Symposium Organizers
Arthur J. Frank National Renewable Energy Laboratory
Nam-Gyu Park Sungkyunkwan University
Tsutomu Miyasaka Toin University of Yokohama
(and Peccell Technologies, Inc.)
Laurie Peter University of Bath
Songyuan Dai Chinese Academy of Sciences
R1: Novel Inorganic, Organic and Inorganic-organic Hybrid Solar Cells
Session Chairs
Arthur Frank
Prashant Kamat
Monday PM, November 30, 2009
Room 312 (Hynes)
9:30 AM - **R1.1
Low Cost ``Plastic” Solar Cells: Self-Assembly of Bulk Heterojunction Nano-Materials by Spontaneous Phase Separation.
Alan Heeger 1
1 , UC Santa Barbara, Santa Barbara, California, United States
Show AbstractSolar cells --- Power from the Sun --- can provide and must provide --- a significant contribution to our future energy needs. The challenge is clear; we must create the scientific foundation and the technology to enable fabrication of high efficiency solar cells at low cost.I will describe the discovery of ultrafast photoinduced electron transfer as the scientific foundation for the creation of a technology for low cost “plastic” solar cells. This initial charge separation occurs at a time scale two orders of magnitude faster than the first step in photo-synthesis in green plants. Charge collection at the electrodes is accomplished through self-assembly of bulk heterojunction (BHJ) nano-materials by spontaneous phase separation. Recent results include the achievement of 6% power conversion efficiency and the demonstration of quantum efficiencies approaching 100%: Each photon absorbed leads to a (positive and negative) pair of mobile charge carriers, and all the photo-generated charge carriers are collected at the electrodes. Higher efficiencies will come from improved harvesting of the photons from the solar spectrum using new semiconducting polymers designed and synthesized for use in “plastic” solar cells. We see a clear technology pathway to high efficiency “plastic” solar cells with lifetimes sufficient for a wide range of applications including portable electronics, semi-transparent solar cells for windows in homes and buildings, and rooftop installation. I will discuss lifetime issues and progress toward manufacturing plastic solar cells by printing/coating technology. I will demonstrate that the dream of low cost plastic solar cells is becoming reality.
10:00 AM - **R1.2
Silicon Nanowire Photovoltaics.
Peidong Yang 1 , Erik Garnett 2 , Sarah Brittman 1
1 , Univ. Calif. Berkeley, Berkeley, California, United States, 2 , Stanford University, Stanford, California, United States
Show AbstractTraditional planar silicon p-n junction photovoltaics are already capable of producing power at an efficiency of 20% in large-scale production. However, because of silicon’s low absorption coefficient in the red and IR spectral regions, standard solar cells must be around 100 microns thick to maximize light absorption, which requires long minority carrier diffusion lengths and expensivehigh-purity silicon wafers. Crystalline silicon offers many advantages including long-term stability, established fabrication procedures and a simple design, so reducing the required silicon material quality and quantity while maintaining high efficiency is one promising route to rapid, large-scale implementation of solar energy. Silicon nanowire photovoltaics could provide a solution to reach both of these goals simultaneously. Nanowires with a radial p-n junction relax the material quality requirement by orthogonalizing the light absorption and charge separation directions, while the geometry allows for improved light scattering and trapping. This light trapping effect can be amplified by coupling the nanowires with metal nanoparticles that have a strong plasmon resonance in the red and IR spectral regions. This talk will discuss our recent progress in fabricating silicon nanowire photovoltaics – both arrays and single nanowire devices – and investigating their output and light trapping characteristics.Arrays are fabricated with simple, rapid processes to assess the feasibility of implementing nanowires in large-scale production, while single nanowire devices remove ensemble effects and allow for more fundamental studies. The results demonstrate that the nanowire geometry provides distinct benefits over planar cells for thin silicon absorber layers.
10:30 AM - **R1.3
Charge Separation and Extraction in Polymer Blend Solar Cells.
Christopher Groves 1 , James Blakesely 1 , L. Koster 1 , Neil Greenham 1
1 Department of Physics, University of Cambridge, Cambridge United Kingdom
Show AbstractExciton dissociation in bulk heterojunction solar cells forms bound charge pairs, and separation of these charge pairs before they can recombine is vital to achieve efficient device operation. I will describe simulations of photovoltaic device operation using Monte Carlo techniques, combined with drift-diffusion and analytical approaches to deal with bulk transport and charge extraction. Particular questions to be addressed include (i) the role of microstructure in constraining charge-pair separation; (ii) the importance of correlated disorder in providing pathways for efficient charge separation; (iii) the interaction between energetic disorder and structural disorder; and (iv) the origin of band-bending at polymer/metal contacts and its effect on open-circuit voltages. Simulations will be compared with results of polymer blend device measurements over a wide temperature range.
11:30 AM - R1.4
Synthetic Approaches Toward the Development of Highly Efficient Molecular Photovoltaics.
Hiroshi Imahori 1
1 iCeMS, Kyoto University, Kyoto Japan
Show AbstractSolar energy conversion is a long-term research interest, and various kinds of photovoltaic and/or photoelectrochemical devices have been explored. In this talk we will present a variety of synthetic approaches toward efficient solar energy conversion. First, we have constructed various porphyrin-fullerene multilayer structures on nanostructured semiconducting electrode (i.e., SnO2, TiO2, ZnO). In particular, we have developed a novel strategy for constructing the vertical arrangement of bicontinuous donor-acceptor (D-A) arrays on a flat SnO2 electrode. The relationship between the film structure and photoelectrochemical properties has been elucidated as a function of the number of donor layer for the first time. These results will provide fundamental clue for the molecular design of high-performance organic photovoltaics. Second, conjugated polymers with alternating main chain structures of zinc porphyrin-furan (PZnPF) and zinc porphyrin-thiophene (PZnPT) have been synthesized by palladium(0)-catalyzed Stille coupling reaction for the first time. The optical, electrochemical, photophysical, and photovoltaic properties of PZnPF and PZnPT were investigated to elucidate the effects of the heterole bridges (i.e., furan vs. thiophene) in the porphyrin polymers. The bulk heterojunction solar cells based on the porphyrin-furan alternating copolymer and PCBM exhibited the power conversion efficiency of 0.048%, which is two times as large as that of the cell using the porphyrin-thiophene alternating copolymer and PCBM. The difference in the power conversion efficiencies may be caused by the plausible higher hole-transporting ability of the porphyrin-furan alternating copolymers and the thinner thickness of the active layer consisting of the porphyrin-furan alternating copolymers. Finally, we have synthesized novel asymmetrically pi-elongated porphyrins for dye-sensitized TiO2 cells. The TiO2 cell exhibited a power conversion efficiency up to 6.3%. These results manifest that our strategy is a useful tactic for highly efficient dye-sensitized solar cells.[1] H. Imahori, T. Umeyama, J. Phys. Chem. C (Feature Article), 2009, 113, 9029. [2] H. Hayashi, A. Kira, T. Umeyama, Y. Matano, P. Charoensirithavorn, T. Sagawa, S. Yoshikawa, N. V. Tkachenko, H. Lemmetyinen, H. Imahori, J. Phys. Chem. C, in press. [3] A. Kira, T. Umeyama, Y. Matano, K. Yoshida, S. Isoda, J.-K. Park, D. Kim, H. Imahori, J. Am. Chem. Soc. 2009, 131, 3198. [4] T. Umeyama, T. Takamatsu, N. Tezuka, Y. Matano, Y. Araki, T. Wada, O. Yoshikawa, T. Sagawa, S. Yoshikawa H. Imahori, J. Phys. Chem. C, in press. [5] H. Imahori, T. Umeyama, S. Ito, Acc. Chem. Res. 2009, 42, in press.
11:45 AM - R1.5
Nanowire Structured Hybrid Cell for Concurrently Scavenging Solar and Mechanical Energies.
Chen Xu 1 , Xudong Wang 1 , Zhong lin Wang 1
1 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractOur living environment has an abundance of energies in the forms of light, thermal, mechanical (such as vibration, sonic wave, wind, and hydraulic), magnetic, chemical, and biological. Harvesting these types of energies is of critical importance for long-term energy needs and sustainable development of the world. Over the years, rationally designed materials and technologies have been developed for converting solar and mechanical energies into electricity. Photovoltaic relies on approaches such as inorganic pn junctions, organic thin films, and organic-inorganic heterojunctions. Mechanical energy generators have been designed on the basis of principles of electromagnetic induction and piezoelectric effect. These existing approaches are developed as independent technologies and entities that are designed on the basis of drastically different physical principles and diverse engineering approaches to uniquely harvest a particularly type of energy. Conversion cells for harvesting solar energy and mechanical energy are usually separate and independent entities that are designed and built following different physical principles. A solar cell works only under sufficient light illumination; a mechanical energy generator is applicable if there is significant mechanical movement/vibration. Innovative approaches have to be developed for conjunctional harvesting of multiple types of energies using an integrated structure/material so that the energy resources can be effectively and complementary utilized whenever and wherever one or all of them are available. We report a hybrid cell that is designed for simultaneously harvesting solar and mechanical energies using nanotechnology. Our approach relies on aligned ZnO nanowire arrays grown on surfaces of a flat substrate, a dye-sensitized solar cell is built on its top surface to convert solar energy, and a piezoelectric nanogenerator is built on its bottom surface for harvesting ultrasonic wave energy from the surroundings. The two energy harvesting approaches can work simultaneously or individually, and they can be integrated in parallel and serial for raising the output current and voltage, respectively, as well as power. It is found that the voltage output from the solar cell can be used to raise the output voltage of the nanogenerator, providing an effective approach for effectively storing and utilizing the power generated by the nanogenerator. This prototype demonstrates a new approach for concurrently harvesting multiple types of energies using an integrated hybrid cell so that the energy resources can be effectively and complementary utilized whenever and wherever one or all of them is available.[1] X.D. Wang, J.H. Song J. Liu, and Z.L. Wang, Science, 316 (2007) 102.[2] C. Xu, X.D. Wang and Z.L. Wang, J. Am. Chem. Soc., 131 (2009) 5866.[3] Research supported by DARPA, DOE and NSE.[4] For more information: http://www.nanoscience.gatech.edu/zlwang/
12:00 PM - R1.6
New Planar and Textured Heterojunction Organic Photovoltaics Based on Titanyl Phthalocyanine (TiOPc) and Chloroindium Phthalocyanine (ClInPc) Donor Layers: Enhanced Near-IR Response, Large Open-circuit Photopotentials and Improved Device Efficiencies.
