany
Show AbstractInorganic semiconductor nanocrystals (NCs) such as CdSe NCs, with tunable bandgaps and high intrinsic charge carrier mobilities can act as good electron acceptors and be incorporated into conjugated polymers to form bulk-heterojunction hybrid solar cells. Nevertheless their power conversion efficiencies (PCEs) are still lagging behind the PCEs of fullerene based devices. Here we report on the efficiency enhancement of CdSe NC based devices due to different postsynthetic treatments of the NCs, the use of low-bandgap polymers and optimized device structures including active layer thickness, electrode materials, novel NC hybrid structures and approaches to control the nanomorphology.PCEs approaching 3% and above are available and further enhancement can be expected by exploring and combining the above mentioned approaches.
4:30 PM - B4.6
Nanostructured Solar Cells based on Patterned Catalyst-free GaAs Nanopillars.
Giacomo Mariani 1 , Joshua Shapiro 1 , Yue Wang 2 , Richard Kaner 2 , Diana Huffaker 1
1 Electrical Engineering, University of California, Los Angeles, Los Angeles, California, United States, 2 Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California, United States
Show AbstractIn this work, we present nanostructured core-shell solar cells based on patterned GaAs nanopillars grown by MOCVD. The patterns are photolithographically defined and center-to-center pitch, hole size and mask arrangement can be precisely pre-determined at nanometric resolution. Our inherently catalyst-free growth mode eliminates any metal (i.e. Au) diffusion into the nanopillars that could hinder the electron-hole pair extraction, paramount in photovoltaics. The lattice-matched growth capability also avoids threading dislocations that normally act as recombination centers, worsening the leakage current in pn-junction based devices. The study aims at evaluating different contact workfunctions for the transparent top electrode applied to GaAs nanopillar photovoltaic devices. Electronic transport along the photo-junction and carrier extraction can be hugely limited by a non-purely ohmic contact, resulting in low power conversion efficiencies. Aluminum-doped Zinc Oxide (AZO), Indium-doped Tin Oxide (ITO) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) are chosen as front electrodes. Comparisons are made in terms of photocurrent density-voltage (J-V) characteristics (under dark and standard AM 1.5 conditions) and external quantum efficiency (EQE), both standard figures of merit in the photovoltaic field. Devices with ITO as top contact are measured along with devices with AZO as front electrode. The best device with AZO as top contact exhibits an open circuit voltage (VOC) of 0.2V, short circuit current density (JSC) of 6.6 mA/cm2 and a fill factor (FF) of 37% whereas the best device with ITO as top electrode showed a VOC=0.39V, JSC=17.8 mA/cm2 and a FF above 34% with a power conversion efficiency of 2.4% , the highest achieved to date in GaAs nanowire photovoltaics. The main figures of merit (J-V characteristics, external quantum efficiencies) are presented in a final comparative study between metal oxides and highly-conductive organic polymers.
4:45 PM - B4.7
Transfer of Preformed 3D Photonic Crystals onto Highly Porous Dye Sensitized Solar Cell Electrodes.
Agustin Mihi 1 2 , Chunjie Zhang 2 , Paul Braun 1 2
1 Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show AbstractWe demonstrate the transfer of preformed self-assembled 3D photonic crystals onto optoelectronic/photovoltaic devices. Because the photonic crystals are preformed, rather than grown directly on the final device, the surface chemistry, topography, and porosity of the final device does not impact the quality of the photonic crystal. As an example of this process, high quality, 3D silica, inverse TiO2, and silicon opals are fabricated and transferred onto dye sensitized solar cell working electrodes through a polymer infiltration, transfer, and wet etching process. This procedure allows the incorporation of 3D photonic structures onto porous structures such as porous titania networks or zinc oxide nanowires, which otherwise is not possible. During this process, the high quality of the photonic structure and the porosity of the electrode are preserved. Interfacing of these photonic crystals with porous structures commonly employed as working electrodes in dye sensitized solar cells, results in an increase in the efficiency of a model system from 1.016% to 1.66%.
5:00 PM - B4.8
Using Atomistic Pseudopotential Theory to Screen Candidate Systems for III-V Based Quantum Dot Intermediate Band Solar Cells.
Voicu Popescu 1 2 , Alex Zunger 2
1 , Colorado School of Mines, Golden, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractSelf-assembled quantum dots (QDs) made of III-V materials represent one of the possibilities to realize intermediate band solar cells (IBSC) yet the choice of materials and parameters - dot material and geometry, matrix and substrate - which lead to an optimal positioning of the impurity band, is currently unknown.We have previously shown [1] that the standard choice of (In,Ga)As dots in GaAs or Ga(As,P) matrix is far from optimal. We have applied our pseudopotential Linear Combination of Bloch Bands (LCBB) method tested previously on numerous III-V dot problems to study lens-shaped dots of (i) In(As,P) embedded in (In,Ga)P and (ii) InAs embedded in Ga(As,Sb) for various alloy compositions and geometries in search for a combination that affords strain-symmetrization as well as the positioning of the electron and hole bands in appropriate energies in the gap of the wide band matrix.Valence force field is used to calculate the strain-balanced condition on an atomistic level. We will discuss the energies of the confined dot levels and strained matrix band edges relative to the expected values for ideal IBSC operation.[1] V. Popescu, G. Bester, M. C. Hanna, A. G. Norman and A. Zunger - Phys. Rev. B 78, 205321 (2008)
5:15 PM - B4.9
Chemically Specific Dynamic Characterization of Photovoltaic and Photoconductivity Effects of Surface Nanostructures.
Okan Ekiz 1 , Hasan Guner 1 , Burak Turker 1 , Aykutlu Dana 1
1 Institute of Material Science and Nanotechnology, National Nanotechnology Research Center, Ankara Turkey
Show AbstractWe report characterization of photovoltaic and photoconductivity effects on nanostructured surfaces through light induced changes in the X-ray photoelectron spectra (XPS). The technique combines the chemical specificity of XPS and the power of surface photovoltage spectroscopy (SPV), with the addition of the ability to characterize photoconductivity under both static and dynamic optical excitation. A theoretical model that quantitatively describes the features of the observed spectra is presented. We demonstrate the applicability of the model on a multitude of sample systems, including homo- and heterojunction solar cells, CdS nanoparticles on metallic or semiconducting substrates, and carbon nanotube films on silicon substrates. In this article we demonstrate an approach that allows the study of photovoltaic and photoconductivity effects using the XPS, under static or modulated illumination. Surface potentials of domains are internally modulated owing to both photovoltaic and photoconductivity effects. We show that a circuit model can be used to estimate the changes in spectra under static and dynamic illumination conditions. Light-induced surface potential differences due to photovoltaic and photoconductive effects can be identified. The technique allows contactless characterization of photoinduced effects on nanostructured materials.
5:30 PM - B4.10
Development of Small-sized PbSe Colloidal Nanocrystals with High-yield and High-quality for Photovoltaic Applications.
Kui Yu 1
1 Steacie Institute for Molecular Sciences (SIMS), National Research Council Canada, Ottawa, Ontario, Canada
Show AbstractSmall-sized PbSe nanocrystals (NCs) were synthesized with high reaction yield (~100%), high quality, and high synthetic reproducibility. These small-sized PbSe NCs with their first excitonic absorption in wavelength shorter than 1200 nm (corresponding to size < ~3.7 nm) were developed for photovoltaic (PV) applications requiring a large quantity of materials. These colloidal PbSe NCs, also called quantum dots (QDs), are high quality, in terms of narrow size distribution with a typical standard deviation of ~7-9%, excellent optical properties with high quantum yield (QY) of ~50-90% and small full width at half maximum (FWHM) of ~130-150 nm of their bandgap photoemission peaks, and high storage stability. A formation mechanism of the PbSe monomer is proposed, which involves two pathways of Pb-Se (Route a) and Pb-P (Route b) complexes; the larger the reactivity of the complex is, the faster the monomer forms, the larger the degree of nucleation and the more the nuclei form. Depending on the method used and its experimental conditions, either pathway may dominate. The understanding of the monomer formation mechanism helps reproducible syntheses of various desired QDs with high yield. The present study addresses two challenging issues in the NC community, the monomer formation mechanism together with whether Pb(oleate)2 reacts with tertiary phosphine selenide and the reproducible syntheses of small-sized NCs with high yield and high quality and large-scale capability, bringing insights into the fundamental understanding on the optimization of nanocrystal yield and quality via the control of the complex reactivity and thus nucleation/growth. Such advances on colloidal science should, in turn, promote the development of next generation low-cost and high-efficiency solar cells. Schottky type solar cells using our PbSe NCs as the active material have achieved the highest power conversion efficiency (PCE) of 2.82%, in comparison with the same type of solar cells using other known PbSe NCs, under Air Mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm2.
5:45 PM - B4.11
Photoelectric Effect in a Densely Packed Array of Vertical Si Nanowires.
Rufi Kurstjens 1 , Jan Wera 1 , Frederic Dross 1 , Jozef Poortmans 1 , Robert Mertens 1
1 , IMEC/KULeuven, Heverlee Belgium
Show AbstractThe concept of an all-Si tandem solar cell can be realized in different manners. Our approach is to engineer the bandgap of Si by 2D-quantum confinement in low-dimensional Si nanowires to create a higher-bandgap c-Si material. The last dimension is not confined to allow easy current flow. This material can then in the future be used to fabricate the top cell. The nanowires need to have a small diameter (< 5 nm) to induce quantum confinement of excitons and increase the bandgap. To support the feasibility of this concept, we have fabricated a bipolar device consisting of a densely packed, vertically integrated array of Si nanowires.As a starting point, a 6x6 mm2 array of vertical 30-nm-diameter nanowires with a 90-nm pitch were made by DUV lithography and anisotropic reactive ion etching. These arrays are subsequently oxidized by in-situ steam generation at 1100°C. The oxidation process is stopped before the stress at the foot of the nanowire causes damage, after which the oxide is removed to release the stress. A second oxidation is then performed to fine-tune the final diameter. To be able to integrate the nanowires into a device, the voids between the oxidized nanowires are filled up by a pulsed PECVD oxide. The overfill oxide is removed by a chemical-mechanical polishing step which also removes the top of the thermal oxide, revealing the top of the Si cores, allowing them to be contacted. A contacting layer in the form of a thin metal or ITO layer is then deposited on top in order to perform electrical measurements. In this way the dark and light currents can be compared. Due to the size of the nanowires, forming a junction by conventional doping is unlikely to be effective, both from a technological and a physical point of view. The technological issues are mostly due to the low number of doping atoms present in such small volumes of Si and the high-temperature oxidation steps. The physical issues are mostly due to the segregation of the doping to the Si-oxide interface and the change in activation energy in low-dimensional systems, very similar to the issues encountered in doping quantum dots. Alternative means to form a junction will be discussed, chief among them the use of a doped a-Si layer deposited on top of the planarized layer containing the Si nanowires.Measurements on the device are on-going, and first results show an influence of the light. Further investigations of these observations might clarify the functioning of the device.
B5: Poster Session: Next Generation Photovoltaics V
Session Chairs
Thursday AM, April 28, 2011
Salons 7-9 (Marriott)
9:00 PM - B5.1
Correlations of Photoluminescence and Size Evolution of Si Quantum Dots in Amorphous Silicon Carbide.
Geng-rong Chang 1 , Fei Ma 1 , Da-yan Ma 1 , Ke-wei Xu 1
1 State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, Shaanxi, China
Show AbstractNovel materials consisting of silicon quantum dots (Si QDs)embedded in a dielectric matrix are of considerable interest for the third generation photovoltaic solar cell. In this work, silicon quantum dots with high density embedded in amorphous SixC (x>1) thin films were fabricated by magnetron sputtering deposition and post-annealing process. Photoluminescence measurement and high-resolution transmission electron microscopy were used to characterize the band-gap configuration and microstructural properties of silicon quantum dots, respectively. The results of photoluminescence indicate a multi-band configuration in the range from ultraviolet to green. It is demonstrated that the multi-band characteristics is closely related with the size distribution of Si QDs due to quantum confinement effect, as evidenced by high-resolution transmission electron microscopy. The Stokes shift further indicates that there exist α-Si QDs and c-Si QDs forming a multi-band configuration. Moreover, the density and size distribution of Si QDs can be improved further by optimizing the ratio of Si and C atoms as well as annealing parameters. This multi-band configuration would be extremely advantageous to improve the photoelectric conversion efficiency all-Si tandem solar cell.
9:00 PM - B5.11
Zinc Oxide Nanorod-based Inorganic-organic Hybrid Solar Cell.
Ahmad Mozafari 1 , Kai Zhang 1 , Jianyuan Sun 1 , Messaoud Bahoura 1 , Aswini Pradhan 1
1 Optical Engineering Dept., Norfolk State University, Norfolk , Virginia, United States
Show AbstractThis essay concerns fabrication of a hybrid solar cell using a conjugated polymer and well aligned Zinc Oxide nanorods (phase separated bi-layer). In this type of solar cell the polymer absorbs photons and donates excitons and the zinc oxide dissociates excitons at the junction and transfers electrons to the ITO glass. The 1X1.5 inch indium tin oxide (ITO) substrates were prepared by acid etching the sides, then cleaning in acetone, ethanol and deionized water (DI) for ten minutes each step. The zinc oxide seed layer was developed on the substrate by spin coating. The zno nanorods were grown on the seed layer using the sol-gel method. Poly 3-hexylthiophene (P3HT) has been used as an active layer and it was spin coated and annealed. Two nanorods samples were coated with a mixture of P3HT and PCBM [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) for comparison of the nanorods effects on the mixture. The process was finished by depositing aluminum electrodes on the top of the polymers in a thermal evaporator glove box. Two difficulties observed during the fabrication process as follows:1-The seed layer was not covered the ITO substrate 100 percent, which caused shortening in the circuit. This problem was solved by reducing spin coating speed.2-The aluminum electrodes touched the zinc oxide nanorods because of nanorods surface roughness. This problem was resolved by increasing the polymer thickness.
9:00 PM - B5.12
Enhanced Performance of Inverted Polymer Solar Cells with ITO Interfacial Tuning via Water-soluble Polyfluorenes.
Seok-In Na 1 , Tae-Soo Kim 2 , Seung-Hwan Oh 2 , Junkyung Kim 1 , Seok-Soon Kim 3 , Dong-Yu Kim 2
1 , Korea Institute of Science and Technology (KIST), Jeollabuk-do Korea (the Republic of), 2 , GIST, Gwangju Korea (the Republic of), 3 , Kunsan National University, Kunsan Korea (the Republic of)
Show AbstractEnhanced performance of inverted organic solar cells (PSCs) is demonstrated by indium tin oxide (ITO) interfacial tuning via a water-soluble polyfluorene (WPF-6-oxy-F). The effects of the WPF-6-oxy-F layer as the ITO interfacial modifier on inverted PSCs, based on poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM), were investigated. Kelvin probe studies demonstrated that the WPF-6-oxy-F layer reduces the ITO work-function because of the relatively strong interfacial dipole of WPF-6-oxy-F. In addition, the dark current-voltage curves of inverted PSCs with the WPF-6-oxy-F layer showed smaller leakage current and better charge transport. As a result, introduction of the WPF-6-oxy-F by simple solution processing into the inverted PSCs dramatically raised open-circuit voltage to 0.65 V, fill factor to 59 %, and power conversion efficiency to 3.56 %, approaching the original values of P3HT:PCBM based PSCs with the normal structure. More detailed studies on the relationship between performance of inverted PSCs and various conditions such as heat treatment, thickness and work-function were also investigated.
9:00 PM - B5.13
Nanostructured Electrodes for Efficient Organic Bulk Heterojunction Solar Cells.
Chang-Yong Nam 1 , Qin Wu 1 , Dong Su 1 , Chien-yang Chiu 2 , Noah Tremblay 2 , Colin Nuckolls 2 , Charles Black 1
1 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York, United States, 2 Department of Chemistry, Columbia University, New York, New York, United States
Show AbstractWe demonstrate improved photovoltaic power conversion efficiency of organic bulk heterojunction solar cells by introducing nanostructured electrodes into the device architecture. This new device geometry increases optimal organic blend active layer thickness and subsequent light absorption without compromising charge collection efficiency, thereby resulting in improved overall device performance. In this demonstration, we have employed networks of carbon nanotubes as efficient hole conduction pathways in poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend solar cells. The photovoltaic power conversion efficiency of P3HT:PCBM bulk heterojunctions containing nanotube network electrodes increased by ~11%, from 3.4% to 3.8%, compared to control devices with planar metal contacts – a striking increase given the small amount of added nanotubes (<0.01 wt%). The improvements originate largely from commensurate increases in the device short circuit current density, resulting from a ~20% increase in optimal blend layer thickness from 90 nm to 110 nm upon addition of the nanotube network. Nanotube network electrodes increase the average P3HT hole conductivity by 20-27%, but leave the optical absorption unaffected. The addition of nanotube network contacts to P3HT:PCBM bulk heterojunction devices increases the external quantum efficiency by ~10% uniformly across the P3HT absorption spectrum, consistent with the nanotubes improving the charge carrier collection efficiency. This research was supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886
9:00 PM - B5.16
Determining the Effect of Selenium Substitution on Low Bandgap Conjugated Polymer-fullerene Solar Cells By Optoelectronic and Morphological Characterization.
Eric Peterson 1 , Robert Coffin 1 , Chris MacNeil 1 , David Carroll 1
1 , Wake Forest University, Winston-Salem, North Carolina, United States
Show AbstractThe substitution of selenium for sulfur into an electron poor monomer serves to lower the bandgap of donor-acceptor copolymers. In this work, we present two polymers: C11Benzodithiophene-benzothiophene (C11BDTBT) and its selenium containing analog C11Benzodithophene-benzoselenophene (C11BDTBSE). We illustrate the decrease in band-gap via ellipsometry and external quantum efficiency (EQE) measurements. Photovoltaic performance is compared between optimized devices constructed from both polymers. Using a combination of TEM, ellipsometry, and scattering techniques (X-Ray), we draw conclusions on how the substitution of selenium effects the morphology of bulk heterojunction devices constructed from the polymer when compared to the analog polymer. From these data, we draw initial conclusions on the effect of selenium substitution and possible routes to increasing the performance of this class of polymers.
9:00 PM - B5.19
The Impossibility of Fabricating Solution Processed P3HT/PCBM Bilayers.
Chris Rochester 1 , Lilian Chang 1 , John Roehling 1 , Adam Moule 1
1 , UC Davis, Davis, California, United States
Show AbstractIn this study we show that it is not possible to form polymer/fullerene bilayer structures using orthogonal solvents. Various solvents that are selective for PCBM are used to cast bi-layers of P3HT/PCBM films. We found (1) that PCBM penetrated the P3HT film in high volume fractions and (2) that the PCBM could be removed from films that are cast from a mixed solution. These effects occur because of the P3HT film becomes swollen by the PCBM selective solvents. Based on these observations we develop a morphological model for P3HT/PCBM mixtures formed using selective solvents. This model is validated using 3D-TEM-tomography of P3HT with C80@(Lu3N)PCBEH and dynamic scanning calorimetry.
9:00 PM - B5.2
Photovoltaic Study of p-type NiO/PC70BM Hybrid Solar Cells.
Sudam Chavhan 1 , Ruben Abellon 2 , Tom Savenije 2 , Ellen Moons 1
1 Department of Physics and Electrical Engineering, Karlstad University, Karlstad Sweden, 2 Optoelectronic Material Section, Department of Chemical Engineering, Delft University of Technology, Delft Netherlands
Show AbstractGenerally, hybrid solar cells are fabricated by using electron donating conducting polymers or molecules and electron accepting inorganic material e.g. metal oxide nanoparticles, such as TiO2, ZnO or SnO2. Inorganic metal oxides posses interesting physical properties like high electron mobility, transparency in the visible spectrum and high dielectric constant. However, there are very few reports on hybrid solar cells fabricated with p-type metal oxide and n-type organic molecules. We have studied photovoltaic properties of bilayer hybrid solar cells constituted of p-type NiO and [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM) molecule. The thin films of NiO were prepared on fluorine doped SnO2 (FTO) substrates by RF sputtering in Ar/O2 mixture atmosphere. To fabricate hybrid solar cells, a PC70BM solution was spin coated on top of the smooth and uniform layer of NiO, having thickness of 90 nm. Current-voltage characteristics were measured in dark and under illumination with monochromatic light of wavelength 460 nm and an incident illumination power of 9 mW/cm2. A short circuit current density of 0.15 mA/cm2, open circuit voltage of 0.23 V, and fill factor of 0.26 were found. To understand the photovoltaic mechanism of this type of hybrid solar cells we studied also the bulk heterojunctions made up of p-type NiO nanoparticles with different PCBM molecules. Key words: NiO, Photovoltaic, Hybrid solar cells, PCBM
9:00 PM - B5.20
In-situ Observation and Manipulation of Polymer-fullerene Self Assembly During Thin Film Drying for Organic Solar Cells.
Benjamin Schmidt-Hansberg 1 , Monamie Sanyal 2 , Michael Klein 3 , Alexander Colsmann 3 , Philip Scharfer 1 , Uli Lemmer 3 , Esther Barrena 2 4 , Wilhelm Schabel 1
1 Thermal Process Engineering, Thin Film Technology, Karlsruhe Institute of Technology, Karlsruhe Germany, 2 , Max-Planck-Institute of Metals Research, Stuttgart Germany, 3 Light Technology Institute, Karlsruhe Institute of Technology, Karlsruhe Germany, 4 , Institut de Ciencia de Materials de Barcelona, Barcelona Spain
Show AbstractControl of polymer-fullerene-blend morphology is crucial for optimizing power conversion efficiencies of polymer solar cells (PSCs). Most material combinations comprise a polymer as electron donor and a fullerene derivative as electron acceptor. For a better understanding of the solidification process of the solution cast blend and the control of the fabrication process both the drying and the crystallization kinetics are investigated. In this work a fundamental study of the evolution of film morphology was accomplished for the system P3HT:PCBM, which was subsequently extended to amorphous and semi-crystalline low band gap polymers. Phase diagrams of the binary systems P3HT-Solvent and PCBM-Solvent, were determined and compared with in-situ grazing incidence x-ray scattering (GIXS) measurements of the crystallization within thin films during solvent evaporation. This comparative study afforded a fundamental knowledge of the interaction forces of the ternary polymer-fullerene-solvent mixture. The dynamics and growth mechanisms of film morphology development could be derived.With different drying temperatures and drying velocities, which can be accurately adjusted in a dedicated film coating/drying setup, distinct morphologies could be fabricated as investigated by GIXS, atomic force microscopy and scanning transmission electron microscopy. Different properties of morphology can be related with the drying temperature and overall drying time. Hence, these properties can be tailored by appropriate drying conditions. This is facilitated by a spatially resolved model for the film drying kinetics simulation which could be established for single and multiple solvents. The impact of changes in film morphology induced by the tunable drying process on the optoelectronic properties of PSCs is discussed in a comparative structure-property study of solar cells comprising P3HT:PCBM and other material systems.Acknowledgement of support:This work is funded by the German Research Foundation within the framework of the priority program SPP 1355. “Elementary Processes of Organic Photovoltaics”.
9:00 PM - B5.21
The Fabrication of Polymer Solar Cells with Solution Processed Interfacial Buffer Layers.
Won Suk Shin 1 , Jung Min Cho 1 , Jin-Uk Park 1 , Won-Bae Byun 1 , Sang-Jin Moon 1
1 , Korea Research Institute of Chemical Technology, Daejeon Korea (the Republic of)
Show AbstractRecently, power conversion efficiencies(PCEs) in the conventional bulk hetero-junction(BHJ) polymer solar cells(PSCs) architecture consisting of polymer donor and fullerene acceptor blends were achieved with about 6 % - 7 %. The PSCs have great attraction on low-cost, flexible, and easy applications to large area devices. The general device architecture of BHJ solar cell consists of multilayer stacks of transparent conducting oxide/hole blocking layer/active layer/metal electrode. Usually, the metal electrodes are fabricated by thermal evaporation or sputtering methods. The vacuum process needs long time and high costs, but the solution process has merits for fabrication of BHJ PSC devices.In this study, we investigated the polymeric BHJ solar cells by solution processing. Efficient polymer solar cells were fabricated with the structure of patterned indium tin oxide (ITO) on glass/titanium oxide (TiOx)/polymer blend/tungsten oxide (WOx) or poly(ethylenedioxythiophene)poly(styrenesulfonate)(PEDOT:PSS)/silver (Ag). In here, interfacial buffer layers such as a hole transport layer or an electron transport layer between active layers and electrodes play important roles to collect selected electrons or transport holes. We demonstrated the polymer solar cells with titanium oxide(TiOx) as hole blocking layers, P3HT(poly(3-hexylthiophene)):PCBM([6,6]-Phenyl-C61-butyric acid methyl ester) blend as active layers and tungsten oxide (WOx), or PEDOT:PSS as hole transport layers prepared by spin casting. And then nano-particle(NP) silver (Ag) solution was used to metal electrodes also prepared by spray method. All PSC devices were measured under 100 mWcm-2 AM 1.5 G illumination in ambient air.The more results will be reported and the details will be discussed.
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Characterization and Devices with New Conjugated Polymers for Organic Photovoltaics.
Andrew Stuart 1 , Sam Price 1 , Huaxing Zhou 1 , Liqiang Yang 2 , Wei You 1
1 Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States, 2 Curriculum in Applied Sciences and Engineering, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Show AbstractWe have synthesized several low bandgap intramolecular charge transfer copolymers using benzo[2,1-b:3,4-b']dithiophene as the donor moiety. Coupling this weak donor moiety with two flanking thiophene units and strong acceptor unit has yielded exceptionally high efficiency ( >5%) photovoltaic polymers. Photovoltaic devices were fabricated from these new materials in order to characterize and compare each of the polymers. Detailed studies have elucidated the structure/properties relationship, which helped summarize the design rationale and will facilitate the future synthesis of new materials.
9:00 PM - B5.25
Critical Role of Carrier Transporting Layers in Bulk Heterojunction Organic Solar Cells.
John Tumbleston 1 , Yingchi Liu 1 , Christoph Kirsch 3 , Abay Gadisa 1 2 , Mukti Aryal 1 2 , Edward Samulski 2 , Sorin Mitran 3 , Rene Lopez 1
1 Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States, 3 Mathematics, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States, 2 Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Show AbstractOrganic bulk heterojunction (BHJ) solar cells are traditionally fabricated as a sandwich of the active layer between two metal electrodes. This simple design led to a description of device performance in terms of a metal/insulator/metal picture. However, insertion of carrier transporting layers, usually in the form of metal oxides, between the active layer and electrodes has gained in popularity. This technique has allowed for the fabrication of more complex device architectures such as inverted and nanopatterned solar cells. The range of metal oxides that have been attempted as carrier transporting layers is expansive even though there has been little fundamental work on the link between the electrical properties of these layers and device performance. Herein, we present a combinatorial study of various metal oxides as electron and hole transporting layers in BHJ solar cells and the relationship between the electrical properties of these layers (i.e. mobility, doping, energy levels) and BHJ device performance. We also present a device model that is an extension of the metal/insulator/metal picture and allows for the inclusion of carrier transporting layers described by their measured electrical properties.
9:00 PM - B5.26
Dye Molecular 1D-assembly for Organic Thin Film Solar Cells.
Osamu Yoshikawa 1 , Yoshiaki Kobuke 1 , Susumu Yoshikawa 1
1 , Kyoto University, Uji, Kyoto, Japan
Show AbstractSelf assembly of dye molecule has attracted much attention in molecular electronics, as the molecular orientation makes the key role in transporting exciton and charge in organic thin film. In this work, Bis(imidazolyethynyl zinc porphyrin) (BIEZP), which forms a supramolecular structure[1] was used as an electron donor materials in active layers of OPV. A porphyrin dimer of a slipped cofacial orientation is formed using a complementary coordination between central metal Zn of Porphyrin and the imidazolyl ligand. A supramolecular porphyrin nanowire and a cyclic assembly by this self-organization has been investigated [2], [3], [4], showing continuous formation of a slipped cofacial porphyrin dimer provides a supramolecular nanowire structure, which may lead to effective exciton diffusion and charge transport Mn porphyrin, which is electron acceptor, was coordinated at the edge of a porphyrin nanowire [5]. The nanowire provides an effective pathway for energy transfer through the porphyrin unit of approximately 100 nm while general organic materials show exciton diffusion length of approximately 10nm. The substrates were sonicated with acetone and ethanol,and exposed ozone under UV irradiation. PEDOT: PSS (Baytron P) was spincoated onto the substrate and then annealed at 200 degree for 10 min. BIEZP layer dissolved in tetrachloroethane was spincoated and annealed at 120 degree for 10 min. PCBM dissolved in chlorobenzene was spincoated.. The device structure used is ITO/PEDOT: PSS/BIEZP/BIEZP: PCBM/TiOx/Al. Energy conversion efficiency was measured at A.M. 1.5, 100 mW/cm2, which is standard condition in solar cell measurement.Maximum energy conversion efficiency of 0.81 % was achieved. In comparison with the device with the bulkheterojunction layer based on ZnTPP and PCBM (0.009%), much higher efficiency was obtained. Exciton diffusion length of BIEZP showed approximately 30 nm, which value was longer than that of the general organic materials. In addition, the formation of increased nanowire lead increase of photocurrent generation. These result indicates that BIEZP nanowire is the promising materials for organic solar cells. The construction of more higher ordered orientation of nanowire will allow high efficiency. References [1] A. Satake, M. Fujita, Y. Kurimoto, and Y. Kobuke, Chem. Commum., 1231 (2009)[2] K. Ogawa and Y. Kobuke, Angew. Chem. Int. Ed., 39, 4070, (2000)[3] A. Nomoto and Y. Kobuke, Chem. Comm., 1104, (2002)[4] Joanne, T. Dy, K. Ogawa, A. Satake, A. Ishizumi, and Y. Kobuke, Chem. Eur. J., 13, 3491, (2007)[5] D. Furutsu, A. Satake, and Y. Kobuke, Inorg. Chem,. 44, 4460, (2005)
9:00 PM - B5.27
Improvement of the Power Conversion Efficiency of Polymeric Solar Cells Based on P3HT:PCBM through Interfacial Modification.
Shizhao Zheng 1 , King Wong 1
1 Physics, Chinese University of Hong Kong, Hong Kong Hong Kong
Show AbstractThe power conversion efficiency (PCE) of a bulk heterojunction solar cell is very sensitive to the processing conditions used in forming the film, e.g. the solvents or additives used and the rate of solidification and thermal annealing time. In particular, it was found that the PCE can be significantly improved for a solar cell employing the poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) combination if the solvent evaporation time was increased [1]. It is believed that by decreasing the solvent removal rate, the ordering and crystallinity of the regioregular P3HT chains within the blend can be improved. In this presentation we report the study of a possible effect of solvent induced surface modification which enhance the PCE of solar cells employing P3HT:PCBM. We fabricated a series of devices with the same solvent evaporation time of the P3HT:PCBM film, so that the ordering, crystallinity and morphology of the P3HT chains within the blended film are kept the same. Before the starting of the evaporation, the coated P3HT:PCBM films were maintained in the liquid state for a range of ‘soaking times’ in a saturated vapor. We observed a significant enhancement of PCE with ‘soaking’. The enhancement was more than 10% from a device without soaking to a device with 60 min of soaking time. Since the solvent evaporation times are the same for all the devices such that the ordering of the P3HT chains within the bulk are presumably the same, we believe the enhancement may originated from the surface modification of the interface with the underlying PEDOT:PSS layer.[1] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Mater. 2005, 4, 864.
9:00 PM - B5.28
Metal Grids/Conduction Polymer Hybrid Transparent Electrode for Inverted Polymer Solar Cells.
Jingyu Zou 1 , Hin-Lap Yip 1 2 , Steven Hau 1 , Alex Jen 1 2
1 Materials Science and Engineering, University of Washington, Seattle, Washington, United States, 2 , Institute of advanced materials and technology, University of Washington, Seattle, Washington, United States
Show AbstractPolymer solar cells have been vigorously explored recently as an alternative for low-cost renewable energy due to the feasibility of fabricating them through simple printing or coating technologies for large-area devices. Indium tin oxide (ITO) as the most commonly used transparent electrode in photovoltaic devices, has its intrinsic limitations, such as poor mechanical properties of ITO-coated plastic substrates, limited conductivity for fabricating high-efficiency and large-area solar cells, limited availability of indium and complicated vacuum sputtering process tend to increase the cost for ITO. These limitations set a potential barrier for the commercialization of low-cost organic solar cells. To alleviate this problem, alternative materials for transparent conducting electrode are needed. A simple method is developed here, by combining soft lithography and chemical etching, to fabricate metal grids as an alternative for transparent electrode. The transparency and conductivity could be tuned easily by varying the width and separation of the metal grids. By combining the appropriate metal grid geometry with a thin conductive polymer layer (poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)), substrates with comparable transparency and sheet resistance to those of ITO could be achieved. We also demonstrate using metal grid/conducting polymer hybrid transparent electrode instead of ITO for the fabrication of inverted structure polymer solar cells. Polymer solar cells fabricated using this hybrid electrode with optimized geometry show efficiencies as high as ~3.2%. This method provides a feasible way for fabricating low-cost, large-area organic solar cells.
9:00 PM - B5.29
Density of Electronic States Model for Organic Solar Cells.
Robert Street 1 , Katherine Song 1 , John Northrup 1
1 , Palo Alto Research Center, Palo Alto, California, United States
Show AbstractThe analysis of the transport and recombination in organic bulk heterojunction (BHJ) solar cells requires knowledge of the electronic density of states. The electronic structure of the interface between the polymer donor and the fullerene acceptor is of particular importance because of its role in carrier generation and recombination. Data from electronic transport, photoconductivity spectral response and theoretical calculations are described, and we show that these provide consistent and quantitative information about the electronic structure. In particular, the three techniques each give independent evidence for an approximately exponential band tail of localized states. The photoconductivity spectral response shows an exponential Urbach tail at low energy characteristics of band tail optical transitions, and also provides a measure of the interface band gap and the band offset between the polymer and fullerene. Time-of-flight photoconductivity shows dispersive transport characteristic of multiple trapping in a band tail and also measured the recombination lifetime. The theoretical calculations demonstrate that an exponential band tail arises from disorder in the pi-pi stacking of the polymer chains, and also gives the density of mobile states. Together the results provide the information to construct a quantitative density of states model. We discuss how the model is used to understand carrier transport and recombination properties of BHJ solar cells.
9:00 PM - B5.3
Solution Processed NiO Hole Transporting Layer in P3HT:PCBM Bulk Heterojunction Organic Solar Cells.
Sudam Chavhan 1 , Ellen Moons 1
1 Department of Physics and Electrical Engineering, Karlstad University, Karlstad Sweden
Show AbstractDespite of low fabrication cost and flexibility, limited life time is the major disadvantage of bulk heterojunction organic solar cells (OSCs). The main causes of short life time of OSCs is the degradation of active layer in presence of light and O2 and the effect of the acidic PEDOT:PSS on the ITO electrode. To overcome this problem PEDOT: PSS can be replaced by a p-type metal oxide hole transportation layer, such as NiO. There are reports on vacuum deposited NiO hole transporting layers in OSCs. Here, we have used the low-cost spin coating technique to deposit the NiO layer from a dispersion of NiO particles in methanol. Concentration and deposition parameters were optimized to obtain 40-50 nm thick layer of NiO as observed by tapping mode atomic force microscopy. The device performance of P3HT:PCBM solar cell with ITO/PEDOT: PSS and ITO/NiO electrodes was compared.Key word: NiO, ITO, Organic solar cell.
