10:15 AM - H1.3
High Open Circuit 1.1 eV Dilute Nitride III-V Quantum Well Solar Cell.
Aristotelis Fotkatzikis 1 2 , Andenet Alemu 1 , Lekhnath Bhusal 1 2 , Alex Freundlich 1 2 3
1 Photovoltaics and Nanostructures Laboratories, Center for Advanced Materials, University of Houston, Houston, Texas, United States, 2 Physics Department, University of Houston, Houston, Texas, United States, 3 Electrical and Computer Engineering Department , University of Houston, Houston, Texas, United States
Show AbstractIII-V dilute nitrides have attracted much interest over the past few years due to their captivating properties, which include a substantial reduction of the band gap and an increase of the electron effective mass upon incorporation of small amounts of N. These properties make this family of materials very attractive for a wide range of optoelectronic applications, including high-efficiency solar cells. The use of ~1.0 to 1.25 eV GaInNAs subcells, lattice matched to GaAs and Ge, has been suggested to enhance the efficiency of existing triple and quadruple junction solar cells. However unfavorable effects resulting from the addition of N in III-V semiconductors, such as poor minority carrier properties have thus far limited the success of this approach.(1)The use of III-V dilute nitride multi quantum wells (MQWs)inserted in the i-region of conventional III-V solar cells is a promising method to increase their efficiency, while alleviating many of the problems associated with bulk III-V based solar cells. Indeed, the insertion of multi quantum wells within the intrinsic region of conventional GaAs p-i-n solar cells has been predicted to yield practical efficiencies exceeding 35% at AM0. 2 III-V-N MQWs should extend the photon absorption range while alleviating minority carrier issues encountered in thick bulk-like GaInNAs. An added advantage of the use of III-V dilute nitride quantum wells is the stronger absorption coefficient, a result of the increase of the electron effective mass. In this work we discuss the development of 1.1 eV GaAsN/GaAs MQW solar cells grown by Chemical Beam Epitaxy (CBE). Device structures were fabricated on n-type GaAs (001) substrates by radio frequency (rf) nitrogen plasma-assisted CBE. 10 to 20 period MQWs of not intentionally doped strained GaAs1-xNx/GaAs quantum wells were inserted in the i-region of conventional GaAs solar cell structure at relatively low temperatures (440C
10:30 AM - **H1.4
Photogeneration and Carrier Collection in Quantum Confined p-i-n Solar Cells.
Alex Freundlich 1 2
1 Center for Advanced Materials, University of Houston, Houston, Texas, United States, 2 Physics and ECE, University of Houston, Houston, Texas, United States
Show AbstractWith staggering efficiency projections, quantum confined solar cells have been suggested and investigated to overcome the fundamental efficiency limitation of single junction solar cells. Thus far most practical realizations have relied on the introduction of a periodic array of low dimensional narrow bandgap semiconductor (wells, wires, dots) within the intrinsic (i) region of a wider bandgap p-i-n diode. While the approach has been shown to successfully extend the spectral sensitivity of the device, one major practical shortcoming resides in the difficulty of extracting photo-generated carriers from the quantum-confined region, which leads to important recombination losses and marginal (if any) efficiency enhancement. After a brief review of past and recent developments in quantum confined p-i-n diodes, the presentation will focus on the present understanding of recombination and escape mechanisms of photo-generated carriers in these devices and will discuss the importance of quantum confined region design parameters (well depth, hole and electron band discontinuities, carrier effective masses and i- region width,…) in maintaining an efficient collection of the photogenerated carriers. Finally the presentation will evaluate few emerging strategies (single confined-carrier devices, intermediate state assisted carrier escape) toward significantly enhancing practical efficiencies in quantum confined p-i-n solar cells.
11:00 AM - H1: Inter Band
BREAK
H2: Inorganic Quantum Dot and Nanowire Solar Cells
Session Chairs
Monday PM, November 26, 2007
Republic B (Sheraton)
11:30 AM - **H2.1
Photoluminescent Spectra of Nanostructured InAs Quantum Dots Enveloped in A GaAsSb Matrix.
Michael Levy 1 , Stephan Bremner 1 , Christiana Honsberg 1
1 Electrical and Computer Engineering, University of Delaware, Newark, Delaware, United States
Show AbstractMaterial properties of nanostructured InAs quantum dots enveloped in a GaAsSb matrix are presented here. This material system is of interest for it is identified as an absorbing medium for a multi-transition solar cell, in principle, capable of achieving greater than 50% efficient photovoltaic solar energy conversion. The structural properties of the samples are investigated with X-ray diffractometry and atomic force microscopy. The optical properties of the samples are investigated with photoluminescent spectroscopy and absorptance measurements. The matrices that were grown include GaAs0.88Sb0.12 matrices. Experimental studies reveal radiative transitions resulting from two distinct photo-induced transitions. These transitions result from interactions with photons with energies greater than and less than the bandgap of the barrier matrix.
12:00 PM - H2.2
Controlled Formation of Nanocrystal Superlattices in Conjugated Polymer Composites.
Alexandros Stavrinadis 1 , Richard Beal 1 , Jason Smith 1 , Hazel Assender 1 , Andrew Watt 1
1 Department of Materials, University of Oxford, Oxford United Kingdom
Show AbstractWe report the formation of nanocrystal superlattices during the post-synthesis processing of PbS nanocrystals/ MEH-PPV composites. Composites were synthesized by a single-step surfactant free route yielding well dispersed nanocrystals with broad size distribution (1-10nm) as characterized by transmission electron microscopy (TEM). When the composites were precipitated from solution by the rapid injection of methanol cubic and triangular colloidal particles, ~200nm in size were formed. Despite apparently monocrystalline electron diffraction patterns bright and dark field imaging shows the particles consist of individual nanocrystals which have self assembled with a high degree of crystallographic alignment. Absorption spectroscopy is used to show that nanocrystals retain their quantum confined properties when assembled. Using a higher alcohol to induce colloid precipitation such as 1-hexanol results in a modification of the superlattice structures observed and improved dispersion properties.
12:15 PM - H2.3
Atomistic Theoretical Modeling of Auger Recombination in Si and Ge Nanocrystals.
Cem Sevik 1 , Ceyhun Bulutay 1
1 Physics and Institute of Materials Science and Nanotechnology, Bilkent University, Ankara Turkey
Show Abstract12:30 PM - H2.4
Photon Management for Solar Cells.
Andries Meijerink 1 , Peter Vergeer 1 , Linda Aarts 1 , Bryan van der Ende 1
1 Chemistry, Debye Institute, Utrecht Netherlands
Show AbstractThe largest energy loss in solar cells is related to the spectral mismatch between the absorption spectrum of the semiconductor material and the solar spectrum. For the most widely used c-Si solar cell, the main loss is related to the relaxation of high energy charge carriers created after absorption of relatively high energy photons (2-4 eV) to the 1 eV bandgap of c-Si. Alternatively, wider bandgap solar cells (like the Gratzel cell) experience a large energy loss due to the fact that the low energy part of the solar spectrum is not absorbed. In this contribution new strategies to reduce these losses will be presented based on photon management using the well-defined energy levels of lanthanides. The underlying mechanisms for the spectral conversion involve up- and downconversion processes where one higher energy photon is split into two lower energy photons (downconversion) or two lower energy photons are added up to one higher energy photon (upconversion). Recently we have shown that it is possible with downconversion to split high energy vacuum ultraviolet photons into two visible photons with a190% efficiency [1]. Here it will be shown that both resonant energy transfer and cooperative sensitization are promising for downconversion processes resulting in a conversion efficiency from visible to infrared close to 200%. For the couple Tb-Yb efficient downconversion from green to infrared is observed in Y1-xYbxPO4:Tb1%. Upon excitation in the 5D4 level of Tb3+ energy transfer to two neighboring Yb3+ ions occurs. The energy transfer mechanism is shown to be cooperative energy transfer via dipole-dipole interaction. In the fully concentrated material (x=1) the theoretical limit is 185% conversion efficiency. The actual quantum efficiency of the Yb3+ emission is significantly reduced by concentration quenching. Downconversion by resonant energy transfer is studied for a number of couples of lanthanide ions: Er-Yb, Ho-Yb, Pr-Yb and Tm-Yb. In a number of oxide and fluoride matrices efficient two-step energy transfer is observed resulting in the 1000 nm emission from Yb3+ after excitation in the 300-500 nm energy levels of the other lanthanide ion. Again, concentration quenching due to energy migration over the Yb-sublattice limits the output of the IR radiation by Yb3+. The quenching can be reduced by improving the synthesis procedure (less quenching centers) or by creating small nanoclusters of lanthanide ions (no energy migration).[1] R.T. Wegh, H. Donker, K.D. Oskam and A. Meijerink, Science 283 (1999) 593.
12:45 PM - H2.5
Large Area (> 1 cm^2) Arrays of Vertically-aligned Si Wires for Photovoltaic Device Applications.
Brendan Kayes 1 , Michael Filler 1 , Morgan Putnam 1 , Michael Kelzenberg 1 , Nathan Lewis 1 , Harry Atwater 1
1 , California Institute of Technology, Pasadena, California, United States
Show AbstractPhotovoltaic devices designed to achieve high cell efficiency in low quality materials must enable collection of low diffusion length charge carriers in optically thick absorber layers. An attractive approach to this challenge is a solar cell designed as a large array of vertically-aligned semiconducting wires with radial pn junctions that facilitate carrier collection over distances that are short relative to the optical thickness of the material [1]. Such designs require the achievement of large area arrays of vertically-oriented semiconductor wires. We demonstrate here the growth, by the vapor-liquid-solid (VLS) growth technique, of large arrays of silicon wires with diameters of 1.5 um and lengths of up to 75 um, with very low defect densities over areas greater than 1 cm^2, by exploiting photolithography to pattern arrays of selectively deposited Au catalyst at sites in hole arrays formed in a silicon oxide thin film.A key conclusion of our previous device design work is that the wires in the array should optimally have a radius approximately equal to the minority-carrier diffusion length in the material. This would provide an optimal tradeoff between increased current collection, and the loss of open-circuit voltage due to the increased junction area and surface area that is produced when the wire radius is decreased. Calculations based upon the equilibrium solubility of gold in silicon and the inferred mid-gap trap density, as well as near-field scanning optical microscope measurements of minority carrier diffusion length in actual wires grown with a Au VLS catalyst [2], indicate that the minority carrier diffusion length is 1 - 5 um for such Si wires. Vertically-oriented wire arrays were fabricated by patterning photoresist with holes 3 um in diameter and with a 7 um pitch, on Si (111) wafers with a 290 nm oxide coating. The oxide was then etched through the pattern. 500 nm of gold was evaporated, followed by liftoff, leaving gold only in the holes in the oxide. Silicon wires were then grown in a tube furnace at atmospheric pressure at 1000 oC, by the catalytic decomposition of SiCl4 at the surface of the gold particles, in a H2 ambient (via the VLS mechanism). Wires were vertically oriented from the substrate, and had diameters of 1.5 um and lengths up to 75 um. The pattern fidelity was maintained over areas greater than 1 cm^2. Transmission electron microscopy (TEM) indicated that these wires were single-crystalline and grew along the <111> direction. Comparison of wires grown on samples with and without an oxide layer revealed the importance of the oxide in maintaining pattern fidelity, by preventing catalyst diffusion and agglomeration that compete with wire growth in the initial stages of wire nucleation and growth. [1] B. M. Kayes, H. A. Atwater, and N. S. Lewis, J. Appl. Phys., 97, 114302 (2005) [2] M. D. Kelzenberg, unpublished results
H3: Multiexciton Generation
Session Chairs
Monday PM, November 26, 2007
Republic B (Sheraton)
2:30 PM - **H3.1
Multiple Exciton Generation in Semiconductor Quantum Dots: Applications to Third Generation Solar Photon Conversion.
Arthur Nozik 1 2 , Matt Beard 1 , Randy Ellingson 1 , Joseph Luther 1 , Justin Johnson 1 , Qing Song 1 , Pingrong Yu 1 , Kathrine Gerth 1 , Matt Law 1
1 , NREL, Golden, Colorado, United States, 2 Department of Chemistry, University of Colorado, Boulder, Colorado, United States
Show AbstractIn order to utilize solar power for the production of electricity and fuel on a massive scale, it will be necessary to develop solar photon conversion systems that have an appropriate combination of high efficiency (delivered watts/m2) and low capital cost ($/m2). One potential, long-term approach to high efficiency is to utilize the unique properties of quantum dot nanostructures to control the relaxation dynamics of photogenerated carriers to produce either enhanced photocurrent through efficient photogenerated electron-hole pair (exciton) multiplication or enhanced photopotential through hot electron transport and transfer processes. To achieve these desirable effects it is necessary to understand and control the dynamics of hot electron and hole relaxation, cooling, charge transport, and interfacial charge transfer of the photogenerated carriers with femtosecond (fs) to ns time resolution. We have been studying these fundamental dynamics in various bulk and nanoscale semiconductors (quantum dots (QDs), and quantum wells) using fs transient, photoluminescence, and THz spectroscopy. We have observed very efficient and ultrafast multiple exciton generation (MEG) from absorbed single high energy photons in Group IV-VI and in Si QDs; the former system yields 3 excitons/photon at photon energies 3 times the QD bandgap. Such efficient MEG has the potential to greatly enhance the conversion efficiency of solar cells that incorporate QDs for both solar electricity and solar fuel (i.e. H2) production. Selected aspects of this work will be summarized and recent advances will be discussed, including work to measure MEG in the photocurrent. Various possible configurations for quantum dot solar cells that could produce ultrahigh conversion efficiencies for the production of electricity and solar fuels will be presented, along with progress in developing such new types of solar cells.
3:00 PM - H3.2
ZnO Nanorods Solar Cells Sensitized with PbS Quantum Dots.
Hung Liao 1 , Chin Lin 1 , San Chen 1
1 Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu Taiwan
Show Abstract3:15 PM - H3.3
Space-separated Multiexciton Generation in Si Nanocrystals.
Dolf Timmerman 1 , Ignacio Izeddin 1 , Tom Gregorkiewicz 1
1 Van der Waals-Zeeman Institute, University of Amsterdam, Amsterdam Netherlands
Show AbstractIn the past, the phenomenon of multiple exciton generation has been reported for nanocrystals (NCs) of II-VI materials. In this case, formation of up to 7 excitons in a nanocrystal upon absorption of a single photon of sufficiently high energy has been shown. The threshold energy for this process is usually found to be equal or close to hν ≥ 3 EG, where EG denotes the HOMO-LUMO optical bandgap of the NC. In this contribution, we report on investigations of multiple exciton generation in Si nanocrystals. The study is performed on solid dispersions of small Si-NCs (diameter 2.7 – 3 nm) embedded in a SiO2 matrix. In the experiment, we follow efficiency of the exciton-related photoluminescence band at ~900 nm as a function of excitation wavelength. We then compare it with the separately measured absorbance. We conclude that when quantum energy of the incoming photons exceeds twice the bandgap of Si-NCs, two excitons are generated per one absorbed photon. Moreover, in contrast to the earlier reported multiple exciton generation in PbSe and CdSe, the two excitons are localized in two neighboring nanocrystals. The microscopic aspects and physical mechanisms of this process are discussed, and similarity with the earlier investigated energy transfer between Si-NCs and Er3+ ions [1] are discussed.[1] I. Izeddin et al., Phys. Rev. Lett. 97, 2007401:1-4 (2006).
3:30 PM - H3.4
Multiple Exciton Generation and Extraction from Hydrazine Treated PbSe Thin Film Device.
Sung Jin Kim 1 , Won Jin Kim 3 , Yudhisthira Sahoo 3 , Alexander Cartwright 1 3 , Paras Prasad 2 3
1 Department of Electrical Engineering, University at Buffalo, the State University of New York, Amherst, New York, United States, 3 Institution of Lasers, Photonics and Biophotonics, University at Buffalo, the State University of New York, Amherst, New York, United States, 2 Department of Chemistry, University at Buffalo, the State University of New York, Amherst, New York, United States
Show AbstractWe report Multiple Exciton Generation (MEG) and electrical extraction from a photoactive device using a thin film of PbSe nanoparticles with an excitonic absorption at 0.7 eV. The PbSe nanocrystal film was spin-cast onto an ITO coated glass substrate. This layer was subsequently dipped in a 1M solution of hydrazine in acetonitrile to remove the resident oleic acid ligand. The resulting hydrazine treated film was smooth and a mirror-like surface profile was observed. After the hydrazine treatment, an aluminum electrode was deposited on the top side of the device. To quantify the effects of MEG and extraction (Electrons collected/Photon absorbed), a wavelength tunable laser setup (Coherent Verdi 18, Mira 900, Rega 9000 and OPA9400) was used to scan the incident photon energy from 1.55eV to 3.1eV. The electrically measured external quantum efficiency was similar to the reported optically measured MEG results. Specifically, multiple electrons were extracted for high energy photons with energy above 2.8 times the PbSe nanocrystal excitonic absorption. At 4.35 times Eg(PbSe), 200% more electrons were extracted.
3:45 PM - H3.5
Charge Transport in Arrays of Lead Selenide Nanocrystals.
Tamar Mentzel 1 , Scott Geyer 2 , Venda Porter 2 , Kenneth MacLean 1 , Moungi Bawendi 2 , Marc Kastner 1
1 Physics, MIT, Cambridge, Massachusetts, United States, 2 Chemistry, MIT, Cambridge, Massachusetts, United States
Show AbstractH4: Organic Solar Cells
Session Chairs
Monday PM, November 26, 2007
Republic B (Sheraton)
4:30 PM - **H4.1
Attainable Efficiencies in Organic Solar Cells.
Sean Shaheen 1 , Nikos Kopidakis 2 , Matthew Reese 2 , William Rance 3 , Jao van de Lagemaat 2 , David Ginley 2 , Garry Rumbles 2 , Brian Gregg 2 , Arthur Nozik 2
1 Dept. of Physics, University of Denver, Denver, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Physics, Colorado School of Mines, Golden, Colorado, United States
Show AbstractOrganic and nanostructured/hybrid solar cells rely on dissociation of an exciton at a donor-acceptor interface for charge carrier generation. In nearly all reported devices to date, there is a substantial (~ 1 eV) energy loss between the energy of the absorbed photon and the energy difference between the final separated electron and hole (which is considered to be the effective electronic band gap of the donor-acceptor couple). This talk begins by examining the role of the exciton binding energy, if any, in determining this energy loss and ultimately in determining the theoretically attainable efficiency. Practical issues relating to efficient carrier extraction are then addressed, with consideration of the competing mechanisms of carrier transport to the electrodes versus carrier recombination in the bulk. In particular, the size distribution and morphology of the donor and acceptor domains is shown to be critical in determining what fraction of the photogenerated carriers ultimately exit the device. Finally, incorporation of 3rd generation mechanisms, such as multiple junctions and frequency up-conversion, into organic device structures is discussed. Even if incorporation of these does not lead to efficiencies above the Shockley-Queisser limit, their incorporation may make sense economically, as is demonstrated by multijunction amorphous silicon devices. Here we briefly discuss practical issues as to their implementation and what sort of efficiency gains one might reasonably expect.
5:00 PM - **H4.2
``Solvent Annealing" Effect in Plastic Solar Cells.
Yang Yang 1 3 , Gang Li 1 3 , Yan Yao 1 , Hoichang Yang 2 , Vishal Shrotriya 3 , Guanwen Yang 1
1 Materials Sci. & Eng., UCLA, Los Angeles, California, United States, 3 , Solarmer Energy Inc., El Monte, California, United States, 2 Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, New York, United States
Show Abstract5:30 PM - **H4.3
Additives Provide Kinetic Control of Morphology in Bulk Heterojunction Materials Processed via Spin Coating.
Gui Bazan 1 , Jeff Peet 1 , Jin Young Kim 1 , Daniel Moses 1 , Alan Heeger 1 , Thuc-Quyen Nguyen 2
1 Materials, University of California-Santa Barabara, Santa Barbara, California, United States, 2 Chemistry, University of California-Santa Barabara, Santa Barbara, California, United States
Show AbstractHigh charge separation efficiency combined with the reduced fabrication costs associated with solution processing (printing and coating) and the potential for implementation on flexible substrates make “plastic” solar cells a compelling option for tomorrow’s photovoltaics. The control the donor/acceptor morphology in bulk heterojunction materials as required for achieving high power conversion efficiency have is therefore of primary concern. We recently showed that by incorporating a few volume percent of alkanedithiols in the solution used to spin cast films comprising a low bandgap polymer and a fullerene derivative, the power conversion efficiency of photovoltaic cells (AM 1.5 conditions) is increased from 2.8% to 5.5% through altering the bulk heterojunction morphology. This approach provides an operationally simple and versatile tool available for the tailoring of the heterojunction solar cell morphology in systems where thermal annealing is not feasible and works even on a system in which long range polymer crystallinity is not observed. Increases in the hole mobility were measured by using FET devices. Modifiers other than alkanedithols work well, although the mechanism by which the heterojunction morphology is affected remains poorly understood. The effect of processing on charge mobility, photoresponse and other conjugated polymer structures will also be discussed.
Symposium Organizers
Mike McGehee Stanford University
David Ginger University of Washington
Christiana Honsberg University of Delaware
Jenny Nelson Imperial College London
Jiangeng Xue University of Florida
H5: Organic Bulk Heterojunctions
Session Chairs
Tuesday AM, November 27, 2007
Republic B (Sheraton)
9:30 AM - **H5.1
Correlation of Solar Cell Performance with Nanoscale Hole and Electron Transport in Bulk Heterojunction Conjugated Polymer/Fullerene Blends.
ThucQuyen Nguyen 1 , Mark Dante 1 , Jeffrey Peet 1
1 Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California, United States
Show AbstractIn the past few years, conjugated polymer (CP) based photovoltaic devices have attracted a great deal of attention. In this work, we applied the various scanning probe techniques to characterize conjugated polymer/fullerene blend materials typically used to fabricate polymer solar cells. Atomic force microscopy (AFM) was used to characterize the film morphology whereas conducting AFM (C-AFM) was used to study the charge transport properties at the nanoscale. C-AFM in two different measurement modes was used to study the nanoscale charge transport of films containing poly(3-hexylthiophene)/[6,6]-phenyl C61-butyric acid methyl ester blend (P3HT:PCBM) and poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3 benzothiadiazole)](low bandgap polymer)/[6,6]-phenyl-C71butyric acid methyl ester (PCPDTBT:PC70BM). In C-AFM, the local current map at a constant bias and the surface topography are obtained simultaneously. Alternatively, local current-voltage curves can be measured from which the mobilities are calculated. Hole current images and mobilities were obtained using Pt-coated silicon probes whereas Mg-coated silicon probes were used to measure nanoscale electron mobilities. Variations in electrical properties and surface topography were examined. Nanoscale hole and electron mobilities increase upon annealing for P3HT:PCBM blends, consistent with increased internal order. A direct correlation between the C-AFM determined nanoscale properties and device efficiencies is observed.
10:00 AM - H5.2
Quantitative Nanoscale Composition and Efficiency Maps for Organic Solar Cells.
Benjamin Watts 1 , Daniel Queen 2 , A.L. David Kilcoyne 3 , Harald Ade 1 , Frances Hellman 2
1 Physics, North Carolina State University, Berkeley, California, United States, 2 ALS, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Physics, University of California, Berkeley, California, United States
Show AbstractSolar cells based on thin blend films of conjugated polymers and/or fullerene derivatives are a promising alternative to the currently available silicon-based solar cells. However, these systems tend to display complex segregation of the organic components during film formation, with the degree of segregation observed to correlate with device efficiency and shown to depend on fabrication parameters such as spin-casting spin-speed, solvent type and annealing conditions. However, the fundamental details of the relationship between the morphology of conjugated polymer blend films and their performance in photovoltaic devices is still not clear and techniques to quantitatively map these parameters with nanoscale resolution are required to enable further progress in this field.Recent work at the ALS in Berkeley, California, has utilized Scanning Transmission soft X-ray Microscopy (STXM) to produce quantitative, nanoscale composition maps of segregated conjugated polymer blend films [1], providing one half of the nanoscale composition-efficiency puzzle. In STXM, a thin sample is rastered across a focused soft X-ray beam at a number of photon energies to produce transmission images at resolutions down to 35 nm. By choosing photon energies corresponding to the molecular-structure dependent Near Edge X-ray Absorption Fine Structure (NEXAFS) resonances, chemical composition maps can be calculated via comparison of the transmission images to known NEXAFS spectra of the component chemical species. Because the NEXAFS resonances depend heavily on the types of bonding present, strong contrast can be achieved between materials with highly similar elemental and isotopic compositions, such as polymers.Now, we have developed a Soft X ray Beam Induced Current (SoXBIC) technique, in which a STXM instrument is extended to image a fully fabricated polymer solar cell device in a novel manner. While the transmitted light is measured and the device composition maps calculated as in normal STXM operation, the absorbed soft X-ray photons create charge carriers in the pixel-volume of the active polymer layer. Therefore, measuring the short-circuit current between the device electrodes, provides a second, current signal to complement the transmission (composition) signal for each pixel in the measured image. Hence, both the local composition and efficiency of a polymer solar cell device are simultaneously mapped at nanoscale resolution approaching the exciton diffusion length.We present details of polymer solar cell device fabrication on X-ray transparent substrates, the SoXBIC technique and new quantitative, nanoscale composition and efficiency maps of solar cell devices based on segregated PFB:F8BT blend films.This work was supported by the U. S. Department of Energy under Contract Nos. DE-FG02-98ER45737 and DE-AC02-05CH11231.[1] C.R. McNeill, B. Watts, L. Thomsen, H. Ade, N.C. Greenham, and P.C. Dastoor Macromolecules, 40, 3263–3270 (2007) DOI: 10.1021/ma070132d
10:15 AM - H5.3
Understanding P3HT Crystallite Orientation and Morphology in P3HT:PCBM Blends.
Alex Mayer 1 , Michael Toney 2 , Shawn Scully 1 , Michael McGehee 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 , Stanford Synchrotron Radiation Laboratory, Menlo Park, California, United States
Show AbstractOrganic bulk heterojunction solar cells based on poly(3-hexylthiophene) (P3HT) and phenyl-c61-butyric acid methyl ester (PCBM) have demonstrated a drastic increase in their power conversion efficiency as researchers gain control of the nanostructured morphology. In order to improve the efficiency further, a detailed understanding of the crystallographic orientation and morphology is required. Recently, researchers have found that decreasing the evaporation rate of the solvent in P3HT:PCBM solar cells greatly enhances power conversion efficiency of the device due to an increase in the hole mobility in the polymer phase. The mechanisms behind the mobility increase remain a puzzle. We employ 2D grazing incidence x-ray scattering (2D GIXS) and rocking curves of films spun from a slow and a fast drying solvent. We show for the first time that the orientation of the P3HT domains is highly dependent on the evaporation rate of the solvent and that the crystallographic orientation increases through the use of a slowly drying solvent. Interestingly, we find that the polymers preferentially orient with their insulating alkyl chains, normally thought to be the direction of slow transport, along the direction of charge transport. We rationalize this observation by noting that a percolating charge encounters many grain boundaries while traversing the cell that affect the charge to a varying degree. In a randomly oriented sample, the angle between adjacent grains can be higher than that found in an oriented sample, leading to a larger hopping barrier and thus a decreased mobility. Another puzzle that remains is that the P3HT photoluminescence is quenched in blends spun from both slow and fast drying solvents. But the internal quantum efficiency is ~100% for the quickly dried blends and <80% for the slowly dried blends even at reverse bias. This is a huge loss mechanism that needs to be explained if further optimization is possible. In order to explain this phenomenon, small angle x-ray scattering, surface x-ray scattering, and device modeling are employed.
10:30 AM - H5.4
Structure and Diffusion in P3HT:PCBM Blends.
Benjamin Watts 2 , Lars Thomsen 1 , Warwick Belcher 1 , Harald Ade 2 , Paul Dastoor 1
2 Physics, North Carolina State University, Raleigh, North Carolina, United States, 1 Physics, University of Newcastle, Callaghan, New South Wales, Australia
Show AbstractNear-edge X-ray absorption fine structure spectroscopy (NEXAFS) and, its 2D mapping counterpart, scanning transmission X-ray microscopy (STXM) are extremely powerful; techniques for probing the structure, alignment and morphology of thin organic films on the nanometer scale [1]. Previous work by us has shown how these techniques can be applied to reveal insights into the morphology and property relationships for the typical semi-conducting polymer blends commonly used in organic electronic applications [2]. Most recently we have been able to use X-ray absorption techniques to probe the alignment and dynamic properties of polymer blends used in organic photovoltaic devices. Here we present our latest results on the structure and diffusion of P3HT:PCBM blends with a focus on their diffusion and phase segregation behaviour.References:1. Kilcoyne, A. L. D.; Tyliszczak, T.; Steele, W. F.; S., F.; Hitchcock, P.; Franck, K.; Anderson, E. H.; Harteneck, B. D.; Rightor, E. G.; Mitchell, G. E.; Hitchcock, A. P.; Yang, L.; Warwick, T.; Ade, H. J. Synchrotron Rad. 2003, 10, 125-136.2. McNeill, C. R.; Watts, B.; Thomsen, L.; Belcher, W. J.; Greenham, N. C.; Dastoor, P. C., Nano Lett. 2006, 6, 1202-1206.
10:45 AM - H5.5
Investigation of Loss Mechanisms in Polymer Blend Photovoltaics via Simultaneous Probing of Charge Separation and Charge Transport.
Astrid Gonzalez-Rabade 1 , Arne Morteani 1 , Richard Friend 1
1 Cavendish Laboratory, Cambridge University, Cambridge United Kingdom
Show AbstractHeterojunctions between organic semiconductors allow efficient operation of photovoltaic diodes. We investigate, via electromodulation spectroscopy, the photoluminescence and the photovoltaic behavior of electron- and hole-transporting polyfluorene derivatives at a broad range of dopant concentrations and at various electric fields. A strong inverse correlation between the exciplex emission and the photocurrent efficiency demonstrates that they both have the same origin and that exciplex formation is the main loss mechanism for charge generation. In addition, we find a quantitatively one-to-one correspondence between the external field induced reduction in exciplex formation (detected as quenching of exciplex luminescence) and the increase in current collected at the external circuit. Interstingly, eventhough the charge collection efficiency and the exciplex quenching are morphology and electric field-dependent, the ratio between these two quantities is constant (and equal to 1) for all morphologies. These findings show, not only that non-geminate recombination can be neglected for the charge collection efficiency, but also that the one-to-one relation is universal amongst substantially dissimilar morphologies, implying that morphology is not relevant for charge collection. Our results demonstrate that the losses during the geminate pair dissociation are morphology-dependent, while, strikingly, the losses during charge collection are not morphology-dependent and are therefore negligible.
