Symposium Organizers
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
Session Chairs
Teresa Barnes
Jennifer Nekuda
Monday PM, 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
1 Physics and Astronom, University of Denver, Denver, Colorado, United States, 2 Measurements and Characterization Division, National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractMinority-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
1 , Martin-Luther University, Halle Germany, 2 , Institute of Physical Hightechnology, Jena Germany, 3 , Max-Planck Institute of Microstructure Physics, Halle Germany
Show AbstractMulticrystalline 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 [1]. 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.[1] 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
1 , Hahn-Meitner-Institut Berlin, Berlin Germany, 2 , United Solar Ovonic Corporation, Troy, Michigan, United States
Show Abstract9:45 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
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
Show Abstract10:00 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
1 Centro de Investigacion en Energia, Universidad Nacional Autonoma de Mexico, Temixco, Morelos, Mexico
Show Abstract10:15 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
1 , Hahn-Meitner-Institut, Berlin Germany, 2 , United Solar Ovonic Corporation, Troy, Michigan, United States
Show Abstract11:00 AM - CC1.7
Intentional Interface Defects as a Means of Band Gap Engineering and the Applications to Photovoltaics
Marcie Black 1 , James Maxwell 2
1 , LANL, Newton, Massachusetts, United States, 2 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractInterfaces 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
1 Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States
Show AbstractA 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
1 , LAVM Inc., Los Alamos, New Mexico, United States
Show Abstract11:45 AM - CC1.10
Material Issues for Terawatt Level Deployment of Solar Cells.
Alex Freundlich 1 , Andrea Feltrin 1
1 Center for Advanced Materials, University of Houston, Houston, Texas, United States
Show Abstract12:00 PM - CC1.11
An Inorganic Approach to Wet-Chemically Fabricated Tandem Cells.
Meng Tao 1
1 Department of Electrical Engineering, University of Texas at Arlington, Arlington, Texas, United States
Show AbstractThe 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
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)
Show Abstract12:30 PM - CC1.13
Bulk-heterojunction Photovoltaics from Solution-processed Oligoacene:Fullerene Blends.
Matthew Lloyd 1 , Alex Mayer 1 , John Anthony 2 , George Malliaras 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 2 Chemistry, University of Kentucky, Lexington, Kentucky, United States
Show Abstract12:45 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
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
Show AbstractOrganic 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
Session Chairs
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
1 Photovoltaics, National Renewable Energies Labratory, Golden, Colorado, United States, 2 Metallurgy and Materials Engineering, Colorado School of Mines, Golden, Colorado, United States
Show AbstractNewly 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
1 , National Renewable Energy Lab, Golden, Colorado, United States, 2 , Eikos, Inc., Franklin, Massachusetts, United States
Show Abstract 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
1 , National Renewable Energy Lab, Golden, Colorado, United States, 2 Materials Science and Engineering, Iowa State University, Ames, Iowa, United States
Show AbstractRecent 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
1 Chemistry Department, University of Toledo, Toledo, Ohio, United States, 2 Physics Department, University of Toledo, Toledo, Ohio, United States
Show AbstractSystems 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
1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show Abstract5:00 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
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
Show AbstractThin 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
1 Chemistry, Louisiana State University, Baton Rouge, Louisiana, United States, 2 Chemistry, University of Colombo, Colombo Sri Lanka
Show AbstractThe 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
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
Show AbstractThe 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 [3]. 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.[1] J. Xue et al, Appl. Phys. Lett. 85, 5757 (2004).[2] G. Li et al, Nature Mat. 4, 864 (2005).[3] 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
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
Show AbstractThe 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.
Symposium Organizers
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
Session Chairs
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
1 Molecular Electronics, Materials Science Centre, University of Groningen, Groningen Netherlands, 2 , Dutch Polymer Institute, Eindhoven Netherlands
Show AbstractRecently, 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.[1] 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%.[1] 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
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
Show Abstract9:45 AM - **CC3.3
Progress in Vapor Deposited Photovoltaic Cells.
Stephen Forrest 1
1 Department of EECS & Physics, University of Michigan, Ann Arbor, Michigan, United States
Show Abstract10:15 AM - **CC3.4
Photoconversion Processes in Bulk Heterojunction Solar Cells of Single-Wall Carbon Nanotubes and Conjugated Polymers and Dendrimers.
Garry Rumbles 1
1 Chemical and Biological Sciences Center, National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractThe 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
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Dept. of Physics, Colorado School of Mines, Golden, Colorado, United States
Show Abstract11:30 AM - **CC3.6
Charge Generation and Recombination in Polymer / Fullerene Blend Solar Cells.
James Durrant 1
1 Chemistry, Imperial College, London, London United Kingdom
Show Abstract12:00 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
1 SMO/M2N, Eindhoven University of Technology, Eindhoven Netherlands, 2 , Dutch Polymer Institute (DPI), Eindhoven Netherlands
Show AbstractAt 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
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show Abstract12:30 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
1 Solar Energy, ECN, Petten Netherlands, 3 , Dutch Polymer Institute, Eindhoven Netherlands, 2 , Holst Centre, Eindhoven Netherlands
Show Abstract12:45 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
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
Show AbstractThe 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
Session Chairs
Martin Lenes
Garry Rumbles
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
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
Show AbstractThin-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
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
Show Abstract3:00 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
1 , National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractHybrid 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
1 chemistry, University of Hasselt, Diepenbeek Belgium, 2 IMOMEC, IMEC, Diepenbeek Belgium
Show AbstractLow 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
1 Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States
Show AbstractWe 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
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
Show Abstract4:30 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
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
Show AbstractConjugated 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
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
Show Abstract5:00 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
1 Chemistry, USC, Los Angeles, California, United States
Show AbstractOrganic 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
1 Applied Chemistry, The University of Tokyo, Tokyo Japan
Show AbstractCompared 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 [1].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[2]. 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[1] L. Schmidt-Mende, A. Fechtenkotter, K. Mullen, E. Moons, R. H. Friend, J. D. MacKenzie, Science, 293, 1119-1122 (2001)[2] 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
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
Show AbstractSubphthalocyanines 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
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
Show AbstractOrganic 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.[1] 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)[2] 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,[2] 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.[1] R. G. Gordon, MRS Bull. 2000, 25, 52.[2] F. Yang, M. Shtein, S. R. Forrest, Nat. Mater. 2005, 4, 37.
Symposium Organizers
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
Session Chairs
Michael Heben
Peter Persans
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
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
Show AbstractThe 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
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
Show Abstract9:45 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
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
Show Abstract10:00 AM - **CC5.4
Efficient Carrier Multiplication in Semiconductor Nanocrystals for Generation III Photovoltaics.
Richard Schaller 1
1 Chemistry Division, Los Alamos National Lab, Los Alamos, New Mexico, United States
Show AbstractThe 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,[1] 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.[2] 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 [3] and have proposed a mechanism for this process that is consistent with all experimentally available data.[4]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
1 Nnaotech Institute, University of Texs at Dallas, Richardson, Texas, United States, 2 , University of South Florida, Tampa, Florida, United States
Show Abstract10:45 AM - CC5.6
Titania-Germanium Nanocomposites for Quantum Dot Solar Cells.
Sukti Chatterjee 1 2 , Amita Goyal 1 , S. Ismat Shah 2 3
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
Show AbstractSeveral 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
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
Show AbstractTiO2-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
1 Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Show Abstract12:00 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
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
Show AbstractWe 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
1 Center for Advanced Materials, University of Houston, Houston, Texas, United States
Show Abstract12:30 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
1 , Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractCC6: Nanostructures for Photovoltaics II
Session Chairs
Matthew Beard
Randy Ellingson
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
1 Physics, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Physics, Temple University, Philadelphia, Pennsylvania, United States
Show AbstractPbS 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
1 Department of Engineering Science and Mechanics, Penn State University, University Park, Pennsylvania, United States
Show Abstract3:00 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
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
Show Abstract3:15 PM - CC6.4
Nanostructured Solar Cells.
Ozgur Yavuzcetin 1 , Cheol-Soo Yang 1 , Thomas Russell 2 , Mark Tuominen 1
1 Physics, UMass , Amherst, Massachusetts, United States, 2 Department of Polymer Science and Engineering, UMass, Amherst, Massachusetts, United States
Show AbstractIn 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
1 , National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractCarbon 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 [4]. 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 [5]. However, QY assessments done by comparing the measured photoluminescence lifetime, τPL, to calculated [6] 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:[1] du Pasquier et al., Appl. Phys. Lett. 87, 203511 (2005)[2] van de Lagemaat et al., Appl. Phys. Lett. 88, 233503 (2006)[3] Rowell, et al., Appl. Phys. Lett 88, 233506 (2006)[4] Guldi, et al., Acc. Chem. Res. 2005, 38, 871-878[5] Jones, M., et al., Phys. Rev. B, 2005. 71(11): p. 115426.[6] 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 Erlangen, Erlangen Germany
Show AbstractNanoscale carbon materials (i.e., fullerenes and nanotubes) have become the focus of considerable interest, because they represent ideal systems for testing fundamental ideas about the roles of dimensionality and confinement in materials of greatly reduced size. Among the wide range of nanostructures available, carbon nanotubes and, in particular, single-wall carbon nanotubes (SWNT), stand as unique materials not only in fundamental but also in applied research. In fact, the extraordinary electronic, mechanical and adsorption properties of carbon nanotubes suggest many possible applications. For example, carbon nanotubes are the stiffest known materials and exhibit novel electronic properties that bridge the bulk and molecular states and represent a flexible starting point for preparing new nanocomposites.SWNT are good electron acceptors as well electron donors and, at the same time, one-dimensional nanowires. They are ready to accept electrons, which are then transported under nearly ideal conditions along the tubular axis.6 Notably, the expected electrical conductivity associated with the tubular structure and good chemical stability opens new promising scenarios for their use as “molecular wires” with high surface areas. Considering this in the context of photovoltaics, SWNT may find their prominent place in electro- and photoactive nanocomposites – just as other carbon modifications / allotropes (i.e., fullerenes) have been tested successfully as electron acceptors in recent research.7 Whereas different approaches towards the design of donor-acceptor nanohybrids have provided interesting and promising results, the use of carbon nanotubes is expected to lead to new breakthroughs.8 However, several obstacles need to be properly addressed when integrating SWNT into functional nanohybrids and testing them in practical applications. Controlled modification of their surface with multifunctional groups – chromophores, electron donors, biomolecules, etc. – is required to fully realize their potential in nanotechnology.I will discuss recent advances in the design, synthesis, characterization, and potential applications of new multifunctional carbon nanotube materials as two- or three-dimensional architectures for electron donor-acceptor chemistry, high mechanical strength, and photoelectrochemical devices.