9:00 PM - B5.30
Solution-processed CdTe Nanoparticle Solar Cells.
Shreyoshree Chakrabarty 1 , Jay Yamanaga 1 , Qing Song 2 , John Bass 2 , Ho-Cheol Kim 2 , J. Campbell Scott 2 , Robert Miller 2 , Xin Ai 2 , Gregory Young 1
1 Chemical and Materials Engineering, San Jose State University, San Jose, California, United States, 2 , IBM Almaden Research Center, San Jose, California, United States
Show AbstractThe last decade has witnessed a surge of research in the area of nanoparticle thin film solar cells devised from metal chalcogenides to achieve improved price-performance ratios for economical solar power generation. In this presentation, we report a set of impact studies on solution-processed CdTe nanoparticle solar cells towards improving their power conversion performance without compromising on cost effectiveness of device manufacturing. CdTe nanoparticles are synthesized using highly reproducible solution processing method involving simple precursors and solvents. The solar cells are devised by fabricating CdTe nanoparticle thin films on glass/ITO substrates having n-type metal oxide layers using economical deposition techniques. Resulting CdTe nanoparticle thin film quality and its optoelectronic properties are studied using XRD, SEM, PL and absorption spectrum characterization techniques. Effects of different processing conditions such as thin film thickness, back contact type, and back contact thickness are studied to determine optimal solar cell parameters for better power conversion efficiency performance.
9:00 PM - B5.31
Fermi Level Pinning via Trap States in a Polymer-fullerene Bulk Heterojunction Solar Cell.
Sarah Cowan 1 , Wei Lin Leong 1 , Robert Street 2 1 , Alan Heeger 1
1 Center for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, California, United States, 2 , Palo Alto Research Center, Palo Alto, California, United States
Show AbstractSmall amounts of impurity, even one part in one thousand, in polymer bulk heterojunction solar cells can dramatically alter the electronic properties of the device, including pinning the open circuit voltage at the energy of the impurity state. Steady state and transient photocurrent studies show dramatic increases in monomolecular (first-order) recombination when [6,6]-phenyl C84 butyric acid methyl ester (PC84BM) is introduced as a trap site in polymer bulk heterojunction solar cells made of a blend of the copolymer poly[N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT) and the fullerene derivative [6,6]-phenyl C61 butyric acid methyl ester (PC60BM). This study reveals the dramatic importance that the purity of polymer and small molecule materials play in limiting the efficiency of these devices.
9:00 PM - B5.32
Photovoltaic Properties and Ultrafast Carrier Dynamics of CdS Quantum Dot-sensitized Solar Cells.
Sojiro Hachiya 1 , Qing Shen 1 2 , Taro Toyoda 1
1 Department of Engineering Science , The University of Electro-Communications, Tokyo Japan, 2 , PRESTO, Japan Science and Technology Agency (JST), Saitama Japan
Show AbstractRecently, quantum dot-sensitized solar cells (QDSSCs) have received much attention because of their specific advantages, such as tunable absorption spectra and multiple electron-hole generation [1]. In QDSSCs, the photoexcited carrier dynamics is very important for improving the conversion efficiencies. To study ultrafast dynamics of electron and hole, we use an improved transient grating (TG) method [2]. The improved TG method can measure both of electron and hole dynamics and suitable for the measurement of sample with rough surfaces [3]. In this work, we report the photovoltaic properties and ultrafast carrier dynamics of CdS QD-sensitized solar cells. CdS QDs were grown directly on a TiO2 surface by chemical bath deposition (CBD) method for several deposition times [4]. Photovoltaic properties were studied with an illumination of AM 1.5 for sandwich structure solar cells. The maximum photovoltaic conversion efficiency of 1.3% has been obtained for CdS QD-sensitized solar cells. We found that electron and hole decay times depend on the deposition time of CdS QDs with the TG measurements. With the increasing deposition time, electron decay time became shorter (180 ps → 170 ps), but hole decay time became longer (6 ps → 8 ps). More detailed investigation on the correlation between carrier dynamics and photovoltaic properties is in progress now.[1] A. J. Nozik, Physica E 14, 115 (2002). [2] K. Katayama, M. Yamaguchi, T. Sawada, Appl. Phys. Lett. 82, 2775 (2003) [3] Q. Shen, K. Katayama, M. Yamaguchi, T. Sawada, T. Toyoda, Thin Solid Films 486, 15 (2005).[4] O. Niitsoo, S. K. Sarkar, C. Pejoux, S. Rühle, D. Cahen, G. Hodes, J. Photochem. Photobiol. A: Chem. 181, 306 (2006).
9:00 PM - B5.33
Effect of ZnS Coating on the Photovoltaic Properties of PbS Quantum Dot-sensitized Solar Cells.
Sojiro Hachiya 1 , Qing Shen 1 2 , Taro Toyoda 1
1 Department of Engineering Science , The University of Electro-Communications, Tokyo Japan, 2 , PRESTO, Japan Science and Technology Agency (JST), Saitama Japan
Show AbstractQuantum dot (QD)-sensitized solar cells have attracted much attention because the use of semiconductor QDs have some advantages in solar cell applications [1]. PbS QDs have a possibility to obtain high conversion efficiency for solar cell applications, because it has shown the potential to generate multiple excitons by one photon absorption of UV or visible light [2]. On the other hand, ZnS coating on the QDs has shown extreme contributes to improve the photovoltaic properties of QD-sensitized solar cells [3]. In this work, we report the photovoltaic properties of PbS QD-sensitized solar cells with ZnS coating under various conditions. PbS QDs were adsorbed onto nanostructured TiO2 electrodes using successive ionic layer adsorption and reaction (SILAR) method for two cycles. After adsorption of PbS QDs, the surfaces of the samples were coated with ZnS by SILAR method for several cycles under various conditions (solvents and concentrations). Finally, sandwich structure solar cells were prepared. The counter electrode was a Cu2S film on brass [4]. Mixing of the 1M S and 1M Na2S solution (polysulfide redox system) was used as the regenerate redox couple. Photovoltaic properties were studied by using a solar simulator (AM 1.5). We found that there was an optimum condition of ZnS coating for the photovoltaic conversion efficiency. The maximum photovoltaic conversion efficiency of 2.1% has been obtained for PbS QD-sensitized solar cells. [1] A. J. Nozik, Physica E 14, 115 (2002). [2] A. J. Nozik, Chem. Phys. Lett. 457, 3 (2008). [3] Q. Shen, J. Kobayashi, L. J. Diguna, and T. Toyoda, J. Appl. Phys. 103, 084304 (2008). [4] G. Hodes, J. Manassen, and D. Cahen, J. Electrochem. Soc. 127, 544 (1980).
9:00 PM - B5.34
Pulsed Laser Deposition of Silicon Nanostructures.
Valeria Russo 1 , Tushar Salve 1 , Paola Bruno 2 , David Dellasega 1 , Giuseppe Filoni 1 , Carlo Casari 1 2 , Carlo Bottani 1 2 , Andrea Li Bassi 1 2
1 Department of Energy, Politecnico di Energia, Milano Italy, 2 Center for Nano Science and Technology - IIT@PoliMi, Italian Institute of Technology, Milano Italy
Show AbstractSemiconductor quantum dots (QDs) are attracting a renewed interest for their potential application in third-generation photovoltaics, e.g. for Quantum Dot-sensitised Solar Cells (QDSC) or solar cells based on QD arrays [A.J. Nozik, Physica E 14, 15 (2002)]. In particular silicon is generally considered an interesting materials for this kind of applications.We here present the growth of nanostructured silicon layers by a physical deposition technique, namely Pulsed Laser Deposition (PLD). The structural and optical properties were controlled by varying deposition parameters and annealing temperature.The nanoscale morphology of the silicon layers has been varied from compact to open-porous by varying background gas pressure in the range 0.01 Pa - 100 Pa, both for inert (Ar) and reactive gas (O2). Raman spectroscopy suggests that as deposited samples are mainly constituted of amorphous silicon (or silicon oxide in the case of deposition in O2) and highly defective, partially oxidized, nanoparticles, while ex-situ annealing in vacuum at 400°C and 1000°C seems to induce the growth of less defective silicon nanocrystals. This is confirmed by the increase in the intensity of photoluminescence (PL) after annealing at high temperatures. PL measurements, performed with excitation at 2.41 eV, have also shown a strong dependence on morphology. Moreover different absorption and PL properties are observed for depositions in inert or reactive atmospheres. A multi-component fitting procedure of PL bands was performed and different contributions in the range 1.6-2.0 eV were highlighted. The results are interpreted in terms of size distribution (in the 2 - 5 nm range), crystallinity and surface oxidation of the silicon nanostructures.Finally, the capability of producing a system composed by hierarchically organized nanostructured titanium oxide [F. Di Fonzo et al., Nanotechnology 20, 105604 (2009)] decorated with silicon QDs by means of Pulsed Laser Deposition (PLD) has been explored.
9:00 PM - B5.35
Highly Ordered Conformation of Single Conjugated Polymer Chains.
Takuji Adachi 1 , Johanna Brazard 1 , Paresh Chokshi 2 , Robert Ono 1 , Zicheng Li 1 , Joshua Bolinger 1 , Venkat Ganesan 2 , Christopher Bielawski 1 , Paul Barbara 1
1 Chemistry and Biochemistry, Universtity of Texas at Austin, Austin, Texas, United States, 2 Chemical Engineering, University of Texas at Austin, Austin, Texas, United States
Show AbstractBy using the single molecule fluorescence polarization spectroscopy technique, we discovered that the majority of single polymer chains of the prototypical conjugated polymer MEH-PPV and P3HT adopt a highly ordered conformation in a PMMA host matrix. Detailed analysis of the excitation polarization anisotropy histogram shows that a large fraction (>70 %) of single MEH-PPV chains have high anisotropy values (> 0.6). A comparison of experimental results with the coarse-grain beads on a chain molecular dynamics simulations reveals that thermally induced sharp-bending defects cause a polymer chain to fold into highly ordered rod-shaped conformation composed of only a few turns. The polarization anisotropy distribution of single P3HT chains is much narrower and higher-peaked with no molecule possessing an anisotropy value less than 0.5, indicating that every single chain of P3HT folds into a highly ordered conformation. However, the anisotropy distribution from P3HT is significantly broadened when the regioregularity of the sample decreases from 95 % to 64 %. The regioregularity dependence is not observed in MEH-PPV. We propose that the folding of P3HT single chains is mainly driven by the side-chains, while that of MEH-PPV single chains is driven by the polymer backbone. Despite the complexity and heterogeneity of the materials, single-molecule spectroscopy is a useful tool to understand the mechanism of morphology formation in conjugated polymer films.
9:00 PM - B5.36
A Simple Method of Evaluating the Degree of Miscibility of Poly(3-hexylthiophene) with a Methanofullerene.
Abay Dinku 1 2 , John Tumbleston 2 , Doo-Hyun Ko 1 , Rene Lopez 2 , Edward Samulski 1
1 Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States, 2 Department of Physics, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Show AbstractA bulk heterojunction (BHJ) blend of poly(3-hexylthiophene) (P3HT) and an electron acceptor methanofullerene molecule [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) has been extensively studied and applied in solar cell applications. The morphology of spin coated films of P3HT/PCBM BHJ is usually modified through thermal annealing in order to enhance both charge generation and transport. Unlike any other amorphous conjugated conducting polymers, P3HT has a tendency to relax during thermal annealing thereby allowing PCBM to diffuse through the film volume. In general, annealing processes lead to regions rich in P3HT chains organized partly in orderly fashions, and regions occupied for the most part by aggregated PCBM molecules. Herein we present an indirect way of evaluating the miscibility of P3HT with PCBM through investigation of the effect of temperature on P3HT/PCBM bi-layer films. We demonstrate that thermal annealing of P3HT/PCBM bi-layer films converts the pure P3HT and PCBM layers into inter-mixed phases, which morphologically, optically, and electrically mimic the characteristics of traditional BHJ films. Such films, when integrated into solar cells, deliver high fill factors (generaly more than 70%), due to formation of efficient, and bi-continuous electrical pathways for photo-generated electrons and holes. We also show that the inter-mixed BHJ film is characterixed by low bi-molecular recombination losses from the saturation to the open-circuit region as investigated through excitation light power dependent photo-current generation. The inevitable diffusion of PCBM through the P3HT matrix, intiated by relaxation of P3HT chains, can also be considered as an indicator of the degree of stability of the P3HT/PCBM BHJ solar cells when exposed to environments with severe temperature conditions.This work is supported by the NSF SOLAR Grant (DMR-0934433).
9:00 PM - B5.38
Polythiophene/poly(fluorene-alt-dithienylbenzothiadiazole) Block Copolymers for Photovoltaic Devices.
Rhiannon Mulherin 1 , Stefan Jung 2 , Sven Huettner 1 , Nils Koenen 2 , Kerr Johnson 2 , Sybille Allard 2 , Ullrich Scherf 2 , Neil Greenham 1
1 Physics, Cambridge University, Cambridge, Cambridgeshire, United Kingdom, 2 Chemie, Bergische Universitaet Wuppertal, Wuppertal Germany
Show AbstractDonor-acceptor block-copolymers, composed of a poly(fluorene-alt-dithienylbenzothiadiazole) donor block and polythiophene acceptor block, are demonstrated in high open-circuit voltage photovoltaic devices. Device efficiency as a function of nano-phase separation and molecular self-assembly is characterised via a range of processing conditions. The effect on blend morphology and device performance for poly(fluorene-alt-dithienylbenzothiadiazole)/polyfluorene blends doped with varying concentrations of block-copolymer is also examined. These nano-structure/performance effects are interpreted in the light of ultra-fast transient absorption spectroscopy examining the dynamics of photoluminescence quenching and charge generation.
9:00 PM - B5.39
Quantum Dot Additive for Enhancing Efficiency of Poly(3-hexylthiophene):[6,6]-Phenyl-C61-Butyric Acid Methyl Ester Bulk Heterojunction Solar Cell.
Hsueh Chung Liao 1 , Tsung-Han Lin 1 , Cheng-Si Tsao 2 , Chih-Min Chuang 2 , Meng-Huan Jao 1 , Yang-Fang Chen 3 , Wei-Fang Su 1
1 Materials Science and Engineering, National Taiwan University, Taipei Taiwan, 2 Institute of Nuclear Energy Research, Atomic Energy Council, Taoyuan Taiwan, 3 Department of Physics, National Taiwan University, Taipei Taiwan
Show AbstractQuantum dots: Cu2S and CdSe were systematically studied respectively as an additive in the photoactive layer to improve the power conversion efficiency of poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61- butyric acid methyl ester (PCBM) bulk heterojunction (BHJ). A 23% enhancement of power conversion efficiency (from 3.5% to 4.3%) was obtained by incorporating 2.8 x 10-4 % by weight in the photo active layer of Cu2S quantum dots with a size of 4-5 nm in diameter. Under such low concentration of additive, the contribution of photocurrent from the quantum dots was ruled out from the external quantum efficiency measurement. The enhancement was due to the controlled morphology of photoactive layer using quantum dot additive. Quantitative investigations of the morphology changes were performed by Synchrotron incidence gracing wide angle X-ray diffraction (GIWAXD) and small angle X-ray scattering (GISAXS). By using the model fitting of SchulzSphere form to analyze the data, the results indicate the quantum dot additive decreases the size and broadens the size distribution of PCBM cluster. Additionally, larger specific surface areas between the two phases were also elucidated by Porod’s law analysis without the expense of the percolated networks. Atomic force microscope observation and space charge limited current (SCLC) measurement of the photoactive layer further confirmed the results of GIWAXD and GISAXS analysis. The 2.8 x 10-4 %wt. CdSe quantum dots additive exhibited the similar effect with a significant efficiency improvement from 3.5% to 4.2%. This study provides a noble strategy to optimize the thin film morphology in the application of BHJ solar cell.
9:00 PM - B5.40
Electrophoresis Deposition of CdSe, CdTe and CdSe/CdTe Layers from Nanoparticles in Solutions.
Bjorn Eckhardt 1 , Elisabeth Zillner 1 , Thomas Dittrich 1 , Ahmed Ennaoui 1 , Martha C. Lux-Steiner 1 2 , Jaison Kavalakkatt 1 2
1 Institute Heterogeneous Material Systems , Helmholtz-Center Berlin for Materials and Energy, Berlin Germany, 2 , Freie Universitaet Berlin, Berlin Germany
Show AbstractNanochemists have reached a high degree of control in synthesizing nanoparticles (NPs) of various materials and despite this enormous progress in the synthesis, the application of colloidal nanocrystals into well-defined mono- or multilayer functional elements such as junctions, arrays and devices to further scale well to industrial fabrication represents some of the major challenges: (1) Developing methods that allowed minimizing wasting; (2) Accurately maintaining quantum scale structures over large area; (3) Removal of highly insulating organic capping molecules to obtain optimal physical properties of assembled colloidal nanoparticles. To overcome these challenges we explored the applicability of electrophoresis deposition (EPD) for the fabrication of all-inorganic nanocrystal solids. We used CdSe and CdTe as model system and a simple ligand-exchange procedure. The layer quality is assessed using Atomic Force Microscopy, Scanning Electron Microscopy, Transmission Electron Microscopy, UV-VIS absorption spectroscopy, X-ray diffraction and X-ray fluorescence. We find that the EPD technique is an excellent tool for the production of films from nanoparticles in solution with a well-controlled number of layers although the resulting layer structure is influenced by the substrate quality. The method is required for large-scale production of well-defined multilayer functional elements from nanoparticles in solution by simply applying a high voltage that moves mostly all particles onto the substrate with a minimum of waste. Evidence of charge separation of photoexcited charge carriers probably due to the type II alignment of two constituting semiconductor materials is investigated using surface photovoltage (SPV) spectroscopy.
9:00 PM - B5.42
Synthesis and Characterization of Rare Earth (Tb3+ and Yb3+) Doped CdS/ZnS Core/Shell Nanocrystals for Enhanced Photovoltaic Efficiency.
Sandip Das 1 , Krishna Mandal 1
1 Department of Electrical Engineering, University of South Carolina, Columbia, South Carolina, United States
Show AbstractCdS host nanocrystals with 3.5-5.1 nm diameter have been doped with rare earth (RE) ions, Terbium (Tb3+) and Ytterbium (Yb3+), prepared from green precursors via chemical synthesis method. Doped CdS cores were shelled by ZnS layers of different thickness. The resulting RE doped core/shell nanocrystals show a complete broadband absorption below 400-460 nm to the deep UV region depending on the size of the cores which act as the energy receptor and sensitizer for the RE ions. The photoluminescence spectra revealed for the first time, the visible/NIR emission from the RE doped quantum dots. The nanocrystals were further characterized by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and secondary ion mass spectroscopy (SIMS). The theoretical aspects of the quantum cutting in RE doped nanocrystals, experimental procedures and the characterization results will be presented. Application of the free-standing RE doped CdS/ZnS core/shell nanocrystals for solar spectral matching to enhance photovoltaic efficiency will also be discussed.
9:00 PM - B5.45
Exciton Diffusion Measurements in Narrow Band Gap Polymers for Applications in Solar Cells.
Oleksandr Mikhnenko 1 2 , Yaroslav Aulin 1 , Hamed Azimi 3 , Markus Scharber 3 , Mauro Morana 3 , Alexander Sieval 4 , Jan Hummelen 1 , Maria Antonietta Loi 1
1 Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands, 2 , Dutch Polymer Institute, Eindhoven Netherlands, 3 , Konarka Austria , Linz Austria, 4 , Solenne BV , Groningen Netherlands
Show AbstractExciton diffusion is one of the physical processes that determine the performance of several kinds of organic optoelectronic devices. Here we estimated the singlet exciton diffusion lengths in a novel low band gap dithiophene-benzothiadizole polymer that show superior performance in bulk heterojunction solar cells [1]. Near infrared time-resolved photoluminescence (PL) spectroscopy revealed decrease of the PL lifetime with polymer thickness. We relate the thickness dependence of the PL lifetime with the quenching processes at the polymer-vacuum interface. The experimental data are supported by theoretical modeling resulting in values of exciton diffusion length shorter than 10 nm. [1] Scharber, M. C.; Koppe, M.; Gao, J.; Cordella, F.; Loi, M. A.; Denk, P.; Morana, M.; Egelhaaf, H.; Forberich, K.; Dennler, G.; Gaudiana, R.; Waller, D.; Zhu, Z.; Shi, X.; Brabec, C. J. Adv. Mater. 2010, 22, 367-370.
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Photon Upconversion for Increased Solar Spectrum Utilization in Photovoltaics.
Melinda Han 1 , Andrew Ferguson 1 , Jeffrey Blackburn 1 , Tanya Singh-Rachford 2 , Felix Castellano 3 , Garry Rumbles 1 , Andrew Norman 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , The Dow Chemical Company, Midland, Michigan, United States, 3 , Bowling Green State University, Bowling Green, Ohio, United States
Show AbstractWe report a novel approach to improving the utilization of the solar spectrum for photovoltaic energy conversion. Triplet-triplet annihilation upconversion is a molecular approach to converting low energy photons to higher energy photons that does not require coherent light. Thus, this technique can be used to harness sub-bandgap components of sunlight that otherwise go unused in single-junction solar cells. The upconversion chromophores can be embedded in a host material to create a solid-state upconverting film, allowing for the design of a photovoltaic architecture with an integrated upconversion system. Here we study photocarrier generation from upconverted light in a P3HT:PCBM bulk heterojunction organic photovoltaic active layer under long-wavelength excitation. We address the relationship between the morphology of the upconverting film and the detailed properties of the upconversion process as compared to solution-based methods. Finally, we present a photon upconversion system coupled to an organic photovoltaic device, allowing for increased efficiency of this rapidly developing solar cell technology.
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Molecular Photovoltaics: Optimizing Energy-level alignment at the Donor / Hole-extracting Electrode Interface.
Ross Hatton 1 , Robert Cook 1 , Sophie Kinnear 1 , Lara-Jane Pegg 1 , David Morris 2
1 Chemistry, University of Warwick, Coventry, Warwickshire, United Kingdom, 2 Department of Physics, University of Warwick, Coventry, Warwickshire, United Kingdom
Show AbstractWe report a solvent free protocol for the derivatization of the surface of the indium-tin oxide (ITO) coated transparent substrates with a covalently bound silane mono-molecular layer. Unlike deposition from solution this method is scalable and a potentially very low cost means of enhancing the functionality of ITO and other conducting oxide electrodes for application in organic optoelectronics. This method is used to address an important fundamental question relating to the design of the hole-extracting electrode in discrete heterojunction organic photovoltaics (OPV) based on undoped molecular semiconductors: When the work function of the hole-extracting electrode is greater than the ionisation potential (Ip) of the donor material, how does the extent of spontaneous ground-state charge transfer from the donor to the electrode impact OPV performance? This question has not been convincingly addressed to date because of the difficulty in achieving large differences in the work function of electrodes without modifying other properties of the electrode known to be critical determinants of interfacial energetics at organic/electrode contacts. Measurements of the interfacial energy level alignment at the contact formed between high work function ITO electrodes (5.2-5.7 eV) and the low ionization potential molecular semiconductor pentacene (Ip ~ 4.9 eV) are used to construct true pictures of the extent of charge transfer upon contact formation as a function of ITO electrode work function. These measurements are correlated directly with the performance of discrete heterojunction photovoltaics with the structure ITO/pentacene/C60/bathocuproine/Al from which an electrode design rule is proposed.
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Plasmon-enhanced Upconversion for Improved Photovoltaics.
Ashwin Atre 1 , Jennifer Dionne 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractMany processes that harvest the sun’s energy are unable to utilize low-energy sub-bandgap photons. Moreover, light with energy just above the bandgap is often transmitted through the material as well due to the low absorption efficiency at these energies. To reduce transmission losses in solar cells, upconversion can be used to convert the low-energy photons to higher-energy photons, which then can be absorbed by the cell. While it has been established that upconversion will not significantly improve the efficiency of an ideal solar cell under one sun illumination, we theoretically demonstrate that efficiencies can increase substantially for cells with non-ideal absorption and recombination. The geometry considered is a single-junction solar cell backed by an electrically isolated upconverting layer. The upconverter is modeled as three series-connected cells, two of which represent the low-energy intermediate absorptive transitions, and the third representing the high-energy radiative emission. By accounting for non-ideal absorption and radiative recombination efficiencies of the cell and upconverter, we determine which photovoltaic technologies exhibit the highest efficiency enhancements from incorporation of an upconverter. For example, using detailed balance equations, substantial efficiency improvements can be expected with poorly absorbing large bandgap solar cells, such as thin film amorphous silicon, dye-sensitized, and polymer solar cells. A 1.6eV solar cell absorbing 20% of the incident light becomes 9.8% efficient when upconversion is used, compared to only 5.7% without upconversion. We also demonstrate the importance of improving the radiative recombination efficiency of the upconverter, which can be accomplished by coupling the upconverter with a field-enhancing plasmonic nanostructure. For a poorly absorbing large bandgap cell, the relative improvement in power conversion efficiency due to upconversion increases from 6% to 75% when the radiative recombination efficiency increases from 10% to 90%. Full-field simulations are used to determine the absorption and radiative-rate enhancements of an upconverter-doped plasmonic nanocrescent composed of a 100-nm-diameter silica core and crescent-shaped gold coating. Near the tip of the crescent, we calculate field enhancements of more than 50x for wavelengths between 800nm and 1000nm, and a Purcell factor (i.e., radiate-rate enhancement) greater than 2000 over the same range. As the absorption by Er3+ at 980nm leads to emission of upconverted photons at 550nm, we expect this broad enhancement in the near-infrared to significantly improve the upconversion process of this example system. Our presentation will discuss the advantages of various upconverter systems and will introduce plasmonic geometries to accomplish broadband upconversion at low incident power.
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The Influence of Interface Defects on the Optical Properties of Silicon Nanocrystals.
Daniel Hiller 1 , Margit Zacharias 1
1 Nanotechnology Group, University of Freiburg - IMTEK, Freiburg Germany
Show AbstractThe next generation of photovoltaics requires high performance thin films made of cheap and abundant materials. Here, silicon nanocrystals (SiNC) in insulating matrix are a promising candidate to realize the so called all-silicon tandem cell [1]. In contrast to bulk materials the properties of quantum dots can be dominated by their interfaces. For instance a NC of 2 nm in diameter has every 3rd Si atom is on its surface. The most prominent interface defect of SiNCs is the dangling bond (Pb center) which is known to quench the SiNC luminescence. Recently, we demonstrated how less than one monolayer of N-atoms incorporated at the Si/SiO2 interface during annealing in N2 ambient influences the photoluminescence (PL) properties and defect densities [2]. The study presented here provides an overview how Pb-type defects influence the optical emission and absorption properties of size controlled SiNCs. In addition, the question will be addressed how the dangling bond densities can be minimized. Initially, the correlation of NC size with defect densities and species (Pb(0), Pb1, etc.) measured by electron spin resonance (ESR) will be presented. Optical external quantum efficiency (QE) measurements reveal a clear dependence on the amount of non-radiative defects. By means of a defect eliminating H2 post-annealing the QE could be increased from less than 4% to 15% [3]. The inability to achieve 100% QE despite the virtual absence of Pb defects is attributed to PL intermittency (ON–OFF blinking) [4]. However, the essential process for solar energy conversion is the absorption of light. Therefore, reflection-transmission measurements (UV-Vis) and photothermal deflection spectroscopy (PDS) was carried out [3]. It will be demonstrated how different NC sizes and interface qualities influence the absorption in different spectral regions. [1] M.A. Green, Third Generation Photovoltaics, Springer, 2003[2] D. Hiller, M. Jivanescu, A. Stesmans, and M. Zacharias, J. Appl. Phys. 107, 084309 (2010); D. Hiller, S. Goetze, F. Munnik, M. Jivanescu, J.W. Gerlach, J. Vogt, E. Pippel, N. Zakharov, A. Stesmans, and M. Zacharias, Phys. Rev. B 82, 195401 (2010)[3] B.G. Lee, D. Hiller, M. Zacharias, O.E. Semonin, M.C. Beard, and P. Stradins, to be published[4] J. Valenta, A. Fucikova, F. Vácha, F. Adamec, J. Humpolíčková, M. Hof, I. Pelant, K. Kusová, K. Dohnalová, and J. Linnros, Adv. Funct. Mater. 18, 2666 (2008)
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Improved Hole Collection in Polymer Heterojunction Photodiodes with DNA/pt-DNA Layers.
Ali Guvenc 1 , Emre Yengel 1 , Shirui Guo 2 , Hayri Akin 1 , Cengiz Ozkan 3 4 , Mihrimah Ozkan 1
1 Electrical Engineering, University of California, Riverside, Riverside, California, United States, 2 Chemistry, University of California, Riverside, Riverside, California, United States, 3 Mechanical Engineering, University of California, Riverside, Riverside, California, United States, 4 Materials Science and Engineering , University of California, Riverside, Riverside, California, United States
Show AbstractIn this study, we investigated the effects of DNA/pt-DNA strands as hole collecting layers in polymer heterojunction solar cells based on ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al structure. We demonstrated that by introducing DNA or pt-DNA layers between the polymer electrode (PEDOT:PSS) and the active layer (P3HT:PCBM), lead to an improvement in the hole collection efficiency and power conversion efficiency. The shift in the C-V measurements showed that spray coated DNA formed a negatively charged layer which increases the hole collection.
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Au/CIS/CdS Coaxial Heterojunction (Single) Nanowire based Solar Cells Fabricated Using LPNE Method: Photovoltaic Characteristics and Promising Applications for Energy Harvesting.
Somnath Ghosh 1 , Justin Hujdic 1 , Alfredo Villicana-Bedolla 1 , Alan Sargisian 1 , Erik Menke 1
1 School of Natural Sciences, University of California, Merced, California, United States
Show AbstractDevelopment of engineered heterostructure nanomaterials provides novel solutions critical for harvesting solar radiation and conversion to electrical energy (solar cells). Possibility to exercise precise control over the size, shape, composition, and spatial orientation during the growth process gives rise to combination of tailored functionalities and advanced properties not feasible in single-component materials. Presently, nanostructured based solar cells are the most promising candidates for clean and renewable energy sources and provide a potential for monolithic incorporation into electronic devices as compact power sources, and an additional advantage in terms of low-temperature processing of photovoltaic devices. Moreover, recent theoretical studies have indicated that coaxial nanowire (NW) structures could improve carrier collection and overall efficiency with respect to single-crystal bulk semiconductors of the same materials. Coaxial heterostructured semiconductor NWs (1D) attract great deal of interest due to their unique physical properties as well as their potentially wide applications in electronic, optical structures and devices. Currently, two important compound semiconductor materials for use in solar cells are CuInSe2 (CIS) and CdS. CIS (p-type absorber) is commonly paired with CdS (n-type) to form the p-n heterojunction that separates the photo-generated electron-hole pairs. A successful fabrication of a precise interface between CIS and CdS is of key importance to the performance of the solar cell device. In the present work, we describe a novel method to pattern coaxial Au/CIS/CdS heterostructure NWs. The fabrication process for the growth of coaxial NW consists of two phases. In the first part, Gold (Au) core NWs are patterned on the glass substrate by lithographically patterned nanowire electrodeposition (LPNE) technique [1]. Second step in the process comprises fabrication of the heterostructure by electro depositing copper indium diselenide (CIS) shell structure on top of the Gold core NWs. This is followed by CdS shell electrodeposition on top of Au/CIS NW heterostructure. The structural and compositional characterization of these coaxial NWs was carried out by TEM, SAED, SEM, EDS and XRD techniques. Photoresponse of Au/CIS core shell absorber NW was done by measuring the photocurrent generated by chopping the white light (AM1.5G) at different frequencies. On and off behavior of the photocurrent shows that Au/CIS NWs are photoconductive. Furthermore the dark and light transport measurements on Au/CIS/CdS (p-n) heterojunction NW solar cell were performed and it showed Short circuit current (Isc) and open circuit voltage (Voc ), which provides an evidence of the photovoltaic effect in the fabricated NW device.[1] E. J. Menke, M. A. Thompson, C. Xiang, L.C.Yang, R.M. Penner, Nature Materials, 5, 914 (2006)
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Electrospun Rice Grain-shaped TiO2 Mesostructures Sensitized by CdS Quantum Dots for Photovoltaic Application.
Yang Shengyuan 1 , A. Sreekumaran Nair 2 , Seeram Ramakrishna 2
1 NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore Singapore, 2 NUS Nanoscience and Nanotechnology Initiative, National University of Singapore, Singapore Singapore
Show AbstractA rice grain-shaped TiO2 mesostructure was produced by electrospinning a polymeric solution containing titanium isopropoxide, polyvinyl acetate and acetic acid in N,N-dimethyl acetamide followed by calcination at 500 oC. CdS quantum dots were then deposited on the mesoporous ‘rice grains’ by successive ionic layer adsorption and reaction (SILAR) method. The resultant composite material was found to have superior photovoltaic applications in quantum dot-sensitized solar cells (QDSCs) compared to TiO2 nanoparticles and nanofibers respectively. We attribute the superior performance to the compromise of the high surface area of spherical nanoparticles and the directed electron transport capability of continuous nanofibers due to the unique inter-connected rice grain-like structure. Without any elaborate and complicated fabrication procedures typically involved in dye-sensitized solar cells (DSCs), the present methodology is also believed to provide a promising mass production way for alternative low-cost solar cells in view of the fact that both electrospinning and SILAR are generally considered as simple and scalable techniques.
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The Influence of Purification on Quantum Dot Sensitization of Photoanodes.
Laurie King 1 , Jason Riley 1
1 Materials, Imperial College London, London, London, United Kingdom
Show AbstractCadmium selenide quantum dots (CdSe QDs) have been utilized as light harvesting assemblies on photoanodes for application in quantum dot sensitized solar cells following the established methodology of Kamat et al.[1] In brief, a titanium dioxide (TiO2) thin film is prepared on conductive glass. The bifunctional linker molecule 3-mercaptopropionic acid (3-MPA) is utilized to connect the QDs (sensitization) to the TiO2. CdSe QDs are synthesised following a trioctylphosphine oxide (TOPO) stabilised hot injection method.[2] The prepared QDs have been characterized with UV-vis and fluorescence spectroscopy enabling calculation of the size and concentration of QD solutions. Recent studies by Weiss et al.[3] have investigated the influence of the purification procedure (washing) of CdSe QDs, on the identity of ligands at the surface of “TOPO-capped” CdSe QDs synthesised with 90% purity TOPO. The work focussed on the presence of X- type (impurities from 90% TOPO) and L- type (TOPO and other chemicals intentionally present in the synthesis) present in the QD dispersion. It was quantitatively determined that the number of L- type ligands remains approximately constant throughout the procedure employed, whilst, the number of X- type is significantly reduced. Studies on the influence of the washing procedure on sensitization of the photoanode have begun with work on single crystal surfaces by Parkinson et al.[4] Our work has been conducted to develop further the influence of the washing procedure on the degree of QD sensitization of the photoanodes beyond the scope of the single crystal. Visual observations have been made and suggest that the greater the number of washing cycles the QD solution is subjected to prior to sensitization, the greater the colouration (and hence sensitization) of the photoanode. Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES) has quantitatively confirmed that as the extent of washing of the QDs is increased, the ratio of Cd:Ti is enhanced. Thus it is implied that the extent of the washing procedure on the QDs enhances the sensitization of CdSe on TiO2 bound with the linker molecule 3-MPA. Experiments are currently underway to determine the influence of this apparently enhanced sensitization on the photocurrent of the prepared photoanodes. Investigation for appropriate imaging techniques are also in progress with a view to determining the nature of the enhanced sensitivity. [1]Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385.[2]Mekis, I.; Talapin, D. V.; Kornowski, A.; Haase, M.; Weller, H. J. Phys. Chem. B 2003, 107, 7454.[3]Morris-Cohen, A. J.; Donakowski, M. D.; Knowles, K. E.; Weiss, E. A. J. Phys. Chem. C 2010, 114, 897.[4]Sambur, J. B.; Riha, S. C.; Choi, D.; Parkinson, B. A. Langmuir 2010, 26, 4839.