11:00 AM - H5:Organic BHJ
BREAK
H6: Organic Solar Cell Device Physics I
Session Chairs
Tuesday PM, November 27, 2007
Republic B (Sheraton)
11:30 AM - **H6.1
Bimolecular Recombination in Polymer / Fullerene Solar Cells.
James Durrant 1
1 , Imperial College London, London United Kingdom
Show AbstractPolymer / fullerene blend films are attracting extensive interest for photovoltaic energy conversion. Several groups have now reported organic solar cells based on a blend of poly(3-hexylthiophene) (P3HT) and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) with power conversion efficiencies of over 4%. The blending of the P3HT and PCBM species on the nanometer scale (the ‘bulk heterojunction’) is essential to enable efficient exciton dissociation. However a further consequence of this blending is that the charge carriers generated by exciton dissociation are not physically well separated, raising the possibility for significant interfacial bimolecular recombination of these carriers. Such recombination losses would compete with charge transport and collection at the device electrodes.In this study, we employ a range of methodologies to determine the charge carrier densities and bimolecular recombination rate constants for P3HT/PCBM films and solar cells, focusing on devices operating at open circuit, the conditions where bimolecular recombination losses are most likely to be important. Three different methodologies are employed to determine the charge carrier densities: photocurrent transients, charge extraction and transient absorbance. Three different methodologies are employed to determine recombination rate constants: transient photovoltage decays and transient absorption decays of both devices and thin films. Excellent agreement is found between all methodologies. The recombination rate constant is found to be strongly carrier density dependent, attributed to the effects of charge trapping in the film. From these measurements we determine the bimolecular recombination flux in devices under operation, and conclude that bimolecular recombination is indeed a key loss pathway limiting device performance.
12:00 PM - H6.2
Free Carrier Generation and Recombination in P3HT/PCBM Bulk Heterojunctions Studied by Time-Resolved Microwave Conductivity.
Nikos Kopidakis 1 , Andrew Ferguson 1 , Sean Shaheen 1 , Garry Rumbles 1
1 , National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractPoly(3-hexylthiophene) (P3HT) is the prototypical polymer used as light absorber and exciton and hole transporter in efficient bulk-heterojunction organic photovoltaic devices. The study of photophysics of P3HT is facilitated by its rather unique property that photoexcitation produces free carriers without the presence of an acceptor, albeit with low yield. Extensive previous studied of TRMC in pristine P3HT has produced a wealth of experimental data and understanding of exciton and free carrier generation in this material. In the work presented here, we build upon these previous studies and extend them to include photoconductivity measurements in bulk heterojunctions of P3HT blended with varying amounts (1%-80% b.w.) of the acceptor fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester (PCBM). We demonstrate how previous models for free carrier generation in pristine P3HT need to be modified to account for the presence of the acceptor and the corresponding high yield for free carrier generation by exciton dissociation at the donor-acceptor interface. We develop a kinetic scheme to describe free carrier generation and recombination that is consistent with our experimental data for both pristine P3HT and P3HT/PCBM blends. In the framework of this scheme, we discuss higher order processes that limit exciton generation in P3HT/PCBM bulk heterojunctions and show how these processes also limit free carrier generation. Implications of our findings on the operation and optimization of organic photovoltaic devices based on P3HT/PCBM bulk heterojunctions are discussed.
12:15 PM - H6.3
Monte Carlo Investigation of Bimolecular Recombination, Geminate Recombination and Space-charge Effects in Nanostructured Polymer Photovoltaic Devices.
Chris Groves 1 , R. Marsh 1 , Neil Greenham 1
1 Optoelectronics, Cambridge University, Cambridge, Cambridgeshire, United Kingdom
Show AbstractThe performance of organic photovoltaic devices (OPVs) is limited by a number of processes relating to the transport of charge carriers. Geminate pairs must first avoid mutual recombination, and thereafter avoid bimolecular recombination with other free carriers en-route to the collecting contacts. Furthermore, at high intensities, differences in carrier mobilities may lead to the accumulation of the least mobile carrier, giving space-charge limited photocurrents. Here we use a Monte Carlo model to examine these limiting factors and find their relative importance. The Monte Carlo model simulates the transport of polarons through a Cartesian geometry of electron- and hole-conducting sites, each with an associated Gaussian disorder. Electrostatic attractions between carriers and image charges at the electrodes are also included.In our simulations we keep one carrier mobility constant, and vary the other carrier mobility by two orders of magnitude in either direction. We find that the degree of geminate recombination at a given field is dependent largely upon the mobility of the most mobile carrier type, as might be expected. Bimolecular recombination increases with intensity and is of the order ~30% for some device configurations at the highest intensities considered. It is found that, over the operating voltage range of an OPV device, the degree of bimolecular recombination increases with increasing mobility. We ascribe this result to carriers diffusing further and so sampling more of the device, which in turn leads to increased opportunity for recombination. For the devices and conditions examined we do not find that the degree of geminate recombination falls with increasing intensity, suggesting the current is not space-charge limited.
12:30 PM - **H6.4
Dual Electron Donor/Electron Acceptor Character of a Conjugated Polymer in Efficient Photovoltaic Devices.
Chris McNeill 1 , Agnese Abrusci 1 , Jana Zaumseil 1 , Richard Wilson 2 , Mary McKiernan 2 , Jonathan Halls 2 , Neil Greenham 1 , Richard Friend 1
1 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 2 , Cambridge Display Technology Ltd., Cambridge United Kingdom
Show AbstractWe report efficient photovoltaic operation of a polymer blend consisting of poly(3-hexylthiophene) (P3HT) and the red polyfluorene poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2’,2’’-diyl) (F8TBT). Power conversion efficiencies of nearly 2% and external quantum efficiencies of over 25% are demonstrated, representing some of the highest values for all-polymer blends. These results are also significant as they demonstrate that F8TBT, previously used in efficient solar cells as an electron donor and hole transporter, can also be employed effectively as an electron acceptor and electron transporter. This dual nature of F8TBT is confirmed by the operation of F8TBT-based light-emitting ambipolar field-effect transistors. The dual electron donor/electron acceptor operation of F8TBT in photovoltaic devices, however, demonstrates that the ambipolar character of conjugated polymers is also valid in low charge density environments. Finally, the observation that efficient photovoltaic operation can result when P3HT is used in combination with F8TBT instead of the often-used (6,6)-phenyl C61-butyric acid methyl ester (PCBM), which has an electron mobility several orders of magnitude higher than F8TBT, questions the assumption that charge carrier mobility is limiting the development of organic photovoltaic devices. We present evidence that the separation of geminate electron-hole pairs is limiting the performance of this class of device rather than recombination resulting from poor charge transport.
H7: Organic Solar Cell Device Physics II
Session Chairs
Tuesday PM, November 27, 2007
Republic B (Sheraton)
2:30 PM - **H7.1
Charge Transport and Photogeneration in All-polymer Solar Cells.
Magda Mandoc 1 , Welmoed Veurman 1 , Jan Anton Koster 1 , Bert de Boer 1 , Paul Blom 1
1 Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands
Show AbstractThe photogeneration mechanism in blends of poly[2-methoxy-5-(3', 7'-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) and poly[oxa-1,4-phenylene-(1-cyano-1,2-vinylene)-(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene)-1,2-(2-cyanovinylene)-1,4-phenylene] (PCNEPV) is investigated. The photocurrent in the MDMO-PPV:PCNEPV blends is strongly dependent on the applied voltage as a result of a low dissociation efficiency of the bound electron-hole pairs. The dissociation efficiency is limited by the low carrier mobilities, low dielectric constant and the strong intermixing of the polymers, leading to a low fill factor and a reduced photocurrent at operating conditions. Additionally, electrons trapped in the PCNEPV phase recombine with the mobile holes in MDMO-PPV phase at the interface between the two polymers, thereby affecting the open-circuit voltage and increasing the recombination losses. At an intensity of one sun, Langevin recombination of mobile carriers dominates over the trap-assisted recombination.
3:00 PM - H7.2
Defect Engineering in π-Conjugated Polymers.
Dong Wang 1 , Nikos Kopidakis 1 , Xin Ai 1 , Brian Gregg 1
1 , National Renewable National Lab, Golden, Colorado, United States
Show AbstractDefects, whether purposely added as dopants or not, usually control the electrical behavior of inorganic semiconductors. Defect engineering is thus a major field of research. In the newer field of organic semiconductors (OSCs), however, little is known about the number, influence or chemical identity of defect states. In OSCs with covalent disorder, deformations in a small fraction of π-bonds can have a profound influence on the electronic properties of the solid. We study the ostensibly pure π-conjugated polymers poly(3-hexylthiophene), P3HT, and poly(1-decyloxy-3-methoxy-2,5-phenylenevinylene), PPV. These commonly employed materials exhibit 9 - 12 orders of magnitude more charge carriers at equilibrium than they would if they were defect-free. Here we introduce chemical treatments, both of the polymers in solution and in solid films, that modify these electrical defects. The chemical reactions are chosen such that they can occur only at destabilized sites on the polymers. Treatment with nucleophiles such as hydride ion or methoxide ion leads to a decrease in film conductivity, an increase in both fluorescence intensity and lifetime, a greater exciton diffusion length, and Fermi level motion toward the middle of the bandgap. The addition of hydrogen or methanol across a strained double bond, converting unstable sp2 carbons into electroinactive sp3 carbons, is proposed as the mechanism. Treatment with electrophiles such as methyl cation, on the other hand, causes an increase in conductivity, Fermi level motion toward the valence band edge, and, again, an increase in exciton diffusion length. This is consistent with the addition of a methyl cation to the polymer backbone and concommittant elimination of an electron, leaving the counter anion unbound. The changes in optoelectronic properties of the π-conjugated polymers upon treatment are self-consistently explained by the chemical nature of the addition reactions.
3:15 PM - H7.3
Photoexcitations and Charge Generation Dynamics in P3HT/PCBM Blended Films Studied by Ultrafast Transient Absorption Spectroscopy.
Jorge Piris 1 , Thomas Eisenmayer 1 , Minh Trinh 1 , Juleon Schins 1 , Laurens Siebbeles 1
1 Optoelectronic Materials Section, Delft University of Technology, Delft Netherlands
Show AbstractWe studied the ultrafast processes following photo-excitation of thin films of the conjugated polymer poly(3-hexylthiophene) (P3HT) mixed with the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) by means of ultrafast transient absorption spectroscopy. We observed in the ps time domain exciton quenching and the subsequent formation of charge carriers as a function of the PCBM fraction in the films. We found that a relative large amount of charges are generated in the pure polymer. Addition of small quantities, ca. 2wt%, of the fullerene derivative reduces the fluorescence yield and lifetime by a factor 2. Nevertheless, it does not increase the initial yield for charge carrier generation significantly but results in longer-lived charge carriers. This is indication of the acceptor taking preferentially the place of the already existing charge separation centers in the pristine polymer. Increasing the PCBM content further increases the charge carrier yield but only up to double of that observed for the neat polymer at early times, reaching a maximum at ca. 20wt%. Higher acceptor concentrations result in reduced yield. In contrast, the most efficient solar cells contain ca. 50wt% of each component. This indicates that higher fullerene concentrations are necessary to transport the electrons than the optimum for charge generation.
3:30 PM - H7.4
Magnetic Studies of Singlet and Triplet Related Photovoltaic Processes in Organic Bulk Heterojunction Solar Cells.
Zhihua Xu 1 , Bin Hu 1
1 Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States
Show AbstractThe dynamic processes of photoexcitation generated excited states are critically important to the photovoltaic response in organic solar cells. Although the spin selection rule only allows the formation of singlet excitons under photoexcitation, both singlet and triplet excitons can coexist in organic materials due to the spin mixing from spin-orbital coupling or hyperfine interaction. When the different binding energies, lifetimes, and diffusion lengths are considered, the singlet and triplet excitons must have different contributions to the photovoltaic response. Therefore, understanding the singlet and triplet photovoltaic processes can influence the improvement of photovoltaic response in organic solar cells. In this report we will present our recent magnetic studies of singlet and triplet photovoltaic processes in organic bulk-heterojunction solar cells. Our experimental results of magnetic field-dependent photocurrent reveal that the singlet and triplet excitons undergo different photovoltaic processes, namely exciton dissociation and exciton-charge reaction, respectively. This finding indicates how the photovoltaic efficiencies can be further improved by optimizing the relative photovoltaic contributions from singlet and triplet excitons through changing the singlet and triplet ratios. The presentation will discuss in detail the magnetic responses of singlet and triplet photovoltaic processes, and the optimization of singlet and triplet photovoltaic contributions in organic solar cells.
3:45 PM - H7.5
Self-Organized Electroactive Polymer Blends and Nanocomposites with Enhanced Carrier Mobility for Organic Vertical Diodes.
Cheng Huang 1 , Byung Jung 1 , Howard Katz 1 , James West 2
1 Materials Science and Engineering Department, Johns Hopkins University, Baltimore, Maryland, United States, 2 Electrical and Computer Engineering Department, Johns Hopkins University, Baltimore, Maryland, United States
Show Abstract4:00 PM - H7: org sol II
BREAK
H8: Hybrid and Dye Sensitized Solar Cells
Session Chairs
Tuesday PM, November 27, 2007
Republic B (Sheraton)
4:30 PM - H8.1
Oxide Engineering for Enhanced Performance in Conjugated polymer / Nanostructured ZnO Photovoltaic Devices.
Dana Olson 1 , Yun-Ju Lee 1 , Matthew White 3 2 , Robert Grubbs 1 , Erik Spoerke 1 , Erica Fang 1 , Nikos Kopidakis 2 , Sean Shaheen 4 2 , David Ginley 2 , James Voigt 1 , Julia Hsu 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 3 Physics, University of Colorado, Boulder, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 4 Physics, University of Denver, Denver, Colorado, United States
Show AbstractNanostructured oxide semiconductor / conjugated polymer composites are promising systems for use in low cost organic photovoltaic devices. The use of ordered nanostructures increases the area of the heterojunction, resulting in increased dissociation of photogenerated excitons and collection of charges. These hybrid devices take advantage of the high electron mobilities found in metal oxide semiconductors such as ZnO, while largely retaining the solution-based processing available to organic semiconductor devices. However, initial versions of hybrid devices, such as those based on ZnO nanorod arrays intercalated with a conjugated polymer, have demonstrated low open circuit voltages (VOC) and short-circuit current densities (JSC), which have limited the power conversion efficiencies (η) of such hybrid devices. Defects at the ZnO surfaces are suspected to contribute to the observed poor performance. Here, we explore the effects of oxide engineering on the device performance. The ZnO sol gel films or nanorod arrays are coated with a number of metal oxide coatings through atomic layer deposition (ALD), and solution-based methods. Additionally, the band offset between the ZnO and the polymer can be modified by alloying the ZnO through the interdiffusion of a suitable metal oxide coating.We have fabricated arrays of ZnO nanorods by solution growth on ITO substrates. The ZnO nanorod arrays are subsequently infiltrated with poly(3-hexylthiophene) (P3HT), and the devices are completed by depositing Ag electrodes. We examine the effects of coating the ZnO with varying thicknesses of Al2O3, or TiO2 on the electron transfer efficiency and polymer ordering at the ZnO/P3HT interface. Cross-sectional SEM, UV-vis absorption, X-ray diffraction, Kelvin probe, time resolved microwave conductivity, and photoluminescence quenching are employed to characterize each component as well as the composites. Finally, these results are correlated with device data to observe the effects of oxide engineering on device performance.The authors would like to thank the IC Post Doctoral Fellowship and the Sandia and NREL LDRD programs for funding this research. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
4:45 PM - H8.2
Photovoltaic Properties of Hybrid Bulk Heterojunction Solar Cells from P3HT and Dye Sensitized ZnO Nanorods.
Guillaume Poize 1 , Aurore Said 2 , Cyril Martini 1 , Julien Hocq 3 , James Durrant 3 , Johann Boucle 4 , Jenny Nelson 4 , Wladimir Marine 2 , Suzanne Giorgio 2 , Frederic Fages 1 , Jorg Ackermann 1
1 Laboratoire des Matériaux Moléculaires et des Biomatériaux, GCOM2 UMR CNRS 6114, , Université de la Méditerranée, Faculté des Sciences de Luminy, Case 901, 163, avenue de Luminy, Marseille France, 2 Centre de Recherche en Matière condensée et Nanosciences UPR CNRS 7251, , Campus de Luminy, 163 Avenue de Luminy,, Marseille France, 3 Department of Chemistry, Centre for Electronic Materials and Devices, Imperial College London, London United Kingdom, 4 Department of Physics, Centre for Electronic Materials and Devices, Imperial College London, London United Kingdom
Show AbstractHybrid organic-inorganic bulk heterojunction solar cells represent a promising alternative to all plastic solar cells by combining solution processibility of p-type semiconducting polymers with efficient electron transport existing in inorganic semiconducting nanoparticles. Amongst the inorganic n-type semiconductors, high-band gap metal oxides such as ZnO nanoparticles are of particular interest due to their low cost synthesis, high degree of nanoparticle shape control (1D and 2D) as well as high electron mobility. Spherical and rod-like ZnO nanoparticles have been successfully applied to the fabrication of hybrid bulk hetorojunction solar cells and solar cell conversion efficiencies up to 1.6 % have been reported.(ref.1) Grafting of organic molecules such as dyes or organic semiconductors on the surface of nanoparticles allows tailering their optoelectronic, electrochemical and self-assembling properties. Therefore the use of dye sensitized ZnO nanoparticles in bulk heterojunction solar cells could lead to increased photocurrent generation due to additional light absorption of the dye, improved charge carrier seperation at the polymer-nanoparticle interface (ref2) as well as a improved distribution of the ZnO inside the blend. In this communication we will present novel hybrid bulk heterojunction solar cells from regioregular poly(3-hexylthiophene) (P3HT) and ZnO nanorods. First we will discuss the influence of the nanorod length on the solar cell performance. Furthermore the assembly and the optical properties of the dye sensitized ZnO nanorods in solution as well as the photovoltaic properties of bulk heterojunction solar cells based on blends of P3HT and dye sensitized ZnO nanorods will be presented. By using different dyes such as porhyrines, an amphiphilic polypyridil ruthenium complex (Z907) as well as modified oligothiophenes as surface modification we will show the influence of the dye on the photovoltaic characteristics of the hybrid solar cells. Results of external quantum efficiency (EQE) measurements and transient absorption spectroscopy (TAS) will be presented to demonstrate the beneficial effect of the dye on photocurrent generation and charge carrier seperation at the hybrid interface, while results of transmission electron microscopy (TEM) analysis on the P3HT/ZnO blends will show the impact of the dye on distribution of the ZnO nanorods inside the polymer matrix. Reference:[1] W.Beek, M. Wienk, M. Kemerink, X. Yang, R. Janssen, J. Phys. Chem. B 2005, 109, 9505.[2]P. Ravirajan, A. Peiro, M. Nazeeruddin, M. Graetzel,D. Bradley,J. Durrant, J. Nelson, J. Phys. Chem. B. 2006 110, 7635
5:00 PM - H8.3
TiO2 Surface State Control for Dye Sensitized Solar Cells with High Efficiency and the Solidification -Fabrication of Charge Carrier Path.
Shuzi Hayase 1 , Yusuke Noma 1 , Yoshitaka Hara 1 , Fumi Inakazu 1 , Daisuke Ogata 1 , Takeshi Kougo 1 , Yuhei Ogomi 1
1 , Kyushu Institute of Technology, Kitakyushu, Fukuoka, Japan
Show AbstractImproving the photo-voltaic performance for dye sensitized solar cells (DSC) are discussed. First of all, we focus on the passivation of TiO2 surface states (electron traps). This corresponds to the fabrication of the electron path. The other item we focus on is the fabrication of all-solid-dye sensitized solar cells and quasi-solid dye sensitized solar cells where the charge carrier path is fabricated in the solid and the quasi-solid medium. This is the approaches toward all-solid organic solar cells from DSC. The surface trap on the porous TiO2 electrode decreases the electron diffusion coefficient and increases the opportunity for the recombination reaction between the electron in the TiO2 electrode and the iodine molecules in the electrolytes. Our trial is to passivate the surface state fully with dye molecules. It is not easy to passivate the nano-porous surface of the TiO2 layers with dye molecyles because of the diffusion problem in the highly porous layer. The dye adsorption on the nano-porous TiO2 surface was carried out under pressurized CO2 atmosphere where dyes expected to diffuse easily. The CO2 process promoted the dye up-take and shortened the dye adsorption time to 1/10 to 1/100. Solar cells prepared by the CO2 process had higher Voc and Jsc than those prepared by the conventional dipping process. The improvement was experimentally explained by the better electron diffusion coefficient in TiO2 layers and by the longer electron life time in TiO2 layers. Thermally stimulated current measurement supported that surface electron traps on TiO2 nano-particles were passivated by the sufficient dye adsorptions on TiO2 surfaces. We achieved 10.4% efficiency.The other topic on nano-interface study is the fabrication of the ion paths in the quasi-solid type DSCs. The solidification decreases the photovoltaic performance because of the retardation of the ionic diffusion coefficient in the solid state. High performance quasi-solid DSCs were fabricated by preparing straight ionic paths in the quasi-solid electrolyte layers. The quasi-solid films consist of anodically oxidized Al2O3 film surface modified with imizasolium moieties bearing long alkyl chains and ionic liquids. The ionic path was fabricated on the straight nano-walls in a porous Al2O3 membrane, where, ions were expected to diffuse by Grötthuss mechanism. We observed higher photovoltaic performances for the quasi-solid DSC than for the parent liquid type DSC (an ionic liquid type DSC). For all-solid-dye sensitized solar cell, in order to retard the electron recombination from TiO2 to the charge charier medium, a passivation layer was fabricated on the TiO2 layer of the cell consisting of TiO2 nanoporous layer/dye/P3HT. The effect of the passivation was made clear by using a thermally stimulated current method. By using the passivation method, the Jsc and Voc increased drastically.
5:30 PM - H8.5
Nanostructured Silicon Layers for use in Hybrid Inorganic/polymer Solar Cells.
Pierre-Jean Alet 1 , Serge Palacin 1 , Remi de Bettignies 3 , Yassine Djeridane 2 , Pere Roca y Cabarrocas 2
1 Chemistry of Surfaces and Interfaces, CEA, Gif sur Yvette France, 3 INES, CEA, Le Bourget du Lac France, 2 LPICM, Ecole Polytechnique, Palaiseau France
Show AbstractMany hybrid organic/inorganic devices have been proposed to enhance the power conversion efficiency of organic solar cells. Our work focuses on silicon nanowires-based devices. As compared to other inorganic electron acceptors, like TiO2 or ZnO, which are also used as nanostructured layers in hybrid solar cells, silicon has a major advantage of absorbing light in the visible range. It contributes thus directly to the generation of photo-current.A 3-step approach has been used to investigate this structure. First, devices made of flat thin films of n-doped and intrinsic silicon covered with spin-coated poly(3-hexylthiophene) (P3HT) have been studied. The crystalline structure of silicon has been varied from amorphous to micro-crystalline in order to modify the bandgap and the conductivity of the silicon layers. Without any chemical treatment of their surface, a power conversion efficiency of 1.6 % has been reached. The combination of spectral responses, UV-visible light absorption spectroscopy and I-V characterisation both in dark and under illumination shown that the separation of charge carriers occurs at the interface between the silicon and the polymer, and that both materials can contribute to the photocurrent.Then, nanostructured micro-crystalline silicon layers with a 50-nm average roughness have been introduced to study the effect of the increase in the contact surface between the two materials. Filling properly the silicon layers with the polymer is a major challenge in such a structure, especially by spin-coating. Thermal annealing has been used to enhance the filling ratio. Its effect on the materials was studied by spectroscopic ellipsometry. Electrical characterizations have shown that for high enough temperatures, the short-circuit current is almost doubled, without any degradation of other parameters.Finally, silicon nanowires have been grown directly on ITO for use in hybrid solar cells. Their typical dimensions are 30 nm in diameter and 500 nm in length. The growth relies on the VLS (Vapor-Liquid-Solid) process, which involves droplets of metal catalysts (gold as well as copper). In all of these devices, the evaporated top electrode is the cathode, whereas it is the anode in most organic solar cells. Despite the good agreement between the work function of gold (5.1 eV) and the HOMO level of P3HT (4.8 eV), the contact between the metal electrode and the polymer is non-ohmic in many samples. It is thus necessary to add an intermediate layer, e.g. PEDOT:PSS, to improve the injection of holes. The deposition and effect of such a layer is investigated.
5:45 PM - H8.6
Hybrid Solar Cells from Polymers and Silicon Nanocrystals.
Chin-Yi Liu 1 , Uwe Kortshagen 1
1 Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractThin film hybrid solar cells based on mixtures of conjugated polymers and inorganic semiconductor nanocrystals have shown promising efficiencies. By mixing conjugated polymers and inorganic semiconductor nanocrystals, efficient exciton dissociation can be achieved at the interface of the polymer and the nanocrystals. A percolated network of semiconductor nanocrystals in such hybrid thin film devices provides high mobility pathways for electrons, enhancing transport. We report devices based on silicon nanocrystals (Si NCs). Si NCs were synthesized in a non-thermal RF plasma via dissociation of silane precursor. Si NCs were dispersed in chloroform without further surface functionalization. A conjugated polymer, poly-3(hexylthiophene) (P3HT), was then dissolved in the chloroform solution which contained 5-8 nm Si NCs. Thin-film hybrid solar cells were made by spin-coating the solution of P3HT and Si NCs onto substrates pre-coated with poly(3,4-ethylenedioxy thiophene)/poly(styrenesulfonate) (PDOT:PSS) on indium tin oxide (ITO) on glass. Aluminum (Al) electrodes were evaporated on top of the P3HT/Si NC film. P3HT is a light absorbing and effective hole transporting material. Si NCs also absorb light, and their absorption spectrum may be tuned by altering the NC size. Efficient exciton dissociation can be achieved at the interface of P3HT and Si NCs due to the relative alignment of their energy band edges. We believe that the network of Si NCs in the film provides a high mobility pathway for electrons to the Al contact. P3HT conducts holes to the ITO. Under Air Mass (A.M.) 1.5 Global solar conditions (100mW/cm2), preliminary solar cells made by spin-coating a solution of 50% Si NCs (by weight) showed a 0.23 mA/cm2 short circuit current density and a 0.46 V open circuit voltage. Scanning electron microscope image analysis shows that Si NCs form connected networks in the film, but are not uniformly distributed throughout the film. This non-uniform morphology is expected to decrease the efficiency of the solar cells. To distribute Si NCs uniformly in the film, it is necessary to functionalize the Si NC surfaces with short ligands which provide solubility in chloroform but only minimally impede electron transport between Si NCs. Gas-phase functionalization of Si NCs was achieved by adding a second plasma downstream of the Si NC synthesis plasma. Propene was injected between the first and second plasma to graft these short ligands onto the Si NC surfaces. Fourier-transform infrared spectroscopy shows that the nanocrystal surfaces are terminated by hydrogen and hydrocarbons. After this treatment, Si NCs are soluble in chloroform. Studies of cells from surface-functionalized Si NCs and P3HT will be reported. This work was supported by NSF under NIRT grant CBET-0506672, MRSEC grant DMR-0212302, and IREE grant LG-C5-2005.
H9: Poster Session: Nanostructured Solar Cells I
Session Chairs
Wednesday AM, November 28, 2007
Exhibition Hall D (Hynes)
9:00 PM - H9.1
Polymer Free Carbon Nanotube Solar Cells.
Jinquan Wei 1 , Yi Jia 1 , Qinke Shu 1 , Kunlin Wang 1 , Zhicheng Wang 1 , Jianbin Luo 1 , Wenjin Liu 1 , Dehai Wu 1
1 , Tsinghua University, Beijing China
Show AbstractCarbon nanotubes (CNTs) are considered as promising candidates for organic solar cell applications due to their unique electronic also optoelectronic properties, also due to the high aspect ratios and large surface area. Recently, organic solar cells associated with CNTs have been developed by several groups, where CNTs can improve the power efficiency of solar cells slightly. However, CNTs play only accessorial roles, such as electrode conducting, holes collecting and transparent electrode layer, in the organic solar cells. Here, we describe a polymer free solar cell basing directly on the heterojunction between p-type CNTs and n-type silicon, where CNTs are used as energy conversion materials. Initial test under solar simulator (AM 1.5) have shown that the open-circuit voltage (Voc) of the CNT/Si can reach to 0.4~0.5 V, short-circuit current (Isc) can reach to 13.8 mA/cm2 and the power conversion can reach to about 1.3%. For a CNT bundle, evident photocurrent phenomena have been observed when they were illuminated by a laser varies from visible to far-infrared, which indicates that CNTs may play an important role as photogeneration in the solar cells. And the n-type silicon was used to create a high density of heterojunctions between nanotubes and n-Si to favor charge separation and extract electrons. Our progress in the solar cells shows that CNTs have attractive potential application in energy conversion materials in the future.
9:00 PM - H9.10
Fundamental Research of Nanohybrid Carbon Nanotube-Polymer Solar Cells.
Tingying Zeng 1 , Tzu-Fan Chen 1 , Jordan Norris 1
1 Chemistry, Western Kentucky University, Bowling Green, Kentucky, United States
Show AbstractResearch concerning about the interaction between carbon nanotubes(CNTS) and conjugated organic molecules including polymers is attracting great interest in sensing and photovoltaics. Carbon nanotubes are very promising to improve the performance of polymer organic solar cells due to their unique nanostrctures, especially their remarkable electrical, thermal, and mechanical properties. This presentation will present our recent progress in the fundamental research of nanohybrid CNTs-polymer solar cells.
9:00 PM - H9.11
PLD Inorganic Layers in Inverted bulk-heterojunction Devices and as Acceptors in Polymer/inorganic Bilayer Hybrid Devices.
