George Crabtree Argonne National Laboratory
Arthur J. Nozik National Energy Renewable Laboratory
Paul Alivisatos University of California-Berkeley
Michael Wasielewski Northwestern University
Rene Janssen Eindhoven Technical University
Nathan S. Lewis California Institute of Technology
CC1: Inorganic Photovoltaics I
Monday AM, November 27, 2006
Independence W (Sheraton)
9:00 AM - CC1.1
Comparison of Techniques for Measuring Recombination Lifetime in Photovoltaic Materials.
Richard Ahrenkiel 1 2 , Steven Johnston 2 , Wyatt Metzger 2 Show Abstract
1 Physics and Astronom, University of Denver, Denver, Colorado, United States, 2 Measurements and Characterization Division, National Renewable Energy Laboratory, Golden, Colorado, United States
Minority-carrier or recombination lifetime is a critical photovoltaic parameter, and is directly related to performance and efficiency. For measuring this parameter, a variety of lifetime techniques are used in the research laboratory and in the production environment. Here, we will analyze four techniques that are currently used for the lifetime characterization of photovoltaic materials at NREL. These are: (A) time-resolved photoluminescence (TRPL); (B) microwave reflection (μPCD); (C) quasi-static photoconductance (QSSPC); (D) resonant-coupled photoconductive decay (RCPCD). All of these methods measure the decay rate of electron-hole pairs after optical excitation. The first three techniques are commercially available, while RCPCD, developed at the National Renewable Energy Laboratory (NREL), has been extensively used in-house to support the national photovoltaic program. Here we will present data and compare results on a variety semiconductor materials, measured by the four techniques. The samples range from direct-bandgap GaAs to indirect-bandgap silicon. Techniques A, B, and D are currently suitable for direct-bandgap semiconductors such as GaAs and InGaAs. Techniques B, C, and D are applied to single-crystal and multicrystalline silicon. Comparisons of the data and the application of transport theory show that both photoconductance techniques (C and D) are especially sensitive to shallow (temporary) trapping effects. When shallow traps are present, the data must be properly massaged in order to extract the recombination lifetimes. Three of the techniques (A, C, and D) are suitable for measuring lifetime over a wide range of injection levels; i.e. high-injection lifetimes may be extracted with these technique. We will show by both experiments and theoretical simulation that the μPCD signal is proportional to excess carrier density only at very low-injection levels, and misleading results may occur at high-injection levels. The photoconductance techniques (C and D) must include an algorithm to account for the variation of carrier mobility with injected carrier density in order to extract the true recombination lifetime at high-injection levels. Although the TRPL technique provides the most unambiguous results for high-injection studies, it is a functional lifetime-measurement technique only for the direct-bandgap semiconductors. The transient techniques A, B, and D are readily adaptable to the measurement of temperature-dependent lifetime. Temperature-dependent lifetime has proven to be valuable in identifying the dominant recombination defect or mechanism in some materials. In summary, we will compare all of the above techniques over a range of materials and measurement conditions and discuss the strengths and weaknesses of each. This information is valuable to those who use these techniques to evaluate photovoltaic materials.
9:15 AM - CC1.2
Mechanical stress measurements with Micro-Raman Spectroscopy in Multi-Crystalline Thin film Solar Silicon
Michael Becker 1 , Gudrun Andrä 2 , Fritz Falk 2 , Silke Christiansen 1 3 Show Abstract
1 , Martin-Luther University, Halle Germany, 2 , Institute of Physical Hightechnology, Jena Germany, 3 , Max-Planck Institute of Microstructure Physics, Halle Germany
Multicrystalline thin film solar cells are characterized by grain sizes, of several ten micrometers in diameter, with grain boundaries between grains. The grain boundary population contains essentially twin boundaries of first and second order, but also random grain boundaries can be observed. The twin boundaries are usually benign in terms of degradation of electrical properties and also usually no long range stresses/strains are related to these grain boundaries, while random large angle grain boundaries are often decorated with dislocation networks and are prone to stress fields of several ten to several hundred MPa. These stresses bring the material close to its theoretical strength limit and materials failure during solar cell processing may easily occur. Therefore, a non-destructive method to locally determine stress-concentrations in multi-crystalline solar cells is of technological importance. In this paper we will present such a method based on micro-Raman spectroscopy to experimentally determine the components of stress tensors within grains of multicrystalline thin film solar cell materials. Currently, micro -Raman spectroscopy is used to measure stresses in silicon wafers and structures of known crystallographic orientation (usually Si(001)). For these cases, the determination of stresses from Raman peak shifts is straightforward. In multicrystalline silicon, however, arbitrary grain orientations unease the determination of stress tensor components. The intensity dependence of Raman intensities on orientation is used to determine the crystallographic grain orientations (accuracy of 1 to 2 degree). Once the orientation is determined, the components of the stress tensor (minimal detectable mechanical stress of around 15 MPa) can be calculated numerically from the Raman peak shifts. As examples, we measure the stress components of grain boundary configurations, e.g. triple points in thin film silicon on glass as obtained by layered laser crystallization of amorphous silicon layers . Independently, we measure grain orientations by electron backscattering diffraction in the scanning electron microscope and perform transmission electron microscopy and analytics to fully characterize the multicrystalline real structure including grain boundaries, dislocations and precipitates. G. Andrä, J. Bergmann, F. Falk, E. Ose, Proc. 19th Europ. Photovoltaic Solar Energy Conf., Vol. I, 873 (2004)
9:30 AM - CC1.3
Transport Properties of Laser-crystallized Polycrystalline Silicon-germanium Thin Films.
Lars-Peter Scheller 1 , Moshe Weizman 1 , Norbert H. Nickel 1 , B. Yan 2 Show Abstract
1 , Hahn-Meitner-Institut Berlin, Berlin Germany, 2 , United Solar Ovonic Corporation, Troy, Michigan, United States
9:45 AM - CC1.4
Evaluation of Polycrystalline Silicon for Solar Cells by Small p-n Diode Array.
Satoshi Tanaka 1 , Keita Imai 1 , Atsushi Ogura 1 , Koji Arafune 2 , Hideaki Kawai 2 , Futoshi Kusuoka 2 , Yoshio Ohshita 2 , Masaaki Inoue 3 , Michio Tajima 3 Show Abstract
1 School of Science and Technolgy, Meiji University, 1-1-1 Higashimita, Tama-ku, Kawasaki, Kanagawa 214-8571, Japan, 2 , Toyota Technological Institute, 2-12-1 Hisakata, Tenpaku, Nagoya, Aichi 468-8511, Japan, 3 , The Institute of Space and Astronautical Science (ISAS)/ Japan Aerospace Exploration Agency(JAXA), 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
10:00 AM - CC1.5
All-chemically Deposited Thin Film Solar Cells using Inorganic Compound Semiconductors.
P. Karunakaran Nair 1 , Guadalupe Delgado 1 , Harumi Moreno 1 , Yolanda Peña 1 , Jose Campos 1 , M. T. Santhamma Nair 1 Show Abstract
1 Centro de Investigacion en Energia, Universidad Nacional Autonoma de Mexico, Temixco, Morelos, Mexico
10:15 AM - CC1.6
Investigation of Dangling Bond Defects in Laser-crystallized Polycrystalline SiGe Thin Films.
Moshe Weizman 1 , Ekaterina Terukova 1 , Norbert H. Nickel 1 , Baojie Yan 2 Show Abstract
1 , Hahn-Meitner-Institut, Berlin Germany, 2 , United Solar Ovonic Corporation, Troy, Michigan, United States
11:00 AM - CC1.7
Intentional Interface Defects as a Means of Band Gap Engineering and the Applications to Photovoltaics
Marcie Black 1 , James Maxwell 2 Show Abstract
1 , LANL, Newton, Massachusetts, United States, 2 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Interfaces can be designed to break the periodic boundary conditions in a crystal; thus interfaces can be used as a new type of band gap engineering. Once the effects of interfaces on the band energies of a crystal are well understood, new materials can be designed in which the properties are optimized for certain applications. In this study, the finite lattice band diagram for silicon is calculated using the tight binding theory. The electronic structure of the finite lattice differs significantly from its bulk counterpart. A surface state forms which is a result of the mixing of the L and Γ points electronic pockets of bulk silicon. The presence of such surface states potentially makes nano-scaled silicon material a good candidate for an intermediate band solar cell. This talk will explain the finite tight binding model implemented, highlighting key differences in the band energies of a finite lattice of silicon compared to bulk silicon. The presentation will also explore the uses of this material for photovoltaic applications.
11:15 AM - CC1.8
Control and Manipulation of Efficiency-Limiting Nanodefects in Multicrystalline Silicon Solar Cells
Tonio Buonassisi 1 , Matthias Heuer 1 , Andrei Istratov 1 , Matthew Pickett 1 , Eicke Weber 1 Show Abstract
1 Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States
A growing 50% of solar cells produced worldwide are made of multicrystalline silicon (mc-Si), a very different material from the more costly, single-crystalline variety. As unfortunate by-products of low-cost feedstocks and crystal growth methods, mc-Si typically contains a plethora of metal impurities in parts-per-billion to parts-per-million concentrations. While metallic impurities are severely detrimental to solar cell conversion efficiencies when not properly controlled, recent work has demonstrated the potential to manipulate impurity distributions, concentrations, and chemical states on the nanometer scale, thus reducing their global impact on solar cell performance and potentially enabling the use of lower-cost, lower-purity silicon feedstock.We have applied synchrotron-based X-ray analytical microprobe techniques to investigate interaction phenomena between multiple metal species in mc-Si. Initially, X-ray fluorescence microscopy (µ-XRF) was employed to develop a classification scheme that divides metallic impurities into general classes of impact. Faster diffusing species such as copper and nickel tend to accumulate preferentially at the most energetically favorable sites (e.g., grain boundaries), reducing their detrimental impact. On the other hand, more slowly diffusing species such as iron and titanium tend to be more homogeneously distributed, forming highly recombination-active point defects and smaller precipitates.It is possible to quantify the effects of different metal nanodefect distributions on solar cell material performance via the spectrally resolved X-ray beam-induced current (SR-XBIC) technique, which uses the synchrotron X-ray beam to generate minority carriers collected by a Schottky diode or pn-junction. It was found that concentrating efficiency-limiting impurities in select locations (e.g., isolated precipitates) is far preferred over more homogeneous distributions. Furthermore, by manipulating metal nanodefect distributions, e.g., by controlling relaxation phenomena, it is possible to vary the minority carrier diffusion length by up to a factor of four, without removing a single metal atom from the material. Further control over metal distributions can be garnered by harnessing multiple metal interactions. It is commonly known, but often overlooked, that many of the faster diffusing species in silicon form metal-silicon eutectics at temperatures several hundred degrees below the melting temperature of silicon. Under the proper conditions, tiny metal-silicon liquid droplets can be formed inside silicon, which have the potential to absorb other (more dangerous) metal impurity species. Using X-ray absorption microspectroscopy (µ-XAS) and computer modeling, we have intentionally formed and identified a novel multiple-metal compound in mc-Si, providing direct evidence for the hyper-accumulation of multiple metallic species.