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Anatase TiO2 Beads Having Ultra-fast Electron Transport Rates and Excellent Optical Scattering Effects for Use in Flexible Dye-sensitized Solar Cells.
Chun-Ren Ke 1 , Jyh-Ming Ting 1
1 Department of Materials Science and Engineering, National Cheng Kung University, Tainan Taiwan
Show AbstractSpherical beads consisting of TiO2 nanoparticles (14 to 20 nm) have been synthesized using a two-step method involving a sol-gel and a hydrothermal process. The diameters of the resulting beads vary from 250 to 750 nm, depending on the sol-gel condition. The crystallinity and the oxidation state were found to increase with the hydrothermal temperature. The beads were then fabricated into photoelectrodes on flexible polyethylene naphthalate substrates for the first time. The fabrication was performed at room temperature. Dye-sensitized solar cells (DSCs) were subsequently assembled and evaluated. The dye used was N719. It was found that the photoelectrodes exhibit the desirable scattering effect around the wavelengths near 500 nm where the major absorbing band of N719 dye occurs. Intensity modulated photocurrent spectroscopy analysis shows that the electron diffusion rate of the photoelectrodes made from the beads is at least 15 times higher than that of the photoelectrodes made from commercial TiO2 powders (P25, Degussa). The diffusion rate is even higher than some of the high-temperature (typically 450 oC) fabricated photoelectrodes. This is attributed to the phase purity and unique structure of the beads, and will be discussed.
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Vertically Aligned Single Crystal TiO2 Nanowires for CdS/CdSe/ZnS Quantum Dot-sensitized Solar Cells.
Eui-Hyun Kong 1 , Yoon-Cheol Park 1 , Yong-June Chang 1 , Byung-Gon Kum 1 , Hyun M. Jang 1 2
1 Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang Korea (the Republic of), 2 Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang Korea (the Republic of)
Show AbstractAs for a light-harvesting sensitizer, various narrow band-gap semiconductor quantum dots (QDs) such as CdS and CdSe have been used over the nanostructured TiO2 by the chemical bath deposition (CBD). In the present study, CdS and CdSe quantum dots were sequentially assembled onto the as-synthesized vertically aligned TiO2 nanowires, and ZnS passivation was subsequently performed to prepare CdS/CdSe/ZnS co-sensitized photoelectrodes. The QD-sensitized electrode was combined with a generative redox couple, a polysulfide electrolyte in a liquid type, and their photocurrent-voltage (I-V) curves and incident photon to current conversion efficiencies (IPCE) were tested. The morphology and the microstructure of TiO2 nanowire photoanodes were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Kinetic parameters were characterized by electro-chemical impedance spectroscopy (EIS). As a result, we achieved considerable photoconversion efficiencies with TiO2 nanowire QDSCs under one sun AM1.5 illumination.
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Characterization of Dye-sensitized Solar Cell With Different Nanoparticle Sizes.
Aung Htun 1 , Lakshmi Munukutla 1 , Arunachalanadar Kannan 1
1 Engineering Technology, Arizona State University, Mesa, Arizona, United States
Show AbstractThe Dye-sensitized Solar Cell (DSSC) has been regarded as the next-generation solar cell because of its simple and low cost fabrication process. A DSSC comprises of a working electrode made of dye-sensitized TiO2 nanoparticles fabricated on a transparent conducting oxide (TCO), a Pt counter electrode, and an electrolyte containing iodide/trididode (I-/I3-) redox couple. The photo conversion efficiency of DSSC is significantly dependent on the quality of TiO2 electrodes, hence it is important to optimize the thickness of TiO2 film and nanoparticles’ size in the film. The experiments conducted in this study are accomplished in two phases to study the cell performance in terms of cell efficiency. Phase one is designed to determine optimum thickness of the TiO2 layer followed by second phase to identify the nanoparticle size for enhanced DSSC performance. The TiO2 electrode or working electrode was fabricated using screen printing technique with the Coatema tool. A variety of drying methods were introduced to prevent peeling and cracking of the coated TiO2 films with thicknesses ranging from ~20 to ~60 µm. After carrying out experiments using different working electrodes with various TiO2 layers having with different film thicknesses, it was observed that both open circuit voltage and photocurrent found to have measurable dependence on the TiO2 layer thickness. Photovoltage ranged from 0.77 to 0.82 V and correspondingly photocurrent ranged from 18 to 28 mA depending on the TiO2 layer thickness. It was observed that the thicker TiO2 layer provided lower photovoltage with higher photocurrent. Following completion of cell fabrication the DSSCs were characterized to determine cell efficiency and continued periodic monitoring of the cell efficiency for a period of several weeks. It was observed that the cell with 40 µm TiO2 thickness showed ~9% photo conversion efficiency compared to ~6.5% and 8% efficiency obtained by the cells with 20 µm and 60 µm TiO2 thicknesses. The second phase of the experiment to identify optimum nanoparticle size was conducted by optimizing the TiO2 film thickness at 40 µm. The study of nanoparticle size for enhancing the photo conversion efficiency of the DSSC was carried out using the particle sizes of 13 nm, 20 nm and 37nm in the TiO2 layer. Comparing the efficiency of DSSC for different particle sizes, 20 nm nanoparticle size showed better performance in terms of efficiency. The cell with 20 nm nanoparticle, in combination with 40 µm TiO2 thickness showed 11.2% efficiency. Photo I-V curves; efficiency trend charts and Nyquist plot are performed to quantify the performance of DSSC cells. SEM images are presented in addition, to illustrate the thickness of these nanocrystalline-TiO2 layers. The work described in this paper showed both optimum TiO2 layer thickness and an optimum TiO2 nanopartcle size for obtaining 11.2% cell efficiency.
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Multi-stacked InGaAs/GaNAs Self-organized Quantum Dot Arrays for Use in Intermediate Band Solar Cells.
Yasushi Shoji 1 2 , Takayuki Morioka 1 3 , Katsuhiro Akimoto 2 , Yoshitaka Okada 1 3
1 Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Tokyo Japan, 2 Institute of Applied Physics, University of Tsukuba, Tsukuba Japan, 3 Graduate School of Engineering, The University of Tokyo, Tokyo Japan
Show AbstractRecently, studies on self-assembled quantum dots (QDs) have drawn increasing attention because of their potential for a variety of applications. In the quantum dot intermediate band solar cell (QD-IBSC), homogeneous QDs should ideally be positioned periodically, which as a result, lead to the formation of an intermediate band (IB) or mini-band. Previously, we reported QD structures grown on GaAs (311)B substrate can be made higher in density with in-plane spatial ordering, in which formation of mini-band structure was observed. In this work, we investigated on the optical properties of multi-stacked layers of strain-compensated InGaAs / GaNAs QDs grown on GaAs (311)B substrates.We fabricated p-i-n QDSC on n+-GaAs (311)B substrate by atomic hydrogen-assisted molecular beam epitaxy with a radio frequency plasma as the nitrogen source. The 10 stacked pairs of 5.8 monolayers In0.4Ga0.6As QDs / 20 nm-thick GaNAs strain compensating layer (SCL) were inserted to the intrinsic layer. The growth rate of QD layer and GaNAs spacer layer was 0.1 μm/h and 1.0 μm/h. The growth temperature was 460°C.We obtained high QD densities with in-plane spatially ordered structures, which were maintained even after 10 layers of stacking. The mean diameter and height are determined to be 32.7 nm and 4.9 nm, respectively. The total QDs density amounts to the order of 1012 cm-2 after 10-layer staking. A vertical alignment of QDs along the growth direction is maintained from one layer to the next without degrading the structure and size uniformity. The external quantum efficiency (EQE) measured at room temperature extends beyond the bandgap of GaAs (880nm) to 1100 nm. This response is attributed to the contribution from InGaAs / GaNAs QDs layer. As the result, an extended current generation 2.1 mA/cm2 is achieved. In order to evaluate absorption of QDs layer, we have performed photoluminescence excitation (PLE) measurement at 10 K. The excitation energy was varied from 1.19 to 1.33 eV. It is observed the absorption trend is changed at around 1.29 eV. From the viewpoint of bandgap energy, the PLE spectrum in the excitation energy ranged from 1.29 to 1.33 eV is thought to be the absorption by GaNAs SCL. We believe that the absorption in the range of 1.19 - 1.29 eV is attributed to InGaAs QDs. Since QD diameter is not small enough, the absorption by QDs shows continuous state-like behavior due to existence of many energy states in QDs layer. Because energy states in the QDs layer should be separated from barrier material band for use in IBSCs, further investigation on the growth of small and uniform QDs is required.
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CdSe Quantum Dot-sensitized Solar Cell Employing TiO2 Nanotube Working-electrode and Cu2S Counter-electrode.
Qing Shen 1 2 , Akari Yamada 1 , Satoru Tamura 1 , Taro Toyoda 1
1 Department of Applied Physics And Chemistry, The University of Electro-Communications, Tokyo Japan, 2 PRESTO, Japan Science and Technology Agency (JST), Saitama Japan
Show AbstractWe proposed a CdSe quantum dot (QD)-sensitized solar cell (QDSSC), which is constructed with a CdSe QD adsorbed TiO2 nanotube working electrode on a Ti substrate, a ring shaped Cu2S counter electrode, prepared on a brass substrate having a glass window, and polysulfide electrolyte [1]. Highly ordered TiO2 nanotube film was prepared on a Ti foil using an electrochemical anodizing method. The 1-demensional tubular structure of TiO2 nanotubes is useful for separating and transporting electrons to the Ti substrates. In addition, Ti has higher electronic conductivity and is much cheaper than the FTO substrate, which will be advantageous for the production of low-cost large area solar cells. One key factor in our QDSSC is that a Cu2S film with a ring shape, simply prepared on a brass sheet, was used as the counter electrode. Optical absorption, incident photon to current conversion efficiency (IPCE) and photovoltaic properties of the samples were all characterized. An IPCE value as high as 65 % and a photovoltaic conversion efficiency as high as 1.8 % under one sun have been achieved. We have demonstrated less costly QDSSC, without the requirement for both a transparent conductive electrode or a platinum film. Acknowledgment: Part of this research was supported by JST PRESTO program, Grant in Aid for Scientific Research (No. 21310073) from the Ministry of Education, Sports, Science and Technology of the Japanese Government. [1] Q. Shen, A. Yamada, S. Tamura, and T. Toyoda, Appl. Phys. Lett. 97, 123107 (2010).
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Photosensitization of ZnO Single Crystal Electrodes with CdSe and PbSe Quantum Dots.
Yongqi Liang 1 , Bruce Parkinson 1
1 Department of Chemistry, School of Energy Resources, University of Wyoming, Laramie, Wyoming, United States
Show AbstractUnderstanding charge transfer across the interface between quantum dots (QDs) and oxide substrates is of crucial importance for improving the efficiency of QD sensitized solar cells. The charge transfer dynamics have been investigated with various spectroscopic techniques, such as transient absorption, time-resolved photoluminescence, and tetrahertz time-domain spectroscopy. However, the interpretation of spectroscopic data is often challenging due to the similarities between the signatures of charge injection, nonradiative recombination and energy transfer within QD aggregates. Electron extraction from photo-excited QDs, measured as an electrical current, is more convincing and directly related to solar cell applications. In this study bifunctional linkers, containing both a thiol and carboxylate groups with varying chain lengths, were used to attach the CdSe QDs to ZnO (0001) single crystal surfaces. In the presence of a redox couple (Sx2-/S2-) in the aqueous electrolyte, charge injection from adsorbed photoexcited CdSe QDs into ZnO was demonstrated. The dependence of the length of the linker on the efficiency of change injection from the CdSe QDs into ZnO will be discussed. The possible extraction of the multiple excitons from PbSe QDs was also explored.
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Synthesis and Characterization of a Series of Alternating Polyfluorenes.
Patrik Henriksson 1 , Stefan Hellstroem 1 , Mats Andersson 1
1 Polymer Technology, Chalmers University of Technology, Goeteborg Sweden
Show AbstractA series of seven polymers with the donor-acceptor-donor structured were obtained via Suzuki coupling. The structure of the polymers vary by the length of the alkoxy chain in the meta position of the 5,7-bis(5-bromothiophen-2-yl)-2,3-bis(3-methoxyphenyl)thieno[3,4-b]pyrazine monomer and the length of the alkyl chain of the 2,2'-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)monomer. The polymers were characterized using UV-VIS spectroscopy, Square Wave Voltammetry, Size-exclusion chromatography and device fabrication with PCBM.
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Quantum Dot Sensitized Solar Cells with Improved Efficiency Prepared Using Electrophoretic Deposition.
Asaf Salant 1 , Menny Shalom 2 , Idan Hod 2 , Adam Faust 1 , Arie Zaban 2 , Uri Banin 1
1 Institute of Chemistry and the Center for Nanoscience and Nanotechnology, Hebrew University, Jerusalem Israel, 2 Faculty of Exact Sciences, Bar Ilan University, Ramat Gan Israel
Show AbstractFabricating quantum dot sensitized solar cells (QDSSC) has drawn attention due to the ability to tune the QDs band gap and optical properties with respect to their size and composition. Previous works made on QDs sensitized solar cells (QDSSC) using mono-disperse QDs needed pretreatment processes of the TiO2 with a bifunctional linker molecule and long deposition times, restricted to slow diffusion rates. These solar cells have also relatively low photovoltaic properties due to low surface coverage and the spacer molecule which binds the QDs to the TiO2 surface. It was previously shown that QDs can be deposited on conductive surfaces using electrophoretic deposition (EPD). We have developed a method for fabrication of QDSSC by EPD CdSe QDs into TiO2 mesoporous FTO electrodes. In contrast to the former method; no pretreatment of the TiO2 was needed and the deposition time was short as 2 hours. EDS analysis shows that the atomic ratio between Ti and Cd is almost constant at different depths of the TiO2 which manifests the efficiency of the method. Measuring the short circuit current as a function of the illumination intensity shows linear behavior over the entire intensity range unlike former systems. Finally photovoltaic properties were also greatly boosted, receiving quantum efficiencies of up to 80% in the absorbing range of the electrode (400-600nm), short circuit current of ~8mA/cm2, open circuit voltage of 554mV, a fill factor of ~35% reaching a total yield of 1.7% under 1 sun illumination.
9:00 PM - B5.61
Fabrication of Silicon Nanowires via Cost-effective Methods for PV Applications.
Jaswinder Mann 1 2 , Philippe Vereecken 1 , Dries Schroos 2 , Frederic Dross 1 , Jef Poortmans 1 2
1 , IMEC, Leuven, Leuven Belgium, 2 , Katholieke Universiteit Leuven, Belgium, Leuven Belgium
Show AbstractThe major drivers for photovoltaics research at the moment are the cost and the efficiency. In order to increase the efficiency, a dual-junction tandem device has the potential to reduce the losses due to carrier thermalization. To bring the costs to acceptable levels, working fully with Si-based material can take benefit of the years of developments for microelectronics. At Imec, we are working towards a tandem structure where the bottom cell is made of optimized crystalline silicon structure and the top cell consists of quantum-confined small-dimension (<5-nm-diameter) Si nanowires (Si NWs). A proof-of-concept with high-precision and high-cost processes (DUV lithography and anisotropic reactive ion etching) is being pursued [1]. Although this technique is viable but most probably ineffective to reach the PV cost targets. We present here embodiments of the same concepts using economically-viable processes for Si NWs fabrication. Herein, we propose a cost-effective approach: a combination of patterning process and metal-assisted wet-chemical etching of Si substrates as a promising solution to achieve precise control over the diameter, length and pitch of fabricated Si NWs. This method possesses a few advantages over the aforementioned ongoing parallel work [2]. It replaces the DUV with anodized aluminium oxide (AAO) template/ colloidal lithography (CL) for patterning and also replaces dry-etching with wet-chemical etching process. Using these techniques Si NWs can be fabricated without using complex/costly equipment to create patterns with features on a scale of few tens of nanometers. The diameter of the fabricated Si NWs can be controlled simply by changing the pore size in AAO or by changing the size of the colloidal particles in CL. Another major advantage is to down scale the pitch as compared to DUV (193 nm) which is so far limited to 90 nm.In the present work, two different patterning techniques have been followed. The first technique uses AAO as a template with high pore density up to ~1011/cm2 (compared to ~1010 /cm2 for DUV) for pattering Si substrates. AAO pores with narrow diameter distribution using H2SO4 at 4 V (9-14 nm) and at 26 V (22-30 nm) are fabricated. In continuation with the aim to achieve low-cost Si NW fabrication techniques, colloidal lithography has been used as a second technique. Colloidal beads of ~100 nm uniformly dispersed on Si substrate were used as a patterning mask of a thin metal (Ag) film. Etching was performed in a solution of H2O2 and HF resulting in Si NW fabrication. This is a first demonstration of the capabilities of colloidal lithography for our purpose; smaller beads are under investigation.A detailed study, including the process parameters responsible for pore size distribution in AAO template and colloid deposition on substrate which in turn are responsible for the highly controlled growth of aligned Si NWs, is described.
9:00 PM - B5.63
Role of Humidity on Indium and Tin Migration in OPV's.
Anirudh Sharma 1 , Gunther Andersson 1 , David Lewis 1
1 Flinders Centre for NanoScale Science and Technology, Flinders University, Adelaide, South Australia, Australia
Show AbstractThe stability of a common interface used in organic photovoltaic cells, between the transparent electrode of IndiumTinOxide (ITO) and a buffer layer of poly(3,4 ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS) is known to be unstable with some controversy over whether indium, Tin or both migrate into the PEDOT:PSS buffer layer. Furthermore, the process parameters that drive the process are not well understood.In this paper, migration was observed to start almost immediately after spin coating and annealing of the aqueous PEDOT:PSS solution at 140C, and reaches a saturation level within twenty four hours. The indium and tin was always uniformly distributed over the sampling depth of almost one-third of the thickness of the PEDOT:PSS layer by Impact Collision Ion Scattering Spectroscopy (NICISS), indicating that diffusion of these species is fast compared with their generation.Migration was found to be strongly influenced by the presence of humidity during processing. Exposure to 95% relative humidity following annealing resulted in the highest concentration (1.8 x 10-3 mol/cm3) of Indium or Tin species, corresponding to about one Indium or Tin moiety per 4.7 monomer units in the PEDOT:PSS. The maximum bulk concentration of Indium is about two orders of magnitude higher after exposure to humid conditions compared to vacuum dried conditions. XPS measurements confirms the presence of both Indium and Tin into the PEDOT:PSS and the formation of salts with the metal ions as cations. This research has significant ramifications for processing and the long term durability and usefulness of these devices.
9:00 PM - B5.66
Optimization of CdSe/CdS Core/Shell Heterostructures for Luminescent Solar Concentrator Devices.
Arnaud Demortiere 1 , Chunxing She 1 , Matthew A. Pelton 1 , Seth B. Darling 1 , Elena Shevchenko 1
1 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractWe will report a systematic study on optimization of type-II CdSe/CdS core/anisotropic shell nanocrystals embedded in a polymer matrix. This system has been chosen as a model to explore the applicability of inorganic materials for luminescent solar concentrator (LSC). In CdSe/CdS core/anisotropic shell nanocrystals, due to the band alignment at the interface, the energy levels of the valence and the conduction band edges of the CdSe core are located inside the band gap of the CdS shell material [i]. This behavior induces both carriers highly confined in the CdSe core and enhances the probability of their radiative recombination [ii] leading to the quantum yield up to 80%. Furthermore, these heteronanocrystals have a high nonresonant Stokes shift (up to 175 nm) that is a useful behavior for the photovoltaic devices, especially for LSC because this feature reduces energy losses associated with the reabsorption effect [iii,iv]. Robust polymer matrix allows the optimization of the concentration of optically active material and refractive index by changing type of polymer and conditions of embedding. We will present the systematic study of the effect of the reaction conditions on the kinetic, structure and optical properties of CdSe/CdS heterostructures in a broad size range. A study has been carried both on the growth mechanism and structural properties of nanocrystals according to the size of the CdSe core (2-6 nm) and the length of the CdS rod shell (reaction time). As-synthetized CdSe nanoparticles have QY less than 5%. Variation of reaction conditions drastically affects the structure and optical properties. The formation of CdS shell enhances QY up to 80% for shell excitation (430-450nm). The evolution of the PL QY and the Stokes shift with the size of the CdS shell was investigated. Different analyses have been used to characterize these nanostructures such as HRTEM, XANES and EDX. In addition, we investigated the optical properties in function of different core/shell configurations; in particular the PL decay and TA measurements. Finally, we showed the influence of the polymer matrix on the optical properties for the optimization of solar concentrator devices.[i] M. Saba et al. Advanced Materials, 2009, 21, 4942-4946.[ii] M.G. Lupo et al. Nanoletters, 2008, Vol. 8, 12, 4582-4587.[iii] D.V. Talapin et al. Nanoletters, 2007, Vol. 7, 10, 2951-2959.[iv] L. Carbone et al. Nanoletters, 2007, Vol. 7, 10, 2942-2950.
9:00 PM - B5.67
Engineering Composition Gradients for Efficient Organic Photovoltaic Cells.
Richa Pandey 1 , Russell Holmes 1
1 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractOrganic photovoltaic cells (OPVs) have drawn significant interest as a source of renewable energy due to their compatibility with high throughput, roll-to-roll fabrication processes. The excitonic nature of organic semiconductors necessitates the use of an electron donor-acceptor (D-A) heterojunction for efficient exciton dissociation and the generation of photocurrent. In many organic semiconductors, the optical absorption length is much larger than the exciton diffusion length. Consequently, not all the photogenerated excitons can reach the D-A interface, limiting the overall cell efficiency. This work presents an approach to realize optimized film morphology and maximize cell efficiency for small molecule organic materials using a highly tunable, continuously graded D-A heterojunction (GHJ). The GHJ allows for an increase in the D-A interface area for an enhanced exciton diffusion efficiency, while also preserving the charge collection efficiency, leading to a significant improvement in device performance relative to that of optimized planar and uniformly mixed OPVs. In this work, we demonstrate a power conversion efficiency of ηP=(4.2±0.1)% under 100 mW/cm2 AM1.5G simulated solar illumination for cells containing boron subphthalocyanine chloride (SubPc) and C60 as the electron donating and accepting materials, respectively. This work will correlate the optimized D-A composition gradient to the underlying film morphology and electrical behavior of D-A mixtures.
9:00 PM - B5.68
Photovoltaics Incorporating Anisotropic CdSe/CdTe Nanocrystal Heterostructures at a Hybrid TiO2/P3OT Interface.
Hunter McDaniel 1 , Moonsub Shim 1
1 Materials Science and Engineering, University of Illinois at Urbana-Champaign, Champaign, Illinois, United States
Show AbstractIn the pursuit of a sustainable future with reduced carbon footprint, many researchers have focused their efforts on clean renewable solar energy. Due to their large scale processability from solutions and their uniquely tunable properties, nanocrystals (NCs) may allow us to envision a simultaneous reduction in cost and improvement in efficiency in photovoltaics. A common strategy has been to form an active layer by mixing NCs and an organic hole transport layer to form a type II heterointerface. In these bulk heterojunctions improvements have been made by incorporating anisotropic nanostructures, particularly CdSe tetrapods embedded in P3HT, into these devices. Ultimately, the efficiency of this device geometry is limited by the inefficient exciton dissociation and poor transport through interconnecting NC network that forms the electron transport layer. Type II anisotropic nanocrystal heterostructures (NCHs) exhibiting efficient photo-induced charge separation and long carrier lifetimes may lead to fundamentally new advances. Due to properties that are strongly dependent on size and shape, and furthermore, due to the necessity for physically and chemically accessing both components of the heterostructure for carrier extraction, the key step towards functional multi-component systems is the ability to control not only the size and shape but also spatial orientation of each component with respect to each other. Having achieved a large degree of synthetic control of NCHs in recent years, we now have a better understanding of growth mechanisms leading to enhanced anisotropy and charge separation in NCHs synthesized from monodisperse NC seeds. We have examined various factors that contribute to structural diversification in the type II CdSe/CdTe system. Highly Stokes-shifted emission that arise from heterointerfacial recombination are shown to be enhanced or suppressed via controlled positioning of CdTe on CdSe nanorod seeds. We have also explored a new direction taking advantage of the attractive qualities of anisotropic type II NCHs in solar cells. We avoid the problems associated with transport through NC films by depositing a very thin layer of NCHs at the interface between efficient wide gap electron (TiO2) and hole (P3OT) transport layers. Such a structure seeks to promote absorption and exciton dissociation in the type II NCHs which then pass off carriers to adjacent transport layers for collection at the contacts. Our initial results are promising with open circuit voltages up to 0.7 V, short circuit current densities up to 4 mA/cm2 and fill factors as high as 0.5 under simulated AM 1.5 light. We demonstrate for the first time that the performance of CdSe/CdTe NCHs can exceed single phase CdSe and CdTe nanorods in an optimal thin film device geometry.
9:00 PM - B5.69
Light Trapping in Ultra-thin a-Si:H Plasmonic Solar Cells.
Claire van Lare 1 , Karine van der Werf 2 , Marc Verschuuren 3 , Ruud Schropp 2 , Albert Polman 1
1 Center for Nanophotonics, FOM institue AMOLF, Amsterdam Netherlands, 2 Physics of Devices, Depertement of Physics and Astronomy, Utrecht University, Utrecht Netherlands, 3 , Philips Research Laboratories, Eindhoven Netherlands
Show AbstractIn conventional amorphous Si (a-Si:H) cells the thickness is determined by a tradeoff between efficient carrier collection and sufficient light absorption. This limits device efficiency. In this work we use periodic arrays of metal nanoparticles, either on the front or on the back of the solar cell, and show that light scatters from the arrays into waveguide modes of the solar cell. This results in an increased effective optical path length in the a-Si:H.We show, using finite difference time domain (FDTD) simulations, that a well engineered array of Ag-particles in combination with an ITO layer provides perfect impedance matching between incident light and the aSi:H cell, resulting in a reflectivity similar to that of a standard ITO antireflection coating. Calculations of the of angular distribution of light scattered into the cell show that, depending on the geometry of the particle array, up to 95% of the light is transmitted at angles beyond the critical angle for total internal reflection (14o for a-Si:H/air). We have used substrate conformal imprint lithography (SCIL) to fabricate nanoparticles on solar cells. SCIL is an inexpensive, scalable nanoimprint replication technique that reliably reproduces structures with a resolution comparable to e-beam lithography. The a-Si:H cells are grown with 13.56 MHz plasma-enhanced chemical vapor deposition (PECVD). The patterns are tiled across a substrate with repetition to control for inhomogeneities during deposition.Spectral response (SR) measurements on 150 nm thick n-i-p a-Si:H cells with Ag-particles on the front clearly show strongly enhanced photocurrent in the 650-750 nm spectral range, indicating coupling to optical waveguide modes. Preliminary data show that the photocurrent is enhanced by as much as 24% integrated over the solar spectrum compared to cells without particles, demonstrating efficient light trapping.For 150 nm thick a-Si:H p-i-n cells with nanoparticles on the back an efficiency enhancement of 10% is observed. Angle-resolved photocurrent spectroscopy is used to characterize the coupling to guided modes. The use of nanoparticles on the back of thin solar cells, rather than using a closed metal film, reduces carrier recombination, and potentially enhances light trapping. Furthermore it is economically advantageous, since it requires only small amounts of Ag. The concepts described here are also applicable to other thin film solar cell designs, including polycrystalline Si and CdTe.
9:00 PM - B5.7
Graphene-CdSe Quantum Dot Electrodes Fabrications for Solar Cells using Electrophoretic Deposition.
Mi-Hee Jung 1 , Man Gu Kang 1
1 Thin Film Solar Cell Technology Research Team, Advanced Solar Technology Research Department, Convergence Components and Materials Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon Korea (the Republic of)
Show AbstractQuantum dots (QDs) solar cells offer significant advantages over dyes sensitized solar cells. QDs can match the solar spectrum better because their absorption spectrum can be tuned to the particle size. In addition, QDs have recently been shown to generate multiple excitons, which can improve the device efficiency. However, despite the advantages of using QDs as a sensitizer, QD cells are less efficient than dye cells 9 because significant carrier loss can be caused by nonradiative recombination processes occurring at the interfaces between the QDs and the electrolyte, the oxide electrode, and adjacent QDs. To enhance the photocurrent generated by these semiconductor-matrices systems. It is essential to retard the recombination of electron-hole species in the semiconductors by efficient electron-transport matrices, such as conducting polymer films or carbon nanotubles. Here, Nanocrystal CdSe QDs/graphene hybrid materials were synthesized, assembled photoanode of solar cells via electrochemical deposition. To prepare the QDs/graphene hybrid composite, we present a simple, noncovalent method for anchoring CdSe QDs onto graphene sheet through a ligand-exchange approach. Trioctylphosphine oxide capped CdSe QDs were prepared and the cap was then exchanged for pyridine. The pyridine moiety functioned as a short, noncovalent linker between the QDs and graphene and allowed more efficient carrier transfer through the assemblies without deleteriously altering electronic structures. A one-step process of solubilization of QDs/graphene in a organic solvent has enabled us to polarized them asymmetrically in a dc electric field. At low dc applied field(~ 30V), all of the CdSe QDs/graphene hybrid materials from the suspension are deposited on the electrode, thus providing a simple methodology to design robust CdSe QDs/graphene films on the transparent conducting electrodes. The possibility of CdSe QDs/graphene films in an electric field opens new ways to achieve the replacement dye/TiO2 system in the dye sensitized solar cells.
9:00 PM - B5.8
The Fabrication of Core-shell ZnO Nanowire/TiO2 Nanotubes Structures for the High Efficiency Solar Cells.
Mi-Hee Jung 1 , Man Gu Kang 1
1 , Electronics and Tecommumications Research Institute, Daejeon Korea (the Republic of)
Show AbstractTitanium (IV) dioxide (TiO2) is one of the most attractive d-block transition metal functional oxides. Many applications of TiO2 such as dye-sensitized solar cells and photocatalyst have been widely investigated. To utilize solar energy efficiently, TiO2 should be well-aligned with a high surface area and promote the charge separation as well as electron transport. Recent years have seen explosive research on the well-aligned TiO2 nanotubes (NTs), since a number of properties, including photoelectronics and photocatalysis. Perpendicularly oriented low dimensional TiO2 NTs as electron acceptors can be coupled with hole-transport materials to produce interdigitated solar-cell devices in which photon-induced exciton can be separated and transported more quickly. To improve charge separation, TiO2/semiconductor composite have proved successful, but effort to enhance electron transport are less so due to the inefficient interface between TiO2 and the semiconductor. Herein we present the new composite structure of TiO2/ZnO. First, we prepared the perfect alignment of NTs with very-high aspect ratios by a two-step anodization process. The TiO2 NTs arranged in a perfectly parallel way, and were perpendicularly orientated to the substrate surface. The ZnO seed layer was deposited on the TiO2 nanotubes. The substrate was immersed in the ZnO precursor solutions. The ZnO nanowires were successfully growned in the TiO2 NTs. We expected this core-shell ZnO nanowires/TiO2 NTs structures improved the solar cell efficiency by providing the direct path way of electrons to the collecting electrodes.
9:00 PM - B5.9
Selective Electron or Hole Transport Enhancement in Bulk-heterojunction Organic Solar Cells with N- or B-Doped Carbon Nanotubes.
Ju Min Lee 1 , Ji Sun Park 1 , Sun Hwa Lee 1 , Hoyeon Kim 2 , Seunghyup Yoo 2 , Sang Ouk Kim 1
1 Department of Materials Science and Engineering, KAIST, Daejeon Korea (the Republic of), 2 Department of Electrical Engineering, KAIST, Daejeon Korea (the Republic of)
Show AbstractWe present the remarkable performance improvement of organic solar cells upon incorporating N- or B-doped carbon nanotubes (CNTs) into the organic semiconductor active layer. A small amount (0.2-5 wt%) of doped multi-walled CNTs are added to the bulk-heterojuction of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61 (PCBM). Unlike undoped metallic multi-walled CNTs, which cause undesired electron-hole recombination, N- or B-doped CNTs uniformly dispersed in the active layer selectively enhance electron or hole transport, respectively, and eventually help carrier collection. Specifically, the incorporation of 1 wt% B-doped CNTs results in a balanced electron and hole transport and accomplishes a power conversion efficiency improvement from 3.0 % (conventional control cells without CNTs) to 4.1 %.
Symposium Organizers
Yalin Lu University of Colorado
MarkT. Lusk University of Colorado
JohnM. Merrill Air Force Research Laboratory
Sheila Bailey NASA Glenn Research Center
Alberto Franceschetti National Renewable Energy Laboratory
Symposium Support
National Renewable Energy Laboratory
Naval Research Laboratory, Solid State Devices Branch
B8: Poster Session: Next Generation Photovoltaics VII: Thin Films, Plasmonics, and Emerging Strategies
Session Chairs
Thursday PM, April 28, 2011
Salons 7-9 (Marriott)
B6: Next Generation Photovoltaics VI: Thin Films and Plasmonics
Session Chairs
Thursday PM, April 28, 2011
Room 2001 (Moscone West)
9:30 AM - B6.1
Solar Energy Harvesting High-resolution Plasmonic Color Filters.
Hui Joon Park 1 , Ting Xu 2 3 , L. Jay Guo 1 2
1 Macromolecular Science & Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Electrical Engineering & Computer Science, University of Michigan, Ann Arbor, Michigan, United States, 3 State Key Laboratory of Optical Technologies for Microfabrication, Institute of Optics and Electronics, Chinese Academy of Science, Chengdu China
Show Abstract Color filters are widely used in all color displays. Apart from transmitted colors, light of other wavelengths are completely absorbed by the colorant and inevitably wasted. If such energy can be utilized to generate electrical power instead, innovative energy-efficient electronic media can be envisioned. Recently, as a method to scavenge light energy from the environment for self-powering systems, photovoltaic cells are highlighted as one of the most promising green energy sources. In this context, we introduce a new color filter concept that can harvest the absorbed light to generate electric power as a PV cell while preserving the color filtering function. Recently we demonstrated metal-insulator-metal (MIM) plasmonic resonators with periodic gratings to implement novel color filters with high efficiencies and high resolutions [1]. Realizing that the top and bottom metal layers of the MIM structure can function as electrodes in an electro-optic system, we propose reflective color filters by replacing the insulating layer with organic semiconductors and demonstrate electrical energy conversion with such color filters. Our energy-generating plasmonic MIM color filters are capable of filtering white light into individual colors across the entire visible band. We focused on the reflectance type color filtering devices because they are capable of working under bright ambient light; and even better under direct sun light. Organic photovoltaic (OPV) cell concept was applied to the plasmonic color filter to achieve dual-function, which also preserve unique advantages of OPV such as low cost, easy fabrication, and compatibility with flexible substrates over a large area. Since the reflective type color filters act similar to the color paint, i.e. absorbing some colors corresponding to specific wavelengths but reflecting the others, we focused on the CMY color scheme (cyan, magenta and yellow) in this work. The reflection spectra of designed CMY color filters showed expected color filtering behavior, and the experimentally measured spectrum was well-matched with the simulation results. Furthermore, photovoltaic function was achieved for all three color filters that were characterized under AM 1.5G simulated sun light (at 100 mW/cm2 intensity). Compared with the filters made by the traditional chemical pigments, our dual-function devices overcome the defects such as fading and embrittlement, and they maximally absorb solar energy and convert it to electricity by using photoactive semiconductors. This unique energy-generating property could lead to energy-efficient electronic displays.[1] T. Xu, Y.-K. Wu, and X.-G. Luo and L. J. Guo, “Plasmonic Nano-resonators for Color Filtering and Spectral Imaging,” Nat. Commun. 1, 59 (2010).