Matthew White 1 2 , Joseph Berry 2 , Dana Olson 3 , Julia Hsu 3 , Jim Voigt 3 , Sean Shaheen 2 , David Ginley 2
1 Physics, University of Colorado, Boulder, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractRecent advances in the field of organic photovoltaics have shown that hybrid organic/inorganic device architectures provide reasonable device performance while removing the need for low work function metals and for PEDOT:PSS, leading to potential increases in electrode stability. The inorganic material can function directly as an acceptor in the device, or as an electron extraction layer in a bulk heterojuction device. This study explores a variety of inorganic materials and combinations of materials for these two applications. Inorganic layers are deposited directly onto an ITO substrate via pulsed laser deposition (PLD), a reproducible technique capable of depositing a wide variety of materials. ZnO films deposited via PLD produce very similar devices to solution processed ZnO films, with VOC of roughly 300 mV. Devices with TiO2 acceptor layers have shown VOC greater than 700 mV, but demonstrate lower short circuit currents than ZnO devices. Combinations of ZnO, TiO2, and other metal oxide layers are also explored to optimize JSC, VOC, and fill factor in bilayer and inverted bulk heterojunction devices.
9:00 PM - H9.12
Inverted Organic-Inorganic Hybrid Solar Cell Based on ZnO Nanowires using Sol-Gel Method.
Hey Jin Myoung 1 2 , SunYoung Lee 1 , Sunglyul Maeng 1 , In-Joo Chin 2 , Sang-Hyeob Kim 1
1 Cambridge-ETRI Joint R&D Center, Electronics and Telecommunications Research Institute, Daejeon Korea (the Republic of), 2 Department of Polymer Science and Engineering, Inha University, Incheon Korea (the Republic of)
Show Abstract9:00 PM - H9.13
Fluorescence Energy Transfer in Hybrid Semiconductor Heterostructures: An Alternative Way of Pumping Fluorescent Beads and Organic Emitters.
Soontorn Chanyawadee 1 , Stefan Rohrmoser 1 , Julia Baldauf 1 , Sameer Sapra 2 , Alexander Eychmuller 2 , Ian Watson 3 , Mohamed Henini 4 , David Lidzey 5 , Richard Harley 1 , Pavlos Lagoudakis 1
1 School of physics and Astronomy, University of Southampton, Southampton United Kingdom, 2 Physikalische Chemie, TU Dresden, Dresden Germany, 3 Institute of Photonics, University of Strathclyde, Glasgow United Kingdom, 4 School of Physics and Astronomy, University of Nottingham, Nottingham United Kingdom, 5 School of Physics and Astronomy, University of Sheffield, Sheffield United Kingdom
Show AbstractColour-conversion via radiative energy transfer from an inorganic quantum well (QW) to inorganic phosphor materials is widely used to produce full-colour and white nitride-based light emitting diodes. Another form of energy transfer possible from inorganic QWs is via a non-radiative process termed fluorescence energy transfer (FET), best known in purely molecular systems. This process can be used as an alternative way of pumping energy in materials with poor carrier mobility such as colloidal quantum dots (QDs) and organic fluorophores (or inversely for draining energy out of those). Hybrid heterostructures offer the exciting prospect of creating new types of hybrid optoelectronic devices and photovoltaics that exploit the complementary properties of the constituent semiconductors. Here we report the observation of exciton energy transfer in two hybrid configurations: from a blue emitting QW to an overlayer of colloidal QDs and from a QW emitting at the near infrared to an overlayer of an organic semiconductor. We investigate the temperature dependence of FET in a QW/QDs hybrid configuration. Time and spectrally resolved measurements reveal that the transfer efficiency is dominated by the interplay between exciton localisation and non-radiative recombination intrinsic to the QW. Whereas the onset of non-radiative recombination channels in the QW compete with FET and eventually dominate the transient QW photoluminescence dynamics, the rate of FET is monotonically increasing with temperature as the exciton localisation length increases. At 60K we obtain an optimum transfer efficiency of 65%, an order of magnitude larger than current electrical injection pumping schemes.We also fabricate a QW/organic semiconductor hybrid heterostructure, where we use pump-probe, time and spectrally resolved fluorescence measurements to demonstrate FET directly at the acceptor site in the near infrared (NIR). We investigate the dynamics of the process on the donor/acceptor spectral overlap by adjusting the width of the QW structures, while using a thin barrier to controllably separate electron-hole pairs in the QW and the organic semiconductor and we obtain transfer efficiencies up to 39%.
9:00 PM - H9.14
Poly-(thiophene)- CdSe Nanoparticle Composite Thin Films Electrochemically Tethered to Indium Tin Oxide Substrates.
R. Shallcross 1 , Steven Bowles 1 , Jeffrey Pyun 1 , Neal Armstrong 1
1 Chemistry, University of Arizona, Tucson, Arizona, United States
Show AbstractThis talk will detail a new approach to the creation of nano-composite thin films based on poly-(thiophenes) and CdSe nanoparticles. The key enabling step in the development of this technology is the creation of new electroactive ligands for the CdSe NP, which can be incorporated into a growing polymer chain via electrochemical polymerization. This talk will focus on polymers with a range of ionization potentials, and a range of nanoparticle diameters, both of which control the energetics for photoinduced charge transfer. Experiments to date suggest that electron transfer from the polymer host to the ligand-capped NP is rate limiting, in photoelectrochemical applications of these materials. We will also present more recent studies which vary the driving force for electron transfer to solution electron acceptors, as a function of the excess free energy for that process. Finally, preliminary photovoltaic devices will be discussed which utilize the NP as the light absorbing layer, using the poly-(thiophene) as the hole-transport layer, and fullerenes as the electron transport/selective top contact.
9:00 PM - H9.16
Broadband Optical Absorption Enhancement in Thin Film Photovoltaic Cells using Coherent Light Trapping.
Mukul Agrawal 1 , Peter Peumans 1
1 Electrical Engineering, Stanford University, Stanford, California, United States
Show AbstractHigh-reflectivity dielectric mirrors are used in resonant-cavity-enhanced photodetectors to obtain a narrowband enhancement of optical absorption by exploiting constructive interference. On the other hand, dielectric anti-reflection coatings are used in solar cells to enhance the incoupling of solar radiation into the absorbing layer over a broad spectral range. We show that these approaches are the limiting cases of the same principle and develop a method to design dielectric mirrors to obtain the maximum possible optical absorption for a given thin-film over a broad spectral range, effectively increasing the optical pathlength of the active layers. This approach is a general way to improve the efficiency of nanostructured solar cells in which the internal quantum efficiency is traded off against optical absorption by tuning of the film thickness. The required optical microcavities enhance the optical transmittance in the high absorptivity region while decreasing the optical transmittance and increasing the coherent standing wave-effect in the low absorptivity regions. Such cavities can be implemented using a specially designed, partially reflecting mirror that exhibit negative phase dispersion. For the concrete example of a nanostructured small molecular weight organic solar cell with a 25nm-thick blend of 1:1 (by weight) copper phthalocyanine (CuPc) and 3,4,9,10-perylene tetracarboxilic bis-benzimidazole (PTCBI) with strong optical absorption for λ=550nm-700nm, the average absorptivity for λ=350-800nm is only 47% in a standard device on ITO/glass with a Ag cathode optimized to exploit optical interference. An ideal optical cavity would result in an average absorptivity of 81% for the same spectral region. The enhancement of the optical absorption is not only broadband, but also extends over a broad angular range. Based on our earlier work on the design of dielectric stacks for improved outcoupling in organic light-emitting devices [1], we demonstrated a systematic approach to design the required dielectric mirrors as one-dimensional non-periodic dielectric stacks. For the specific case of a 25nm-thick CuPc-PTCBI 1:1 mixture, we have designed TiO2/MgF2 dielectric stacks that improve the average optical absorption for λ=350-800nm 1.5-fold from 47% to 70%, approaching the theoretical limit of 81%. This approach can be applied to a broad range of state-of-the-art nanostructured solar cells without modification of the active layers to improve their efficiencies significantly.[1] Mukul Agrawal, Yiru Sun, S. R. Forrest and Peter Peumans, Appl. Phys. Lett. 90, 241112 (2007). Mukul Agrawal and Peter Peumans, to be published in Opt. Express (2007).
9:00 PM - H9.17
Plastic Near-Infrared Photodetectors Utilizing Low Band Gap Polymer.
Yan Yao 1 , Yongye Liang 2 , Luping Yu 2 , Yang Yang 1
1 Materials Science and Engineering, UCLA, Los Angeles, California, United States, 2 Department of Chemistry, University of Chicago, Chicago, Illinois, United States
Show AbstractHigh performance near-infrared photodetectors are of tremendous importance in scientific and industrial applications. Here we report the first plastic near-infrared photodetector using a new low band gap polymer. By utilizing an ester group modified polythieno[3,4-b]thiophene, we have successfully lowered the highest occupied molecular orbital energy level of the polymer, so that it can match the energy level of (6,6)-phenyl C61-butyric acid methyl ester and has good solubility. When composing the device in a donor-acceptor type energy band structure and operating it in the reverse bias condition (less than 5 V), we show that the photodetector has external quantum efficiency exceeding 38%, bandwidth of 4 MHz and the noise equivalent power of 3.85 ×10-12 W/Hz1/2 at 850 nm. With the promising results demonstrated here, we believe low band gap materials will open up a new perspective in the near-infrared detection.
9:00 PM - H9.19
Using Nanosphere Templating to Engineer Organic Photovoltaic Device Architectures.
Martyn McLachlan 1 , Sarah Berhanu 1 , Tim Jones 2 , David McComb 1
1 Materials, Imperial College London and London Centre for Nanotechnology, London United Kingdom, 2 Department of Chemistry, University of Warwick, Warwick United Kingdom
Show AbstractColloidal crystals exhibit three-dimensionally periodic and porous structures which can be used as templates to direct the formation of 3DOM solids. There is wide-scale interest in 3DOM materials owing to their unique physical and structural properties, the most widely studied being the photonic band gap (PBG). Additionally, 3DOM structures have inherently large surface to volume ratios, a property which has been explored for applications including chemical sensing and photocatalysis. 3DOM structures also contain two interpenetrating lattices, usually air and a solid material. The combination of high surface to volume ratio and the interconnectivity has prompted interest for other applications such as high performance cathodes in batteries.A performance limitation in organic photovoltaic (OPV) devices based on donor (D) - acceptor (A) heterojunctions is the low exciton diffusion length in organic semiconductors. Considerable efforts are being directed at producing structured D-A composites for OPV applications by creation of polymer blends or intermixed molecular layers. However these random composite structures rely on the fortuitous creation of interfaces to improve device performance. We present the results of a recent study out discussing the progress made towards the preparation of nanostructured D-A arrays using colloidal crystal templating. The applicability of the templating process for the preparation of thin films with pore diameters ranging from 50 to 500nm will be demonstrated. We will show that the formation of 3-DOM films of, copper phthalocyanine can be carried at room temperature using solution processing. Characterisation of the structures formed using scanning/transmission electron microscopy and x-ray diffraction will be presented, confirming that a low temperature route can be used to prepare highly ordered structures. The applicability of the process for a range of organic small molecules and binary metal oxides will be discussed. References1.Rand, B.P., J. Xue, S. Uchida, and S.R. Forrest, Journal of Applied Physics, 2005. 98: p. 124902.2.Gregg., B.A., Journal of Physical Chemistry B, 2003. 107: p. 4688-4698.3.Brabec, C.J., N.S. Sariciftci, and J.C. Hummelen, Advanced Functional Materials, 2001. 11(1): p. 15-26.4.Peumans, P., A. Yakimov, and S.R. Forrest, Journal of Applied Physics, 2003. 93(7): p. 3693-3723.5.Xue, J., B.P. Rand, S. Uchida, and S.R. Forrest, Advanced Materials, 2005. 17: p. 66.6.Sullivan, P., S. Heutz, S.M. Schultes, and T.S. Jones, Applied Physics Letters, 2004. 84(7): p. 1210-1212.7.McComb, D.W., B.M. Treble, C.J. Smith, D.L. Rue, R. M, and N.P. Johnson, Journal of Materials Chemistry, 2001. 11: p. 142-148.8.McLachlan, M.A., N.P. Johnson, R.M. DeLaRue and D.W. McComb, Journal of Materials Chemistry, 2004. 14(2): p. 144-150.
9:00 PM - H9.2
Electrode Optimizations of Organic Photovoltaic Devices.
Matthew Reese 1 , Matthew White 1 , Garry Rumbles 1 , David Ginley 1 , Sean Shaheen 1
1 , National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractA blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) is used as the active layer in a series of bulk heterojunction organic solar cells. This polymer blend serves as a test-bed to explore the effects on device performance of using low work function metals and/or alkali metal halides as the top, negative electrode. A series of contact materials is investigated including Al, Ca/Al, Ba/Al, LiF/Al, Ag, and Mg:Ag/Ag; many devices are prepared with each contact type to validate the statistical significance of the results. Device stability measurements are also reported. Furthermore, the effects of treating poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) at the interface of the positive electrode are investigated. We demonstrate sizeable fill factor enhancement with appropriate surface treatment.
9:00 PM - H9.20
Preparation and Elecyro-Optical Property of Novel Discotic Liquid Crystals and Poly(acryamide) Dispersed LC with Application to Organic Solar Cells.
Fan Chris 1 2
1 , Institute of Electro-Optical Engineering, National Sun Yat-sen University, Kaohsiung Taiwan, 2 , Institute of Electro-Optical Engineering, National Sun Yat-sen University, Kaohsiung Taiwan
Show Abstract9:00 PM - H9.21
Charge Extraction in Polymer Diodes and Polymer: Fullerene Solar Cells - Influence of Thermalisation and Recombination.
Carsten Deibel 1 , Andreas Baumann 1 , Jens Lorrmann 1 , Vladimir Dyakonov 1
1 Experimental Physics VI, University of Würzburg, Würzburg Germany
Show AbstractIn polymer:fullerene solar cells, electronic transport of photogenerated carriers is governed by hopping within a gaussian density of states distribution. Accordingly, the current flow in the composite material is influenced by carrier recombination and thermalisation. A versatile experimental technique for investigating charge transport in these disordered materials is the CELIV method, charge extraction by linearly increasing voltage [1]. Applying it, the charge carrier mobility in the low-carrier concentration regime of thin diodes is easily accessible, which is directly relevant to organic photovoltaic devices. In our study, the bulk transport behavior of poly(3-hexyl thiophene) (P3HT) diodes, as well as solar cells of P3HT blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM), is investigated with the photoinduced CELIV method. Charge carriers are generated by a short laser pulse, the extraction of the charges commences after a variable delay time. We observe thermalisation, represented by a time-dependent mobility, as well as bimolecular recombination of the photogenerated charges. Dispersive transport, induced by the carrier thermalisation, is found to be dominant at the temperature and field ranges investigated. As dispersive transport is not considered in the original derivation of the CELIV analysis, we use Monte-Carlo calculations of carrier extraction in order to support the distinction between the effects of thermalisation and recombination.[1] G. Juska, K. Arlauskas, M. Viliunas, and J. Kocka, Phys. Rev. Lett. 84 (2000) 4946.
9:00 PM - H9.22
Polymer Bulk Homojunction Light-Emitting Electrochemical Cells and Photovoltaic Cells Based on Gold Nano-Islands.
Wayne Bonnet 1 , Jun Gao 1 , Corey Tracy 1 , Guillaume Wantz 2
1 Department of Physics, Queen's University, Kingston, Ontario, Canada, 2 IMS Laboratory, Université Bordeaux, Bordeaux France
Show AbstractPolymer light-emitting electrochemical cells (LECs) have been shown to operate with a fundamentally unique mechanism as compared to traditional polymer light-emitting devices. An LEC is formed by the in situ doping of a luminescent polymer and solid-state electrolyte blend that leads to the creation of a p-n junction. The high conductivity of the doped regions makes the device relatively insensitive to the electrode spacing, making the planar LEC structure possible with spacings on the order of millimeters. These planar devices offer great insight into the formation of the light-emitting p-n junction, but typically have an emission zone that is only a small fraction of the electrode spacing, and thus have low efficiencies as both light-emitters and photovoltaic cells.The bulk homojunction LEC has addressed the issue of specific emitting area by mixing metallic particles into the LEC solution before casting the film. The result is thousands of micrometer-sized p-n junctions spread uniformly throughout the bulk of the device. An added benefit of this configuration is that a bulk homojunction device can achieve very high open-circuit voltages when the photovoltaic response is measured. Unfortunately, the short circuit current has thus far been quite small in these devices. A limiting factor in the number of junctions that can be formed using this fabrication technique has been the unfavorable aggregation of the metallic particles in the LEC film. In recent work, aggregates of up to 50 microns were found when nominally sub-micron particles had been used.We report on the fabrication and imaging of bulk homojunction LECs with metallic particles on the order of 20 nm. This new method of fabrication takes advantage of the clustering and island formation of gold on sapphire. By depositing a small amount of gold on a sapphire substrate at a low rate of evaporation, nanoscale islands are formed in a non-continuous film that is non-conducting on its own. Casting a regular LEC film on top of these islands forms the basis of a bulk homojunction device potentially capable of forming upwards of 10,000 p-n junctions formed across the electrode spacing. Despite the asymmetry of the device structure (approximately 5 nm tall gold islands covered by an LEC film approximately 200 nm thick), the gold islands are still able to sufficiently block the propagation of the electrochemical doping, and thus many p-n junctions are formed. Similar devices have also been demonstrated where the gold was deposited on top of an LEC film.The scale of and control over the size distribution obtained using this work have not been achieved in bulk homojunction devices. These factors will help in the investigation of how the bulk homojunction phenomenon changes with miniaturization, as an ultimate goal of a power efficient sandwich structure bulk homojunction photovoltaic cell is sought.
9:00 PM - H9.23
Environmental Passivation and Temperature Cycling of PCBM - Polymer Solar Cells.
Annick Anctil 1 2 , Andrew Merrill 3 , Cory Cress 2 4 , Brian Landi 2 , Ryne Raffaelle 1 2 3
1 Material Science and Engineering, Rochester Institute of Technology, Rochester, New York, United States, 2 NanoPower Research Laboratories, Rochester Institute of Technology, Rochester, New York, United States, 3 Physics, Rochester Institute of Technology, Rochester, New York, United States, 4 Microsystems Engineering, Rochester Institute of Technology, Rochester, New York, United States
Show AbstractThe development in recent years of higher efficiency polymer solar cells (exceeding 5%) has progressed to a point where issues involving environmental and thermal stability need to be addressed. Conventional polymer devices generally employ a PEDOT/PSS layer and a derivatized polythiophene or polyphenylenevinylene (PPV) composite blend sandwiched between two electrodes. It is well established that these devices are susceptible to degradation in device performance with ambient exposure on the order of hours to days. Practical implementation of these devices will rely upon the development of efficient passivation designs to maintain performance under typical environmental conditions: thermal gradients (250 K – 325 K), humidity, and oxygen. In the present work, polymer solar cells comprising PCBM[60] and PCMB[70] were dispersed in derivatized PPV (MEH and MDMO) and used as active layers in an ITO/PEDOT-PSS/Al sandwich. Optimization of device performance has been performed through variation in PCBM:polymer ratios and with systematic thermal annealing before and after aluminum deposition. Power conversion efficiencies will be reported from current-voltage measurements for these different processing conditions under simulated air mass 1.5 illumination. In general, the devices exhibit a positive temperature coefficient for the short-circuit current density which dominates the overall efficiency of the device over the measured range of 80 K – 350 K. Results from thermal cycling between 250 K – 325 K under vacuum will be compared to degradation rates measured using dry air, nitrogen, and argon environments. Finally, the development of device passivation approaches using multiple polymer coatings which enhance the resilience to water and oxygen will be discussed. Results using a combination of parylene and polymethylmethacralate show a dramatic improvement to device passivation under ambient conditions compared to non-passivated devices.
9:00 PM - H9.24
Investigation of Recombination of Dissociated Electrons and Holes in Organic Bulk Heterojunction Solar Cells.
Jaime Sullivan 1 , Zhihua Xu 1 , Bin Hu 1
1 Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States
Show AbstractThe dissociation of photoexcited states is a critical process in organic solar cells. Bulk heterojunction structures formed in donor-acceptor polymer blends are an effective approach to dissociate the photoexcitation generated excited states for the generation of photocurrent. However, we found from our recent studies of photoluminescence that the dissociated electrons and holes can recombine and reform excitons. This recombination decreases the photovoltaic response in organic solar cells. We further studied how singlet and triplet excitons are involved in the recombination of dissociated charge carriers by using temperature-dependent photoluminescence and magnetic field-dependent photocurrent. We selected two typical polymers: MEHPPV named as singlet polymer and P3HT named as triplet polymer, to examine the recombination of dissociated charge carriers in their respective PCBM doped solar cells. Our experimental results show that the singlet photovoltaic system of PCBM doped MEHPPV experiences significant recombination as compared to the triplet photovoltaic system of PCBM doped P3HT. This phenomenon suggests a new pathway to enhance the photovoltaic response of organic solar cells by reducing the recombination of dissociated charge carriers through tuning singlet and triplet ratios. The presentation will also discuss how the electron spin correlation affects the recombination of dissociated electrons and holes.
9:00 PM - H9.25
The Nature of the Charge Carriers in Conducting Polymers: A Detailed Study by in situ Techniques.
Beatriz Meana-Esteban 1 , Fredrik Sundfors 1 , Andreas Petr 2 , Carita Kvarnstrom 1 , Lothar Dunsch 2 , Ari Ivaska 1
1 , Åbo Akademi University, Åbo-Turku Finland, 2 , Institute of Solid State and Materials Research Dresden, Dresden Germany
Show AbstractConducting polymers and oligomers are potential candidates for many different applications, such as actuators, corrosion protection, sensors, energy storage, solar cells, and light-emitting diodes. However, this requires a detailed knowledge of these materials and their redox processes, also called doping reactions. Doping of conducting polymers and oligomers leads to the formation of different types of charge carriers, non-linear excitations, which are delocalized along the polymer chain and exhibit peculiar vibrational, electronic, magnetic and optical properties. Evidence for the existence of charge carriers of different nature has been obtained by different spectroscopic and electrochemical methods due to their characteristic optical and magnetic signatures. These organic materials are promising candidates as the light-harvesting unit in solar cells. Thus, the conducting material should fulfill some requirements, like a suitable band gap which matches the photon flux from solar irradiation. This will increase the photon harvesting, and thus, the efficiency of the solar cellIn this work, in situ Electron Spin Resonance (ESR) and UV-visible-NIR (UV-vis-NIR) spectroscopy have been used to study the nature of the charge carriers during electrochemical p-doping of 2-methoxynaphthalene films electrosynthesized in an organic solvent (TBAPF6-NB). Furthermore, the value of the band gap together with the energy levels of poly(2-methoxynaphthalene) has been calculated by Electron Voltage Spectroscopy (EVS).
9:00 PM - H9.26
Structural Study of P3HT and P3HT:PCBM Thin Films by using Synchrotron X-ray Diffraction.
Takeaki Sakurai 1 , Susumu Toyoshima 1 , Masato Kubota 2 , Toshihiro Yamanari 3 , Tetsuya Taima 3 , Kazuhiro Saito 3 , Katsuhiro Akimoto 1
1 Institute of Applied Physics, University of Tsukuba, Tsukuba Japan, 2 Photon Factory, High Energy Accelerator Research Organization (KEK), Tsukuba Japan, 3 Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Japan
Show AbstractThe significant progress in the development on the polymeric photovoltaic cells has been made in this decade, and the energy conversion efficiency of polymer-based bulk-heterojunction cell, which is composed of a mixture of poly-hexylthiophene (P3HT) and 6,6-Phenyl-C61 butyric acid methyl ester (PCBM), got over 5%. However, the structural properties of the P3HT:PCBM mixture film, which strongly affect the performance of the photovoltaic cells, have not been clearly understood yet because the structure of the inside of the film is much complicated. In this work, we report on the structural characterization of the pristine P3HT film and P3HT:PCBM mixture film by using the synchrotron X-ray diffraction (XRD) techniques. The synchrotron XRD measurements (λ=0.154 nm) were carried out at the beam line 4C in KEK photon factory. In the out-of-plane XRD analysis, we observed the P3HT (100) and (200) diffraction peaks corresponding to the spacing of the main chain layers of 1.64 nm for both P3HT and P3HT:PCBM films, while we did not find noticeable difference between the XRD profiles of these samples even though these samples were annealed. These results suggest that the crystallization between the main chains of P3HT occurred smoothly nevertheless PCBM was blended in P3HT. In the in-plane XRD analysis, on the other hand, a large difference between the profiles of P3HT and P3HT:PCBM was observed. For the pristine P3HT film, we observed the P3HT (010) peak caused by π-π stacking of the thiophene rings in both profiles of as-deposited and annealed samples. For the P3HT:PCBM mixed film, there was no P3HT(010) peak in the profile of as-deposited sample, while this peak was gradually appeared with increasing the annealing temperature. These results suggest that P3HT was prevented from forming the π-π stacking structure when PCBM was blended in P3HT, although such stacked structures were formed after annealing due to the mass transport of P3HT, which is caused by its glass transition [1]. It is well known that π-π stacking of the thiophene rings is important for the carrier transport, further, the anneal treatment of the P3HT:PCBM cell is required for improving its efficiency [1]. Therefore, the formation of the π-π stacking structure of P3HT may induce the improvement of the electrical properties in P3HT:PCBM photovoltaic cells.[1] Y.Kim, S.Choulis, J.Nelson, D.C.Bradley, S.Cook, J.Durrant, Appl. Phys. Lett. 86, 063502 (2005).
9:00 PM - H9.27
In-situ Spectroscopy of Solvent Vapor Annealed Polymer-Fullerene Blends for Organic Photovoltaics.
Steve Miller 1 , Giovanni Fanchini 1 , Manish Chhowalla 1
1 Materials Science and Engineering, Rutgers University, Piscataway, New Jersey, United States
Show AbstractOrganic photovoltaics (OPVs) are promising alternatives to their inorganic counterparts because they can be prepared simply from solution on cheap substrates. However, cheap, flexible plastic substrates maybe damaged during thermal annealing which has been shown to be critical for improving efficiencies of OPVs. Room temperature solvent vapor annealing has been recently demonstrated to improve the power conversion efficiency of organic poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) photovoltaic devices. We have used in-situ photoluminescence (PL) and Raman spectroscopies, in conjunction with optical absorption data and atomic force microscopy (AFM), to provide insight into the nanoscale morphological changes of the P3HT:PCBM blend. The suppression of PL, decrease in Raman line-width, modifications in optical absorption spectra, and agglomerated morphology observed by AFM suggest de-mixing of PCBM and coplanar stacking of P3HT, similar to what is observed during thermal annealing.
9:00 PM - H9.28
Pt-acetylide Organometallic Polymers as Active Materials for Organic Solar Cells.
Jianguo Mei 1 2 , Katsu Ogawa 1 2 , Young-Gi Kim 1 2 , Kirk Schanze 1 2 , John Reynolds 1 2
1 Department of Chemistry, University of FLorida, Gainesville, Florida, United States, 2 , Center for Macromolecular Science and Engineering, Gainesville, Florida, United States
Show AbstractConjugated polymers (CP) have drawn great attention as active layer materials for bulk heterojunction (BHJ) “plastic” solar cells due to cost effectiveness, low density, and mechanical flexibility. The desire to enhance the solar cell power conversion efficiency (PCE) has been driving synthetic organic chemists to design various polymers potentially suitable for serving as active layer materials. Organometallic complexes have been well known in the field of dye-sensitized solar cells. In contrast, organometallic polymers have seldom been used for BHJ solar cells. Our previous research on a thienylene-linked Pt acetylide polymer (p-Pt-Th)/PCBM blend as an active layer for BHJ cells showed a power-conversion efficiency of 0.3%. The low efficiency can be explained by the mismatch between the absorption of the polymer and the solar spectrum. Longer wavelength absorbing Pt-acetylide organometallic polymers p-Pt-BTD-Th and p-Pt-BTD-EDOT bearing lower energy absorbing chromophore have been synthesized, which have maximum absorption around 550 nm and 600 nm, respectively. Operating under standard solar simulator conditions (i.e. AM1.5, 100 mW cm-2), the p-Pt-BTD-Th/PCBM based devices exhibited an open circuit voltage (Voc) of 0.54 V, a short circuit current density (Isc) of 7.17 mA-cm-2, and a fill factor of 36%. The overall power conversion efficiency (η) of the device reached 1.4%. For the device based on the p-PtBTD-EDOT/PCBM blend, values are 0.50 V, 4.56 mA-cm-2, 35%, and 0.8% respectively. The p-PtBTDTh/PCBM based device showed IPCE of ~55% at 380 nm and ~36% at 570 nm. The p-PtBTD-EDOT/PCBM based devices presented similar IPCE characteristics with slightly lower efficiency.
9:00 PM - H9.29
New C60 Derivatives as Electron Acceptor for Organic Solar Cells.
HsinHan Tsai 1 , Leeyih Wang 2 3 , Syang-Peng Rwei 1 , Chinwei Liang 2
1 institue of organic and polymeric materials, National Taipei University of Technology, Taipei Taiwan, 2 Center for Condensed Matter Science, National Taiwan University, Taipei Taiwan, 3 Institute of Polymer Science and Engineering , National Taiwan University, Taipei Taiwan
Show AbstractOrganic solar cells using the mixture of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) and poly(3-hexylthiophene) (P3HT) as photoactive material have attract considerable interests in recent years because a power conversion efficiency around 5% has been demonstrated by carefully tuning the fabrication process. In this study, two C60 derivatives, di(butyl)(1,2-methanofullerene C60)-61,61-dicarboxylate and (butyl-2-thienylmethyl)(1,2-methanofullerene C60)-61,61-dicarboxylate, were synthesized by the Bingel cyclopropanation of malonic ester-bearing molecules and C60 cage. The products were characterized using 1H NMR, 13C NMR, UV-vis, EA and MALDI TOF-MS. These new C60 derivatives possess much better solubility than PCBM in many organic solvents such as THF and chloroform, allowing the use of higher concentration of P3HT and C60 derivatives in the cell fabrication process. The effect of solvent and ingredient concentration on the performance of the solar cells of ITO/PEDT:PSS/P3HT:C60 Derivatives/Al will be reported.