11:30 AM - CC1.9
Environment Related Conditions Imposed to Solar Cells Materials.
Liviu Popa-Simil 1 Show Abstract
1 , LAVM Inc., Los Alamos, New Mexico, United States
11:45 AM - CC1.10
Material Issues for Terawatt Level Deployment of Solar Cells.
Alex Freundlich 1 , Andrea Feltrin 1 Show Abstract
1 Center for Advanced Materials, University of Houston, Houston, Texas, United States
12:00 PM - CC1.11
An Inorganic Approach to Wet-Chemically Fabricated Tandem Cells.
Meng Tao 1 Show Abstract
1 Department of Electrical Engineering, University of Texas at Arlington, Arlington, Texas, United States
The widely-cited advantage of organic solar cells is their low cost, enabled by the wet-chemical fabrication process, but their efficiency is low due to the inferior properties of organic semiconductors. Inorganic crystalline solar cells have demonstrated the highest efficiencies, but they are expensive due to the vacuum-based fabrication process. In this talk, the author will present an inorganic approach to wet-chemical fabrication of tandem cells, which promises a new generation of solar cells as cheap as organic ones but as efficient as III-V tandem cells. The class of materials which allows wet-chemical fabrication of inorganic tandem cells is semiconducting metal oxides. Their ability to be synthesized by wet chemistry but still possess excellent electrical and optical properties for solar cells is attributed to the ionic bonding in these materials, which is more flexible to accommodate structural defects than the rigid covalent bonding in silicon. Experimental results will be shown for metal oxide thin films synthesized by wet chemistry, which show a polycrystalline nature with carrier mobility between 10 and 50 cm2V-1s-1 at a deposition temperature of 40 to 80oC. An analysis will be presented on the challenges of utilizing metal oxides for low-cost high-efficiency solar cells, including the band gap, which tends to be larger than the optimum 1.4 eV for single-junction cells, and native point defects, which often dominate their electrical properties. The author proposes mixed-valence metal oxides to achieve the desired band gap in these materials for solar-cell applications. The need for doping in these materials to combat native point defects is reiterated and the author uses molecular dynamic simulation to understand the native point defects. Copper oxides are of interest for single-junctions cells with their band gap between 1.0 and 2.1 eV. For 50% efficient third-generation tandem cells, metal oxide hetero-valence multi-junctions (HVMJ) are proposed. Titanium oxides are particularly attractive. When the oxygen content, x, in TiOx changes from 1 to 2 (or the valence of Ti changes from 2+ to 4+), the band gap of TiOx goes from 0 to 3.5 eV, covering the entire solar spectrum! This allows wet-chemical fabrication of inorganic tandem cells with an arbitrary number of junctions in the cell and an arbitrary band-gap value for each junction.
12:15 PM - CC1.12
Silicide Mediated Grown Silicon Thin Films for Photodiodes.
Joondong Kim 1 2 , Wayne Anderson 1 , Chang-Soo Han 2 , Eung-Sug Lee 2 Show Abstract
1 Electrical Engineering, University at Buffalo, Buffalo, New York, United States, 2 Nano-Mechanical Systems Research Center, Korea Institute of Machinery and Materials, Daejeon Korea (the Republic of)
12:30 PM - CC1.13
Bulk-heterojunction Photovoltaics from Solution-processed Oligoacene:Fullerene Blends.
Matthew Lloyd 1 , Alex Mayer 1 , John Anthony 2 , George Malliaras 1 Show Abstract
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 2 Chemistry, University of Kentucky, Lexington, Kentucky, United States
12:45 PM - CC1.14
Radical Salt-Doped Hole Transporters In Organic Photovoltaic Devices
Santhisagar Vaddiraju 1 , Mathew Mathai 1 , Emmanuel Kymakis 2 , Fotios Papadimitrakopoulos 1 3 Show Abstract
1 Nanomaterials Optoelectronics Laboratory, Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States, 2 Technological Educational Institute (T.E.I) of Crete, School of Applied Technology, Estavromenos, Crete, Greece, 3 Department of Chemistry, University of Connecticut, Storrs, Connecticut, United States
Organic photovoltaics are the focus of intense research efforts due to the low-cost of processing and their potential applications in flexible electronics. Various materials and device architectures have been proposed to achieve higher power conversion efficiencies. Oxidized transport layer (OTL) is typically a three component system comprising of a charge transport molecule, an oxidized charge transport molecule and a polymer binder. Our group had investigated the use of OTL in organic light emitting devices due to its homogenous and tunable conductivity. We herein report on the possibility of the use of this radical salt doped polymer in organic photovoltaic devices. Single layer organic photovoltaic devices based on a poly carbonate of N, N’-bis(3-hydroxymethyl)-N,N’-bis(phenyl) benzidine and diethylene glycol (PC-TPD) as a photoactive material were fabricated and studied. An organic dopant, SbF6- salt of N,N,N’,N’-tetrakis (4-methylphenyl) benzidine (TMPTD.+ SbF6-) was used to dope the polymer. Device comprising of this polymer were fabricated with various amounts of salt in the polymer. The short circuit current as well as the open circuit voltage increased with increasing amount of the salt dopant in the polymer which was attributed to the formation of a heterojunction between the salt (TMPTD.+ SbF6-) and the polymer (PC-TPD). Along with these results, we would be discussing results from the bulk heterojunction devices based on this polymer and buckminster fullerene (C60). Photovoltaic devices based on the bulk heterojunction concept, containing a blend of this OTL polymer (with varying salt concentration) and Buckminster fullerene (C60) were studied. It was found that the short circuit current increased by 1 - 1.5 orders of magnitude suggesting the classical case of photo induced electron transfer from the polymer to the fullerene. This short circuit current was further increased to 2 orders of magnitude (0.48 mA/cm2) by increasing the salt concentration to 10%. The power conversions of these devices also increased with increasing salt concentration with the maximum of 1.5% achieved for the device containing a 1:1 blend of OTL and C60 wherein the salt concentration is fixed at 10%.
CC2: Inorganic Photovoltaics II and Organic Photovoltaics
Monday PM, November 27, 2006
Independence W (Sheraton)
2:30 PM - CC2.1
Atmospheric Pressure Synthesis of CuInSe2 Thin Films from Liquid Precursors.
Jennifer Nekuda 1 2 , Maikel van Hest 1 , Alex Miedaner 1 , Calvin Curtis 1 , John Perkins 1 , Dennis Readey 2 , David Ginley 1 Show Abstract
1 Photovoltaics, National Renewable Energies Labratory, Golden, Colorado, United States, 2 Metallurgy and Materials Engineering, Colorado School of Mines, Golden, Colorado, United States
Newly developed liquid In-Se and Cu-Se metal organic decomposition (MOD) precursors were used in conjunction with atmospheric pressure rapid thermal processing (RTP) to make crystalline CuInSe2 (CIS) thin films. This synthesis approach does not require either expensive vacuum deposition systems or the use of toxic gases (H2Se), therefore, it holds great promise for economically and environmentally viable production of CIS thin film solar cells. Two different precursor treatments have proven successful in producing CIS thin films. The first method involves a layering sequence of indium-and copper-based precursors with rapid thermal processing between the layers. The indium-based precursor is first spin coated onto Mo-coated glass and then rapid thermal processed in flowing argon at atmospheric pressure. X-ray diffraction (XRD) analysis shows that several indium-based layers can be stacked using this method to produce γ-In2Se3 with the desired thickness. In order to produce CIS, a copper-based precursor was then spin-coated on the In2Se3 film. Upon rapid thermal processing, this precursor reacted with the In2Se3 film to produce CIS, observed via XRD. In these layering experiments, an unidentified intermediate phase is formed before full conversion to CIS occurs, but disappears when full conversion is reached. The second method which proved successful in fabricating CIS thin films on Mo-coated glass was through the mixture of the indium- and copper-based precursors in the liquid phase. This mixture was then spun onto Mo-coated glass substrates and rapid thermal processed under the same conditions as the previous experiments. XRD analysis confirmed that a homogeneous CIS thin film is produced via this method. Details of the precursor chemistry and rapid thermal process conditions as well as scanning electron microscope (SEM) and XRD materials characterization will be presented.
2:45 PM - CC2.2
p-type Carbon Nanotube Transparent Contacts for Photovoltaics.
Teresa Barnes 1 , Jao van de Lagemaat 1 , Miguel Contreras 1 , Garry Rumbles 1 , Sean Shaheen 1 , Timothy Coutts 1 , Chris Weeks 2 , Igor Levitsky 2 , David Britz 2 , Jorma Peltola 2 , Paul Glatkowski 2 Show Abstract
1 , National Renewable Energy Lab, Golden, Colorado, United States, 2 , Eikos, Inc., Franklin, Massachusetts, United States
Transparent electrodes are an integral part of photovoltaic (PV) devices. However, the transparent conducting oxide (TCO) films currently in use are not ideal for all PV applications. Nanostructured bundles of single-wall carbon nanotubes (SWCNTs) can be deposited from solution to form highly conductive and transparent thin films. The optoelectronic properties of the SWCNT films approach those of commonly used TCO films. But unlike ZnO and other traditional TCOs, the SWCNT films have little absorption in the visible or near-infrared. Furthermore, Seebeck measurements show that the SWCNT films are hole conductors. In this paper, we demonstrate the utility of SWCNT-based transparent contacts on several different types of solar cells, including CIGS and organic PV cells. First, these contacts successfully replaced ZnO in high-efficiency (13%) CIGS devices. Second, the SWCNT coating replaced ITO and PEDOT:PSS in a bulk heterojunction device, resulting in an exceptionally high efficiency of 1.44% for a device without indium or PEDOT:PSS. We will explore the use of SWCNT coatings as p-type transparent contacts to other PV materials, including Si and CdTe. We will also present data on fundamental electrical and optical properties of the SWCNT thin films. This abstract is subject to government rights.
3:00 PM - CC2.3
Transparent Conducting Nb Doped TiO2 Films Grown by Pulsed Laser Deposition.