9:45 AM - **B6.2
Broadband Light Absorption Enhancement in Thin-film Silicon Solar Cells.
Wei Wang 2 , Shaomin Wu 2 , Kitt Reinhardt 3 , Yalin Lu 4 , Shaochen Chen 1
2 Materials Science and Engineering, The University of Texas at Austin, Austin, Texas, United States, 3 , United States Air Force Office of Scientific Research, Arlington, Virginia, United States, 4 , United States Air Force Academy, United States Air Force Academy, Colorado, United States, 1 Nanoengineering, he University of California, San Diego, La Jolla, California, United States
Show AbstractA major problem of current silicon thin film solar cells lies in low carrier collection efficiency due to short carrier diffusion length. Instead of improving the collection efficiency in a relatively thick solar cell, increasing light absorption while still keeping the active layer thin is an alternative solution. In this research, large, broadband, and polarization-insensitive light absorption enhancement was realized via integrating with periodic metallic nanopattern. Through simulation, three possible mechanisms were identified to be responsible for such an enormous enhancement. A test for totaling the absorption over the solar spectrum shows an up to ∼30% broadband absorption enhancement when comparing to bare thin film cells.
10:15 AM - B6.3
Aligned Nanorod Photovoltaics.
Jessy Rivest 1 5 , Sarah Swisher 2 5 , A Paul Alivisatos 3 4 5
1 Mechanical Engineering, UC Berkeley, Berkeley, California, United States, 5 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Electrical Engineering, UC Berkeley, Berkeley, California, United States, 3 Materials Science, UC Berkeley, Berkeley, California, United States, 4 Chemistry, UC Berkeley, Berkeley, California, United States
Show AbstractColloidal nanoparticles hold great promise for inexpensive optoelectronics, particularly in photovoltaic applications where the ability to solution-process a high-quality semiconductor film could enable an order-of-magnitude cost reduction. By leveraging device-scale self-assembly of nanorods as well as in-film cation exchange, we are able to produce a monolayer film of nanorod heterojunctions; a massively parallel array of individual p-n junctions. We take electrical measurements of these monolayer devices and assess future prospects of nanocrystal solar cells through this test platform.
10:30 AM - B6.4
Gold Nanolayers Embedded in Zinc Oxide for Large Area Flexible Photovoltaics.
Karthik Sivaramakrishnan 1 , Terry Alford 1
1 School of Materials, ASU, Tempe, Arizona, United States
Show AbstractTransparent conducting ZnO/Au/ZnO films were grown by the magnetron sputtering technique on flexible PEN substrates. The films were found to undergo a seven orders of magnitude drop in resistivity from 200 Ω-cm to 5.2×10-5 Ω-cm upon increase of the gold layer thickness from 0 nm to 12 nm. The sheet resistance also showed a substantial decrease to a low 6.5 Ω/sq. The films displayed a photopically average transmittance between 75% and 85% depending upon the gold thickness, and a peak transmittance of up to 93%. The best Haacke figure of merit was 15.1×10-3 Ω-1. Transmission electron microscopy was used to systematically observe the growth of initially discontinuous gold islands into films. The conduction changed from conduction through the substrate when the nanometal islands are small and far apart to activated tunneling between discontinuous islands, and finally to direct tunneling between larger islands and metallic conduction through a near-continuous layer. Optical transmission behavior of the films was understood to comprise gold’s absorption due to interband electronic transitions in the shorter visible wavelengths, and free carrier absorption losses at the longer red wavelengths. This was combined with the limitation of the mean free path in discontinuous films in the longer visible wavelengths. The dielectric constant of the small gold particles exhibits a size dependence that enhances absorption. Low temperature thermal processing of the multilayer ZnO/Au/ZnO films for up to 96 hours at 85 oC leads to no deterioration in the properties while the films also remained intact when subject to room temperature static bend testing and high temperature static bend testing. Thus the films are also shown to be robust to environmental harshness making them a suitable choice for commercial exploitation in large-area flexible photovoltaics.
10:45 AM - B6.5
Plasmonic Thin-film Solar Cells with Metallic Nanostructures.
Wallace C.H. Choy 1 , Wei Sha 1 , L. Qiao 1 2 , S. He 1 2 , Weng Chew 1
1 Department of Electrical & Electronic Engineering, The University of Hong Kong, Hong Kong, ----------------, China, 2 Centre for Optical and Electromagnetic Research, Zhejiang University, Hangzhou , 310058, China
Show AbstractA theoretical and experimental study of the plasmonic thin-film solar cell with the metallic nanostructures is presented in this paper. For periodic metallic nanostructures, the finite-difference frequency-domain method is employed to discretize the inhomogeneous wave function for modeling the solar cell. In particular, the hybrid absorbing boundary condition and the one-sided difference scheme are adopted. The parameter extraction methods for the zero-order reflectance and the absorbed power density are also discussed: they are important for testing and optimizing the solar cell design. Furthermore, we propose the phase and attenuation constants conditions of the surface plasmon polariton for lossy materials. For the numerical results, the physics of the absorption peaks of the thin-film solar cell is explained by electromagnetic theory and correspond to the waveguide mode, Floquet mode, surface Plasmon resonance, and the constructively interference between metallic nanostructures. The work is therefore important for the theoretical study and optimized design of the plasmonic thin-film solar cell. Meanwhile, we have also conducted experimental work in plasmonic polymer thin film solar cells by using conventional blend polymer materials of poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM). Our results show that the incident photon to charge-carrier efficiency (IPCE) and current density (J)-voltage (V) characteristics are improved. The power conversion efficiency of the polymer thin film solar cells can be enhanced by about 20-30% through introducing appropriate metallic nanoparticles.
11:30 AM - B6.6
Photocurrent Enhancement in Single Nanowire Silicon Solar Cells Using Shape-controlled Silver Nanocrystals.
Sarah Brittman 1 , Erik Garnett 2 , Peidong Yang 1
1 , University of California, Berkeley, Berkeley, California, United States, 2 , Stanford University, Stanford, California, United States
Show AbstractMuch recent work has demonstrated that the plasmonic properties of metal nanoparticles can be used to increase a solar cell's absorption of light. In metals, the electrons oscillate collectively when excited by an electric field. These oscillations, known as surface plasmons, lead to increased scattering cross-sections and can also create intense electric fields within tens of nanometers of the particle's surface and particularly at corners and edges. In order to study the interaction between metal nanoparticles and a solar cell in detail, core-shell single nanowire silicon solar cells have been fabricated and coated with shape-controlled silver nanocrystals. The performance of the single nanowire solar cells was characterized before and after deposition of the nanocrystals by current-voltage measurements under simulated sunlight and by scanning photocurrent mapping. Photocurrent enhancements under simulated sunlight have been observed, while scanning photocurrent mapping has been used to visualize changes in the spatial distribution of the photocurrent as well as its dependence on the wavelength of the incident light.
11:45 AM - B6.7
Fabrication of Subsurface Metallic Nanoparticles For Enhanced Carrier Generation in Silicon-based Photovoltaics.
Nirag Kadakia 1 , Hassaram Bakhru 1 , Mengbing Huang 1
1 Ion Beam Laboratory, State University of New York - Albany, Albany, New York, United States
Show Abstract Due to the low absorption coefficient of silicon in the bulk of the solar spectrum, the majority of silicon-based photovoltaic cells are at least 300 micrometers thick, limiting their economic feasibility. To achieve cost parity with conventional sources of energy, silicon-based photovoltaics have begun to move towards thinner substrates, on the order of tens of micrometers; such thinner cells necessitate the use of light-trapping methods to increase the optical path length. Much research has begun investigating the use of trapped electromagnetic waves, or surface plasmons, to increase light scattering and interband carrier transition rates in the surrounding material. In one scheme, plasmonic modes can be supported through polarization of metallic nanoparticles. Past research has focused on the deposition of silver nanoparticles on the surface, and has shown that light absorbance can be increased for certain bandwidths by several factors. Here, the relevant mechanism is increased lateral light scattering, which tends to guide the light into directions that are then totally internally reflected. When such metallic nanoparticles are polarized by incoming radiation, in addition to increased light scattering, the electric field in the vicinity of the nanoparticle is highly magnified. Such high fields can increase the carrier transition rates, and therefore the absorbance, in the surrounding silicon by orders of magnitude. The enhancement however decreases exponentially with the radial distance and is virtually diminished by 10 nm from the nanoparticle surface. Deposition on the surface of the solar cell, therefore, cannot exploit this effect, being isolated from the silicon itself by the passivating layer. It has been suggested that instead, such particles might be embedded into the silicon itself. To that end, we have developed a method to create subsurface silver nanospheres, using implantation followed by thermal deposition of a thin silver layer and subsequent thermal annealing. By tailoring the implantation parameters, we can localize the layer of nanospheres and even create various bands at various depths. Through Rutherford backscattering characterization, we have found that the Ag has indeed annealed into the desired location. To ensure that the Ag has agglomerated to nanoparticles, we have confirmed this with TEM images and selected area diffraction patterns that indicate that such nanoparticles are indeed bulk phase silver and have diameters of 30-50 nanometers. Furthermore, Rutherford backscattering channeling measurements indicate that the crystal lattice, while damaged by the implant, is fully recrystallized by thermal annealing. This method can help realize more efficient surface plasmon-enhanced Si-based photovoltaics.
12:00 PM - B6.8
Novel Plasmonic Structures for Absorption Enhancement in Organic Photovoltaics.
Wenli Bao 1 2 3 , Qiaoqiang Gan 1 , Guofeng Song 2 , Lianghui Chen 2 , Filbert Bartoli 1 , Zakya Kafafi 4 1
1 1 Electrical and Computer Engineering , Lehigh University, Bethlehem, Pennsylvania, United States, 2 Institute of Semiconductors , Chinese Academy of Sciences, Beijing China, 3 Electronic Engineering , Tsinghua University, Beijing China, 4 Division of Materials Research, National Science Foundation, Virginia, Virginia, United States
Show AbstractPlasmonic structures are integrated with organic photovoltaics (OPVs) to concentrate the electromagnetic field inside the photoactive layer(s) and improve their photon absorption in the solar spectrum. For instance, a novel double plasmonic nanostructure consisting of metallic nanodiscs on one side of the OPV and a nanopatterned thin metal film on the other side can lead to a large, easily tunable photon absorption enhancement due to the excitation of localized surface plasmon polaritons on the metallic nanodiscs and short-range surface plasmon polaritons on the nanopatterned metallic film. Three-dimensional finite-difference time domain calculations show that these plasmonic structures can enhance the optical absorption of OPVS by more than 100% depending on the nature of the organic material(s) used as the active layer(s). These results are promising for the design of organic photovoltaics with enhanced performance.
12:15 PM - B6.9
Improved Solar Cell Absorption through Surface Plasmon Excitation on Nanostructured Interfaces.
Eadaoin McClean 1 , Dominic Zerulla 1
1 SFI-Stategic Research Cluster in Solar Energy Conversion, UCD School of Physics, University College Dublin, Dublin 2 Ireland
Show AbstractSurface Plasmon Polaritons generated along a metallic/dielectric interface give rise to an enhanced electromagnetic field close to this boundary, which decays exponentially away from the surface. Designing a solar cell, which includes such an interface along the active layer, leads to electromagnetic field enhancement experienced within the absorbing section of the cell. Increasing the field at this location boosts the active layers exposure to photons, and therefore greatly increases the likelihood of photon absorption. These improved absorptive properties will lead to a reduction in the required thickness of the active layer within the cell, which greatly reduces recombination of electron-hole pairs, a significant problem in both silicon and polymer-based solar cells. Numerical simulations of new solar cell designs, which implement a new metallic plasmonic layer, are presented for both poly-3-hexylthiophene-phenyl-C61-butyric (P3HT:PCBM) and Sensitized Titanium-Dioxide based solar cells. Different aspects regarding the inclusion of a new metallic layer are discussed, with special attention to the plasmon excitation requirements.Surface Plasmon excitation is highly dependent of the direction and wavelength of the incoming radiation, the properties of the materials involved and the surface geometry of the interface. As is typical for solar cells, the angle of incidence of the light was assumed to be close to normal to the surface, and the typical AM1.5 Solar intensity spectrum was used. As either the active layer of the solar cells or the glass contacts can be utilised as the dielectric side of the interface, a new metallic layer must be inserted directly next to either of these to ensure plasmon excitation. New surface geometries were investigated in order to broaden and tailor the spectral response of the plasmon across the solar spectrum.
12:30 PM - B6.10
Design Criteria For Surpassing the Classical Light-trapping Limit in Thin Film Solar Cells.
Jeremy Munday 1 , Dennis Callahan 1 , Harry Atwater 1
1 Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California, United States
Show AbstractWe describe a methodology for designing thin film solar cells that have light-trapping intensity and absorption enhancements that exceed the classical, ergodic light-trapping limit using a wave optics approach. From thermodynamic arguments, Yablonovitch and Cody determined the maximum absorption enhancement in the ray optics limit for a bulk material to be 4n2, where n is the index of refraction of the absorbing layer [1]. Stuart and Hall expanded this approach to study a simple waveguide structure; however, for the waveguide structures they considered, the maximum absorption enhancement was <4n2 [2]. Using a combination of analytical and numerical methods, we describe why these structures do not surpass the ergodic limit and show how to design structures that can. We present here a physical interpretation in terms of the waveguide dispersion relations and describe the necessary criteria for surpassing the classical limit. In particular, we show that the wavevector needed for a mode to surpass the ergodic limit is given by β>(2n2h ω2)/(Γ π c2), where Γ is the waveguide confinement factor and h is the waveguide thickness. Finally, we provide examples of waveguide structures with absorption enhancements in excess of 4n2 based on the above criteria.[1] Yablonovitch and Cody. IEEE Trans. Elect. Dev. 29 300 (1982)[2] Stuart and Hall J. Opt. Soc. Am A 14 3001 (1997)
12:45 PM - B6.11
Effective Medium Analysis of Silver Nanoparticle Films for Plasmonic Light Trapping in Solar Cells.
Rudi Santbergen 1 , Jeroen Sap 1 , Tristan Temple 1 , Sergey Solntsev 1 , Rene van Swaaij 1 , Miro Zeman 1
1 , Delft University of Technology, Delft Netherlands
Show AbstractAn efficient light trapping scheme that maximizes the absorption in the active layer of thin-film solar cells is essential for obtaining high energy conversion efficiency. Conventional light trapping schemes rely on substrate surface roughness to achieve scattering of incident light. A novel light trapping approach is based on the application of metal nanostructures. These structures can scatter visible and near infrared light more efficiently due to surface plasmon resonance. In addition, this plasmon mediated light scattering is highly tunable through manipulation of the nanostructure geometry. Empirical evidence presented in literature demonstrates that a sub-monolayer of silver nanoparticles can improve the performance of photo detectors and simple solar cell structures. However, there is still some debate on which mechanisms play a role in the absorption enhancement.We shed some light on this issue by showing that the macroscopic optical properties of metal nanoparticle films can be described in terms of a uniform film with an ‘effective’ dielectric function. This effective medium approximation (EMA) was validated experimentally. For this we fabricated films of silver nanoparticles with a diameter on the order of 100 nm. The particle shape and size distributions were analyzed by means of scanning electron microscopy and the optical properties were determined using spectral reflectance-transmittance measurements and ellipsometry. In addition, finite-difference time-domain simulations were performed to study the effect of nanoparticle shape on the effective dielectric function in more detail. The insight gained in this way is implemented in an optoelectrical solar cell simulation program. Simulation results of the external quantum efficiency of amorphous silicon solar cells with embedded silver nanoparticles will be presented.Unique to or our approach is the use of the Bergman EMA to determine the effective dielectric function. The well known Maxwell-Garnett and Bruggeman EMAs are special cases of this more general Bergman EMA. Input for the Bergman EMA is the dielectric function of nanoparticles and of the surrounding medium and a so-called spectral density function. This spectral density function links the topological details of a nanoparticle film to its macroscopic optical properties and therefore provides valuable insight.
B7: Next Generation Photovoltaics VII: Thin Films and Plasmonics I
Session Chairs
Thursday PM, April 28, 2011
Room 2001 (Moscone West)
2:30 PM - B7.1
Cu2ZnSnS4, and Mo Annealing by Rapid Thermal Annealing, and Its Effects on Grain Growth and Cell Performance.
Jeffrey Johnson 1 , Michael Scarpulla 1 2
1 Electrical and Computer Eng, University of Utah, Fruit Heights, Utah, United States, 2 Material Science and Eng., University of Utah, Salt Lake City, Utah, United States
Show AbstractMany reports have been made for the annealing of Cu2ZnSnS4 (CZTS) for the absorber layer of thin film type solar cells at many different temperatures. We have investigated many of these as well. Since a temperature of 550 oC or greater is needed to grow grains of CZTS, and since substrates become stressed or softened at temperature over 600 oC, the required annealing temperature range of CZTS is small. In dealing with the issues of annealing CZTS, which include substrate stress, substrate softening/deformation, small grain size and poor cell performance, we investigated the use of rapid thermal annealing (RTA). Substrate stress, substrate softening and deformation happen when the annealing temperature of the deposited film surpasses the annealing temperature of the substrate. When the annealing temperature is surpassed, the substrate will deform to relieve the stress between the substrate and the just deposited film. Also if the cooling of the substrate is not controlled after surpassing the annealing temperature, the substrate will be stressed and brittle. With the use of RTA, we have achieved large grain growth on the order of 700 nm on boro-aluminosilicate (BSG) (corning 1737) without the use of Na, and without the stress, softening or distortion of the substrate. More than 60% of grains have similar orientation (112) in the normal direction as determined by X-Ray Diffraction (XRD) and electron backscatter diffraction (EBSD). In addition we have developed a two step process for depositing and annealing the CZTS layer. The two step process consists of depositing a thin layer of CZTS, next RTA is used to crystallize this layer, followed by the deposition of the rest of the CZTS layer and completed with another annealing step. This two step process allows us to have crystallized and well oriented base material to pattern the growth of the rest of the deposition. We compare the improvements of this two step deposition method with standard annealing and RTA by XRD, EBSD, SEM and AFM. As far as we know, no other group has looked at annealing the Mo back contact before deposition. For Mo annealing before deposition of CZTS, we evaluated the Mo to substrate stress, Mo resistance, Mo crystallization, and the changes to grain structure of the CZTS film. These results, using the following methods: 4-point probe, XRD, EBSD and SEM will be shown.
2:45 PM - **B7.2
Fundamental Processes and Limits of Small Molecule Organic Solar Cells.
Wolfgang Tress 1 , David Wynands 1 , Toni Mueller 1 , Ronny Timmreck 1 , Martin Hermenau 1 , Moritz Riede 1 , Karl Leo 1
1 Institut fuer Angewandte Photophysik, Technische Universitaet Dresden, Dresden Germany
Show AbstractDue to their advantageous properties such as low materials usage, flexibility, and transparency, organic solar cells have been recently investigated intensively. However, to be serious contenders for power generation applications, organic solar cells still need a significant improvement in terms of efficiency and lifetime. In this talk, we discuss our recent work on the investigation of some of the relevant processes which need to be understood to reach these goals. For instance, organic solar cells often suffer from distorted IV-curves due to charge carrier mobility imbalances and energy barriers at interfaces. Furthermore, the efficient charge carrier generation requires a careful adjustment of deposition conditions of the photoactive layers in form of a bulk heterojunction and corresponding optical spacers. Finally, optimized tandem devices realized in collaboration with our partner Heliatek have achieved efficiencies beyond 8% on application-relevant device areas.
3:15 PM - B7.3
Antimony Surfactant Mediated Improvement of the Properties of GaInNAsSb Films for Multi-junction Tandem Solar Cells.
Naoya Miyashita 1 , Nazmul Ahsan 1 , Kin Yu 2 , Wladek Walukiewicz 2 , Yoshitaka Okada 1
1 Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Meguro-ku, Tokyo, Japan, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show Abstract[Introduction] The dilute nitride semiconductor alloys, GaInNAs, have received much attention because of their potential applications for high-efficiency multi-junction tandem solar cells. Incorporation of nitrogen into InGaAs allows growth of GaInNAs films lattice matched to GaAs with bandgap of ~1eV. However addition of N has been found to degrade both the optical and electrical characteristics of these alloys. Recently, effectiveness of antimony (Sb) surfactant for improving optical properties of MBE grown GaInNAs thin films was reported. In this work, we focus on the electrical properties of GaInNAsSb, such as carrier concentrations and mobilities.[Results and Discussion] MBE grown GaAs, GaInxNyAs, and GaInxNyAsSb films (x ~0.045, y ~0.015) used in this study contain almost the same amount of Si donor impurity, which was confirmed from a separate SIMS measurement. Compared to GaAs (n = 6.5×1017 cm−3), introduction of N into GaInAs significantly reduces activation efficiency of Si donors (n = 3.4×1016 cm−3 in GaInNAs). This carrier reduction in GaInNAs can be attriuted to trapping of electrons in N-related defects. A supply of Sb suppresses the defects, which improves Si donor activation with n = 1.9×1017 cm−3 in GaInNAsSb. This interpretation is further supported by the temperature dependence of the electron concentration, n. As expected in degenerately doped GaAs, there is very small change of the n in the temperature range of 7-296K. In contrast, a rapid decrease of n with decreasing temperature is observed in GaInNAs where the conduction band electrons are localized in the defects. A much weaker temperature dependence of n found in GaInNAsSb is fully consistent with a considerable reduction of the defects in samples grown with Sb. Furthermore, it has been found that adding Sb improves electron mobilities from 222cm2/Vs (no Sb) to 419cm2/Vs (Sb flux = 5×10−8 Torr). Si donors in GaInNAs alloys can be passivated by N through the formation of N and Si nearest neighbor pairs. These pairs can also act as deep traps in Ga(In)NAs material. To study the effect of Sb on the formation of N-Si pairs, we grew two sets of samples (with and without Sb) highly doped with 5×1018 cm-3 of Si donors. In order to enhance N-Si nearest neighbor pair formation, rapid thermal annealing (RTA) at several temperature below 1000°C were carried out. No significant change in the carrier concentrations up to a RTA temperature of 850°C was observed. However, the concentration decreases significantly at RTA temperatures >900°C in both samples. This suggests that our GaInNAs(Sb):Si samples are thermally stable and no significant diffusion of the Si and N in the samples occurs below 850°C.[Acknowledgements] Part of this work is performed at Lawrence Berkeley National Laboratory (LBNL), and is supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This work is performed under SOLAR QUEST program supported by NEDO and METI, Japan.
3:30 PM - B7.4
Effects of Sodium Addition on Cu(In,Ga)Se2 Thin-film Solar Cells: Microstructure, Texture, and Cell Performance.
Tao Liu 1 , Oana Cojocarcu-Miredin 1 , Pyuck-Pa Choi 1 , Dierk Raabe 1 , Roland Wuerz 2
1 Microstructure Physics and Metal Forming, Max-Planck-Institut fuer Eisenforschung, Duesseldorf, NRW, Germany, 2 , Zentrum für Sonnenenergie- und Wasserstoff-Forschung, Stuttgart Germany
Show AbstractPolycrystalline Cu(In,Ga)Se2 (CIGS) thin films deposited on Mo-coated steel substrates with and without Na doping were analyzed by means of high-resolution electron back-scattering diffraction (EBSD), atom probe tomography (APT), and X-ray fluorescence (XRF) combined with secondary ion mass spectrometry (SIMS). Using EBSD, Primarily {110}tet/{102}tet and {112}tet fiber textures were observed for both Na-free and Na-containing samples. High angle, low angle and twin boundaries were observed in the CIGS layers. The evolution of the texture components and the grain boundary types was studied from the initial Mo-CIGS interfaces to the CIGS growth surfaces. The concentration and distribution of Na and Fe impurities in CIGS were measured by APT and SIMS. For the Na-doped sample, Na was found to be segregated and clustered at the grain boundaries, while Cu was depleted. It is suggested that Na atoms at the grain boundaries partly occupy Cu vacancies and suppresses the formation of compensating InCu antisite defects. Also, the influence of Na doping on crystallographic textures and grain size and shape evolutions in CIGS thin films was investigated. The phenomena are associated with a growth selection mechanism.
3:45 PM - B7.5
Nanomolding of Zinc Oxide Nanostructures and Its Application to Efficient Light Trapping in Thin-film Solar Cells.
Corsin Battaglia 1 , Jordi Escarre 1 , Karin Soederstroem 1 , Franz-Josef Haug 1 , Matthieu Despeisse 1 , Christophe Ballif 1
1 IMT PV-Lab, EPFL, Neuchatel Switzerland
Show AbstractThe ability to pattern functional materials at the nanometer scale has become of crucial importance in many fields of nanoscience and nanotechnology. In particular for photovoltaics, nanopatterning has gained tremendous importance, as absorption of sunlight may be enhanced drastically by proper engineering of photonic nanostructures ultimately enabling thinner absorber layers. Thin cells not only reduce the amount of raw material required, but also tolerate materials with shorter carrier diffusion length.Currently zinc oxide (ZnO) is one of the key functional materials for advanced optoelectronic and photonic applications, including photovoltaics, due to its high transparency across the solar spectrum, excellent electrical properties, and the possibility to synthesize a rich variety of nanostructures. Approaches, so far explored to increase light trapping in solar cells, include the growth of ZnO films with randomly-oriented pyramids via chemical vapor deposition [1] or wet etching of crater-like structures into sputtered ZnO films [2]. Also solution-based methods have been extensively investigated for the synthesis of nanopillar-type ZnO structures [3].Although these approaches provide a certain degree of freedom in designing the surface morphology of ZnO films, the basic feature morphology, i.e., pyramids, craters or pillars, is dictated by the underlying growth and etch kinetics. Experimentally, one desires a method capable of implementing arbitrarily designed surface morphologies. Here we demonstrate that nanomolding of ZnO provides exactly such a platform [4].We use ultraviolet nanoimprint lithography (UV-NIL) [5,6] for mold fabrication from a given master structure. ZnO films are subsequently grown on the mold via chemical vapor deposition and anchored on glass substrates using an UV-curable sol-gel lacquer. Finally the mold is detached, leaving a nanostructured ZnO film. We demonstrate the power of our method by fabricating one-dimensional periodic gratings, two-dimensional quasi-periodic dimple arrays, and randomly-oriented pyramid networks.We further present first experimental results on high-efficiency thin-film silicon solar cells grown on these substrates. We anticipate a strong potential for other thin-film technologies, including cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), and organic solar cells, as well as for other optoelectronic and photonic applications as our method provides a versatile tool to study nanophotonic effects of a large variety of nanostructures directly on the device performance.[1] S. Faÿ et al., Thin Solid Films, 515, 8558 (2007)[2] M. Berginski et al., Thin Solid Films, 516, 5836 (2008)[3] D. Lincot, MRS Bulletin, 35, 778 (2010)[4] C. Battaglia et al., submitted (2010)[5] J. Escarre et al., submitted (2010)[6] C. Battaglia et al., Appl. Phys. Lett. 96, 213504 (2010)
4:30 PM - B7.6
Impact of Junction Area on the Performance of Micro/Nano Pillar-based III-V Solar Cells.
Loucas Tsakalakos 1 , Joleyn Balch 1 , Albert Byun 1 , Ahmed Elasser 1 , Jody Fronheiser 1 , Ted Kreutz 1 , Oleg Sulima 1 , Suraj Rawal 2 , Justin Likar 3
1 Micro & Nano Structures Technologies, GE Global Research, Niskayuna, New York, United States, 2 , Lockheed-Martin Space Systems Company, Littleton, Colorado, United States, 3 , Lockheed-Martin Space Systems Company, Newtown, Pennsylvania, United States
Show AbstractSolar cells based on elongated nanostructures have been recently demonstrated in silicon, InP, GaAs, and CdTe nanowires, as well as amorphous silicon (a-Si:H) nanodomes, with power conversion efficiencies of up to 6% having been obtained for CdTe and a-Si:H nanodevices. While the use of silicon is compelling due to its abundance and non-toxic nature, III-V materials offer the promise of higher efficiency due to their direct bandgap, and the potential to implement advanced high efficiency schemes such as multi-junctions. The wire array structure provides several additional distinct benefits, including inherent anti-reflective properties, efficient lateral carrier collection leading to enhanced Jsc, and also enables growth of high quality single crystal arrays on unconventional substrates due to the combination of relaxing biaxial stresses imposed by epitaxy and the use of the catalytic vapor-liquid-solid (VLS, and related vapor-solid-solid) growth mechanism. Therefore, III-V nanowires offer the potential to develop a low-cost, high efficiency (>18%) photovoltaic module with a flexible form factor. Here we demonstrate the basic operational principles of III-V elongated micro and nanostructured devices by first performing extensive modeling studies of GaAs and InP nanowire-based solar cells. We show that the performance of these devices scales with pillar diameter and that within the modeled regime the pillar length has a minimal effect on performance, though the pillar spacing (area fraction) has a more direct impact on efficiency. We fabricated prototype single junction devices in bulk n and p-type <100> oriented GaAs wafers by a combination of photolithography and dry etching, followed by epitaxial growth of conformal p-n junctions with and without an AlGaAs passivation layer. Pronounced faceting of the conformal layers was observed by scanning electron microscopy. The solar cells showed a clear improvement in efficiency with pillar diameter (0.7-2 mm) with the best performing devices achieving an active-area power conversion efficiency of ~11% (8.6% total-area efficiency, Voc = 781 mV, FF = 0.67), whereas planar control samples showed an active-area efficiency of ~12.7% (~10% total area efficiency, Voc = 878 mV, FF = 0.75). Importantly, the Jsc was found to be higher for the pillar device than the planar control (20.9 vs. 19.3 mA/cm2), suggesting that pillar devices provide improved carrier extraction, as expected theoretically. The impact of junction area on dark and illuminated device parameters will be described, as well as the potential effects of implementing MOCVD grown nanowire devices.
4:45 PM - B7.7
Mesoporous TiO2 Thin Films Prepared by Flame Stabilized on a Rotating Surface - Application to Dye Sensitized Solar Cells.
Saro Memarzadeh 1 , Jason Walker 1 , Denis Phares 1 , Hai Wang 1
1 , University of Southern California, Los Angeles, California, United States
Show AbstractDye sensitized solar cells (DSSC) offer great promise as an inexpensive alternative to conventional silicon and thin film solar cells. One of the primary advantages of DSSCs is that they do not contain rare elements that may limit their future mass production. The components of the cell include a photosensitive dye coated onto a mesoporous titania film and submerged in an iodide/triiodide electrolyte. The role of the titania layer is to receive electrons from the photo-excited dye molecules, transmit the electrons out of the cell, and allow the electrolyte to diffuse throughout the film to replenish the electrons in the dye. The authors have developed a premixed stagnation flame synthesis methodology that produces and deposits titania films in a single step, and with a very high level of control over particle size and crystallinity. Thin films were fabricated using the flame synthesis approach and assembled into DSSCs. The cells were characterized by their photoconversion efficiencies among other properties as a function of fabrication parameters. The data demonstrate that the cells fabricated using the flame synthesis method are comparable to those fabricated using the conventional method.
5:00 PM - B7.8
Photo-modulated Reflectance of GaNAs Grown by Atomic Hydrogen-assisted MBE.
Nazmul Ahsan 1 , Naoya Miyashita 1 , Tooru Tanaka 2 3 , Kin Yu 2 , Wladek Walukiewicz 2 , Yoshitaka Okada 1
1 Research Center for Advanced Science and Technology, The University of Tokyo, Meguro-ku, Tokyo, Japan, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Electrical and Electronic Engineering, Saga University, Saga Japan
Show Abstract[Introduction] Dilute nitride semiconductor alloys, in particular GaNAs and GaInNAs, have attracted considerable attention due to their potential for applications in the next-generation ultra-high efficiency solar cell. Here, we focus on the evolution of the energy states of a series of GaNAs layers, coherently strained to the GaAs substrate. For this purpose, we applied photo-modulated reflectance (PR) spectroscopy, which yields higher-order transitions, and provides detailed information about the electronic structure. In order to facilitate 2D growth mode and suppress formation of vacancy-type defects, an optimized atomic H flux was applied during the MBE growth of the films.[Experimental] The investigated samples were 0.5 μm-thick GaNxAs1-x epilayers with nitrogen composition in the range x ≈ 0.70-1.75%. The layers were grown on semi-insulating GaAs(001) substrates. PR measurements were performed at room temperature utilizing a mechanically chopped 325 nm line of a 50 mW He-Cd laser.[Results] Each of the PR spectrum measured in the 1.0-2.0 eV energy range contains three distinct energy features. The low energy feature E- below 1.4 eV and the high energy feature E+ above 1.5 eV originate from the GaNAs epilayers. The middle energy feature at about 1.4 eV is independent of N composition and can be attributed to the band gap transition in the GaAs substrate. With increasing N composition, the E- and E+ transitions show red- and blue-shift, respectively. This is consistent with the prediction of band anti-crossing (BAC) model. In addition, fine features around the E- transitions were also observed. Unlike the E- features, which decrease in energy with increasing N, these fine transitions increases with increasing N. However, when GaNAs sample is lattice matched to the GaAs substrate with optimal introduction of In atoms, these features disappear. These observations suggest that the fine features can be attributed to strain induced splitting of the light- and heavy-hole bands. They are well resolved in the room temperature PR spectra, and reflect good crystalline quality and compositional homogeneity in our samples. If a bowing effect in the deformation potential of GaNAs is assumed, the amount of valence-band splitting with respect to the N composition can be well explained. The PR spectrum of a GaNAs sample (x~1.4%), annealed at 900 °C for 10 seconds, show a blue-shift of E- features, while E+ remains almost unchanged. This is presumably due to a reduction of N-related defects, which influenced the DOS tails, and thereby resulting in the blue-shift of E- features.[Acknowledgements] Part of this work is performed at Lawrence Berkeley National Laboratory (LBNL), and is supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This work is performed under SOLAR QUEST program supported by NEDO and METI, Japan.
5:15 PM - **B7.9
New Concepts and Materials for Solar Power Conversion.