Weining Wang 1 , Diogenes Placencia 1 , Neal Armstrong 1
1 Department of Chemistry, University of Arizona, Tucson, Arizona, United States
Show AbstractOne of the key challenges in the optimizing of organic solar cell (OPV) technologies is to create donor/acceptor (D/A) heterojunctions with excellent near-IR photoactivity, without sacrifice of open-circuit photopotential (VOC). We have recently demonstrated that solvent-annealed titanyl phthalocyanine (TiOPc) donor layers, with C60 as the acceptor layer, greatly improve the near-IR response (versus previous Pc/C60 heterojunctions), can be nanotextured to enhance the short-circuit photocurrent (JSC), and provide VOC near 0.6 volts. This talk will focus on our more recent studies where we change the acceptor layer in TiOPc OPVs to C70, which further enhances OPV efficiency, and move to chloroindium phthalocyanine as the donor layer, which for ClInPc/C60 heterojunctions provides VOC near 0.8 volts, with no loss of JSC. In our hands the best AM1.5 device efficiencies have risen to ca. 4%, versus CuPc/C60 efficiencies of ca. 2% and pentacene/C60 efficiencies of ca. 2%. We focus here on the characterization of the frontier orbital energies for these heterojunctions using photoemission spectroscopies, and the origin of the large increases in VOC in the context of much lowered reverse saturation currents (J0) for both TiOPc and ClInPc OPVs, versus OPVs with donor layers with lower ionization potentials (CuPc and pentacene).
12:15 PM - R1.7
Photovoltaics Made Out of Liquid Crystals and ZnO Nanoparticles and Nanowire Arrays.
Kaitlin Traister 2 , Iriselies Melendez 2 , Luz Martinez-Miranda 1 , Lourdes Salamanca-Riba 1
2 REU participant, Materials Research Science and Engineering Center, University of Maryland, College Park, Maryland, United States, 1 Materials Science and Engineering, University of Maryland, College Park, Maryland, United States
Show AbstractSmall molecules used in organic photovoltaics (PV) have the advantage of easy and cheap processing and flexibility. Also, they have higher carrier mobilities and are monodisperse as opposed to polymers [1, 2]. Liquid crystals in addition exhibit molecular orientation in some of their phases that can provide a path for the electrons or holes to move from one electrode to the other. There have been several studies of mixtures of discotic liquid crystals as the donor and a soluble perylene dimide derivative as the acceptor material in photovoltaic systems.[3] We have mixed a smectic A liquid crystal (8CB) with varying concentrations of ZnO nanoparticles of ~3 nm in diameter and have observed a photovoltaic effect as a function of concentration of ZnO. The liquid crystal is made of small molecules of about 3 nm in size and is believed to enhance the alignment of the nanoparticles and aid in the diffusion of electrons through the particles to the collection electrode. We have also made PV cells of ZnO nanowire arrays grown on Au layers. The ZnO nanowires act as the absorbers in the PV cells and as the conducting path for the electrons. The nanowire arrays are covered with 8CB liquid crystal for hole conduction. We compare the absorption of the PV cells as a function of wavelength of the light for the ZnO nanoparticle and the ZnO nanowire cells. 1. M. T Lloyd, J. E. Anthony, G. G. Malliars, Photovoltaics from soluble small molecules, Materials, 10, 34-41 (2007). 2. N. C. Greenham, Xiaogang Peng, and A. P. Alivisatos, Charge separation and transport in conjugated-polymer/semiconductor-nanocrystal composites studied by photoluminescence quenching and photoconductivity, Phys. Rev. B, 54, 17 628-37 (1996)). 3. L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, E. Moons, R. H. Friend J. D. Mackenzie, Self-Organized Discotic Liquid Crystals for High-Effieciency Organic Photovoltaics, Science, 293, 1119-1122 (2001).*This research is supported by the National Science Foundation under the University of Maryland MRSEC DMR 0520471.
12:30 PM - R1.8
Electrochemical Self-Assembly of Inorganic/Organic Nano-Hetero Interfaces for Photovoltaic Systems.
Tsukasa Yoshida 1 , Lina Sun 1 , XiaoFeng Wang 1 , Kazumasa Funabiki 1 , Takashi Sugiura 1 , Servane Haller 2 , Daniel Lincot 2
1 Graduate School of Engineering, Gifu University, Gifu, Gifu, Japan, 2 , IRDEP, Paris France
Show AbstractElectrodeposition of compound semiconductor thin films from chemical solutions is an attractive synthetic method to obtain electrode materials for nanostructured solar cells, in contrast with the expensive and energy demanding process to achieve ultrapure crystalline silicon that is used in today’s photovoltaic panels.We have introduced a new concept to electrochemically self-assemble (ESA) nanostructured hybrid materials of inorganic semiconductors and organic dye molecules to process photoelectodes for dye-sensitized solar cells (DSSCs) without high temperature heating, thus being perfectly compatible with plastic electrodes. Conversion efficiency close to 8% could be achieved by such materials and further studies for the improvement of the efficiency and stability are on their way with the industrial partners for commercialization of such devices.The reference system for ESA of hybrid thin film is that employing cathodic electrodeposition of ZnO from aqueous solutions of Zn salts. It is one of the rare examples in which the film growth operates in a perfectly atom-by-atom fashion, thus achieving heteroepitaxial growth of single crystalline ZnO thin films when suitable electrode such as GaN single crystal is used. Simply by adding surface adsorbates such as eosinY, one of the family of xanthene dyes, one can obtain nanostructured ZnO/eosinY hybrid thin film having 1D porous crystalline structure consisting of interconnected ZnO nanowires to retain the structure of crystalline ZnO. Complex chemistry between eosinY and ZnO is involved in the process of ESA of such materials.Replacing eosinY with zwitter ionic RhodamineB completely changes the picture, achieving a 2D nanostructured ZnO/RhodamineB hybrid thin films consisting of folded nano-curtains of ZnO. Because pores with average gap of some tens of nm are oriented vertically to the substrate, such a 2D nanostructure is expected to be perfectly matched with solid absorber/hole conductors such as P3HT, while the 1D nanostructure obtained with eosinY is ideal for DSSCs.As the counter part of the n-ZnO hybrid system, p-CuSCN/dye hybrids can also be obtained by ESA when cationic dyes containing amino groups such as Oxazine1 is used. Oxazine1, however, does not hybridize with ZnO as it lacks carboxylic acid group which usually is a good anchor to attach the molecule to ZnO. When both eosinY and Oxazine1 are added to the deposition bath, these two molecules self-assemble and loaded together both to ZnO and CuSCN. Amino acids have both carboxylic acid group and amino group. When Gluthathione, a tripeptide molecule is added, formation of nanostructured hybrid ZnO and CuSCN was confirmed. Such selective and cooperative ESA of hybrid nanosctructure is therefore the key process to achieve photoactive nano-hetero interfaces (NHIs) in a configuration such as n-ZnO/dye/p-CuSCN to achieve efficient solid-state DSSCs.Yoshida et al., Adv. Func. Mater., 19 (2009) 17.
12:45 PM - R1.9
Electronic and Optical Properties of Organic Photovoltaics: Fullerene and PCBM from GWW and TDDFT.
Xiaofeng Qian 1 , Paolo Umari 2 , Davide Ceresoli 1 , Nicola Marzari 1
1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Theory at Elettra Group, CNR-INFM Democritos, Basovizza (Trieste) Italy
Show AbstractDensity functional theory is a ground-state theory, and inadequate to describe excited states. We use here many-body perturbation theory (GW) and time-dependent density-functional theory (TDDFT) to study electronic and optical excitations, respectively, in organic photovoltaics. For the former, we employ a very efficient quasiparticle approach using optimal basis sets (GWW) that we recently developed. We apply these methods to the electron acceptors often used in bulk heterojunction solar cells: fullerene C60 and its derivative phenyl-C61-butyric acid methyl ester (PCBM). We found that GW greatly improves the DFT results for the description of ionization potential and electron affinity, and provides much more accurate photoemission and inverse photoemission spectra. Meanwhile, the optical absorption spectra for both fullerene and PCBM from real-time TDDFT calculations are also found in good agreement with experimental data.
R2: Third-generation Solar Cells
Session Chairs
Brian O'Regan
Yoshiharu Sato
Monday PM, November 30, 2009
Room 312 (Hynes)
2:30 PM - **R2.1
Third Generation Solar Photon Conversion to Electricity and Fuel: Multiple Exciton Generation in Colloidal Quantum Dots; Quantum Dot Arrays and Solar Cells.
Arthur Nozik 1 2
1 , NREL, Golden, Colorado, United States, 2 , University of Colorado at Boulder, Boulder, Colorado, United States
Show AbstractOne potential, long-term approach to more efficient future generation solar cells is to utilize the unique properties of quantum dot (QD) nanostructures to control the relaxation pathways of excited QD states to produce enhanced conversion efficiency through efficient multiple exciton generation (MEG) in QDs.We have observed very efficient multiple exciton generation (MEG) in PbSe, PbS, PbTe, and Si QDs at threshold photon energies of 2-3 times the HOMO-LUMO transition. We have studied MEG in close-packed QD arrays where the QDs are electronically coupled in the films and thus exhibit good carrier mobility. We have developed a simple, all-inorganic metal/QD/metal sandwich solar cell that produces a large short-circuit photocurrent (~25-35 mA/cm2 - equivalent to crystalline Si) via a Schottky junction at the negative electrode, without the need for QD sintering, superlattice order or separate phases for electron and hole transport. We have demonstrated that the MEG efficiency in conductive Pb chalcogenide QD films after certain chemical treatments can be comparable to isolated QDs in colloids, but the QY varies greatly depending upon the specific chemical treatment. and subsequent QD surface chemistry. Selected aspects of this work will be summarized and recent advances will be discussed. Various possible configurations for novel QD solar cells that could produce high conversion efficiencies for the production of electricity and solar fuels. like H2 via water splitting, will be presented, along with progress in developing such new types of solar cells. Recent controversy about MEG will also be addressed.
3:00 PM - **R2.2
Understanding the Physics of Carrier-multiplication in Nanostructures – What’s Going On?