9:00 PM - H9.30
Synthesis of a Low Band Gap Polymer Having a Novel Fused Aromatic Thieno[3,4,b]pyrazine Moiety and Its Solar Cell Performance.
Nobuyuki Miyaki 1 , Toshihiro Okamoto 1 , Alex Mayer 2 , Michael McGehee 2 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Sunnyvale, California, United States, 2 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show Abstract Recently, low band gap polymers have attracted much attention for bulk heterojunction solar cells because they harvest longer wavelength of light (around 700 nm) and gives improved efficiency. In order to achieve low band gap in a polymer, it is necessary that both an electron-rich (donor) unit and an electron-deficient (acceptor) unit are incoporated into a conjugated polymer chain. In our design of low band gap polymers, thienopyrazines are utilized for as the acceptor units. They were chosen because their chemical structures are easy to modify so that we can fine tune their electrical physical properties. We design a novel fused aromatic thieno[3,4,b]pyrazine structure as an acceptor unit to control an interaction between polymer chains and its electron density. The resulting polymer has a relatively low band gap (1.71 ev. from cyclic voltammetry) and shows signs of crystalline order. We will report the synthese and solar cell performance of this class of novel low gap polymers.
9:00 PM - H9.31
Interaction of Coronene Polymers with Electron Deficient Molecules.
Suresh Valiyaveettil 2 , Vajiravelu Sivamurugan 2 , Ganpathi Balaji 2
2 Department of Chemistry , National University of Singapore, Singapore, Singapore, Singapore
Show Abstract9:00 PM - H9.32
Novel Polythiophene/titania Hybrids with Tunable Absorption Spectra for Polymer Photovoltaic Cells.
Leeyih Wang 1 2 , Yi-Ming Chang 2 , Wei-Fang Su 2
1 Center for Condensed Matter Sciences, National Taiwan University , Taipei Taiwan, 2 Institute of Polymer Science and Technology, National Taiwan University, Taipei Taiwan
Show AbstractRecently, many studies have focused on the use of TiO2 as electron acceptor in fabricating polymer photovoltaic cells because it is a safe, environmental stable and cheap material. However, the inherent incompatibility between the hydrophilic surface of TiO2 and hydrophobic conjugated polymers frequently causes macroscopic phase separation, thus significantly reducing the donor/acceptor interface and carrier dissociation efficiency. In this study, we present a new class of hybrids with tunable absorption wavelengths and excellent miscibility which were synthesized via the in-situ sol-gel condensation of titanium (IV) n-butoxide in the presence of a hydroxyl-bearing poly(3-hexylthiophene) , P3HT-OH. The images from optical microscope, tunneling electron microscope and atomic force microscope clearly indicate the functional P3HT-OH effectively prevents the macroscopic aggregation of TiO2 during the sol-gel reaction, creating a uniform distribution of TiO2 throughout the polymer-TiO2 hybrid solution and film. Moreover, both UV-vis absorption and photoluminescence spectra reveal strong interactions between the polymer and titania particles. The color of P3HT-OH can be fine-tuned from yellow-green to blue and their bandgaps can be changed from 1.9 eV to 2.2 eV by simply varying the TiO2 content in hybrids from 10 to 40 wt%. Most importantly, these P3HT-OH-TiO2 hybrids exhibit much higher photo-induced electron transfer efficiency compared with the P3HT-TiO2 hybrids prepared at the same synthetic procedure. The performance of photovoltaic cells based on these new materials will be also presented and discussed.
9:00 PM - H9.33
Polymer Solar Cell Using Poly(3,4-ethylenedioxythiophene-mathanol) as Anode Electrode.
Youn Soo Kim 1 , Young Kwan Lee 2 , Jai Kyeong Kim 1
1 , Korea Institue of Science and Technology, Seoul Korea (the Republic of), 2 , Sungkunkwan University, Suwon Korea (the Republic of)
Show Abstract9:00 PM - H9.34
Hole Mobility Studies on Thiophene-Based Conjugated Polymers and Oligomers Developed for Use in Organic Electronic Devices.
N. Heston 1 , C. Nielsen 2 , C. Grenier 2 , D. Tanner 2 , J. Reynolds 1
1 Department of Physics, University of Florida, Gainesville, Florida, United States, 2 Department of Chemistry, University of Florida, Gainesville, Florida, United States
Show AbstractIn optimizing organic electronic devices, such as solar cells and field effect transistors, the mobility plays a crucial role affecting many aspects of performance, including: charge separation efficiencies, carrier densities, and drain currents. By fabricating hole-dominated devices and fitting the measured current-voltage characteristics to the field-dependent space-charge-limited mobility model we were able to measure hole mobilities in a set of conjugated polymers and oligomers. The hole mobilities of poly(phenylenedioxythiophene-(C12H25)2) were found to reach 2.0 x 10-5(cm2V-1s-1) and those of poly(phenylenedioxythiophene-EtHx(C12H25)) reached 1.8 x 10-5(cm2V-1s-1). We found hole mobilities as high as 3.6 x 10-3(cm2V-1s-1) in a thiophene/phenylene/EDOT (TPEEPT) based hybrid diacrylate oligomer. These materials show promise as active layers in organic solar cells, light-emitting diodes, and field effect transisitors.
9:00 PM - H9.35
TCSPC SNOM of Device Applicable Blends of Photovoltaic Conjugated Polymers.
Ashley Cadby 1 , David Lidzey 1 , Gamal Khalil 1
1 Physics Department, University of Sheffield, Sheffield, South Yorkshire, United Kingdom
Show AbstractScanning near field optical spectroscopy (SNOM) is a scanning probe microscopy (SPM) that allows the characterization of the optical properties of a surface at length scales below the classical diffraction limit, in this case around 50nm. It allows many different forms of optical spectroscopy to be performed at very high spatial resolutions. We have used this versatility to combine time correlated single photon counting spectroscopy (TCSPC) with SNOM to map the photoluminescence lifetime of a variety of systems with a temporal resolution of 10ps. In this talk I will show how this experimental technique has been used to measure energy transfer in photo-voltaic device applicable blends of the conjugated polymers poly(9,9-dioctylbenzothiadiazole) [F8BT] and poly(9,9,-dioctylrene-co-bis-N,N-(4-butylphenyl)-bis-N,N-phenyl-1,4 phenylenedi-amine) [PFB] . I will show that these polymer blends show a very high level of photoluminescence quenching at very small length scales (< 100nm), even though phase separation produces features several microns in length. We also find that unlike the polymer blends used in light emitting diodes phase separated features in these blends show a reduced contrast in the PL lifetime images of PFB and this reduction is also independent on polymer concentration within each phase. We see a large difference in the PL quenching rates between PFB and F8BT. This asymmetry can be explained by a combination of mechanisms. This work has important applications for the development of photovoltaic device from conjugated polymers.
9:00 PM - H9.36
Efficient Thin Polymer Solar Cells with Post-annealing.
Shun-Wei Liu 1 2 , Chi-Jiun Liao 3 , Chin-Chien Lee 3 , Chin-Ti Chen 1 , Juen-Kai Wang 4 5
1 Institution of Chemistry, Academia Sinica, Taipei Taiwan, 2 Department of Electrical Engineering and Graduate Institute of Electro-Optical Engineering, National Taiwan University, Taipei Taiwan, 3 Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei Taiwan, 4 Center for Condensed Matter Science, National Taiwan University, Taipei Taiwan, 5 Institute of Atomic and Molecular Science, Academia Sinica, Taipei Taiwan
Show AbstractThe development of high-performance organic solar cells with low-cost fabrication processes has become one of the most important tasks in the vast endeavors of releasing the world-wide energy demand from fossil fuels. Nowadays, the power-conversion efficiency of polymer solar cells in excess of 5% has been demonstrated, but they involve complicated film formation mechanisms of thick active layers or delicate design of spacer layers. These approaches, therefore, may increase the series resistance of the devices and complicate fabrication procedure. In this report, we present a highly efficient polymer solar cells with a bulk heterojunction layer of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methylester (P3HT:PCBM) which is annealed at 130°C for 5 min. in a nitrogen environment (O2 < 0.1 ppm and H2O < 0.1 ppm) before cathode deposition. The annealing temperature is much lower and the annealing time is shorter than previous works. The device exhibits conversion efficiency of 4.6%, fill factor of 53 %, and open-circuit voltage of 0.67 V. These values are still comparable with the highest values reported previously. The annealing process is expected to modify the network morphology of the P3HT:PCBM layer. The nanoscale morphology and crystallinity of thin polymer films were analyzed and will be discussed. Finally, the thickness of the active layer is reduced to 50 nm which is much thinner than previously reported values, may facilitating the fabrication of tandem photovoltaic structures.
9:00 PM - H9.37
New Solution Processable Conjugated Dendrimers and Star-Shaped Molecules for Photovoltaic Device Applications.
Benjamin Rupert 1 , Matthew Reese 1 , William Rance 1 , Nikos Kopidakis 1 , David Ginley 1 , Garry Rumbles 1 , Sean Shaheen 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractThere has been recent interest in the use of pi-conjugated based dendrimers for the light absorbing and charge transporting components in organic solar cells. Dendrimers offer the promise of the same inexpensive processing methods that are used with polymers combined with the high purity and relatively simple modification of small molecules. This talk will focus on the synthesis and characterization of a new generation of materials that build upon and refine our previous efforts with phenyl-cored thiophene dendrimers. These new materials eliminate the branch points of our previously reported ones, and so they can be considered zeroth-generation dendrimers or simply star-shaped molecules. This design is intended to minimize steric repulsion of neighboring molecules pi faces, thereby improving film morphology and intermolecular ordering. Chemical synthesis and characterization are reported, along with initial characterization of structural and electronic properties of thin films of the new dendrimers. Additional modifications to the dendrimers such as inclusion of thienopyrizine units to reduce the optical band gap toward 1.4 eV as well as strategic placement of electron withdrawing groups to control localization of the LUMO will also be discussed.
9:00 PM - H9.38
NEXAFS Spectra of Conjugated Polymers: Contrast Mechanisms for Soft X-ray Characterization of Electronic Polymer Structures.
Benjamin Watts 1 , Harald Ade 1 , Jan Luning 2 3
1 Physics, North Carolina State University, Raleigh, North Carolina, United States, 2 , Stanford Synchrotron Radiation Laboratory, Marseille France, 3 LCP-MR, Université Pierre et Marie Curie, Paris France
Show AbstractElectronic devices based on conjugated polymers promise to revolutionize both display and solar energy technologies with significant advantages over conventional inorganic based devices. However, progress in this field is hampered by the fact that many conventional characterization techniques, such as electron microscopy and neutron and hard X-ray scattering and reflectivity, are difficult to apply to polymer blends or layered structures due to poor contrast between materials. On the other hand, polymer heterojunction and layered device structures can be characterized via a variety of soft X-ray techniques that have a strong intrinsic contrast mechanism based on the near edge X-ray absorption fine structure (NEXAFS) resonances of the component polymers or the corresponding changes in the index of refraction [1,2].Here, we present a database of calibrated NEXAFS spectra of conjugated polymer materials, including polythiophene, poly phenylenevinylene and polyfluorene based polymers. These spectra determine the level of contrast that is achievable with soft X-ray techniques as a non-trivial function of photon energy and so the utilization of specific photon energies will be demonstrated with some representative experiments.This research was carried out at SSRL, a national user facility operated by Stanford University on behalf of the US Department of Energy, Office of Basic Energy Sciences. Work at NCSU is supported by the U. S. Department of Energy (DE-FG02-98ER45737).[1] C. Wang, T. Araki, B. Watts, S. Harton, T. Koga, S. Basu and H. Ade, J. Vac. Sci. Technol. A 25(3) 575–586 (2007) DOI: 10.1116/1.2731352[2] C.R. McNeill, B. Watts, L. Thomsen, H. Ade, N.C. Greenham, and P.C. Dastoor Macromolecules, 40, 3263–3270 (2007) DOI: 10.1021/ma070132d
9:00 PM - H9.39
Synthesis and Characterization of Novel Arylene Ether Polymers.
Wen-Yao Huang 1 , Chun-Che Lee 1 , Mei-Ying Chang 1
1 , institute of Electro-Optical Engineering, Kaoshiung,Taiwan Taiwan
Show Abstract9:00 PM - H9.40
Energy Migration and Excimer Formation in Discotic Liquid Crystalline Derivitives: dibenzo[a,c]phenazines.
Wen-Yao Huang 1 , Chun-Che Lee 1 , Duo Zheng Fan 1 , Yao wei Chuang 1
1 , institute of Electro-Optical Engineering, Kaoshiung,Taiwan Taiwan
Show Abstract9:00 PM - H9.41
Charge Carrier Dynamics in Regioregular Polythiophene-perylene Derivatives for Photovoltaic Applications.
Francoise Provencher 1 , Simon Gelinas 1 , Ricardo Izquierdo 2 , Natalie Stingelin-Stutzmann 3 , Carlos Silva 1
1 Department of Physics, Université de Montréal, Montreal, Quebec, Canada, 2 Department of Informatics, Université du Québec à Montréal, Montréal, Quebec, Canada, 3 The Centre for Materials Research, Queen Mary University of London, London United Kingdom
Show AbstractWe present results of femtosecond transient absorption and time-resolved photoluminescence spectroscopies to unravel charge photogeneration and recombination dynamics on blends of regioregular poly(3-hexylthiophene) and N,N'-Di-(pent-3-yl)-perylenetertracyvlocarboxylic acid bisimide designed for applications in photovoltaic diodes. We investigate time windows ranging from a few picoseconds to microseconds to explore all the relevant dynamics that limit the efficiency of the device. We examine the dependence on blend microstructure on the carrier dynamics. We find that charge photogeneration occurs from subpicosecond to several picosecond timescales and that charge recombination on nanosecond to microsecond timescales produces singlet and triplet excitons on the polymer. We correlate the kinetics extracted from the spectroscopic measurements with photovoltaic efficiency measurements and discuss the interplay of charge photogeneration and transport efficiencies and their dependence on film microstructure.
9:00 PM - H9.42
Morphological Effects on Exciton Dissociation and Charge Transport in Polymer Blend Photovoltaics.
David Ostrowski 1 , David Vanden Bout 1 2 3
1 Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas, United States, 2 Center for Nano and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas, United States, 3 Texas Materials Institute, The University of Texas at Austin, Austin, Texas, United States
Show AbstractNear-field scanning optical microscopy (NSOM) techniques have been used to analyze polymer photovoltaic devices. Many polymer blend systems phase separate into a complex morphology when deposited from solution. In the system of poly(9,9’-dioctylfluorene-co-bis-N,N’-(4-butylphenyl)-bis-N,N’-phenyl-1,4-phenylenediamine) [PFB] and poly(9,9’-dioctylfluorene-co-benzothiadiazole) [F8BT] there exists phase separation of the polymers on both the micron and nanometer length scales. Data will be presented from time-resolved fluorescence and photoconductivity NSOM measurements to yield insight towards the effect that this complex morphology has on important device processes such as exciton dissociation and charge transport.
9:00 PM - H9.43
Influence of Solvent-Vapor Exposure on the Morphology and Device Performance of Bulk Heterojunction Organic Solar Cells.
Ye Zhang 1 , Debra Mascaro 2
1 Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah, United States, 2 Department of Mechanical Engineering, University of Utah, Salt Lake City, Utah, United States
Show Abstract9:00 PM - H9.44
New Electron-Deficient (N-Type) Semiconducting Polymers.
Shawn Sapp 1 , Silvia Luebben 1
1 , TDA Research, Inc., Wheat Ridge, Colorado, United States
Show AbstractThin-film, organic electronic devices like OLEDs and organic PVs both require at least two semiconducting materials with offsets in their molecular orbital (HUMO-LUMO) energetic levels. In the organic semiconductor world, one can create such an energy offset by forming an interface between an electron-rich (p-type) semiconductor and an electron-poor (n-type) material. It is therefore important to have a wide variety of p-type and n-type materials to choose from. There are a number of available classes of relatively electron-rich, p-type semiconducting molecules and polymers. In contrast, there are few electron-poor, n-type semiconducting molecules, like metalloporphyrins and methanofullerenes. Even rarer are the n-type semiconducting, π-conjugated polymers like cyano-PPVs.
Our group at TDA Research, Inc. has been working to develop and produce new n-type semiconducting, π-conjugated polymers. Our approach is quite similar to what has been done for many years to produce electron-rich, p-doped conducting polymers; we introduce a heteroatom to the π-conjugated backbone that can alter the electron density of the overall polymer. The heteroatom that we add is boron, whose vacant p orbitals are conjugated to the π electronic system of unsaturated repeat units of the polymer. Because of the absence of electrons in the boron p orbitals, the overall π electronic system of the polymer becomes inherently electron deficient and, therefore, the polymer has n-type electronic properties.
Our group has prepared a number of both new and previously reported π-conjugated organoboron polymers and oligomers. Over the past few years we have refined their synthesis and purification, characterized their properties as organic semiconductors, and evaluated their performance in thin film devices. All the prepared organoboron polymers are colored and the majority are strongly photoluminescent in the blue to green region of the visible spectrum. Air-stability has not been fully assessed yet, but preliminary evidence indicates that it varies with the polymer structure.
The electronic band structure of selected organoboron polymers was characterized via ultraviolet photoelectron spectroscopy at Colorado State University. These results conclusively proved that our polymers are in fact n-type semiconductors and that their valence band (VB) resides at a similar energy to the VB of common n-type organic semiconductors including methanofullerenes (PCBM) and cyano-PPV. Additionally, results from photoluminescence quenching experiments carried out at the National Renewable Energy Laboratory, indicated that our polymers efficiently quenched the excited state of a typical p-type semiconductor (MDMO-PPV) with efficiencies up to 83%.
9:00 PM - H9.45
Controlled Surface Area CuPc Thin Films.
Michael Fleischauer 1 , Ryan Tucker 2 , Michael Brett 1 2
1 , NRC National Institute for Nanotechnology, Edmonton, Alberta, Canada, 2 Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta, Canada
Show AbstractOrganic photovoltaic devices incorporating bulk heterojunctions have demonstrated considerable efficiency improvements over planar junction devices (1). Co-deposited small molecule bulk heterojunctions are often limited by a disordered interface, which creates indirect pathways to both electrodes, and leads to reduced efficiencies. Forrest et al. have made considerable progress towards reducing serial resistance in small molecule bulk heterojunctions using the organic vapour-phase deposition (OVPD) technique, which can be used to either produce, and then fill, high surface area CuPc thin films (2), or produce nanocrystalline donor / acceptor domains using multiple donor / acceptor deposition steps (3). Here, we introduce a relatively simple high-vacuum thermal evaporation technique to produce CuPc thin films of similar surface area with a relatively ordered CuPc morphology. The film structure can be engineered to consist of a thin (5-10 nm, or more) planar layer of CuPc followed by ~30 nm dia. ‘bumps’ of independently-adjustable height and density. At a suitably high aspect ratio the film could be considered to consist of vertical CuPc columns. High surface area films of CuPc on ITO and PEDOT:PSS coated ITO will be demonstrated, although the technique is compatible with most materials and substrates suitable for vacuum deposition. Further refinement of the technique is likely possible with substrate temperature control and variable deposition pressure, although neither is required.1) P. Peumans, S. Uchida, and S. R. Forrest, Nature, 425, 158 (2003).2) F. Yang, M. Shtein, and S.R. Forrest, Nature Mat., 4, 37 (2005).3) F. Yang, K. Sun, and S.R. Forrest, APS March Meeting, Denver, CO, P24.7 (2007).
9:00 PM - H9.46
Homeotropic Alignment of Columnar Materials in Open Films for use in Organic Solar Cells.
Ioana Gearba 1 , Charles Black 1 , Ronald Pindak 1
1 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York, United States
Show AbstractMaterials forming 2D columnar structures are promising materials for organic photovoltaic (PV) applications. Recently-demonstrated organic solar cells with efficiencies of 5.7% have a columnar phase active material [1], however the material columns were randomly oriented with respect to device electrodes. We may expect further device efficiency enhancements if the material columns are uniformly oriented with axes perpendicular (homeotropic) to the device electrode surface. For use in PV devices the columnar active material layers should be thin and air-stable in order to facilitate processing of additional device layers. This poses a fabrication challenge as the material columns tend to naturally orient parallel to the columnar phase/air interface.Our recent results [2] have demonstrated one possible solution by incorporating substrates with friction applied polytetrafluoroethylene (PTFE) as an alignment treatment for subsequent columnar material deposition. A difficulty of this technique is achieving large-area defect-free surfaces. We are exploring a promising approach for homeotropic alignment of columnar materials using thin lamellar-phase diblock copolymer films. These materials can uniformly cover large sample areas with a network of a network of 30nm to 40 nm wide tracks using only simple solution processing. A further advantage of these surfaces is that unlike friction-applied PTFE, there is no insulating material between the columnar phase filling the tracks and the device electrode, such that we expect more efficient charge transfer through the device. Moreover, we can modify the diblock copolymer alignment track aspect ratio (width vs height) by plasma etching. We demonstrate the effectiveness of this technique through polarized optical microscopy and X-ray diffraction measurements of alignment of a copper phthalocyanine (CuPc) derivative on such patterned substrates, and we discuss the factors responsible for the homeotropic alignment. References:1.Forrest et al., Appl. Phys. Lett. 85, 5757 (2004).2.Gearba et al. Adv. Mater. 19, 815 (2007).
9:00 PM - H9.47
Effect of Molecular Packing on the Exciton Diffusion Length in Organic Solar Cells.
Seung-Bum Rim 1 , Jan Schoeneboom 2 , Peter Erk 2 , Peter Peumans 1
1 Electrical Engineering, Stanford University, Stanford, California, United States, 2 , BASF Gmbh, Ludwigshafen Germany
Show AbstractThe exciton diffusion length (LD) of organic molecular crystals is a major factor that limits the efficiencies of organic bilayer and nanostructured solar cells. Increased exciton diffusion lengths would allow for thicker bilayer solar cells with improved optical absorption or coarser nanostructures with improved charge transport. However, the molecular parameters that determine LD are still incompletely understood. Here, we show that LD and the efficiency of photocurrent generation in bilayer organic solar cells increase when molecular order is improved. This effect is studied in solar cells using copper phthalocyanine (CuPc) as the donor material and pure cis and trans isomers of the acceptor material 3,4,9,10-perylene tetracarboxylic bisbenzimidazole (PTCBI). X-ray diffraction studies show that the molecular π-π stacking direction of the acceptor lies in the substrate plane for both isomers and that the trans-isomer exhibits improved molecular order in the out-of-plane direction. The improved stacking leads to an increased LD for trans-PTCBI (LD=4.3nm) compared to cis-PTCBI (LD=2.8nm) and the cis-trans mixture (LD=3.0nm). The increase in LD leads to an increase in external quantum and power conversion efficiencies. The increase in LD is confirmed by photoluminescence quenching experiments and by fitting the external quantum efficiency of CuPc/PTCBI cells to model calculations. Based on the crystal structure of trans- and cis-PTCBI, we also calculated the theoretical values of LD using the Complete Active Space Self-Consistent Field method with double zeta basis and compared the results to our experimental observations. The calculated values of LD are ~30,000-fold larger than the experimentally observed values, indicating that extrinsic causes such as exciton traps caused by chemical impurities or structural disorder play a dominant role in determining LD. This data provides concrete evidence that structural disorder is the main limiter to exciton diffusion.
9:00 PM - H9.48
Study of Boron Subphthalocyanine Chloride based Photovoltaic Cells.
Junbo Wu 1 , Seung Rim 2 , Peter Peumans 2
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Electrical Engineering, Stanford University, Stanford, California, United States
Show AbstractThe typical open circuit voltage (VOC) of copper phthalocyanine (CuPc) -based cells (combined with a perylene derivative or C60) is 0.4 to 0.5V at room temperature under 1 sun AM1.5G illumination. Since increasing VOC would lead to reduced efficiency losses, Boron Subphthalocyanine Chloride (SubPc) is an interesting replacement for CuPc because of its favorable energy level alignment. SubPc also exhibits strong absorption in the visible region and is suitable for use in tandem structures due to its relatively narrow absorption spectrum. Optimized SubPc/C60 cell have efficiencies that are nearly double that of comparable CuPc/C60 reference cells, mainly due to an enhancement in VOC from 0.45V to ~1V. If the physics of SubPc can be further elucidated, power conversion efficiencies nearly double that of today’s best devices might be achievable. In this work, we explore combinations of the donor materials CuPc and SubPc with various perylene-based acceptors including 3,4,9,10-perylene tetracarboxylic bisbenzimidazole (PTCBI), 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), and tetrachloro-3,4,9,10-perylene tetracarboxylic diimide (TC-PTCDI), and C60. The dependence of VOC and the exciton dissociation efficiency at the donor-acceptor (DA) interface on the HOMO-LUMO gap at the DA interface will be discussed. Our work indicates that a minimum energy level offset of 0.4-0.5eV is required for sufficiently fast exciton dissociation. The exciton diffusion length (LD) of SubPc was measured by photoluminescence (PL) quenching. The LD was found to depend strongly on crystalline order, varying from 1.5nm to 5nm. Our results also show that the VOC of SubPc-based cells decreases with decreasing thickness of SubPc and with increasing deposition temperature. Intensity-dependent photocurrent, bias-dependent external quantum efficiency and capacitance-voltage measurements were performed to elucidate the main loss mechanisms in SubPc-based cells.
9:00 PM - H9.49
Morphology Control of CuPc and its Application on Organic Solar Cells.
Yu-Sheng Hsiao 1 , Wha-Tzong Whang 1 , Shich-Chang Suen 1 , Jau-Ye Shiu 2
1 Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu Taiwan, 2 , Center of Applied Sciences, Academia Sinica, Taipei Taiwan
Show AbstractOrganic solar cells using copper phthalocyanine (CuPc) as donor layer and fullerene (C60) as acceptor layer were prepared. This research is using self-assemble CuPc molecules on different material surfaces to construct CuPc by thermal evaporator and controlling morphology to increase the diffusion length of excitions in active layer. The morphology of CuPc can be controlled to form highly folded donor-acceptor interfaces with large increase in area. On the other hand, the processing treatments are discussed to minimize the shadow effects between donor-acceptor interfaces.
9:00 PM - H9.5
Carbon Nanotubes Grown Directly on Indium Tin Oxide Coated Glass to Facilitate as a Large Area Electrode for Organic Solar Cells.
Anthony Miller 1 , Ross Hatton 1 , G. Chen 1 , Ravi Silva 1
1 Nano-electronics Centre, Advanced Technology Institute, University Of Surrey, Guildford, Surrey, United Kingdom
Show Abstract9:00 PM - H9.50
Organic Rigid-Rod Sensitizers on Semiconductor Nanoparticles.
Olena Taratula 1 , Elena Galoppini 1 , Piotr Piotrowiak 1 , Gerald Meyer 2
1 Chemistry, Rutgers University, Newark, New Jersey, United States, 2 Chemistry and Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractRigid-rods with organic chromophores (Pyrene and Coumarin 6H) having a oligophenylenethynylene (OPE) rigid linker (linear or branched) were synthesized and bound to MOn (TiO2, ZrO2, and ZnO) nanoparticle thin films through the COOH groups anchoring groups. Influence of the OPE linker length and branching on the photophysical properties of the compounds has been studied in solution and on MOn surfaces. All compounds showed high extinction coefficients and high quantum yields. The nature of the binding of organic sensitizers to MOn was characterized by FTIR-ATR and found to be dependent on the pH pretreatment of the semiconductor surface. Aggregation phenomena of the organic dyes were studied on insulating ZrO2 films through UV-Vis and fluorescence emission measurements (fluorescence emission is not quenched on ZrO2). Photoelectrochemical studies of the pyrene rigid-rods in regenerative solar cells showed near quantitative conversion of absorbed photons into electricity.
9:00 PM - H9.52
Synthesis and Characteristics of Starburst Fullerene-Chromophore Pentaads as Ultrafast Photoresponsive Charge Generators.
Mohamed E. El-Khouly 1 , Robinson Anandakathir 2 , Sea Ho Jeon 2 , Long Y. Chiang 2 , Osamu Ito 1 , Loon-Seng Tan 3
1 Chemisry, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai, Japan, 2 Chemistry, University of Mass. -Lowell, Lowell, Massachusetts, United States, 3 Air Force Research Laboratory, MLBP, Wright-Patterson Air Force Base, Dayton, Ohio, United States
Show AbstractConstruction of starburst C60(DPAF-C9)4 pentads was coupled with the use of highly fluorescent DPAF-C9 addends as donor components in conjunction with the fullerene acceptor during single-photon excitation processes. High quantum yields (ΦCS) of charge-separation processes in a value of 0.83 0.90 for C60(PAF-C9)n(n = 1, 2, or 4) were obtained in the formation of C60(DPAF-C9)(DPAF-C9)'n 1 transient states. Lifetime of the radical ion-pairs (τRIP) was found to be 900 ns for starburst C60(DPAF-C9)4 samples in 6-folds longer than that of the linear analog C60(DPAF-C9) 1, with ca. 2 times increase of the charge-separation rate (kCS) from that of 1. These data implied the important role of sterically hindered DPAF-C9 pendants arranged in starburst-like environment encapsulating the central C60 core on extending the lifetime of the radical ion-pairs. We interpreted the phenomena by the occurrence of intramolecular migration or exchange of electron or positive charge among multiple DPAF-C9 pendents of C60(DPAF-C9)(DPAF-C9)'n 1 giving an increased rate in charge generation and the delay of charge recombination.
9:00 PM - H9.53
Water-splitting Nanodevices from Self-assembled Porphyrin-based Nanostructures.
Zhongchun Wang 1 , Craig Medforth 1 2 , James Miller 1 , John Shelnutt 1 3
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico, United States, 3 Department of Chemistry, University of Georgia, Athens, Georgia, United States
Show Abstract9:00 PM - H9.54
Determination of Nanomaterial Energy Levels for Organic Photovoltaics by Cyclic Voltammetry.