Wladek Walukiewicz 1
1 , LBNL, Berkeley, California, United States
Show AbstractIn the presentation I will introduce various new concepts for high efficiency conversion of solar energy. The core presentation will focus on two research areas: group III-nitrides for high efficiency multijunction solar cells and highly mismatched alloys for multiband solar and photoelectrochemical cells.The discovery of the low band gap of InN greatly expanded the range of the direct gaps of group III-nitride alloys from 0.64 in InN to 6.1 eV in AlN. Therefore In1-xGaxN and In1-yAlyN alloys are promising materials for high efficiency solar cells, as their band gaps are continuously tunable across the solar spectrum. The potential of In1-xGaxN as a solar cell material has been well established and there has been a significant worldwide effort aimed at practical application of this material system for photovoltaic devices. The large electron affinity (5.8 eV) of InN offers a unique opportunity of matching the conduction band edge of group III-In-nitride alloys to the valence band of standard semiconductors such as Si and Ge. Thus n-type In1-xGaxN with x=0.55 or In1-yAlyN with y=0.3 form “ideal” low resistance junctions with p-type Si. Calculations show that two junction InGaN/Si or AlInN/Si tandem cells could have practical power conversion efficiencies exceeding 35%. The low resistance contact between n-InGaN and p-Si has been demonstrated for InGaN films grown on Si with GaN as well as Al thin buffer layers. Latest results on practical realization of InGaN/Si tandem cells will be presented and remaining challenges will be discussed.The second part of the presentation will be devoted to recent progress in synthesis of highly mismatched alloys (HMAs). Such alloys exhibit unusual optical and electrical properties and with a proper choice of component materials allow for an independent engineering of band gaps and band offsets. We have synthesized group II-VI dilute oxides and group III-V dilute nitride HMAs. The alloys have a unique band structure with a narrow intermediate band in the band gap of the host material. The band serves as a “stepping stone” allowing pairs of sub-bandgap photons to contribute to the solar light induced photo-current, leading to better utilization of the full solar spectrum. Recent progress on demonstration of simple, single junction intermediate band solar cell based on these alloys will be discussed. I will also present most recent results on successful synthesis of GaNAs alloys in the whole composition range and discuss potential use of these materials in photoelectrochemical cells for solar water dissociation.*In collaboration with Solar Energy Materials Research Group (http://emat-solar.lbl.gov/) **Supported by the Division of Materials Science and Engineering, US DOE
5:45 PM - B7.10
Nanostructured Thin Film Solar Cells: A Heterojunction of PbS Colloidal Quantum Dots and TiO2 Nanopillars.
Ho-Cheol Kim 1 , Illan Kramer 2 , John Bass 1 , Teya Topuria 1 , Leslie Krupp 1 , Philip Rice 1 , Ratan Debnath 2 , Lukasz Brzozowski 2 , Larissa Levina 2 , Edward Sargent 2
1 , IBM Almaden Research Center, San Jose, California, United States, 2 Electrical and Computer Engineering, Univeristy of Toronto, Toronto, Ontario, Canada
Show AbstractColloidal quantum dot (CQD) has been recognized as a promising solar cell material that offers tunable band gap and inexpensive solution process. Recent report demonstrated the power conversion efficiency (PCE) of above 5% (AM 1.5) using thin films of PbS colloidal quantum dots and TiO2 nanoparticles. This so-called depleted-heterojunction-CQD solar cells have overcome limitations of CQD Schottky devices and promised potential for further improvement of solar cell performance. In this paper, we report the effect of nanostructures of TiO2 on the performance of heterojunction CQD solar cells. Well-defined nanopillars of TiO2 were prepared on top of F:SnO2 substrate using micro-transfer molding (MTM) technique. TiO2 nanopillars of 70 nm in diameter (half-width), 340 nm in height and 275 nm in center-to-center distance were used for subsequent layer-by-layer spin coating of PbS CQD. PCE of >5% was measured for the nanopillar solar cells without extensive optimization, which is mainly due to improved short circuit current (20.5 mA/cm2) compare to that of previous work (16.2 mA/cm2). Detailed studies on the microstructure of materials, surface properties, optical and electrical properties and optimization will be discussed along with performance of flat TiO2-PbS CQD solar cells.
B8: Poster Session: Next Generation Photovoltaics VII: Thin Films, Plasmonics, and Emerging Strategies
Session Chairs
Friday AM, April 29, 2011
Salons 7-9 (Marriott)
9:00 PM - B8.1
Synthesis and Characterization of ZnO/ZnTe Nanowire-based Heterojunctions for Photovoltaic Applications.
Ebraheem Azhar 1 , Jhih-Hong Peng 1 , Sandwip Dey 2 , Ronald Roedel 1 , Hongbin Yu 1
1 School of Electrical, Computer and Energy Engineering, Arizona State University, Phoenix, Arizona, United States, 2 School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona, United States
Show AbstractDirect band gap II-VI nanowire-based semiconductors are attractive for photonic applications owing to their appeal to the capability of modulating optoelectronic properties of the material and relative ease and low cost of synthesizing. A core-shell configuration of intrinsic n-type Zinc Oxide (ZnO) and p-type Zinc Telluride (ZnTe) hetero-nanowires are being studied for potential applications in photovoltaics. In this work, the ZnO/ZnTe heterojunction was fabricated by the following procedure: Vertically aligned, high-quality low-defect ZnO nanowires with 200 nm diameter and 5 μm lengths were grown on a low resistance Si (100) substrate with sputtered ZnO thin film using a high temperature vapor-liquid-solid (VLS) deposition at 700oC. The ZnTe shell was subsequently grown using a lower temperature VLS deposition chamber, the thickness of which was increased until the hybrid structure was formed. The deposition of ZnTe on ZnO nanowires can be controlled through the growth temperature, and their morphology was examined by scanning electron microscope and the presence of ZnTe was confirmed using EDS, XRD, and photoluminencese measurements. A top contact can then be formed by sandwiching indium tin oxide (ITO) covered glass over the hybrid nanowire array and using the Si substrate as a bottom contact.
9:00 PM - B8.11
Flexible Carbon-nanofiber Counter Electrode for Quasi-solid State Dye-sensitized Solar Cells.
Veerappan Ganapathy 1 , Bojan Karunagaran 1 , Shi-woo Rhee 1
1 Chemical Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show AbstractDye sensitized solar cells (DSSC) have become the most promising alternative to the conventional silicon-based solar cells. Despite the high energy conversion efficiency and moderate stability, platinum (Pt) based counter electrodes (CE) suffer from high cost and high temperature sintering and the development of a low cost and stable electrocatalyst for tri-iodide reduction is required. Moreover, the use of fluorine doped tin oxide (FTO) glass substrate and liquid electrolyte in DSSCs makes it more difficult for large scale applications, due to its brittleness and electrolyte leakage. Here, we report the successful application of carbon nanofibers (CNFs) as I3- reduction electrocatalyst for flexible CE both with liquid and quasi-solid electrolytes. Catalytic property of the CNFs was measured using cyclic-voltammetry (C-V) and electrochemical impedance spectroscopy (EIS). It was found that RCT of doctor bladed CNF electrode in iodide/tri-iodide redox electrolyte is smaller than the Pt electrode, which might be from the nanofiber stacking morphology. Its herringbone and antler structure with defect rich edge planes and a large diameter in CNFs facilitate the electron transfer kinetics, resulting in lower RCT. Current-voltage characteristic of the fabricated DSSC was measured using a solar simulator under one sun illumination (100mW/cm2, AM 1.5) with CNF counter electrode and liquid electrolyte and energy conversion efficiency of > 6% and 7% was obtained for flexible and glass based devices, respectively. DSSCs made with quasi-solid electrolyte for both flexible and glass based devices showed promising performances. The energy conversion efficiency achieved with CNF CE is comparable to that of the Pt CE in all the above mentioned devices.
9:00 PM - B8.13
Low-cost Tandem Solar Cells.
Erik Garnett 1 , Mark Brongersma 1 , Michael McGehee 1 , Yi Cui 1
1 Materials Science, Stanford University, Stanford, California, United States
Show AbstractSolar cells have emerged as a leading candidate to replace fossil fuels for large-scale renewable power generation. Due to the large balance of systems costs associated with solar installations, the Department of Energy has estimated that for this transformation to occur photovoltaics must reach 25% efficiency at a cost of $0.50 per peak Watt power output. It is unlikely that traditional single junction solar cells, which have a fundamental efficiency limit below 31% (depending on materials properties), can satisfy these criteria. However, combining a thin-film silicon and organic solar cell in tandem increases the theoretical efficiency to 43% and should be able to reach very low fabrication costs. Here we will outline progress towards realizing a silicon/polymer tandem solar cell that uses only low-temperature processing. We fabricated a Si thin-film/PEDOT Schottky junction solar cell with an open-circuit voltage (Voc) up to 0.63V, superior to standard p-n junction control cells. Stacking a polymer solar cell on top of this cell requires an inverted geometry where electrons move to the substrate and holes to a transparent top contact. We tuned the electron and hole contact energy levels to reverse the device polarity while maintaining a high Voc (up to 0.9V). These inverted solar cells showed comparable efficiencies to the best standard polymer cells (5.6%) even though they had a lower fill factor, suggesting further improvements are possible. Finally, we have fabricated tandem cells using two different substrates and demonstrated the individual cell voltages add to within an error of 20 mV.
9:00 PM - B8.14
Chalcogenide Glass Thin Films for Nanodipole Junctionless Photovoltaics.
Sakina Junaghadwala 1 , Daniel Georgiev 1 , Victor Karpov 2 , Rossen Todorov 3 , Nanke Jiang 1
1 Electrical Engineering and Computer Science, University of Toledo, Toledo, Ohio, United States, 2 Physics and Astronomy, University of Toledo, Toledo, Ohio, United States, 3 Institute of Optical Materials and Technologies , Bulgarian Academy of Sciences, Sofia Bulgaria
Show AbstractJunctionless photovoltaic (PV) devices that employ the electric field of aligned nanodipoles, instead of junctions, for the separation of photogenerated carriers were proposed recently [1]. The active PV material consists of nano particles that possess uncompensated electric dipole moment and are embedded in a photoconductive host. Such PV material structures would offer potential benefits in terms of simplicity of fabrication, environmental friendliness, and cost.As part of this project, which is in progress, we studied the potential of chalcogenide glass films as materials for these new PV devices. The idea was to obtain films consisting of a dispersed nanocrystalline phase in a glassy host material that has suitable optical and electrical properties. This approach is based on observations that most nano-particles possess significant dipole moments related to their asymmetric shapes [2]. We synthesized Ge-Se-Bi glasses, which absorb in visible and near-infrared spectral regions, in quartz ampoules from high-purity Bi, Ge, and Se elements by a conventional melt quenching technique.. This material was then used to deposit thin films with different thicknesses on various substrates by thermal evaporation under high-vacuum conditions. The original bulk glasses and layers were characterized by EDS, XRD, Raman, differential scanning calorimetry, and spectrophotometry. The analysis of the transmittance and reflectance spectra showed that the optical bang gap of the bulk glass is 1.2 eV and decreases to 0.99 eV for thin film. The calculated values for the refractive index were 2.68 and 2.73 (at λ = 2.0 µm) for bulk materials and thin layer respectively. The thin films were annealed in the nitrogen atmosphere to induce crystal nucleation and partial crystallization. Open-circuit voltage (Voc) readings under illumination were obtained from as-deposited and annealed films on conducting substrates. The as deposited films of thickness 1.3 μm showed Voc values above 100mV. However, the same films showed similar or even higher absolute values of Voc but of reversed polarity upon 10 min annealing at temperatures just above the glass transition temperature. These observations can be interpreted in terms of nanodipole conception and alignment upon annealing (i.e. upon glass host softening). The efficiency of such PV material structures remains below practical interest at this time. Yet, the observed strong effect on Voc represents encouraging evidence in favor of the junctionless PV where the built-in field is due to nanodipoles. We discuss the corresponding current-voltage characteristics and examples of other implementations.[1] D. Shvydka and V. G. Karpov, Appl. Phys. Lett., 92, (2008) 053507[2] S. Shanbhag and N.A. Kotov, J. Phys. Chem. B, 110 (2006) 12211; Z.Y. Tang et al., Science, 314 (2006) 274
9:00 PM - B8.16
DSC Applying Vertically-aligned ZnO Nanorods: Modification of the Hydrothermal Synthesis to Enhance Ppower Conversion Efficiency.
Irene Gonzalez-Valls 1 , Belen Ballesteros 1 , Frank Guell 2 , Monica Lira-Cantu 1
1 Laboratory of Nanostructured Materials for Photovoltaic Energy, Centre d'Investigacio en Nanociencia i Nanotecnologia (CIN2, CSIC), Bellaterra, Barcelona, Spain, 2 M-2E, IN2UB, Departament d’Electronica, Universitat de Barcelona, C/Martí I Franques 1. Barcelona (SPAIN), 08028., Barcelona, Barcelona, Spain
Show AbstractThe application of vertically-aligned ZnO nanostructures it is thought to improve contact between the donor and acceptor material in organic solar cells (OSCs), or improve electron injection in Dye sensitized solar cells (DSCs) [1-5]. Up to date, DSC based on ZnO nanoparticles has achieved promising power conversion efficiency values larger than 5% [6]. Nevertheless, the efficiency of DSC applying vertically-aligned ZnO nanorods is still low, with power conversion efficiencies not higher than 2.4% [7]. For this reason, many research efforts are currently focused on the synthesis of ZnO nanostructures like hierarchical ZnO nanoplates, nanosheets, disk-like nanostructures and aggregates that can achieve about 5-6% when applied in DSCs [8]. In an effort to understand the factors that limit the power conversion efficiency of DSC based on vertically-aligned ZnO NRs, we compare in this work, the properties of ZNO NRs obtained from two different synthesis methods. One of these methods is the most used hydrothermal synthesis technique, where the growth of ZnO NRs is made at low temperature in aqueous solutions. Thus, we report in this work a slight modification of the hydrothermal synthesis technique. Our initial results show that, for the same NR growth time, shorter NR length but higher power conversion efficiencies are obtained with the new synthesis method. The latter is attributed to the nanocrystalline properties of the ZnO NRs that allow for higher dye loading capacity when obtained with the modified synthesis technique. An example is shown in Table 1. Our findings could guide the path towards thinner electrodes for DSC with higher power conversion efficiencies. References[1] I. Gonzalez-Valls and M. Lira-Cantu, Energy Environ. Sci., 2009, 2, 19.[2] B. O’Regan and M. Grätzel, Nature, 1991, 353, 737.[3] M. Lira-Cantu, F.C. Krebs, P. Gomez-Romero and S. Yanagida, Mater. Res. Soc. Symp. Proc., 2007, 1007-S14-04.[4] M. Law, L.E. Greene, J.C. Johnson, R. Saykally and P. Yang, Nat. Mater.,2005, 4, 455.[5] I. Gonzalez-Valls and M. Lira-Cantu, Energy Environ. Sci., 2010, 3, 789.[6] K. Keis, E. Magnusson, H. Lindström, S.-E. Lindquist and A. Hagfeldt, Solar Energ. Mater. & Solar Cells, 2001, 73, 51.[7] M. Guo, P. Diao, X. Wang and S. Cai, J. Sol. St. Chem., 2005, 178, 3210.[8] Y.-C. Qiu, W. Chen and S. Yang, J. Mater. Chem., 2010, 20, 1001.
9:00 PM - B8.17
Si Nanosponge Embedded in Silica – A More Efficient Thin-film PV Cell Semiconductor?
Karl-Heinz Heinig 1 , Bernd Schmidt 1 , Karl-Heinz Stegemann 2 , Arndt Muecklich 1 , Bartosz Liedke 1 , David Friedrich 1
1 Inst. Ion Beam Physics & Materials Res., Research Center Dresden-Rossendorf, Dresden Germany, 2 , Signet Solar GmbH, Mochau Germany
Show AbstractNanostructured thin-film PV materials are expected to become more and more important due to their high competitiveness in cost reduction. Assemblies composed of quantum dots and/or wires have been reported in which quantum confinement is used as a design parameter. However, there are still problems related to the low-cost fabrication of such structures, and, in case of quantum dots embedded in a dielectric matrix, to charge carrier separation.Here, we present Si nanosponge embedded in silica as a new nanostructured active PV cell material which could overcome such problems. The Si nanosponge has typical feature sizes of 2…4 nm. This is much smaller than the ~100nm of electrochemically etched porous Si, which was studied intensively several years ago. Thus, the nanosponge shows a band gap widening by quantum confinement which allows band gap engineering for optimum adjustment to the solar spectrum. Furthermore, the Si sponge/SiO2 matrix interface is electrically passive which lowers losses. And, the Si sponge is electrically percolated, resulting in an efficient charge carrier separation.Si nanosponge is expected to replace easily a-Si in thin-film PV cell production lines. The PECVD equipment will be used to deposit SiOx instead of a-Si. The Si nanosponge is formed by thermally activated spinodal decomposition of SiOx. The large glass panels of thin-film PV cells allow a low thermal budget only, therefore scanned laser processing with ms dwell times has to be used. EFTEM images of Si nanosponge formed by co-sputtering of SiOx followed by rapid thermal processing are in full agreement with atomistic simulations of the spinodal decomposition process. Electrical and optical properties measured so far are in agreement with the expectations. Studies on the morphology of sponges form by very rapid thermal processing are under way.
9:00 PM - B8.19
Advanced Optical Modeling of Thin-film Silicon Solar Cells with 1-D Periodic Gratings.
Serge Solntsev 1 , Olindo Isabella 1 , Diego Caratelli 2 , Marilena Kyriakoy 1 , Olexander Yarovyi 2 , Miro Zeman 1
1 PVMD/DIMES, Delft University of Technology, Delft Netherlands, 2 IRCTR, Delft University of Technology, Delft Netherlands
Show AbstractIn thin-film silicon solar cells (TFSSC) an efficient light trapping is needed in order to maximize the absorption in the active layer and, hence, achieve high energy conversion efficiency. One-dimensional (1-D) periodic gratings represent an alternative solution [1] to randomly textured morphologies, since they can efficiently scatter light into large discrete angles resulting in a favorable increase of the optical path length inside the absorber layer. The control of design of the grating parameters such as shape, period, height, duty circle gives a possibility to enhance light scattering phenomena over a broad wavelength range. Periodically textured micro- and nano-features can be fabricated by using, for example, the technology for CD/DVD manufacturing. The optical modeling of TFSSC is not a trivial task because of their complex geometry, consisting of different layers with thicknesses spanning several orders of magnitude (from nanometers up to several microns) and conformal shapes relevant to the textured substrate carrier. In order to determine the light intensity distribution in such devices it is necessary to solve Maxwell’s equations numerically. By using a freely available software package MEEP [2], based on the finite-difference time-domain (FDTD) method [3], the optical simulations of single-junction TFSSC on 1-D periodic gratings were performed. In order to accurately evaluate the absorption in each layer in TFSSC, a challenge was to obtain a precise matching of the complex permittivity of films constituting the cell to their experimental values. We applied an adaptive approach to precisely fit the dispersive behavior of the films and we validated the developed numerical procedure by comparing the simulation results of optical properties of a reference TFSSC with flat interfaces with the results from the device simulator ASA [4]. In order to assess the performance of the MEEP-based procedure in terms of numerical stability, computational time, meshing, and dispersive material properties handling, a comparison with the commercial solvers CST [5] and HFSS [6], based on the finite integration technique (FIT) and the finite element method (FEM) respectively, was carried out.In this contribution we present an extensive study aimed at the optimization of the geometrical parameters of 1-D gratings for the maximal absorption (i.e. photocurrent) in amorphous and microcrystalline silicon solar cells both in p-i-n and n-i-p configurations. [1] O. Isabella, et al., 23rd EUPVSEC Proceedings, 2320-2324 (2008).[2] A. F. Oskooi, at al., Computer Physics Communications 181, 687–702 (2010).[3] K. S. Yee, IEEE Trans. Antennas Propagat., Vol. 14, No. 3, pp. 302–307, May 1966.[4] M. Zeman and J. Krc, J. Mater. Res., Vol. 23, No. 4, Apr. 2008.[5] Microwave Journal, Vol. 52, No. 11, 110 (2009)[6] HFSS v.12, HFSS, Ansoft Corporation, Pittsburgh, PA, U.S.A.
9:00 PM - B8.2
New Concepts for Dye-sensitized Solar Cells.
Udo Bach 1 , Torben Daeneke 2 , Dongchuan Fu 1 , Andrew Nattestad 1
1 Department of Materials Engineering, Monash University, Clayton, Victoria, Australia, 2 School of Chemistry, Monash University, Clayton, Victoria, Australia
Show AbstractDye-sensitized solar cells (DSCs) are viable low-cost alternatives to conventional silicon solar cells. A number of novel concepts are currently being developed that promise to improve the manufacturability and efficiency of these devices. Here we will summarize our recent results, relating to the development of tandem DSCs, back-contact DSCs, as well as non-corrosive electrolytes for DSCs. Tandem solar cell concepts have successfully been applied in the field of conventional solid-state semiconductor solar cells to achieve efficiencies that are beyond the theoretical efficiency limitation of single junction solar cells. Here we report on the recent development of tandem DSCs based on dye-sensitized NiO photocathodes, coupled with dye-sensitized TiO2 photoanodes and describe strategies towards their improvement. Back-contact solar cell concepts have also been originally developed to improve the performance of silicon solar cells. Here we show that their application towards dye-sensitized solar cells allows to reduce optical transmission losses and permit the realization of monolithic DSC architectures on opaque substrates. Finally we will report on DSCs employing novel ferrocene-based electrolytes that yield efficiencies of up to 7.9% under simulated sunlight (AM1.5; 1,000 W/m2).
9:00 PM - B8.20
Admittance Spectroscopy Study of MBE Grown Undoped-GaInNAsSb Thin Film for Multi-junction Tandem Solar Cell.
Muhammad Islam 1 , Naoya Miyashita 1 , Nazmul Ahsan 1 , Takeaki Sakurai 2 , Katsuhiro Akimoto 2 , Yoshitaka Okada 1
1 Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Tokyo, Tokyo, Japan, 2 The Institute of Applied Physics, The University of Tsukuba, Tsukuba, Ibaraki, Japan
Show AbstractGaInNAs has been considered as a promising material to realize third sub-cell in multi-junction tandem solar cell due to its low band gap of ~ 1eV and lattice matching to GaAs. However, crystal quality deteriorates for higher concentration of incorporated nitrogen leading to the degradation of optical properties. Use of antimony (Sb) in GaInNAs material has been found to improve the surface morphology and crystalline quality. In this work, we have performed admittance spectroscopy measurement of MBE grown GaInNAsSb thin film to investigate the effect of Sb in relation to the deep level defects in this material. Three p-i-n diodes, consisting of 150nm p-GaAs (~ 1018 cm-3)/300nm i-GaInNAs(Sb)/ 500nm n-GaAs (~ 1017 cm-3), were subsequently grown on (001) n+ GaAs substrate by RF-MBE with atomic hydrogen irradiation. On the top of the p layer, a heavily doped 30nm-thick p+ GaAs (5× 1018 cm-3) cap layer was grown to facilitate ohmic contact. Undoped i-GaInNAs(Sb) layers without and with Sb were grown in which two Sb fluxes were set. Ohmic contacts with 500 μm of diameter were made from AuZn. Admittance measurement was carried out in the temperature range between 20K to 350K with ac frequencies between 1kHz to 1MHz using Agilent 4284A LCR meter. From the depth profile, measured by C-V method,carrier type for the undoped GaInNAs was assumed as p-type with concentration of ~ 1016 cm-3. It suggests that trap information comes from the GaInNAs(Sb) layer in the structure. In admittance spectrum, GaInNAs sample without Sb shows two dominant hole traps, H1 and H2 at an energy level 0.12 and 0.41 eV above the valance band with trap density 1.5×1015 cm-3 and 2.4×1015 cm-3, respectively. Relatively large capture cross section of 1.2×10-13 cm2 for the H2 defect indicates that it acts as an efficient trap. A significant reduction of defect distribution was observed for the GaInNAs film when grown with Sb having flux of 3×10-8 Torr. Intensity of H1 defect reduced drastically, while the broader deep defect H2 deconvoluted into two defects of reduced intensity naming H3 (~0.5 eV) and H4 (~0.6 eV) with estimated trap density of 4.3×1014 cm-3 and 1.0×1015 cm-3, respectively. This phenomenon suggests the improvement of material quality for Sb irradiated samples. For GaInNAsSb film, grown with higher Sb flux of 10×10-8 Torr, defect distribution was almost similar to that of lower Sb irradiated sample except a shift in the H4 level to more higher energy at ~0.7 eV. Besides these traps, a shallow defect was observed in films included with Sb. The origin of defects has been discussed.
9:00 PM - B8.21
Solid State DSSC Based on SnO2 Nanorods Grown by Low Temperature Hydrothermal Route.
Aruna Ivaturi 1 , Pablo Docampo 2 , Giorgio Divitini 3 , Caterina Ducati 3 , Henry J. Snaith 2 , Mark E. Welland 1
1 Nanoscience Centre, Department of Engineering, University of Cambridge, Cambridge United Kingdom, 2 Department of Physics, Clarendon Laboratory, University of Oxford, Oxford United Kingdom, 3 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge United Kingdom
Show AbstractSolid-state dye sensitized cells (SDSCs) employing molecular hole-transporter (spiro-OMeTAD) instead of the volatile and corrosive liquid electrolyte have been emerging as a potential alternative for the future generation cost-effective photovoltaics to the conventional and more widely studied liquid electrolyte cells. In the present study, SnO2 nanorods grown by low temperature hydrothermal route directly on FTO substrates covered with a sputtered tin oxide compact layer have been explored as photoanode in SDSCs. Detailed X-ray diffraction (XRD), High-resolution transmission electron microscopy (HRTEM) and Scanning electron microscopy (SEM) analysis revealed the rutile structure of as-grown SnO2 nanorods having square cross-section with the direction of growth to be along c-axis. SDSCs employing spiro-OMeTAD as the hole-transporter have been fabricated with as-grown and annealed SnO2 nanorods with and without insulating shell (e.g MgO). The presence of insulating shell in addition to high temperature treatment resulted in substantive efficiency improvement of ~ 500 % for SDSCs fabricated from D102 sensitized SnO2 nanorods (200 – 300 nm in length) with typical VOC ~ 600 - 700 mV, JSC ~ 0.6 – 1 mA/cm2 and FF > 65 %. The improved cell performance is understood in terms of reduced charge recombination losses which are probed using detailed small perturbation transient photovoltage and photocurrent measurements.
9:00 PM - B8.22
Effect of Annealing Electrochemically Grown ZnO Nanorod Arrays in Hybrid Photovoltaic Performance.
Diana Iza 1 , Xin Ren 2 1 , Suman-Lata Sahonta 1 , Chuan-hai Jiang 2 , Judith MacManus-Driscoll 1 , David Munoz-Rojas 1
1 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge United Kingdom, 2 School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai China
Show AbstractNanostructured hybrid solar cell devices are promising structures to achieve a balance between efficiency and cost of photovoltaic devices. Nanorod arrays as hole blocking layers in bulk heterojunction solar cell devices offer the advantage of direct pathways to the electrode as well as an increased interfacial area with the bulk heterojuction layer. In this work, a simple one step seed-layer free electrodeposition method is used to grow ZnO nanorod arrays, which are incorporated into P3HT:PCBM devices. The architecture produced in this work consists of the nanowire arrays coated by a thin layer of the polymer blend, and subsequent coating of the back electrode. This architecture allows for very small quantities of polymer to be utilised, thus minimising the distance charge carriers have to travel to reach either the back electrode or ZnO nanorod, providing a high extraction of photogenerated charges from the device. The very thin layer of polymer used also allows for a better characterisation of the actual role nanorods play in the final cell efficiency. A photoluminescence (PL) study to determine the effect of annealing the nanorod arrays prior to device preparation had on the overall performance of the device was also undertaken. It was determined that a considerable part of the PL emission in the as-deposited samples resulted from a Zn(OH)2 layer. The effect of OH desorption from the arrays when annealed to temperatures equal or higher than 200 °C had a marked impact on device performance. Higher annealing temperatures are shown to result in improved photovoltaic response. By coating the arrays with atmospheric atomic layer deposited TiO2 it was determined that some defects may exist on the surface of the rods, but that most of these exist at the indium tin oxide/ZnO interface.
9:00 PM - B8.23
Fabrication and Performance of Dual Back-Contact 3-Dimensionally Structured Thin Film Photovoltaic Devices.
Daniel Josell 1 , Carlos Hangarter 1 , Behrang Hamadani 2 , Suyong Jung 2 , Jonathan Guyer 1
1 Metallurgy Division, NIST, Gaithersburg, Maryland, United States, 2 CNST, NIST, Gaithersburg, Maryland, United States
Show AbstractI will describe the fabrication and performance of photovoltaic devices with a dual back contact geometry created by depositing semiconductors onto two interdigitated comb-electrodes that have been previously patterned lithographically on insulating substrates. Widths and spacings of the individual wires in the combs are on the order of one micron, with the interdigitated comb electrodes in the 4 mm square area of the test devices each comprising thousands of parallel wires. The active devices are fabricated by electrochemical deposition of a semiconductor on one of the electrodes, independent control of the applied potentials on the electrodes permitting selective deposition, followed by deposition of a second semiconductor over the entire device. I will describe homojunction CdTe devices fabricated from a single electrolyte. I will also describe heterojunction devices, including CdS/CdTe devices with CdS electrodeposited on one electrode followed by CdTe electrodeposited over both the coated and uncoated electrode through impingement of the CdTe deposits to create active devices. I will detail the performance of devices, including i-V and external quantum efficiency, including the impact of electrode geometry, device processing, and electrode(contact) material. I will also present modeling results that explain the observed device performance. Finally, I will detail the positive aspects of the process, including elimination of light-blocking front contact metals and UV-light blocking conducting oxides and the inherent absorption enhancing surface contouring as well as materials related issues arising from the requirement that all processing be done with both electrodes present.D. Josell, C. R. Beauchamp, S. Jung, B.H. Hamadani, A. Motayed, L.J. Richter, M. Williams, J.E. Bonevich, A. Shapiro, N. Zhitenev, T.P. Moffat, Three-Dimensionally Structured CdTe Thin Film Photovoltaic Devices with Self-Aligned Back-Contacts: Electrodeposition on Interdigitated Electrodes, Journal of the Electrochemical Society 156(8), H654-H660 (2009).D. Josell, C. R. Beauchamp, B.H. Hamadani, S. Jung, J.E. Guyer, A. Motayed, C. Hangarter, N. Gergel-Hackett, H. Xu, N. Zhitenev, Three-Dimensionally Structured Thin Film Heterojunction Photovoltaic Devices on Self-Aligned Back-Contacts, Transactions of the Electrochemical Society, Vol. 28(2), 521-532 (2010).
9:00 PM - B8.24
Control of the Interface between Amorphous Silicon Cell and Polymer Cell of Hybrid Tandem Solar Cell by Graphene Thin Layer.
Heesuk Kim 1 , Chulki Kim 1 3 , Taehee Kim 2 , Seunghee Han 2 , Ilwon Kim 3 , Sang-Soo Lee 1 , Kyungkon Kim 2
1 Polymer Hybrids Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 3 Department of Chemical and Environmental Engineering, Soongsil University, Seoul Korea (the Republic of), 2 Solar Cell Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractCompared to crystalline silicon solar cell, amorphous silicon solar cell has the narrow solar absorption spectrum ranged from 350 to 600nm. To achieve the wide range of absorption with a simple process, development of tandem solar cell by stacking a solution processible polymer solar cell based on a low bandgap semiconductiong polymer onto the amorphous silicon solar cell can be an excellent solution. Recently, we demonstrated the photovoltaic property of the tandem cells composed of a-Si solar cells and polymer solar cells for the first time. It was found that the hydrophilicity of PEDOT:PSS used as the buffer layer leads to difficulty in spin coating of PEDOT:PSS layer onto the hydrophobic amorphous silicon surface. In this study, we have controlled the interface between amorphous silicon surface and PEDOT:PSS buffer layer by graphene thin layer. The graphene layer spray-coated onto the amorphous silicon surface is transparent, conductive and hydrophilic, thereby achieving the improvement of PEDOT:PSS coating and efficiency of hybrid tandem solar cell. The graphene thin film coated on the amorphous silicon surface has been characterized by contact angle measurement, four point probes, UV spectrometer and scanning electron microscopy (SEM).
9:00 PM - B8.25
Single Crystalline Mesoporous Silicon Microwire Solar Cells.
Jae Hyun Kim 1 , Ho-Jin Choi 1 , Seongho Baek 1
1 Division of Nano & BioTechnology, DGIST, Daegu Korea (the Republic of)
Show Abstract The enormous potential of solar energy utilization is widely recognized due to environmental and shortage issues of fossil fuels and great effort is being devoted to developing new solar energy conversion systems of low cost and high efficiency. Recently, solar energy conversion systems based on one-dimensional semiconductor nano/micro wires have attracted much interest for that kind of objective. Especially, using solar cells with wire array structured radial p-n junctions offers the unique advantages of orthogonalizing the direction of light absorption and charge separation while allowing for improved light scattering and trapping. For fabrication of vertical silicon wire arrays, metal-assisted etching has excellent method which is free of metal catalyst contamination compared with vapor-liquid-solid approach. We used metal assisted chemical etching method that is composed of two steps to make silicon wire arrays. Au metal was deposited on photolithographycally defined Si surface by sputtering and then etched in various HF and H2O2 solutions. The Si substrates were p-type (10 ~ 20 Ω cm). The effect of Au thickness and concentration of H2O2 on wire array fabrication were systematically studied. Solid microwire arrays having smooth surface were obtained when the Au thickness is about 10 nm at etching solution of 4.6 M BOE and 0.22 M H2O2. Below Au metal thickness of 3 nm and over 20 nm, clearly defined wire arrays were not fabricated. Mesoporous micro wires were obtained as the concentration of H2O2 was increased from 0.22 M to 0.66 M. Also, The wider gap between wire with constant diameter was, the more the porosity of wires were increased. The different morphologies obtained are explained in terms of the relative catalytic activity of the metal ions that determine the porosification rate and the catalytic activity of the nanoparticles that determine the silicon wire etch rate. Radial p-n junction wire arrays were fabricated by spin on doping (phosphor), starting from chemical etched p-Si wire arrays. In/Ga eutectic metal was used for contact metal. The energy conversion efficiency between the solid microwire array solar cell and mesoporous silicon wire photovoltaic cell were compared. The effect of meso pores at surface of microwires on charge collection and absorption and finally the cell efficiency are discussed.
9:00 PM - B8.26
Increasing Light Absorption of Thin Film Hematite Deposited by Atomic Layer Deposition.
Benjamin Klahr 1 , Thomas Hamann 1
1 Chemistry, Michigan State University, East Lansing, Michigan, United States
Show AbstractHematite (α-Fe2O3) has long been considered as a potential material for photo-catalytic water oxidation because of a favorable valence band edge, reasonably low band gap, high stability and potentially low cost. However, only modest conversion efficiencies have been achieved, which has generally been attributed to the low diffusion length of minority carriers through the semiconductor. While many methods of making nanostructured hematite electrodes have been examined to overcome the small diffusion length – by maximizing absorption of light and minimizing the distance over which holes must travel to be collected – the unique self limiting, non-directional, gas phase mechanism of atomic layer deposition (ALD) allows the conformal deposition of high aspect ratio substrates. We utilized ALD to deposit nanostructured hematite electrodes in order to increase light harvesting and reduce the diffusion length. We will present in-depth photoelectrochemical studies of characterized nanostructured hematite electrodes deposited via ALD in contact with one electron outersphere redox couples.
9:00 PM - B8.28
Effects of Thermal Treatment in the Preparation of CdTe Thin Films Using a Low Cost Process.