4:30 PM - CC6.7
Semiconducting Silicide Nanowires for Ecologically Sustainable High-Performance Photovoltaics
Song Jin 1 , Jeannine Szczezh 1
1 Chemistry, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractIron disilicide (β-FeSi2) can be a direct band gap semiconductor (Eg= 0.8 eV) with efficient light absorption, but only when under appropriate lattice stress. The nontoxic silicon and iron elements are respectively the second and fourth abundant elements in earth’s crust. When materials problems are properly solved, β-FeSi2 can prove to be higher performance photovoltaic materials than the traditional crystalline silicon yet more ecologically sustainable than those direct bandgap semiconductors. We are developing chemical synthesis of free standing one-dimensional nanowire materials and heterostructures of β-FeSi2 and other semiconducting silicides such as CrSi2 (Eg= 0.4 eV), to investigate if the lattice strains created by the surface tension due to the nanoscale dimension(s) and chemically synthesized heterostructured silicide/silicon nanowirecan, together with quantum confinement can cause nanoscale materials of β-FeSi2 to become direct bandgap materials, therefore making them high-performance yet ecologically sustainable materials for photovoltaic applications. We will report the synthesis and structural characterization of single crystal nanowires of CrSi2 prepared via chemical vapor transport. We will also report synthesis and structural characterization of single crystal nanowires of FeSi prepared using chemical vapor deposition of the single source precursor and our progress towards nanowires of β-FeSi2. We will also discuss our preliminary physical property investigation of these novel nanomaterials.
4:45 PM - CC6.8
Synthesis and Characterisation of Porphyrin Modified Gold Nanoparticles for Solar Energy Conversion
Brenda Long 1 , Osman Bakr 1 , Francesco Stellacci 1
1 DMSE, MIT, Cambridge, Massachusetts, United States
Show AbstractPhotosynthesis is natures way of converting solar energy into chemical energy in the cells of green plants and photosynthetic bacteria. This process is facilitated by the presence of a series of protein-pigment complexes within the cell. It is the unique ring-like arrangement of pigment molecules (chlorophyll) within these complexes that permits the extremely efficient and high yielding energy conversion. In order to mimic this process in vitro, it is necessary to take the salient components of natures apparatus, and incorporate them into a system that mimics natures design but at a level of complexity that is feasible to fabricate in the laboratory. It was discovered by this group that nanoparticles synthesised using mixtures of two types of ligands result in phase separation into sub-nanometer ring-like domains. Gold nanoparticles, stabilised by porphyrin molecules (same family as chlorophyll) and a secondary ligand, have been synthesised. For the first time, porphyrin aggregation on the nanoparticle has been observed. 1H NMR spectroscopy has revealed very interesting structural details both in the monomeric and aggregated state of the porphyrin on the nanoparticle unique to what we see for the free molecule. Quantum yield and emission lifetime measurements highlight the potential for the use of these nanoparticle-pigment systems in future solar energy cells
5:00 PM - CC6.9
Understanding Nanostructured Solar Cell Performance with Time-Resolved Electrostatic Force Microscopy
David Coffey 2 , David Ginger 1
2 Physics, University of Washington, Seattle, Washington, United States, 1 Chemistry, University of Washington, Seattle, Washington, United States
Show AbstractOrganic composite solar cells comprising blends of conjugated polymers with fullerenes, nanocrystals, or other polymers make promising materials for low cost photovoltaic applications. Different processing conditions are known to impact the efficiency of these blended solar cells by creating a variety of nanostructured film morphologies. However, the relationship between local film structure and device efficiency is not fully understood. We use time-resolved electrostatic force microscopy (EFM) as a means to measure charge generation in thin films of semiconducting polymers under photoexcitation, and show that we can probe charge generation with a resolution of 100 nanometers and 100 microseconds. The EFM measurements correlate well with the external quantum efficiencies for a series of polymer photovoltaic devices, providing a direct link between local morphology, local optoelectronic properties, and device performance. The EFM data show that film regions away from the visible domain interfaces can account for the majority of the photoinduced charge collected in model polymer blend solar cells. These results underscore the importance of controlling not only the length-scale of phase separation, but also the composition of the domains, when optimizing nanostructured composite solar cells.
5:15 PM - CC6.10
Surface Area Analysis of Low Temperature Nanostructured Anatase Films.
Adam Froimovitch 1 , Steve McGarry 1 , Tom Smy 1 , Jeff Fraser 2
1 Electronics, Carleton University, Ottawa, Ontario, Canada, 2 , National Research Council Canada, Ottawa, Ontario, Canada
Show AbstractNanostructured titania (NST) is an excellent wide bandgap semiconductor for a diverse range of applications. Large specific surface area anatase and rutile films have been shown to be effective as photo-catalysts for the production of hydrogen and oxidation of pollutants, in the production of efficient dye-sensitized solar cells, as gas sensors and in biomedical or dental applications. Presented here for the first time, the Brunauer, Emmett and Teller (BET) method of surface area analysis is used to investigate the porosity and surface area of NST grown by the oxidation of titanium metal in H2O2 . The specific surface area is determined to be nearly 600m2/g. NST films can be formed using a number of techniques. Any preferred method of producing NST films should produce large specific surface areas, be highly ordered for enhanced electron transport and have low processing temperatures (less than or equal to 300C) because of the potential for flexible, heat stabilized polyester or polyimide substrates and roll to roll processing. Such a process was reported by Wu et. al. [1] who formed NST films by reacting bulk titanium sheets with 10 percent hydrogen peroxide at 80C for one hour, and then annealing at 300C for 1 hour. The resulting NST was a highly porous honeycomb like structure. Such a process is potentially ideal for dye-sensitized solar cells because it is low cost and low temperature. In the paper, Ti films deposited on silicon and glass substrates by RF magnetron sputtering at 2.5 to 25 mTorr are reacted with 10% hydrogen peroxide at 80C for 45 minutes. The surface area of the resulting NST films is measured before and after annealing at 300C for 10 hours. It is demonstrated that the surface area can be controlled by the process pressure. The largest post annealed specific surface area measured was found to be 592m2/g, among the highest reported for nanostructured titania. X-ray diffraction patterns for the NST films show that a well crystallized anatase phase can be obtained by annealing at temperatures lower than previously reported, as low as 240C. This opens the door to roll to roll processing on flexible, economical and transparent substrates such as heat-stabilized polyester and polyimide. Optical characterization of the NST films confirm that they are nearly 90% transparent between 400 and 1100nm. To demonstrate the potential use of the NST in dye-sensitized solar cells annealed films are dyed in 10-4 M eosin-y ethanol. The dyed films exhibit deep absorption of light in the visible spectrum.[1] J.-M. Wu, S. Hayakawa, K. Tsuru, and A. Osaka, "Porous titania films prepared from interaction of titanium with hydrogen peroxide solution," ScriptaMaterialia, vol. 46, 2002.
5:30 PM - **CC6.11
Novel Nanoscale Organic Materials for Optimal Photovoltaic Functions
Lin Chen 1 , Dmiitri Polshakov 1 , Shengqiang Xiao 2 , Yongye Liang 2 , Luping Yu 2
1 Chemistry, Argonne National Laboratory, Argonne, Illinois, United States, 2 Chemistry, The University of Chicago, Chicago, Illinois, United States
Show Abstract
Symposium Organizers
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
CC7: Dye-Sensitized Solar Cells I
Session Chairs
Thursday AM, November 30, 2006
Independence W (Sheraton)
9:00 AM - **CC7.1
Rational Molecular Design for Organic Solar Cells
Hiroshi Imahori 1
1 Department of Molecular Engineering, Kyoto University, Kyoto Japan
Show Abstract9:30 AM - CC7.2
Dynamics of Charge Transport and Recombination in Dye-Sensitized TiO2 Nanotube Solar Cells: Nanotube Arrays versus Random Nanoparticle Networks.
Kai Zhu 1 , Nathan Neale 1 , Alexander Miedaner 1 , Arthur Frank 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractIn traditional nanoparticle-based dye-sensitized solar cells, photoinduced electrons diffuse through a random nanoparticle network before reaching the charge-collecting electrode. This torturous conducting pathway results in slow charge transport. One-dimensional nanotube arrays aligned perpendicular to the substrate could potentially provide a continuous electron-conducting pathway to facilitate the charge-collection process. In this talk, the electron dynamics in dye-sensitized TiO2 nanotube arrays, the relation between charge transport and recombination, and the morphological characterizations of the nanotube films are discussed. Also, the device performances of dye-sensitized solar cells based on nanotube arrays and the traditional randomly-packed-nanoparticle films are compared.
9:45 AM - CC7.3
Extending the Spectral Response of Dye-sensitized, Organic and Polymer Solar Cells.
Arie Zaban 1 , Elad Koren 1 , Igor Lubomirsky 2 , David Cahen 2
1 Chemistry Department, Bar-Ilan University, Ramat-Gan Israel, 2 Dept. of Materials & Interfaces, Weizmann Institute of Science, Rehovot Israel
Show Abstract10:00 AM - CC7.4
Light Harvesting Using Oligothiophene-Sensitized Nanocrystalline TiO2 Films.
David Officer 1 2 , Wayne Campbell 1 2 , Sanjeev Gambhir 1 2 , Pawel Wagner 1 2 , Yvonne Ting 1 2 , Mohammad Nazeeruddin 3 , Michael Grätzel 3
1 Nanomaterials Research Centre, Massey University, Palmerston North New Zealand, 2 MacDiarmid Institute for Advanced Materials and Nanotechnology, Massey University, Palmerston North New Zealand, 3 Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Swiss Federal Institute of Technology, Lausanne Switzerland
Show AbstractSolar cells comprised of dye-sensitised wide band-gap inorganic semiconductors have gained widespread attention as low-cost, lightweight alternatives to conventional silicon-based photovoltaic devices. To date, ruthenium polypyridyl complexes have proved to be the most efficient TiO2 sensitizers, with the N3 dye demonstrating incident photon-to-electron conversion efficiencies (IPCEs) of up to 85% from 400-800 nm and an overall photovoltaic cell energy conversion efficiency of 11%. The photosensitization of wide-bandgap semiconductors such as TiO2 by organic dyes is being actively pursued and impressive efficiencies have been reported for porphyrin (5.6%), coumarin (7.4%), and indoline (8%) dyes. There is also increasing interest in the use of polymers such as polythiophenes as sensitizers given their broad absorption characteristics, excellent hole transport properties and high stability. However, the results to date have not been promising. In order to better evaluate the potential of polythiophenes as light harvesting materials, we have begun a systematic and staged approach by investigating the use of oligothiophenes as sensitisers for nanocrystalline TiO2. To this end, we have synthesised a series of oligothiophenes and oligothienylenevinylenes with cyanoacrylic acid or malonic acid linkers attached to either the terminal α-position or middle ring β-position. The absorption spectra of the α-linked thiophenes attached to TiO2 generally exhibit peak broadening and blue shifts consistent with the formation of H-aggregates on the surface. The resulting increased spectral coverage is reflected in good light harvesting efficiency for such simple molecules.The thiophene materials were investigated for conversion of sunlight into electricity by constructing liquid junction dye-sensitized TiO2 solar cells using an I-/I3- electrolyte. External quantum efficiencies (η) of up to 3.1% were obtained depending on the nature and position of the binding group, the oligomer length and electrolyte used. The extension of these oligomers to polymers holds promise for the development of efficient polythiophene-sensitised TiO2 solar cells.