Matthew Dabney 1 , Maikel van Hest 1 , Meagen Gillispie 2 1 , Jeff Alleman 1 , John Perkins 1 , David Ginley 1 Show Abstract
1 , National Renewable Energy Lab, Golden, Colorado, United States, 2 Materials Science and Engineering, Iowa State University, Ames, Iowa, United States
Recent work has shown that substitutionally doped anatase TiO2 may be a viable low-cost Indium free transparent conductor. For Ti0.97Nb0.03O2 films grown by pulsed laser deposition (PLD) on SrTiO3, a conductance of 3500-5000 S/cm for 40 nm thick transparent films was reported. Here, we report on the growth and characterization of a set of films with varying thickness to test the published hypothesis that the high conductivity might be due to a conductive layer at the film-substrate interface. For this work, films ranging in thickness from 20 nm to 500 nm were grown in 1x10-5 Torr O2from a Ti0.95Nb0.0502 target by PLD on SrTiO3 (STO), LaAlO3 (LAO), and fused silica substrates with a substrate temperature of 550°C. The films grown on single crystal substrates exhibited conductivity three orders of magnitude higher than those grown on the fused silica (2000 S/cm on STO and LAO versus 1 S/cm on fused silica). The conductivity was determined from sheet resistance measured using a 4-pt probe and thickness measurements made with a Dektak profilometer. For the Ti0.95Nb0.0502 films grown on STO, the conductance remained the same independent of the film thickness. This indicates that the observed conductance is due to bulk conductivity not an interfacial effect. X-ray diffraction (XRD) studies showed crystalline anatase TiO2 phase on the STO and LAO, but the films grown on fused silica were amorphous. Transmission electron microscopy (TEM) showed that the films on STO and LAO were epitaxial in a cube on cube orientation. In addition, for the films grown on highly twinned LAO, the conductivity was both variable and anisotropic. These results indicate that the conductivity depends strongly on the crystallinity and microstructure.
3:15 PM - CC2.4
Metal Oxide Thin Films for solar conversion devices
Ahmad Taha 1 , Dean Giolando 1 , Xuming Deng 2 Show Abstract
1 Chemistry Department, University of Toledo, Toledo, Ohio, United States, 2 Physics Department, University of Toledo, Toledo, Ohio, United States
Systems combining solar energy and water electrolysis (PV-hydrogen systems) are a very promising option for the production of hydrogen that could be used as a fuel in the future. In general, a PV-hydrogen system uses solar energy to directly electrolyze water to produce hydrogen gas. One design of the PV-hydrogen system has the photovoltaic device immersed in an electrolyte, where the surfaces of the photovoltaic device act as electrodes of an electrolysis cell. Light passing through the electrolyte is absorbed by the photovoltaic device, which then generates sufficient electricity to electrolyze water. However, the photovoltaic device is severely degraded on contact with the electrolyte, which is usually a high concentration basic solution. To overcome the degradation, a corrosion resistant layer is needed to coat the photovoltaic device.Metal oxide thin films such tin oxide, being chemically stable, conductive and transparent, are good candidates to act as a corrosion resistant layer. Tin oxide thin films deposited, by spray pyrolysis at a temperature of 220 – 280 °C, on glass, tec-15 (commercial tin oxide coated glass) and PV cells were tested in an electrolysis cell. We found that the presence of tin oxide thin film prolonged the lifetime of the PV cell under electrolysis in the presence of a high pH electrolyte. Furhtermore, tin oxide coated glass samples were immersed in electrolyte in an attempt to mimic the PV hydrogen system. This setup allows studying the optical properties of the electrolyte-TCCR (transparent conductive corrosion resistant layer) interface, and comparing it to that of air-TCCR. The electrolyte-TCCR showed higher % transmittance than that of air-TCCR. This illustrates another advantage of immersing the PV cell in electrolyte for use as a PV-hydrogen system.Moreover, the described metal oxide thin film can be applied as a buffer layer in the CdTe/CdS solar cell. A resistive buffer layer is needed to prevent contact between CdTe and TCO layer due to pinholes present when using a very thin CdS layer. Using a thin CdS layer would decrease the absorption of light passing to the CdTe layer, thus may improve the efficiency of the solar device. Buffer layer, resistive tin oxide, is deposited on tec-15 and processed to construct CdTe/CdS solar cells that are then characterized and compared to CdTe/CdS solar cells constructed without a buffer layer to study the effect of the buffer layer on the solar cell properties.In conclusion, tin oxide thin films prepared by spray pyrolysis at low temperature can be of great advantage when used in both PV-hydrogen system and CdTe/CdS solar cells.
4:30 PM - **CC2.5
New Semiconductor Materials for High Efficiency Solar Cells.
Wladek Walukiewicz 1 Show Abstract
1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
5:00 PM - CC2.6
High Efficiency Si-on-insulator Solar Cells with Textured Photonic Crystal Backside Reflector
Lirong Zeng 1 , Yasha Yi 1 , Peter Bermel 2 , Bernard Alamariu 3 , Ching-yin Hong 1 , Mark Schattenburg 4 , Xiaoman Duan 1 , Lionel Kimerling 1 Show Abstract
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Microsystems Technology Laboratories, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Center for Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Thin film solar cells (TFSC) are the leading candidates for next generation PV applications. Current efficiency of TFSC, however, is very low due to its insufficient absorption of long wavelength photons. To tackle this problem, we invented a new light trapping scheme using textured photonic crystal as a backside reflector with a reflectivity as high as 99.98%, which can enormously elongate the optical path length for complete light absorption. It is composed of a reflection grating and a distributed Bragg reflector (DBR). In this paper, optimization of the back reflector is systematically designed and conducted using Si-on-insulator (SOI) wafers to avoid complication of materials issues. Interdigitated top metal contacts are employed with lateral p-i-n junctions. Both 1D and 2D submicron gratings are developed and SiO2/Si DBR is used. The backside reflector parameters are optimized based on simulation using scattering matrix method. Specifically, as for the grating, period and etch depth as well as the duty cycle are studied in detail and the best combination is realized. In the case of 2D gratings, the influence of variations in two perpendicular directions is explored. As for DBR, the period is optimized to make the stopband as wide as required by different solar cell thickness. The highest power conversion efficiency is achieved for a given solar cell thickness with an optimized back reflector structure. The optimized back reflector design can be readily applied to polycrystalline and amorphous Si thin film solar cells.
5:15 PM - CC2.7
Supramolecular Self-Assembly of Photovoltaic Molecules: a Novel Strategy to the Efficient Organic Solar Cells.
Evgueni Nesterov 1 , K. M. Nalin de Silva 1 2 , Euiyong Hwang 1 Show Abstract
1 Chemistry, Louisiana State University, Baton Rouge, Louisiana, United States, 2 Chemistry, University of Colombo, Colombo Sri Lanka
The development of organic-based solar cells is a highly promising area as these devices will benefit from inexpensive, high-volume production techniques. Despite some major recent achievements, the light-to-current conversion efficiency of the currently available devices is still low. We are working towards a new supramolecular approach to organic photovoltaics that may allow overcoming some of the problem associated with traditional bulk heterojunction solar cells. The key design feature includes a new “bottom up” strategy based on the stepwise self-assembly of two types of molecular building blocks with rigid rod-like geometry and opposite electronic demands. Each building block consists of a central core equipped with a supramolecular connector. The connectors at the electronically opposite building blocks possess the complementary “lock-key” architectures, and are responsible for correct nanoscale supramolecular self-assembly of the opposite building blocks into a photovoltaic molecule. The high macroscale order and organization, which are ultimately required for the high photocurrent generation efficiency, can be achieved through the use of self-assembled monolayer technique. This approach yields a highly ordered and well-organized framework which incorporates heterojunction between n- and p-sublayers on the intramolecular level. As a result, the conditions for an efficient exciton formation with subsequent effective charge separation and transfer along the straight current paths can be achieved. Synthesis, self-assembly, and preliminary studies of a proof-of-concept system designed along this way, will be reported in this presentation.
5:30 PM - CC2.8
The Source of the Maximum Open Circuit Voltage in Molecular Photovoltaic Devices and Implications for Efficiency Improvements.
Barry Rand 1 , Diana Pendergrast 1 , Stephen Forrest 2 Show Abstract
1 Electrical Engineering and Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University, Princeton, New Jersey, United States, 2 Electrical Engineering & Computer Science, Physics, and Materials Science & Engineering, University of Michigan, Ann Arbor, Michigan, United States
The interest in organic materials for solar energy conversion has grown rapidly due to the demand for inexpensive and renewable energy sources. The power conversion efficiency of organic photovoltaic (PV) cells based on polymeric and small molecular weight materials has steadily improved, reaching values of over 4% [1, 2], but higher efficiencies are needed for practical PV systems. Given that open circuit voltage (VOC) and short circuit current (JSC) are directly proportional to device efficiency, a complete understanding of the fundamental parameters that govern these devices is crucial to stimulate future efficiency gains. In this work, we focus on understanding the source of VOC in molecular PV cells.The device architecture that we use is the double heterostructure, consisting of a donor (D), acceptor (A), and exciton blocking layer (EBL) deposited on an indium tin oxide anode, with a metal (Ag, Al, or Au) cathode on the EBL. Upon absorption of a photon in the D (A) layer, a bound electron-hole pair, or exciton, is formed, and is rapidly dissociated at the DA interface, provided that the difference in electron affinity [EA] (ionization potential [IP]) between the DA layers is larger than the exciton binding energy. Here, we monitor VOC as a function of light intensity and temperature for 14 DA pairs. We find VOC increases with light intensity and inversely with temperature. Additionally, we find that VOC saturates at low temperatures (~175 K) for many of the heterojunctions studied. The data show that the saturated value of VOC correlates with the difference between the donor IP and acceptor EA (Δ), minus the binding energy of the dissociated, geminate electron-hole pair.We compare our photocurrent data with a model for charge collection efficiency as a function of voltage previously applied to organic PV cells, but extended to include temperature and asymmetric DA junctions . We find this model accurately predicts the experimental phenomena, specifically the saturation of VOC with intensity and decreasing temperature, and the increase of VOC with Δ. Using the Shockley equation with series and shunt resistances to accurately describe the diode dark current along with a transfer matrix based model to determine JSC, we predict the ultimate efficiency of double heterostructure organic PV cells, using the optical gaps of the donor and acceptor layers, and Δ as parameters. When combined with mixed layers to increase photocurrent and stacked cells to increase VOC, we show that efficiencies for the realistic use of organic PV cells for power generation are attainable. J. Xue et al, Appl. Phys. Lett. 85, 5757 (2004). G. Li et al, Nature Mat. 4, 864 (2005). J. Nelson et al, Phys, Rev. B 69, 035337 (2004).