Alex Zunger 1
1 , NREL, Golden, Colorado, United States
Show AbstractThe concept of Carrier multiplication (“two electron-hole pairs from one photo”) based on nanostructures, have created significant interest and excitement, but, at the same time, raised questions pertaining to the approaches used to argue their validity. In part, confusion arose because the initial arguments made were based on rather qualitative, if not naive, physical models. Here we propose to use the tools of modern theory of nanostructures for examining these concepts. Simple, but surprising physical pictures emerge from such complex atomistic calculations. In this talk I will review and discuss the underlying concepts and experiments surrounding Carrier Multiplication, delineating fact from fiction, and pointing to some new, important unanswered questions.Supported by DOE, Office of Science, and in collaboration with J.W. Luo, J. An and A. Franceschetti.
3:30 PM - **R2.3
Carrier Multiplication in Semiconductor Nanocrystals and Competing Processes.
Victor Klimov 1
1 Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractCarrier multiplication (CM) is a process in which absorption of a single photon produces multiple electron-hole pairs (excitons). Potentially, CM can produce an appreciable improvement in a power conversion efficiency of solar cells through increased photocurrent. The first experimental observation of CM in quantum-confined nanocrystals (NCs) was reported in ref. [1], where this effect was detected on the basis of a distinct decay component due to Auger recombination of multiexcitons. Later, spectroscopic signatures of CM were observed in NCs of other compositions including PbS, PbTe, CdSe, InAs, and Si. However, in addition to a large body of experimental data demonstrating high-efficiency CM in NCs, several recent reports have questioned the claim of enhanced CM in NCs and even its existence, in at least some NC systems. In this talk, I will address this apparent controversy regarding CM in nanocrystalline materials and analyze potential reasons for observed discrepancies, including sample-to-sample variations, differences in detection techniques, and the influence of extraneous effects that could lead to CM–like signatures in spectroscopic measurements. Further, I will describe the results of our recent studies of CM in PbSe NCs in which we apply two different spectroscopic techniques: time-resolved photoluminescence and transient absorption [2]. Both techniques show clear signatures of CM with efficiencies that are in good mutual agreement. CM yields measured under conditions when extraneous effects are suppressed via intense sample stirring and the use of extremely low pump levels (0.02 - 0.03 photons absorbed per NC per pulse) indicate that both the electron-hole creation energy and the CM threshold in NCs are reduced compared to those in bulk solids. These results are consistent with a confinement-induced enhancement of the CM process in NC materials. Further optimization of CM performance should be possible by utilizing more complex nanostructures (for example, shaped-controlled or heterostructured NCs) that allow for facile manipulation of carrier-carrier interactions and single- and multiexciton dynamics.[1]Schaller, R. D.; Klimov, V. I. Phys. Rev. Lett. 2004, 92, 186601.[2]McGuire, J. A.; Joo, J.; Pietryga, J. M.; Schaller, R. D.; Klimov, V. I. Acc. Chem. Res. 2008, 41, 1810.
4:30 PM - R2.4
Efficient Carrier Multiplication in InP Nanoparticles.
David Binks 1 , Stuart Stubbs 1 , Samantha Hardman 1 , Darren Graham 1 , Wendy Flavell 1 , Nigel Pickett 2 , Steve Daniels 2 , Ben Spencer 1
1 Physics and Astronomy, University of Manchester, Manchester, Lancashire, United Kingdom, 2 , Nanoco Technologies Ltd, Manchester, Lancashire, United Kingdom
Show AbstractEfficient carrier multiplication offers a means by which photon energy in excess of the band gap may be utilised to enhance photocurrent and thus increase the efficiency of photovoltaic cells. Here we report its observation in InP nanoparticles for the first time. An ultrafast transient absorption experiment was used to determine the average number of excitons created per absorbed photon in dilute suspensions of colloidal InP nanoparticles. The suspensions were pumped by 100 fs laser pulses with a range of photon energies and the resulting photo-induced change in transmission through the sample was measured with much weaker 100 fs pulses at a wavelength of 620 nm, corresponding to the 2.0 eV band gap of the nanoparticles. A variable delay could be introduced between the pump and probe pulses which enabled the lifetime of the photo-induced absorption change to be found. For each pump photon energy, the average number of excitons per photoexcited nanoparticle was found for a range of pump fluences. At high pump fluences, the absorption transient had an initial mono-exponential decay feature which was present for all pump photon energies. The ~40 ps lifetime of this initial decay was much shorter than the single exciton lifetime of 8.5+/-0.2 ns, as determined previously for these nanoparticles from photoluminescence decay measurements. For photon energies of 2 times band gap or less, the feature reduced in magnitude as the pump fluence was decreased and eventually disappeared, and was thus attributed to the decay of biexcitons created by successive photon absorption events. In contrast, for higher photon energies a biexcitonic decay feature persisted even in the limit of vanishing pump fluence and was thus attributed to carrier multiplication. At the greatest photon energies used, corresponding to 2.6 times the nanoparticle band gap, the carrier multiplication factor was found to be 1.20+/-0.03. The photon energy threshold for the onset of carrier multiplication was calculated to be 2.04+/-0.08 times the band gap of the nanoparticles, in good agreement with theoretical prediction.
4:45 PM - R2.5
Thermal and Spectral Characteristics of Quantum Dot Solar Cells.
Seth Hubbard 1 , Jamie Gardner 1 , Eric Albers 1 , Christopher Bailey 1 , David Forbes 1 , Ryan Raffaelle 1
1 NanoPower Research Laboratory, Department of Physics, Rochester Institute of Technology, Rochester, New York, United States
Show AbstractRecently, quantum dots (QDs) have been gaining attention in the solar cell community for possible device enhancement purposes. Specifically, with the cost per Watt savings available by using high concentration PV (HCPV), QD solar cells could make a potential impact. Due to the current matching requirement of the state-of-the art HCPV cells (InGaP/GaAs/Ge multi-junction III-V devices), excess current in the bottom cell is wasted, leading to a lower overall efficiency. In order to harness this excess bottom cell current and improve overall efficiency, the middle cell bandgap can be lowered to ~1.2eV at 500 sun concentration. One method to engineer the middle cell is to insert low dimensional nanostructures such as quantum dot superlattice (QD-SL) in the middle GaAs cell of a conventional triple junction concentrator cell. Bandgap engineering of the middle cell could lead to AM1.5d cell efficiency over 55% at 500 suns.However, the complexity of the QD growth makes realization of enhanced devices a delicate process. The development of successful QD growth techniques requires the correlation of device properties with specific growth parameters. A vital task in this cycle is the detailed electronic, optical, and thermal characterization of quantum dots and their effects on device performance. Additionally, as CPV are predicted to operate between 50-80C°, the thermal dependence of QDSCs must also be studied. A combined application of two spectroscopic techniques, electroluminescence (EL) and photo-reflectance, is performed on InAs QD enhanced GaAs solar cells. Results demonstrate the complementary relationship between absorptive and radiative spectroscopy and provide a full description of the major optical transitions in a QDSC. Temperature dependence of the EL signal allows for extraction of various activation energies. These activation energies are correlated to calculated quantum confined transition energies. Additionally, the temperature dependence of the solar cell parameters (short circuit current, open circuit voltage and fill factor) are measured and fit with a physics-based model to yield a quantitative evaluation of the thermal stability of QDSCs.
5:00 PM - R2.6
Multiple Exciton Generation in Single-walled Carbon Nanotubes.
Marat Khafizov 1 , Shujing Wang 1 , Xiaomin Tu 3 , Ming Zheng 3 , Todd Krauss 1 2
1 Department of Chemistry, University of Rochester, Rochester, New York, United States, 3 , Du Pont Central Research, Wilmington, Delaware, United States, 2 Institute of Optics, University of Rochester, Rochester, New York, United States
Show Abstract Multiple exciton generation (MEG) is a process by which a single high energy photon can be converted into more than one electron-hole pair, thereby improving light to charge carrier conversion efficiency. It has been proposed that MEG is more efficient in nanoscale-sized semiconductors due to a relaxation of strict momentum conservation in many-particle interactions, as well as a longer carrier cooling time because of well-spaced energy levels and reduced coupling to vibrations. MEG has been observed for colloidal semiconductor nanocrystals of several materials, although the relative efficiency of the MEG process is currently debated.The exceptional optical and electronic properties of single walled carbon nanotubes (SWNTs) make them promising materials for photovoltaic applications. SWNTs are strong absorbers throughout the visible and near infrared, and they are able to convert photons into electron-hole pairs, separate charge carriers, and transport these carriers long distances. Thus, they have the potential to serve as the photoactive element in an inexpensive solar cell. We will present studies of MEG in isolated SWNTs using ultrafast transient absorption (TA) spectroscopy. SWNTs were dispersed using single stranded DNA and were highly enriched in a single chirality by ion-exchange chromatography. A pure chirality sample free of residual nanotube bundles and metallic nanotubes is vital to identify the correct physical mechanism behind observed TA signals, as sample inhomogeneity complicates the interpretation of results. When photoexciting SWNTs with low energy photons and with low excitation fluence, we observe the excited excitonic state decays in tens of picoseconds according to a power-law, indicative of diffusion limited interactions. Upon increasing the photon energy while keeping the fluence well below an average of one electron-hole per SWNT, we detect an additional fast component in our TA transients, which is a signature of multiple exciton interactions. Our preliminary estimate of the MEG efficiency in (6,5) SWNTs excited at 335 nm (hν/Eg = 2.96) is approximately 20%.
5:15 PM - R2.7
InAs Quantum Dot Enhancement of GaAs Solar Cells.
Ryne Raffaelle 1 , Seth Hubbard 1 , Christopher Bailey 1 , Stephen Polly 1 , David Forbes 1
1 NanoPower Research Labs, Rochester Institute of Technology, Rochester, New York, United States
Show AbstractThe use of vertically stacked InAs quantum dots (QDs) inserted into the i-region of a GaAs p-i-n solar cell has been explored as a means of bandgap tuning for III-V multiple junction solar cell sub-cell current enhancements for space applications, and in terms of the potential that these structures may hold for the realization of intermediate band solar cells (IBSC). When grown with thin enough barriers, InAs QD arrays can be used to create sub-GaAs bandgap energy mini-bands in the band structure of the solar cell. This may allow for sub-GaAs bandgap photons to be converted with high efficiency with minimal loss in open-circuit voltage of the device. However, the complexity of QD growth kinetics makes the realization of solar cell enhancement a challenging endeavor. While theoretical models predict significant gains in solar cell performance, undesirable factors related to the QD growth, such as strain-induced defects and capping-layer intermixing, often result in improved spectral characteristics at the expense of overall device performance. Using MOCVD-grown InAs QD/GaAs p-i-n solar cells, we explain these difficulties using solar cell device parameters and structural results of modification of strain balancing thicknesses and material, QD delta doping and current multiple junction results including Ge substrat/junction activation. High resolution X-ray diffraction (HRXRD) and transmission electron microscopy (HRTEM), and conventional optoelectronic solar cell characterization has been used to improve the overall cell performance. The trends for increasing QD contribution to the overall device photocurrent and improvement as a function of light intensity (for terrestrial applications) with increasing number of quantum dot layers will be presented and minimized loss in open circuitr voltage (tens of mV) is shown. The impact on these results as a function of temperature will also be given and discussed in terms of the IBSC. Finally, we will discuss the prospects that these results hold for high efficiency solar cells and future concentrator photovoltaic systems.