Roberta DiLeo 1 2 3 , Nathan Darling 3 , Annick Anctil 2 3 , Cory Cress 3 4 , Brian Landi 3 , Ryne Raffaelle 1 2 3
1 Physics, Rochester Institute of Technology, Rochester , New York, United States, 2 Material Science and Engineering, Rochester Institute of Technology, Rochester , New York, United States, 3 NanoPower Research Laboratories, Rochester Institute of Technology, Rochester , New York, United States, 4 Microsystems Engineering, Rochester Institute of Technology, Rochester, New York, United States
Show AbstractA wide variety of nanomaterials and associated nanomaterial/polymer composites are being developed in the effort to produce higher efficiency organic solar cells. This development requires a fundamental understanding of the energy levels or band structure of the individual materials and their composites to enable device designs which incorporate appropriate energy level matching. This will allow not only the tuning of the materials to the solar spectrum, but will also enable efficient exciton disassociation, carrier transport, and ultimately carrier collection. Cyclic voltammetry (CV) allows for determination of the energy levels or band edges of these various nanomaterials and composites by measuring their oxidation and reduction potentials. These potentials correspond to a given material’s ionization potential (IP) and electron affinity (EA), respectively. The material’s bandgap (Eg) is represented by the difference in the IP and EA. It has been well established that nanomaterials such as quantum dots (QD) exhibit size-dependent changes in optical bandgap, however, the underlying shifts in the valence band edge (i.e., IP) and conduction band edge (i.e., EA) with respect to the vacuum level are not well established. The results for the EA, IP, and Eg determined by CV for a variety of nanomaterials including derivatized fullerenes (PCBM), cadmium selenide (CdSe), and lead selenide (PbSe) QDs measured in isolation and in conjugated polymers composites with MEH-PPV, P3HT, and MDMO-PPV will be presented. The results for both the CdSe and PbSe QDs show a shift in the IP that is consistent with the change in Eg as a function of size with little or no change in the EA for these materials. These results enable a better understanding of the magnitude of interaction (based upon shifts in the energy levels) and extent of charge transfer that occurs between the polymer and nanomaterials in a device. Finally, CV measurements conducted under dark and illuminated conditions will be used to show the relationship between energy levels within a composite and how they correlate to measured photovoltaic device performance.
9:00 PM - H9.55
Solution-processed Organic Tandem Solar Cells with Imbedded Optical Spacers.
Afshin Hadipour 1 , Bert Boer 1 , Paul Blom 1
1 MEPOS, University of Groningen, Groningen Netherlands
Show AbstractWe demonstrate a solution-processed polymer tandem solar cell in which the two photoactive single cells are separated by an optical spacer. The use of an optical spacer allows for an independent optimization of both the electronic and optical properties of the tandem cell. The optical transmission window of the bottom cell is optimized to match the optical absorption of the top cell by varying the layer thickness of the optical spacer. The two bulk heterojunction sub cells have complementary absorption maxima at λmax ~ 850 nm for the top cell and λmax ~ 550 nm for the bottom cell. The sub cells are electronically coupled in series or in parallel using four electrical contacts. The series configuration leads to an open-circuit voltage of >1 V, which is equal to the sum of both sub cells. The parallel configuration leads to a high short-circuit current of 92 A/m2, which is equal to the sum of both sub cells. The parallel configuration results in a much higher efficiency compared to the series configuration.
9:00 PM - H9.6
Birefringence in Transparent and Conducting Single Walled Carbon Nanotube Thin Film Electrodes and its Effects on the Conversion Efficiency of Organic Photovoltaics.
Giovanni Fanchini 1 , Steve Miller 1 , Bhavin Parekh 1 , Manish Chhowalla 1
1 Materials Science and Engineering, Rutgers University, Piscataway, New Jersey, United States
Show AbstractTransparent and conducting thin films of single walled carbon nanotubes (SWNTs) have been demonstrated to be a suitable alternative to Indium-Tin Oxide (ITO) in organic photovoltaics [1]. For such applications, understanding the optoelectronic properties of transparent and conducting nanotubes networks in the visible-near UV photon energy range is of great relevance. We report measurements of the optical anisotropy and the birefringence of single walled carbon SWNT thin film networks as a function of their density. By combining absorption spectroscopy at different incidence angles and different types of substrates with spectroscopic ellipsometry data, we obtain the real and imaginary parts of the in-plane (//) and out-of-plane (_/_) complex dielectric functions of the films. We observe that in the lowest density films, the two components of the imaginary part of the complex dielectric constant can differ by a factor ε2// / ε2_/_ = 1.5 at 2.25 eV photon energy. The resulting angular dependence of transmittance at incidence angles varying from 0-700 results is reflected in the power conversion efficiency of organic solar cells using SWNT thin films. [1] A. DuPasquier et al. APL 87 (2005) 203511
9:00 PM - H9.7
Tripodal Chromophores with Large Footprint for Aggregation Effect Studies on Metal Oxide Surfaces.
Sujatha Thyagarajan 1 , Elena Galoppini 1 , Jovan Giaimo 2 , Gerald Meyer 2
1 Chemistry, Rutgers University, Newark, New Jersey, United States, 2 Chemistry and Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractTripodal pyrene sensitizers with a oligophenyleneethylene (OPE) spacer and three COOH binding groups in para (1) or meta (2) position were prepared to study the effect of the anchoring group position and the footprint size on the binding and aggregation processes. The photophysical properties of compounds 1 and 2 were studied in solution and bound to metal oxide surfaces. The UV-vis absorption and fluorescence emission spectra of 1 and 2 in solution were red-shifted with respect to pyrene due to the extended π-conjugation. Higher extinction coefficients and near unity quantum yields were also observed. Binding of 1 and 2 to base pre-treated anatase TiO2 films resulted in quenching of the fluorescence indicating efficient electron injection into the semiconductor. A carboxylate bidentate binding mode (FT-IR-ATR) indicates that all the three acid groups are bound to the TiO2 surface. Tripods with large footprint, by design, should isolate the pyrene molecule from the nearest neighbors and avoid the formation of excimers on ZrO2 films (ZrO2 is an insulator and the fluorescence is not quenched). However, we observed excimer emission at 505 nm. This suggests contacts between pyrene chromophores from adjacent nanoparticles and necking regions. The effect of aggregation of pyrene chromophores on ZrO2 films were studied as a function of surface coverage, solvents, temperature, dyeing times and addition of additives.
9:00 PM - H9.8
Comparison Study of Various Blocking-layers at the Interface of Conductive Substrate and Nano-porous TiO2 for Dye Sensitized Solar Cells.
Ho-Gyeong Yun 1 , Yong-Seok Jun 1 , Seung-Yup Lee 1 , Jong-Hyeok Park 1 , Hun-Kyun Pak 1 , Man-Gu Kang 1 , Jong-Dae Kim 1
1 IT Convergence & Components Lab., ETRI, Daejeon Korea (the Republic of)
Show Abstract The efforts to improve the photon-to-electron conversion efficiency of dye sensitized solar cells (DSSCs) have been made over a couple of decades. Blocking layers formed at the interfaces between conductive substrates and nano-porous TiO2 have been regarded as one of the essential components. By decreasing electron/hole recombination happening at the interface between conductive substrates and electrolyte, these blocking layers improve light-to-energy conversion efficiency of DSSCs. There have been a few well known studies for the realization of the efficient blocking-layer. Examples include spin-coating of Ti-isopropoxide solution or immersing conductive substrates in aqueous TiCl4. These methods cover the conductive substrates surfaces and reduce electrons leakage consequently. In this work, considering the uniformity of the blocking-layers, we have developed a different method for a blocking-layer formation of the DSSCs. The substrate has been covered with Ti metal by either electrochemical method or ebeam evaporation. For a comparison, we have also prepared blocking-layers on the conductive substrates by conventional Ti-isopropoxeide spin-coating and immersing conductive substrates in aqueous TiCl4. The characteristics of these blocking-layers have been analyzed using SEM, XPS and XRD. The electrical properties of DSSCs with these various blocking-layers have also been examined with IV measurement and electrochemical impedance spectroscopy (EIS). EIS characterizes the impeding elements at each interface such as Dye/TiO2/substrate with electrolyte, and electrolyte/Pt counter electrode. For the substrates, we have employed metal substrates as well as conducting glass substrates. Metal substrates are one of the best candidates for flexible DSSCs. Thickness of the Ti layers, prepared by ebeam evaporation or electrochemical method, ranges between 50 Å and 500 Å. When the Ti layer is thermally oxidized in the air condition around 500 °C, titanium oxide layer has been completed. Compared with the cells without any blocking-layers, the cells with blocking-layers show efficiency increases by 27% at 200Å thickness. Over this thickness, Ti layer is not fully oxidized and the efficiency decreases accordingly. EIS data are also collected and analyzed to see relations between efficiency and impedance elements. In addition, SEM images are compared to see morphologies of blocking-layers. When the blocking-layer is prepared by ebeam evaporation, its morphology is most homogeneous. However, spin-coating Ti-isopropoxide solution makes neither clear nor even blocking-layer, and TiCl4 treatment is less efficient to prepare. Metal substrates, especially stainless steel, desperately require blocking-layer and its performance improvement ability in the metal substrates will also be presented through IV characteristic and EIS.
9:00 PM - H9.9
Spray Coated Semi Transparent Nano-carbon Counter Electrode for Dye-Sensitized Solar Cell.
Easwaramoorthi Ramasamy 1 2 , Won Jae Lee 1 , Dong Yoon Lee 1 , Jae Sung Song 1
1 Advanced materials and application research division, Korea Electrotechnology Research Institute , Changwon Korea (the Republic of), 2 , University of Science and Technology, Daejeon Korea (the Republic of)
Show Abstract Dye sensitized solar cells(DSSC) appear as potential alternative to expensive solar to electric energy conversion technologies. Although high energy conversion efficiency and good stability was achieved by using Pt as counter electrode, cost and high temperature sintering necessitate the development of low cost and stable electrocatalyst for tri-iodide reduction. In this respect, here we fabricated DSSC with semi-transparent carbon counter electrode. Carbon powders with an average particle size of 40nm were dispersed in alcohol solution and spray coated on FTO glass substrate. Transmittance of the carbon electrode was controlled by varying the solution concentration and spray time. The catalytic performance of carbon electrode was evaluated by measuring the charge transfer resistance (RCT) in symmetric thin layer cell configuration. It was found that RCT of carbon electrode in iodide/tri-iodide redox electrolyte is two times less than the case of Pt electrode. Using such carbon counter electrode and sensitized TiO2 working electrode, DSSCs were fabricated and current-voltage performance was measured in simulated solar light. Under one sun condition (AM1.5, Pin: 100mW.cm-2) carbon counter electrode DSSC shows comparable energy conversion efficiency and stability to the Pt counter electrode based DSSC.
Symposium Organizers
Mike McGehee Stanford University
David Ginger University of Washington
Christiana Honsberg University of Delaware
Jenny Nelson Imperial College London
Jiangeng Xue University of Florida
H10: Organic Solar Cells II
Session Chairs
Wednesday AM, November 28, 2007
Republic B (Sheraton)
9:30 AM - **H10.1
Controlling the Morphology of Small Molecular Weight Organic Thin Film Bulk Heterojunctions Using Organic Vapor Phase Deposition.
Stephen Forrest 1 , Richard Lunt 2 1 , Fan Yang 2 1
1 EECS and Physics, University of Michigan, Ann Arbor, Michigan, United States, 2 EE and Chem Eng., Princeton University, Princeton, New Jersey, United States
Show AbstractOrganic vapor phase deposition has proven to be a useful technique for the growth of small molecular weight thin films. We have found that the presence of a carrier gas during deposition provides significant control over the morphology of the thin films on both the micro- and nanometers length scales. This has led to exploration of the structure in bulk heterojunction solar cells, whereby strain is used to create very large surface area interpenetrating regions of donor and acceptor molecules. By growing nanocrystalline donor/acceptor junctions, an enhancement of the efficiency of CuPc/fullerene cells is increased by a factor of 3 beyond that attainable using a conventional planar junction. We also have found that crystalline layers with very long range order are also achieved using this growth process. Long range order is essential to achieve high mobility layers with long exciton diffusion lengths, both of which are key to the realization of stable, high efficiency organic solar cells. In this talk, we will review the process of OVPD and recent advances in the growth of organic heterostructures for solar energy conversion applications.
10:00 AM - **H10.2
Improved Light Absorption, Exciton Diffusion and Carrier Separation in Small Molecular Weight Organic Solar Cells.
Peter Peumans 1
1 Electrical Engineering, Stanford University, Stanford, California, United States
Show AbstractLight absorption, exciton diffusion and electron-hole pair separation are three processes whose efficiency limits the overall efficiency of organic solar cells. In this talk, I will discuss approaches that render these steps more efficient, resulting in increased overall power conversion efficiencies. Improved light absorption is achieved using generalized light trapping schemes that work either in the wave or geometric optics regime. We have developed practical schemes that, for low-index materials, perform better than Lambertian light trapping. The application of these schemes to thin-film solar cells to obtain higher efficiencies will be discussed. Exciton diffusion was studied using structural, electrical and optical characterization coupled to theoretical models. I will show that exciton diffusion is dominated by structural disorder and that improved molecular order immediately translates into improved device performance. The effect of molecular structure on molecular packing and hence, exciton diffusion, will be discussed and guidelines for small molecule design for optimal exciton diffusion will be provided. Finally, I will show experimentally that geminate pair dissociation at the donor-acceptor interface is a critical problem, especially for small molecular weight organic solar cells. Increased separation efficiencies can be obtained by providing a energy stepping stone at the donor-acceptor interface or by the use of electrical doping to concentrate the built-in potential drop at the donor-acceptor interface.
10:30 AM - **H10.3
Variable Gap Conjugated, Organometallic and Hyperbranched Polymers in Hybrid Photovoltaic Devices.
John Reynolds 1 , Kirk Schanze 1 , Hui Jiang 1 , Young-Gi Kim 1 , Jianguo Mei 1 , Katsu Ogawa 1 , Qiquan Qiao 1 , Prasad Taranekar 1
1 Chemistry, Univ. of Florida, Gainesville, Florida, United States
Show AbstractSuccessful development of organic photovoltaic materials requires optimization of a combination of optoelectronic, physical and interfacial properties. By controlling the energy levels of the HOMO and LUMO states, along with the magnitude of the electronic band gap, the properties of a series of light absorbing and electron donating variable gap conjugated organic and platinum acetylide organometallic polymers, along with linear and hyperbranched conjugated polyelectrolytes, have been developed for electron transfer to PCBM and TiO2 acceptors. Understanding the detailed mechanisms of exciton lifetime and diffusion lengths, charge carrier creation efficiencies, back electron transfer to annihilate the charge separated state, and complete charge separation to allow high photocurrents has been the focus of an effort utilizing a new class of thienylene linked platinum-acetylide (p-PtTh) organometallic polymers which effectively form long-lived triplet states. We have demonstrated photovoltaic cells that contain p-PtTh and PCBM and by broadening the spectral absorption using donor-acceptor chromophores their solar efficiencies are greatly enhanced. Cationically and anionically charged hyperbranched conjugated polyelectrolytes (HB-CPEs) have been synthesized via the Heck coupling method using an A3+B2 type approach. TiO2 hybrid solar cells have been fabricated with the HB-CPEs as self-assembled sensitizers with bilayer films showing higher overall efficiencies when compared to their respective monolayers.
11:30 AM - **H10.4
Recent Advances in Organic Photovoltaic Cells and Integrated Modules for Portable Power.
Bernard Kippelen 1
1 School of ECE, Georgia Tech, Atlanta, Georgia, United States
Show AbstractIn recent years, organic solid-state photovoltaic cells (OPV) have emerged. Electrical energy provided by paper-thin solar cells will be essential to power the wide range of new portable devices and circuits including radio-frequency identification tags (RFID), and smart sensors that are used for environmental and structural monitoring. The strong excitonic nature of excited states in organics generally limits the photovoltaic conversion efficiency of multilayer organic solar cells because exciton diffusion lengths are generally short and smaller than the optical penetration depth needed to absorb all incident radiation. Recently, we showed that pentacene combined with fullerenes in multilayer geometries can result in an efficient carrier generation with external quantum efficiencies as high as 68% due to large exciton diffusion lengths in pentacene. In this talk, we will report on encapsulation of these solar cells that resulted in degradations of less than 6% of their initial performance after 6,000 hours. Most applications require specific operating electrical specifications that can be met by connecting multiple solar cells in parallel and in series and to integrate them into modules. In this talk, we will also discuss the performance of integrated organic photovoltaic (OPV) modules with scalable open-circuit voltage fabricated from blends of poly(3-hexylthiophene) (P3HT) and a soluble C70 derivative, [6,6]-phenyl C71 butyric acid methyl ester (PCBM-70).
12:00 PM - H10.5
Control of Electric Field Strength and Orientation at the Donor-Acceptor Interface in Organic Solar Cells.
Albert Liu 1 , Shanbin Zhao 1 , Seung Rim 2 , Martin Konemann 3 , Peter Erk 3 , Peter Peumans 2
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Electrical Engineering, Stanford University, Stanford, California, United States, 3 , BASF Gmbh, Ludwigshafen Germany
Show AbstractMany small molecular weight materials are unintentionally doped by byproducts of the synthesis process that are hard or impossible to remove. The presence of such unintentional doping controls the built-in electric field at the donor-acceptor interface in bilayer organic solar cells and has an effect on device performance that dominates other material parameters such as purity and band offset. Using capacitance-voltage (C-V) profiling and current-voltage (I-V) measurements matched to electrical modeling, we demonstrate that a broad range of donor-acceptor pairs used successfully to build bilayer organic solar cells (e.g. CuPc/C60, CuPc/PTCBI, and others) are unintentionally electrically doped such that the built-in electric field is concentrated at the donor-acceptor interface, assisting geminate charge pair separation. We propose that this is an essential feature of efficient small molecular weight organic solar cells that must be considered in device design. Using mass spectroscopy, C-V profiling and I-V measurements, we show that in the case of the acceptor material N,N’-bis(2-phenylethyl)perylene-3,4:9:10-bis-(dicarboximide) (BPE-PTCDI), the unintentional doping can be removed by repeated thermal gradient purification. Surprisingly, we find that when the unintentional electrical doping is removed from the acceptor, device performance is impacted negatively. The introduction of intentional n-type doping is required for a favorable orientation of the electric field at the donor-acceptor interface to assist geminate electron-hole pair separation. We also show that device lifetime is mainly impacted by a gradual removal of electrically active doping sites, and that the exciton diffusion length is negatively impacted by the presence of doping. Using engineered doping profiles in the acceptor layer, we have fabricated devices that exhibit improved lifetimes and improved exciton diffusion lengths. We will discuss the consequences of our findings on the efficiency prospects of organic solar cells.
12:15 PM - H10.6
Planar Heterojunction Photovoltaics Consisting of a Printed Colloidal Quantum Dot Film and a Small Molecule Hole Transport Layer.
Alexi Arango 1 , Matt Panzer 1 , Vladimir Bulovic 1 , David Oertel 2 , Scott Geyer 2 , Moungi Bawendi 2
1 EECS, MIT, Cambridge, Massachusetts, United States, 2 Chemistry, MIT, Cambridge, Massachusetts, United States
Show AbstractA planar heterojunction between cadmium selenide (CdSe) colloidal quantum dots (QDs) and a thermally evaporated N,N'-Bis(3-methylphenyl)-N,N'-bis-(phenyl)-9,9-spiro-bifluorene (spiro-TPD) transparent hole transport layer is achieved by printing the CdSe layer onto spiro-TPD with an elastomeric stamp. In a photovoltaic structure, the planar interface facilitates charge separation and promotes directed charge diffusion toward the electrodes, resulting in a high open circuit voltage (0.8V) and a high built potential (1.3V). We show that charge generated within the CdSe film is subject to transport losses due to the insulating QD capping ligands and, therefore, the internal quantum efficiency reaches a peak value of 17% at a film thickness of roughly two monolayers.
12:30 PM - H10.7
Nanostructured Organic Heterojunctions for Photovoltaic Applications: An Integrated Experimental and Computational Study.
Ying Zheng 1 , Sharon Pregler 1 , Susan Sinnott 1 , Jiangeng Xue 1
1 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractIt is widely accepted that a nanoscale percolation of donor and acceptor phase can significantly enhance the efficiency of organic photovoltaic (OPV) cells due to the creation of a spatially distributed interface, which enhances exciton dissociation, and the presence of continuous conducting paths for efficient charge collection.[1,2] However, ideal nanoscale percolation is not readily achievable and experimental characterization and theoretical description of its formation is challenging. Here, classical molecular dynamics simulations are used to examine the phase separation process of a donor-acceptor molecular mixture consisting of pentacene donors and C60 acceptors. In particular, phase separation with different starting conditions is considered. At the same time, experiments are conducted to examine the validity of simulation results to determine the photovoltaic properties in actual OPV devices. Simulation results show that the pentacene and C60 molecules maintain a certain degree of aggregation in mixtures, and that the system temperature and the initial molecular arrangements of two species of molecules strongly influence the aggregation process. X-ray diffraction patterns of the mixed film grown by vacuum thermal evaporation show that stronger phase separation can be induced by increasing the substrate temperature during deposition. Scanning electron microscope and atomic force microscope images reveal the changing of surface morphology of the mixed films during the deposition process. These results suggest that the degree of phase separation of molecular mixtures can be controlled by varying the process conditions, which may lead to new pathways to generate nanoscale percolation for application in OPV cells. [1]P. Peumans et al., Nature 425, 158 (2003).[2]J. Xue et al., J. Appl. Phys. 98, 124903 (2005).
H11/F7: Joint Session: Interfacial Issues in Organic Photovoltaics
Session Chairs
Wednesday PM, November 28, 2007
Back Bay C (Sheraton)
2:30 PM - **H11.1/F7.1
Charge Photogeneration at Nanostructured Organic and Organic/inorganic Interfaces.
James Durrant 1
1 , Imperial College London, London United Kingdom
Show AbstractExcitonic solar cells, photovoltaic devices based on molecular or polymer light absorbers, are attracting increasing academic and commercial interest. The function of such solar cells is typically based upon electron transfer dynamics across donor / acceptor interfaces. Such interfaces are typically highly reticulated on the nanometer length scale, enabling the high interfacial surface area necessary for efficient exciton dissociation. In my talk I will discuss a range of different approaches to achieving such interfaces, including nanoparticle / polymer; dye sensitised heterojunctions and polymer / small molecule blend films, and will emphasis the similarities and differences of these approaches in terms of interface function. My talk will focus on the ability of such interfaces to dissociate molecular excited states, or excitons, into long lived charge separated species, and their utilisation for photovoltaic energy conversion.Issues addressed in my lecture will include: A comparison of organic / organic and organic / inorganic interfaces; Materials and molecular approaches to multilayer interfaces designed to achieve efficient charge generation and minimise recombination losses; The role of interface dipoles and surface charge; Coulombic attractions resulting in the formation of interfacial bound radical pair states.
3:00 PM - H11.2/F7.2
Effect of Surface Modification on the Polymer Disorder at the P3HT/ZnO Interface.
Dana Olson 1 , Erik Spoerke 1 , Yun-Ju Lee 1 , Matthew Lloyd 1 , Todd Alam 1 , Nolanne Chang 1 , David Wheeler 1 , Cary Allen 2 , Darick Baker 2 , Thomas Furtak 2 , Rueben Collins 2 , James Voigt 1 , Julia Hsu 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 Physics, Colorado School of Mines, Golden, Colorado, United States
Show AbstractHybrid conjugated polymer/metal oxide photovoltaic devices offer a low-cost alternative to current inorganic PV technologies through the use of solution processed organic and inorganic composite materials. In nanostructured poly(3-hexylthiophene) (P3HT)/ zinc oxide (ZnO) solar cells, an electron is transferred from P3HT to ZnO upon photoexcitation. We found that the P3HT layer at the ZnO interface is disordered, as evident by a blue shift (up to 55 nm) in the absorption spectra. A blue shift indicates a disruption in the conjugation length in the polymer chains, which leads to an increased band gap and degrades the transport properties of P3HT. This effect depends on the substrate on which the P3HT was deposited, the solvent of the P3HT solution, the solution concentration, and subsequent annealing and cooling processes. We determined that P3HT films less than 10 nm thick are disordered when deposited on solution-based ZnO films or nanorods. Annealing the composite P3HT/ZnO films further increases the disorder. The blue shift has not been observed on single crystal ZnO or a variety of other oxide substrates, and it is believed to be the result of defects at the surface of the solution deposited ZnO.To understand the origin of this disorder, the P3HT films are characterized by optical absorption, photoluminescence, and solid-state nuclear magnetic resonance. Additionally, modification of the P3HT/ZnO interface using molecular monolayers is observed to reduce or eliminate the blue shift of the polymer. Finally, the effects of such modifiers on the optical and morphological properties of the P3HT and the hybrid device performance have been studied.The authors would like to thank the IC Post Doctoral Fellowship for partial funding this research. A portion of this material is based in part upon work supported by the National Science Foundation under Grant No. DMR 0606054. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
3:15 PM - H11.3/F7.3
The Role of Bottom Contact Electrical Uniformity at the Nanometer Scale in the Performance of Organic Solar Cells.
Peter Veneman 1 , Neal Armstrong 1 , Michael Brumbach 1
1 Chemistry, University of Arizona, Tucson, Arizona, United States
Show AbstractNon-uniformity in the electrical properties of TCO bottom contacts in organic solar cells affects the overall performance of these devices, by increasing recombination rates at the donor/acceptor interface. We have previously shown that the surface of Indium-Tin Oxide (ITO) is not chemically or electronically uniform at the nanometer length scale. Conducting-Tip Atomic Force Microscopy (C-AFM) showed the magnitude of current injected from ITO into Copper Phthalocyanine (CuPc) and Titanyl Phtlalocyanine (TiOPc) can vary drastically for different areas within nanometers of each other on a given sample. This nanoscopic electrical non-uniformity is observable in millimeter scale OPV devices as an area dependence on device performance, increased diode quality factors and a decrease in the repeatability of production of these devices. The standard equivalent circuit model for Organic Photovoltaics (OPVs) is a diode with parallel (Rp)and series (Rs) resistors. The diode component arises because of the energy band offset at the Donor-Acceptor (D-A) interface. The series resistance is due to charge transfer resistance at electrodes as well as the bulk resistance of electrodes and semiconductor materials. The parallel resistance is due to shunts in the device. This model can be modified by the addition of a second diode, parallel to the first, that is operative in the light. We assert that this extra diode is a perturbation of the initial diode’s dark behavior due to an increase in the free carrier concentrations at the Donor-Acceptor (D-A) interface caused by the splitting of excitons at that interface when the device is under illumination. We find that ITO interface composition and activation procedure and Pc film thickness control the magnitude of these extra recombination currents.
3:30 PM - H11.4/F7.4
Electronic Structure at the Interface Between Bathocuproine and Metal Electrode.
Susumu Toyoshima 1 2 , Takeaki Sakurai 1 , Taima Tetsuya 2 , Hiroo Kato 3 , Kazuhiro Saito 2 , Katsuhiro Akimoto 1
1 Institute of Applied Physics, University of Tsukuba, Tsukuba Japan, 2 Recerch Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Japan, 3 Faculty of Science and Technology , Hirosaki University, Aomori Japan
Show AbstractThe electrical properties of the organic/metal interface are sensitive to the performance of organic devices, so it is important to understand and control their properties. In organic photovoltaic cells composed of Phthalocyanine and C60, bathocuproine (BCP) is used as the buffer layer between C60 and metal electrode to improve the cell efficiency[1]. The highest occupied molecular orbital (HOMO) level of the BCP is situated at 6.5 eV from the vacuum level and the energy difference between HOMO level and the lowest unoccupied molecular orbital (LUMO) level is ~3.5 eV. The role of BCP, however, on the performance of photovoltaic cells has not been clearly understood yet. In this work, we studied the electronic structure at the interface between BCP and various kinds of metal (K, Ca, Mg, Al, Ag, Cu, Au) by ultraviolet photoemission spectroscopy (UPS). The various kinds of metal were deposited on Au/Si substrates and then BCP was deposited in a vacuum chamber. The specimens were transferred to the UPS measurements chamber without exposing air. The UPS measurements were carried out with the photon energy of 21.2 eV. It was found that the energy difference between the HOMO of BCP and the Fermi level of metal was almost constant with 3.7 eV for the case of K, Ca, Mg, Al, Ag, whose work function were relatively low. Considering the HOMO-LUMO energy difference in BCP, the energy position of the LUMO level almost accords with the Fermi level of these metals. Further, we observed new peaks at around 1 eV below the Fermi level which may be due to the formation of the interface states. These results suggest that electrons can through from LUMO of BCP to metal via interface state, that is, the barrier height for electrons is lowered by the interface state. For the specimens of BCP deposited on Cu, Au, whose work function are relatively high, the position of the HOMO level from the Fermi level was varied depending on the work function of the metals, suggesting the increase of the barrier height at the interface. No new peak was observed for the case of BCP on Cu and Au. We fabricated solar cells, whose structure is ITO/PEDOT/ZnPc/ZnPc:C60/C60/BCP/metal, with varying metal electrode. The performance of the cells strongly depended on the Fermi level of the electrode metal and higher efficiency was obtained using metal electrode with lower work function. From these results, it is considered that the interaction between BCP and metal depends on the work function of metal, and the close position of the LUMO level to the Fermi level is important to make an interaction between BCP and metal. The interface states induced by the interaction may play an important role in the electrical properties at the interface. [1] P. Peumans and S. R. Forrest, Appl. Phys. Lett. 79, 126 (2001).
3:45 PM - H11.5/F7.5
Interelectrode Morphology of Bulk Heterojunction Photovoltaic Devices as Revealed in-situ by Grazing-incidence X-ray Scattering.
Brian Pate 1 2 , Michael Durstock 1
1 Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio, United States, 2 Engineering Division, Universal Technology Corporation, Dayton, Ohio, United States
Show AbstractInterfacial structure is a critical determinant of most if not all of the processes underlying the energy conversion exhibited by bulk heterojunction photovoltaic devices. However, detailed morphological characterization of these devices has mostly been limited to the areas peripheral to the interelectrode (i.e. functional) space. Recently, grazing-incidence X-ray scattering has been employed to characterize the interelectrode morphology of poly(3-hexylthiophene) (P3HT) and its blends with 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM) within simulated photovoltaic device environments. These studies and coupled microscopic investigations reveal a systematic dependence of phase behavior and structural anisotropy on interfacial composition, blend ratio, annealing conditions, and applied fields.