10:15 AM - CC7.5
Photoelectrochemical Performance of Sensitized ZnO Thin Films - Dependence on the Crystalline Orientation of ZnO in Electrochemically Deposited Hybrid Materials
Kazuteru Nonomura 1 , Daisuke Komatsu 2 , Tsukasa Yoshida 2 , Hideki Minoura 2 , Derck Schlettwein 1
1 Institute of Applied Physics, Justus-Liebig-University Giessen, Giessen Germany, 2 Graduate School of Engineering, Gifu University, Gifu Japan
Show AbstractPorous yet crystalline thin films of ZnO can be formed by electrochemical deposition of ZnO in the presence of appropriate structure- directing agents. In the presence of Eosin Y (hereafter EY) in the deposition solution, ZnO / EY thin films with the ZnO c-axis perpendicular to the growth direction are obtained [1]. In the presence of Coumarin 343 (hereafter C343), however, ZnO / C343 with the c-axis of ZnO parallel to the growth direction are obtained. The dye molecules are desorbed from the hybrid thin films to yield crystalline porous matrices of ZnO. These serve to adsorb different sensitizer molecules [2]. ZnO films prepared in the presence of C343 show faster electron transport as seen in photocurrent transients and IMPS (Intensity Modulated Photocurrent Spectroscopy), speaking for an increased charge carrier mobility in ZnO perpendicular to the c-axis and faster cation transport in the pores since electron transport in such nanostructured electrodes is coupled to the cations in solution. The lifetime of injected electrons was found at quite comparable values by IMVS (Intensity Modulated photoVoltage Spectroscopy) at open circuit. The results are rationalized by a decreased recombination rate in ZnO prepared in the presence of C343 and a higher electrode efficiency can be expected for such a material. Up to now, however, the specific surface area of the ZnO / EY electrodes turned out to be superior leading to a higher overall electrode efficiency.1. T. Yoshida, T. Oekermann, K. Okabe, D. Schlettwein, K. Funabiki, H. Minoura, Electrochemistry, 70, 470-487 (2002). 2. T. Yoshida, M. Iwaya, H. Ando, T. Oekermann, K. Nonomura, D. Schlettwein, D. Wöhrle, H. Minoura, Chem. Commun., 2004, 400-401.3. Measurements performed in cooperation with J. Rathousky, Heyrovsky Institute, Prague.
10:30 AM - CC7.6
Dye Sensitized Solar Cells (DSC) with Templated Nanocrystalline TiO2 Films having Dual Size-Scale Porosity
Lai Qi 1 , Judith Sorge 1 , Dunbar Birnie 1
1 Ceramic and Materials Engineering, Rutgers University, Piscataway, New Jersey, United States
Show AbstractThe interpenetrating TiO2 network junction plays a key role in the DSC operation. The morphology of TiO2 photoelectrode will largely decide the ability to balance and optimize the coupled charge transportation, by which DSC functions. In this work, we present a novel coating technique and resulting microstructures for DSC’s. The demonstrated structures are nanocrystalline TiO2 (using Degussa P25) films templated with polystyrene (PS) nanospheres of 200nm to 1μm in diameter. SEM and BET investigation show that this structure combines the high surface area of nanocrystalline films with a secondary interconnected larger-sized pore network. Contrary to the known inverse opals, the produced coating is almost free of cracking due to the pre-crystallized nature of the P25 particles, which gives much less shrinkage than organic Ti precursors. DSC cells based on the templated nanocrystalline structures were tested under simulated AM1.5 sunlight. Short-circuit current of 9.44mA/cm^2, open-cell voltage of 0.73V and fill factor of 55% have been recorded for a film thickness of 12.7 μm, which gives an efficiency of 4.5%. Comparison between templated and non-templated P25 structures in their specific surface area, dye adsorption, porosities, and pore size distribution indicates that the dual porosities in templated structure can render a better charge transport rate in the electrolyte, which may possibly improve the dye regeneration, reduce the charge recombination rate and pay off the reduction of surface area from non-templated structures.
10:45 AM - CC7.7
Alignment of Sensitizer Dye Molecules N3 on TiO2 in Dependence on the Polarity of Coadsorbed Solvent Molecules.
Konrad Schwanitz 1 , Eric Mankel 1 , Ralf Hunger 1 , Thomas Mayer 1 , Wolfram Jaegermann 1
1 Surface Science Division, Materials Science, Darmstadt University of Technology, Darmstadt Germany
Show AbstractThe electronic structure of the dye sensitized solar cell interface TiO2/Ru dye/solvent has been investigated with highest surface sensitive photoemission using the Solid Liquid Analysis System SoLiAS at the BESSY synchrotron. TiO2 was prepared in a MOCVD process using the single precursor titanium tetra isopropoxide (TTIP). The Ru dye was adsorbed from ethanolic solution under pure nitrogen atmosphere and transferred to UHV without contact to ambient air. The polar and unpolar solvents acetonitrile and benzene respectively were condensed out of the gas phase in vacuum on the liquid nitrogen cooled sample.With coadsorption of acetonitrile remarkable changes of line shape, intensities and binding energies of Ru dye core and valence levels occurred. While the emission of the "dry" dye is broad, the line width is reduced with coadsorbed acetonitrile. The binding energy of the S2p emission and the HOMO level shifted to higher values. Initially the emission was damped whereas with further addition of acetonitrile the intensity increased distinctly. All these processes were reversed upon desorbing acetonitrile. In contrast the coadsorption of unpolar benzene has not shown such peculiarities. The intensity of the S2p emission is damped with increasing dosage of benzene and the line width does not change remarkably.A simple geometric model can be deduced from these investigations: In the absence of acetonitrile the N3 sensitizer molecules lie disordered on the TiO2 surface with their NCS groups interacting with the surface and neighbouring sensitizer molecules causing statistical broadening of the S2p level. Coadsorbed acetonitrile molecules evidently penetrate the sensitizer layer, insulate the molecules from each other and force the N3 molecules to point their NCS groups away from the substrate, thereby reducing the statistical broadening. Thus acetonitrile serves not only as the medium in which the hole transporting redox species can move but has a distinct and important influence on the adsorption geometry. Comparison with benzene evidences that the effects observed with acetonitrile depend on the polarity of the solvent molecules.
11:30 AM - CC7.8
Photoinduced Charge and Energy Transfer in Dye-Doped Conjugated Polymers.
Dirk Veldman 1 , Stefan Meskers 1 , René Janssen 1
1 Molecular Materials and Nanosystems, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractThe open-circuit voltage of bulk heterojunction polymer solar cells reflects the energy of the charge-separated state formed in the photoinduced charge transfer between the donor (D) and acceptor (A) in the blend. Consequently, the open circuit voltage can be raised by increasing the energy difference between the oxidation potential of the donor and the reduction potential of the acceptor. Because photoinduced electron transfer will no longer occur when the energy of the charge separated state approaches that of the lowest singlet state of the pure materials, this increase is limited by the excited state energies of donor and acceptor. To establish experimentally the limiting conditions for photoinduced electron transfer, we have studied a large variety of blends of conjugated donor polymers (polyphenylene vinylenes and fluorene copolymers) with acceptor molecules (bodipy dyes, perylenediimides, and fullerenes) with (time-resolved) fluorescence and photoinduced absorption spectroscopy. The results show that in solid state blends charge transfer occurs when the energy of the lowest excited singlet state of either donor or acceptor is higher than the difference between the oxidation and reduction potentials (as determined from cyclic voltammetry in solution) augmented with an additional energy of 0.35-0.50 eV. This result provides an important guideline for the design of polymer solar cells with optimized open circuit voltage and represents a restriction to the maximum power conversion efficiencies that can be obtained.
11:45 AM - CC7.9
Synthesis, Characterization, Photocatalytic Activity and Ddye-sensitized Solar Cell Performance of Nanorods/nanoparticles TiO2.
Sorapong Pavasupree 1 , Supachai Ngamsinlapasathian 1 , Yoshikazu Suzuki 1 , Susumu Yoshikawa 1
1 Institute of Advanced Energy, Kyoto University, Uhi, Kyoto, Japan
Show Abstract12:00 PM - CC7.10
High-efficiency Dye-sensitized Solar Cell Based on ZnO Nanorod Arrays Electrode.
Patcharee Charoensirithavorn 1 , Susumu Yoshikawa 1
1 , Kyoto University, Uji, Kyoto, Japan
Show Abstract12:15 PM - CC7.11
Influence of Nitric Oxide Adsorption on Energy Conversion Efficiency of Dye-Sensitized Solar Cells.
Hyun Suk Jung 1 , Jung-Kun Lee 1 , Michael Nastasi 1 , Sang Wook Lee 2 , Jing-Young Kim 2 , Kug Sun Hong 2
1 Materials Physics and Application, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 , Seoul National University, Seoul Korea (the Republic of)
Show AbstractSince the application of dye-sensitized solar cells (DSCs) in solar energy conversion was first reported in 1991, the increase in conversion efficiency has been primarily due to improvements in organic sensitizers and liquid electrolytes. In contrast to these improvements, little work has been done in improving the TiO2 nanoparticles based photoelectrode. However, inefficient carrier transport in the photoelectrode such as backward electron transfer and carrier trapping at grain boundaries between nanoparticles, have limited DSCs from achieving higher efficiencies. In this study, we improved the carrier extraction and transport properties of TiO2 nanoparticles based photoelectrode by acid treatment and explored the physics underlying the change in the DSCs performance. The TiO2 nanoparticles photoelectode was treated in nitric acid before adsorbing dyes and then, the influence of surface nitric oxide groups on the DSCs performance was investigated. The energy conversion efficiency for the nitric acid-treated TiO2 photoelectrode was significantly improved from 8.6 % to 9.6 % as compared to bare TiO2 photoelectrode. The enhanced solar cell performance is mainly attributed to the increase in photocurrent. Transient photovoltage and impedance analysis of the nitric acid-treated solar cell demonstrates that the acid-treated photoelectrodes retard back electron transfer at the interface with the electrolyte and increase the amount of adsorbed dyes. This phenomenon is explained by the presence of nitric oxide and hydroxyl groups on the TiO2 surface, which are quantitatively analyzed using X-ray photoemission and FT-IR spectroscopy studies.