5:30 PM - R2.8
Effects of Barrier Width on Spectral Response of Strained Layer Superlattices for High Efficiency Solar Cells.
Geoffrey Bradshaw 1 , Conrad Carlin 1 , Salah Bedair 1
1 Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractStrained layer superlattices (SLS) consisting of alternating layers InxGa1-xAs and GaAs1-yPy can be incorporated into the intrinsic layer of a p-i-n structure in order to improve the overall efficiency of multijunction solar cells (S.M. Bedair et al, Solar Cells 21, 413 (1987)). By tailoring the composition and thickness of both the InGaAs quantum wells and GaAsP barriers, the effective bandgap of a SLS can be lowered to achieve extended spectral response in the near infrared region. An appropriate balance of composition and thickness of each layer allow for the growth of a defect free SLS and enhanced electrical properties. Thick layers of InGaAs (>100 Å) are preferable for increasing absorption at wavelengths of λ >900 nm. The thicker layers also help avoid quantum size effects that counteract the bandgap reduction of the super lattice structure. Layers of GaAsP balance the lattice strain of the InGaAs wells that result during MOCVD growth but also act as quantum barriers. The strain resulting from thick InGaAs layers with high In% can only be accommodated by a high P% composition GaAsP or thicker layers of GaAsP. However, thick quantum barriers can result in an increase of tunneling lifetime and decrease tunneling probabilities. We have successfully grown balanced SLS with InGaAs wells of x=15% and GaAsP barriers of y=85% with different thicknesses of InGaAs/GaAsP. We will report on the effect of thickness of GaAsP barrier layers on solar cell performance using I-V and spectral response. We have studied devices with 20 period SLS (x=0.15, y=0.84) consisting of barrier thicknesses ranging from LGaAsP=25-45 Å. Well-designed, balanced SLS with thin GaAsP barriers showed spectral responses that match GaAs for λ<900 nm and extend the absorption to 970 nm, beyond that of GaAs. Current-Voltage measurements show only a slight reduction in open circuit voltage (<5%) and up to a 16% increase in the short circuit current with the addition of the SLS. Devices with SLS barrier thicknesses LGaAsP < 30 Å indicate that tunneling through the barrier is close to 100%. For thicker barriers LGaAsP > 30 Å, there is a reduction in the spectral response in the UV and visible wavelength regions. This decrease in spectral response is dependent to barrier thickness. We will report on the probability of tunneling through the SLS barrier at different barrier thickness and the effects on the efficiency of the solar cell.
R3: Poster Session: Novel Inorganic, Organic, Inorganic-organic Hybrid Solar Cells / Third Generation Solar Cells
Session Chairs
Tuesday AM, December 01, 2009
Exhibit Hall D (Hynes)
9:00 PM - R3.1
The Role of Surface Chemistry on Multiple Exciton Generation in PbSe Nanocrystals.
Aaron Midgett 1 2 , Matthew Beard 1 , Hugh Hillhouse 3 1 , Octavi Semonin 1 4 , Barbara Hughes 1 2 , Justin Johnson 1 , Tieneke Dykstra 1 , Arthur Nozik 1 2
1 Chemical Sciences and Nanoscience, National Renewable Energy Laboratory, Golden , Colorado, United States, 2 Chemistry, University of Colorado, Boulder, Colorado, United States, 3 Chemical Engineering, Purdue, West Lafayette, Indiana, United States, 4 Physics, University of Colorado, Boulder, Colorado, United States
Show AbstractMultiple exciton generation (MEG) is a process that occurs in semiconductor nanocrystals where one highly energetic photon produces multiple charge carriers and can significantly enhance solar energy conversion efficiencies. We report different MEG QYs for films of PbSe NCs treated with a variety of chemicals. In some cases, the QY was almost completely quenched and in others it was increased. These different chemical treatments had a variety of effects on the films and it is unclear which ones govern MEG. First, the treatments remove different percentages of the oleic acid cap. Second, the NCs are brought closer together as some of the cap was removed. And finally, some of the treatments are thought to stay on the NC surface and the ligand-NC interactions may dope the NCs in different ways. We are performing one study in order to better understand the role that the NC surfaces have on MEG yield, photoluminescence and the possible existence of long-lived charge states in PbSe quantum dot solutions. Differences due to the presence of a small amount of cadmium oleate in solution show changes in the surface interactions that lead to different MEG dynamics and photoluminescence quantum yield. This study focuses on how the surface can affect the NC dynamics in both static and flowing solutions. We find that flowing does change Auger decay dynamics, but to a different extent for the solutions treated with Cd(oleate)2 and those left as synthesized. The reason, or mechanism for these findings remains unclear. Both of these studies help to shed light on the controversy surrounding MEG and the factors that affect it.
9:00 PM - R3.10
Plasmonic Dye-Sensitized Solar Cells.
Byoung Koun Min 1 , Jung Eun Heo 1 , Oh Shim Joo 1
1 , Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractLocalized surface plasmon resonance (LSPR) is the electron oscillation in spatially confined small metallic nanoparticle coupled to an external electromagnetic field (e.g. light) and has been experimentally and theoretically demonstrated to enhance light absorption; thus, enable efficient light harvesting in the vicinity of the metal nanoparticles. Dye-sensitized solar cells (DSSCs) in which mesopourous and transparent semiconducting material (typically TiO2), sensitized with molecular dye is employed would be good candidates to apply LSPR to improve light harvesting efficiencies since metal nanoparticles would act as absorption amplifiers of dyes near the metal nanoparticles. In this study TiO2 films incoporated with gold nanoparticles were applied instead of TiO2 aiming at enhancing solar conversion efficiencies by virtue of the LSPR effect. To control the LSPR effect, various Au-TiO2 films with different size and density of gold nanoparicles were applied, and their light absorption and photoelectron generation behaviors were investigated. Notably, little difference in dye adsorption amount was observed in Au-TiO2 films compared with TiO2 films in the absence of gold nanoparticles. However, both light absorption and photoelectron generation were enhanced resulting in overall increase of photoconversion efficiencies. The details of the study will be discussed in the presentation.
9:00 PM - R3.11
Fabrication of Nanostructured Hybrid Solar Cells Using Soft Imprint Lithography.
Neil Treat 1 3 , Luis Campos 3 , Michael Chabinyc 1 , Craig Hawker 1 2 3
1 Materials Department, Univsersity of California, Santa Barbara, Santa Barbara, California, United States, 3 Materials Research Laboratory , University of California, Santa Barbara, Santa Barbara, California, United States, 2 Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California, United States
Show AbstractRecently, there has been a renewed interest in affordable sources of renewable energy. Organic solar cells offer great potential to fill this need due to the low cost of materials and fabrication techniques. With the aim of better understanding the effects of active layer structure on solar cell performance, we have fabricated, using soft imprint lithography, various patterns of TiO2. This was achieved using our stamping materials which can pattern sub-100 nm features in a fast and reliable manner. The generated pattern was then infiltrated with a blend of poly(3-hexylthiophene) and fullerene derivative followed by the evaporation of a high work function metal electrode generating an inverse organic solar cell. We are currently studying the effects of changing the aspect ratio (from 1 to 3) of TiO2 structures having pores of 50 nm in diameter and period of 90 nm on solar cell performance and comparing these results to a nonpatterned TiO2 layer.
9:00 PM - R3.12
Structure and Optical Characterization of Nanocrystalline Silicon Thin Films for Solar Cells.
Ryo Morisawa 1 , Akira Shirakura 1 , Chen-Chung Du 2 , Jen-Rong Huang 2 , Muh-Wang Liang 2 , David Ch Wu 2 , Tetsuya Suzuki 1
1 Center for Science of Enviroment, Resources, and Energy , Keio University, Yokohama-shi Japan, 2 Mechanical and Systems Research Laboratories, Industrial Technology Research Institute, Hsinchu Taiwan
Show Abstract Crystalline silicon (c-Si) solar cells made of the second most abundant element in the Earth’s crust, have a desirable band-gap (1.1 eV) and long-term stability. For these reasons, the research on c-Si has advanced with expanding usage of c-Si solar cells, and the resulting performance continues to improve year by year. Recently, amorphous silicon (a-Si:H) and hydrogenated nanocrystalline silicon (nc-Si:H) have attracted much attention as promising materials for photovoltaic (PV) solar cells. By using thin film solar cell compared to bulk c-Si, reduction of many resources is possible to avoid high costs. The deposition of a-Si:H or nc-Si:H thin films by using VHF-plasma enhanced chemical vapor deposition (VHF-PECVD) is highly system dependent and influenced by process parameters. The optimization of these parameters and their impact on optical properties has been investigated by many researchers. The structure and optical properties of various Si:H films are determined mainly by the substrate temperature and hydrogen concentration. However, the influence of RF-power on the microstructure and optical properties of nc-Si:H films deposited by VHF-PECVD with pure silane has not been sufficiently studied. To prepare the nc-Si:H film at a higher diluted silane condition and low RF power, it is necessary to investigate the effect of these parameters to the microstructure and optical properties. This study aims to investigate the characteristic features of the nc-Si:H films prepared at the different RF powers with pure silane at a high hydrogen dilution ratio. In this study, instead of using low purity silane, ultrapure silane at a higher dilution ratio (R>30) was used to deposit nc-Si:H film. The nc-Si:H films were prepared at RF power from 50 to 300 W. The prepared nc-Si:H were characterized by X-Ray diffraction analysis, Raman analysis and high-resolution transmission electron microscopy (HRTEM) analysis. The results clearly indicate that an amorphous layer grew in the films using highly diluted pure silane prepared below 100 W. On the other hand, a crystalline layer grew in the films prepared above 150 W. The a-Si:H to nc-Si:H transition was observed between 100 W and 150 W. The nucleation mechanism toward nc-Si:H near the transition point of amorphous phase was discussed based on HRTEM analysis with atomic scale. From HRTEM analysis, similar columnar crystalline phase existed for all films prepared 150 W. However, under 100 W, the films were completely amorphous agreeing with XRD results. Above results showed that the microstructure of the films prepared in the range of over 150 W was very similar to each other, but from UV-visible spectroscopy analysis, the film prepared at 150 W contributed the highest optical absorption in a wide photon energy range. Further, it is suggested that nc-Si:H films with the best optical properties would be obtained near the transition point from the amorphous phase to the crystalline phase.