5:45 PM - CC2.9
Porphyrin-based Nanostructures by Self-assembly.
Zhongchun Wang 1 , Craig Medforth 1 , John Shelnutt 1 2 Show Abstract
1 Surface and Interface Science Department, Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 Department of Chemistry, University of Georgia, Athens, Georgia, United States
The design and preparation of functional materials through self-assembly processes, in which molecules associate spontaneously into ordered aggregates as a result of noncovalent interactions, is becoming one of the primary frontiers of materials research. Porphyrins and their analogues are particularly attractive building blocks for self-assembly because of their rigid and planar geometry and wide spectrum of desirable optical, electronic, and catalytic properties. In our recent efforts to synthesize well-defined porphyrin-based nanostructures, enhanced self-assembly has been achieved by the introduction of ionic, H-bond donor/acceptor, and/or coordinating groups as the peripheral substituents of the porphyrin rings or as axial ligands to the central metals. Thus, a series of porphyrin-based nanostructures with various dimensions have been prepared, namely pseudo-0D nanospheres, 1D nanofibers, nanorods and nanotubes, and 2D nanosheets. The porphyrin molecular tectons and self-assembly parameters have been altered to optimize the morphology and functional properties of the nanostructures. Due to strong and extensive excitonic interactions, the nanostructures typically exhibit a broad absorption profile in the UV-visible spectrum, which is potentially advantageous for their use as efficient solar light harvesters. Furthermore, highly organized internal structures may facilitate charge separation and transport as well as energy transfer inside the nanostructures. The nanostructures containing tin or antimony porphyrins are found to be efficient photocatalysts, and photocatalytic self-metallization provides a novel and facile means to fabricate porphyrin-metal composite nanostructures. The porphyrin-based nanostructures and their metallized products have been investigated for potential applications in sensing, electrocatalysis, non-linear optics, photovoltaics, and solar hydrogen production. Relevant results will be presented and discussed in this talk. For example, efficient solar hydrogen evolution has been demonstrated via an energy transfer mechanism using platinized porphyrin nanorods as the solar light harvesters.This work was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy (DE-FG02-02ER15369). 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.
George Crabtree Argonne National Laboratory
Arthur J. Nozik National Energy Renewable Laboratory
Paul Alivisatos University of California-Berkeley
Michael Wasielewski Northwestern University
Rene Janssen Eindhoven Technical University
Nathan S. Lewis California Institute of Technology
CC3: Organic, Molecular, and Hybrid Photovoltaics I
Tuesday AM, November 28, 2006
Independence W (Sheraton)
9:00 AM - **CC3.1
Balanced Charge Transport in Polymer-fullerene Bulk Heterojunction Solar Cells.
Jan Anton Koster 1 2 , Mihailetchi Valentin 1 , Kees Hummelen 1 , Paul Blom 1 Show Abstract
1 Molecular Electronics, Materials Science Centre, University of Groningen, Groningen Netherlands, 2 , Dutch Polymer Institute, Eindhoven Netherlands
Recently, we have developed a device model that quantitatively describes the operation characteristics of poly(2-methoxy-5-(3,7-dimethoxyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) solar cells. For PPV-based compounds with lower hole mobilities we have demonstrated that the photocurrent in BHJ cells reaches a fundamental space-charge limit, which is detrimental for the fill factor and efficiency. The relevance of space-charge formation is strongly dependent on the mobility difference between the electrons and holes. For MDMO-PPV:PCBM the mobility difference of a factor of ten limits the thickness of the active layer to typically 100 nm, at which only 60% of the incoming light is absorbed. Increasing device thickness results in a lower overall power conversion efficiency, mainly due to a lowering of the fill factor due to a combination of charge recombination and space charge effects. In the last 2-3 years attention has shifted from PPV-based devices towards BHJ solar cells based on regioregular poly(3-hexylthiophene) (P3HT). The hole mobility in the P3HT phase of the blend is dramatically affected by thermal annealing. It increases more than three orders of magnitude, to reach a value up to ≈2×10-8 m2/Vs after the annealing process, as a result of an improved crystallinity of the film. It has recently been demonstrated that the efficiency can exceed 4% by controlling the growth rate of the active layer. Slowing down the drying process of the wet films leads to an enhanced self-organization. The origin of the enhanced performance of bulk heterojunction solar cells based on slowly dried films of P3HT and PCBM is investigated, combining charge transport measurements with numerical device simulations. We find that slow drying leads to a 33-fold enhancement of the hole mobility up to 5.0×10-7 m2V-1s-1 in the P3HT phase of the blend as compared to the annealed devices, thereby balancing the transport of electrons and holes in the blend. The resulting reduction of space-charge accumulation enables the use of thick films (~300 nm), absorbing most of the incoming photons, without losses in the fill factor and short-circuit current of the device. We use the device model to exploit the potential of polymer/fullerene bulk heterojunction solar cells. As a starting point devices based on P3HT and PCBM, reaching 3.5% efficiency, are modeled. Lowering the polymeric band will lead to a device efficiency exceeding 6%. Tuning the electronic levels of PCBM in such a way that less energy is lost in the electron transfer process enhances the efficiency to values in excess of 8%. Ultimately, with an optimized level tuning, band gap and balanced mobilities polymeric solar cells can reach power conversion efficiencies approaching 11%. G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, and Y. Yang, Nature Materials 4, 864 (2005).
9:30 AM - CC3.2
Charge Dissociation in Polymer:Fullerene Bulk Heterojunction Solar Cells with Enhanced Permittivity.
Martijn Lenes 1 2 , Floris Kooistra 1 , Kees Hummelen 1 , Paul Blom 1 , Ineke Van Severen 3 , Laurence Lutsen 4 , Dirk Vanderzande 3 4 , Thomas Cleij 3 Show Abstract
1 Materials Science Centre, University of Groningen, Groningen Netherlands, 2 , Dutch Polymer Institute, Eindhoven Netherlands, 3 Institute for Materials Research, Hasselt University, Diepenbeek Belgium, 4 Division IMOMEC, IMEC, Diepenbeek Belgium
9:45 AM - **CC3.3
Progress in Vapor Deposited Photovoltaic Cells.
Stephen Forrest 1 Show Abstract
1 Department of EECS & Physics, University of Michigan, Ann Arbor, Michigan, United States
10:15 AM - **CC3.4
Photoconversion Processes in Bulk Heterojunction Solar Cells of Single-Wall Carbon Nanotubes and Conjugated Polymers and Dendrimers.
Garry Rumbles 1 Show Abstract
1 Chemical and Biological Sciences Center, National Renewable Energy Lab, Golden, Colorado, United States
The bulk heteojunction solar cell comprising a blend of conjugated polymer with either C60 or colloidal quantum dots has been shown to demonstrate moderate solar cell power conversion efficiencies. In this presentation, the role that single-wall carbon nanotubes (SWNTs) might play in enhancing the efficiencies these devices will be examined. Two different aspects will be discussed: (i) the potential for SWNTs to act as a replacement for ITO as the transparent conducting electrode, and (ii) the role that SWNTs might play as a replacement for C60 in the photoactive component of the device. In both cases, the importance of the specific nanotube species (chirality, diameter etc.) will be explored in detail, and the importance of isolation and bundling examined.Using a number of conjugated polymers, including poly (3-hexylthiophene) (P3HT) and a variety of soluble poly (phenylene vinylenes) (PPVs), we have successfully solubilized and isolated SWNTs into thin films. Using two-dimensional, steady-state PL/PLE spectroscopy the properties of the SWNTs have been studied and their photophysical properties examined. Time-resolved microwave conductivity (TRMC) and transient terahertz spectroscopy (TTS) have been used to study photoinduced electron transfer processes at the polymer/nanotube interface, allowing the quantum yield of free carrier production and their respective mobilities to be estimated. The results will be discussed in terms of the impact that these fundamental studies have on solar cell device performance.The use of indium tin oxide (ITO) as a hole-collecting electrode in most organic solar cells is not ideal, with the interface with the hole-transporting component of the photo-active material less than ideal. We will report some recent results on using thin layers of SWNTs as a replacement for ITO as the transparent conducting electrode, where power conversion efficiencies are very close to those using ITO. The SWNTs provide not only a means of making the device flexible, but they also offer a better molecular-level substrate into which the holes can be injected.
10:45 AM - CC3.5
Pi-conjugated Dendrimers for Organic and Hybrid Photovoltaic Devices.
Sean Shaheen 1 , Nikos Kopidakis 1 , William Mitchell 1 , Muhammet Kose 1 , William Rance 2 , David Ginley 1 , Garry Rumbles 1 Show Abstract
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Dept. of Physics, Colorado School of Mines, Golden, Colorado, United States
11:30 AM - **CC3.6
Charge Generation and Recombination in Polymer / Fullerene Blend Solar Cells.
James Durrant 1 Show Abstract
1 Chemistry, Imperial College, London, London United Kingdom
12:00 PM - CC3.7
New Low-band Gap Polymers in Polymer-fullerene Solar Cells.
Arjan Zoombelt 1 2 , Marta Fonrodona 1 2 , Martijn Wienk 1 2 , René Janssen 1 2 Show Abstract
1 SMO/M2N, Eindhoven University of Technology, Eindhoven Netherlands, 2 , Dutch Polymer Institute (DPI), Eindhoven Netherlands
At present one of the limiting factors for efficient photovoltaic energy conversion is the mismatch of the absorption spectrum of the active layer and the solar emission. Low band gap materials afford an improved overlap of the polymer absorption with the solar emission spectrum, which peaks around 700 nm (1.77 eV), leading to an increase of absorbed photons and photocurrent. A successful and flexible strategy to achieve low-band gap conjugated polymers involves the alternation of electron-rich and electron-deficient units in the polymer chain. Here we present new copolymers with exceptionally low band gaps for use in bulk heterojunction solar cells together with fullerenes. We have used a bithiophene moiety as the electron-rich unit and varied the electron-deficient unit to tune the band gap. Using thienopyrazine afforded a low band gap polymer with an optical band gap in thin films of 1.2-1.3 eV. By replacing thienopyrazine with a more electron-deficient thiadiazoloquinoxaline moiety a further reduction of the band gap was achieved. The resulting alternating copolymer of bithiophene and thiadiazoloquinoxaline has an optical band gap well below 1 eV and represents one of lowest band gaps ever obtained for a soluble conjugated polymer. The performance of these low band gap polymers in bulk heterojunction solar cells with fullerene derivatives is presented.