12:30 PM - **CC7.12
Molecular Engineering of Sensitizers for Solar Cell Applications.
Mohammad Nazeeruddin 1
1 LPI, ISIC, SB, EPFL, Lausanne, vd, Switzerland
Show AbstractThursday, 11/30New Invited Abstract11:30 *CC7.12Molecular Engineering of Sensitizers for Solar Cell Applications. Mohammad Khaja Nazeeruddin, LPI, ISIC, SB, EPFL, Lausanne, Switzerland. Nanocrystalline TiO2 based Dye-Sensitized Solar Cells (DSSC) are potentially low in cost compared to the conventional silicon solar cells. The DSSC consists of a working electrode, which is a sensitizer derivatized mesoporous TiO2 film, and a counter electrode, sand-witched with an iodide/triiodide (I-/I3-) redox electrolyte. The immobilized sensitizer absorbs a photon to produce an excited state, which transfers efficiently its electron into the TiO2 conduction band. The oxidized dye is subsequently reduced by electron donation from the iodide/triiodide redox system. The injected electrons flows through the semiconductor network to arrive at the back contact and then through the external load to the counter electrode. At the counter electrode, reduction of triiodide in turn regenerates iodide, which com-pletes the circuit. [1]In these cells the sensitizer is one of the key components for high power conversion efficiency. [2] In this talk I will discuss the various design strategies of sensitizers consisting of different ligands with specific functionality and their power conversion efficiencies.Reference[1]. Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Grätzel, M. J. Am. Chem. Soc. 1993, 115, 6382.[2]. Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Le, C.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C.A.; Graetzel, M. J. Am. Chem. Soc. 2001, 123, 1613Acknowledgement. We acknowledge financial support of this work by the Swiss Science Foundation, Swiss Federal Office for Energy (OFEN) and U. S. Air Force Research Office under contract number F61775-00-C0003.
CC8: Dye-Sensitized Solar Cells II
Session Chairs
Derck Schlettwein
Arie Zaban
Thursday PM, November 30, 2006
Independence W (Sheraton)
2:30 PM - CC8.1
TiO2 Nanotubes for Solar Energy Conversion.
Jan Macak 1 , Andrei Ghicov 1 , Patrik Schmuki 1
1 Dep. of Materials Science, Chair for Surface Science, Erlangen, Bavaria, Germany
Show AbstractThe presentation deals with self-organized high aspect ratio titanium oxide nanotubes that can be produced by tailored electrochemical anodization. Tubes can be grown to the length of several micrometers with single tube diameter of several tens of nm and with the possibility to modify the geometry (1-4). We will discuss some specific functional properties, for instance, the use in conversion of solar light (5) and enhanced photocatalytical performance after the N-doping (6). Thus, present work exploits these highly ordered self-organized TiO2 nanotubular layers for the solar energy conversion. We demonstrate that significant light-to-electricity conversion efficiencies can by achieved by dye-senzitization of the tubes (7) or by doping the tubes using thermal treatment (8, 9) or ion implantation (10, 11) of N. Growth, properties and critical issues will be discussed in details. References:1. R. Beranek, H. Hildebrand and P. Schmuki, Electrochem. Solid-State Lett., 6, B12 (2003).2. J. M. Macak, K. Sirotna and P. Schmuki, Electrochim. Acta, 50, 3679 (2005).3. J. M. Macak, H. Tsuchiya and P. Schmuki, Angew. Chem., 44, 2100 (2005).4. J. M. Macak, H. Tsuchiya, L.V. Taveira, S. Aldabergerova, P. Schmuki, Angew. Chem. 44, 7463 (2005)5. B.O. Regan and M.Grätzel, Nature 353, 737 (1991).6. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293, 269 (2001).7. J.M. Macak, H. Tsuchiya, A. Ghicov, P.Schmuki, Electrochem. Comm. 7, 1133 (2005).8. R.P. Vitiello, J. M. Macak, A. Ghicov, H. Tsuchiya, L.F.P. Dick, P. Schmuki, Electrochem. Commun. 8, 544 (2006).9. J.M. Macak, A. Ghicov, R.Hahn, H. Tsuchiya and P. Schmuki, J. Mater. Res., accepted.10. A. Ghicov, J. M. Macak, H. Tsuchiya, J. Kunze, V. Haeublein, S. Kleber, P. Schmuki, Chem. Phys. Lett. 419, 426 (2006). 11. A. Ghicov, J. M. Macak, H. Tsuchiya, J. Kunze, V. Haeublein, L. Frey, P. Schmuki, NanoLetters 6, 1080 (2006).
2:45 PM - CC8.2
Increasing the Efficiency of Dye Sensitized Solar Cells by Modification of the Titania Electrode.
Rachel Caruso 1 , Fuzhi Huang 1 , Yi-Bing Cheng 2
1 School of Chemistry, The University of Melbourne, Melbourne, Victoria, Australia, 2 Department of Materials Engineering, Monash University, Melbourne, Victoria, Australia
Show AbstractThe efficiency of the Dye Sensitized Solar Cell (DSSC) can be enhanced by tailoring the composition, crystal phase and morphology of the titanium dioxide-based electrode. In this study templating techniques and the addition of a second metal oxide have been combined during the preparation of the titanium dioxide-based material. Mixed alkoxide precursor solutions of the titania and the second metal oxide were prepared with contents of 0 to 10 wt. % of the second metal oxide. Infiltration of these solutions within a porous template, an agarose gel, followed by sol-gel chemistry and a heating step resulted in the formation of porous inorganic structures. The materials were characterised by electron microscopy to gain an understanding of the morphology, and X-ray diffraction to obtain the crystallization characteristics as a function of both the second metal oxide wt. % and heating temperature. The surface properties of the samples were studied by both nitrogen sorption and monitoring the adsorption of the sensitizing dye, N719. When the materials were used in the DSSC it was found that the stage at which the second oxide was added during synthesis, as well as the type of metal being added, influenced the overall efficiency of the electrode. In some cases the open circuit voltage was improved, while in other cases the short circuit current increased. A discussion highlighting the electrodes of highest efficiency will be presented.
3:15 PM - CC8.4
Solar Cells Using Solution-derived Oxide Films as Photoelectrodes.
Yanfeng Gao 1 , Masayuki Nagai 1
1 , musashi institute of technology, Tokyo Japan
Show AbstractSolar cells such as dye-sensitized and polymer/metal-oxide hybrid types are employing an oxide semiconductor layer as a means to transport electrons to collecting electrodes. The nanostructures, crystallinity, interfaces between semiconductors and electrodes/polymers are representative components which have great effects on the performance of cells. Solution-based processes are capable to control all technique factors mentioned above by understanding crystallization/nucleation chemistry and surface/interface physics and chemistry. We present here our efforts to develop TiO2/ZnO based photoelectrodes with tunable microstructures by treatment of the transparent conducting substrates in the corresponding aqueous salt solutions.In the case of TiO2, we realized a novel aqueous peroxotitanate solution to deposit TiO2 films under normal conditions in terms of pressures and temperatures. We have achieved microstructure-tuned TiO2 films either amorphous or crystalline by regulating the deposition parameters including pH values and temperatures of solutions. These films were integrated to assemble dye-sensitized solar cells; mean conversion efficiencies of these cells are in the scope of 1.3 – 4% for TiO2 films with thicknesses of 2 – 3 μm.The pathways for the electron transportation are constructed by interconnected TiO2 nanoparticles. Electrons may recombine with holes during the transportation, especially at the grain boundaries where exist a large amount of defects. Ordered, direct pathways provided by nanowire or nanorod arrays are probably the way to resolve the above problem. However, preparation of such an ordered nanostructure of TiO2 usually requires pre-produced moulds, such as anodic aluminum oxides. At the preliminary stage of our study, we tend to fabricate assembled ZnO nanowire arrays and investigate how the arrangement of nanocrystals affects on the properties of resulted devices. At present, we have successfully obtained ZnO films of nanowires, 150 nm in diameter and 15 – 20 μm in length, by a solution deposition method. We can actually tune the aspect ratio from 8 – 100 by regulating the solution parameters including the molar ratio of starting materials, the concentration of zinc (II) and kinds/amounts of additives. Assembly of solar cells of both hybrid and dye-sensitized types is being conducted. We hope that the efforts we are making in this study may contribute to understand how to develop a low-cost, simple process for the fabrication of dye-sensitized solar cells with high efficiencies.
3:30 PM - CC8.5
Laser Sintering of Titanium Oxide for Low Temperature Processing of Photoelectrochemical Cells
Nicholas Kattamis 1 , Christina Peabody 1 , Guodan Wei 1 , Nan Yao 2 , Craig Arnold 1 2
1 Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States, 2 Princeton Institute for Science and Technology of Materials, Princeton University, Princeton, New Jersey, United States
Show AbstractWide bandgap, mesoporous TiO2 continues to attract vast interest for its applications in dye sensitized solar cells (DSSC), chemical sensors and phototcatalysis. For applications in these fields, there exists a common requirement that the size of the TiO2 particles remain on the nanoscale to optimize surface area and optical absorption. However, in many cases, such as DSSC, there is an added requirement that the TiO2 material must be sintered in order to generate a continuous conduction band for transport of electrons through the system. Typical sintering occurs at temperature above 450 oC for several hours, thereby eliminating the option of inexpensive, low temperature polymer substrates. In this work, we present an alternative laser sintering approach that reduces the thermal budget at the substrate, enabling a wider choice of substrates for these materials. In contrast to traditional laser sintering of thin films, we employ a laser forward transfer technique that allows for laser interactions during the deposition process itself. This approach enables a greater surface area of material to be processed and eliminates the need for subsequent irradiation on the substrate. Furthermore, we can directly vary the energy input through secondary irradiation during transfer. High resolution TEM imaging of deposited material shows local regions of sintering for laser fluences above 15 J/cm2. In contrast, low laser fluence (< 1 J/cm2) and blading techniques produce a film that does not exhibit similar structure. Optical absorption studies confirm the microscopy results by showing an increase in absorption as a function of laser energy during deposition. This increase can be correlated to a change in the band gap consistent with an increase in particle size. Other experimental techniques such as XRD and Raman show similar broadening in average particle size while maintaining the Anatase phase. The general laser transfer technique will be discussed in detail along with results on laser sintering of TiO2 in the context of device fabrication and efficiency.