Jin Young Lee 1 , Soo Yeun Hwang 1 , Tae Jin Lee 1 , Chih Hung Chang 2 , Si Ok Ryu 1
1 School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk, Korea (the Republic of), 2 School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, Oregon, United States
Show AbstractCadmium telluride (CdTe) thin film is one of the most promising candidates in the family of ΙΙ-VΙ type binary compounds for the photovoltaic applications because of its unique physical, optical, and electronic characters. The direct band gap of CdTe in the range of 1.4-1.5 eV is near to the optimum conversion efficiency(~31%). Absorption coefficient of CdTe is about 10-5. Up to now, a number of methods such as evaporation, close spaced sublimation, and sputtering have been studied for the formation of CdTe thin films. However, those technologies cause the high production costs owing to their requirements of an expensive vacuum system and a high temperature condition. Solution-based chemical deposition processes have many important advantages due to their low cost and low temperature processing surroundings. Those processes are able to be used to fabricate large area thin films on various substrates.The purpose of this study was to investigate the effect of thermal treatment on CdTe thin films deposited by non-vacuum spray process in low cost and in low temperatures for photovoltaic solar cells. For CdTe thin film deposition, the precursor solutions were prepared by mixing stream A and B. Stream A consisted of tellurium oxide, hydrazine hydrate, and ammonium hydroxide. Stream B consisted of cadmium chloride and ammonium hydroxide. The mixed solutions were sprayed on the substrates, which were heated at 130°C .XRD measurement was used for the crystalline orientation of CdTe films. The oxidations of precursors during the films deposition under atmospheric conditions were observed in the thermal treatment process from 150 to 350°C, in consequence, which resulted in the formation of CdTeO3 (JCPDS 49-1757). In this study, we observed that the intensity of CdTeO3 peaks in XRD decreased as the annealing temperature increased. However, the diffraction peaks annealed at 450°C had the major peaks at 2-theta = 23.80°, 39.31°, 46.53°, 56.84°, 62.54°, 71.27° and 76.42°, which were corresponding to the (111), (220), (311), (400), (331), (422), and (511) crystal planes of the CdTe structure. These diffraction patterns were consistent with the reference (JCPDS 15-0770) without any CdTeO3 peaks. A SEM was employed to examine the morphology and particle sizes of the CdTe thin films. From the images, the films were well formed with uniform grain sizes. The optical properties were measured using a UV-visible spectrophotometer. The estimated optical band gap of CdTe thin film annealed at 450°C was 1.45 eV. From the experimental results, we confirmed that the thermal treatment affected the crystallization, morphology, optical band gap, and synthesis of the CdTe films.Following are results of a study on the “Human Resource Development Center for Economic Region Leading Industry” Project, supported by the Ministry of Education, Science & Technology (MEST) and the National Research Foundation of Korea (MRF).
9:00 PM - B8.29
Tailoring the Shape and Surface Properties of Chalcopyrite Nanostructures by Intrinsic Defect Chemistry Control.
Sebastian Lehmann 1 2 , Sascha Sadewasser 1 , David Fuertes Marron 3 , Robert Baier 1 , Juergen Albert 1 , Sebastian Schmidt 1 , Marcus Baer 1 , Regan Wilks 1 , Lothar Weinhardt 4 , Clemens Heske 5 , Martha Ch. Lux-Steiner 1
1 Institute for Heterogeneous Materials Systems, Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin Germany, 2 Solid State Physics, Lund University, Lund Sweden, 3 Instituto de Energia Solar-ETSIT, Universidad Politecnica de Madrid, Madrid Spain, 4 Experimentelle Physik VII, Universitaet Wuerzburg, Wuerzburg Germany, 5 Department of Chemistry, University of Nevada, Las Vegas, Nevada, United States
Show AbstractNanofabrication is a versatile tool for the exploration of novel approaches in fields like photovoltaics, energy storage, photonics, and catalysis. Within the field of photovoltaics, Cu-containing chalcopyrite materials have already proven their superior performance with energy conversion efficiencies above 20% [1]. Notwithstanding, chalcopyrites do offer advantageous intrinsic material properties, which may widen their range of application, including 3rd generation solar cell concepts in the form of nanostructures [2,3].
In this contribution we explore the first steps toward the fabrication of chalcopyrite nanostructures by means of expitaxial growth of CuInSe2 and CuGaSe2 onto crystalline substrates. Chalcopyrite nanodots have been grown by means of metal-organic vapor phase epitaxy (MOVPE) onto <111>-oriented Si and GaAs wafers - with both surface terminations for the case of GaAs A and B. We have found different growth regimes of the nanostructures as a function of the type of substrate and processing parameters, which include 2D- and 3D-growth modes. We propose thereof a model, in which the intrinsic defect chemistry rules the preferred growth mode. In order to test the model, a series of experiments have been performed, whereby the enthalpies of formation of prominent defect complexes have been varied by means of control over the copper chemical potential, while group-III- and Se chemical potentials were kept constant. The control over the defect chemistry in turn leads to a change in surface energies of the most favorable surface termination of the nanostructures, {112}A and {-1-1-2}B, by which 2D- or 3D-growth type is activated in the case of the growth on III-V substrates with a {111}A termination. Scanning electron microscopy, transmission electron microscopy, and X-ray emission spectroscopy, the latter by means of high-brilliance synchrotron radiation, were used to analyze sample morphology and composition, respectively. Additionally, the surface work functions were determined by UHV-Kelvin probe force microscopy by transferring samples under inert gas conditions from the growth to the analysis chamber. This allows – via the different work function values – the identification of the facet terminations, which is difficult to access by other methods but essential for the interpretation. Finally, the developed growth model was tested and successfully validated by MOVPE growth of CuInSe2 and CuGaSe2 on various substrates and substrate orientations.
[1] M. A. Green, K. Emery, Y. Hishikawa, und W. Warta, Prog. Photovolt: Res. Appl. 18, 346-352 (2010).
[2] D. Fuertes Marrón, E. Cánovas, M. Levy, A. Martí, A. Luque, M. Afshar, J. Albert, S. Lehmann, D. Abou-Ras, S. Sadewasser, und N. Barreau, Solar Energy Materials and Solar Cells 94, 1912-1918 (2010).
[3] D. F. Marrón, A. Martí, und A. Luque, Phys. Status Solidi (a) 206, 1021-1025 (2009).
9:00 PM - B8.3
Sol-gel Synthesized Gel Organo-oligosiloxane Electrolyte for Dye-sensitized Solar Cells.
Jun-Young Bae 1 , Dae-seop Lim 1 , Joon-Soo Kim 1 , KyungHo Jung 1 , Byeong-Soo Bae 1
1 , KAIST, Daejeon Korea (the Republic of)
Show AbstractSince, low fabrication cost as well as comparable efficiency is expected, Dye-Sensitized Solar Cells (DSSC) have attracted much interest among the future solar cells. To achieve high energy conversion efficiency, development of electrolyte with both highly conductive and long-term stability is an important issue. Hence highly viscous quasi-solid state (gel) electrolyte is extensively fabricated due to its non-volatility and low leakage for DSSC stability. Gel electrolyte shows better contact with electrode than solid electrolyte and it has comparable efficiency to liquid electrolyte. So far, various methods have been tried for gelation by addition of nanofillers (silica, CNT etc) or cross-linkers. However, these gelators can interfere in good contact between electrolyte and electrode. Moreover, there are still leakage and volatility problems in the presence of solvent.In this study, solvent-free gel electrolyte is fabricated by sol-gel condensation reaction of trimethoxysilane with various functional groups (epoxy, fluorine etc) and diphenylsilanediol (DPSD). Imidazolium iodide and iodine were incorporated in liquid mixture of the silane precursors to form I-/I3- redox couple for ion conduction. After that, the mixture was injected into DSSC cell for easy penetration into TiO2 pores in order to achieve good contact with electrode. Finally, it became highly viscous gel electrolyte by formation of sol-gel oligosiloxane network at low temperature. Because of this low temperature process, there is less degradation of organic dye during the gelation. Fabricated sol-gel oligosiloxane electrolyte shows better contact with electrode since we use chemical reaction without any gelators. Furthermore the electrolyte has low leakage and volatility due to its solvent-free atmosphere. For practical applications, high ionic conductivity is required to design efficient DSSC. In order to increase ionic conductivity, various functional groups of oligosiloxane are applied and compositions of silane, imidazolium iodide, and iodine are optimized. As well as high conductivity up to 10-3 S/cm, high condensation degree over 80% and Si-O-Si bond formation are verified by FT-IR and 29Si NMR and formation of I/I3- redox couple is confirmed by FT-Raman. To fabricate long-term stable electrolyte with low leakage, it requires high viscosity over 20000cP and viscosity increases when condensation degree increases. In this paper, viscosity and condensation degree was controlled by varying the composition of silanet. After all, high viscosity over 20000cP was achieved. Finally, DSSC cell will be fabricated using sol-gel oligosiloxane electrolyte and photovoltaic parameters such as open circuit voltage, short circuit current, fill factor and energy conversion efficiency will be measured by varying organic functional groups.Keyword : Sol-gel, Organo-oligosiloxane, Ionic conductivity, Dye-sensitized Solar cell, Gel electrolyte
9:00 PM - B8.30
Application of a Sol-gel Templated Hybrid Blocking Layer in Solid State Dye-sensitized Solar Cells.
Philipp Lellig 1 , Jochen Gutmann 2
1 , Max Planck Institute for Polymer Research, Mainz Germany, 2 , University of Duisburg-Essen, Duisburg-Essen Germany
Show AbstractIn TiO2 based solar cells a blocking layer between the transparent electrode and the mesoporous TiO2 film is used to prevent short circuits. The conventional approach is to use a compact layer of TiO2 prepared by spin coating or spray pyrolysis. The thickness of the blocking layer is critical. On one hand the layer has to be thick enough to enclose the rough substrate completely. On the other hand with increasing film thickness the serial resistance increases due to the formation of voids and impurities.Using an amphiphilic block copolymer as a functional template we were able to create an alternative hybrid blocking layer that is thinner than the conventional compact TiO2 film thus possesses a higher conductance. Still this type of blocking layer covers the rough electrode material completely avoiding current loss through charge recombination. The novel blocking layer is built up using a PEO-MA(PDMS) block copolymer in combination with sol-gel chemistry. While the hydrophilic PEO part of the polymer coordinates with the titania precursor to form a network of TiO2 nanoparticles the hydrophobic PDMS part turns into an insulating ceramic layer. This way defined charge percolation pathways for current flow are created.Conductive scanning probe microscopy of the hybrid films revealed conductivity through the crystalline TiO2 network sourrounded by an insulating material. The application in solar cell devices shows a 38% improvement of efficiency compared to the same device without blocking layer.
9:00 PM - B8.32
Heterogeneous Light Distribution and Equivalent Circuit in Three Dimensional Photovoltaics.
Yuan Li 1 , Wanyi Nie 1 , Qi Li 3 , Huihui Huang 4 , Mingjun Wang 4 , Eric Peterson 1 , Coffin Robert 1 , MacNeill Christopher 2 , David Carroll 1
1 Center for Nanotechnology and Molecular Materials, Department of Physics, Wake Forest University, Winston-Salem, North Carolina, United States, 3 Physics, Wake Forest University, Winston-Salem, North Carolina, United States, 4 Department of Electronic Science and Technology, Wuhan University, Wuhan, Hubei, China, 2 Chemistry, Wake Forest University, Winston-Salem, North Carolina, United States
Show AbstractRecently, there have been introduced several types of 3 dimensional organic photovoltaics based on fiber architectures. Basically, the intention is to confine light in a fiber-based cavity to enhance the optical absorption, also named as Optical Confinement Geometry Photovoltaics (OCGPV). Examples include Fiber-based PV, Tube-based PV, Fiber bundle PV, stamped fiber PV, and other Fiber-like PV. In these types of architectures, their optical paths are extended to improve absorption and avoid some of the limitations of planar OPVs by using relatively thin absorbing layers. However, because of the 3 dimensional nature of the architecture, the optical distribution within the structure is heterogeneous and thus it cannot be described by the traditional equivalent circuit model of used in organic photovoltaics. In this work, we introduce a novel equivalent circuit model that separates the whole cell into many subunits with responses based on the cavity modes of the fiber. These are then re-integrated to form a single photovoltaic response. Through this model, it can be shown the open-circuit voltage is not severely limited by the heterogeneity. Exploring further we can show that the model exactly matches trends in Voc with light intensity measured on fiber-based devices.
9:00 PM - B8.34
Hybrid Photovoltaic Cells Based on ZnO/Sb2S3/P3HT Heterojunction.
Chao-Ping Liu 1 2 , Hong-En Wang 1 2 , Zhen-Hua Chen 1 2 , Guang-Wei She 2 3 , Igor Bello 1 2 , Ludvik Martinu 1 4 , Juan Antonio Zapien 1 2 , Wen-Jun Zhang 1 2
1 Department of Physics and Materials Science, City University of Hong Kong, Hong Kong China, 2 Center of Super-Diamond and Advanced Films, City University of Hong Kong, Hong Kong China, 3 Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing China, 4 On Leave from the Department of Engineering, Ecole Polytechnique, Montreal, Quebec, Canada
Show AbstractRecent development in photovoltaic devices shows that much attention was given to inorganic-organic hybrid photovoltaic cells in order to simplify the technology and cut down the production cost of the photovoltaic devices. In this work, hybrid photovoltaic (PV) cells based on n-i-p heterojunctions were fabricated. The hybrid cell structure comprises glass substrate coated with tin indium oxide (ITO), n-type ZnO, intrinsic Sb2S3, p-type conjugated polymer, poly (3-hexylthiophene) (P3HT), and silver electrode. The Sb2S3 thin layer and P3HT serve as the absorber layers. Study of the device structure and functional layer shows that annealing temperature has significant effect on both the crystallinity and optical absorption of Sb2S3 films. The near-intrinsic film of Sb2S3 annealed at 300 degree, exhibits dark conductivity of 1.42×10-7 Scm-1. The performance of PV cells is greatly dependent on the thermal treatment of Sb2S3, the layer thickness of Sb2S3 and P3HT. The fabricated PV cell achieved a power conversion efficiency of 2% under illumination of air mass 1.5 global (AM 1.5G) with an intensity of 100 mW cm-2.
9:00 PM - B8.35
Facile Hydrothermal Growth of ZnO Nanorods and Their Decoration with Aqueous CdSe and CdS Quantum Dots for Photovoltaic Applications.
Chunyan Luan 1 2 , Aleksandar Vaneski 1 , Andrei S. Susha 1 , Hong-En Wang 1 2 , Xue Chen 1 2 , Jun Xu 1 2 , Wenjun Zhang 1 2 , Igor Bello 1 2 , Chun-Sing Lee 1 2 , Juan Antonio Zapien 1 2 , Andrey L. Rogach 1
1 Department of Physics and Materials Science, City University of Hong Kong, Kowloon Hong Kong, 2 Center of Super-Diamond and Advanced Films (COSDAF),Department of Physics and Materials Science, City University of Hong Kong, Kowloon Hong Kong
Show AbstractDye sensitized solar cells based on ZnO are being intensively studied due to the similar band gap of ZnO compared to TiO2, but higher electron mobility. Furthermore, ZnO is readily available in different nanostructures such as ordered nanowire arrays which could provide a direct electrical pathway and thus favorable electronic transport for even higher solar cell performance. In this report, vertically aligned single crystalline ZnO nanorods (~2 µm in length and 50 – 450 nm in diameter) have been grown by a simple solution approach on a Zn foil substrate. The cathodoluminescence (CL) properties including intensity and distribution of the band-to- band and defect emission of the ZnO nanostructures have been investigated. A high coverage of ZnO nanorods with CdS and CdSe quantum dots has been achieved using water-soluble nanocrystals capped with a short-chain bifuncional linker thioglycolic acid. The photovoltaic performance of these hybrid nanostructures has been evaluated in a photoelectrochemical solar cell configuration with a liquid triiodide/iodide electrolyte. Power conversion efficiencies of up to 2.24% have been demonstrated for solar cells co-photosensitized with both CdS and CdSe quantum dots.
9:00 PM - B8.36
Spectroscopic Charge-generation Imaging of Molecular Interfaces in Organic Solar Cells via Variable-wavelength Electric Force Microscopy.
Justin Luria 1 , Alon Gorodetsky 2 , Andrew Jacobs 1 , Colin Nuckolls 2 , John Marohn 1
1 Chemistry and Chemical Biology, Cornell University, Ithaca, New York, United States, 2 Chemistry, Columbia University, New York, New York, United States
Show AbstractExperiments imaging the prototypical polymer-blend photovoltaic material PFB:F8T2 under fixed-wavelength illumination have led to contradictory conclusions about whether charge generation occurs more efficiently at the organic/organic interface [1] or in the bulk [2]. To resolve this controversy, we have developed an electric force microscope in which samples can be illuminated from above with variable-wavelength light using a fiber-optic cable. This innovation allows us to record and image spectra of surface photovoltage in thin-film solar cells. When compared to signal from reference samples, our surface photovoltage spectra provide new, more direct evidence of vertical phase separation in the PFB:F8T2 films. Our data moreover make clear how both the material and the morphology determine the efficiency of charge generation.We have used our hybrid instrument to study organic photovoltaic devices that take advantage of geometrical complementarity between the donor (contorted hexabenzocoronene, HBC) and acceptor (buckminsterfullerene, C60) molecules [3]. These devices feature electronically intimate, self-assembled organic/organic interfaces and exhibit stable operation under ambient conditions. We have investigated efficient nanostructured photovoltaic devices from a related class of contorted donor molecules with thiophene groups incorporated within the HBC framework. We will describe the insight that our white light and variable-wavelength electric force microscope studies have provided into the operating mechanism and long-term ambient stability of these devices.1. M. Chiesa et al. Nano Lett. (2005) 5, 559.2. D. C. Coffey et al. Nano Lett. (2007) 7, 738.3. A. A. Gorodetsky et al. Chem. Mater. (2009) 21, 4090.
9:00 PM - B8.38
Exploring the Limits of Phosphor-Based Spectral Management for Photovoltaic Applications.
Komal Magsi 1 , Charles Fortmann 1
1 , SUNY Stony Brook, Stony Brook , New York, United States
Show AbstractOptical techniques for increased photovoltaic solar cell efficiency have traditionally focused on light trapping through the use of textured surfaces. More recently phosphor based up and/or down conversion of poorly converted portions of the solar spectrum for increased photovoltaic cell performance has been proposed (e.g., see Shalav [1]). Here the limits of phosphor-based solar efficiency gain are examined.Phosphor based spectral conversion is a well-known tool for laser applications requiring spectral conversion. Importantly, the spatial and temporal incoherence of sunlight needs carefully consideration before the lessons of laser physics are applied to solar cells. This work will explore the dynamics and limitations of multi-level atomic systems under low intensity illumination (relative to laser irradiation – low intensity for the purpose of this work ranges from one to one hundred sun concentrations).The main issue of board spectrum sun light illumination is the relative amounts of parasitic absorption and unwanted light scattering in spectral regions where a particular solar cell platform already converts light with near 100% efficiency. Calculations show that phosphor-based spectral up and down conversion can guide poorly used or un-used portions of spectrum to photon energies that are more easily converted into electrical power. However, the calculations also show that losses associated with parasitic absorption and re-emission of the visible spectrum (where light is already converted well) can more than offset any gains form lesser used portions of the solar spectrum. Light scattering losses are related to phosphors absorbing and then re-emitting photons to all solid angles with equal probability. Subsequently, a significant portion (~ 20 to ~30% depending on the refractive index of the matrix used for the phosphor) of the re-emitted and/or scattered photons will have trajectories that result in the photon being reflected back out the front of the solar cell rather than into the absorber region of the cell. Raman-based rear reflector systems (described in a second submission Rose Ping Lee et. al.) do not have these intrinsic limitations.[1] Shalav A, IEEE 248-250 (2003), 306.
9:00 PM - B8.4
Performance Evaluation of Blended Dyes for Use in Luminescent Solar Concentrators.
Benjamin Balaban 1 , Yvonne Rodriguez 1 , Sue Carter 1
1 , UC Santa Cruz, Santa Cruz, California, United States
Show AbstractWe present a study which optimizes the concentration of DCM* within polymethyl methacrylate (PMMA) based luminescent solar concentrators (LSCs). The study also examines the viability of DCM as a complement to the organic dye Lumogen Red 305 within PMMA LSCs. We compare the results of blended Lumogen 305:DCM films to graded layers and separated films. LSCs offer the potential for low cost improvements to existing silicon solar systems. Made from inexpensive dye-doped polymers, LSCs absorb and guide diffuse light while also producing a red shift determined by the spectroscopic properties of the dye(s) used. Thus, LSCs allow for reduced cost solar power by red shifting the solar spectrum to more closely match the peak absorption of silicon, as well as allowing light collected over a large area to be concentrated onto a minimal amount of solar cells without the need for expensive tracking systems. DCM is of interest due to its large Stokes shift and high quantum yield. Similarly, Lumogen Red 305 is of interest because of its broad absorption spectrum and long wavelength emission within a polymer matrix. DCM doped PMMA films have absorption and emission peaks at approximately 460nm and 570nm, respectively. Emission intensity and wavelength are both affected by self absorption within DCM films and can be optimized as a function of concentration. Additionally, DCM’s absorption peak compliments a dip in the absorption spectrum of Lumogen Red 305. *([2-[2-[4-(dimethylamino)phenyl]ethenyl]-6-methyl-4H- pyran-4-ylidene]-propanedinitrile)
9:00 PM - B8.41
The Dye Sensitized Solar Cell Stability and Performance Study Using Different Electrolytes.
Sailaja Radhakrishnan 1 , Lakshmi Munukutla 1 , Arunachalanadar Kannan 1
1 Engineering technology, Arizona State University, Mesa, Arizona, United States
Show AbstractThe overarching goal of Dye Sensitized Solar Cells (DSSCs) is to improve photovoltaic performance and their long term stability for use in practical applications because of their simple fabrication technology at a reasonable cost. The DSSCs are fabricated using a wide band gap semiconductor material such as TiO2 and a redox couple electrolyte. Many investigations have been carried out with the aim of improving the photovoltaic performance and stability by converting solar energy into electrical energy reaching efficiency levels as high as 11.2% for the laboratory based solar cells in the literature. The focus of our research, presented here, is to achieve cell stability as our first goal and then to improve solar energy conversion efficiency. The work presented in this abstract focused on three different electrolytes, Iodolyte R-150, Iodolyte AN-50, Iodolyte PN-50 and Iodolyte MPN-100, and standard electrolyte Iodolyte R-50 to fabricate the dye sensitized cells and evaluated their stability over time along with its efficiency by characterizing the cells. The basic principle of the DSSC is to generate photocurrent by electron injection from the excited dye into the conduction band of the semiconductor, TiO2 and restoring the excited state of the dye to its original state to continue this process for long time. The restoration of the dye is achieved by accepting the donated electrons from the electrolyte. Therefore, the electrolyte’s role is critical to sustain the DSS cell performance over time to instill cell stability. The key electrolyte parameters are: low viscosity for easier injection into the cell, low vapor pressure and high boiling point to minimize electrolyte evaporation, wide redox window to generate sufficient donating electrons to the dye, low cost and non-toxicity. Electrolytes with higher concentration of Iodolyte were chosen for this study to widen redox potential window. The above-mentioned electrolytes were purchased from Solaronix for this work. These are Iodide based redox electrolytes and are made with 100 mM of tri-iodide in 3-methoxypropionitrile. Our results from investigation revealed that the cell with Iodolyte R-150 gave improved performance with efficiency of 10.2% when compared to the reference cell with Iodolyte R-50, which showed efficiency of 8.4%. The other three electrolytes, Iodolyte AN-50, Iodolyte PN-50 and Iodolyte MPN-100 showed efficiencies of 8.79 %, 7.5% and 8.1% respectively. These cells stabilized over a time of 4 weeks. The fill factor of the cell changed about 10% and the internal resistance decreased from 6.7 Ω to 4.3 Ω. The results of our work demonstrated reduced internal resistance, and improved fill factor leading to cell stability. Additionally, replacing the standard electrolyte with the Iodolyte R-150 yielded enhanced cell efficiency compared to the reference electrolyte (Iodolyte R-50).
9:00 PM - B8.42
The Temporal Response of a Dye Sensitised Solar Cell.
Thomas O'Reilly 1
1 , University College Dublin, Dublin Ireland
Show AbstractA dye-sensitized solar cell (DSSC) consists of a mesoporous, nanoparticle film of titanium dioxide onto which a monolayer of dye is adsorbed, an electrolyte and two electrodes. Light energy incident on the cell is converted to electricity on a molecular level, similar to the natural photosynthesis process, and this is a regenerative, photoelectrochemical process by way of the electrolyte. The temporal response of a DSSC is critically linked to the operating environment. When a DSSC is partially illuminated and the incoming light is of low intensity, the response time of the cell is prolonged dramatically. Temporal timescales reported here show a strongly varying cell response in the order of µs to tens of seconds. Temporal response analysis was employed to fully understand the underlying processes (e.g.: electronic excitations, electron conduction and injection) although these processes happen individually, typically in the ultra-fast time scale (ns-fs). The nanoparticle film has been characterized for thickness, average grain size and internal structure using imaging techniques; e.g.: scanning electron microscopy (SEM) and focused ion beam (FIB). Hence, the performance and structure of a DSSC may be investigated by the above-mentioned techniques to yield information about the underlying physical mechanisms of the device, i.e.: electron generation, injection, transport, and the effect of grain size and thickness upon these electronic processes.
9:00 PM - B8.44
Quantification of III-Related Modulation in InGaP/AlGaAs/Ge Multi-junction Solar Cell and Their Influence on Electronic Properties of the Device.
Marina Gutierrez 1 , Carlo Enzo Pastore 1 , Daniel Araujo 1 , Ergbert Rodriguez 2
1 , Cadiz University, Puerto Real Spain, 2 , Isofoton, Malaga Spain
Show AbstractTo reduced cost and improve efficiencies, the sun concentrator is one of the most attractive approach. Indeed, multi-junction solar cells based on Ge substrate, offer the widest direct energy gaps in the III–V alloys, apart from N based compounds and demonstrate very high conversion efficiencies; each active junction is composed of an alloy with a different lattice constant chosen to maximize the theoretical efficiency (M. Yamaguchi, Sol. Energy Mater. Sol. Cells 75, 261-269 (2003)). However the epitaxial growth of III–V alloys entails other types of difficulties as disorder in alloy composition. In this sense, three situations are possible: (a) random distribution of atoms, (b) ordering and (c) composition modulation or phase separation. It is fundamental to control the semiconductor alloy composition since these properties affect device optoelectronic properties, as carrier mobility reduction and so its technological applications (Gutierrez et al. J.N.N. 10, 1-5 (2010)). The three junction approach includes active layers of InGaP/AlGaAs/Ge with band gaps around 1.88eV/1.41eV/0.68eV respectively. AlGaAs/GaAs tunnel junctions allow their electrical connections. The present contribution, describe, the micro and nanostructure of such monolithic cascade-type InGaP/InGaAs/Ge junctions connected in series, by means of different mode of electron microscopy. The In0.49Ga0.51P top subcell, the In0.01Ga0.99As middle subcell and Ge bottom subcell were all lattice-matched and grown on a p-type Ge substrate. The InGaP subcell was connected to the InGaAs subcell by a p-AlGaAs/n-GaAs tunnel junction. The InGaAs subcell was connected to the Ge subcell by a p-AlGaAs/n-GaAs tunnel junction.Diffraction contrast mode of transmission electron microscopy (CTEM) allowed to reveal the layer structure and composition modulation of InGaP layers with a high sensitivity; however to quantify the In-related variation, high angle annular dark field (HAADF) is able to reach atomic resolution in STEM (scanning transmission electron microscopy) and its signal is highly sensitive to the atomic number. HAADF is here used to quantify the In vs Ga modulation, fitting the experimental contrast (intensity profiles) to the numerically simulated one.To corroborate and confirm results obtained with the HAADF-STEM technique we used the CL technique (Cathodoluminescence). The CL results confirm the presence of modulation.
9:00 PM - B8.45
Guideline for Material Selection in Metal-insulator-metal Diodes for Energy Harvesting Applications.
Prakash Periasamy 1 2 , Steven Christensen 2 , Paul Ndione 2 , Philip Parilla 2 , Joseph Berry 2 , David Ginley 2 , Ryan O'Hayre 1
1 Department of Metallurgical and Materials Engineering, Colorado School of Mines/National Renewable Energy Laboratory, Golden, Colorado, United States, 2 NCPV, National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractRectennas represent an intriguing yet often overlooked potential 3rd generation solar-cell technology. Experimental demonstrations of rectennas operated at microwave frequencies have reported conversion efficiencies (DC power out / microwave power in) above 80%. Theoretical calculations suggest that similar efficiencies may be achieved at infrared and optical frequencies. In practice, however, most experimental results of state-of-the-art rectennas operated at infrared frequencies have exhibited power conversion efficiencies (PCE) below 1%. Impedance mismatch between the two major components of the rectenna, namely, the antenna and the rectifier, results in poor coupling of the photon energy thereby limiting possible PCE. Additionally, the rectifier suffers from poor performance and further limits the conversion efficiency. Metal-Insulator-Metal (MIM) diodes are one of the rectifier candidates for rectennas owing to their ultra-fast electron transport (via tunneling) mechanism. Desired properties from such MIM diodes are low resistance and minimal parasitic capacitance combined with highly efficient rectification. Although advances in nanoscale fabrication now enable the fabrication of ultra-small (down to 100s of nm2) MIM diodes, scientific understanding of the materials property/performance relationships governing MIM behavior (i.e. design rules) is lacking. In this work, we begin addressing this issue through systematic studies that can shed light on influence of material properties on diode behavior. One such study was done to understand the role of the workfunction difference (Δφ) of the two metals in the MIM structure on the rectification properties. The template MIM combination for this study was based on Nb/Nb2O5. Although all rectification properties in general showed an increasing trend with increasing Δφ, the device asymmetry (defined as |Iforward/Ireverse|) was most profoundly influenced by Δφ. In contrast, the nonlinearity (defined as dI/dV × V/I) and the responsivity showed a relatively weak dependence on Δφ. This study also identified an optimized device based on Nb/Nb2O5/Pt with superior rectification values and a remarkably low turn-on-voltage (< 0.1 V). Asymmetry as high as 30 with nonlinearity and responsivity values of around 5 and 20 V-1 at 0.5 V were achieved in this device. In another related study, the role of metal/insulator interface barrier height was investigated using a Nb/Pt template with varying insulators including Nb2O5, TiO2, MgO, Al2O3 and ZrO2. Superior diode performance was exhibited in devices in which the barrier height at one of the metal/insulator interfaces was close to 0 eV. Other devices exhibited linear or symmetric current-voltage responses. Detailed characterization results from these studies will be presented. Finally, design guidance to fabricate MIM diodes with optimized rectification properties will be summarized.
9:00 PM - B8.46
Disordered Nanowire Based Photovoltaics.
Sourobh Raychaudhuri 1 , Rene Lujan 1 , Katherine Song 1 , Chris Paulson 1 , Robert Street 1
1 , Palo Alto Research Center (PARC), Palo Alto, California, United States
Show AbstractIn planar photovoltaic structures increasing the device thickness will increase the absorption efficiency of the material but will reduce the carrier collection efficiency. Vertical solar cells provide a means to circumvent this trade-off, but also pose a considerable manufacturing challenge. In this paper we present experimental results incorporating traditional amorphous silicon (a-Si) photovoltaic devices with a three-dimensional, disordered nanowire (NW) mat using approaches that are compatible with current large area a-Si processing techniques. We are able to grow disordered mats on a variety of substrates including silicon, glass and flexible stainless steel foil.We have devised a scheme where a thin a-Si cell is deposited over a disordered NW mat. The disordered geometry of the NW mat scatters light causing a photon to interact with multiple nanowires increasing the probability that it will be absorbed by the a-Si cell coating the nanowires. Thus the NW mat makes it possible to improve the effective absorption efficiency of the cell without increasing the cell thickness and compromising the carrier collection efficiency.We fabricated and studied these disordered nanowire structures. The mats are grown using the VLS technique with nanowire lengths of 5-10 microns. The nanowires are coated with 100-200 nm a-Si PIN solar cell structures. Top contact is achieved by using a transparent conducting oxide. We discuss optical characterization data exploring how different NW mat geometries can be used to enhance light absorption. We present spectral response curves and current-voltage characteristics of NW mat based photovoltaics to demonstrate how mat geometries can be optimized to affect cell performance. Our experiments focus on device geometries that can be fabricated using techniques that are compatible with a-Si processing technologies.
9:00 PM - B8.47
Optimization and Characterization of Nanostructured Surfaces for Photon-enhanced Thermionic Emission and Photoemission Cathodes.
Daniel Riley 1 3 4 , Vijay Narasimhan 1 2 , Joel Jean 5 , Igor Bargatin 5 , Jared Schwede 1 3 4 , Zhi-Xun Shen 1 3 4 , Yi Cui 1 2 , Nicholas Melosh 1 2 3
1 Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California, United States, 3 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California, United States, 4 Department of Physics and Applied Physics, Stanford University, Stanford, California, United States, 2 Department of Materials Science and Engineering, Stanford University, Stanford, California, United States, 5 Department of Electrical Engineering, Stanford University, Stanford, California, United States
Show AbstractThe dimensions of a cathode in an energy converter based on photon-enhanced thermionic emission (PETE) must be small, as photoexcited carriers may need to encounter the emissive surface numerous times before having sufficient thermal energy to escape into vacuum. However, in a traditional planar geometry, a thin cathode results in incomplete light absorption. Nanostructuring has the potential to increase light capture and boost emission by decoupling the lengths associated with photon absorption and electron emission. Nanostructures may complicate the properties of the emissive surface; therefore, the effect of nanostructuring on emission efficiency needs to be studied.We have recently reported preliminary theoretical results from a suite of simulation tools to capture the full photoemission process: photon absorption, carrier transport within the active material, and electron ballistics following emission. In this work we use the simulation suite to optimize nanostructures for applications including PETE-based solar energy converters, photodetectors and electron sources. The samples are then characterized, and the emission efficiency measured in an ultra-high vacuum test chamber under application-centric conditions. The results presented here will contribute to the development of future PETE- and photoemission-based devices.
9:00 PM - B8.49
Light Emitting Polymers in Fluorescent Concentrator Systems.
Yvonne Rodriguez 1 , Jeremy Olson 2 , Donez Horton-Bailey 1 , Glenn Alers 1 2 , Sue Carter 1
1 Physics Department, UC Santa Cruz, Santa Cruz, California, United States, 2 , APV Research, Sunnyvale, California, United States
Show AbstractWe have done a comparative study of the performance of light emitting polymers and Lumogen Red 305 (LR305) in luminescent solar concentrators (LSC). LSCs are emerging as a viable, low cost alternative to current solar energy conversion technologies, because they absorb both direct and diffuse light, eliminating the need for expensive tracking systems. Additionally, LSCs have the capacity to concentrate light onto much smaller photovoltaic cells than are currently used in traditional concentrating systems. LR305 is a promising material for use in LSCs and has been widely studied because of its near unity quantum yield and broad absorption spectrum. However, if appropriate matches for the responsivities of available solar cells are to be achieved, more materials need to be investigated. Semiconducting polymers, with some quantum yields reported at 0.67 and large stokes shifts, are good candidates for LSCs. Both neat films and films made with dyes dispersed in varying concentrations in 3 different host matrices were fabricated. Films on the order of 10 microns were doctor-bladed onto glass substrates. Concentrations were optimized via absorption, surface emission and edge emission. Best results for LR305 were obtained in a PMMA matrix (n=1.5), with significant losses of PL emission in all other host materials. Neat LR305 films exhibited near zero emission due to self-absorption effects. PL of the polymer films was optimized in a PVK (n=1.7) matrix. Although the neat polymer films did photoluminescence at the edges, this was enhanced upon dilution in PVK. Under AM1.5 illumination near the edge of the film, the ratios of the light detected at the edge of the neat polymer film LSC and diluted film to that collected at the LR305:PMMA LSC edge were 0.53 and 0.66 respectively. Silicon solar cells were coupled to the edges of 2” x 2” substrates with diluted polymer films and LR305 films and were both found to have ~4% efficiency. Additionally, polymer films have been found to be stable under light soaking and elevated temperatures. These results suggest that light emitting polymers have potential for commercial applications.