12:15 PM - CC3.8
Exciton Transport in Organic Photovoltaic Cells.
Michael McGehee 1 , Shawn Scully 1 , Melissa Summers 1 Show Abstract
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
12:30 PM - CC3.9
Improving Polymer Based Photovoltaic Devices by Reducing the Voltage Loss at the Donor-Acceptor Interface.
Sjoerd Veenstra 1 3 , D. Moet 1 , L. Slooff 1 , J. Kroon 1 3 , J. Sweelssen 2 3 , M. Koetse 2 3 Show Abstract
1 Solar Energy, ECN, Petten Netherlands, 3 , Dutch Polymer Institute, Eindhoven Netherlands, 2 , Holst Centre, Eindhoven Netherlands
12:45 PM - CC3.10
High Efficiency Solar Cells Based on Nanocomposites ofRegioregular Polymers with Short and Long Chains
Kanzan Inoue 1 2 , Kamil Mielczarek 1 2 , Brian Wang 1 2 , Dean Hsu 1 2 , Elena Sheina 3 , Darin Laird 3 , Shawn Williams 3 , Sergey Lee 1 , Anvar Zakhidov 1 2 Show Abstract
1 NanoTech Institute, Universty of Texas at Dallas, Plano, Texas, United States, 2 Physics Department, University of Texas at Dallas, Richardson, Texas, United States, 3 , Plextronics, Pittsburgh, Pennsylvania, United States
The polymeric nanocomposites of regioregular P3HT of low molecular weight and medium/high molecular weight are created. The optimal polymeric composition, procedure and architecture are determined experimentally, which lead to a highest energy conversion efficiency of 4.3% under AM1.5 100 mW/cm2 for bulk heterojunction P3HT/PCBM solar cells, which can be further increased by ~30 % under lower light intensity of 10 mW/cm2. It is demonstrated that the increase of light harvesting and energy conversion is due to improved overall charge collection and transport in nanocomposites of optimal morphology, achieved by combining the good intracrystallite and intercrystallite transport of photogenerated holes. Upon appropriate heat-treatment, short chain polymer component easily forms highly crystalline clusters, which have high charge mobility within each cluster, while medium /high molecular weight polymer provides connectivity between the clusters with their disordered longer chains. Together with optimum PCBM concentration and heat-treatment for obtaining the morphology of interconnected network, the device performance is significantly higher than current polymeric photovoltaic devices with narrow chain length distribution.
CC4: Organic, Molecular, and Hybrid Photovoltaics II
Tuesday PM, November 28, 2006
Independence W (Sheraton)
2:30 PM - CC4.1
Device and Scanned-probe Microscopy Studies of Transparent Carbon-nanotube Networks Used in Organic Solar Cells.
Mark Topinka 1 3 , Michael Rowell 1 , David Goldhaber-Gordon 3 , Michael McGehee 1 , Liangbing Hu 2 , David Hecht 2 , George Gruner 2 Show Abstract
1 Material Science & Engineering, Stanford University, Stanford, California, United States, 3 Physics, Stanford University, Stanford, California, United States, 2 Physics, UCLA, Los Angeles, California, United States
Thin-film based photovoltaics show promise as a low-cost, flexible, printable alternative to conventional silicon solar cells. A critical aspect of these photovoltaics is the conduction of current out of the cell across the illuminated side of the device through the layer known as the transparent conductor (TC). In this talk we will discuss our work on improving the transparent-conductor layer in organic solar cells, by replacing the commonly used In2O3:Sn (ITO) layer with a conductive, transparent carbon nanotube (CNT) film. The traditional use of ITO in these solar cells is potentially problematic for a number of reasons- high cost due to indium scarcity, relatively low flexibility, and the requirement of vacuum and high-temperature processing for high quality ITO films, to name a few. CNT-films offer an alternative to ITO which could potentially perform equally well or better, but be low-cost, flexible, and solution processible. We have successfully fabricated and investigated the performance of P3HT-PCBM blend bulk heterojunction polymer cells using both ITO and CNT-films as the transparent conductor and have succeeded in attaining similarly high efficiency levels using either transparent conductor, (3.0% with ITO, 2.5% with CNT-films). The small difference in performance between the ITO and CNT-film devices can be explained with a straight-forward model by the higher sheet resistance currently present in CNT-films, around 200ohms/sqr (with 85% optical transparency) compared to around 10ohms/sqr for the ITO. Using a simple model, it can be shown that CNT-films should theoretically be able to reach roughly 1ohm/sqr at 90% transmission levels. We have investigated the performance of CNT-films using several electrically sensitive scanning probe microscopy (SPM) techniques, including electric force microscopy (EFM) and scanned Kelvin-probe microscopy (SKPM), to better understand the current gap between real-world CNT-film performance and the theoretical limits. From the resulting high-resolution (better than 30nm) electrical images of current flow through CNT-films we have found that most of the potential drop occurs at tube-tube contacts, meaning that the dominant source of resistance in the films studied seems to be the inter-tube tunneling resistance, as opposed to scattering due to phonons or defects. The detailed, spatial information resulting from these studies can be used to understand the underlying physics of conduction through real-world CNT-films, and to find ways to push CNT-film performance (conductivity/absorptivity ratios) even closer to the theoretical limit than can currently be obtained.
2:45 PM - CC4.2
Alternating Copolymers of Fluorene for Wide Spectral Coverage and Good Electrical Transport in Polymer Solar Cells.
Olle Inganas 1 2 , Mats Andersson 3 2 , Fengling Zhang 1 2 , Erik Perzon 3 2 , Abay Gadisa 1 2 , Mattias Andersson 1 2 Show Abstract
1 Biomolecular and organic electronics, IFM, Linkoping University, Linkoping Sweden, 2 Center of Organic Electronics (COE), Linkopings Universitet, Linkoping Sweden, 3 Materials Chemistry/Polymer technology, Chalmers University of Technology, Göteborg Sweden
3:00 PM - CC4.3
Time-Resolved Microwave Conductivity study of Zn1-xMgxO/poly(3-hexylthiophene) bilayers for photovoltaic applications
Nikos Kopidakis 1 , Jorge Piris 1 , Dana Olson 1 , Sean Shaheen 1 , David Ginley 1 , Garry Rumbles 1 Show Abstract
1 , National Renewable Energy Lab, Golden, Colorado, United States
Hybrid photovoltaic devices based on metal oxide-conjugated polymer composites have attracted much attention both from the applied and basic science communities due to their well-defined structure and the ability to tune the device properties by modifying the nanostructure and/or composition of the metal oxide. In these devices the polymer is the light-absorbing and hole-transporting layer while the oxide serves as the electron acceptor.A fundamental aspect of these devices that is not well understood is the significance of the oxide-polymer interface in the separation of excitons photogenerated in the polymer film. The present study aims to address this issue in a subgroup of these devices composed of bilayers of solution-cast Mg-doped ZnO and poly(3-hexylthiophene) (P3HT). Doping ZnO with Mg increases the band gap of the material and has recently been used to enhance the open circuit voltage and short circuit current of ITO/Zn1-xMgxO/P3HT/Ag photovoltaic devices. We use the contactless Time-Resolved Microwave Conductivity (TRMC) technique to study Zn1-xMgxO thin films, P3HT films and Zn1-xMgxO/P3HT bilayers, with x between 0 and 0.4. We show that the photoconductivity of solution-cast Zn1-xMgxO thin films decreases exponentially with x. The decrease is attributed to decreasing charge carrier mobility, consistent with observations in the respective photovoltaic devices. TRMC measurements on Zn1-xMgxO/P3HT bilayers show that at low Mg-doping (x<0.2) the photoconductivity signal is dominated by electrons in the oxide, while at higher doping it is dominated by charge carriers in P3HT. By comparing the data for pure P3HT and Zn1-xMgxO/P3HT bilayers we determine the locus of exciton dissociation in the bilayers and evaluate the importance of the oxide/polymer interface in terms of charge separation. Implications of our findings on the operation of the respective photovoltaic devices are discussed.
3:15 PM - CC4.4
The use of Poly(Thienylene Vinylene) derivatives as Low Band gap p-type conjugated polymer in organic photovoltaics.
Dirk Vanderzande 1 2 , Fateme Banishoeib 1 , Thomas Cleij 1 , Laurence Lutsen 2 Show Abstract
1 chemistry, University of Hasselt, Diepenbeek Belgium, 2 IMOMEC, IMEC, Diepenbeek Belgium
Low band gap conjugated polymers stayed an intriguing research topic since the first low band gap polymer Poly(IsoThiaNaphthene) (PITN) was synthesized. However in time it became clear that the chemistry associated with their synthesis was not always simple. That made it difficult to develop with success synthetic routes that yielded a material with all wanted specifications on band gap, solubility, stability, molecular weight, etc. In the research field of organic photovoltaics a specific interest in low band gap conjugated polymers emerged as a potential method to improve the spectral response of the solar cell. Poly(Thienylene Vinylene) (PTV) derivatives showed interesting potential as low band gap polymer, but the high reactivity of the corresponding monomers and/or the harsh conditions (strong acid) for conversion of the precursor precluded important progress. Over the last five years, efforts were started in our research group to develop an improved synthesis for PTV derivatives. The approach we have chosen makes use of the polymerization behaviour of p-quinodimethane systems. Dithiocarbamate derivatives of bis(chloromethylene) thiophenes proved sufficient stable to be used as monomers in said polymerization scheme. Initial experiments with LDA as a base proved to yield disappointing results. The introduction of another base, Lithium Bis(trimethylsilyl)amide, leads to a versatile and efficient polymerization reaction. A precursor polymer of high Mw (> 100 000) toward plain PTV was synthesized and converted at 180°C. Electrochemical, optical and electronic data are consistent with the formation of a high quality PTV. The procedure was extended toward the synthesis of substituted PTV’s, e.g. the soluble poly(3,4-bis(4-butylphenyl)-2,5-thienylene vinylene). Most recently a high Mw ( = 97 000) poly(3-hexyl-2,5-thienylene vinylene) was synthesized. The polymer with an optical band gap of 1.7 eV, a HOMO level of -5.1 eV and a LUMO level of -3.1 eV shows a high mobility of charge carriers (µ = 10-2-10-3 cm2/Vs) and in line with this data, characteristics interesting for photovoltaic applications. An overview will be given in this contribution of material characteristics and their performance in photovoltaic device.