CC9: Solar Hydrogen
Session Chairs
Brian Cole
Christian Richter
Thursday PM, November 30, 2006
Independence W (Sheraton)
4:15 PM - CC9.1
Dye-Sensitized Solar Cells Based on Anatase TiO2 Nanoparticle/Nanowire Composites
Bing Tan 1 , Yiying Wu 1
1 Department of Chemistry, Ohio State University, Columbus, Ohio, United States
Show AbstractThursday, 11/30Transferred to CC7.12 to CC9.1 3:15 PMDye-Sensitized Solar Cells Based on Anatase TiO2 Nanoparticle/Nanowire Composites. Yiying Wu
4:30 PM - CC9.2
Nanoporous Titania-Based Nanomaterials for Advanced Photochemical Devices
Christiaan Richter 2 , Zhen Wu 1 , Latika Menon 1
2 Chemical Engineering, Northeastern University, Boston, Massachusetts, United States, 1 Physics, Northeastern University, Boston, Massachusetts, United States
Show AbstractThe development of technologies that could make the renewable production of hydrogen an economically reality is both a scientific, political and environmental priority. In the case of producing solar hydrogen by means of the photochemical splitting of water, two of the most promising avenues of innovation are: 1) the design of hybrid cells capable of harvesting a broad spectrum of solar energy, and 2) the use of nanostructured materials. In this regard, we have fabricated nanoporous titania templates by electrochemical methods. The solar harvesting potential of these templates is studied by growing co-catalyst nanowires (such as CdS) that are active in the visible spectrum within the titania templates. In addition to the co-catalysts, promoters which enhance proton reduction (hydrogen evolution) such as Pt, NiO are also incorporated. These prototype devices are perfect for studying the impact of nano-scale features on the individual catalytic steps in the water splitting process. Our results on absorption, gas-chromatography structures on these hybrid structures will be discussed.
4:45 PM - CC9.3
GaN-based Semiconductor Alloys for Photoelectrochemical Cells.
R. Jones 1 2 , K. Alberi 1 2 , W. Walukiewicz 1 , J. Ager III 1 , K. Yu 1 , E. Haller 1 2 , O. Dubon 1 2 , H. Lu 3 , W. Schaff 3 , A. Kimura 4 , H. Tang 4 , T. Kuech 4
1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Dept. of Materials Science and Engineering, UC Berkeley, Berkeley, California, United States, 3 Dept. of Electrical and Computer Engineering, Cornell University, Ithaca, New York, United States, 4 Dept. of Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show Abstract5:00 PM - CC9.4
Role of Nitrogen Doping on the Optical and Structural Properties of WO3 for Photoelectrochemical Applications.
Brian Cole 1 , Bjorn Marsen 1 , Eric Miller 1
1 Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, Hawaii, United States
Show AbstractMetal oxide semiconductors have found renewed interest for application in photoelectrochemical (PEC) cells for solar production of hydrogen. Within this class of materials, tungsten trioxide (WO3) has been shown to have properties favorable for PEC, including suitable electronic properties and good corrosion resistance in aqueous electrolytes. However, for WO3 to remain a viable candidate for PEC hydrogen production, its bandgap (~2.6 eV) must be reduced to provide better overlap with the solar energy spectrum. Nitrogen doping has been identified as one viable method to reduce the bandgap for WO3 via valence band modification. Pure and nitrogen-doped WO3 thin-film materials, both amorphous and polycrystalline, have been fabricated using reactive RF magnetron sputtering to study the influence of nitrogen on the optical, structural, and photoelectrochemical properties. Key process parameters included oxygen and nitrogen partial pressure in the sputtering ambient, as well as substrate temperature. Through these studies, bandgap reductions in excess of 0.2 eV have been achieved, however it was found that the net reduction competes with grain-size effects as a contributing mechanism. For certain process conditions, the substitutional doping of nitrogen for oxygen acts to inhibit grain growth, resulting in higher bandgaps consistent with a more amorphous WO3 phase. X-ray diffraction studies show fundamental changes in structure as nitrogen is incorporated into the films. By careful tailoring of the growth process, we have been able to isolate the effects of nitrogen incorporation and grain size on bandgap reduction. As a practical application, these films have demonstrated water-splitting photocurrents exceeding 3 mA/cm2 in acidic electrolyte, measured under AM1.5G simulated sunlight. This talk will address the specifics of sputter process conditions and the detailed impact on structure, bandgap, and photoelectrochemical performance.
5:15 PM - CC9.5
Chalcopyrite Film Photocathodes for Direct Solar-Powered Water Splitting.
Bjorn Marsen 1 , Susanne Dorn 1 , Brian Cole 1 , Richard Rocheleau 1 , Eric Miller 1
1 Hawaii Natural energy Institute, University of Hawaii at Manoa, Honolulu, Hawaii, United States
Show Abstract5:30 PM - CC9.6
Photocatalytic Hydrogen Evolution over Tantalate Photocatalysts.
Jaturong Jitputti 1 , Sorapong Pavasupree 1 , Yoshikazu Suzuki 1 , Susumu Yoshikawa 1
1 , Institute of Advanced Energy, Kyoto University, Uji, Kyoto, 611-0011 Japan
Show AbstractMuch attention has been paid to the photocatalytic decomposition of water into H2 and O2 over semiconductor materials since the first invention of photochemical water splitting over titania photoelectrode proposed by Fujishima and Honda [1]. Since hydrogen is recognized as an environmental friendly and a highly efficient fuel, the production of H2 by direct water splitting attainable without polluted by-products is one of potential alternatives to produce fuel for future energy supply [2]. In recent years, various ultraviolet and visible-light photocatalysts for water splitting reaction were reported, such as NiO/NaTaO3, TaON, SrTiO3 and (Ga1-xZnx)(N1-xOx)solid solution.[3-4]In this study, tantalate photocatalyst was prepared by solid-state reaction at 1000oC using SrCO3, TiO2, and Ta2O5 as starting material. The prepared solid catalyst was characterized using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and N2-adsorption/desorption analysis. The hydrogen production of this catalyst by water splitting reaction under ultraviolet irradiation was carried out in an inner-irradiation type reactor with the cooling water temperature of 30oC. The preliminary results revealed that this tantalate photocatalyst show high photocatalytic hydrogen production by water splitting without co-catalyst loading. The hydrogen production rate of this catalyst was found to be 15 μmolh-1, which was about 10 times higher than that of commercial TiO2 (Ishihara ST-01). Moreover, the effect of calcination temperature and time on the photocatalytic H2 production was also investigated. The photocatalytic activity of this catalyst under visible-light will be performed and reported later.References:[1] A. Fujishima and K. Honda, Nature, 238 (1972), 37.[2] Hatamachi, T. Kodama, M. Sato, and K. Toda, Chem. Mater., 17 (2005), 5161.[3] A. Kudo, International Journal of Hydrogen Energy, 31 (2006), 197.[4] D. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen, Brief Communication. Nature, 440 (2006), 295.
CC10: Poster Session
Session Chairs
Doreen Edwards
Stephen Sanford
Judith Sorge
Friday AM, December 01, 2006
Exhibition Hall D (Hynes)
9:00 PM - CC10.1
Hybrid Photovoltaic Device Based on ZnO Nanostructure
Kazuko Takanezawa 1 , Kouske Hirota 1 2 , Keisuke Tajima 1 , Kazuhito Hashimoto 1
1 , The University of Tokyo, Tokyo Japan, 2 , Mitsui chemicals Inc., Chiba Japan
Show Abstract Organic photovoltaic devices have many advantages such as easiness of fabrication, low cost, flexibility, and possibility to obtain a large device area. The idea of “bulk-heterojunction” has been proven to be very promising. The power conversion efficiency (PCE) of the devices drastically increased. However it is possible that the domains where the carriers transport are not connected to the electrodes, and the structure might not be optimized for efficient carrier transport. To overcome this problem, we report the fabrication of a photovoltaic device with the nanostructured ZnO. ZnO is a good electron transport material compared to the organic semiconductors and works as an electron-acceptor. Rod-like structure and uniaxial crystallinity of the material could help the efficient electron transport in the active layer. We have successfully fabricated a highly uniform and densely packed array of ZnO nanorods on Indium tin oxide (ITO). The average height of nanorods could be controlled by changing the reaction time for ZnO nanorod growth. Poly(3-hexylthiophene) (P3HT) was spin-coated on the substrate and silver electrode was vacuum deposited on the polymer surface subsequently. From the electric measurement, external quantum efficiency (EQE) increased as the length of the ZnO nanorods increased. This improvement could be attributed mainly to the increase of short circuit current. This result shows that the ZnO nanorods accept the electron from the P3HT and then the electron moves smoothly through the ZnO. We also succeeded to fabricate ZnO nanostructure/polymer-fullerene bulk heterojunction photovoltaic devices. In contrast to the previous case, the bulk heterojunction devices showed improved fill factor as the length of the ZnO nanorods increased, leading to higher PCE. Role of the ZnO nanorod array will be discussed.
9:00 PM - CC10.10
Evanescent Wave Phenomena in Solar Cells as a Promising Way Towards Extremely Efficient Energy Conversion.
Elad Koren 1 , Snir Dor 1 , Arie Zaban 1
1 , Bar Ilan University, Ramat Gan Israel
Show AbstractDye sensitized solar cell is a photoelectrochemical cell based on high surface area nanocrystalline TiO2 film that allows adsorption of dye molecules. Such nanocrystalline film mediated by a redox couple to a counter electrode can show solar conversion efficiency of ~ 11% in the laboratory conditions. These kinds of solar cells are very much attractive due to their simple and low cost preparation compared to the conventional silicon solar cells. However, there are some losses due to a limited life time of free electrons before they are recombined. Regarding to the specific absorption pattern, the most dominant losses are obtain in the red part of the visible spectrum due to low absorption coefficient of the N3 dye molecule. The red photons are being absorbed far from the current collector so they have less chances to collect and much more chances to recombine.Using a wave guide solar cell that is based on the evanescent wave phenomena, we are demonstrating the obtained enhanced photocurrent in dye sensitized solar cell. In this approach, we are benefiting from both efficient photon collections throughout the solar spectrum, as well as high electron transport efficiency due to the short distance that the electron has to travel towards the current collector.