9:00 PM - R3.13
Chemical Vapor Annealing Processing in Nanocrystal-Conductive Polymer Hybrid Solar Cells.
Yue Wu 1
1 School of Chemical Engineering, Purdue University, West Lafayette, Indiana, United States
Show AbstractSemiconductor nanostructure-based solar cells are promising candidates for future-generation photovoltaic devices due to their advantages in low temperature processing, scalability, and low cost. Among the different types of nanostructure-based solar cells, hybrid solar cells possess a unique architecture of a solid-state blend of interpenetrating and percolating inorganic nanostructures and organic polymers. In principle, hybrid solar cells should demonstrate a better performance when compared with organic polymer bulk heterojunction solar cells due to the higher absorption coefficient of inorganic semiconductor nanomaterials. However, so far there has been no report of higher performance observed in hybrid solar cells than organic bulk heterojuction solar cells in the real experiment, which can be mainly attributed to the existence of non-conductive surfactant molecules with long alkyl chain, for example, phosphonic acids, on the surface of nanocrystals. A few recent reports on the surface ligand exchange reactions tried to address this issue, however, all of them ended up with aggregated nanocrystals after the replacing of long alkyl chain surfactants with short chain molecules. Once the aggregation happens, the nanocrystals will precipitate out of the solution and can not be mixed uniformly with conductive polymer any more for solution process of hybrid solar cell fabrication. In our research, we have demonstrated that a chemical vapor annealing process can be used to significantly improve the performance of CdSe/P3HT hybrid solar cells though a surface ligand exchange reaction. This method represents a new approach to enhance the charge separation/transport for solution-processed high-performance photovoltaic solar cells by replacing bulky surfactants binded on nanocrystal surface without the sacrifice of solubility of nanocrystals in organic solution and polymer blend.
9:00 PM - R3.14
Block-copolymer Self-assembly in Processable Mesoporous Metal Oxide Films: Application in Efficient Hybrid Solar Cells.
Pablo Docampo 1 , Stefan Guldin 2 , Priti Tiwana 1 , Chris Orilall 3 , Ulrich Wiesner 3 , Ullrich Steiner 2 , Henry Snaith 1
1 Condensed Matter Physics, University of Oxford, Oxford, Oxfordshire, United Kingdom, 2 Physics, University of Cambridge, Cambridge, Cambridgeshire, United Kingdom, 3 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractLow cost organic and ceramic semiconducting materials have bulk electronic properties typically orders of magnitude inferior to conventional semiconductors. However, introducing nano to mesostructure into low cost hybrid composites can introduce novel electronic functionality. In the case of hybrid solar cells an intricate mix on the tens of nm scale of an organic hole-transporter, a light absorbing chromophore and an electron transporting oxide, facilitates an improvement of orders of magnitude in solar light to electrical conversion efficiency as compared to planar devices. Correctly matching the electronic and morphological nature of the materials employed is critical to ensuring efficient device operation.In this study we explore different meso-morphologies of a range of metal oxides structured by self-assembling di-block copolymers and their influence on charge transport, recombination and ensuing performance of solid-state dye-sensitized solar cells. A detailed analysis of the hole-transporter, 2,2', 7,7'-tetrakis(N,N-di-p-methoxypheny-amine)-9,9'-spirobifluorene (spiro-OMeTAD), pore filling is also undertaken. The mesoporous TiO2 was self-assembled through sol-gel chemistry using poly(isoprene)-block-poly(ethylene oxide) (PI-b-PEO) as the structure-directing-agent. Different ratios of inorganic and organic components were varied to yield very different morphologies.Maximum solar power conversion efficiencies of above 3 % were achieved for the best performing composite, directly comparable to devices made from state-of-the-art nanoparticle pastes. Interestingly, the charge transport and recombination characteristics did not appear to correlate with trends in device performance. However, an increasingly deep and broad density of sub band-gap states correlates well with increasing photocurrent collection, suggesting that a high density of sub-bandgap states is critical for efficient photo-induced electron transfer and separation.
9:00 PM - R3.15
Inorganic Photo-sensitized Transparent Conductive Oxide (TCO) Nanocomposite Thin Films for Photovoltaic (PV) Energy Conversion.
Cary Allen 1 , G. Shih 1 , B. Potter, Jr. 1
1 , University of Arizona, Tucson, Arizona, United States
Show AbstractNanophase semiconductor composites are widely researched for third-generation PV devices. Through quantum size effects and phase assembly manipulation, the optical absorption and carrier transport properties exhibited by the nanocomposite films can be influenced. We investigate the potential for improved PV energy conversion efficiency by examining the photo-sensitization of indium-tin-oxide (ITO) with nanophase germanium (Ge). Nanocomposite films are produced by a sequential, RF-magnetron sputter deposition technique. Deposition parameter control and post-deposition annealing are shown to influence the structural characteristics of the Ge phase, including, average Ge quantum dot size, volume fraction and spatial distribution. Optical absorption and carrier transport characteristics were correlated to variations in the composite film structure confirmed by Raman spectroscopy, X-ray diffraction, and transmission electron microscopy. In addition to structure-dependent variation in spectral absorption, Hall-effect measurements and spectrally resolved photoconductivity confirmed the introduction of additional free carriers, associated with the incorporation of the Ge phase into the ITO host. These results support the further evaluation of such nanocomposite TCO materials as heterojunction components in PV devices.
9:00 PM - R3.17
Organic Solar Cell Based on PCBM and P3HT with Various Functional Groups.
Youngmin Lee 1 , Anthonysamy Arockiam 1 , Jin Kon Kim 1
1 National Creative Research Initiative Center for Block Copolymer Self-Assembly, Depatment of Chemical Engineering, POSTECH, Pohang, Gyeongbuk, Korea (the Republic of)
Show AbstractPoly (3-hextylthiophene)/[6,6]-phenyl-C61 butyric acid methyl ester (P3HT/PCBM) bulk heterojuction solar cells have been extensively investigated. Power convergence efficiency (PCE) with 5~6 % was achieved by judiciously control of nano-scale phase separation between P3HT and PCBM by using thermal or solvent annealing. Here, chemically modified P3HTs are synthesized and the blend morphology with PCBM is investigated. For this purpose, various end functional groups (-OH, -Br(reference), -C2H5, and –C3F7) covering from hydrophilic to hydrophobic group were attached to P3HT by Grignard metathesis method (GRIM). We found that among four different end-functional groups, P3HT-C3F7/PCBM pair give ~ 40% increase in PCE compared with reference P3HT (P3HT-Br)/PCBM. The large increase in PCE is due to very uniform nano-sized phase separation.
9:00 PM - R3.18
Photovoltaic Properties of Polyaniline-Titania Composite for Hybrid Solar Cells Applications.
Michael Ibrahim 1 2 , Maria Bassil 1 3 , Umit Demirci 2 , Georges El Haj Moussa 1 , Mario El Tahchi 1 , Philippe Miele 2
1 LPA-GBMI, Lebanese University, Jdeidet Lebanon, 2 LMI, University of Claude Bernard Lyon 1, Lyon France, 3 IMP-LMPB, CNRS, UMR5223, University of Claude Bernard Lyon 1, Lyon France
Show AbstractSolar energy harvesting has been extensively studied in the last three decades to provide a green energy source. Hybrid photovoltaics (HPV) based on titania (TiO2) are researched for their easiness of production and low cost. Nanostructured mesoporous titania films and conductive polymers were used recently to form hybrid solar cells [1]. TiO2, mainly an n-type semiconductor with a band gap of 4.2 eV, is employed in several applications from which paints form the highest world use of titania making it an attractive material to use in HPV industry. On the other side, our targeted conductive polymer is polyaniline (PANI), a hole conductor polymer, which is used in such HPV cells due to its high charge-carriers mobility, absorption coefficient in the visible range and environmental stability. PANI and nanocrystalline TiO2 films fabricated using spin coating or layer by layer assembly techniques behave as a p-n heterojunction diode and can be used as solar cells [2-4].Precursor solutions are prepared by polymerizing aniline-HCl inside an aqueous solution of titania. To study the effect of the precursor concentration on the PANI-TiO2 composite, polymerization of aniline is held in diverse TiO2 concentrations in water. Industrial grade TiO2 powders with particle size ranging from 200 nm to several μm are used. PANI-TiO2 precursor solutions are dip coated or slot dyed on various substrates such as PMMA, PET and PP, all with metal oxide conductive coatings. Bulk PANI-TiO2 pellets are prepared for comparison. The electrical and photovoltaic properties of the obtained films and pellets are investigated to choose the optimum blend composition for HPV cell. Finally a theoretical study and an analytical model of the HPV cell are presented relating the size of TiO2 and PANI particles and their respective geometrical distribution inside the blend to the transport characteristics of charge carriers and the overall efficiency of the HPV cell.[1] M. McGehee, MRS Bulletin, Vol. 34, No. 2, February 2009.[2] Z. Liu, W. Guo, D. Fu and W. Chen, Synthetic Metals, Vol. 156, pp. 414–416, 2006.[3] Z. Liu, J. Zhou, H. Xue, L. Shen, H. Zang and W. Chen, Synthetic Metals, Vol. 156, pp. 721–723, 2006.[4] X. Zhang, G. Yan, H. Ding and Y. Shan, Materials Chemistry and Physics, Vol. 102, pp. 249–254, 2007.
9:00 PM - R3.2
GaAs Based InAs/InGaAs Quantum Dots-in-a-Well Solar Cells and Their Concentration Applications.