3:30 PM - CC4.5
Improvement of Photovoltaic Response Based on Spin-Orbital Coupling Increased Triplet States in Organic Solar Cells
Zhihua Xu 1 , Bin Hu 1 Show Abstract
1 Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States
We report an enhancement of photovoltaic response by dispersing phosphorescent Ir(ppy)3 molecules in organic solar cell of poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene] doped with surface-functionalized fullerene 1-(3-methyloxycarbonyl)propy(1-phenyl [6,6] C61 (PCBM). It is known that photoexcitation generates both singlet and triplet states through intersystem crossing caused by hyperfine or spin-orbital coupling. Due to long diffusion length the triplet excitons can migrate from their generation sites to the interfaces of donor-acceptor interaction and directly dissociate into charge carriers. We found, based on the studies of magnetic field-dependent photocurrent and electroluminescence, that the dispersed Ir(ppy)3 molecules increase the spin-orbital coupling strength and triplet density in the MEHPPV matrix due to the penetration of MEHPPV pi electrons into the large field of orbital dipoles of the Ir(ppy)3. Especially, the triplet excitons facilitate the direct dissociation into charge carriers at the donor-acceptor interacting interfaces in the composite of MEHPPV and PCBM, and consequently improve the photovoltaic response in organic solar cells.
3:45 PM - CC4.6
Tuning from Singlet State to Charge-transfer Excited States in Styryl-substituted Terthiophenes: An Ultrafast and Steady-state Emission Study.
Keith Gordon 1 4 , Tracey Clarke 1 4 , David Officer 3 4 , David Phillips 2 , Wai Kwok 2 Show Abstract
1 Chemistry, University of Otago, Dunedin New Zealand, 4 , MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington New Zealand, 3 Chemistry, Massey University, Palmerston North, North Island, New Zealand, 2 Chemistry, University of Hong Kong, Hong Kong Hong Kong
4:30 PM - CC4.7
Hybrid Conjugated Polymer / Nanostructured ZnO Photovoltaic Devices
Dana Olson 1 , Yun-Ju Lee 1 , Erik Spoerke 1 , Matthew White 2 3 , Sean Shaheen 2 , David Ginley 2 , James Voigt 1 , Julia Hsu 1 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Physics, University of Colorado, Boulder, Colorado, United States
Conjugated polymer / nanostructured oxide semiconductor composites are promising systems for use in low cost organic photovoltaic devices. Incorporation of a conjugated polymer into a nanostructured array of an oxide semiconductor can result in a bulk heterojunction device in which photogenerated charges are more effectively captured and transported to the electrodes. These hybrid solar cells can 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. We have fabricated arrays of ZnO nanorods by low-temperature solution growth on patterned ITO substrates. The dense ZnO nanorod arrays are subsequently infiltrated with poly(3-hexylthiophene) (P3HT), and the devices are completed by depositing Ag top electrodes. Depending on the seeding conditions, we can control the alignment of ZnO rods: ordered (aligned perpendicular to the substrate) versus disordered. The polymer infiltration process consists of depositing P3HT films from solution either through spin coating or vacuum drying. We investigated the effect of subsequent annealing at various temperatures and found improved penetration of the polymer into the ZnO nanorod array. To observe the effects of the infiltration process on both the extent of infiltration and the polymer morphology, the polymer/ZnO nanorod composites were characterized by cross-sectional SEM, UV-vis absorption, X-ray diffraction. We also examined the effect of treating ZnO nanorods with UV-ozone prior to polymer infiltration. The electron transfer efficiency of the various composite films was studied using photoluminescence quenching. We found that PL quenching depends on the alignment of ZnO nanorods as well as whether the nanorods have been treated with UV-ozone. Finally, these results are correlated with the device data to observe the effects of ZnO nanorod ordering, interfacial treatment, and the infiltration process on the device performance.
4:45 PM - CC4.8
Charge Separation in Polyfluorene:PCBM Blend Films and the Role of Electric Bias.
Jessica Benson-Smith 1 , Hideo Ohkita 3 , Chris Shuttle 2 , James Durrant 2 , Donal Bradley 1 , Jenny Nelson 1 Show Abstract
1 Department of Physics, Imperial College London, London United Kingdom, 3 Department of Polymer Chemistry, Kyoto University, Kyoto Japan, 2 Department of Chemistry, Imperial College London, London United Kingdom
5:00 PM - CC4.9
Role of the Organic-Cathode Interface on the Performance of Organic Solar Cells.
Maria Perez 1 , Krystal Sly 1 , Elizabeth Mayo 1 , Mark Thompson 1 Show Abstract
1 Chemistry, USC, Los Angeles, California, United States
Organic photovoltaics have the potential for development of inexpensive and efficient solar cells. Processes occurring after the absorption of a photon within the solar cell are still not completely understood. In particular, the process by which the free electron is collected at the metal organic interface is still under debate. It is usually considered that charge collection at this interface is a very efficient process and its effect on cell performance is generally disregarded. This work illustrates that charge collection is highly dependant on the nature of the metal-organic interface. It has been shown that different metals (e.g. Al vs Ag) result in significantly different cell efficiencies, with cells using Ag as the cathode performing better than those using Al. In an effort to understand this effect, different compositions of these two metals have been tested as cathodes. In order to elucidate differences between the role of the EBL/cathode interface from the role of the cathode/electrode contact, Al and Ag were deposited sequentially with variation in the layer thicknesses, as well as the order of deposition. The cathode materials were also codeposited to form a mixed Al/Ag cathode. The thickness of the overall metallic layer was maintained at 1000 Å throughout all experiments. Results will be presented that show the clear dependence of solar cell performance on the cathode architecture and composition. These striking observations suggest that not only do the bulk properties of the metals strongly influence the nature of cell performance, but also that the molecular properties of the metal-organic interface can not be ignored.
5:15 PM - CC4.10
Design and Synthesis of Liquid Crystal Molecules for Organic Photovoltaic Device Application.
Keisuke Tajima 1 , Takeshi Nishizawa 1 , Hady Kesuma 1 , Kazuhito Hashimoto 1 Show Abstract
1 Applied Chemistry, The University of Tokyo, Tokyo Japan
Compared to conventional silicon solar cells, organic photovoltaic devices (OPVs) are attractive due to their advantages such as low cost of production, flexibility, processability, and possibility of large area devices. In order to obtain a high performance in OPVs, it is very important to achieve an efficient charge separation and subsequent transportation of the charge carriers in the active layer. The most common strategy for making efficient OPVs is to create so-called bulk heterojunctions, in which the electron donor and the electron acceptor are blended to form one mixed layer. In this system, the formation of phase separation in the nanoscale enhances the charge separation and transportation. However, the structural control of the films so far depends only on the miscibility of the components, using post-casting processes such as thermal annealing. For a more sophisticated strategy, it has been suggested that liquid crystalline materials may realize spontaneous formation of ordered structure in OPVs .Herein we report the design and the synthesis of liquid crystalline molecules for photovoltaic devices. We designed new T-shaped amphiphilic triblock molecules with a rigid aromatic core of a p-type semiconductor, hydrophobic alkyl chains and a hydrophilic group attached to methanofullerene as an n-type semiconductor. This class of T-shaped molecules has been reported to show unique polygonal columnar liquid crystalline phases with three incompatible parts segregated into 1-D domains because of their polarity difference. Oigothiophene or oligo(phenylenevinylene) were introduced as p-type semiconductors. The structures and the thermal behaviors of the materials were investigated by XRD and DSC. The photovoltaic effect was observed under monochromatic light irradiation and the comparison to the bulk-heterojunction system was made.References L. Schmidt-Mende, A. Fechtenkotter, K. Mullen, E. Moons, R. H. Friend, J. D. MacKenzie, Science, 293, 1119-1122 (2001) B. Chen, X. Zeng, U. Baumeister, G. Ungar, C. Tschierske, Science, 307, 96-99 (2005).
5:30 PM - CC4.11
Organic Photovoltaic Cells Exhibiting Increased Voc with Subphthalocyanine Donor-like Materials.
Kristin Mutolo 1 , Erin Morrison 1 , Elizabeth Mayo 1 , Barry Rand 2 , Stephen Forrest 2 3 , Mark Thompson 1 Show Abstract
1 Chemistry, University of Southern California, Los Angeles, California, United States, 2 Department of Electrical Engineering and Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University, Princeton, New Jersey, United States, 3 Departments of Electrical Engineering and Computer Science, Physics and Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Subphthalocyanines exhibit strong absorption in the visible region and extinction coefficients similar to that of copper phthalocyanine (CuPc). Oxidation and reduction potentials of subphthalocyanines indicate they may be a suitable donor-like material in organic photovoltaic (PV) cells employing C60 as the acceptor-like material. A double heterostucture organic photovoltaic (PV) cell has been fabricated using boron subphthalocyanine chloride (SubPc) as the donor-like material and C60 as the acceptor-like material. The SubPc/C60 cell showed a more than doubling of Voc compared to a conventional CuPc/C60 cell under 1 sun AM1.5G simulated illumination. The substantial increase in Voc is attributed to an increase in the energy difference between the lowest unoccupied molecular orbital (LUMO) of the acceptor-like material and the highest occupied molecular orbital (HOMO) of the donor-like material (referred to as the interface gap, Ig). In addition, peripheral substitution of SubPc has been shown to alter the reduction and oxidation potentials of the material, which, in turn, alters the HOMO and LUMO energies. Incorporating the substituted SubPcs as the donor-like material in PV cells with C60 as the acceptor-like material should allow us to vary the value of Ig and further explore the significance of Ig on Voc.
5:45 PM - CC4.12
Organic Solar Cells Using Transparent SnO2:F Anodes.