9:00 PM - CC10.12
Nano Ceria-Based Variable Bandgap Semiconductors for Solar Cells
Yong Jun Seo 1 , Yong Soo Cho 1
1 Materials Science and Engineering, Yonsei University , Seoul Korea (the Republic of)
Show AbstractSolar cells using nano-structures have been widely reported for the last decade. Especially, photovoltaic cells using semiconductor metal oxides have been intensively investigated to improve energy and cost efficiency. Titanium oxide has been one of the most widely used semiconductor metal oxides particularly for dye-sensitized solar cells. Nano titanium oxide powder provides a large surface area and promising electric contacts so that electron transfer and diffusion in electrolyte easily occur. Despite these merits, the life time of the solar cell has been questionable because dye as a major constitute of the solar cell system is easily degradable. This work demonstrates the potential of none dye-based solar cells utilizing ceria nanopowders. Undoped and doped cerium oxide nanopowders with less than 50 nm particle size were prepared by hydrothermal synthesis of common solutions at 200 oC utilizing different valence ions such as Ca2+, La3+, and Zr4+. X-ray diffraction patterns of doped cerium oxide powder showed no secondary phase up to 10 mol% addition level, but corresponding peaks shifted largely depending on the doping level. Optical band gap was measured as ~ 2.9 eV for undoped CeO2 and < 2.8 eV for doped CeO2. These values are lower than the reported band gap of 3.2 eV for bulk CeO2. This inconsistency with related quantum size effects may be associated with oxygen vacancies and defects on the powder surface. The main purpose of this work is to demonstrate the controllable bandgaps of the nanopowder system by adjusting the dopant level and nanosynthesis conditions. Subsequent testing results applied for the solar cells are another subject of this presentation with focused correlations between the nanopowders and photovoltaic performance.
9:00 PM - CC10.13
Morphologies of ZnO Nanocrystals in Citric acid - Assisted Hydrothermal Synthesis with the Controlled Alkali Solution Route for the Dye Sensitized Solar Cells
Shaheer Akhtar 1 , Alam Khan 1 , Hyun Lee 1 , Ki Ju Kim 1 , O-Bong Yang 1
1 Environment and Chemical Engineering , Chonbuk National University, Jeonju, Chonbuk, Korea (the Republic of)
Show AbstractThe ability to tailor the properties of materials by constraining their physical dimensions has become an important tool in nanotechnology. Depending upon the particular system, the optical, electrical, chemical, and magnetic properties can be tailored by synthesizing particles of specific size. ZnO is an exceptionally important material for applications in pigments, rubber additives, gas sensors, and optical devices. Controlled synthesis of inorganic nanostructures in terms of size and shape has been strongly motivated by their size and shape dependent properties. The effect of alkaline solution on the morphologies of zinc oxide has been elucidated and various morphologies of high quality hexagonal sheets, nanoballs and nanoflowers are obtained by mere changing the basicity of the reaction solutions and in the same reaction conditions. The synthesized product was characterized by FE-SEM (FE-SEM, Hitachi 4700) TEM (JEOL JEM-2010), XRD (Rigaku, with Cu Kαradiation), Raman spectra, Photoluminescence and Photocurrents. Because of various morphologies have various electronic properties owing to different electron flow rate from conical tips of flower petals and hexagonal nanoballs. The Zinc oxide has been expected to compatible with TiO2 nanomaterials because of its higher electronic mobility, similar electron affinity and energy level of the conduction band. These morphologies of the zinc oxide electrodes are the obvious choice as the dye sensitized solar cells and applied as a photoanode for the DSSCs other than the TiO2 for the improvements of DSSCs efficiencies. Various morphologies of zinc oxide have potential advantage in the sense of charge transport improvement, recombination and minimize the grain boundaries in between the interfaces. For that reason ZnO is an alternative candidate as an effective photoanode electrode for DSSCs and further substituting the nanoparticles network film electrode with different morphologies of zinc oxide nanomaterials such as nanorods, nanoflowers, nanoplates would be advantageous for the solar cell efficiency and cost effective technology. In the present work, we develop cost effective method to avoid copious use of different costly capping agents to synthesize various morphologies of ZnO such as nanosheets, nanoflowers and spheres in controlled alkali solution under hydrothermal conditions. These morphologies provide a good pathway for high photocurrent generation and offer an alternative novel semiconductor network surface for DSSCs which also enhanced the light harvesting and improved photo to electric performance. Dye sensitized solar cell fabricated from these synthesized nanoplates, nanoflowers and nanospheres of the ZnO nanomaterials, and the maximum short circuit current obtained is from the nanospheres of 12.28 mA/cm2 with open circuit voltage of 0.557 volt and the overall conversion efficiency of 2.61%.
9:00 PM - CC10.14
The Effect of Interfacial Layers Between Nanocrystalline TiO2 Layer and TCO in DSC.
Dong Hyun Cha 1 , Yoon Hee Lee 1 , Young Seok Kim 1 , Wan In Lee 1
1 Department of Chemistry, Inha University, Incheon Korea (the Republic of)
Show Abstract9:00 PM - CC10.15
Conjugated Polymer-Based Flexible Photovoltaic Cells with Controlled Nanostructures
Myung-Su Kim 1 , Jin-Sung Kim 4 , Jae Cheol Cho 1 , Max Shtein 1 3 , L. Jay Guo 4 3 , Jinsang Kim 1 2 3
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 4 Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, United States, 3 Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Chemical Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show Abstract9:00 PM - CC10.16
MeV Ion Beam Bombardments Effects on the Thermoelectric Properties of Zn4Sb3 / CeFe(4-x)CoxSb12 nano-layered superlattices
S. Budak 1 , B. Zheng 1 , C. Muntele 1 , R. Zimmerman 1 , D. Ila 1
1 Center of Irradiation for Materials, Alabama A.&M. University, NORMAL, Alabama, United States
Show Abstract9:00 PM - CC10.17
Screening of Tunneled Titanates Photocatalysts for Environmental Remediation and Hydrogen Generation.
Stephen Sanford 1 2 3 , Jake Amoroso 1 2 3 , Doreen Edwards 1 2 3
1 Materials Science Division, Alfred University, Alfred, New York, United States, 2 , New York State College of Ceramics, Alfred, New York, United States, 3 , Kazuo Inamori School of Engineering, Alfred, New York, United States
Show AbstractThe anatase form of titanium oxide has been the most commonly and extensively researched material for photocatalysis over the past few decades. Tunneled titanates have not been fully explored as photocatalysts but show promising results. Inuoe et al. studied the water-splitting properties of the tunneled titanates, BaTi4O9 and M2Ti6O13 (M = Na, K, Rb) loaded with a RuO2 promoter, in the 1990’s. The results of the investigation displayed the near-stoichiometric production rates of hydrogen and oxygen gases required for ongoing photocatalysis. The unique photocatalytic behavior of these materials was attributed to the structural features of the tunnels, but is currently not well understood. There are numerous other tunneled titanates which exhibit different structures and tunnel geometries that have not been explored for photocatalysis. The current investigation is aimed at synthesizing tunneled titanates of differing structures and geometries and screening their photocatalytic potential for use in environmental remediation and hydrogen generation. The various tunneled titanates in this study have been prepared using solid state synthesis and possess tunnel geometries and structures such as hexagonal tunnels in β-gallia rutile intergrowths, 2 x 2 tunnels in hollandites, octagonal-shaped tunnels in alkaligallotitogallates, etc. The photocatalytic behavior of the materials was examined using UV-Vis spectroscopy and gas chromatography. The correlation between structure and photocatalytic behavior of the materials will be discussed.This work was supported by the National Science Foundation (DMR-0093690).
9:00 PM - CC10.18
Fabrication of Dye-Sensitized Solar Cells using TiO2 Nanocomposite Films
Jih-Jen Wu 1 , Gaun-Ren Chen 1 , Chia-Chun Lu 1 , Chen-Hao Ku 1
1 Department of Chemical Engineering, National Cheng Kung University, Tainan Taiwan
Show AbstractTwo types of the TiO2 nanocomposite films on ITO substrates, which are composed of P25-TiO2 nanoparticles (NPs) and anatase-TiO2 nanowires (NWs) with the separated (type I) and the mixed (type II) configurations, have been employed to be the anodes of the N3 dye-sensitized solar cells (DSSCs). Preliminary results show that the efficiencies of the DSSCs are enhanced using type I anodes in comparison with the P25-NP anodes as the total thickness of the film are lower than 10 μm. On the other hand, the type II DSSCs possess superior performances than the NP DSSCs when the anode thickness is larger than 15 μm although the efficiencies of the type II DSSCs with thinner mixed film are low.
9:00 PM - CC10.19
Characterization of Intrinsic a-Si:H Thin Films Prepared by ICP-CVD for HIT Solar Cell Applications.
Chaehwan Jeong 1 , Seongjae Boo 1 , Minsung Jeon 2 , Koichi Kamisako 2
1 Gwanju Research Center, Korea Institute of Industrial Technology, Gwangju Korea (the Republic of), 2 Electrical and Electronic Engineering, Tokyo University of Agriculture and Technology, Tokyo Japan
Show Abstract9:00 PM - CC10.2
Characteristics of Chemically Deposited Thin Film Solar Cells using SnS and Sb2S3 Absorbers.
M. T. Santhamma Nair 1 , David Avellaneda 1 , Sarah Messina 1 , P. Karunakaran Nair 1
1 Centro de Investigacion en Energia, Universidad Nacional Autonoma de Mexico, Temixco, Morelos, Mexico
Show Abstract9:00 PM - CC10.20
Photovoltaic Devices with TiO2 Nanostructure
Qingshuo Wei 1 , Kouske Hirota 1 2 , Keisuke Tajima 1 , Kazuhito Hashimoto 1
1 Applied Chemistry, The University of Tokyo, Tokyo Japan, 2 Electronic and Engineered Materials Laboratory, The Mitsui Chemicals, Inc., Chiba Japan
Show AbstractCompared to conventional silicon solar cells, organic photovoltaic devices (OPVs) are attractive due to their advantages such as low cost of production, flexibility, processability, and availability of large area. 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 microphase separation in the nanoscale enhances the charge separation. However, the uncontrolled structure of bulk heterojunction could prevent the efficient charge transport to the electrodes. A promising structure for photovoltaic device could be an ordered interdigitating nanostructure of electron acceptor and donor. Structural dimension could be optimal around 10∼20 nm since the diffusion length of excitons in semiconducting polymers is around 10 nm.Herein we report the fabrication of a novel inorganic nanostructure and its application to inorganic/polymer photovoltaic devices. We have chosen titanium dioxide (TiO2) because of its good electron-conducting and accepting properties. By sol-gel synthesis in a reversed micelle system, we successfully obtained a TiO2 nanostructure on flat sol-gel TiO2 substrate. FE-SEM images show the array of projections with the vertical height around 50 nm uniformly covered the flat so-gel TiO2. TEM cross-section images with higher magnification show that the individual structure consists of TiO2 nanorods with the diameter of several nanometers, similar to the ones formed in the solution phase. We investigated the current-voltage properties of the photovoltaic devices with this TiO2 nanostructure under monochromatic and simulated solar light (AM 1.5). The EQEs of the device with the nanostructure reached 17%, which is more than double of the one with the flat TiO2 (6%). We consider that the improvement can be attributed to larger interface area of polymer and TiO2 and an efficient electron transport in the oriented TiO2 nanostructure.