Kai Yang 1 , Mohamed El-Emawy 1 , Tingyi Gu 1 , Andreas Stintz 1 , Luke Lester 1
1 Center for High Technology Materials, University of New Mexico, Albuquerque, New Mexico, United States
Show AbstractQuantum dot (QD) solar cells have been actively investigated recently since they have been theoretically shown to have the potential for high conversion efficiencies. However, very little research has analyzed the effect the dots have on the transport or recombination effects in the device. In this paper, we report the I-V and spectral response characteristics of InAs/InGaAs “dots-in-a-well” (DWELL) solar cells and compared them with GaAs control cells. The QD cells show higher short circuit density and better long-wavelength efficiency compared to the control cell. By comparing the dark current behavior of the QD cells to the GaAs control cells, we have conservatively estimated the concentration level at which the QD solar cells would surpass the conventional GaAs devices.The nanostructured solar cells are grown by MBE with multiple DWELL layers, which contain InAs QDs embedded in InGaAs quantum wells (QWs), sandwiched in the i-layer of a standard GaAs pin structure. The control cell is the same structure except without the DWELL layers. The I-V characteristics were measured under AM1.5G, and it was found that the DWELL cell has about a 33% larger short circuit current density. The spectral response data show that the GaAs control cell and the DWELL cell have similar external quantum efficiency (EQE) in the visible to near-IR range (400-870nm). Beyond the GaAs absorption edge (870nm), the DWELL solar cell shows extended response with much higher EQE up to the ~1200 nm (IR wavelength range). We have found that the InGaAs QWs are actually the primary contributor to the increased short circuit current density. However, the QDs significantly modify the ideality factor performance as a function of voltage due to the dramatically increased efficiency of spontaneous radiative recombination compared to the GaAs control cell.Whereas the GaAs control cell shows the typical monotonically decreasing ideality factor from 0.3 to 0.7V, a linearly increasing ideality factor up to about 2 is observed in the DWELL. This behavior has significant potential benefits for concentration. Based on the measured dark currents, and neglecting series resistance, we extrapolated the IV curves to higher voltages and found that they intercept at ~2x104 mA/cm2. Dividing this value by the short circuit current density of the DWELL cell conservatively estimates the light concentration (~1400X) above which the DWELL cell would have a higher Voc and superior efficiency compared to the GaAs cell, assuming additivity applies.
9:00 PM - R3.20
The Role of a MoO3 Hole-extracting Layer on the Performance of Small Molecule Photovoltaic Devices.
Ian Hancox 1 , Paul Sullivan 1 , Virendra Chauhan 1 , Ross Hatton 1 , Tim Jones 1
1 Chemistry, University of Warwick, Coventry United Kingdom
Show AbstractThe effect of the addition of a thin MoO3 hole-extracting layer inserted between the indium tin oxide (ITO) electrode and the organic donor material in small molecule heterojunction photovoltaic (PV) cells has been studied for four different donor materials in conjunction with a C60 acceptor. When using donor materials where the MoO3 interlayer causes a favourable energy level alignment for hole-extraction, for example chloro-aluminium phthalocyanine (ClAlPc) and boron subphthalocyanine (SubPc), the device performance is greatly enhanced. Of particular note is the large increase in open circuit voltage (Voc) in these systems, with increases of up to 60% observed for both ClAlPc and SubPc devices containing the MoO3 layer, for both planar and mixed heterojunction devices. In the case of a pentacene donor layer, where an unfavourable energy level alignment leads to a barrier to hole-extraction, the device performance is found to decrease with insertion of the MoO3 layer. Devices containing a copper phthalocyanine (CuPc) donor layer resulted in a marginal improvement in device performance with a negligiblebarrier to hole-extraction. The MoO3 layer also brings other advantages to these devices, in particular more reproducible performance and a reduction in device degradation, a consequence of a more electrically homogeneous electrode surface, as shown by scanning Kelvin Probe microscopy. The reasons behind these improvements are discussed in terms of energy level alignments of the systems as well as interfacial properties.This work was supported by the Engineering and Physical Sciences Research Council (EPSRC), UK, under the SuperGen Excitonic Solar Cell Consortium. Financial support from Asylum Research UK is also acknowledged. RAH is grateful to the Royal Academy of Engineering for the award of a Fellowship.
9:00 PM - R3.21
Improvement of the Light-Harvesting Efficiency in P3HT/PCBM Solar Cells with Phthalocyanine Molecules.
Hideo Ohkita 1 , Satoshi Honda 1 , Seiichirou Yokoya 1 , Hiroaki Benten 1 , Shinzaburo Ito 1
1 Department of Polymer Chemistry, Kyoto University, Kyoto Japan
Show AbstractCurrently, a regioregular poly(3-hexylthiophene) (P3HT) and a fullerene derivative (PCBM) are the most effective materials employed in polymer/fullerene bulk heterojunction solar cells, which exhibit around 4–5% power conversion efficiencies. In such devices, the efficiency can be improved by thermal annealing because it considered to promote crystallization of the P3HT domain. The photoluminescence efficiency of the blend has been reported to increase after the thermal annealing, suggesting that the P3HT domain is grown up upon the thermal annealing. This result indicates that there is still room to improve the light-harvesting efficiency in this device because some P3HT excitons cannot reach the interface after the thermal annealing. Herein we employed two phthalocyanines with different structures as a dye sensitizer in P3HT/PCBM solar cells to address how molecular structures impact on the light-harvesting efficiency. For a ternary blend device of P3HT/PCBM with a planar zinc phthalocyanine (ZnPc), neither Jsc or EQE was improved but FF significantly decreased, suggesting that dye aggregates suppress the charge transport in the ternary blend device. For a ternary blend device of P3HT/PCBM with a silicon phthalocyanine attached with bulky axial substituents (SiPc), on the other hand, both Jsc and EQE substantially increased and hence the device performance was improved by 20%. The increase in EQE was observed at wavelengths not only for the absorption band of SiPc but also for the absorption band of P3HT, suggesting the following two points. One is that most of SiPc molecules are likely to be located at the P3HT/PCBM interface and therefore can contribute to the photocurrent. The other is that P3HT excitons can be dissociated into charge carriers more efficiently in the presence of SiPc molecules at the interface by energy transfer from P3HT to SiPc molecules. In other words, there are two origins for the increase in the photocurrent by the introduction of SiPc: SiPc molecules serve not only as a light-harvesting photosensitizer but also as an energy funnel for P3HT excitons at the P3HT/PCBM interface.
9:00 PM - R3.23
Photoelectrical Study of the Effect of Thermal Annealing and X-ray Irradiation on the Density of Defect States in Organic Solar Cells based on P3HT:ThCBM[60] and P3HT:ThCBM[70] Mixtures.
Adam Purkrt 1 2 3 , Ales Poruba 1 , Jiri Hybler 1 , Milan Vanecek 1 , Vera Cimrova 2
1 Department of Optical Materials, Institute of Physics of the AS CR, v. v. i., Prague 6 Czechia, 2 Department of Bioanalogous and Special Polymers, Institute of Macromolecular Chemistry of the AS CR, v. v. i., Prague 6 Czechia, 3 Department of Solid State Engineering, Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Prague 2 Czechia
Show AbstractWe report on a comparative photoelectrical study of bulk-heterojunction polymer solar cells based on poly(3-hexylthiophene) mixed with two types of ThCBM (fullerene derivative), subjected to several thermal annealing and X-ray irradiation steps.ThCBM[60], a 1-(3-methoxycarbonyl) propyl-1-thienyl-[6,6]-methanofullerene, has been recently used as a thienyl analog of PCBM, in the mixture with P3HT - a poly(3-hexylthiophene-2,5-diyl), with promising results [2]. ThCBM[70] is an analog of ThCBM[60], based on a fullerene with 70 carbon atoms. We have successfully prepared samples of bulk heterojunction organic solar cells based on P3HT mixed with ThCBM[60] and ThCBM[70]. Power conversion efficiencies of our samples were about 2%, with the best ones exceeding this value.We used the Fourier Transform Photocurrent Spectroscopy (FTPS) [1] to study the optical absorption processes, connected with the collection of free photo-generated charge carriers, in the 0.8 – 2.8 eV spectral range. FTPS is very sensitive, its dynamic range reaches 8 orders of magnitude in organic solar cells [3]. FTPS spectra of complete solar cells are complemented with current-voltage characteristics, both in dark and under illumination, and furthermore with UV-VIS absorption spectra of the active layer alone, measured in an inert (nitrogen) atmosphere and undergoing identical treatment as the whole cell.Thermal annealing is found to be a key step in attaining high efficiency of our cells, similarly as in the already thoroughly studied case of P3HT:PCBM mixture [4]. Moreover, we subjected our samples to X-ray irradiation and compared the changes in FTPS spectra due to X-ray with those induced by thermal annealing. According to [5], X-ray irradiation induces defect states in the polymer semiconductor. In support of [5], trend of change observed by us in the low absorption region of the FTPS spectra can be interpreted as reflecting the change of the density of defect states. FTPS signal in the relevant region (1.3-1.9 eV) increases with X-ray irradiation and decreases with proper thermal treatment. This insight into the changes occurring in the bulk-heterojunction during the annealing establishes FTPS as a valuable tool for the study of organic solar cell materials.Acknowledgement: We would like to acknowledge the support of the Ministry of Education, Youth and Sports of the Czech Republic (grant No.1M06031) and of the Grant Agency of the Academy of Sciences of the Czech Republic (grant No. IAA4050409). Presented work was also supported by the Center for Nanotechnology (project LC510).[1] M. Vanecek, A. Poruba, APL 80 (2002) 719[2] L. M. Popescu et al., APL 89 (2006) 213507[3] L. Goris et al., APL 88 (2006) 052113[4] A. Purkrt et al., Proceedings of the 22nd EU PV SEC, Milan, Italy, 3-7 September 2007, pp. 487-490, ISBN: 3-936338-22-1[5] G. Li et al., Nanotechnology 19 (2008) 424014 (4pp)
9:00 PM - R3.25
Fast-Switching Photovoltachromic Cells with Tunable Transmittance.
Jih-Jen Wu 1 , Min-Da Hsieh 1 , Wen-Pin Liao 1 , Wei-Ting Wu 2 , Jen-Sue Chen 2
1 Department of Chemical Engineering, National Cheng Kung University, Tainan Taiwan, 2 Department of Materials Science and Engineering, National Cheng Kung University, Tainan Taiwan
Show AbstractIn this study, we demonstrate a photovoltachromic cell (PVCC) which is a solar cell and able to take solar energy to stimulate chromic behavior with the characteristic of tunable transmittance. The cell is composed of a patterned WO3/Pt electrochromic electrode and a dye-sensitized TiO2 nanoparticle photoanode. Compared to reported photoelectrochromic cells (PECC) with non-patterned WO3 electrochromic electrodes, PVCC achieves a much faster bleaching time by blocking the light at short circuit. When PVCC is bleached under illumination at open circuit, an exceedingly short bleaching time is achieved. Furthermore, PVCC has photovoltaic characteristics comparable to those of dye-sensitized solar cells (with Pt as the counter electrode). In contrast to conventional photochromic devices, the transmittance of PVCC under a constant illumination can be adjusted by the resistance of a load in series with the cell. The mechanisms of coloration and bleaching of PVCC will be discussed in the presentation.