Fan Yang 1 , Stephen Forrest 2 Show Abstract
1 Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 2 Departments of Electrical Engineering & Computer Science, Physics, and Materials Science & Engineering, University of Michigan, Ann Arbor, Michigan, United States
Organic solar cells offer low production cost and compatibility with flexible substrates. Conventional organic molecular photovoltaic (PV) devices and light-emitting diodes (OLEDs) are typically grown on transparent indium-tin-oxide (ITO) anodes. Replacing ITO with F-doped SnO2 (SnO2:F)-coated glass, which is only 30% as costly, therefore has obvious potential as a route to inexpensive solar power conversion. Nevertheless, organic small molecule or polymeric devices, with active layers typically <1000 Å thick, can readily be shorted due to pronounced surface roughness (approximately 40 nm RMS and a height variation of 300 nm) characteristic of SnO2:F anodes. In this work, we circumvent this problem by using the conformal growth characteristics of organic vapor phase deposition (OVPD) to demonstrate thin film copper phthalocyanine (CuPc)/C60 heterojunction PV cells on the SnO2:F anodes The diffusive deposition of organic molecules via OVPD completely covers the rough oxide surface, effectively preventing shorts between opposing cathode and anode contacts. The SnO2:F / CuPc (240 Å) / C60 (490 Å) / 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline BCP (100 Å) / Ag solar cell shows a rectification ratio >104 at ±1.0 V in the dark, implying a continuous p-n junction between the SnO2:F anode and the Ag cathode. The power conversion efficiency reaches 1.3 ± % at 1 sun simulated AM 1.5 G illumination, with a peak external quantum efficiency (EQE) of 19% at 620 nm. In addition, we show that by controlling the organic film morphology, we can grow the donor-acceptor (D-A) interface into a three-dimensional interdigitated bulk heterojunction (BHJ) structure, where the donor layer is comprised of a 120 Å planar plus rough CuPc layer, followed by a 490 Å thick C60 acceptor layer which planarizes the underlying CuPc layer. The BHJ solar cells have a power conversion efficiency of 2.5%, and peak EQE of 31%, nearly twice those of analogous devices with a planar heterointerface on SnO2:F. R. G. Gordon, MRS Bull. 2000, 25, 52. F. Yang, M. Shtein, S. R. Forrest, Nat. Mater. 2005, 4, 37.
George Crabtree Argonne National Laboratory
Arthur J. Nozik National Energy Renewable Laboratory
Paul Alivisatos University of California-Berkeley
Michael Wasielewski Northwestern University
Rene Janssen Eindhoven Technical University
Nathan S. Lewis California Institute of Technology
CC5: Nanostructures for Photovoltaics I
Wednesday AM, November 29, 2006
Independence W (Sheraton)
9:00 AM - **CC5.1
Single-Photon Generation of Multiple Excitons in Colloidal Nanocrystalline Semiconductor Quantum Dots.
Randy Ellingson 1 , Matthew Beard 1 , Justin Johnson 1 , Kelly Knutsen 1 , Qing Song 1 , James Murphy 1 , Andrew Shabaev 2 , Alexander Efros 2 , Arthur Nozik 1 Show Abstract
1 Center for Chemical and Biological Sciences, National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , Naval Research Laboratory, Washington, District of Columbia, United States
The importance of efficient and affordable solar energy conversion technology has been well-established. Although nanostructured materials have yet to play a major role in developed energy conversion technologies, certain characteristic properties establish them as promising candidates to facilitate the efficient conversion of sunlight to either fuels or electricity. For example, high surface-to-volume ratios typically result in a close proximity of the absorber to the charge-separating interface, reducing the propensity for recombination loss. In addition, size-dependent optical properties and nanostructure shape-control enable customization of both energetics and function.Quantum-confined semiconductor nanocrystals, or quantum dots (QDs), exhibit uniquely size-dependent electronic structure. By reducing the QD diameter below the Bohr exciton radius, one increases the threshold photon energy for light absorption. Such strong confinement also results in strong interactions of photoexcited charge carriers within a single QD. While a single exciton within a PbS QD may recombine with a time constant of ~ 1 μs, a state consisting of two excitons within a single QD shows a lifetime of ~ 0.1 ns, dominated by Auger recombination and yielding a single exciton state. This efficient exciton-exciton annihilation interaction suggests the likelihood of the reverse process also occurring efficiently.Our work shows that the quantum-confined environment of a QD facilitates efficient production of multiple excitons following absorption of a single photon. We have termed this process occurring in QDs Multiple Exciton Generation (MEG), and have studied the photon energy dependence for three lead-salt QDs: PbSe, PbS, and PbTe. For each of these types of semiconductor QDs, we observe efficient MEG; for PbTe and PbSe, MEG yields three excitons per absorbed photon at a photon energy of four times the band gap. We will discuss recent work to measure and characterize MEG in Si QDs. In addition, we will discuss the impact of MEG on solar energy conversion efficiency.
9:30 AM - CC5.2
Coherent Generation of Mmulti-excitons in Semiconductor Nanocrystals.
Andrew Shabaev 2 , Alexander Efros 1 Show Abstract
2 School of Computational Sciences, George Mason University, Fairfax, Virginia, United States, 1 Center for Computational Material Science, NRL, Washington, District of Columbia, United States
9:45 AM - CC5.3
Photoconductivity and Multiple Exciton Generation in Arrays of Coupled Semiconductor Nanoparticles.
Matthew Beard 1 , James Murphy 1 2 , Joseph Luther 1 3 , Arthur Nozik 1 2 Show Abstract
1 Basic Energy Sciences, National Renewable Energy Laboratory, Golden , Colorado, United States, 2 Department of Chemistry, University of Colorado, Boulder, Colorado, United States, 3 , Colorado School of Mines, Golden, Colorado, United States
10:00 AM - **CC5.4
Efficient Carrier Multiplication in Semiconductor Nanocrystals for Generation III Photovoltaics.
Richard Schaller 1 Show Abstract
1 Chemistry Division, Los Alamos National Lab, Los Alamos, New Mexico, United States
The efficiency with which photons are converted into charge carriers determines the ultimate efficiencies of various photo-induced physical and chemical processes including photo-generation of electricity (photovoltaics) and solar fuels, optically pumped lasing, generation of nonlinear-optical responses, etc. Normally it is assumed that the absorption of a single light quantum (a photon) by a semiconductor produces a single electron-hole pair (an exciton), meaning that the quantum efficiency (QE) in generating charge carries is 100%. However, as we demonstrated recently, quantum-confined semiconductor nanocrystals (NCs) of Pb-salts can produce two or more excitons in response to a single absorbed photon via the process of carrier multiplication (CM). Generation of multiexcitons from a single photon absorption event is observed to take place on an ultrafast (sub-picosecond) timescales and occurs with up to unity efficiency depending upon the excess energy of the initially generated exciton. This process has the potential to considerably increase the power conversion efficiency of NC-based solar cells, increase detector sensitivities, and also lower the lasing threshold of NC-based optical amplifiers. Since performing this initial work, we have begun to investigate the generality of CM to other materials as well as the mechanism for this phenomenon via comparative studies of CM in PbSe and CdSe NCs that are characterized by significant differences in both electronic structures and carrier relaxation behaviors. Despite these differences, both compositions exhibit CM with comparable efficiencies (defined in terms of the slope of the QE dependence on photon energy above the CM threshold), which is indicative of the generality of this phenomenon to quantum-confined, semiconductor nanoparticles. CdSe NCs exhibit a lower activation threshold for CM than PbSe NCs (~2.5 vs. ~2.9 energy gaps), which can be explained using simple carrier effective-mass arguments. Furthermore, we observe a monotonic increase in QE with increasing excess energy above the CM threshold. We have observed formation of as many as seven excitons from a single photon  and have proposed a mechanism for this process that is consistent with all experimentally available data.Our finding of efficient CM in quantum-confined materials also presents new challenges that must be overcome in order to improve the efficiency of photovoltaic devices. However, these challenges should be surmountable meaning that NC materials should provide a realizable path toward low-cost, high efficiency Generation III solar cells.1. R.D. Schaller and V.I. Klimov, Phys. Rev. Lett. 2005, 92, 186601.2. R.D. Schaller, M.A. Petruska, and V.I. Klimov, Appl. Phys. Lett. 2005, 87, 253102.3. R.D. Schaller, M. Sykora, J.M. Pietryga, and V.I. Klimov, Nano Lett. 2006, 6, 424.4. R.D. Schaller, V.M. Agranovich, and V.I. Klimov, Nature Phys. 2005, 1, 189.
10:30 AM - CC5.5
Enhanced Photoreponse from Nanocomposite of Surface Modified PbS nanocrystals and Functionalized polyhexylthiophene
Xiaomei Jiang 1 2 , Miaoxin Zhou 1 , Erica Neiser 1 , Sergey Lee 1 , Anvar Zakhidov 1 Show Abstract
1 Nnaotech Institute, University of Texs at Dallas, Richardson, Texas, United States, 2 , University of South Florida, Tampa, Florida, United States
10:45 AM - CC5.6
Titania-Germanium Nanocomposites for Quantum Dot Solar Cells.
Sukti Chatterjee 1 2 , Amita Goyal 1 , S. Ismat Shah 2 3 Show Abstract
1 Materials Science and Engineering, University of Delaware, Newark, Delaware, United States, 2 Physics and Astronomy, University of Delaware, Newark, Delaware, United States, 3 Center for Compositte Materials, University of Delaware, Newark, Delaware, United States
Several new photovoltaic semiconductor materials and technologies have been developed due to increasing need for renewable energy sources. Quantum dot (QD) based solar cell is potentially one of the best contenders. We have developed a thermodynamically stable nanocomposite (stable up to 900°C) titania-germanium (TiO2-Ge) which shows promise as the active layer for QD solar cells. In TiO2-Ge nanocomposites Ge nanodots are distributed in a TiO2 matrix. Due to the 3-D quantum confinement effect, tailoring of the optoelectronic properties is relatively easily done by simply varying the Ge nanodots size. Ge is particularly advantageous since the Bohr radius of Ge is relatively large, 25 nm. In this paper results of the variation of the optoelectronic properties of TiO2-Ge nanocomposites as a function of the nanostructural parameters are presented. TiO2-Ge nanocomposite thin films were synthesized using RF magnetron sputtering. The nanocomposite films were sputtered from TiO2-Ge composite targets with varying Ge concentrations. RF power and processing temperatures were varied in the range of 100-200 Watt and 500-700°C, respectively. The morphological studies (by XRD, HRTEM, and Raman) established the fact that the Ge concentration in the composite target governs the size of the Ge nanodots whereas RF sputtering power controls the density of Ge nanodots in the nanocomposite films . The optical spectroscopic studies showed that the variation of the size of Ge nanodots in the TiO2-Ge films shifts the absorption edge from ~ 0.6 eV (infrared) to ~ 2.2 eV (blue-green). Similarly, dark conductivity also varies in a wide range of 10-7- 10-2 Scm-1 by altering the concentration as well as the morphological phase (amorphous or crystalline) of Ge. Our interest is to exploit these novel nanocomposites for photovoltaic application. Preliminary results on the device fabrication and characterization will also be presented.