4:00 PM - H11/F7Interfaces
BREAK
H12: Device Issues: Light Trapping, Electrodes, Device Structures
Session Chairs
Wednesday PM, November 28, 2007
Republic B (Sheraton)
4:30 PM - H12.1
Multiple Bandgaps in Reflective Polymer Solar Cells.
Kristofer Tvingstedt 1 2 , Viktor Andersson 1 2 , Fengling Zhang 1 2 , Olle Inganas 1 2
1 Biomolecular and organic electronics, IFM, Linkoping University, Linkoping Sweden, 2 Center of Organic Electronics, Linköping University, Linköping Sweden
Show AbstractAlternating copolymers of polyfluorene (APFO’s) are promising materials for the production of inexpensive and flexible polymer photovoltaic cells. With molecules design of different APFOs we can tune the absorption within a large spectral range, from 400-1000 nm. This enables the construction of tandem photovoltaic cells consisting of several absorbers. We here demonstrate reflective tandem cells, where single cells are reflecting the non absorbed light upon another adjacent cell. Such a photovoltaic reflective tandem system is constructed by folding two planar cells towards each other, forming a V. The folded structures with cells of different bandgap materials on each side give a new manufacturing compromise. They can be printed on a flat flexible surface, side by side, and the plastic support may subsequently be buckled under compression to form the V geometries. Reflective tandem cell exploiting one, two or more different conjugated polyfluorenes blended with fullerenes as the active layers are demonstrated and characterized. Blends with a low bandgap on one side are complemented with blends with a high bandgap on the other side. An improvement of the overall solar light absorption is demonstrated, well correlating with the sum of the absorbing materials. Light trapping leads to improved power conversion efficiency by a factor of 1.8.
4:45 PM - H12.2
An Effective Light Trapping Configuration for Thin-film Solar Cells.
Seung-Bum Rim 1 , Shanbin Zhao 2 , Shawn Scully 2 , Michael McGehee 2 , Peter Peumans 1
1 Electrical Engineering, Stanford University, Stanford, California, United States, 2 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractMany thin-film solar cells make a compromise between achieving complete optical absorption using films that are thicker than the optical absorption length and achieving efficient conversion of the absorbed photons into current, which is favored in thinner structures. This is especially the case for organic solar cells, where exciton diffusion takes place over distances much shorter than the optical absorption length. Light trapping techniques such as surface texturing increase the optical pathlength manifold and can be used to enhance the optical absorption for a given film thickness. We analyze a simple V-shaped light-trapping structure that does not require modification of the active layers and achieves significant absorption enhancement. We show that the optical pathlength increase using the V-shaped light trapping scheme is larger than that achieved using Lambertian light trapping [1] for low refractive index (n<2) materials. Moreover, this light-trapping geometry lends itself to low-cost fabrication techniques. Using optical modeling, we show that the V -fold configuration substantially increases the optical path length for all angles of incidence and evaluate its performance for a broad class of thin-film solar cells, including small molecular weight, polymer-blend and thin-film silicon solar cells. Experiments using copper phthalocyanine (CuPc)/3,4,9,10-perylene tetracarboxylic bisbenzimidazole (PTCBI) organic solar cells show that the power conversion efficiency can be improved by 24% using a V-shaped light trap. The performance of spin-coated poly-(3-hexylthiophene) (P3HT)/[6,6]-phenyl C60 butyric acid methyl ester (PCBM) blend solar cells was increased by 53% using this approach. The improvement in power conversion efficiency and photocurrent is chiefly attributable to a broadening of the optical absorption into regions where the optical absorption is weak. For optimized film thicknesses, this approach affords a 2.6-fold improvement compared to the best planar cells for CuPc/PTCBI cells, and a 27% increase over optimized P3HT:PCBM cells. We therefore anticipate that application of this geometry to today’s most efficient organic solar cells will lead to power conversion efficiencies approaching 10%.[1] E. Yablonovitch and G. D. Cody, IEEE Trans. Electron Devices ED-29,300 (1982)
5:00 PM - H12.3
Scattering-Enhanced Optical Absorption and Energy Conversion Efficiency of Dye-Sensitized ZnO Solar Cells.
Qifeng Zhang 1 , Tammy Chou 1 , Bryan Russo 1 , Samson Jenekhe 2 , Guozhong Cao 1
1 Department of Materials Science and Engineering, University of Washington, Seattle, Washington, United States, 2 Department of Chemical Engineering, University of Washington, Seattle, Washington, United States
Show AbstractMultiple scattering of light in disordered medium has the capability of extending the traveling distance or causing the Anderson confinement of light, and accordingly can be of benefit to the photon capturing efficiency of photoelectrode films as well as the performance of dye-sensitized solar cells. ZnO has the advantage in crystallization and can easily grow to various nanostructures that may result in strong scattering to the light. We report a kind of hierarchically-structured ZnO films as the photoelectrode of dye-sensitized solar cells to joint both the proper structure for light scattering and a porous structure for dye adsorption. The films consist of submicron-sized ZnO aggregates synthesized via the hydrolysis of zinc acetate in diethylene glycol, and a further SEM analysis reveals that the aggregates are formed by nano-sized crystallites with a closely-packed porous structure. The maximal overall energy conversion efficiency as high as 4.6% has been achieved from the photoelectrode film that contains polydisperse ZnO aggregates with the diameter ranging from 200 nm to 360 nm. On account of the relatively large refractive index of ZnO and the fact that the dimension size of aggregates is compared to the wavelength of incident light, we demonstrate that the spherical ZnO aggregates are efficient scatters that can arouse an extremely strong scattering to the light in visible spectrum, and more photons have the probability to be absorbed due to the increased traveling distance of light in the films. That means an enhanced ability of photon harvest as well as optical absorption due to the existence of submicron-sized ZnO aggregates in the films. Meanwhile, the porous structure of ZnO aggregates with nanocrystallites provides a huge internal surface area for the photoelectrode films to adsorb sufficient dye molecules, which is essential for the dye-sensitized solar cells to obtain a considerable current density as well as a high overall energy conversion efficiency. A theoretical simulation based on Mie theory about light scattering by spherical particle was also carried out to support our experimental results. It is anticipated that the enhancement of conversion efficiency through controlled aggregation of nanocrystallites in photoelectrodes would be equally applicable to other semiconductors in dye-sensitized solar cells.
5:15 PM - H12.4
Distance Dependence of Plasmonic Enhancement in Organic Bulk Heterojunction Photovoltaics.
Anthony Morfa 1 2 , Thomas Reilly 2 , Jao van de Lagemaat 2 , Kathy Rowlen 1
1 , University of Colorado, Boulder, Boulder, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractPlasmon-active materials have been used to enhance optical processes including second harmonic generation, surface-enhanced Raman scattering and infrared absorption. These optical processes are enhanced through an increase in the local electromagnetic field at the surface of the surface plasmon active material. Recently, plasmonic enhancements to silicon and small molecule photovoltaic devices have been demonstrated. The electromagnetic field enhancement decays rapidly with distance allowing device performance and local field enhancement to be tuned within a device. We investigated a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) and the distance dependence on enhanced light absorption from plasmon active silver thin films within this blend. By varying the distance between the enhancing layer and the absorbing blend with a layer of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), we were able to vary the amount of enhancement within a device. Photovoltaic conversion efficiency was monitored using internal photon to current conversion efficiency (IPCE), current-voltage measurements as well as UV-visible absorption measurements. We observed that the plasmonic enhancement to conversion efficiencies within an organic bulk heterojunction device decreased with increasing distance from the plasmon-active layer. The relationship between absorption within the spacer layer and thickness was also explored. These studies demonstrate that plasmonic enhancement within a bulk heterojunction device can be successfully controlled.
5:30 PM - H12.5
Surface Plasmon Polariton Mediated Energy Transfer in Organic Photovoltaic Devices.
Timothy Heidel 1 , Jonathan Mapel 1 , Madhusudan Singh 1 , Kemal Celebi 2 , Marc Baldo 1
1 Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractWe enhance the optical absorption of organic photovoltaics (PVs) by fabricating a resonant cavity light-absorbing antenna external to the charge generating PV layers. Resonant antennas are coupled to phthalocyanine-based PV cells, which exhibit a gap in their absorption spectra between the Q and Soret bands. Light absorbed in a rubrene-based antenna is coupled to the PV, using energy transfer via surface plasmon polaritons (SPPs) and radiation into waveguide modes. SPPs are a particularly effective energy transfer mechanism as they propagate in the plane of the PV rather than parallel to the incident radiation. In addition, the SPP mode extends deeply into both dielectric layers, extending the range of energy transfer up to ~ 100 nm. Measurements of the energy transfer efficiency yield (25±10)%, consistent with modeling.[1] To reduce the uncertainties in the measurement of energy transfer, we fabricate an organic superlattice photodetector and antenna without the resonant cavity. This structure also enhances the energy transfer since it allows thicker phthalocyanine layers while maintaining a high internal quantum efficiency, thereby increasing the absorption of SPPs in the charge generating layers. We measure the energy transfer efficiency in this device to be (51±10)%.While the introduction of the antenna necessarily adds a step into the energy transduction process, it can be successfully employed in spectral regions where the absorption fraction of the PV cell drops below the SPP-mediated energy transfer efficiency. Thus, targeting resonant antennas to regions of poor absorption and employing SPP-mediated energy transfer, promises to solve a characteristic deficiency of organic PVs. [1] K. Celebi, T.D. Heidel, and M.A. Baldo, "Simplified calculation of dipole energy transport in a multilayer stack using dyadic Green's functions," Optics Express 15 (4), 1762-1772 (2007).
H13: Poster Session: Nanostructured Solar Cells II
Session Chairs
Thursday AM, November 29, 2007
Exhibition Hall D (Hynes)
9:00 PM - H13.1
III-V Semiconductor Vertical and Tilted Nanowires on Silicon Using Chemical Beam Epitaxy.
Gokul Radhakrishnan 1 2 , Alexandre Freundlich 1 2 3 , Joe Charlson 2 , Bodo Fuhrmann 4
1 Photovoltaics and Nanostructures Laboratories at the Center for Advanced Materials, University of Houston, Houston, Texas, United States, 2 Electrical and Computer Engineering , University of Houston, Houston, Texas, United States, 3 Physics Department, University of Houston, Houston, Texas, United States, 4 Interdisciplinary Center of Materials Science, Martin Luther University of Halle, Halle Germany
Show AbstractNowadays nanostructures play a vital part in the rapidly expanding areas of photovoltaics. The ability of nanowires to transfer photo-generated carriers rapidly across a solar cell has lead our interest in growth of nanowires [1].Currently Vapor Liquid Solid (VLS) epitaxy is the most common method used to grow epitaxial vertical nanowires. A metal particle such as gold is used to form a liquid alloy eutectic with the material of a substrate or with material supplied in the vapor phase. In growing semiconductor wires using metal droplets, it has been shown that the wires grow in the (111) direction and have clean facets [2, 3]. Furthermore these wires generally present an undesirable larger pyramidal base at the bottom [4] and there is also evidence of surface migration of the metal catalyst [5]. Thus far, most of the effort in the development of vertical III-V semiconductor nanowires has been limited to homo-polar combinations (e.g. InAs on InP). The ability to fabricate III-V nanowires on silicon could however pave the way toward the monolithic integration of III-V nanostructured solar cells with Si. Here we demonstrate the growth of GaAs and InP nanowires on silicon (111) using gold as the metal seed particle. An ordered array of gold nano dots is integrated on the surface of a silicon substrate using self-assembled polystyrene nanospheres as the Au evaporation template [6]. The size of the gold dots range from 40 nm to 150 nm and the pitch is about 500 nm. The growth of the wires is done by chemical beam epitaxy under a vapour phase environment. Scanning electron microscopy, photoluminescence and Raman spectroscopy are used to characterize these nanowires. Wire exhibit high crystallinity and there is an absence of the pyramidal base at the bottom of the nanowire using this technique. Furthermore the study also shows evidence of pregrowth motion of some of the gold particles causing coalescence of nanowires and leading to the development of nanopods and tilted (off-normal) nanaowires. Finally in the light of their optical properties the relevance of these wires to photovoltaic applications is discussed.References:[1] A. Freundlich, A.Alemu, S. Bailey, Proc. 31st IEEE PVSC, ISBN: 0-7803-8708-2 (2005) p137[2]Wagner, R. S. & Ellis. Appl Phys Lett Vol 4 89-90 (1964)[3] Linus E. Jensen et al Nanoletters – Vol 4 (2004) p 1961[4] A.I. Persson et al, Journal of Crystal Growth - Vol 272 (2004) p167–174[5] J. B. Hannon et al Nature – Vol 440, (2 March 2006), p 69[6] B. Fuhrmann et al , Nanoletters – Vol 5 (2005) p 2524
9:00 PM - H13.10
Sensitization of TiO2 Nanoparticle Films by Chemical-bath Deposited CdS Nano-domains Studied by Time-Resolved Microwave Conductivity.
Nikos Kopidakis 1 , Jorge Piris 1 , Andrew Ferguson 1 , Garry Rumbles 1 , Don Selmarten 1
1 , National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractQuantum-confined CdS domains were grown as inorganic sensitizers onto TiO2 and ZrO2 nanoparticle films for solar cell applications. The domains were grown using the Successive Ionic Layer Adsorption and Reaction (SILAR) technique. The advantage of SILAR deposition is that CdS grows directly onto the oxide, without a capping group that may obstruct electronic communication between the two materials. Previous work based on optically probing the excited state of CdS provided compelling evidence for charge injection from CdS to TiO2 in a time window extending to 300 ps. In the work presented here we use optical excitation and a microwave probe to investigate the properties of photogenerated carriers in CdS, TiO2 and ZrO2. By choosing the excitation wavelength, we can selectively excite primarily the oxide or the CdS nanocrystals on the surface. We show that charge injection from CdS to TiO2 occurs, but not from CdS to ZrO2. This observation allows us to use the ZrO2/CdS film to study the photoconducting properties of isolated CdS domains and infer on the electron and hole contributions to the photoconductivity. Measurements on TiO2/CdS show that the photoresponse extends to wavelengths over 500 nm, following the absorption curve of the CdS sensitizer. Notably, the CdS nanocrystals do not appear to have an effect on surface trapping of carriers in TiO2. Analysis of the time-dependent photoconductivity signals from TiO2/CdS films allows us to estimate the charge injection efficiency from CdS to TiO2.
9:00 PM - H13.11
Growth of Cadmium Telluride Nanowires via Electrodeposition on Porous Nano Templates.
Subarna Banerjee 1 , Susanta Mohapatra 1 , Glenn Sklar 1 , Matthew Savage 1 , Jeffrey LaCombe 1
1 Chemical and Metallurgical Engineering Department, University of Nevada, Reno, Reno, Nevada, United States
Show AbstractCadmium telluride, with its direct bandgap of 1.4 eV, is a potentially suitable material for use in photovoltaic applications. We report here on CdTe nanowires arrays grown into specially-prepared nanoporous templates using electrodeposition technique. A combined electrochemical and autocatalytic processing scheme is presented which results in CdTe nanowires growing out of pores in a porous oxide template. The template pores, after formation through anodization, serve as sites into which Cd and Te are electrochemically codeposited from a precursor solution, using electrodeposition parameters that are tuned to yield the desired stoichiometry. The template materials investigated in this work include anodic aluminum oxide (AAO) and titanium oxide (ATO). The template preparation steps are performed with the goal of producing templates that are effectively “seeded” for uniform growth of CdTe nanowires. It is desirable to have templates that fill evenly during CdTe nanowire growth, in order to avoid overfilling of some pores (inconsistent nanowire structures were found to readily occur in poorly-controlled experiments). In the reported experiments, we describe CdTe nanowires of ~70nm diameter, although the nanowire dimensions can be controlled. Nanowire array length is primarily controlled by electrodeposition time (with limits). The overall array size can (in principle) be scaled up to industrial scales using well-understood anodizing techniques. In addition to our preparation approach, we will also present here data characterizing the electrodeposition process as well as the template nanostructures and the CdTe nanowires. Studies have also been carried out on the photovoltaic properties of the CdTe nanowires.
9:00 PM - H13.12
Self-Assembly of Cadmium Telluride Nanoparticles into Nanowires for Optoelectronic Devices.
Kevin Critchley 1 2 , Nicholas Kotov 1
1 Chemical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Physics and Astronomy, University of Leeds, Leeds United Kingdom
Show Abstract9:00 PM - H13.13
Spray-casting of Semiconductor Nanorod Thin Films for Photovoltaic Devices.
Artjay Javier 1 , Edward Foos 1
1 Chemistry, Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractSpray-casting organic-soluble semiconductor nanomaterials is a fast and simple route for electronic component preparation, particularly photovoltaic devices. Nanorods provide the dual benefit of strong absorption (quantum confinement) and enhanced charge separation (where the long axis dimension exceeds the exciton radius). Thin, optically transparent films (~50-1000nm thick) were prepared by spray-casting chemically synthesized nanorods of CdTe and CdSe (~4/1 aspect ratio) onto ITO-coated slides using a commercially available airbrush. The top-contact electrode (conductive Ag paint) was deposited in a similar fashion, resulting in a device fabricated entirely through spray techniques. Adjustment of spray-casting conditions controls variation in smoothness, ranging from spin-coated quality (~40nm FWHM roughness) to closely spaced droplet features (~10μm wide, ~400nm FWHM roughness). The conductivity and morphology have been analyzed using voltammetry, profilometry, SEM and solid-state UV-Vis absorbance. A preliminary photoresponse of ~3 μA/cm2 (1-sun illumination) was observed for the typical bilayer cell: ITO | CdTe || CdSe | Ag.
9:00 PM - H13.14
Towards Improved Photovoltaic Conversion using Dilute Magnetic Semiconductors.
Jean-Francois Guillemoles 2 , Par Olsson 1 2 , Christophe Domain 1 2
2 IRDEP, CNRS/EDF/ENSCP joint laboratory, Chatou France, 1 MMC, EDF R&D, Moret sur Loing France
Show AbstractPresent photovoltaic devices, based on p/n junctions, are limited from first principles to maximal efficiencies of 31% (respectively 40% under full solar concentration). However, more innovative schemes may overcome the Schockley-Queisser limit since the theoretical maximal efficiency of solar energy conversion is higher than 85%. To date, the only practical realisation of such an innovative scheme has been multi-junction devices, which presently hold the world record efficiency of nearly 41% at significant solar concentration. It has been proposed to make use of the solar spectrum in much the same way as the multi-junction devices does but in a single cell, using impurity induced intermediate levels to create gaps of different sizes. This intermediate level semiconductor (ILSC) concept has a maximal efficiency similar to that of multi-junction devices but suffers from prohibitively large non-radiative recombination rates. We here propose to use a ferromagnetic impurity scheme in order to reduce the non-radiative recombination rates while maintaining the high theoretical maximum efficiency of the ILSC scheme, that is almost 50%. Using density functional theory calculations, the electronic and energetic properties of transition metal impurities in a wide range of semiconductors has been analysed. Out of the several hundred compounds studied, only a few fulfil the design criteria we here present.
9:00 PM - H13.15
Optical Anisotropy of InGaAs/Ga(As,P) Quantum Dots Grown on GaAs (311)B Substrates.
Yuanchang Zhang 1 , Anup Pancholi 1 , Hassan Shah 1 , Jonathan Boyle 1 , Prasoon Pancholi 1 , Andrew Norman 2 , Valeria Stoleru 1
1 , University of Delaware, Newark, Delaware, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractSelf-assembled semiconductor quantum dots (QDs) appear to have enormous potential in photovoltaics, especially for implementing specific device configurations, such as the intermediate bands, multiple exciton generation, or hot-carrier solar cells. These nanostructured materials exhibit unique properties, dependent on their shape, size, density, and spatial arrangement, which open up opportunities for enhancing and controlling the charge separation, the optical absorption, and the transport processes. The vast majority of (In,Ga)As QDs for optoelectronic device applications are grown on (001) GaAs substrates. However, higher Miller index surfaces provide an alternative approach to influence the QD growth mode. The substrate orientation has been proved to have a large impact on the range of achievable QD shapes, sizes, and ordering, which in turn determine the electronic bandstructure and optical properties of the dots.We report on the optical and structural characterization of In0.47Ga0.53As/Ga(As,P) QD superlattices grown on GaAs (311)B substrates by metal-organic vapor phase expitaxy. The structures consist of 50 periods of QDs formed by the deposition of 6.1 monolayers of In0.47Ga0.53As, separated by 10 nm Ga(As,P) barriers, and covered by 300 nm GaAs cap layers. The phosphorus is introduced in the barriers for strain compensation and its content ranges in the samples from 0.8 to 18 %.We measured low temperature (T=77 K) polarization dependent photoluminescence from the cleaved edges on all samples described above. The QD ground state emission was identified to be TE-mode dominated, indicating the dominant transition from the electron ground state to the heavy-hole ground state. Unlike the dots grown on GaAs (001) substrate, where the transverse electric (TE) mode of the field is parallel to the dot base (i.e. the sample surface), the TE mode of the electric field showed ~5-6° deviation from the sample surface for the dots grown on GaAs (311)B substrate. This can be attributed to the shape anisotropy of the dots resulted by the substrate orientation, as verified by the structural transmission electron microscopy analysis. The dots grown on (311)B substrates are also aligned at about 9° to the surface normal, which is not the case for the dots grown on (001) substrates. Based on a continuous elasticity model, this inclination angle can be explained by a surface strain field driven by the buried dot. Our studies provide information about the optical anisotropy and spatial ordering of QDs which may be useful in designing the optical coupling for solar cells based on QDs grown on off-oriented substrate.
9:00 PM - H13.16
Bandstructure Engineering in InAs/GaAsSb/GaAs Quantum Dots for Optoelectronics and Photovoltaics.
Jonathan Boyle 1 , Valeria Stoleru 1
1 Materials Science and Engineering, University of Delaware, Newark, Delaware, United States
Show AbstractIn addition to their proven advantages for optoelectronic devices such as lasers and detectors, quantum dots (QDs) appear to be promising candidates for photovoltaics, especially for implementing specific device configurations, such as the intermediate bands (IBSC) and the multiple exciton generation solar cells. QDs exhibit unique properties, dependent on their size and shape, which open up opportunities for enhancing and controlling charge separation, and the transport and absorption processes.A proposed prototype IBSC device based on InAs/GaAs QDs highlighted a number of technical challenges, including the fact that the effective bandgap between the intermediate band and the conduction band (0.2-0.3 eV) in this material system was too low and needs to be brought towards the optimum value of 0.71 eV.We report here on the bandstructure engineering of InAs/GaAsSb/GaAs in a dot-in-a-well design, which we identified as a viable solution for bandgap optimization. Structures with 12% Sb in the quantum well barriers, in which the barrier thickness was varied between 10 and 30 nm, were grown by molecular beam epitaxy and analyzed via photoluminescence (PL). We observed an increase in PL intensity with increasing thickness of the GaAsSb strain reducing layer. The radiative recombination efficiency appears to be improved compared to the usual InAs/InGaAs/GaAs strain-uncompensated structures because the InAs/GaAsSb/GaAs heterointerface bandstructure suppresses carrier escape from the dots more effectively.Recent publications show that the emission from InAs QDs capped with GaAsSb can be extended from 1.28 to 1.6 µm by increasing the Sb composition from 14% to 26%. The emission from structures with 18%, 22%, and 26% Sb was shown to exhibit a strong blueshift with increasing excitation power, consistent with a type-II transition between an electron state in the dot and a hole state in the GaAsSb. In contrast, the emission from structures with 14% Sb displayed power independence, consistent with a type-I system. We performed bandstructure calculations for InAs/GaAsSb/GaAs structures in which the Sb content was 12%, 14%, 18%, 22%, 26%, and 30%, respectively, in the framework of an eight-band k.p formalism. The average size for the dots was taken from literature at 40 nm diameter and 5 nm height. Literature reported PL peak wavelengths as a function of Sb composition, as well as our own observations for 12% Sb, are consistent with our calculations. With increasing Sb composition above 14%, the rapid decrease in the valence band energy of GaAsSb confirms the formation of a staggered type-II system with respect to the InAs QD, in which the lowest energy states for an electron and a hole are concentrated in different materials. Incorporation of Indium in the barrier will be explored in the future, as suggested by recent literature. This suggests great flexibility for achieving bandstructure engineered structures for optoelectronics and photovoltaics.
9:00 PM - H13.17
Carrier Dynamics in Multi-Stacked (In,Ga)As/GaAs(1-x)P(x) Quantum Dot Structures for Solar Cell Applications.
Anup Pancholi 1 , Yuanchang Zhang 1 , Hassan Shah 1 , Jonathan Boyle 1 , Prasoon Pancholi 1 , Andrew Norman 2 , Valeria Stoleru 1
1 Materials Science and Engineering, University of Delaware, Newark, Delaware, United States, 2 , National Renewable Energy Laboratory (NREL), Goldon, Colorado, United States
Show AbstractQuantum dot (QD)-based solar cells have been predicted as promising candidates for achieving high efficiency solar energy conversion to electricity by utilizing hot photo-generated carriers to produce higher photo-voltage or higher photo-current. QDs have been proposed as viable candidates for practical implementation of the intermediate band solar cell concept, as they provide flexibility in tuning the position of the energy levels. In such QD-based devices, the intermediate band would arise from the confined energy levels of the electrons in the dot. In this approach, different photon energies would promote absorption from different isolated energy levels and therefore allow for the production of different voltages. The mismatch between the incident energy of the solar spectrum and a single band gap is accommodated by introducing additional energy levels such that photons of different energies can be efficiently absorbed. The radiative lifetime of different transitions is critical parameters for solar cell performance. We present photoluminescence (PL) analysis for multi-stacked strain compensated (In,Ga)As/GaAs(1-x)P(x) QD structures grown on (311)B GaAs substrates by metal organic vapor phase technique. This material system has been chosen as a model system for investigating intermediate band solar cells. The structures consist of 50 layers of (In,Ga)As QDs separated by GaAs(1-x)P(x) barrier layers in which the phosphorous mole fraction was varied from 0.8 to 18 %. The thickness of the barriers was in the order of 10 nm, which insures strong vertical coupling between the dot layers.We performed and analyzed temperature and excitation dependent time-integrated and time-resolved PL. We observed a decrease in the PL linewidth in the samples with phosphorus-containing barriers, as compared with similar structures with no phosphorus in the barriers, indicating more uniform QD size distribution. This may be due to the strain compensation effect, in which the residual compressive strain of buried QD layer does not have strong effect on the growth of next QD layer. Although the dots still form a vertical self-aligned columnar structure, the dot sizes in the subsequent layer are not necessarily larger than those of the dots in previous layer, as normally observed in multi-stacked (In,Ga)As/GaAs uncompensated structures. We also found an increase in the radiative lifetime at room temperature in samples with higher phosphorus content in the barrier layers, which may be due to (1) the lower dislocation density as a result of strain compensation and/or (2) strong carrier confinement in the dots provided by the barrier. Finally, we studied the effect of electronic coupling on radiative lifetimes in the multi-stacked QD structures. An attempt is being made to calculate the energy levels of these structures theoretically, in an eight-band k.p formalism, in which the strain is incorporated, and compare these results with the experimental observations.
9:00 PM - H13.18
Strain Balanced Quantum Dot Intermediate Band Solar Cell Development.
Seth Hubbard 1 , Christopher Bailey 1 , Ryne Raffaelle 1 , Ryan Aguinaldo 1 , David Wilt 2 , William Maurer 2 , Sheila Bailey 2
1 Department of Physics, Rochester Institute of Technology, Rochester, New York, United States, 2 Photovoltaic and Power Technologies Branch, NASA Glenn Research Center, Cleveland, Ohio, United States
Show AbstractInsertion of low dimensional heterostructures into a single junction p-i-n solar cell may lead to formation of an intermediate band (IB) within the bandgap of the host and a resulting efficiency near 63%. Ultra high efficiency III-V solar cells have potential application for both space and terrestrial arrays. Increased efficiency is of the utmost importance for space application to maximize the power to weight ratio. In terrestrial concentrator systems, the more expensive solar cell material is replaced with cheaper concentrator system components. However, in order to be cost competitive with other renewable energy sources, most manufacturers anticipate using high efficiency III-V solar cells at the core.In this paper, Metal Organic Vapor Phase Epitaxy (MOVPE) was used to grow a series of GaAs p-i-n solar cells with InAs Quantum Dots (QD) embedded in the cell structure to form the intermediate band. The InAs QDs were grown by the Stranski-Krastanow (SK) strain-induced method. The optimal InAs QDs were grown at 500°C and had a size and density of 7×40nm and 5(±0.5)×1010 cm-2, respectively. In order to improve the absorption in the QD region of a solar cell, 5 layer of the QDs were grown with a 10 nm GaAs spacer layer between each dot layer. Unfortunately, the SK QD growth process resulted in misfit dislocations that degraded solar cell performance. In order to mitigate this effect, we have investigated the use of strain compensation. The strain compensation (SC) used in our structures was GaP, which has a tensile strain to GaAs of 3.6% to help offset the compressive InAs QD induced strain. The effect of the GaP SC layer was first studied by varying the thickness of the GaP from 8-32Å and measuring the photoluminescence (PL) characteristics of PL test structures. The optimal GaP thickness using PL peak intensity was 26 Å. The peak PL of the optimized dots was centered near 1140nm (1.09eV) with variations of QD size (peak PL wavelength) and coherence (PL intensity) across the 2” wafer of 2% and 17%, respectively.Additionally, a series of SC QD solar cells were grown with varied GaP thickness. An array of 1 cm2 solar cells was fabricated on each type of wafer, IV curves were collected under AM0 conditions and spectral response was measured from 300-1100 nm. Fabricated solar cells gave quite different results than would be expected based on the PL optimization. In this case, the best solar cell performance was observed for a 14 Å GaP SC layer. Open circuit voltage was improved compared to an uncompensated cell and short circuit current recovered to near optimal levels. The improvements in Voc and Isc can be directly related to reduction in recombination driven current (reduced dislocation density) and improved carrier transport through the intrinsic and emitter regions. In addition to GaP thickness, effects of the number of QD stacked layers will be presented as well as further optical and thermal characterization of the strain balanced cells.