9:00 PM - CC10.21
Gallium Doped Zinc Oxide Films as Transparent Electrodes for Organic Solar Cell Applications.
Vikram Bhosle 1 , John Prater 2 , Diana Pendergrast 3 , Steve Forrest 3 , Jagdish Narayan 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Materials Science Division, Army Research Office, Research Triangle Park , North Carolina, United States, 3 Department of Electrical Engineering, Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University, Princeton, New Jersey, United States
Show Abstract9:00 PM - CC10.24
Syntheses of Ordered Titania Nanorod and P3HT Film as Solar Cell Material.
Sze-Ming Yang 1
1 Chemical and Materials Eng, National Central Univ., Jung-Li , Taoyuan, Taiwan
Show AbstractTitania nanorods are prepared by using template method involving the synthesis of titania nanorod within the holes of a nanoholes membrane. Homogeneous films of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) with cyclindrical domain are prepared on top of ITO glass substrates under suitable atmosphere. The nanoholes polymer templates were prepared by selective removal of the poly(ethylene oxide) domain by hydrolyses, the EO group can be hydrolytically degraded because of the unstable character of the ester group. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), fourier transformed infrared spectroscopy (FTIR), UV-vis and PL were used to characterize these films. The PS-b-PEO film formation is studied by atomic force microscopy in tapping mode. TEM was used to confirm the formation of nanoholes, the diameter of the nanohole is 28 nm in hydrolyzed membrane. SEM photographs show the titania nanorods with diameter around 30 nm are formed. The PL measurements also show that most of the excitons in the polymer are quenched, which suggests that electron transfer from the P3HT to the titania.
9:00 PM - CC10.25
Diamond Nanowires and Their Relevance to the Performance of Nanocarbons as Thermoelectrics for Solar Power Conversion.
Paola Bruno 1 , Dieter Gruen 1
1 MSD, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractThe synthesis and characterization of ultrananocrystalline diamond (UNCD) films from argon microwave plasmas containing 1% by volume of carbonaceous molecules such as C60 or CH4 has been extensively described in the literature. Such films are composed of 3-5nm randomly oriented crystallites strongly bonded by carbon atoms across atomically abrupt disordered grain boundaries. When molecular nitrogen (N2) in the range 0-20% by volume is added to the synthesis gas, the initially insulating films show electrical conductivities up to several hundred S/cm. At N2 contents up to about 5%, the conductivities are strongly temperature dependent and can be described in terms of a variable range hopping model. Extensive tight binding-density functional (TBDF) calculations involving high energy, high angle twist grain boundaries reveal that the largely sp2 bonded grain boundary carbon atoms introduce a plethora of electronic states into the 5.5ev band gap of diamond making it likely that the grain boundary electronic structure of UNCD films is intimately connected with their conductivity behavior.When the N2 content reaches about 7% by volume, there is a jump in conductivity of 3-4 orders of magnitude. We have recently studied this phenomenon in detail and find that substrate temperatures in a narrow range, 800-900 Celsius, are required to reach conductivities in the several hundred S/cm regime. The activation energy for conductivity declines precipitously from several tenths of an electron volt to an almost vanishingly small value reminiscent of some metals thus leading to the speculation that one is dealing with an insulator- metal transition. Interestingly, Hall measurements show the conductivity due to the addition of N2 to be n-type while the addition of boron produces p-type UNCD. The films possess very low thermal conductivity which together with their electrical properties suggests their potential use as thermoelectrics. It occurred to us that the sudden increase in conductivity is due to a structural transformation resulting in improved grain boundary connectivity. Films deposited under a wide variety of conditions varying both the N2 content and deposition temperatures were therefore studied using scanning electron microscopy (SEM). A striking correlation was observed between high conductivity behavior and the formation of diamond nanowires with diameters of 5-10nm and lengths of several hundred nanometers. The nanowires are aligned in regions of several square microns but randomly oriented over larger areas.These results as well as recent data on small angle neutron scattering (SANS) and high resolution transmission electron microscopy (HRTEM) will be presented from the viewpoint of their relevance to the use of UNCD as a high efficiency thermoelectric material for solar power conversion. (1) (1) Manuscript submittedThis work is supported by the U.S. Department of Energy, BES-Materials Science, under Contract W-31-109-ENG-38
9:00 PM - CC10.26
Co-Synthesis of H2 and ZnO by in-situ Zn Aerosol Formation and Hydrolysis.
Frank Ernst 1 , Antonio Tricoli 1 , Aldo Steinfeld 2 , Sotiris Pratsinis 1
1 Particle Technology Laboratory, ETH Zurich, Zurich Switzerland, 2 Institut for Energy Technology, ETH Zurich, Zurich Switzerland
Show AbstractFormation of H2 by thermal dissociation of water on metals is the exothermic second step of the so-called two-step water-splitting cycles for storage and use of solar energy, the first being the endothermic reduction of metal oxides to metals using concentrated solar energy.1 In particular, the hydrolysis of Zn to ZnO with release of H2 is most attractive for its potential in achieving high energy conversion efficiencies.2Simultaneous synthesis of H2 and ZnO nanoparticles is investigated by steam-hydrolysis of Zn vapor in a hot-wall aerosol flow reactor with separated evaporation, cooling and reaction zones. Superheated Zn vapor was carried by Ar into a tubular, quartz reactor where it was mixed and quenched by a superheated, equimolar H2O/Ar stream resulting in Zn/ZnO nanoparticles and H2. The Zn(g) vapor was generated by electric heating of a Zn crucible whose weight was continuously monitored and compared to H2 production rate. The reactor was operated at 1 bar and 573 - 1273 K: above and below the Zn saturation vapor pressure suppressing and allowing, respectively, Zn aerosol formation by condensation to take place in parallel with the omnipresent ZnO formation by gas-phase or surface hydrolysis of Zn. The process yielded up to 90% H2 conversion and nanoparticles with Zn and ZnO mean crystallite sizes of 100 and 40 nm, respectively, containing up to 80 wt% ZnO.1. Steinfeld A, Kuhn P, Reller A, Palumbo R, Murray J, Tamaura Y. Solar-processed metals as clean energy carriers and water splitters. International Journal of Hydrogen Energy. 1998; 23: 767-774.2. Steinfeld A. Solar Thermochemical Production of Hydrogen - A Review. Solar Energy. 2005; 78: 603-315.
9:00 PM - CC10.27
Electrical Response of Wet Chemically Grown ZnO Nanorods for Photovoltaic Application.
Julian Tornow 1 , Klaus Schwarzburg 1
1 SE4, Hahn-Meitner-Institut Berlin GmbH, Berlin Germany
Show AbstractUsing arrays of ZnO nanorods instead of a nanocolloide TiO2 network is expected to be advantageous for dye sensitized and extremely thin absorber solar cells. This is first of all due to a well ordered structuring, which results in a better contact of the absorber and the electrolyte even for solid state electrolytes or hole conductors. Secondly, a larger diffusion length is expected in the ZnO nanorods than resulting from the trap limited diffusion in a TiO2 network. This might allow for new electrolytes or hole conductors, even if they do not show extremely slow recombination kinetics as for example present in the I/I- electrolyte.Within this work, complete solar cells are constructed of wet chemically grown ZnO nanorods. The diffusion coefficient of our ZnO nanorods is in fact some orders of magnitude higher than known from a TiO2 nanocolloide network. Nevertheless this cannot not simply be extrapolated into a larger diffusion length, because the response of the ZnO nanorod solar cells is not dominated by a diffusional process but by the RC behavior of the cell. The RC time constant in the cell is in the ms-range due to a large cell capacity arising from an internal space charge layer building up in the ZnO nanorods. This is quite contrary to TiO2 nanocolloide structures where no internal space charge capacity develops. The space charge capacity is assumed to result from the nanorod arrays morphology as well as from the material properties of ZnO and is investigated by transient photocurrent measurements and impedance spectroscopy.
9:00 PM - CC10.28
Microcrystalline SiO and its Application to Solar Cell.
Porponth Sichanugrist 1 , Nirut Pingate 1 , Decha Yotsaksri 1 , Channarong Piromgit 1
1 ISET, NSTDA, Pathumthani Thailand
Show AbstractMicrocrystalline silicon oxide (μc-SiO) was reported to be more promising material than microcrystalline silicon carbide (μc-SiC) for the application to solar cells by author since its microcrystallization could be occurred more easily than μc-SiC. At that time, it was deposited by 13.56 MHz and was applied to the p-layer of nip-type amorphous silicon (a-Si) solar cell fabricated on metal substrate. There is no work done after that on pin-type glass substrate. On the other hand, μc-SiC deposited by ECR has been developed and applied to the device fabricated on glass. Higher efficiency was achieved, but its scale up to the manufacturing was somehow difficult. Recently, Mitsubishi Heavy Industries has developed μc-SiC using VHF (Very High Frequency) glow discharge and has applied to the microcrystalline bottom cell, but there is no work done with the top a-Si cell. In our present work, we have developed μc-SiO p-layer, buffer interface layer μc-SiO n-layer using VHF for the first time and have applied them to both top and bottom cells with a-Si and μc-Si i-layer in a-Si/μc-Si tandem type solar cell, respectively. A cluster-type, multi-chamber system in which various films (Ag, ZnO, a-Si, μc-Si and μc-SiO films) can be deposited on 30 cm x 40 cm area without breaking the pressure has been used. High VHF frequency of 60 MHz and carbon dioxide gas are used for μc-SiO deposition while TMB and phosphine is used here as the doping gas. Thin ZnO layer is coated by DC sputtering on tin oxide coated glass substrate in order to promote the crystallization of the μc-SiO p-layer. Furthermore, glass substrate coated with LPCVD ZnO has also been used. The thick of the top cell is fixed to be around 150-200 nm while the thickness is around 2 μm for the bottom cell. As the results it was found that the top and bottom cell with this novel μc-SiO p-layer, buffer layer and μc-SiO n-layer has higher cell efficiency than the one with conventional one. Cell efficiency of more than 14% has been achieved so far. Furthermore, it was found that this μc-SiO also worked on ZnO coated by LPCVD. The overall results shows that μc-SiO fabricated by VHF plasma is the promising material for the application to a-Si/μc type solar cell fabricated on the glass substrate.