9:00 PM - R3.26
Silicon Nanocrystals Composite Films for Improved Tandem Solar Cells.
Xavier Paquez 1 , Yann Leconte 1 , Olivier Sublemontier 1 , Philippe Thony 3 , Pascal Gentile 4 , Xavier Portier 2 , Mustapha Lemiti 5 , Georges Bremond 5 , Nathalie Herlin-Boime 1 , Cecile Reynaud 1
1 DSM/IRAMIS/SPAM/LFP, CEA-CNRS, Gif sur Yvette France, 3 DRT/DTS/LCS, CEA-INES, Le Bourget du Lac France, 4 DSM/INAC/SP2M, CEA, Grenoble France, 2 DSM/IRAMIS/CIMAP, CEA-CNRS, Caen France, 5 INL, CNRS, Villeurbanne France
Show AbstractThe efficiency of amorphous Si-based tandem solar cells is limited by their poor conduction properties, and the time stability of hydrogenated layers constitutes another drawback. Replacing the amorphous layer by a nanocrystalline Si film could help overcoming these problems. Transport properties can indeed be increased while keeping the bandgap of the layer close to 1.7 eV thanks to the efficient quantum confinement that appears when the crystal size is decreased below 5 nm. Finally, such a nanostructured tandem cell (nanocrystals cell on single-crystalline cell) could reach a theoretical 42% efficiency.Silicon nanoparticles (nc-Si) are produced from silane by laser pyrolysis and collected as powders. The obtained mean grain size can be tuned in the 3 – 5 nm range with high production rates (> 300 mg/h). After surface passivation, the nc-Si show under UV excitation a strong photoluminescence (PL) emission in the visible region, with a PL peak position in the 1.6 – 1.7 eV range that is in good agreement with quantum confinement models.The nanostructured layer deposition is achieved by spin coating of silica sol-gel precursor where the nanoparticles are dispersed. The obtained layers result in a silica matrix containing a high concentration of well dispersed Si nanocrystals (2.6.1012.cm-2) with narrow size distribution. In order to achieve the doping of the nanostructured layers a phosphorus and a boron precursors are separately added in the sol-gel mixture. Activation of the doping element is studied by resistivity measurements after different annealing treatments. The effects of the thermal step, tested for different values between 750 and 1100°C, are observed on the structure and on the optical and electric properties of the composite films. The effects of the nc-Si concentration and of the dopant type and concentration on the resistivity are also presented.Nc-Si can were also deposited in-situ by low energy beam deposition (LEBD) from a supersonic jet created by a pressure gradient between the laser pyrolysis synthesis chamber and the deposition chamber. The obtained layers are highly porous but the deposition rates are very high (up to 250 nm/min) with a substrate kept at room temperature. These layers could also be encapsulated by post deposition of silica and silicon nitride films.We report here the structural characterization of the nanostructured films together with their optical and electric properties. Photocurrent measurements and elaboration of the nanostructured PN junction are in progress.
9:00 PM - R3.27
3D Photonic Crystal Intermediate Reflector for Micromorph Thin-film Tandem Solar Cell.
Ralf Wehrspohn 1 , Johannes Uepping 1 , Andreas Bielawny 1 , Carsten Rockstuhl 2 , Reinhard Carius 3
1 Institute of Physics Institute of Physics, µMD, Martin-Luther-University Halle-Wittenberg, Halle Germany, 2 Institute of Physics, Solid States Optics, University of Jena, Jena Germany, 3 Institute of Energy Research, IEF-5 Photovoltaics, Forschungszentrum Juelich, Juelich Germany
Show AbstractThe concept of 3D photonic intermediate reflectors for micromorph silicon tandem cells toward first prototype cells has been investigated. The reflector enhances the absorption of spectrally selected light in the top cell and decreases the current mismatch between both junctions. A numerical method to predict filter properties for optimal current matching is presented. Our device is an inverted opal structure made of Al doped ZnO and built using self organized nanoparticles and atomic layer deposition coating methods. We present design rules, preparation, and characterization of a 3D photonic thin film device for optimal current matched tandem cells. An integrated prototype is compared to state of the art reference cell. Results to fully integrate and conductivity enhanced 3D photonic interlayers in textured silicon tandem cells using process compatible steps are shown.
9:00 PM - R3.28
Altering the Properties of Elastomer to Optimise its Ability to Remove Nanosized Particles from Solar Cell Substrates.
Sheila Hamilton 1
1 , Teknek Limited, Inchinnan, Renfrewshire, United Kingdom
Show AbstractThe removal of very small loose particles of contaminants from substrates is very challenging in any production environment due to the very high adhesion forces holding the particles. In third generation solar cells particulate contamination affects both the process yield and the cell efficiency. Effective removal of these particles is vital to the commercial viability of the manufacturing process for these solar cells.This paper analyses the interaction of the forces between the particle of contaminant, the substrate, the elastomer cleaning roller and the purging adhesive roller. The results highlight how properties of the elastomer and the adhesive can be altered to increase the removal forces exerted by the elastomer on the particles without compromising the interaction with the adhesive rollThis new optimised cleaning technique is a dry process which offers significant cost benefits to solar cell manufacturers while reducing the environmental impact of the cleaning process.
9:00 PM - R3.29
Next-Generation Photovoltaics Devices Based on Silicon Nanocrystals.
Bhavin Jariwala 1 , Pauls Stradins 2 , Joseph Beach 1 , Reuben Collins 1 , P. Craig Taylor 1 , J. George Radziszewski 1 , S. Kim Williams 1 , Jifang Cheng 1 , Jeremy Fields 1 , Kristin Kiriluk 1 , Cristian Ciobanu 1 , Sumit Agarwal 1
1 , Colorado School of Mines, Golden, Colorado, United States, 2 , National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractSi nanocrystals (NCs) less than 5 nm in diameter exhibit a size-dependent tunable band gap, visible photoluminescence, and multiple exciton generation [1]. These properties of Si NCs have led to an increased interest in their utilization in third-generation photovoltaic (PV) devices [2]. In this presentation, we will discuss the synthesis of Si NCs from a SiH4/Ar plasma. These plasma-synthesized NCs do not agglomerate in the gas phase as they acquire a negative charge in the discharge. The particles are transported out of the plasma by gas flow, and either collected onto a grid or embedded within an amorphous Si matrix. The size distribution, structure, and optical properties of the as-synthesized NCs have been characterized using transmission electron microscopy, infrared and Raman spectroscopy, and excitation-dependent photoluminescence. The TEM measurements show that the NCs have a diameter over the range of 3 to 7 nm: the average size can be controlled by varying the operating conditions such as the residence time in the plasma volume and the plasma power. Photoluminescence from ~7 nm NCs has an emission peak centered at 850 nm, which blue shifts as the crystal size decreases due to oxidation. One of the key challenges in utilizing Si NCs in a PV device is surface passivation. While our infrared measurements show that the surface Si atoms of the as-synthesized NCs are H-terminated with mono-, di-, and tri-hydride species, these NCs oxidize over a few minutes. After removing the native oxide on the NCs in situ or ex situ, we have studied the effect of different passivating layers on their optical properties. Size distributions, tolerances, as well as methods for size separation for future PV applications are also discussed. Support from NSF award number DMR-0820518, and DOE DE-AC36-08GO28308 is gratefully acknowledged.1. M.C. Beard, K.P. Knutsen, P. Yu, J.M. Luther, Q.Song, W.K. Metzger, R.J. Elligson, A.J. Nozik, Nanoletters 7 (2007) 2506.2. E.-C. Cho, S. Park, X. Hao, D. Song, G. Conibeer, S.-C. Park, M.A. Green, Nanotechnology 19 (2008) 245201.
9:00 PM - R3.3
Radiative and Nonradiative Transition Times in InAs/GaAs QD Based Intermediate Band Solar Cells.
Stanko Tomic 1 , Nicholas Harrison 1 2
1 Computational Science and Engineering Department, STFC Daresbury Laboratory, Warrington, Cheshire, United Kingdom, 2 Department of Chemistry, Imperial College, London United Kingdom
Show AbstractIntermediate band solar cells (IBSC) have emerged as an alternative design for third generation solar cells that could lead to dramatically improvements in power conversion efficiencies [1]. Here, it is demonstrated that a k.p multiband theory with periodic boundary conditions can easily be applied to predict electronic and absorption characteristics of semiconductor quantum dot (QD) arrays that produce a mini-band (IB). The IB is located in the forbidden energy gap of the barrier material and is well separated from valence and conduction band of the barrier material. Using model mentioned above, we have calculated influence of QD size and periodic separation on the: electronic structure, absorption spectra, spontaneous emission spectra, quasi-Fermi level variation with carrier density, radiative and nonardiative lifetimes. The dependence of nonradiative lifetimes related to the polar coupling with longitudinal phonons, deformation potential coupling to acoustic phonon, as well as those related to the Auger related processes like electron cooling and bi-exitonic relaxation have also been computed. The nonradiative scattering times related to Auger processes were calculated under standard time-dependent perturbation theory, where Coulomb or exchange matrix elements were not corrected with the Makov-Payne correction, in order to allow for the effects of mirror charges from neighbouring dots. The energy conservation condition for the QD array was approximated with the energy width of the intermediate bands. The scattering rates between higher states in the CB and first exited state in the CB are largely dominated by scattering via LA and LO phonons and are in the range of ~ps. This suggests very fast depopulation of upper CB states down to first excited CB state. The radiative scattering rate between IB and the top of the valence band is of the order ~ns. Although increased for at least one order of magnitude comparing to radiative time on the same transition of a single QD structure, due to delocalisation of the IB related wavefunction, it is still much faster than radiative transitions between first exited state in the QD and IB which is of the order ~μs. The only other scattering time between first exited excited state in the CB and IB that is in the order of ~0.1 ns is via nonradiative Auger cooling process. Analysis of the electronic and absorption structure suggest that the most promising design for an IB material exhibiting a quasi-Fermi level will employ small InAs/GaAs QDs (~6-10 nm QD lateral size) arranged in a periodic array. Using larger (> 20 nm QD lateral size) QDs leads to the extension of the absorption spectra into a longer wavelength region but does not provide a separate IB in the forbidden energy gap unless amount of In is reduced in QD or In grading in the QD region is assumed.[1] A. Luque and A. Marti, Phys. Rev. Lett. 78, 5014 (1997).[2] S. Tomic, T.S. Jones, N.M. Harrison, Appl. Phys. Lett. 93, 263105 (2008).