9:00 PM - B8.5
Increasing the Efficiency of Dye-sensitized Solar Cells by Optimizing the Dye Adsorption Process.
Giorgio Bazzan 1 2 , James Deneault 1 , Tae-Sik Kang 1 , Micheal Durstock 1
1 Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio, United States, 2 , National Research Council, Washington, District of Columbia, United States
Show AbstractIn recent years, dye-sensitized solar cells (DSSCs) have become a viable technology for the conversion of sun light into electricity. The promise for high efficiency, low cost, and light weight devices makes DSSCs a promising system for many different solar power applications.A key component for high power conversion efficiencies in a DSSC is the interface between the Ruthenium complex dye and the surface of the mesoporous titanium dioxide film. Despite many studies aimed at understanding the specific anchoring method of the N3 and N719 molecules on the titania surface, there isn’t a commonly accepted explanation for the dye adsorption mechanism. Furthermore, it is generally believed that a uniform monolayer of strongly attached molecules is required in order to maximize the efficiency of electron injection into the semiconductor, however the amount of dye adsorbed onto the mesoporous film has not been optimized.Here, we have developed a process that significantly increases the surface concentration of the dye molecules adsorbed on the TiO2. By carrying out a partial de-sorption after the initial ad-sorption of the dye, and then repeating this process a series of times, up to a 20% increase in the amount of dye adsorbed and in the measured power conversion efficiency is observed. In addition, vibration spectroscopy studies using surface enhanced resonance raman spectroscopy (SERRS) and FTIR have been used to develop a better understanding of the dye adsorption process and attachment mechanism, and the means to manipulate it to optimize the power conversion efficiency of devices.
9:00 PM - B8.50
Advances in Nanopillar Photovoltaics: Roll-to-roll Processing of Anodic Alumina Templates, Doping of Indium Phosphide with Ammonium Sulfide Monolayers, and Nanopillar Shape Control.
Daniel Ruebusch 1 , Zhiyong Fan 2 , Rehan Kapadia 1 , Onur Ergen 1 , Kee Cho 1 , Ali Javey 1
1 EECS, U.C. Berkeley, Berkeley, California, United States, 2 , HKUST, Hong Kong Hong Kong
Show AbstractSolar energy represents an abundant source of renewable energy, however significant advancements are needed to make photovoltaic energy conversion ubiquitous and economical. Large area arrays of semiconductor nanopillars have shown significant promise as a next generation photovoltaic technology (1,2). We have previously demonstrated a PV structure utilizing a 3D array of CdS nanopillars embedded in a poly-crystalline CdTe film fabricated on low cost Al foil (3). Prior to nanopillar growth the Al foil was anodized to form a porous aluminum oxide film. The vertically aligned nanopillars were used as a template for nanopillar growth. All steps in this process are potentially compatible with high throughput roll-to-roll processing. We report here on the successful production of anodized aluminum oxide templates on aluminum foil using a purely roll-to-roll process scheme. This is the first major step toward implementing completely roll-to-roll fabrication of nanopillar solar cells. Through simulations we have found efficiencies in excess of 20% to be achievable using the nanopillar architecture. Integration of InP into the nanopillar structure is a promising direction toward approaching these efficiencies (4). With this ultimate goal in mind, we report on the successful formation of conformal, ultra shallow, heavily doped junctions in p-type InP nanopillars. Junctions were formed by sulfur doping with self assembled monolayers of ammonium sulfide in solution, a technique we previously demonstrated with InAs (5). Finally, we have demonstrated growth of CdS and Ge nanopillars in porous aluminum oxide templates with controllable square, rectangular, and circular cross sectional shapes (6). Templates were formed by anodizing Al foil that was pre-texturized by a double imprint with silicon diffraction gratings. Subsequent Au catalyzed vapor-liquid-solid growth of nanopillars showed pillar shapes conformed well to the pore geometry.1.Z. Fan, et al. "Challenges and Prospects of Nanopillar Based Solar Cells", Nano Research, 2, 829-843 2009.2.Z. Fan, Ret al. “Ordered arrays of dual-diameter nanopillars for maximized optical absorption”, Nano Letters, 10, 3823–3827, 2010.3.Z. Fan, et al. "Three dimensional nanopillar array photovoltaics on low cost and flexible substrates", Nature Materials, 8, 648-653, 2009.4.R. Kapadia, et al. "Design constraints and guidelines for CdS/CdTe nanopillar based photovoltaics", Applied Physics Letters, 96, 103116, 2010.5.J. C. Ho, et al. "Nanoscale doping of InAs via sulfur monolayers", Applied Physics Letters, 95, 072108, 2009.6.O. Ergen, et al. "Shape-Controlled Synthesis of Single-Crystalline Nanopillar Arrays by Template-Assisted Vapor-Liquid-Solid Process", Journal of the American Chemical Society, 132 (40), 13972–13974, 2010.
9:00 PM - B8.51
Current Density Enhacement on CdTe Based Solar Cells by Deep Level Doping.
Carmen Ruiz Herrero 1 , Edgardo Saucedo Silva 2 , Veronica Bermudez Benito 3
1 , ICMSE-CSIC, Sevilla Spain, 2 , IREC, Barcelona Spain, 3 , NEXCIS, Rousset France
Show AbstractMaximum current generation of a solar cell is limited by the band gap of the absorber material that defines the absorption range of the final device. Lower energy photons are not absorbed and so, they do not contribute to the carrier generation. Different approaches have been used to increase the absorption region of solar cells. Multijunction cells with matched spectral absorption, Intermediate Band devices, by the creation of a band inside the forbidden band that allows the absorption of photons with lower energy. This band can be reached by the growth of quantum wells in the device, or by the generation of an impurity band by doping the semiconductor, preferentially in the deep region.One of the particularities of CdTe is that is a semiconductor where deep donor doping has been extensively studied. In particular deep levels on CdTe can be achieved by using semimetals such as Ge, Sn and Bi. In this work, Bi doped CdTe has been studied. Photoconductive and electrical measurements have been carried on bulk crystals for determining the impact of this deep level in its properties. Photoconductivity in this kind of materials is very high regarding usual values for the same undoped materials. This increasing in the photo sensibility is due to this deep defect structure and is related to their efficient absorption in the infrared region of the spectra.Moreover the electrical transport properties for a dopant concentration about between 1017 and 1018 at/cm3 are maximized diminishing for other concentrations, values of mu*tau = (5-6)x10-3 cm2/V can be obtained (the value is commonly (1-2)x10-3 cm2/V for undoped material) contributing also to the improvement of the short circuit current.Finally, solar cells with Bi doped CdTe have been grown and the results obtained are compared with the bulk values.
9:00 PM - B8.52
Radial Core-shell Metal-metal Oxide Hybrid Nanowires for Efficient Dye Sensitized Solar Cells.
Gayatri Sahu 1 , Scott Gordon 1 , Kai Wang 2 , Haiqiao Su 2 , Weilie Zhou 2 , Matthew Tarr 1
1 Advanced Materials Research Institute and Department of Chemistry, University of New Orleans, New Orleans, Louisiana, United States, 2 Advanced Materials Research Institute, University of New Orleans , New Orleans, Louisiana, United States
Show AbstractHighly aligned Au-TiO2 core-shell hybrid nanowires were synthesized using several approaches to grow Au nanowires as core and crystalline anatase TiO2 as shell. The core-shell nanostructures were characterized using electron microscopy and X-ray powder diffraction. The results show that the wires were highly aligned, well separated, and with desired crystalline phase. Dye sensitized solar cells were fabricated using the core-shell nanowire arrays as photoanode, N535 dye as the sensitizer, I3-/I- as the redox couple, and Pt coated ITO as cathode. The Au nanowires inside the crystalline TiO2 anatase nanoshell provided a direct conduction path from the TiO2 shell to the ITO substrate and improved transport of electrons between the TiO2 and the conducting substrate. This efficient electron conduction out of the oxide enhanced the current generation as well as the power conversion efficiency of the cell. Furthermore, TiCl4 treatment of the Au-TiO2 core-shell nanowire structures resulted in additional efficiency improvements.
9:00 PM - B8.53
Transparent UV Solar Cells Utilizing Simply Fabricated Amophous Selenium p-n Junction.
Ichitaro Saito 1 2 , Wataru Miyazaki 1 , Masanori Oonishi 1 , Yuki Kudo 3 2 , Tomoaki Masuzawa 1 , Takatoshi Yamada 4 , Angel Koh 5 , Daniel Chua 5 , Kenichi Soga 1 , Mauro Overend 1 , Masami Aono 6 , Gehan Amaratunga 1 , Ken Okano 2
1 Department of Engineering, University of Cambridge, Cambridge, Cambridgeshire, United Kingdom, 2 Department of Material Science, International Christian University, Tokyo Japan, 3 Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba Japan, 4 Nanotube Research Center, National Institute of Advancved Industrial Science, Tsukuba Japan, 5 Department of Materials Scinece and Engineering, National University of Singapore, Singapore Singapore, 6 , National Defense Academy, Tokyo Japan
Show AbstractThis research is an attempt to create a solid transparent solar cell from wide band gap material rather than etching away conventional materials such as crystalline or amorphous silicon to make see-through solar cells. This could either be a glass panel module to replace window panels or a film that may be easily applied onto current window glasses. Transparency would obviously mean giving up energy within the visible wavelength, were out attempt focuses on photovoltaic effect towards the ultraviolet region.Since a-Se is semi-transparent towards the visible wavelength and has a gentle absorption tail into the ultraviolet (UV) region, there is a possibility for it to serve as a window shade coating and a UV cut filter that enables electric power. However, in order to fabricate a p-n junction in a-Se, it is necessary to introduce donors and acceptors such as Arsenic (As) and Chlorine (Cl) to a concentration such that it contributes as an electronic carrier, which is technically difficult to accomplish due to the nature of amorphous materials not always being structure sensitive to impurities. Here we report an amorphous selenium (a-Se) p-n junction film fabricated through an inexpensive and simple process of thermal evaporation and electrolysis, and that such film exhibits promising properties towards application for see-through solar cell use in the future.
9:00 PM - B8.54
Growth and Characterization of CuInS2 Thin Films Free from Cu-Au Ordering.
Sunil kumar Samji 1 , Ramachandra Rao 1
1 Physics, IIT Madras, Chennai, Tamil Nadu, India
Show AbstractCuInS2 thin films are well known for their photovoltaic applications. CuInS2 thin films can be grown by various techniques such as sputtering, spray pyrolysis, co-evaporation etc. Since CuInS2 is a ternary compound, 12 types of defects are possible corresponding to vacancies, interstitials and antisite defects [1]. Out of 12 types of possible defects CuInS2 and Cu-Au polymorph (antisite defect) differ in formation energy by only 2 meV/atom [2]. Since defect phases affect conductivity, growth of Cu-Au free CuInS2 films is crucial to photovoltaic applications.In this work CuInS2 thin films have been grown using spin coating by varying preheating temperature from 225 oC to 275 oC. CuAc.H2O and InAc were used as Copper and Indium precursors. The preheating temperature was determined from results obtained from TGA studies. Structural properties have been studied using X-ray diffraction and Raman spectroscopy. The presence of Cu-Au polymorph is attributed to sulphur vacancies and can be identified by the presence of a small hump at 305 cm-1 in the Raman spectra. By replacing the source of sulpharization CS2 by elemental sulphur, films free from Cu-Au ordering have been grown. It is also observed that the films sulpharized with elemental sulphur are more crystalline with FWHM of 0.2o. Microstructure and Compositional analysis is done by FESEM & EDS respectively. It is found that the films free from Cu-Au ordering are Cu rich. References:1. R. Marquez, C.Rincon, Materials Letters 40 (1999) 66-70. 2. J.Alvarez-garcia, B.Barcones, A. Perez-Rodriguez, A.Romano-rodriguez, and J.R.Morante, A.Janotti and Su-Hui-Wei, Phys. Rev. B 71, 054303 (2005).
9:00 PM - B8.55
Development of Size-controlled Metal Nanoparticles for Improvement of Energy Conversion Efficiency of Thin-film Solar Cells.
Rudi Santbergen 1 , Rita Najjar 2 , Noella Lemaitre 2 , Salim Boutami 2 , Marilyn Armand 2 , Srinivas Saranu 3 , Marko Berginc 4 , Toby Meyer 5 , Alistair Kean 3 , Marko Topic 4 , Miro Zeman 1 , Etienne Quesnel 2
1 , Delft University of Technology, Delft Netherlands, 2 , CEA, Grenoble France, 3 , Mantis Deposition, Thame United Kingdom, 4 , University of Ljubljana, Ljubljana Slovenia, 5 , Solaronix, Aubonne Switzerland
Show AbstractSeveral simulation and experimental studies have been devoted to absorption enhancement in thin-film solar cells through a surface plasmon effect. Gold, silver and copper nanoparticles, ranging from 10 to 120 nm have been deposited by different techniques (wet process, thermal evaporation followed by heat-treatment to generate islands). However, even if an increase in short-circuit current has been demonstrated, all the authors underlined the necessity to better control the characteristics of the nanoparticles (size, shape, surface density). This control is required to optimise the characteristics of the nanoparticles to further improve absorption of sunlight in the active layers and to be able to tune the plasmons resonance to a given wavelength range through an optimized localization of those nanoparticles in an appropriate surrounding medium.We present experimental results of the synthesis of nanoparticles using a recently developed room temperature vacuum deposition process that allows depositing pure and alloyed nanoparticles with an unprecedented control of size distribution, shape and surface density. Moreover, this technique is compatible with the current device processing of thin-film silicon solar cells in which such nanoparticles could be integrated and is inherently scalable to mass production.After description of the advanced nanoparticle source that was initially designed for very small nanoparticles, typically 5 nm in diameter, experimental results are presented that demonstrate the potential of the technology in terms of nanoparticle characteristics control and scalability towards larger nanoparticles in the range of 10 to 40 nm. Manipulation of optical and electrical properties of films with embedded nanoparticles will be demonstrated. Finally silver nanoparticles were embedded in thin-film silicon solar cells. The results of the electrical characterisation of these solar cells will be presented.
9:00 PM - B8.56
Photon-enhanced Thermionic Emission for Solar Concentrator Systems.
Jared Schwede 1 2 3 , Igor Bargatin 4 , Daniel Riley 1 2 3 , Brian Hardin 1 5 , Samuel Rosenthal 1 5 , Yun Sun 6 , Felix Schmitt 1 2 3 , Piero Pianetta 6 , Roger Howe 4 , Zhi-Xun Shen 1 2 3 , Nicholas Melosh 1 2 5
1 Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California, United States, 2 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California, United States, 3 Department of Physics and Applied Physics, Stanford University, Stanford, California, United States, 4 Department of Electrical Engineering, Stanford University, Stanford, California, United States, 5 Department of Materials Science and Engineering, Stanford University, Stanford, California, United States, 6 , Stanford Synchrotron Radiation Lightsource, Menlo Park, California, United States
Show AbstractPhoton-Enhanced Thermionic Emission (PETE) is a newly proposed method of solar energy harvesting which combines quantum and thermal processes into a single electricity generating mechanism. A PETE device is based on thermionic emission of photoexcited electrons from a semiconductor cathode and is designed to operate efficiently at elevated temperatures, which would allow the device’s waste heat to power a secondary thermal cycle. In this presentation, we describe the PETE process and show the results of experimental investigations, including high-temperature emission-yield measurements on GaN that show strong evidence for photon-enhanced thermionic emission. Calculated power conversion efficiencies for idealized devices based on the effect are shown to exceed the Shockley-Queisser limit on single junction photovoltaic cells, as PETE can harvest heat due to sub-bandgap photons and carrier thermalization. A combined cycle comprising of a PETE device in tandem with a solar thermal engine is calculated to reach an overall conversion efficiency above 50%.
9:00 PM - B8.57
TiO2 Nanotube Arrays on FTO Glass for DSSC.
Ulugbeck Shaislamov 1 , BeeLyong Yang 1
1 Dept of Information Nano Materials, Kumoh National Institute of Technology, Gumi, Geongbuk Korea (the Republic of)
Show AbstractOver the past several years increasing interest has been focused in research of TiO2 for solar energy harvesting. Many research related to photo-catalytic properties of TiO2 nano powders or thin films has been performed due to their advantages of low cost and chemical stability. However, the efficiency for solar energy harvesting is still low. In this study, we report photocatalytic properties of highly ordered TiO2 nanotube arrays on FTO glasses to address issues of improving the photo-conversion efficiency for dye sensitized solar cells (DSSC). Fabrication of highly ordered TiO2 nanotube-arrays on FTO glass was performed using both techniques of transplanting TiO2 nanotube arrays grown on Ti substrate into FTO glass and anodizing Ti films formed on FTO glass by pressing at high temperature. Dye sensitized solar cells were fabricated using these nanotube arrays on FTO glass as a working electrode. The solar efficiency through measurements of J-V curves, and interface structures of these cells by SEM/TEM will be discussed.
9:00 PM - B8.58
Graphene-silicon Schottky Solar Cells.
Shabnam Shambayati 1 , Derek Lin 1 , David Pulfrey 1 , Peyman Servati 1
1 Electrical & Computer Engineering, University of British Columbia, Vancouver, British Columbia, Canada
Show AbstractGraphene films showcase critical qualities such as low sheet resistance, high optical transparency, and chemical and electronic stability, which make them an interesting option for use in photovoltaic (PV) devices and applications. This paper investigates the light and dark current-voltage characteristics of graphene-silicon Schottky PV devices. Graphene film directly deposited on a silicon wafer acts as a transparent electrode and forms a Schottky junction, which has the potential of reducing the manufacturing steps and cost in comparison to the PN junction devices used in conventional silicon solar cells. Schottky junctions are successfully fabricated by producing graphene using the scotch-tape method, and allowing the graphene to collapse on the underlying p-type silicon after etching the silicon oxide film on the substrate. The forward bias characteristics are analyzed based on the Schottky-Mott relation in order to extract Schottky barrier height and explore the effect of graphene-silicon junction and interfacial properties. The solar cell device properties including short-circuit current, open circuit voltage, fill factor and conversion efficiency under AM 1.5 solar irradiation are investigated for mono-layer, bi-layer, and few-layer graphene, and the number of layers is confirmed by Transmission Electron Microscopy (TEM). The photo-response of the device as a function of irradiation wavelength is explored to observe the impact of graphene absorption on efficiency.
9:00 PM - B8.59
Minority Carrier Lifetime in Ge Film Epitaxially Grown on Si by Nanoscale Interfacial Engineering.
Josephine Sheng 2 1 , Darin Leonhardt 1 , Jeffrey Cederberg 3 , Malcolm Carroll 3 , Qiming Li 3 , Manuel Romero 4 , Steve Johnston 4 , Sang Han 1 2
2 Nanoscience and Microsystems, University of New Mexico, Albuquerque, New Mexico, United States, 1 Chemical & Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico, United States, 3 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 4 , National Renewable Energy Laboraotry, Golden, Colorado, United States
Show AbstractHigh-quality Ge-on-Si (GoS) heterostructures have been actively pursued for many advanced applications, including near-infrared photodetectors, high-mobility field effect transistors, and virtual substrates for integrating III-V multijunction solar cells. However, growing epitaxial Ge on Si poses many engineering challenges, ranging from lattice mismatch, to thermal expansion coefficient mismatch, to non-planar morphological evolution. The lattice mismatch between Ge and Si often leads to a high density of threading dislocations. These dislocations, if not reduced, propagate through the subsequently grown GaAs layer, deteriorating its quality. To reduce threading dislocations, nanoscale interfacial engineering involving a SiO2 template is used. Optimization of our nanoscale interfacial engineering and GoS film growth has led to the elimination of stacking faults in the GoS film, as well as the elimination of anti-phase domains (APD) in the subsequent GaAs layer. Integration of GaAs/AlGaAs/GaAs on GoS substrates leads to photoluminescence as well as cathodoluminescence intensity comparable to that on GaAs substrates. This III-V integration required a slurry-free, chemical-mechanical polish step to planarize the GoS surface, resulting in a Ge surface with a root mean square (RMS) roughness of less than 1 nm. Herein, we focus on the characterization of carrier lifetime in the Ge epilayers grown on Si by the said nanoscale interfacial engineering for the purpose of establishing the baseline quality of epitaxially grown Ge. Using photoconductivity decay techniques, minority carrier lifetime is measured in the GoS substrates to extract effective surface recombination velocity as well as carrier lifetime in bulk Ge. The effective surface recombination velocity, representing both the Ge surface and the Ge-Si interface decorated with chemical SiO2, is approximately 3x103 cm/sec. We observe that the extracted lifetimes, which vary with the Ge film thickness, correlate well with the dislocation density that varies as a function of distance from the Ge-Si interface. The Ge bulk lifetime near the Ge surface was found to be 15.7 ns, comparable to reported lifetimes of Ge substrates with high conductivity.
9:00 PM - B8.6
Enhancing Nanocrystalline Solar Cells with Plasmonic and Photonic Crystal Back Reflectors.
Joydeep Bhattacharya 1 , Rana Biswas 1 2 , Nayan Chakravarty 1 , Vikram Dalal 1
1 Microelectronics Research Center; Dept. of Electrical and Computer Engineering, Iowa State University, Ames, Iowa, United States, 2 Depts. of Physics & Astronomy; Ames Laboratory, Iowa State University, Ames, Iowa, United States
Show AbstractIt is essential to harvest red and near infrared photons in thin film nano-crystalline silicon (nc-Si) solar cells to increase their current and efficiency. These long wavelength photons are poorly absorbed in Silicon due to the photon absorption lengths that are much larger than the absorber layer thickness. We have enhanced thin film nc-Si silicon solar cells by using a photonic-plasmonic crystal back reflector. Rigorous scattering matrix simulations provided an optimized photonic crystal back reflector consisting of a triangular lattice of nano-holes, with a pitch near 800 nm. The photonic crystal back reflector with pitch 800 nm was fabricated on a crystalline silicon substrate by photolithography and reactive ion etching, and coated with silver and zinc oxide. Nano-crystalline silicon solar cells were grown on the patterned substrates. We observed ~7% enhancement of the absorption and photo-generated current relative to cells on a planar Ag/ZnO substrate, with an enhancement ratio of 1.5 near the band edge. Significant enhancement occurred in photon absorption at near infrared wavelengths greater than 700 nm, due to diffraction resonances of the incoming light. We will compare the performance enhancement to other plasmonic crystal approaches.We characterized the optical and electrical properties of these structures and devices. The diffuse and total reflectance of photonic crystal substrates was measured and compared with etched zinc oxide substrates, to compare the periodic texture with the randomly textured substrate. We characterized the electrical properties with forward bias dark-current measurements. The reverse saturation current and diode ideality factor are similar for the photonic crystal device and the flat Ag substrate device. These will be compared to devices on etched zinc oxide to infer the distribution of defect states. The work was partially supported by Iowa Power Fund.
9:00 PM - B8.60
Interfacial Effects on Optical and Electronic Properties in Germanium-transparent Conductive Oxide Nanocomposites.
Grace Shih 1 , Cary Allen 1 , Barrett Potter 1
1 , University of Arizona, Tucson, Arizona, United States
Show AbstractThe effective use of quantum-scale semiconductors is critically dependent upon the nature of the interface between the semiconductor and the embedding phase as well as the associated impact on both optical absorption and carrier transport/extraction. As a means to evaluate the impact of interfacial chemistry and structure on the optoelectronic behavior in such systems, nanocomposites containing germanium (Ge) within either zinc oxide (Ge:ZnO) or indium tin oxide (Ge:ITO) matrices have been studied. Thin film nanocomposites were fabricated using a sequential RF-magnetron sputtering deposition technique which allowed the effect of embedding matrix on optical and electronic behavior to be investigated over a range of semiconductor nanophase spatial distributions produced through both deposition control and post-deposition thermal annealing. The resulting Ge distributions ranged from isolated Ge nanocrystals to two-dimensional-extended structures. Under consistent semiconductor nanostructural conditions, significant differences in the quantum-confinement-induced optical absorption edge energies and the corresponding carrier transport properties of the composites were observed. In this case, an increased blue-shift in absorption onset energy observed in the Ge:ITO system is correlated with the evolution of vibrational structure consistent with the presence of Ge-O bonding. Hall effect measurements further revealed a decrease in carrier mobility with annealing for Ge:ITO while identically processed Ge:ZnO films exhibit increasing carrier mobility. The effects are attributed to the different propensities for GeOx interfacial phase development within nanocomposites produced using different embedding materials. It is anticipated that the development of this interface will contribute to increased carrier confinement in the Ge while also producing enhanced carrier scattering during charge transport within these nanoheterogeneous materials. Understanding the role of interface and phase assembly in phenomena dependent upon both short- and long-range structural characteristics in these semiconductor-based nanocomposite systems is of significant relevance to their successful application in photovoltaics.
9:00 PM - B8.61
Thermally Evaporated High Efficiency Cu2ZnSnS4 (CZTS) Thin Film Solar Cells: Interfacial Reactions between CZTS and Mo Back Contact.
Byungha Shin 1 , Kejia Wang 1 , Oki Gunawan 1 , Nestor Bojarczuk 1 , S. Jay Chey 1 , Teodor Todorov 1 , David Mitzi 1 , Supratik Guha 1
1 , IBM T. J. Watson Research Center, Yorktown Heights, New York, United States
Show AbstractConsisting of earth-abundant elements and having suitable optical properties for photovoltaic applications, Cu2ZnSnS4 (CZTS) has recently emerged as a potential candidate for a light absorbing layer in thin film solar cell devices. Currently, common practice in fabricating CZTS-based solar cells is simply replacing the absorbing layer of well-studied Cu(In, Ga)Se4 (CIGS)-based thin film solar cells with a CZTS layer, while keeping the other constituent layers – a transparent conducting oxide (TCO) layer, a buffer layer, a back contact, and a substrate – the same. The choice of materials and their processing conditions for these other layers in CIGS solar cells are the outcome of extensive studies and gradual optimization that have been undertaken over several decades. Naturally, it is not guaranteed that what have been optimized for CIGS would also work for CZTS. Therefore, a close examination of possible interfacial reactions between CZTS and the surrounding layers is necessary. In this work, we report on device results of thermally evaporated high efficiency CZTS thin film solar cells, and on interfacial reactions between a CZTS layer and a Mo back contact and their consequences in structural and electrical perspectives. CZTS layers were thermally evaporated using elemental sources of Cu, Zn, Sn, and S on a Mo-coated soda-lime glass, which is the most common substrate used in CIGS solar cells. Following the deposition, samples received a brief annealing at > 500 oC in the presence of S vapor. Solar devices were completed by depositing a CdS buffer layer, followed by an intrinsic ZnO layer, a TCO layer (either Al-doped ZnO or ITO), and Ni/Al electrodes. Structural analysis showed that an interfacial MoSx layer of a substantial thickness formed between CZTS and Mo. Elemental profiling across the CZTS/Mo interface revealed out-diffusion of Cu from CZTS into MoSx, the result of which appears to lead to the phase separation of CZTS into CuxSnSy and ZnS near the CZTS/(MoSx)/Mo interface. A temperature-dependent series resistance measurement suggests that this interfacial MoSx essentially acts as a Schottky barrier, whose height amounts to as large as 320 meV. Additionally, wavelength-dependent quantum efficiency (QE) measured at a reverse-bias showed an improvement at longer wavelengths compared to QE at zero-bias, indicating an inefficient collection of charge carriers that are photo-generated deep in the absorbing layer (i.e., close to CZTS/Mo interface). This is consistent with the structural disturbance observed near CZTS/(MoSx)/Mo interface due to the presence of the MoSx layer. Despite these issues at the back-side interface, we demonstrate a CZTS solar cell with the efficiency of > 7%, which is the highest efficiency reported for pure sulfide CZTS solar cells without addition of Se.
9:00 PM - B8.62
Synthesis of Cu2ZnSnS4 Quaternary Nanocrystal Formed by Annealing Precursors Prepared by Microwave-assisted Solution Method.
Seung Wook Shin 1 , Jun Hee Han 1 , Chan Young Park 2 , Jong-Ha Moon 2 , Jeong Yong Lee 1 , Jin Hyeok Kim 2
1 Department of Materials Science and Engineering, KAIST, Daejeon Korea (the Republic of), 2 Department of Materials Science and Engineering, Chonnam National University, Gwangju Korea (the Republic of)
Show AbstractNanocrystals (NCs)-based approach for thin film solar cells (TFSCs) device have been attracted considerable interest to fabricate low cost and high power conversion efficiency (PCE) TFSCs due to its low cost solar cells fabricating process. Cu2ZnSnS4 (CZTS) is a promising candidate material, which contains abundant elementals such as Zn and Sn and it has stable band gap energy of 1.5 eV and high absorption coefficient over 104 cm-1. The solution-based CZTS NCs have been prepared using different organometallic solution-based routes and the PCE for CZTS NCs-based TFSCs have been achieved about 0.8 %. However, the previous literature survey indicates that the hazardous chemicals such as trioctylphosphine, oleylamine and trioctylphosphine oxide have been used to synthesis for well uniform and high quality CZTS NCs and it has resulted in a serious environment problem. In order to solve this problem, we introduced the microwave-assisted solution method. CZTS NCs were synthesized by novel route using two-step process. In the first step, the precursors were obtained by irradiating microwave at 700W for 10 min from aqueous solutions contacting copper, zinc, tin, and sulfur elements. In the second step, the precursors were sulfurized by annealing in the mixture of H2S (5%) + N2 (95%) atmosphere at 550 oC for 1 hour. The structural, compositional, thermal and optical characteristics of CZTS NCs were investigated. X-ray diffraction patterns, X-ray photoelectron spectroscopy and transmission electron microscopy results showed that the sulfurized NCs were a single kieserite CZTS phase without any secondary phase such as Cu2SnS3, ZnS, CuS and SnS. TGA and DTA indicated a weight loss at 840 °C and endothermic peak at same temperature from CZTS nature. Energy dispersive X-ray results showed that CZTS NCs was Cu and Zn rich and S poor. UV-VIS spectroscopy results indicated that the absorption coefficient constant was over 104 cm-1 in the visible region and direct band gap energy of CZTS NCs was estimated about1.5 eV.
9:00 PM - B8.63
High-efficiency Surface Plasmon Energy Harvesting Using Optimized Organic Photovoltaics.
Matt Sykes 1 , Kwang An 3 2 , Max Shtein 1
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 3 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 , GE Global Research, Albany, New York, United States
Show AbstractSurface plasmon polariton (SPP) resonances can be used to enhance absorption in thin-film solar cells, but much of the absorption is parasitic due to rapid quenching at the metal surface, dramatically lowering the net contribution to electricity generation. Previous modeling has shown large potential power conversion efficiencies in hypothetical metal core + thin semiconductor shell nanoparticles [1] (up to 20%), but the means for practical implementation is currently lacking.
To understand the limitations of plasmon-based energy harvesting in physical photovoltaic systems, we fabricated and modeled metal-insulator-metal (MIM) organic photovoltaic devices [2] based on heterojunctions between chloro[subphthalocyaninato]boron(III) (SubPc) and C60, sandwiched between two silver electrodes whose thickness exceeds the skin depth. The structures are designed to exhibit surface plasmon resonance (SPR) at wavelengths matched to the SubPC absorption spectrum. Using the Kretschmann testing configuration, we measured record external quantum efficiencies at SPR wavelengths. [3] The effects of device structure on the efficiency of near-field SPP energy collection while mediating parasitic quenching effects will be discussed.
[1] C. Hagglund & S. Apell, Optics Express 18 (2010), A343.[2] B. O’Connor et al., Applied Physics Letters 93 (2008), 223304.[3] J.K. Mapel et al., Applied Physics Letters 90 (2007), 121102.
9:00 PM - B8.64
Synthesis and Characterization of Hybrid CdS/MEH-PPV Nanocomposites for Photovoltaic Applications.
Anna Laera 1 , Vincenzo Resta 1 , Emanuela Piscopiello 1 , Monica Schioppa 1 , Leander Tapfer 1
1 UTTMATB, ENEA, Brindisi Italy
Show AbstractInorganic-organic nanocomposites, with II-VI or III-V semiconductor quantum dots (QDs) embedded in semiconducting polymer matrix, are very promising materials for photovoltaic applications. In particular, the high photosensitivity of QDs and the possibility of carrier multiplication may contribute to a higher efficiency in solar energy conversion if compared to full organic solar cells. Here, a crucial factor and prerequisite is to achieve a highly dense and uniform network of QDs, avoiding agglomeration and particle clustering, in order to assure a high photon - electron conversion and an efficient collection and transport of the photogenerated carriers through the bulk heterojunction.Here, we present an effective and easy synthesis procedure to obtain a hybrid nanocomposite with CdS nanocrystals (NCs) QDs dispersed in poly[2-methoxy-5-(2-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) conjugated polymer. The formation and growth of CdS NCs suitable unimolecular precursors, containing both the metal and the non-metal part, are specifically synthesized in order to guarantee a stoichiometrical control of the process and to prevent inhomogeneity of multicomponent mixing. The unimolecular precursor is dispersed homogeneously in the polymer and, through a subsequent annealing process at moderate temperature (T<200°C) in controlled inert atmosphere and for a short time (<20 min), QDs are synthesized directly within the matrix. The nanocomposites can be prepared by casting or spin coating, obtaining bulk-like materials or thin film samples. The experimental results obtained by using cadmium-bis(benzylthiolate) (CBz) and a modified methyl-imidazole cadmium-bis(benzylthiolate) (CBz-MI) unimolecular precursor are presented in detail. The synthesized hybrid materials were investigated and characterized by simultaneous thermo-gravimetric analysis (TGA/DTA), small- and wide-angle X-ray scattering, high-resolution transmission electron microscopy, optical absorption and photoluminescence spectroscopy (PL). In particular, we show that the CBz-MI precursor can be easily and homogeneously dispersed within the MEH-PPV polymer. CdS QDs are formed at low annealing temperature (about 180°C) avoiding structural damages and without affecting the functional properties of the MEH-PPV polymer. The NCs diameter ranges between 1.5nm and 4nm depending on the annealing temperature. In addition, no coalescence phenomena of CdS QDs were noticed in TEM observations even at very high particle density (40 wt %).
9:00 PM - B8.66
Simulations of Symmetrically Doped Silicon Nanowire Solar Cells.