9:00 PM - H13.19
Thermal and Radiation Dependence of Quantum Dot Solar Cells.
Cory Cress 1 2 , Hubbard Seth 1 2 3 , Brian Landi 2 , David Wilt 4 , Ryne Raffaelle 1 2 3
1 Microsystems Engineering, Rochester Institute of Technology, Rochester, New York, United States, 2 NanoPower Research Laboratories, Rochester Institute of Technology, Rochester, New York, United States, 3 Physics, Rochester Institute of Technology, Rochester, New York, United States, 4 Glenn Research Center, NASA, Cleaveland, Ohio, United States
Show AbstractWe are reporting our research regarding the thermal and radiation dependence of epitaxially grown III-V quantum dot solar cells (QDSC). An appropriate temperature dependent operation is a key requirement of QDSCs for future space and terrestrial applicability, while a high resilience to ionizing radiation is an additional requirement for space employment. It is not clear how the thermal dependence observed in QD-based structures should impact the performance of QDSCs; however, experimental results have shown QD-based structures and devices to exhibit an enhanced radiation tolerance over their bulk counter parts and is expected to be observed in QDSCs. Therefore, we have measured the thermal and radiation dependence of two 5-layer InAs QD / GaAs p-type / intrinsic / n-type (pin) QDSC devices in reference to a GaAs pin baseline device. All three devices were grown using organo-metallic vapor phase epitaxy and the Stranski-Krastanov growth mode was employed for the InAs QD self assembly. Temperature dependent current – voltage (I-V) measurements under dark and simulated air mass zero illumination resulted in the determination of the normalized temperature coefficients (for efficiency, open circuit voltage, short circuit current, and fill factor) over the temperature range of 80 K to 400 K. The corresponding spectral response (SR) was measured over the same temperature range. Curve fitting of the dark I-V data enabled the temperature dependence of the various solar cell intrinsic parameters (e.g., ideality, saturation current, etc.) to be obtained, and allowed for a better understanding of the fundamental operation of the devices. The I-V and SR performance parameters were also measured as a function of 210-Po alpha-particle fluence. The three devices’ radiation damage coefficients were determined by curve fitting the various diode performance parameters with respect to alpha particle fluence. A similar procedure was performed for the supra-GaAs energy bandgap and sub-GaAs energy bandgap regions of the spectral response data enabling the radiation response of the GaAs and the QDs to be independently investigated.
9:00 PM - H13.2
Energy Band Engineering for Improved Vertical Transport in Quantum Structured III-V p-i-n Solar Cells.
Andenet Alemu 1 , Alex Freundlich 1
1 Center for Advanced Materials, University of Houston, Houston, Texas, United States
Show AbstractThe use of InGaAsN and GaBiAsN quantum structures in the intrinsic region of conventional III-V p-i-n solar cells, lattice matched to GaAs, presents several advantages for photovoltaic (PV) application. First they allow for very shallow to zero valence band offsets thus permitting the free movement of holes. Second, a wide range of band-gap values are made possible due to the large band gap decrease upon the introduction of minute amounts of N and Bi. Using band structure calculations that include the strain effects, conduction and valence band anti-crossing models describing the large band gap bowing and the transfer matrix method, we present the theoretical investigation of optimum design conditions for enhanced vertical transport. The direct quantum mechanical resonant tunneling of electrons out of the quantum structures and into the continuum of the conduction band of the host semiconductor material can be facilitated provided that an adequate choice of material parameters is made. The high electron transmission probability together with the free movement of quasi-3 D holes is predicted to result in enhanced PV device performance. Furthermore, the increase in electron effective mass due to the incorporation of N translates in enhanced absorptive properties, ideal for PV application.
9:00 PM - H13.20
K.P Method for Band Structure Calculation of Intermediate Band Solar Cell.
Som Dahal 1 , Michael Levy 1 , Christiana Honsberg 1
1 Physics and Astronomy, University of Delaware, Newark, Delaware, United States
Show Abstract9:00 PM - H13.21
Germanium Nanocrystal Films for Photovoltaic Applications.
Zachary Holman 1 , Ryan Gresback 1 , Uwe Kortshagen 1
1 Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractSemiconductor nanocrystals have recently been incorporated into solar cell structures, primarily due to their tunable band gap and relatively large mobilities. However, nanocrystal photovoltaics to date have been limited by costly or toxic constituent materials (e.g. cadmium), and processing difficulties. We report on the synthesis of non-toxic germanium nanocrystals (Ge-NCs), and thin films constructed using these crystals which hold promise for photovoltaics.Germanium nanocrystals are synthesized in a non-thermal plasma reactor via the dissociation of germanium tetrachloride (GeCl4). Transmission electron microscopy has verified that this method produces crystals that are free-standing and nearly monodisperse (10-15% mean particle size standard deviation), with diameters which may be tuned from 4-20 nm by varying the plasma parameters. During synthesis, hydrogen gas (H2) is introduced into the plasma along with GeCl4, and Auger electron spectroscopy and Fourier-transform infrared (FTIR) spectroscopy indicates that the surfaces of the crystals can be smoothly varied from chlorine-terminated to hydrogen-terminated by adjusting the H2 concentration.Films of Ge-NCs have been deposited both from the gas and liquid phases. Gas-phase deposited films are grown by placing a glass substrate lithographically patterned with aluminum electrodes (5 μm electrode gap) downstream of the plasma in the path of the Ge-NCs. Thickness characterization with both scanning electron microscopy (SEM) and Rutherford backscattering reveals deposition rates of 150 nm/min with porosities of over 98%. Current-voltage characteristics of these fluffy films have been recorded both in the dark and under simulated Air-Mass 1.5 solar irradiation. The Ge-NC films exhibit conductivities that are photosensitive, field-dependent, and surprisingly high. Despite the low density of the films, conductivities as high as ~10E-5 S/cm are found, and photoconductivities that are an order of magnitude larger than the corresponding dark conductivities have been measured.Liquid-phase deposited films are formed by drop-casting solutions of Ge-NCs passivated with alkene molecules. Ge-NCs are synthesized with hydrogen-terminated surfaces and alkenes such as dodecene and octadecene are then attached to crystal surfaces using hydrogermylation reactions. The resulting passivated Ge-NCs are soluble in a variety of solvents, yielding clear nanocrystal inks. SEM images show that films drop-cast from these inks are uniform, dense (unlike the gas-phase deposited films), and have thicknesses which may be tuned by varying the Ge-NC concentration in the ink. Electrical measurements will be reported.This work was supported by NSF under NIRT grant CBET-0506672 and in part under MRSEC grant DMR-0212302, and by IREE grant LG-C5-2005.
9:00 PM - H13.23
Ab Initio Study of the Stability and Electronic Structure ofSemiconductor Nanowires.
Thomas Sadowski 1 , Rampi Ramprasad 1
1 , University of Connecticut, Storrs, Connecticut, United States
Show AbstractSemiconductor nanocrystals (NCs) have been the subject of intense theoretical and experimental studies for a wide variety of potential applications since their reduced sizes and geometry leads to quantum confinement effects that may give rise to optical and electrical properties that differ greatly from the bulk [1].One application of semiconductor NCs that holds much promise is photovoltaics [2],[3]. Asuccessful photovoltaic device require the following: (i) generation of excitons due to absorption of a photon, (ii) exciton dissociation into an electron and hole, and (iii) transport of the electron and hole toward their respective electrode [2],[4]. Although current thin film solar cellsmeet these requirements, several fundamental drawbacks prevent these devices from being highly efficient [2].Recent studies have shown that in semiconductor NCs exciton decay is slowed due to quantum confinement and the associated discrete energy levels. This results in multiple exciton generation rather than heat loss [2]. Furthermore, there is a growing belief that nanorods, rather than quantum dots or systems with heterojunctions between nanorods of dissimilar materials (e.g. between CdSe and CdTe) [5] provide sufficient opportunity for exciton dissociation (due to a band offset at the heterojunctions), and charge transport (due to lack of quantum confinement along the axisof the rods).The focus of this study is to provide a more comprehensiveunderstanding of the stability and electronic structure of cadmium selenide (CdSe) and cadmium telluride (CdTe) nanorods. In particular, first principles computational methods have been applied to infinitely long nanorods of CdSe in the wurtzite crystal structure over a range of topologies and diameters. A similar analysis was also performed upon the wurtzite and zincblende phasesof CdTe. The dependence of the total energy (i.e. stability) of the nanorods, their electronic band gaps, and the tendency of surface atoms to reorient are assessed as a function of the types of terminating surface facets of the nanorods, their cross-sectional topologies, and diameters. Consistent with our prior work on CdSe quantum dots [6], calculations show that hexagonal cross-sections containing surface atoms with one dangling bond are highly stable, possessing a large electronic band gap and exhibitingminimal surface reorientation. It is also shown that the total energy of a nanorod of arbitrary size can be approximated by an algebraic expression based on ab initio bulk, surface and edge energies.[1] V. Klimov, A. Mikhailovsky, S. Xu, Science 290,314-317 (2000).[2] A. Nozik, Inorg. Chem. 44, 6893-6899 (2005).[3] Z. Tang, N. Kotov, M. Giersig, Science 237, 237-240 (2002).[4] J. Li, L.W. Wang, Nano Lett. 3, 1357-1363 (2003).[5] P. Peng, D.J. Milliron, et al. Nano Lett. 5, 1809-1813 (2005).[6] M. Yu, et al. Appl. Phys. Lett. 88, 231910 (2006).
9:00 PM - H13.24
Enhancement of Silicon Solar Cell Efficiency using Colloidal Semiconductor Quantum Dot Spectral Converters.
Anuj Madaria 1 , Atul Konkar 1
1 Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles , California, United States
Show AbstractColloidal semiconductor nanocrystalline quantum structures are major focus of current research due to a broad range of applications such as for biological labeling, optoelectronic and energy conversion devices. The attractiveness of these nanostructures lies in their relative ease of synthesis, tunability of their optoelectronic properties, and relative ease of incorporation into macroscopic structures while maintaining their individual properties. These nanostructures, in the form of quantum dots, rods, and hyperbranched shapes, have been employed in different types of bulk heterojunction photovoltaic solar cells. Our focus is on the use of colloidal nanocrystal quantum dots (NCQD) as spectral converters to increase efficiency of conventional silicon solar cells [1]. Standard Si (001) n+-p-p+ single crystal silicon solar cells with varying n+ and p+ depth and dopant concentration were used in these studies. II-VI NCQDs with varying sizes and emission wavelengths were synthesized in non-coordinating solvents using literature procedures. The NCQDs, embedded in PMMA, were subsequently deposited on the silicon solar cell. For the CdSe NCQDs, we have achieved up to ~ 10% increase in the efficiency in the solar cell. We will present our results on the effect of varying NCQD layer thickness, size, and emission wavelength on the efficiency of the silicon solar cells. We will discuss the role of various parameters such as: the NCQD and Si absorption spectra, NCQD emission spectra, effective reflectivity of the combined NCQD/Si PV cell that contribute of the towards increasing the cell efficiency. This work was partly funded by the USC Future Fuels & Energy Initiative grant.1. W.G.J.H.M. Van Sark, A. Meijerinkb, R.E.I. Schroppc, J.A.M. van Roosmalend, E.H. Lysen, Sol. Energy Mater. Sol. Cell 87, 395-409 (2005)
9:00 PM - H13.25
Electrophoretic Deposition of CdSe Nanocrystals for Photovoltaic Applications.
Nathanael Smith 1 , Kevin Emmett 2 , Tony Watt 1 , James McBride 1 , Sandra Rosenthal 1
1 Department of Chemistry, Vanderbilt University, Nashville, Tennessee, United States, 2 Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee, United States
Show AbstractCdSe nanocrystals chemically linked to nanocrystalline titanium dioxide substrates form a promising material for nanostructured photovoltaic devices. The usual method for attaching the nanocrystals to the titanium dioxide substrate is by means of a linking molecule (such as mercaptopropionic acid). In this paper, we report the use of an alternative technique, electrophoretic deposition (EPD), to directly deposit CdSe nanocrystals onto the substrate. In EPD, a voltage is established between two electrodes that are immersed in a solution of nanocrystals. At room temperature, a fraction of the nanocrystals are thermally charged, and these charged nanocrystals migrate to the electrodes and adhere to the surface. A significant advantage of EPD over the use of linking molecules is the speed at which the nanocrystals are deposited: EPD takes only a few minutes, compared to the several hours required for chemical linking.EPD has only recently been applied to nanocrystals, but already, uniform, robust films of CdSe nanocrystals have been demonstrated. In this paper, we show that EPD can be used to deposit large, uniform films of CdSe nanocrystals onto titanium dioxide and ITO substrates, which can then be used as the starting point for photovoltaic devices. We have also used EPD to deposit nanocrystals onto porous titanium dioxide, which will allow for the subsequent formation of bulk heterojunction photovoltaic devices. Parameters affecting the deposition, such as voltage, nanocrystal concentration, and nanocrystal ligand coverage, have been investigated with RBS, AFM and TEM. The affects of the geometry of the deposition, including size, shape and spacing of the electrodes, has been investigated. Finally, the photovoltaic performance of initial photovoltaic devices is presented.
9:00 PM - H13.26
Tin Oxide and Niobium Oxide Nanowire Based Dye Sensitized Solar Cells.
Suresh Gubbala 1 , Vivekanand Kumar 1 , Uros Cvelbar 2 , Mahendra Sunkara 1
1 Chemical Engineering, Universty of Louisville, Louisville, Kentucky, United States, 2 Plasma Lab F4, Jozef Stefan Institute, Ljubljana Slovenia
Show AbstractCrystalline nanowires potentially offer advantages over nanoparticle networks possibly by providing a direct and fast pathway for electron transport, thus decreasing the electron lifetime in the semiconductor electrodes for a variety of photoelectrochemical energy conversion applications.1,2 Shorter lifetimes in the semiconductor electrode could reduce the electron recombination reactions with the oxidized species in the electrolyte, increasing the overall conversion efficiencies. Here, we investigated the performance of two types of nanowire materials, three-dimensionally networked nanowire powder made of tin oxide and vertical nanowire arrays made of niobium pentoxide, as electrodes in dye sensitized solar cells.Tin oxide nanowires in mat like formats were synthesized by the self catalytic reactive vapor transport of tin using lean oxygen conditions,3 whereas niobium pentoxide nanowire arrays on niobium metal foils were synthesized by exposing niobium foils to low pressure, weakly ionized, fully dissociated cold oxygen plasma.4 The results show that tin oxide nanowire based cells have similar or better efficiencies than the nanoparticle cells. Transient photocurrent measurements performed on the nanowire based cells showed faster electron transport through the nanowire compared to the nanoparticle networks, which explains their higher open circuit potentials. Further improvement in the performance of the nanowire based cells was achieved by increasing the surface area of the nanowires by coating them with tin oxide and titania nanoparticles. Titania nanoparticle coatings increased the efficiency of the cells from 1.6% to 2.9% at AM1.5 100 mW/cm2 light intensity and 1.8% to 4.1% at 50 mW/cm2 light intensity. Other modifications of the nanowires by coating them with titania using atomic layer deposition (ALD) were also performed. The performance of the unmodified niobium pentoxide nanowire based solar cells was limited to about 0.5%, mainly due to the high density of nanowires and the difficulty in dye sensitization. Further optimization of niobium pentoxide nanowires is being carried out to optimize their performance. References:1.N. Beermann, L. Vayssieres, S.E. Lindquist, and A. Hagfeldtz, J Electrochem. Soc., 147, 2456 (2000)2.S. Gubbaa, J. Thangala, and M.K. Sunkara, Solar Energy Materials and Solar Cells, 91, 813 (2007)3.Rao, Chandrasekaran, Gubbala et al., J. Electronic Materials 35 (5), 941-946 (2006).4.M. Mozetic, U. Cvelbar, M.K. Sunkara and S. Vaddiraju, Advanced Materials, 17, 2138 (2005)
9:00 PM - H13.27
Copper Phthalocyanine Nanowire Based Solar Cells.
Vijay Singh 1 2 , Suresh Rajaputra 1 2 , Sovannary Phok 1 2 , Gautam Chintakula 1 2 , Gayatri Sagi 1 2
1 electrical & Computer Engineering, University of Kentucky, Lexington, Kentucky, United States, 2 Center for Nanoscale Science & Engineering, University of Kentucky, Lexington, Kentucky, United States
Show AbstractPhotovoltaic devices based on organic semiconductors are of interest because of their potential as flexible, lightweight and inexpensive devices. One of the promising devices, involves the heterojunction between copper phthalocyanine (CuPc) and 3,4,9,10-perylenetetracarboxylic bis-benz-imidazole (PTCBI). Earlier, we reported, the highest Voc (1.125V) in a single organic heterojunction solar cell in an ITO/PEDOT:PSS/CuPc/PTCBI/Al structure. Results were interpreted in terms of a modified CuPc-Al Schottky diode for this thin PTCBI layer case and a CuPc-PTCBI heterojunction for the thick PTCBI case. We also reported the device characteristics of Copper phthalocyanine (CuPc)/Aluminum (Al) Schottky diode solar cells. Here, open circuit voltages (Voc) increased from 220 mV at 15 nm to 907 mV at 140 nm. Analysis of the current-voltage characteristics indicated that tunneling and interface recombination mechanisms are important components of the current transport at the CuPc/Al junction.With the objective of higher short circuit current densities in mind, we have extended our earlier work on organic semiconductors to nanowire based designs. In this paper, we report the fabrication, materials and electrical characterization of CuPc nanowire based solar cells in AAO templates by electro-deposition. The CuPc nanowires were electrochemically deposited into the AAO templates using CHCl3 (Chloroform) containing 10-4 M CuPc with 0.5 ml of CF3COOH (Trifluoro acetic acid). The nanowires were characterized by XRD, UV-Vis absorption spectroscopy, electron microscopy and electrical measurements. CuPc nanowires were also electrodeposited onto ITO/glass substrates and compared with templated nanowires for their structural, optical and electrical properties. Effect of the PEDOT: PSS buffer layer on the nanowire based device characteristics was also investigated.
9:00 PM - H13.28
Fabrication of Copper Indium Diselenide Nanowires.
Sovannary Phok 1 , Suresh Rajaputra 1 , Vijay Singh 1
1 electrical & Computer Engineering, University of Kentucky, Lexington, Kentucky, United States
Show AbstractNanostructured semiconductors have received a great deal of attention due to their unusual physical properties. The possibility of controlling these properties by varying the particle size, shape and surface properties is of great interest for nanoscale device applications in microelectronics, non-linear optics and optoelectronics in particular. Copper Indium diselenide (CIS), a p-type semiconductor with a band gap of about 1.01 eV has shown promise as an absorber for photovoltaic cells. Polycrystalline CIS film has been fabricated by various physical and chemical techniques over the past couple of decades. However, the preparation of nanostructured CIS is still challenging due to the fact that CIS tends to agglomerate resulting in large grains. Furthermore, an annealing step is also required to improve the film properties, which results in increased grain size. Here we report a simple, low cost solution to this problem through electrochemical deposition of CIS into the confined nanopores of an anodic aluminum oxide (AAO) template. The nanoporous aluminum oxide template, a few microns thick was prepared by either a one step or a two anodization in oxalic acidic media. CIS was deposited into the nanoporous AAO template by pulsed cathodic electro-deposition from an aqueous mixture. The electrodeposited nanowires had diameter ranging up to 40 nm and length ranging from 600 nm to 5 microns depending on the length of the template. The hybrid nanostructured CIS/AAO was annealed in vacuum at 230 C for several hours to achieve stoichiometric CuInSe2 phase. The embedded CIS nanowires were characterized by X-ray diffraction and scanning electron microscopy. The optical bandgap was deduced from UV-Vis absorption spectroscopy measurements.
9:00 PM - H13.3
Evidence of Sequential Carrier Escape in III-V p-i-n Multi-Quantum Well Solar Cells.
Andenet Alemu 1 , Alex Freundlich 1
1 Center for Advanced Materials, University of Houston, Houston, Texas, United States
Show AbstractSeveral InAsP/InP p-i-n Multi-Quantum Well (MQW) solar cells, only differing by their MQW region composition and geometry, were investigated. For each sample, the Arrhenius plot of the temperature related variation of the photoluminescence intensity was used to deduce the radiative recombination activation energy. The electron and holes confinement energy levels in the quantum wells and the associated effective potential barriers seen by each carrier were theoretically calculated. Carrier escape times were also estimated for each carrier. The fastest escaping carrier is found to display an effective potential energy barrier equal to the experimentally determined photoluminescence activation energy. This not only shows that the temperature related radiative recombination extinction process is driven by the carrier escape mechanism but also that the carriers escape process is sequential. Moreover, a discrepancy in device performance is directly correlated to the nature of the fastest escaping carrier.
9:00 PM - H13.30
Plastic and Solid-state Dye-sensitized Photocells Incorporating Mesoscopic Carbon Materials.
Tsutomu Miyasaka 1 , Nobuyuki Ikeda 1 , Masashi Ikegami 1
1 Graduate School of Engineering, Toin University of Yokohama, Yokohama, Kanagawa, Japan
Show Abstract Dye-sensitized mesoscopic photocell is a sole utility-type solar cell manufactured under atmospheric pressure by simple coating processes. We have developed the low-temperature TiO2 coating method (<150oC) using a binder-free TiO2 paste, which enabled fabrication of plastic DSC of high efficiency (6.4%) using organic redox electrolytes and ITO-coated polyethylene naphthalate (PEN) as conductive plastic substrate.1 Flexible DSC is highly sought after as a ubiquitous power source in consumer electronics. Solidification of DSC by replacing liquid electrolytes with solid conductors is an important issue to ensure safety and durability. We have fabricated solid-state DSCs using clay-like soft conductive materials containing carbon particles. Polyaniline-coated carbon black (PACB) was found to function as efficient hole conductor in junction with dye-adsorbed titania layer when the interior of titania mesopore was filled with ionic liquid iodide. In the interior electron exchange mechanism between ionic liquid molecules drives the charge transport. DSC, sensitized by ruthenium complex dye (N719) and solidified using this paste, achieved 3.5 and 4.1% efficiency for light intensities of 100 and 23 mW cm-2, respectively.2 We applied this method to solidification of plastic DSC. Among various carbonaceous materials examined with plastic electrode single-wall carbon nanotube (SWCNT) yielded highest performance. The SWCNT-incorporated DSC realized conversion efficiency up to 2.3% with maximum photovoltage of 0.66V.3 High-purity SWCNT, when exposed to oxygen (air), tends to have p-type conductivity that effectively rectifies hole transport from dye to carbon. A unique point in the cell construction is that iodine as the electron acceptor is omitted in the solidified conductor layer. We consider that this was made possible by minimizing the distance between the surfaces of working photoelectrode and counterelectrode. The soft carbon layer functions itself as counterelectrode, which closely comes to the surface of titania. Large-area plastic DSCs were fabricated by the above solidification processes. These are a first example of plastic flexible DSC without using liquid electrolyte. The carbon-based DSCs also lead to cost-effective fabrication by roll-to-roll mass production in which all conductors are set by simple printing processes.1. Y. Kijitori, M. Ikegami, T. Miyasaka, Chem. Lett., 36, 190(2007). 2. N. Ikeda, K. Teshima, T. Miyasaka, Chem. Commun., 2006, 1733.3. N. Ikeda, T. Miyasaka,, Chem. Lett., 36, 466(2007).
9:00 PM - H13.31
Comparison Study of Charge Transport and Recombination in Dye-sensitized Solar Cells Prepared with TiO2, ZnO, SnO2, and In2O3.
Shogo Mori 1 , Yosuke Fukai 1 , Yusuke Kondo 1 , Akihiro Asano 1 , Yukiko Uzawa 1 , Masanori Miyashita 1 , Kenji Sunahara 1 , Eiji Suzuki 1
1 Department of Fine Materials Engineering, Shinshu University, Ueda Japan
Show AbstractDye-sensitized solar cells (DSC) have been paid large attention due to the high energy conversion efficiency with an expectation of low production cost. Highly efficient DSCs typically consist of nano-porous TiO2 electrode and Ru complex dyes immersed in electrolytes containing a redox couple. In view of charge transport, the high efficiency has been achieved due to high injection yield from the dye to the semiconductor and due to sufficient electron diffusion length in the electrode. Besides the TiO2, various metal oxides, such as ZnO, SnO2, Nb2O5, and In2O3, have been applied for DSCs. However, none of them have shown comparable efficiency with the DSC using TiO2. For the case of ZnO and SnO2, the reason of the low efficiency was initially assumed due to faster charge recombination caused by higher electron mobility. On the other hand, electron transport in nano-porous TiO2 electrode and electron transfer at TiO2/dye/electrolyte are heavily influenced by the condition of intra-band charge traps and fabrication methods of the nano-porous electrodes, and thus, similar effect may dominate also other oxides. In this study, we prepared electrodes from nano-particles of TiO2, ZnO, SnO2, and In2O3. In order to obtain general trends, at least two different particles were examined for each metal oxide. Then, DSCs were fabricated with the electrodes with a Ru-complex dye. Electron diffusion coefficient, electron lifetime, and electron density were measured and the values were compared among the DSCs using various metal oxides. In the case of electron lifetime, all of them showed similar light intensity dependence. The values of lifetime seem to depend on the fabrication methods rather than the difference of metal oxides. In the case of electron diffusion coefficients in the electrode, except for In2O3, similar light intensity dependence was seen. These results imply that the electron transport and recombination in nano-porous metal oxides are hardly influenced by the nature of the oxides. On the other hand, for example, since the ZnO and SnO2 have lower dielectric coefficient than that of TiO2, there might be non-negligible band bending in the particles and may influence the diffusion coefficient and lifetime. Thus, the mechanism of the electron transport and recombination may differ from that of TiO2. We will discuss the issues with the electrodes prepared with different particle sizes. Then, we will discuss the possibility of the highly efficient DSCs with ZnO, SnO2 and In2O3.
9:00 PM - H13.32
Efficient Flexible Dye-Sensitized Solar Cells based on Anodic Titanium Oxide Nanotube Arrays.
Chien Chon Chen 1 , Wei-Guang Diau 1
1 , Chiao Tung University, Hsinchu Taiwan
Show Abstract9:00 PM - H13.33
Dye-sensitized Solar Cells Based on Anatase, Brookite, and Rutile Nanoparticles.
David Reyes-Coronado 1 , Geonel Rodriguez-Gattorno 1 , Gerko Oskam 1
1 Applied Physics, CINVESTAV - IPN, Merida, Yucatan, Mexico
Show AbstractTiO2 nanoparticles are used in a variety of photochemical applications including catalysis, antibacterial coatings, and dye-sensitized solar cells (DSCs). The phase, morphology, and size of the nanomaterials strongly affect the performance in the applications for reasons that are not well understood. Here we report a method for the synthesis of TiO2 nanoparticles in the three most common phases: rutile, anatase, and brookite. The method allows control over the particle size, providing the opportunity to investigate in detail the relation between the structural and physical properties of nanomaterials.The synthesis method is based on the use of a single base material, consisting of amorphous titania, that can subsequently be transformed to one of the three crystalline phases under the appropriate experimental conditions. Amorphous titania was obtained by adding titanium isopropoxide to isopropanol with a low concentration of water. A paste of amorphous titania was then mixed with an acidic aqueous solution, and hydrothermally treated at elevated temperature. Anatase was obtained using acetic acid, rutile was obtained with HCl, and brookite was obtained also with HCl but at lower concentration and temperature. The nanoparticles were characterized with X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM), Raman spectroscopy, UV-Vis absorbance spectroscopy, and dynamic light scattering (DLS).The results show that the particle size for anatase, brookite and rutile as determined from XRD can be adjusted in the range of 10 nm to 20 nm as needed for DSCs. From TEM it can be concluded that anatase and brookite are prepared as well-dispersed nanoparticle colloids with weakly facetted particles, while the rutile nanoparticles are rod-shaped and show a strong tendency to agglomerate. DLS shows that the colloids contain aggregates with sizes between tens to hundreds of nm, with the average size increasing in the order anatase, brookite, and rutile. The rutile rods align themselves in the aggregates, and at longer times or higher temperatures, larger crystals are formed by recrystallization. From the combined results, we propose mechanisms for the formation of the three phases as a function of the experimental condtions. Solar cells prepared with the three phases show good results for anatase and brookite, while the rutile cells have a very low efficiency, due to a low roughness factor. We will also present preliminary results on transport measurements for the three phases.
9:00 PM - H13.34
Hollow Structured TiO2 Electrode for Quasi-solid Dye-sensitized Solar Cells.
Su Chul Yang 1 , Eun-Sik Kwak 1 , Dae-Jin Yang 2 , Il-doo Kim 1 , Junkyung Kim 1 , Hyunjung Lee 1
1 Materials Science and Technology Division, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of)
Show Abstract9:00 PM - H13.35
Improving the Dye-sensitized Solar Cell Performance by Enhancing the Electrical Property of Tin Doped Indium Oxide.
Sangwook Lee 1 , Shin Tae Bae 1 , Jun Hong Noh 1 , Hyun Suk Jung 2 , Kug Sun Hong 1
1 School of Materials Science and Engineering, Seoul National Univ., Seoul Korea (the Republic of), 2 School of Advanced Materials Engineering, Kookmin University, Seoul Korea (the Republic of)
Show AbstractIn this study, the electrical property of tin doped indium oxide (ITO) was enhanced by heat-treatment at reducing atmosphere. Following the reduction process, the reduced TiO2 film on ITO electrode was oxidized electrochemically. The influences of the processes on the performance of the dye-sensitized solar cells (DSSCs) were investigated. In order to reduce the ITO, the TiO2 coated ITO electrode was annealed at 350oC for 2 hour in H2 atmosphere after annealed at 450oC for 1 hour in O2 atmosphere. The resistance of the ITO was decreased from 8.66×10-4 to 1.42×10-4 Ωcm and the charge carrier concentration was increase from 1.26×1020 to 8.57×1020 cm-3 by the reduction. However, TiO2 film was reduced by the H2-annealing also, which dropped the DSSC characteristic by increasing the back electron transfer from TiO2 to electrolyte. In order to decrease the back electron transfer, the H2-annealed TiO2 film was oxidized again by electrochemical oxidation. Through the previous processes, we could fabricate the ITO based DSSC which exhibits superior performance to the FTO based DSSC. The effects of the processes on the internal resistances of the DSSCs were investigated using the electrochemical impedance analysis, and the change in the surface state of the ITO and the TiO2 film were discussed with the X-ray photoelectron spectroscopy. It was considered that the improved performance of the ITO based DSSC is due to the decreased sheet resistance of the ITO conduction layer and the lowered Schottky barrier at ITO/TIO2 interface even while preventing the deterioration of the diode-like characteristic of DSSC.