9:00 PM - CC10.29
Hybrid Dye-sensitized Solar Cells Based on Ultrathin TiO2 Films with Different Morphologies.
Yajun Cheng 1 , Jochen Gutmann 1 2
1 , Max-Planck Insitute for Polymer Research, Mainz Germany, 2 Inst. of. Physical Chemistry, University of Mainz, Mainz Germany
Show AbstractUltrathin TiO2 films showing rich morphologies are achieved by using sol-gel chemistry coupled with an amphilic polystyrene-block-poly (ethylene oxide) (PS-b-PEO) diblock copolymer as a structure-directing agent. The block copolymer undergoes a good-poor-solvent pair induced phase separation in a mixed solution of 1,4-dioxane, concentrated hydrochloric acid (HCl) and Titanium tetraisopropoxide (TTIP). By adjusting the weight fractions of 1,4-dioxane, HCl, and TTIP, inorganic-block-copolymer composite films containing a variety of different morphologies are obtained. Through calcination the amorphous Titania composite films can be converted to crystalline anatase phase. On the basis of the morphology control, TiO2 films of different morphologies are incorporated into hybride dye-sensitized soalr cells as the electron transport layers. By changing the morphology the performance of the solar cells can be tuned and the performance-morphology relationship of TiO2 films is studied and discussed.
9:00 PM - CC10.3
Transparent MWCNT Decorated with Debundled SWCNT as Electrodes in Graetzel Cells.
Hasan Shodiev 1 , Silva Giordani 2 , Sergey Lee 1 , Ali Aliev 1 , Mei Zhang 1 , Shaoli Fang 1 , Ray Baughman 1 , Anvar Zakhidov 1
1 Nanotech, UTD, Richardson, Texas, United States, 2 School of Physics, University of Dublin, Dublin Ireland
Show Abstract9:00 PM - CC10.30
ZnO Nanowire-nanoparticle Composite Dye-sensitized Solar Cell.
Chen-Hao Ku 1 , Jr-Yuan Lai 1 , Guan-Ren Chen 1 , Jih-Jen Wu 1
1 Department of Chemical Engineering, National Cheng Kung University, Taiwan, Tainan Taiwan
Show AbstractThe single crystalline feature of the nanowire dye-sensitized solar cells (DSSCs) provides a direct electrical pathway for charge transportation. However, the efficiencies of the nanowire DSSCs are not superior to the conventional DSSCs due to that their surface area for dye loading is relatively lower than that of the nanoparticle DSSCs. In the present work, the composite films composed of the ZnO nanowires and nanoparticles formed on FTO substrates are employed to be the anodes of DSSC for improving both surface area of dye-loading and charge transportation. A full sun (AM 1.5, 100 mW/cm2) efficiency of 1.13% using a 5μm-thick mercurochrome-sensitized anode has been achieved so far. The influences of the content of the nanoparticles filling among the ZnO nanowires and the annealing conditions for the composite films on the performance of the nanocomposite DSSCs will be further discussed in this presentation.
9:00 PM - CC10.31
High Deposition Rate of Microcrystalline Silicon for n-i-p Solar Cell by Using VHF PECVD Method.
Jun-Chin Liu 1 , Chih Jeng Huang 1 , TeChi Wong 1 , Jian-Shu Wu 1 , Yih-Rong Luo 1 , Chi-Lin Chen 1
1 Photovoltaics Technology Center, Industrial Technology Research Institute, Hsinchu Taiwan
Show Abstract9:00 PM - CC10.32
New Materials in Luminescent Solar Concentrators for Solar Energy Harvesting.
Veronica Sholin 1 , Jeremy Olson 1 , Sue Carter 1
1 Physics, University of California at Santa Cruz, Santa Cruz, California, United States
Show AbstractRecent developments in visible and infrared-emitting semiconducting polymers and quantum dots have opened up new opportunities for higher efficiency luminescent solar concentrators (LSCs). LSCs are waveguides consisting of a slab of highly transparent material doped with a fluorescent dye. Photons incident on the slab are down-converted by the fluorescent molecules and waveguided toward the edges of the structure and then photovoltaically converted by solar cells coupled to the edges of the LSC. The efficiency of LSCs consisting of liquid solutions of semiconducting polymers encased in glass was measured and compared to the efficiency of LSCs based on small molecule dyes and on a commercial pigment. The highest integrated optical efficiency obtained through Monte-Carlo simulations for the materials tested is 11%, corresponding to an LSC of a liquid solution of polyfluorene semiconducting polymer. These results are in good agreement with our experimental values and they will be compared to the results obtained for infrared quantum dot LSCs. The results of the application of optical efficiency enhancement strategies, such as choosing materials of small absorption and emission band overlap or adding a scattering surface beneath the structure in order to increase photon collection will also be analyzed.
9:00 PM - CC10.33
Transport Properties and Microstructures of Oxide Thermoelectric Materials.
Qing Jie 1 , Qiang Li 1
1 , Brookhaven National Lab, Upton, New York, United States
Show Abstract9:00 PM - CC10.4
Large Area Copper Sulfide Coatings on Plastic Sheets Produced by Chemical Deposition for Energy Conservation through Laminated Solar Control Window Installations.
P. Karunakaran Nair 1 , Oscar Gomezdaza 1 , Jorge Aguilar 1 , Corina Hernandez 1 , M. T. Santhamma Nair 1 , Kenneth Liechti 2
1 Centro de Investigacion en Energia, Universidad Nacional Autonoma de Mexico, Temixco, Morelos, Mexico, 2 Aerospace Engineering and Engineering Mechanics, University of Texas, Austin, Texas, United States
Show Abstract9:00 PM - CC10.5
TEM Characterization of Nanostructured Titanium Dioxide for Use in Dye Sensitized Solar Cells.
Judith Sorge 1 , Lai Qi 1 , Dunbar Birnie 1
1 Materials Engineering, Rutgers University, Piscataway, New Jersey, United States
Show AbstractDye sensitized solar cells utilize a dye that bonds directly to a wide bandgap semiconductor, which is usually titanium dioxide. Recent research by several different groups demonstrates that dye sensitized solar cells (DSSCs) can be produced with titania nanotubes and nanowires instead of spherical particles. This project expands the research in this area by thoroughly characterizing different titania structures using transmission electron microscopy (TEM), and then testing these different structures in the DSSCs.
The variations in crystallography and morphology due to production methods are observed in the TEM while the composition is confirmed by x-ray diffraction. The titania nanowires and nanotubes are produced by hydrothermal growth and surfactant mediated assembly. Templated titanium dioxide nanostructures in a honeycomb array are also characterized and used as a comparison study. Structural information about particle free-surface crystal planes and their interconnectivity can help explain differences in cell efficiency. We will attempt to connect the structural data with data about the dye bonding properties and electron transfer properties of the cell.
9:00 PM - CC10.6
p-type AgSbSe2 Thin Films using a Non-vacuum Process for Photovoltaic Applications.
Bindu Krishnan 1 , Ana Maria Arato 1 , T.K. Das Roy 1 , G.Alan Castillo 1
1 Facultad de Ingenieria Mecanica y Electrica, Universidad Autonoma de Nuevo Leon, San Nicolas de los Garza, Nuevo Leon, Mexico
Show Abstract9:00 PM - CC10.7
Vertically Aligned TiO2 Nanotubes for Photovoltaic Cells.
Seok-In Na 1 , Seok-Soon Kim 1 , Jang Jo 1 , Dong-Yu Kim 1
1 Heeger Center for Advanced Materials & Photonics Polymer Laboratory, Dept. of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of)
Show AbstractVarious fabrication methods for well-ordered nanostructures, specially vertically-aligned to the substrate, have been extensively studied for the optoelectronic applications. These include self-assembly, e-beam lithography, nanosphere lithography, room temperature imprint lithography, electrospinning and other numerous novel approaches. Although various lithographic approaches enable precise control of the spacing and position of nanofeatures, these techniques are typically limited due to high cost and low throughput. Therefore, new approaches that can be applied to the fabrication of controllable nanostructures with more practical and higher throughput approaches are needed. In this work, we report on the new fabrication of vertically well-aligned nanotubes of TiO2, which is a promising material for photonic and photoelectrochemical applications due to a high refractive index, chemical stability, and environmental compatibility. TiO2 nanotubes were fabricated by a simple, controllable, economical and reproducible method, using a spin-on based sol-gel reaction of a Ti-precusor with well-ordered ZnO nanorods as template. Further applications of these TiO2 nanotubes for the fabrication of solar cells will be demonstrated.
9:00 PM - CC10.8
Shape Control of TiO2 Nanocrystals and Their Applications to Dye-Sensitized Solar Cells
Sung Bum Choi 1 , Jin Young Kim 1 , Tae Hoon Noh 1 , Sangwook Lee 1 , Hyun Suk Jung 2 , Kug Sun Hong 1
1 School of 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
Show AbstractNanocrystalline TiO2 dye-sensitized solar cells (DSSCs) have been intensively investigated as a device for inexpensive, large-scale solar energy conversion since their first demonstration by Grätzel in 1990. In particular, DSSCs composed of TiO2 nanocrystals with 1-D structures such as nanorods, nanowires, and nanotubes have been attracted much interest due to their peculiar properties compared to nanospheres. In spite that there have been various studies on the TiO2 nanocrystals with 1-D structures, most of them focused only on the overall solar efficiency and did not explain their mechanisms. In this study, the effects of the shape of TiO2 nanocrystals on the solar conversion efficiency were systematically investigated from the viewpoints of carrier lifetime and electron conduction. TiO2 nanocrystals were synthesized via a two-phase thermal process and the aspect ratios of nanocrystals were controlled with the relative ratio of ethanolamine (EA) to oleic acid (OLA). This shape evolution was ascribed to the selective adsorption of surfactants and the resulting selective stabilization of the crystallographic planes. In order to compare the photovoltaic properties, TiO2 films composed of the nanospheres and nanorods were prepared as the electrodes of DSSCs. DSSCs composed of TiO2 nanorods exhibited reduced charge recombination and longer electron life time, which was confirmed by the Voc transient measurements. On the other hand, impedance analyses revealed that the electron conduction between the nanorods were poorer than the nanospheres. Owing to the compromise between the enhanced carrier life time and the reduced interparticular conduction, the overall photovoltaic conversion efficiency of the nanorod-based DSSCs was similar to the nanosphere-based DSSCs.