Felix Voigt 1 2 , Thomas Stelzner 1 , Silke Christiansen 1 3
1 , Institute of Photonic Technology, Jena Germany, 2 Institute of Physics, University of Oldenburg, Oldenburg Germany, 3 , Max Planck Institute for the Science of Light, Erlangen Germany
Show AbstractSilicon nanowire solar cells were simulated using the Silvaco TCAD software kit. For optimization of speed the simulations were performed in cylinder coordinates with cylindrical symmetry. Symmetric doping was assumed with a dopant density of 1e18 cm-3 in the p-type core and inside the n-type shell. In the implementation a cathode contact was wrapped around the semiconductor nanorod and an anode was assumed below the rod. Optimization of cell efficiency was performed with regard to the rod radius and the rod length. In both optimization processes clear maxima in efficiency were visible, resulting in an optimal radius of 66 nm with the pn junction at 43.5 nm and an optimal rod length of about 24 μm. The maximum of efficiency with respect to the rod radius is due to a decrease of short-circuit current density (Jsc) and an increase of open-circuit voltage (Uoc) with radius, while the maximum with respect to the rod length is explained by the combination of an increase of Jsc and a decrease of Uoc. Fill factors stay rather constant at values between 0.6 and 0.75. Further, the influence of application of a back surface field (BSF) layer was surveyed in simulations. Curiously, the BSF predominantly had an effect on open-circuit voltage and not on short-circuit current. Positioning the BSF next to the cathode contact improved cell efficiency relatively by 19%, while a BSF near the anode of the wire lead to insignificant changes. Implications on the construction of silicon nanowire solar cells concerning experimental viability will be discussed. In addition, simulations with a cathode contact on top of the nanowire structure will be undertaken. No severe deterioration of cell performance with increasing radius was observed so far in this configuration. Hence, nanorods with much larger radii can be used for solar cells with this contact scheme. In comparison to simulations with wrapped cathode contacts, Jsc and Uoc are considerably improved leading to about 50% higher cell efficiency.
9:00 PM - B8.67
A ``Virtual Substrate” Supplies Tailored Lattice Constant Template for High Efficiency Multijunction Solar Cells.
Emily Warmann 1 , Marina Leite 1 , Harry Atwater 1 2
1 , California Institute of Technology, Pasadena, California, United States, 2 , Kavli Nanoscience Institute, Pasadena, California, United States
Show AbstractCurrent III-V multijunction solar cell designs are constrained by the limited range of alloy band gap combinations available with lattice constant at or near that of single-crystal wafers such as Ge, GaAs or InP. Here we present experimental results and mechanical analysis detailing the successful realization of a “virtual substrate.” The virtual substrate consists of a large-area, defect-free single crystal alloy thin film that has undergone coherent strain relief to its native unstrained lattice parameter, bonded to a planar support. This provides a new epitaxial wafer with defined lattice constant that can be tailored to that of any desired alloy, allowing high quality, lattice-matched epitaxial growth absent the critical thickness constraint for non lattice-matched device designs. In particular, a virtual substrate with lattice constant 5.800 Å enables the lattice-matched growth of InAlAs/InGaAsP/InGaAs multijunction solar cells with a theoretical detailed balance efficiency of 54%. Lattice-matched growth of this energy band gap combination, not possible with currently available wafer substrates, prevents strain-induced dislocations from degrading cell performance.Fabrication of a virtual substrate requires managing the strain in a large-area thin single crystal film. Our experiments used 80 nm films of InxGa1-xAs grown coherently strained on InP substrates. High resolution x-ray diffraction (XRD) was used to measure relaxation in the films as-grown, with composition x = 0.43 (5.827 Å lattice constant, 0.7% tensile strain) exhibiting < 1% relaxation and composition x = 0.36 (5.800 Å, 1.2% tensile strain) 3.5% relaxed. This indicates a low dislocation density in the meta-stable films despite exceeding the Matthews-Blakeslee critical thickness (12 nm and 9 nm) for these compositions. To elastically relax the film to its native lattice constant the InP substrate is removed by etching while a ductile wax transfer layer holds the film intact and allows InxGa1-xAs strain relaxation. Subsequent bonding to a Si/SiO2 wafer (100 nm thermal oxide) and wax removal results in a strain-free, mechanically supported InxGa1-xAs film which can serve as template for lattice-matched growth at the native lattice constant of the InxGa1-xAs. XRD measurements verify creation of virtual substrates with lattice constant dictated by the film composition. While successful transfer of relaxed films was achieved for both compositions, optical microscopy reveals cracks in the transferred film, density increasing with as-grown strain. Strain and orientation uniformity in the transferred films were analyzed with Raman spectroscopy. Analysis of the film-wax mechanical system identified more optimal wax properties for increasing the continuous area of transferred films and reducing cracks. Overall, the virtual substrate holds promise for lattice-matched synthesis of photovoltaic and optoelectronic devices at previously inaccessible lattice constants.
9:00 PM - B8.69
Self-assembled Quantum Dots in Silicon Based Nanostructures for Solar Cell Applications.
Che-Ning Yeh 1 , Tri-Rung Yew 1
1 Materials Science and Engineering, National Tsing-Hua University, Hsinchu Taiwan
Show AbstractIn this study, metal sulfide quantum dots (MxSy QDs) were synthesized in silicon-based nanostructures via self-assembly method for third-generation solar cell applications. Silicon-based nanostructures were fabricated and applied to not only provide full-spectrum light absorption but also increase the surface contact with MxSy QDs. Metal sulfide quantum dots, with their quantum size effect and carrier multiplication properties, were utilized to achieve overall sunlight absorption and efficient carrier collection; therefore, the short circuit current and power conversion efficiency (PCE) of the solar cells could be enhanced. The morphology and structure of MxSy QDs and Si-based nanostructures were characterized by scanning electron microscope (SEM), high-resolution transmission electron microscopy (HR-TEM), and X-ray diffraction spectroscopy (XRD). Their band-gaps and absorption spectra were analyzed using UV-visible optical absorption spectroscopy. The PCE and quantum efficiency (QE) of self-assembled QDs in Si-based nanostructured solar cells were measured using solar simulators.
9:00 PM - B8.7
Recombination Barrier Layers Deposited Via Atomic Layer Deposition in Solid-state Dye-sensitized Solar Cells.
Thomas Brennan 1 , Jonathan Bakke 1 , I-Kang Ding 2 , Michael McGehee 2 , Stacey Bent 1
1 Chemical Engineering, Stanford University, Palo Alto, California, United States, 2 Materials Science, Stanford University, Stanford, California, United States
Show AbstractDye-sensitized solar cells (DSSCs) show enormous promise as an inexpensive, efficient next-generation photovoltaic technology. In a DSSC, a dye molecule chemisorbed to a nanoporous TiO2 substrate absorbs a photon, creating an e-/h+ pair in the dye; the excited electron is injected into the TiO2 and diffuses to a transparent anode. While efficiencies in devices where the hole-transporting material is a liquid electrolyte are nearing 12%, those in more practical solid-state devices are only about half that. Solid-state devices suffer from a higher rate of electron-hole recombination, a process which involves injection of an electron from the TiO2 into a hole in the Spiro-OMeTAD, a commonly used molecular hole conductor in solid-state DSSCs. This process, which is much faster than the analogous reaction in liquid devices, results in a short electron diffusion length, thereby limiting device thickness and, in turn, light absorption. One way to slow this recombination process is to build a barrier layer on the TiO2 surface between dye molecules and the TiO2. The barrier layer would serve to passivate the TiO2 surface and serve as a tunneling barrier to electrons in the TiO2. Various metal oxides, e.g. Al2O3, HfO2, Nb2O5, have been explored by others to construct such a barrier layer. While solution methods have shown some success in the deposition of these barrier layers, atomic layer deposition (ALD) is a more attractive route due its high spatial resolution (on the order of angstroms, not nanometers) and a potentially shorter processing time. Here we focus on solid cells where recombination is a more serious concern than in liquid cells. Furthermore, we aim to resolve the discrepancy present in the literature on the optimal barrier layer thickness.We have carried out ALD of several metal oxides including Al2O3, ZnO, ZrO2, and HfO2 to construct barrier layers in DSSCs. Barrier layers have been studied using cross-sectional Auger electron spectroscopy (AES) and transmission electron microscopy (TEM) in order to confirm pore-filling and barrier layer growth rate, respectively. Our ALD-grown barrier layers are found to reduce dark current in the solid-state DSSCs, raising device VOC. The reduction in recombination also enables us to build efficient thick devices. Finally, we investigate the tradeoff between VOC, which improves due to a drop in recombination, and JSC, which can decline due to a slowdown in forward electron injection through the barrier layer.
9:00 PM - B8.70
Influence of Excitation Energy on Interfacial Electron Transfer in Tethered Assemblies of CdSe Quantum Dots and TiO2 Nanoparticles.
Diane Youker 1 , David Watson 1
1 Chemistry, University at Buffalo, Buffalo, New York, United States
Show AbstractQuantum dots (QDs) have many potential advantages over molecular chromophores for light-harvesting applications in dye-sensitized solar cells and photocatalysts. However, certain fundamental aspects of the electron-transfer reactivity at QD-semiconductor interfaces remain unexplored. This presentation will focus on our recent spectroscopic characterization of the influence of excitation energy on the dynamics and quantum yield of electron injection within assemblies of CdSe QDs and TiO2 nanoparticles prepared by linker-assisted assembly. We have used both steady-state and nanosecond time-resolved emission quenching experiments to characterize electron injection. The excitation energy was varied from well above the bandgap of CdSe to below the bandgap of CdSe. Steady-state and time-resolved emission experiments revealed that electron injection gave rise to dynamic quenching. A non-time-resolvable component was also measured. Analysis of emission quenching data suggests that electron injection occurred from band-edge electronic states and surface trap states, and that the quantum yield of electron injection was independent of the excitation energy.
9:00 PM - B8.71
PbS/TiO2 Single-junction and Graded Band-gap Solar Cells.
Guangmei Zhai 1 2 , Alison Breeze 3 , Yvonne Rodriguez 1 , Anna Bezryadina 1 , Chris France 1 , Glenn Alers 1 , Sue Carter 1 , Daoli Zhang 2
1 Department of Physics, University of California, Santa Cruz, Santa Cruz, California, United States, 2 Department of Electronic Science and Technology, Huazhong University of Science and Technology, Wuhan, Hubei, China, 3 , Solexant Corporation, San Jose, California, United States
Show AbstractThe aim of this work is to better understand the effect of size of PbS colloidal quantum dots on PbS/TiO2 heterojunction solar cells and improve the device performance. Here we fabricated and characterized PbS/TiO2 single-junction and graded band-gap solar cells with three different sizes (excitonic peak: 743nm, 1000nm and 1200nm) of PbS quantum dots. For PbS/TiO2 single-junction devices, short circuit current density (Jsc) is maximized above 20mA/cm2 for PbS with excitonic peak in the 1000nm range, while open circuit voltage (Voc) is maximized at 0.65V for the larger band-gap PbS quantum dots. Best energy conversion efficiency (>4%) was obtained from device based on PbS quantum dots with excitionic peak of 1000nm. In an attempt to capture the best of both Jsc and Voc in the same device, we also tried to explore graded band-gap devices. Three types of graded band-gap solar cells (normal, reverse and double gradation) were investigated. Preliminary results show that double graded band-gap solar cells have the bigger Voc than other two graded structures and single-junction devices. In this talk, we will also present the change in performance of these single-junction and graded band-gap devices upon air exposure and temperature with current density-voltage (J-V) and external quantum efficiency (EQE) curves. Probable physical mechanism behind the phenomena will be discussed.
9:00 PM - B8.73
Localized Surface Plasmon-enhanced Organic Photovoltaic Devices with Ag/CFx-modified Anode.
Furong Zhu 2 , Xizu Wang 1 , Jian Wei Ho 1 , Hoi Lam Tam 2 , Gui Xin Li 2 , Kok Wai Cheah 2
2 Department of Physics, Hong Kong Baptist University, Kowloon Tong Hong Kong, 1 , Institute of Materials Research and Engineering, Singapore Singapore
Show AbstractLight trapping through the excitation of plasmonic modes in metal nanoparticles has attracted a lot of attention for application in thin film organic photovoltaic (OPV) devices. In this work, we realized light localization using Ag nanoparticles/CFx modified indium tin oxide (ITO) anode. The plasmonic effect of Ag nanoparticles efficiently concentrates light around the anode and trap light with the device to enhance light absorption in the active layer of the OPV cells. An ultra-thin Ag nanoparticles/CFx–modified ITO anode also has a favorable high workfunction suited for application in OPV devices. The device performance of zinc phthalocyanine (ZnPc):fullerene (C60)-based OPV devices showed a significant improvement when the structural identical cells are made with the Ag nanoparticles/CFx-modified ITO. This work yielded a promising power conversion efficiency of 3.2%, notably higher than that with a bare ITO anode (2.6%).
9:00 PM - B8.74
Bias-voltage Dependence of Charge-collection Property in Dye-sensitized Solar Cells: Relationship between Charge Dynamics and Device Characteristics.
Kai Zhu 1 , Song-Rim Jang 1 , Arthur Frank 1
1 Chemical and Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractThere is interest in understanding the charge-collection properties in dye-sensitized solar cells (DSSCs) based on TiO2 nanocrystalline films. The charge-collection efficiency is a measure of the percentage of photogenerated electrons being collected at the conducting substrate, and it is determined by the competition between charge transport and recombination processes [1]. For well engineered DSSCs, it is usually accepted that the charge-collection efficiency at short circuit is approximately unity because recombination at short circuit is negligible [2,3]. However, the properties of the charge collection process at other points along the photocurrent-voltage (J–V) characteristics, especially at voltages near the open-circuit condition, have not been fully investigated. In this presentation, we discuss the results of our studies on the effect of bias voltage on the charge-collection properties in TiO2 nanoparticle-based DSSCs. Charge transport and recombination properties of DSSCs were studied by frequency-resolved intensity modulated photocurrent/photovoltage spectroscopies (IMPS/IMVS) and electrochemical impedance spectroscopy (EIS). The voltage-dependent incident photon-to-electron conversion efficiency (IPCE) measurements indicate that the charge-collection efficiency depends strongly on the bias voltage, in agreement with our previous results [4]. The results from IMPS/IMVS and EIS measurements are compared. The underlying mechanisms that determine the relationship between the electron dynamics and the shape of J–V characteristics are discussed. The effect of electrolyte composition on the relationship between electron dynamics and charge-collection properties is also discussed. References[1] G. Schlichthörl, N. G. Park and A. J. Frank, Journal of Physical Chemistry B, 1999, 103, 782.[2] N. Kopidakis, E. A. Schiff, N. G. Park, et al, Journal of Physical Chemistry B, 2000, 104, 3930.[3] J. van de Lagemaat and A. J. Frank, Journal of Physical Chemistry B, 2001, 105, 11194.[4] J. van de Lagemaat, N. G. Park and A. J. Frank, Journal of Physical Chemistry B, 2000, 104, 2044.
9:00 PM - B8.75
Kelvin Probe Force Microscopy Study on P3HT/TiO2 Nanorod Bulk Heterojunction Photovoltaic Devices.
Ming-Chung Wu 1 , Yi-Jen Wu 1 , Wei-Che Yen 2 , Hsi-Hsing Lo 1 , Ching-Fuh Lin 3 , Wei-Fang Su 1 2
1 Department of Materials Science and Engineering, National Taiwan University, Taipei Taiwan, 2 Institute of Polymer Science and Engineering, National Taiwan University, Taipei Taiwan, 3 Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei Taiwan
Show Abstract Polymer photovoltaic devices are under great scrutiny for developing solution-processable, low-cost, large-area, mechanically flexible photovoltaic devices. However, the conducting polymer/PCBM system needs annealing to achieve high efficiency which imposes problems in large area fabrication and long term stability. Moreover, the synthesis of C60 and C70 is an energy intensive process with low yield that makes the material quite expensive. Titanium dioxide nanocrystals have potential as an electron accepting material in polymer hybrid photovoltaic device applications because they are safe, environmentally stable, non-toxic and low cost. Many recent studies have utilized Kelvin probe force microscopy (KPFM) to resolve the degree and dimension of the phase separation in polymer bulk heterojunctions. KPFM allows simultaneous mapping of both structural and electronic properties of conjugated polymer based photovoltaic materials. This technique uses a non-contact atomic force microscopy tip with a conductive coating to measure the difference between the tip potential and the local surface potential with a lateral and potential distribution below 70 nm and 10 mV respectively. In addition, Kelvin probe measurements can be applied to study charge carrier generation and electron blocking at inter-layers within polymer solar cells. In this study, we concentrate on the surface potential changes of P3HT/TiO2 nanorod bulk heterojunction thin films. The surface potential changes are affected by interlayer structures, the molecular weight of P3HT, the processing solvents and the surface ligands on the TiO2. The addition of an electron blocking layer and/or the hole blocking layer to the P3HT/TiO2 thin film can facilitate charge carrier transport and result in a high surface potential shift. The changes in surface potential of multilayered bulk heterojunction films are closely correlated to their power conversion efficiency of photovoltaic devices. Changing ligand leads to the largest change in surface potential yielding the greatest effect on the power conversion efficiency. Merely changing the P3HT molecular weight is less effective and varying the processing solvents is least effective in increasing power conversion efficiency. The steric effect of the ligand has a large influence on the reduction of charge carrier recombination resulting in a great effect on the power conversion efficiency. By monitoring the changes in the surface potential of bulk heterojunction film of multilayer structures, we have obtained a useful guide for the fabrication of high performance photovoltaic devices.
9:00 PM - B8.78
Highly Efficiency Inverted Bulk Heterojunction Polymer Solar Cells with UV Ozone-treated Ultrathin Metal Aluminum Interlayer.
HongMei Zhang 1
1 , National University of Singapore , Sigapore Singapore
Show AbstractA highly efficient inverted bulk heterojunction polymer solar cell based on poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl) -propyl-1-phenyl-(6,6)C61 (PCBM) blend was developed by introducing a thermally evaporated 1.2 nm Al film as interlayer between the polymer layer and the indium tin oxide (ITO) cathode. The ultrathin Al layer was treated by UV ozone into an Al2O3 buffer layer to improve electron extraction. The maximum power conversion efficiency of 3.9% was obtained with short-circuit current of 10.7 mA/cm2, open-circuit voltage of 0.59 V, and fill factor of 62% under AM1.5G 100 mW/cm2 irradiation. We attributed the improvement to the reduction of the barrier height for electron extraction due the introduction of UV ozone-treated ultrathin Al layer.
9:00 PM - B8.79
Degradation and Molecular Bond Rupture Kinetics in Transparent Barriers for Solar Energy Technologies.
Fernando Novoa 1 , Reinhold Dauskardt 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractLittle is currently understood regarding the fundamental photochemical and stress-enhanced degradation processes that determine the ultimate life and reliability of second and third generation solar cells. In particular, the coupled effects of mechanical stress, thermal cycling, moisture and chemically active species present in the environment together with incident UV radiation from the higher energy portion of the solar spectrum are simply unknown and have not been quantified or modeled. It is well known, however, that the synergistic effects of stress and environmental species like moisture can greatly accelerate damage processes of materials and interfaces in other device technologies. Understanding such degradation processes are particularly important for the highly transparent barrier coatings that are required for new PV technologies. This work describes the first quantitative characterization of molecular bond rupture kinetics that determine the evolution of defects in a photovoltaic barrier film under multivariable simulated terrestrial environments. We demonstrate how adhesive debonding of a multilayered hybrid barrier coating is influenced by in-situ UV exposure along with temperature, moisture and mechanical loads. The techniques allow debonding rates as low at 1 A/sec to be accurately quantified and related to the rupture of molecular bonds at the tip of the propagating crack. We demonstrate a marked acceleration of molecular bond rupture kinetics occurs in the presence of even very low intensities of selected UV wavelengths. The measured behavior is rationalized in terms of a direct contribution of UV photons to the cleavage of highly strained molecular bonds. Atomistic bond rupture models are employed to provide accurate predictions of the defect evolution rate when modified to include a contribution for bond rupture associated with the photon flux. Such kinetic models form the basis from which improved accelerated testing can be developed and long term reliability assessed.
9:00 PM - B8.8
Numerical Simulations for the Effiency Improvement of Hybrid Dye-microcrystalline Silicon Pin-solar Cells.
Sven Burdorf 1 , Gottfried Bauer 1 , Rudolf Brueggemann 1
1 Institute of Physics, Carl von Ossietzky University Oldenburg, Oldenburg Germany
Show AbstractHybrid solar cells consisting of dye sensitizers incorporated in the i-layer of microcrystalline silicon pin solar cells have been proposed and even recently processed [1,2]. The dye sensitizer molecules are embedded in the matrix and enhance the overall absorption of the dye-matrix system due to their high absorption coefficient in the spectral range interesting for photovoltaic applications. However, the charge transport properties of dyes are quite poor. Microcrystalline silicon on the other hand has acceptable charge transport properties, while the absorption, given a layer thickness in the micron range, is relatively poor. This contribution investigates the effiency improvement of hybrid dye-microcrystalline solar cells compared to pure microcrystalline solar cells by simulation. The results indicate that, under optimal conditions, the effiency can be improved by more than 20 % compared to a pure microcrystalline silicon cell. The thickness reduction for the hybrid system can be as large as 50 % for the same effiency. However, the effiency improvement also depends on the amount of additionally induced defects in the matrix by the embedded dye molecules. Therefore, the simulations investigate the performance of the hybrid solar cell for different absorption enhancements and variable defect density.[1] T. Mayer, U. Weiler, C. Kelting, D. Schlettwein, S. Makarov, D. Wöhrle, O. Abdallah, M. Kunst and W. Jaegermann 2007 Solar Energy Materials and Solar Cells 91 1873-1886. [2] T. Mayer, U. Weiler, E. Mankel, W. Jaegermann, C. Kelting, D. Schlettwein, N. Baziakina and D. Wöhrle 2008 Renewable Energy 33 262-266.
9:00 PM - B8.80
In Situ Annealing of Benzoporphyrin and Fullerene Films with 2D GIWAXS.
Christopher Liman 1 , Justin Cochran 2 , Michael Toney 3 , Michael Chabinyc 1
1 Materials, University of California Santa Barbara, Santa Barbara, California, United States, 2 Chemistry, University of California Santa Barbara, Santa Barbara, California, United States, 3 , Stanford Synchrotron Radiation Laboratory, Menlo Park, California, United States
Show Abstract2D grazing incidence wide angle X-ray scattering was conducted on small molecule thin films while annealing them in situ. Thin films of tetrabenzoporphyrin (BP) and its precursor, the fullerenes PCBM and PCBNB, and blends of BP and fullerenes were examined. By varying annealing temperature and ramp rate, we were able to obtain information about the kinetic behavior of crystallization with regard to crystal texture. We were also able to determine new preliminary unit cells for thin film metastable BP, PCBM, and PCBNB, and confirm that of stable BP. This information is valuable in optimizing the organic solar cells and transistors that these materials are used in.Annealing PCBM or PCBNB films resulted in two different behaviors depending on the ramp rate. When the films were brought from room temperature to 180°C over a time period of about 3 minutes, crystallization occurred but there was no texturing, or preferred orientation of crystallites. When the as-cast films were instead placed on a hot stage that is already at 180°C, they crystallized and became highly textured. In the samples directly annealed at a temperature, a 20° alteration of temperature led to significant differences in texture, with higher temperatures resulting in greater texture. This is thought to be a result of higher nucleation density at higher temperatures, limiting crystallite growth to the direction normal to the substrate. Despite the similarity of the PCBM and PCBNB molecules, PCBM has a cubic unit cell while PCBNB is orthorhombic.On annealing the amorphous solution-processable BP precursor, the crystalline metastable BP phase was briefly seen, followed by a transformation to the crystalline stable BP phase. An annealing temperature of 160°C resulted in a much slower transformation than an annealing temperature of 180°C did. The orthorhombic metastable phase is highly textured, while the monoclinic stable phase is much less textured. In the metastable phase, the unit cells are oriented with the a-axis normal to the substrate, while in the stable phase, they are oriented with the b-axis normal to the substrate. This greatly affects hole mobility in electronic devices, which is anisotropic. Ramping the temperature or directly annealing did not affect crystal structure or texture.When films containing blends of BP precursor and either PCBM or PCBNB were annealed, there were no differences in crystal structure or texture in comparison with the pure materials, indicating that the small molecules phase segregate and do not form co-crystals. This was corroborated by SEM and AFM images. This is in contrast to observations of polymer-small molecule blends of P3HT and PCBM in the literature, in which the texturing of either phase was either absent or weaker compared to the pure materials.
9:00 PM - B8.81
Understanding Semiconductor Nanorod Assembly from Solution: Towards Aligned Nanorod Solar Cells.
Ajay Singh 2 1 , Robert Gunning 2 1 , Ambarish Sanyal 2 , Shafaat Ahmed 2 , Christopher Barrett 2 , Edric Gill 2 , Catriona O’Sullivan 2 , Hugh Geaney 2 , Emma Mullane 2 , Dervla Kelly 2 , Claudia Coughlan 2 , Tadhg Kennedy 2 , Kevin Ryan 2 1
2 Department of Chemical and Environmental Sciences, Materials and Surface Science Institute, Limerick Ireland, 1 SFI-Strategic Research Cluster in Solar Energy Conversion, Materials and Surface Science Institute , Limerick, Limerick, Ireland
Show AbstractSelf or directed assembly of colloidal semiconductor nanorods into vertically aligned arrays offer significant potential where individual rod properties such as single electron charging, wide band-gap absorption and linearly polarised emission can be collectively harnessed and upscaled for bulk device application. It has been highlighted that a significant portion of charge recombination in nanorod based solar cells is due to random nanorod distribution in these devices limiting power conversion efficiencies. As a result, control over the direction and orientation of rods in hybrid nanorod/polymer and all-nanorod based devices – where nanorods are both close-packed and aligned perpendicular – would greatly improve the properties of the donor-acceptor matrix. Here we show a solution based approach to assembly of II-VI nanorods over large areas that can be controllably varied between 1D or 2D assemblies by altering surface charge. Sequential 2D arrays of CdSe and CdS nanorods can be formed into a 3D architecture with a high degree of order creating the possibility of designing the p-n junction in solution. These assemblies can be extended to large area coverage of centimetre scale by electrophoresis for device scale application. The nanorods are extensively characterized by HRTEM, HRSEM and zeta potentiometry
9:00 PM - B8.82
New Quinoxaline and Pyridopyrazine-based Polymers for Solution Processable Photovoltaics.
Renee Kroon 1 , Timothy Steckler 1 , Ergang Wang 1 , Robert Gehlhaar 2 , Afshin Hadipour 2 , Paul Heremans 2 , Mats Andersson 1
1 Chemical and Biological Engineering, Chalmers, Gothenburgh Sweden, 2 OPV-group, IMEC vzw, Leuven Belgium
Show AbstractDuring the last decade, utilizing polymers as the light harvesting material in bulk heterojunction solar cells has been under intensive research. The main driving force for this is that polymers offer the advantage of cost-efficient production of solar cells via, for instance, roll-to-roll processing or inkjet printing. Other advantages of polymer-based solar cells are that they can be made thin, light-weight and flexible, thereby opening up numerous commercial applications. One major hurdle still is the power conversion efficiency of these devices, which has to be increased before the concept becomes commercially viable. Currently, the best polymer-based solar cells achieve an efficiency of about 8% in the lab.Recently, an easy to synthesize quinoxaline-thiophene copolymer has been synthesized by our group. This so-called TQ1 polymer achieved excellent efficiencies (up to 6%) in solar cells that were based on a TQ1:fullerene blend. In an effort to achieve even higher efficiencies and to investigate new possibilities with this type of polymer, the structure of this polymer has been slightly modified. This resulted in a series of new polymers in which their electronic and photophysical properties have been evaluated and compared in respect to the structural changes that have been made.
9:00 PM - B8.84
Aggregation-induced Increases of the Efficiency of Electron Injection from Chalcogenorhodamine Dyes to TiO2.
Kacie Mulhern 1 , Brandon Calitree 1 , Alexandra Orchard 1 , Anthony Smith 1 , Jonathan Mann 1 , Michael Detty 1 , David Watson 1
1 Chemistry, University at Buffalo, Buffalo, New York, United States
Show AbstractWe are investigating the influence of H-aggregation on the electron injection yield, incident photon-to-current efficiency (IPCE), and global energy conversion efficiency (η) of dye-sensitized solar cells (DSSCs) incorporating chalcogenorhodamine dyes on nanocrystalline TiO2 surfaces. By varying the positions of surface-binding carboxyl groups relative to xanthylium cores of dyes, we have achieved control over H-aggregation of chalcogenorhodamine dyes on TiO2.1 This presentation will highlight our time-resolved spectroscopic characterization of electron injection yields and our photoelectrochemical characterization of devices incorporating three series of sensitizers: 2,7-bis(dimethylamino)-9-(2-thienyl-5-carboxy)chalcogenoxanthylium dyes (1-E, where E = O, S, Se); 2,7-bis(dimethylamino)-9-(3-thienyl-2-carboxy)chalcogenoxanthylium dyes (2-E, where E = S, Se); and 2,7-bis(dimethylamino)-9-(n-carboxyphenyl)chalcogenoxanthylium dyes with the carboxylic acid group in ortho- (n = 2), meta- (n = 3), and para- (n = 4) positions (3-O-E, 3-M-E, and 3-P-E, respectively, where E = O, S, Se). Series 1 dyes, 3-M-E, and 3-P-E underwent H-aggregation on TiO2 surfaces, whereas series 2 dyes and 3-O-E adsorbed in amorphous monolayers. H-aggregated dyes exhibited broadened absorption and increased light-harvesting relative to dyes that adsorbed in amorphous monolayers. H-aggregated series 1 dyes also exhibited dramatically increased IPCEs and absorbed photon-to-current efficiencies (APCEs), leading to greatly improved photoelectrochemical performance relative to non-aggregating dyes and suggesting increased electron-injection yields and/or charge-collection efficiencies.Our transient absorption experiments have quantified electron-injection yields and charge-separated-state lifetimes as a function of the extent of aggregation of dyes on TiO2. Notably, H-aggregation of 1-Se resulted in a two-fold increase of electron-injection yield relative to non-aggregated 1-Se or 2-Se. This result provides compelling evidence that the orientation, aggregation, and electron-transfer reactivity of chalcogenorhodamine dyes on surfaces are tunable through structural modification. Ongoing spectroscopic characterization of phenyl-containing dyes will also be discussed.In summary, the results of spectroscopic and photoelectrochemical experiments indicate that, for certain rhodamine derivatives, H-aggregation can lead to increased electron-injection yields in addition to increased light harvesting efficiencies. Thus, controlled aggregation is a promising strategy to increase the efficiencies of organic DSSCs and dye-sensitized photocatalysts.(1) Mann, J. R.; Gannon, M. K.; Fitzgibbons, T. C.; Detty, M. R.; Watson, D. F. "Optimizing the Photocurrent Efficiency of Dye-Sensitized Solar Cells through the Controlled Aggregation of Chalcogenoxanthylium Dyes on Nanocrystalline Titania Films." J. Phys. Chem. C 2008, 112, 13057-13061.
9:00 PM - B8.85
Solution Growth of Functional Nanomaterials for Solar Cells.
Athavan Nadarajah 1 , Rolf Koenenkamp 1
1 Physics, Portland State University, Portland, Oregon, United States
Show AbstractWe report the solution-based growth techniques for nanostructured solar cell window and absorber layers. Doped and undoped ZnO nanowire films are grown electrochemically using aqueous chloride and nitride-based electrolytes and solvable chloride compounds as precursors at temperatures below 90oC. Dopants were added to the electrolyte in form of chlorine or nitride compounds. Our results indicate that the as-grown nanowire structures have considerable internal strain resulting in clearly visible lattice distortions in bright- and dark-field transmission electron micrographs. Photo-electroluminescence studies indicate that the strain induced defects strongly dominate over any dopant-related effects. However, moderate temperature thermal anneal as well as laser annealing induces strain relaxation and concomitant dopant activation, such that the optical and electrical properties of the nanowires become suitable for the fabrication of devices such as solar cells and LEDs. Compound semiconductor quantum dots including CdSe, In2S3, and CdTe were synthesized in a wet chemical route and the growth process and nucleation were studied by absorption and emission spectra analysis. The quantum dot suspensions were subsequently deposited as thin films on planar and nanostructured substrates and converted to extremely thin continuous poly-crystalline films with typical grain diameters of 30-50nm in a surfactant-aided thermal anneal process.Combining both methods, nanowire growth and quantum dot layer deposition, in solar cell devices and optimizing both processes provided solar cells with an open-circuit voltage up to 0.4 V, a short circuit current of ~10 mA/cm2, an external quantum efficiency of 65%, and an energy conversion efficiency above 2%.We will provide a detailed analysis of the optimized ZnO nanowire films in these solar cells addressing dopant concentration, defect characterization and anneal treatments. We will also present a detailed analysis of the quantum dot layer deposition process addressing the conformality of these layers on deeply structured substrates, the effect of surfactants and encapsulates on the fusion of quantum dots and the formation of polycrystalline films. The comparison study between the three absorber materials chosen, i.e CdSe, CdTe and In2S3 quantum dots will be also reported.
9:00 PM - B8.9
Multi Band-gap Metal Oxide Heterostructures for Broadband Photovoltaics.
Laura Droessler 1 , Hazel Assender 1 , Andrew A.R. Watt 1
1 Department of Materials, University of Oxford, Oxford United Kingdom
Show AbstractThe leading second generation thin film solar cells are made of copper indium gallium diselenide and cadmium telluride. Significant market penetration of these systems is hampered by their toxicity and use of expensive rare earth metals. There is growing need to develop new second generation thin film photovoltaic materials which are robust, low energy cost, lower toxicity and recyclable. Metal oxides composed of titanium and zinc have been the cornerstone of excitonic solar cell technologies for more than a decade where they play an electronic rather than photoactive role. This project applies a novel visible light absorbing metal oxide semiconductor as a photoactive material in heterojunction photovoltaic structures.Lead oxides have found a broad range of applications over many centuries ranging from batteries to paint and it is well known that the lead to oxygen ratio determines the band gap and hence colour for example PbO is yellow, Pb3O4 is red and Pb12O19 is brown. The Pb2O3 form of lead oxide has a band gap of around 1.4 eV at 300K which gives a maximum theoretical potential power conversion efficiency of around 20% for a single junction cell. There are a number of studies, both theoretical and experimental, of different forms of lead oxide which show interesting optical and electronic properties pointing towards it being a very useful photovoltaic material with strong absorption cross sections and high carrier mobility and conductivity. Single junction Schottky devices have been made via the route of thermal evaporation of Pb onto ITO substrates and subsequent oxidation of the Pb thin film and via reactive evaporation of Pb and PbO. Al top contacts were also deposited through thermal evaporation. Parameters like evaporation rate, film thickness, oxidation time and temperature were varied in order to optimise the performance of the solar cells.Recently we demonstrated the first polycrystalline thin film lead oxide in a rectifying Schottky barrier junction photovoltaic device.So far, open circuit voltages, VOC, of up to 1.66 V and fill factors, FF, of up to 0.53 have been achieved in these material systems, but short circuit currents are too low at present.Further improvement is expected to be achieved through the determination of the ideal parameters and processing window, the integration of suitable blocking layers and the implementation of reactive evaporation. Ideal processing and heat treatments will improve the homogeneity of the films and increase the grain size. Together with efficient blocking layers that stop current leakage this will lead to higher short circuit currents. A change to reactive sputtering will permit the production of various different stoichiometries of lead oxides and allow the development of multi junction lead oxide solar cells with increased efficiencies.