9:00 PM - H13.36
Efficient Dye Sensitized Solar Cells using ZnO Nanoplants.
Ashutosh Tiwari 1 , Michael Snure 1 , Surabhi Pandey 2
1 Materials Science & Engineering, university of utah, Saltlake city, Utah, United States, 2 , TechnoMat Inc., Sandy, Utah, United States
Show AbstractDye-sensitized solar cells (DSSCs) have received considerable attention as a cost-effective alternative to conventional solar cells. These cells operate on a process similar to photosynthesis, the process by which green plants generate chemical energy from sunlight. A thick semiconductor nanoparticle film (electrode) provides a large surface area for the adsorption of energy by light harvesting organic dye molecules which then “inject” electrons into the nanostructured semiconductor electrode. This process is accompanied by a charge transfer to the dye from an electron donor mediator supplied by an electrolyte, resetting the cycle. A significant increase in the long term stability and the efficiency of DSSCs has been realized during the last few years. However, still the current nanoparticle-based DSSCs suffer from the trap-limited diffusion transport mechanism of electrons, a slow mechanism that limits the device efficiency, especially at longer wavelengths. Recently we have developed a new version of the dye-sensitized cells in which the traditional electrode (sintered nanoparticle film) is replaced by a specially designed ZnO electrode possessing an exotic ‘nanoplant-like’ morphology. This advance fixes a major efficiency limiting factor in current nanoparticle-based DSSCs. We demonstrated that the direct electrical pathway, provided by the interconnected nanoplants, provides rapid collection of carriers generated throughout the device, and significantly enhances the conversion efficiency of the system over that of sintered nanoparticle based devices.
9:00 PM - H13.37
Acid Treated Carbon as Tri-iodide Reduction Electrocatalyst for Dye-sensitized Solar Cell Application.
Won Jae Lee 1 , Easwaramoorthi Ramasamy 1 2 , Dong Yoon Lee 1 , Jae Sung Song 1
1 Advanced materials and application research division, Korea Electrotechnology Research Institute , Changwon Korea (the Republic of), 2 , University of Science and Technology, Daejeon Korea (the Republic of)
Show AbstractDye sensitized solar cells (DSSCs) received considerable interest, due to high energy conversion efficiency and simple fabrication methods. In order to enhance the tri-iodide reduction kinetics, catalytic layer of Pt was coated on transparent conductive oxide (TCO) substrate and used as a counter electrode in DSSCs.Very recently, we have demonstrated that carbon as potential alternative to Pt counter electrode for DSSCs. To further enhance the catalytic performance and simplify the electrode preparation method, carbon powders were acid treated, dispersed in binder added solution and sprayed on TCO glass substrate. Electrocatalytic activity towards I3- reduction and adherence of carbon powders on substrate were evaluated from diffusion limited current and charge transfer resistance in redox electrolyte. The effect of acid treatment on catalytic performance of carbon was studied through assembling such electrodes with TiO2 electrode in sandwich configuration and measuring the I-V performance of the device under illumination.
9:00 PM - H13.38
Improvement on Morphological Properties of Anode Layer in Dye Sensitized Solar Cell Using Ion Beam Modification.
Bangke Zheng 1 , Satilmis Budak 1 , Bopha Chhay 1 , Robert Zimmerman 1 , Daryush Ila 1
1 Center for Irradiation of Materials, Alabama A&M University, Normal, Alabama, United States
Show Abstract9:00 PM - H13.39
Study on Dye Sensitized Solar Cells consisting of Shape-controlled TiO2 Nanoparticles.
Jung-Kun Lee 1 , Hyun Suk Jung 2
1 Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 School of Advanced Materials Engineering, Kookmin University, Seoul Korea (the Republic of)
Show AbstractThe ability to produce nanoscale materials with architectures from 0-dimension to 1-dimension has both scientific and technological importance. In particular, given that the grain boundaries prevent the transport of carriers via scattering, the decreased density of grain boundaries in 1-dimensional nanomaterials are expected to increase carrier mobility and provide a venue for the synthesis of unique nanocomposites for photonic and photovoltaic applications.In this study, we present the effect of the aspect ratio on the photovoltaic properties of dye sensitized solar cells (DSSC). 1-dimensional TiO2 nanorods were synthesized using a combined gel-sol and hydrothermal process. Ellipsoidal TiO2 nanorods of different aspect ratios in the range of 2-10 were prepared by a highly pressurized gel-sol method in the presence of amino acids. The solar cell performance was characterized as a function of aspect ratio. Structural analysis including X-ray diffraction and transmission electron microscopy show that the TiO2 nanorods from the gel-sol method have pure and highly crystalline anatase phase, suited for studying the effect of nanoscale architecture. Transient photovoltage measurement shows that increasing the aspect ratio of TiO2 nanorods up to 10 suppressed the electron-hole recombination and increased the life time of photogenerated carriers in DSSCs, as compared to 0-dimensional TiO2 nanoparticles. This is attributed to the reduced number of grain boundaries and the increased carrier mobility. The physics underlying the effect of shape and grain boundary on the carrier life time and mobility are systematically explored.
9:00 PM - H13.4
To a Question About High-effective Solar Energy Conversion.
Halyna Khlyap 1 , Viktor Laptev 1
1 Physics, University, Stuttgart Germany
Show Abstract9:00 PM - H13.40
Free-standing Grid-like Nanostructures Sssembled into 3-D Open Architectures for Photovoltaic Devices.
Xiangyang Kong 1
1 school of materials science and engineering, shanghai jiao tong univesrity, Shanghai China
Show Abstract9:00 PM - H13.41
Block-copolymer Directed Mesoporous TiO2 for Solar Cell Applications.
Mihaela Nedelcu 1 , Henry Snaith 1 2 , Jinwoo Lee 3 , Scott Warren 3 , Mahendra Orillal 3 , Ulrich Wiesner 3 , Ullrich Steiner 1
1 Department of Physics, University of Cambridge, Cambridge United Kingdom, 2 Department of Physics, University of Oxford, Oxford United Kingdom, 3 Materials Science Department, University of Cornell, Ithaca, New York, United States
Show AbstractDye sensitized solar cells are one of the most promising strategies for the conversion of sun light into electricity. While the most efficient devices use a liquid electrolyte, solid state (polymer based) cells are technologically more interesting, avoiding packaging issues that make liquid-containing devices difficult to deploy. The power conversion efficiency of polymer-based cells is, however, much lower compared to liquid electrolyte analogues. One of the challenges in the manufacture of more efficient cells is the improvement of the TiO2 scaffold that is an integral part of the device. The TiO2 phase has to be finely divided (on the 10-nm length scale) with a high degree of crystalinity – two requirements which are typically difficult to combine. Here, we describe an approach that leads to highly crystalline, highly porous TiO2 and its incorporation into dye-sensitised solar cells. Our approach makes use of micro-phase separated block-copolymer morphology as a template. The block-copolymer was mixed with a titania precursor that selectively swell only one block of the block-copolymer. Mesoporous TiO2 was obtained after pyrolysis at 550-600 oC in an inert atmosphere. This procedure yields mesoporous TiO2 with 20 nm pores size.The mesoporous TiO2 films were utilised to manufacture solar cells using a solid state hole transporter.
9:00 PM - H13.42
Use of Anodic TiO2 Nanotubes in Solar Cell.
Jan Macak 1 , Robert Hahn 1 , Thomas Stergiopoulos 2 , Andrei Ghicov 1 , Dimitris Tsoukleris 2 , Sergiu Albu 1 , Julia Kunze 1 , Doohun Kim 1 , Polycarpos Falaras 2 , Patrik Schmuki 1
1 Dep. of Materials Science, University of Erlangen, Erlangen, Bavaria, Germany, 2 Institut of Physical Chemistry, NCRS Demokritos, Athens Greece
Show AbstractThe presentation deals with use of a high aspect ratio titanium dioxide nanotubes grown by anodization in dye-sensitized solar cells (1). The high surface area and the ordered geometry of the nanotubes represents potentially very suitable architecture for solar energy conversion also due to enhanced electronic properties in comparison with conventional nanoparticulate layers, such electron transport and low recombination rate (2,3). The nanotubes are grown by self-organized anodization (Fig.1a) and by rapid breakdown anodization (Fig.1b). The length of the nanotubes is in the range of several μm up to few dozens of μm with single tube diameter of several tens of nm (4-7). We demonstrate that significant light-to-electricity conversion efficiencies can be achieved by the sensitized nanotubes (2, 8). Growth and properties of the nanotubes and critical issues of their use in solar cells will be discussed in details. References:1. B.O'Regan and M.Grätzel, Nature 353, 737 (1991).2. J.M. Macak, H. Tsuchiya, A. Ghicov, P.Schmuki, Electrochem. Comm. 7, 1133 (2005).3. K. Zhu, N. R. Neale, A. Miedaner, A. J. Frank, Nano Letters 7, 69 (2007).4. J. M. Macak, H. Tsuchiya and P. Schmuki, Angew. Chem., 44, 2100 (2005).5. J. M. Macak, H. Tsuchiya, L.V. Taveira, S. Aldabergerova, P. Schmuki, Angew. Chem. 44, 7463 (2005)6. S. Albu, A. Ghicov, J. M. Macak, P. Schmuki, Phys. Stat. Sol. (RRL) 1, 65 (2007).7. R. Hahn, J.M. Macak, P. Schmuki, Electrochem. Comm. 9, 947 (2007).8. R. Hahn et al., Phys. Stat. Sol. (RRl) 1, 135 (2007).
9:00 PM - H13.43
Heterojunction MEH-PPV:CdS/TiO2 Solar Cells.
Chanchana Thanachayanont 1 , Kroekchai Inpor 2 , Natthawut Temchuen 2 , Somjai Reabanko 3 , Pratya Boonchan 3 , Somboon Sahasithiwat 1 , Visanu Meeyoo 3 , Porponth Sichanugrist 2
1 , National Metal and Materials Technology Center, Pathumthani Thailand, 2 , Institute of Solar Energy Development, Pathumthani Thailand, 3 , Mahanakorn University of Technology, Bangkok Thailand
Show Abstract9:00 PM - H13.44
Titania Nanotubes for Photoelectrochemical and Photovoltaic Applications.
Christiaan Richter 1 , Eugen Panaitescu 2 , Laura Lewis 1 , Ronald Willey 1 , Latika Menon 2
1 Chemical Engineering, Northeastern University, Boston, Massachusetts, United States, 2 Physics, Northeastern University, Boston, Massachusetts, United States
Show AbstractAnodic titania nanotubes [1] have great potential for use in solar energy generation, in particular as a photocatalytic material for the direct solar production of hydrogen via the splitting of water and as the metal oxide support in dye sensitized (photovoltaic) solar cells. The nanotube morphology and detailed chemical composition impact the photocatalytic and photovoltaic activity of this material. For example, nanotubes possess a large catalytically-active surface area, enhanced UV light absorption and reduced charge recombination rates. Titania nanotubes fabricated by electrochemical anodization have superior contact between the oxide (TiO2) and metal (Ti) substrate, allowing much more efficient charge transfer than that furnished by sintered nanoparticles. Recent results have suggested that the TiO2 radiation absorption window can be extended into the visible spectrum by carbon doping [2, 3]. The merits of different techniques for the electrochemical fabrication of nanotube arrays will be discussed and light absorption properties will be evaluated through reflectivity, photocurrent and gas chromatography (GC) measurements. The effect of carbon doping in TiO2 nanotubes doped during synthesis and their water-splitting efficiency under simulated solar light has been evaluated. Recent results concerning the synthesis, characterization and data interpretation of titania nanotubes will be presented. References:[1]Richter, C., et al., Ultra-High-Aspect-Ratio Titania Nanotubes. Advanced Materials, 2007. 19(7): p. 946-948.[2]Wang, H. and J.P. Lewis, Second-generation photocatalytic materials: anion-doped TiO2. Journal of Physics: Condensed Matter, 2006. 18(2): p. 421-434.[3]Park, J.H., S. Kim, and A.J. Bard, Novel Carbon-Doped TiO2 Nanotube Arrays with High Aspect Ratios for Efficient Solar Water Splitting. Nano Lett., 2006. 6(1): p. 24-28.
9:00 PM - H13.45
Hybrid Solar Cell Designed by Aligned Electrospun TiO2 Nanofibers and Conjugated Polymer.
Hee-Sang Shim 1 , Won Bae Kim 1 , Hyo-Jin Ahn 1 , Yong Seok Kim 1 , Sang Hoon Nam 1 , Seung Mok Choi 1
1 MSE, Gwangju Institute of Science & Technology (GIST), Gwangju Korea (the Republic of)
Show AbstractA lot of efforts for making efficient organic photovoltaic cells have suggested. One of the efforts is an introduction of the inorganic materials to improve the transporting rate of dissociated electron at the organic-inorganic interface. These solar cell called a hybrid photovoltaic cell (HPV) which are typically composed of an inorganic semiconductor material as an electron acceptor and a conjugated organic polymer material as an electron donor[1-3]. However, these photovoltaic cells still have problem in electron dissociation and transporting due to the structural limitation of proposed inorganic nanostructure and the unfavorable interface between organic and inorganic materials.[4,5] In this work, highly aligned TiO2 nanofibers were proposed as an electron acceptor material to improve the electron dissociation and transporting in HPVs. Effects of their degree of the alignment and stack of the aligned layer have been investigated through various analyses such as photoluminescence (PL), UV-vis, transmission electron microscopy (TEM) and solar performance test. Titanium (IV) dioxide nanofibers arrays were synthesized by electrospinning technique on patterned indium tin oxide coated transparent glasses for solar cell application. Electrospun titania nanofibers were aligned using rotating collector on substrates. To characterize the photovoltaic properties as a different thickness and mass, organic-inorganic hybrid cells were fabricated. Poly[2-methoxy, 5-(2’-thylhexyloxy)-1,4-henylenevynylene] (MEH-PPV) which acts as an electron donor and light absorber was coated on the flat titania film, one-directionally ordered titania nanofibers and aligned titania nanofibers stacks, respectively. And then, PEDOT:PSS layer was prepared on this as an easy hole transport layer and exciton blocking layer. Finally, an Au top electrode was evaporated thermally in a vacuum at a pressure of the order of 10-6 Torr. Cell performance was measured under 100 mW/cm2 using Xe light source with AM 1.5 global filter and Keithley 4200 source meter. Structural analysis of prepared titania nanofibers electrodes were carried out by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), TEM bright field image. For the calcinated electrospun nanofiber, the XRD pattern corresponded with reference anatase titania (JCPDS #84-1286) and such result got also from XANES spectra. Calcinated electrospun spectra represents the formation of anatase titania as shown in Figure 1. The photovoltaic and microstructural properties of nanocomposite will be described and discussed in detail.References1. W. U. Huynh, A.P. Alivisatos, Science, 295 (2002) 2495.2. K.M. Coakley, M. D. McGehee, Appl. Phys. Lett. 83(2003) 3380.3. W. J. E. Beek, R. A. J. Janssen, J. Mater. Chem. 15(2005) 2985.4. M. Lira-Cantu, F. C. Kerbs., Sol. Energy. & Sol. Mater. 90(2006)2076.5. T. W. Zeng, Wei-Fang Su, Nanotech. 17(2006) 5387.
9:00 PM - H13.46
Nanocrystalline TiO2 films Deposited via Rapid Expansion Supercritical Solution for Dye-sensitized Solar Cells.
Wen-Yao Huang 1 , Chun-Che Lee 1 , Yu Ting Lin 1 , Yuan Hong Lin 1
1 , institute of Electro-Optical Engineering, Kaoshiung,Taiwan Taiwan
Show Abstract9:00 PM - H13.47
Nanostructured Oxide Semiconductor Films with Controlled Morphology by a Flame Aerosol Reactor (FLAR).
Elijah Thimsen 1 , Neema Rastgar 1 , Pratim Biswas 1
1 Energy Environmental and Chemical Engineering, Washington University in Saint Louis, Saint Louis, Missouri, United States
Show AbstractNanostructured oxide semiconductors, such as TiO2, are essential to many low cost solar cells. The morphology of the semiconductor has been identified as an efficiency-limiting component, making rational control over morphology critical to any film synthesis process. An atmospheric pressure, single-step gas phase process to rapidly synthesize robust nanostructured oxide films with controlled morphology has been developed (AIChE J., Vol. 53, No. 7, 2007). The process is based on a flame aerosol reactor (FLAR) and requires about 10 minutes to deposit one film. Control over film morphology is achieved through the process parameters. The key system variables are aerosol-phase particle size and substrate temperature, through their influence on particle sintering on the substrate. These system variables can be controlled though process parameters such as: precursor feed rate, burner-substrate distance and substrate cooling rate. Through judicious adjustment of the process parameters, the film morphology can be tuned to meet the needs of specific applications. Two types of film morphologies are observed. The first is a granular structure consisting of agglomerated particles caked onto the substrate, characterized by a high surface area and poor exciton mobility. The second is a columnar morphology consisting of highly crystalline one dimensional (1D) structures oriented normal to the substrate, characterized by slightly lower surface area and superior charge-carrier mobility. The semiconductor film morphology affects performance. Morphology-performance correlations for two types of solar cells based on TiO2, a dye-sensitized solar cell and a photo-watersplitting cell, are reported. The photo-watersplitting cell achieves uv-light to hydrogen conversion efficiencies in excess of 10%, rivaling the best reported values in the literature.In addition to control over film morphology, the FLAR offers other advantages. It can be used to deposit multiple layers of intermediate band-gap materials or to form solid-state inorganic p/n junctions, through sequential deposition of different semiconducting oxide materials. Preliminary results for such systems will be presented.
9:00 PM - H13.49
Synthesis of Nanostructured SnO2 Thin Film Electrode Using the Nanoimprinted Glass.
Takahiro Watanabe 1 , Akita Yasuyuki 1 , Masahiko Mitsuhashi 2 , Mamoru Yoshimoto 1
1 Department of Innovative and Engineered Materials, Toukyou Institute of Technology, Yokohama, Kanagawa Japan, 2 , Knangawa Industrial Technology Center, Ebina Japan
Show AbstractFor development of large area nanostructured solar cells using small molecules, nanostructured transparent electrode are expected to play an imprint role from the point of device construction. Transparent conducting oxide (TCO) material of SnO|*bsub*|2|*esub*| has chemical stability and high transparency, and SnO|*bsub*|2|*esub*| thin films are used for electrode especially of silicon-based solar cell. Recently, solar cells have nanostructure on the TCO thin film (e.g. TiO|*bsub*|2|*esub*| nanocrystals in Dye-Sensitized Solar Cell), so the properties of nanostructured TCO thin film should be revealed. In this work, the nanostructured SnO|*bsub*|2|*esub*| thin films were fabricated on the nanowire array patterned glass substrate, and electrical and optical properties of the films were characterized. Recently, we have reported that the periodic channel structures on the NiO thin film surfaces were generated [1]. The NiO films were deposited on the atomically smooth sapphire (0001) substrates by pulsed laser deposition (PLD) method at room temperature. NiO films were annealed 3 h in air at 700|*deg*|C, and then the periodic nanogroove-striped structures on the film surfaces could be generated.In experiments, by using the nanogroove-striped NiO thin films as the molds, nanoimprinting was carried out onto the silicate glass plate having the glass transition temperature (Tg) of 521|*deg*|C [2]. The nanopatterned mold was contacted to the surface of the glass plate and then heated them at 600|*deg*|C and pressed at 1 kPa for 1 h in air. When the sample was cooled down to below 40|*deg*|C, the mold was separated from the glass plate. The surface morphology of the specimens was observed by atomic force microscopy (AFM). The nanowire array pattern was obtained on the glass surface through reversely transferring of the mold. The nanowires had an interval of ~80 nm, wire width of ~70 nm, and wire height of ~20 nm.Then, pure and Ge-doped SnO|*bsub*|2|*esub*| thin films were fabricated on nanopatterned glass substrate by pulsed laser deposition method. KrF excimer laser beam (wavelength of 248 nm, duration time of 20ns) was irradiated onto the sintered ceramics targets at the substrate temperature of 300|*deg*|C and 1.0 x 10|*bsup*|-5|*esup*| Torr O|*bsub*|2|*esub*| atmosphere. Sintered ceramics targets of SnO|*bsub*|2|*esub*| and SnO|*bsub*|2|*esub*|/GeO|*bsub*|2|*esub*| = 97/3 mol% were used. Electrical and optical properties of the films are presented.[1] Akiba et al. Nanotechnology 17 (2006) 4053-4056[2] Akiba et al. Appl. Surf. Sci. 253 (2006) 4512-4514
9:00 PM - H13.5
New 3D Configuration for Solid State Inorganic Nanocomposite Solar Cells.
Marian Nanu 2 , Diana Nanu 1 , Barbara van der Zanden 2 , Camelia Joanta 2 , Harry Donker 2 , Ben Meester 2
2 , Advanced Photovoltaic Applications B.V., Bleiswijk Netherlands, 1 Materials Science and Engineering, Delft University of Technology, Delft Netherlands
Show AbstractInexpensive, stable, non-toxic, and efficient solar cells are required to ensure a widespread use of solar power as energy source. With the advent of dye-sensitized (Grätzel) solar cells, C60/polymer, and CdSe/polymer bulk heterojunctions challenging alternatives are offered. A major concern in these alternatives is their poor stability when operating in full sunlight. All-solid, completely inorganic, bulk heterojunctions do not require expensive sealing. In recent years, interest in solar cells based on chalcopyrite-like semiconductors such as Cu(In,Ga)(Se,S)2 has grown. Different configurations of Cu(In,Ga)(Se,S)2 based thin film and nanocomposite inorganic solar cells have been proposed. Although efficiencies up to 19% were obtained for thin film Cu(In,Ga)(Se,S)2 solar cells, much lower efficiencies (3-5%) were reported for nanocomposite solar cells comprising these materials. Responsible for the low efficiency of nanostructured inorganic solar cells is the charge recombination due to defects and interfaces. New concepts of nanostructured heterojunctions are required to minimize charge recombination. Moreover, the use of materials with low toxicity and high stability are desirable for new concepts.Here, we report on a new approach towards an all-solid-state inorganic solar cell with a 3D configuration that ensures fast charge transport and low recombination. In this type of nanocomposite solar cell, a wide bandgap n-type semiconducting oxide (anatase TiO2) and a p-type visible-light sensitive semiconductor (CuInS2) are interconnected on a nanometer scale. Nanotubes of TiO2 are obtained using electro-corrosion of 2 mm titanium foil. After calcination of TiO2 in air at 450 oC for 1 hour, sequentially and without cooling down, a 20 nm In2S3, and a 1 µm CuInS2 layer are sprayed in air with a maximum temperature of 350 oC. To complete the solar cell an ITO layer is also sprayed at the same temperature. Current voltage measurements under AM 1.5 irradiation of such devices shows promising results. This solar cell concept may open a new horizon in the manufacture of next-generation photovoltaic devices.
9:00 PM - H13.50
Improvment of the Efficiency of Dye-sensitive Solar Cell with TiO2 Blocking Layer Prepared by Different Processes.
Iris Wang 1 , Yung-Liang Tung 1 , Song-Yeu Tsai 1
1 Photovoltaic Technology Center, Industrial Technology Research Institute, Hsinchu Taiwan
Show AbstractA compact TiO2 film between TiO2 mesoporous electrode and transparent conducting glass usually serves as an effective blocking layer to retard back electron transfer from transparent conducting oxide to electrolyte in dye sensitized solar cells [1-2]. Wet process such as sol-gel or aerosol pyrolysis was mostly employed to prepare the blocking layer. In the articles, vacuum deposition method such as atomic layer deposition and sputtering will be carried out instead of wet process. The optimization in thickness and process conditions will be studied under the characterization of electrochemical impedance spectroscopy and photocurrent-voltage characteristics. The effectiveness of the TiO2 blocking layers on the photovoltaic performance of DSSC will be discussed for various preparation processes. Ref. 1. J.N. Hart, D. Menzies, Y-B, Cheng, George.P. Simon, L. Spiccia, C.R. Cimie 9 (2006), 622-626.2. L. Kaven and M. Gratzel, Electrochim. Acta 40 (1995), 463.
9:00 PM - H13.51
The Role of Multi-wall Carbon Nanotubes Within the Donor Layer of Bi-layer Organic Solar Cells.
Anthony Miller 1 , Ross Hatton 1 , Ravi Silva 1
1 Advanced Technology Institute, University Of Surrey, Guildford, Surrey, United Kingdom
Show AbstractThe role of multi-wall carbon nanotubes (MWCNTs) incorporated within the donor layer of bi-layer organic solar cells is investigated. We show that low concentrations of MWCNTs (0.25 – 7.5%) can be uniformly distributed within the donor layer without complicating the device fabrication process or reducing the optical density. The MWCNTs are shown to act as a large area interpenetrating electrode having a dramatic effect on the cell series resistance and fill factor. The MWCNTs are also shown to play a role in the open circuit voltage and we demonstrate that the open circuit voltage in said devices can be manipulated by chemical functionalisation of the nanotube surface with polar surface moieties. Two different donor layer material systems are investigated and are based on MWCNTs combined with PmPV and PTEBS respectively. The latter is processed from an aqueous solution which greatly enhances its environmental compatibility. This work clearly demonstrates the potential MWCNTs have as a versatile functional material in organic solar cells.
9:00 PM - H13.6
Multiexciton Generating Nanowires of PbSe and PbS.
Song Jin 1
1 , University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractOne-dimensional nanowire materials of multiexiton generating PbSe and PbS were synthesized using a chemical vapor deposition and structurally characterized. Complex networks of multiple levels of nanowires grow perpendicularly from the previous generation of nanowires in an epitaxial fashion to produce a dense cluster structure of a complex nanowire network. No intentional catalyst was employed for the nanowire synthesis, but it is suggested that lead itself might serve as a vapor-liquid-solid (VLS) catalysts for the anisotropic growth of PbS/PbSe. The Raman scattering and preliminary photophysical properties of these nanoscale multiexciton generating materials will be discussed. While still confined in 2 dimensions, photocarriers can be collected along the axial direction of the wire, thus these complex interconnected networks of nanowire materials of multi-exciton generating materials would provide a direct electrical connection for carrier collection, which is potentially of advantages in solar energy harvesting.
9:00 PM - H13.7
Study of Carrier Multiplication in Semiconductor Nanocrystals by Transient Photoluminescence Spectroscopy.
Gautham Nair 1 , Moungi Bawendi 1
1 Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThe enhancement of carrier multiplication (CM) is an important aim that could increase solar cell performance and widen the range of materials suitable for future solar technologies. Pump-probe measurements have shown evidence of strongly enhanced CM in lead chalcogenide, InAs, and CdSe nanocrystals (NCs). However, the nature of the enhancement mechanism is not well understood. We have carried out an experimental assessment of CM yields in semiconductor NCs by carefully studying exciton and biexciton signatures in transient photoluminescence decays. In the case of CdSe NCs, though the technique is particularly sensitive due to the biexciton’s strong radiative rate, we have found no evidence for CM up to photon energies as high as 3.1 Eg. These are well above previously reported relative energy thresholds for CM in CdSe NCs. The implications of our findings are discussed within a general physical framework.
9:00 PM - H13.8
Development of Double-Gyroid Topology Nanowire Array Solar Cells.
Hugh Hillhouse 1 2 , Rakesh Agrawal 1 2
1 School of Chemical Engineering, Purdue University, West Lafayette, Indiana, United States, 2 The Energy Center, Purdue University, West Lafayette, Indiana, United States
Show Abstract9:00 PM - H13.9
Multi-exciton Generation and Dissociation in Blends of PbSe with Conjugated Organic Materials for High-efficiency Solar Cells.
Minh Tuan Trinh 1 , Juleon Schins 1 , Jorge Piris 1 , Albert Goossens 1 , Arjan Houtepen 2 , Daniel Vanmaekelbergh 2 , Tobias Hanrath 3 , Rene Janssen 3 , Laurens Siebbeles 1
1 DelftChemTech, Delft University of Technology, Delft Netherlands, 2 Debye Institute, University of Utrecht, Utrecht Netherlands, 3 Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractIn a conventional solar cell, absorption of a photon by an inorganic semiconductor leads to formation of electrons and holes with an excess energy equal to the difference between the photon energy and the semiconductor bandgap. This excess energy is wasted in the form of heat. Use of low bandgap nanocrystals as the light-absorbing medium has the potential to enhance the efficiency of a solar cell by utilizing the excess energy to produce multiple electron-hole pairs. The current work aims to provide insight into the mechanism of multi-exciton generation (MEG) in PbSe nanocrystals and to dissociate the multi-excitons in blends with organic materials. Experiments on MEG were carried out using femtosecond optical and terahertz spectroscopic techniques.Pyridine capped PbSe nanocrystals with a band gap of 0.8 eV were excited by 400 nm photons (3.1 eV). The ratio (Nx) of the bleaching of the first exciton transition at short (~ 1ps) and longer times (~ 1ns) is usually interpreted as the number of excitons per photo-excited nanocrystal. For incident pump fluence sufficiently low, so that on average at most one photon is absorbed per nanocrystal, the value of Nx corresponds to the efficiency of MEG. For pumping at 400 nm we find an efficiency for MEG equal to 1.9 excitons per absorbed photon, in agreement with the literature. Increasing the pump fluence causes the value of Nx to approach 8 in agreement with expectations on basis of the 8-fold degeneracy of the first exciton state. However, for pumping at 800 nm (1.55 eV) values of Nx depend on the probe wavelength and can reach values as high as ~ 20. Possible causes of these phenomena will be discussed. In addition, results on charge transfer from photo-excited nanocrystals to conjugated polymers (P3HT and MEH-PPV) will be presented.