11:30 AM - CC5.7
Synthesis of CdSe-TiO2 Nanocomposites and Their Applications to Quantum-Dot-Sensitized Nanocrystalline TiO2 Solar Cells
Jin Young Kim 1 , Sung Bum Choi 1 , Sangwook Lee 1 , Hyun Suk Jung 2 , Kug Sun Hong 1 Show Abstract
1 Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
TiO2-based composite material systems combined with semiconductor quantum dots (QDs) such as CdS, CdSe, and InP have been intensively investigated due to their potential applications including sensing, catalysis, and opto-electronics. Recently, special interest has been focused on their applications in solar energy conversion devices since the quantum yield can be greater than one due to the impact ionization. Furthermore, the absorption spectrum can be adjusted by the size and the shape of QDs. In this study, CdSe-TiO2 nanocomposites were prepared via a modified aminolysis process using CdSe nanocrystals as the seeds for the reaction and a new strategy to fabricate the QD-sensitized nanocrystalline TiO2 solar cells was proposed. Successful formation of CdSe-TiO2 nanocomposites were confirmed by a high-resolution transmission electron microscope (HR-TEM), X-ray diffraction, and Raman spectroscopy. Separation of photoelectrons could also be confirmed by the luminescence quenching and fluorescence spectra which are shifted to lower energies relative to the CdSe seeds. As-synthesized CdSe and CdSe-TiO2 sensitizers were adsorbed on the nanocrystalline TiO2 films from colloid QD solutions and the QD-adsorbed TiO2 films were heat-treated at a mild condition in order to make more electron paths from sensitizers to TiO2 films. TiO2 phase was revealed to play roles as not only separation of photoelectrons but also passivation of CdSe seeds. As a result, feasible QD-sensitized nanocrystalline TiO2 solar cells were demonstrated since they exhibited the short circuit current (Isc) and the open circuit voltage (Voc) increased by 460 % and 81 % compared to the bare-TiO2 film, respectively.
11:45 AM - CC5.8
Plasma Synthesis of Highly Monodisperse Ge Nanocrystals and Self-assembly of Dense Nanocrystal Layers.
Ryan Gresback 1 , Lorenzo Mangolini 1 , Uwe Kortshagen 1 Show Abstract
1 Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
12:00 PM - CC5.9
Heterojunction Photovoltaics Using Printed Colloidal Quantum Dots as a Photosensitive Layer.
Alexi Arango 1 , David Oertel 2 , Moungi Bawendi 1 , Vladimir Bulović 2 Show Abstract
1 Electrical Engineering & Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
We demonstrate an efficient quantum dot photovoltaic device using a thin film of cadmium selenide (CdSe) quantum dots that is printed onto a wide band gap organic hole-transporting molecule N,N’-diphenyl-N,N’-bis(3-methylphenyl)-(1,1’-biphenyl)-4,4’-diamine (TPD) using a non-destructive microcontact stamping method. The resulting hetero junction device yields an internal quantum efficiency of 9 % and an open circuit voltage of 0.8 V, even though the work function asymmetry between the top contact (indium-tin-oxide, ITO) and the bottom contact (Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate, PEDOT:PSS) is only 0.2 eV. Furthermore, measurement of the locked-in photocurrent under bias reveals a strong photocurrent signal extending out to +1.4 V, in excess of the built-in acceptor-donor energy level offset of approximately 0.8 eV. The high built-in potential is consistent with fast rates of charge separation relative to interfacial recombination. The photocurrent response exactly matches the absorption spectrum of the colloidal quantum dot film, suggesting that the photocurrent response spectra can be tailored by selecting quantum dots of the desired absorption.
12:15 PM - CC5.10
GaInAsN Based Multiquantum Well Solar Cell.
Alex Freundlich 1 , Lekhnath Bhusal 1 , Aristotelis Fotkatzikis 1 , Wenkai Zhu 1 , Andenet Alemu 1 , Andrea Feltrin 1 , Gokul Radhakrisnan 1 Show Abstract
1 Center for Advanced Materials, University of Houston, Houston, Texas, United States
12:30 PM - **CC5.11
Surface Reconstruction of Nanoparticles: An Opportunity for Improving Solar Energy Conversion
Tijana Rajh 1 , Zoran Saponjic 1 , Nada Dimitrijevic 1 , Linda da la Garza 1 , Yupo Lin 1 , Seth Snyder 1 Show Abstract
1 , Argonne National Laboratory, Argonne, Illinois, United States
CC6: Nanostructures for Photovoltaics II
Wednesday PM, November 29, 2006
Independence W (Sheraton)
2:30 PM - CC6.1
Exciton Lifetime in PbS Quantum Dots in Glass
Peter Persans 1 , Aleksey Filin 2 Show Abstract
1 Physics, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Physics, Temple University, Philadelphia, Pennsylvania, United States
PbS quantum dots exhibit promising and unique properties for solar energy conversion. The lifetime, efficiency, and saturation behavior of photoluminescence and photoinduced bleaching elucidate the fundamental nature of exciton generation, decay, and interactions. Photoluminescence efficiency for PbS particles in glass with HOMO-LUMO transition of 0.8 eV is temperature-dependent with a peak around 250K and surprisingly long decay time of order 10 microseconds. Similar decay times are observed in pump-probe studies of HOMO-LUMO absorption. The magnitude and saturation of bleaching of the HOMO-LUMO transition indicates that only single-exciton states have such long lifetimes. The effects of particle size, excitation energy, excitation intensity, and temperature will be discussed.
2:45 PM - CC6.2
Design and Fabrication of PbSe Nanocrystal-Polymer Hybrid Photovoltaic Cells to Enhance the Harvest of Near Infrared Solar Energy
Ting Zhu 1 , Fan Zhang 1 , Dehu Cui 1 , Jian Xu 1 Show Abstract
1 Department of Engineering Science and Mechanics, Penn State University, University Park, Pennsylvania, United States
3:00 PM - CC6.3
Novel Solar Cell Device Structure Employing a Tightly Packed Ordered Array of Lead Chalcogenide Quantum Dots.
Joseph Luther 1 2 , James Murphy 1 , Mark Hanna 1 , Kathrine Gerth 1 3 , Arthur Nozik 1 3 Show Abstract
1 , NREL, Golden , Colorado, United States, 2 Applied Physics, Colorado School of Mines, Golden, Colorado, United States, 3 Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, United States
3:15 PM - CC6.4
Nanostructured Solar Cells.
Ozgur Yavuzcetin 1 , Cheol-Soo Yang 1 , Thomas Russell 2 , Mark Tuominen 1 Show Abstract
1 Physics, UMass , Amherst, Massachusetts, United States, 2 Department of Polymer Science and Engineering, UMass, Amherst, Massachusetts, United States
In this work we investigate the use of nanofabrication technique to improve the overall efficiency of silicon solar cells. The efficiency of silicon solar cells strongly depends on the quality of the anti-reflective coating. In this work, the change in the index of refraction on the surface of a substrate can be controlled by the amount of porosity, which is well known in effective medium theory. Also by changing the thickness of the porous layer, the medium can be fine tuned to a specific wavelength as an AR coating. We fabricate the nanoporous layer by using a self-assembled P(S-b-MMA) coating as a mask to etch into the silicon substrate using reactive ion etching. The use of different molecular weight diblock copolymer and different etching time allow us to tune the index of refraction. FT-IR and variable angle ellipsometry provide information about the transmission and reflection properties along with the index of refraction and the thickness of the coating. The investigation of the efficiencies are performed by comparing the I-V plots of conventional and nanostructured cells. Additional research is underway in order to apply this technology to other types of substrates. This work is supported by NSF grants DMR-0306951, DMI-0531171 and MRSEC.
3:30 PM - CC6.5
Solar Energy Conversion Using Carbon Nanotubes.
Wyatt Metzger 1 , Timothy McDonald 1 , Marcus Jones 1 , Chaiwat Engtrakul 1 , Jeffrey Blackburn 1 , Kelly Knutsen 1 , Randy Ellingson 1 , Garry Rumbles 1 , Michael Heben 1 Show Abstract
1 , National Renewable Energy Lab, Golden, Colorado, United States
Carbon nanotubes are finding increasing use in solar energy conversion schemes. For example, solar conversion efficiencies in excess of 1% have been shown when single-wall nanotubes (SWNTs) were used in transparent conducting layers that selectively harvested holes generated in polymer-fullerene solar cells [1-3]. SWNTs have also found use on electrodes as electron acceptors in donor-acceptor nanocomposites . Thus, nanotubes promise to improve solar energy conversion efficiencies by providing charge transporting networks. Current synthetic methods produce a mixture of SWNTs with semiconducting (s-SWNT) and metallic (m-SWNT) character. It is not clear if s-SWNTs or m-SWNTs are desired to optimize the charge transporting networks that have been investigated to date, but, clearly, s-SWNTs will be required if the energy of a nanotube’s photoexcited state is to be available to do useful electric or chemical work in an external circuit. Interestingly, s-SWNTs possess structure dependent band-gaps that provide good overlap with the solar spectrum. The photoluminescence quantum yield (QY) provides a measure of the accessibility of the energy in a s-SWNT excited state. QY measurements on ensembles of nanotubes in solution indicate low values in the range of 10-3 to 10-4 . However, QY assessments done by comparing the measured photoluminescence lifetime, τPL, to calculated  values for the radiative lifetime, τrad , suggest that the QY may be significantly higher, perhaps exceeding 10%. Here we report measurements of τ PL from 15 different SWNT species dispersed in aqueous solution. Room-temperature PL decay curves were measured by time-correlated single-photon counting. We reveal multi-exponential PL decay kinetics with lifetime components ranging over three orders of magnitude from about 100 ps to hundreds of nanoseconds. The results will be discussed in terms of trends in the values of across the series and the correspondence with recently calculated values of τ rad. The nature of intrinsic versus extrinsic nanotube properties in controlling the quantum yield for emission will also be discussed. Finally, results in the area of separating s-SWNTs and m-SWNTs for use in a full range of photoconversion schemes will also be presented. This work was supported by the Solar Photochemistry program within DOE’s Office of Basic Energy Sciences.References: du Pasquier et al., Appl. Phys. Lett. 87, 203511 (2005) van de Lagemaat et al., Appl. Phys. Lett. 88, 233503 (2006) Rowell, et al., Appl. Phys. Lett 88, 233506 (2006) Guldi, et al., Acc. Chem. Res. 2005, 38, 871-878 Jones, M., et al., Phys. Rev. B, 2005. 71(11): p. 115426. Perebeinos, et al., Nano Lett. 5, 2495-2499 (2005)
3:45 PM - CC6.6
Nanoscale Carbon Materials for Solar Energy Conversion.
Dirk Guldi 1
1 , Univeristy of