9:00 PM - R3.30
Three Dimensional Nanopillar Array Photovoltaics on Low Cost and Flexible Substrates.
Zhiyong Fan 1 2 3 , Haleh Razavi 1 2 3 , Jae-won Do 1 2 3 , Aimee Moriwaki 1 2 3 , Onur Ergen 1 2 3 , Yu-lun Chueh 1 2 3 , Paul Leu 1 2 3 , Johnny Ho 1 2 3 , Toshitake Takahashi 1 2 3 , Lothar Reichertz 2 , Steven Neale 1 3 , Kyoungsik Yu 1 3 , Ming Wu 1 3 , Joel Ager 2 , Ali Javey 1 2 3
1 Electrical Engineering and Computer Sciences, University of California at Berkeley, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Berkeley Sensor and Actuator Center, University of California at Berkeley, Berkeley, California, United States
Show AbstractSolar energy represents one of the most abundant and yet least harvested source of renewable energy. In recent years, tremendous progress has been made in developing photovoltaics (PVs) that can be potentially mass employed. Of particular interest to cost-effective solar cells is to utilize novel device structures and materials processing for enabling acceptable efficiencies. In this regard, here, we report the direct growth of highly regular, single-crystalline nanopillar (NPL) arrays of optically active semiconductors in anodic alumina membrane (AAM) on aluminum substrates which are then configured as solar cell modules. The AAMs are fabricated with nanoimprint technique in conjunction with two-step anodization to achieve large scale perfect hexagonal ordering for NPL array growth. To fabricate PV structure on such membranes, we have grown high density 3D single crystalline n-CdS NPLs with chemical vapor deposition method in AAMs, then embedded the 3D NPL structure in poly-crystalline thin films of p-CdTe, to enable high absorption of light and efficient collection of the carriers. The performance of the cells has been evaluated and shown un-optimized energy conversion efficiency ~6% which is mainly limited by the transmission of top contact. Meanwhile, we have successfully embedded the PV structure in highly flexible PDMS substrates to enable bendable PV devices and the electrical measurements have demonstrated robustness of such devices. Through experiments and modeling, we demonstrate the potency of this approach for enabling highly versatile solar modules on both rigid and flexible substrates with enhanced carrier collection efficiency arising from the geometric configuration of the NPLs.
9:00 PM - R3.31
Interdigitated Thin Film Photovoltaics: Cu2S via Atomic Layer Deposition.
Alex Martinson 1 4 , Jeffrey Elam 2 4 , Michael Pellin 1 3 4
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 4 Argonne-Northwestern Solar Energy Research (ANSER) Center, Argonne, Evanston, Illinois, United States, 2 Energy Systems Division, Argonne National Laboratory, Argonne, Illinois, United States, 3 Department of Chemistry, Northwestern University, Evanston, Illinois, United States
Show AbstractThe conformal growth of a plentiful and economical photovoltaic (PV) material, chalcocite (α-Cu2S), is presented. Inexpensive and abundant photovoltaic materials, in general, have high impurity and defect concentrations. The reduced length over which charges may be extracted from these active layers effectively limits their useful thickness. Surmounting the incongruity between the active layer thickness required for sufficient photons absorption and thickness required for efficient charge extraction in affordable materials may be accomplished by decoupling light absorption and carrier extraction into orthogonal spatial dimensions. Atomic layer deposition (ALD), a layer-by-layer surface synthetic technique capable of evenly coating ultra-high aspect ratio features, is ideally suited for fabrication of these devices. To demonstrate the approach, phase-pure Cu2S films are grown by ALD. Optical and electronic studies of the oriented thin films reveal a 1.2 eV band gap and relatively low (6E19) carrier concentration. The properties of preliminary devices incorporating thin films of the as-grown material will also be discussed.
9:00 PM - R3.32
Improved Efficiency P3HT/ZnO Solar Cells via Lithium Incorporation.
Matthew Lloyd 1 , Yun-ju Lee 1 , Robert Davis 1 , Erica Fang 1 , Julia Hsu 1
1 , Sandia National Labs, Albuquerque, New Mexico, United States
Show AbstractWhile ZnO/polymer heterojunctions offer solution-based deposition and low temperature processing, these devices suffer from very low efficiencies stemming primarily from low short-circuit current (Jsc). Typically, hybrid photovoltaics utilize metal oxide films that are not intentionally doped. Consequently, the conductivity or the position of the Fermi-energy level of the acceptor can vary arising from small changes in the detailed synthesis conditions. We incorporate Li in sol-gel derived ZnO films to investigate the effect on the electrical properties of the metal oxide and the performance of poly-3-hexylthiophene (P3HT)/Li:ZnO solar cells. The Li:ZnO sol gel films were characterized using UPS, AFM, surface contact potential, and UV-vis absorption. By loading ZnO films with 15% (atomic) lithium, we observe a more than two-fold increase in Jsc compared to pristine ZnO/P3HT photovoltaic devices. The increased surface roughness of the Li:ZnO film alone cannot adequately account for the Jsc increase. We also observe a more than doubling in the open-circuit voltage, yielding an increase in overall device efficiency by more than a factor of five. We attribute the increased device performance to an enhancement in the diode characteristics, notably a systematic suppression of the reverse bias current and improvement in ideality factor with increased Li concentration. The specific role of Li is further illuminated in measurements of Au/Li:ZnO/ITO diodes. The current-voltage curves of these diodes reveal a higher barrier height with increasing Li concentration consistent with the decrease in work function measured by UPS and Kelvin probe. Overall, the incorporation of Li yields dramatic increases in the efficiency of bilayer photovoltaic devices and represents a technique that may be adapted to nanostructured architectures.
9:00 PM - R3.35
Photovoltaic Properties of Radial p-n Junctions in High Impurity Silicon.
Akram Boukai 1 , Erik Garnett 4 , Alec Talin 3 , Peidong Yang 2
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 4 , Stanford University, Stanford, California, United States, 3 , NIST, Gaithersburg, Maryland, United States, 2 Chemistry, University of California, Berkeley, Berkeley, California, United States
Show AbstractIn this talk I will present photovoltaic data on devices that consist of nanoscale and microscale radial p-n junctions. The junctions are fabricated on a highly impure silicon wafer. The advantage of using a highly impure silicon source is the low-cost associated with its manufacture. The ultimate goal is to use metallurgical grade silicon feedstock to lower cost while maintaining a high efficiency through radial p-n junction geometries. Enhanced photovoltaic efficiencies are obtained using radial p-n junctions over their planar counterparts.
9:00 PM - R3.36
Enhanced Performance of ZnO/P3HT Solar Cells Modified with Interfacial CdS.
Erik Spoerke 1 , Matthew Lloyd 1 , Erica Martin 1 , Yun-ju Lee 1 , Julia Hsu 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractThe zinc oxide/poly(3-hexylthiophene) (ZnO/P3HT) hybrid solar cell is a promising inexpensive and versatile photovoltaic technology. We describe here the affects of introducing a nanocrystalline CdS film at the ZnO/P3HT heterojunction, creating a cascading energy band structure. The versatile, solution-phase nature of the growth processes used allow this process to be applied to a wide range of nanostructured configurations ranging from planar films to complex branched architectures. Current-voltage characteristics produced under simulated solar illumination show that, compared to ZnO/P3HT devices, CdS-modified devices exhibited approximately a doubling of the open-circuit voltage and a mild increase in fill factor without sacrificing any short-circuit current, resulting in a doubling of device efficiency. Meanwhile, external quantum efficiency spectra reveal distinct photocurrent contributions from the P3HT, CdS, and ZnO layers, confirming the unique cascading band structure of this composite system. In addition to descriptions of the materials system assembled and characterizations of device performance, mechanisms behind the influence of CdS on this system will be discussed. 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.
9:00 PM - R3.38
Broadband Optical Absorption Enhancement in Thin Film Si Solar cells using Periodic and Aperiodic Textures.
Ragip Pala 1 , Justin White 1 , Mark Brongersma 1
1 Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California, United States
Show AbstractA combined computational and experimental study optimizing plasmon-enhanced absorption in Si thin film solar cells is presented. A model system consisting of 2-dimensional periodic /aperiodic arrays of Ag nanostructures on a silica coated Si film supported by a silica substrate is used in the simulations. We develop basic design rules for the realization of broadband absorption enhancements for such structures, by simultaneously taking advantage of 1) the high near-fields surrounding the nanostructures close to their surface plasmon resonance frequency and 2) the effective coupling to waveguide modes supported by the Si film through an optimization of the array properties. In order to verify our computational results, we have fabricated Schottky-Barrier solar cells and metal nanowires of varying shape and periodicity on top of the cell. Large enhancements in the short-circuit photocurrent density have been achieved in agreement with our theoretical predictions.
9:00 PM - R3.39
Growth of Highly Symmetrically Aligned Various shaped-ZnO Nanostructures & Their Application to Exitonic Solar Cells.
Ki Seok Kim 1 , Gun Young Jung 1
1 Material Science & Engineering, Gwnagju Institute of Science & Engineering, Gwangju Korea (the Republic of)
Show AbstractFor highly efficient, low cost and environmentally-friendly sources of energy, the conversion of sunlight into electricity has been a interesting issue. Among different photovoltaic devices, excitonic solar cells (XSCs) are promising candidates since they employ low-cost materials and offer large scale commercialization by inexpensive (solution processing) technique. The most important examples of XSC are Hybrid Solar Cells (HSC) and Dye Sensitized Solar Cells (DSC). In these days, many research groups have utilized the nanostructured electrodes to improve the photo conversion efficiency in the XSCs. In HSCs, an inorganic semiconductor oxide nanoparticles are dispersed within the light-harvesting conjugated polymer matrix. The oxide nanoparticles/polymer blend must be annealed to improve connections between the nanoparticles and to enhance electron pathway to the electrodes. Nevertheless, these devices are still limited by incomplete exciton dissociation due to the isolation of some nanoparticles, which alleviates the efficient electron transport and thus reduces overall device efficiency. The proposed vertically aligned various shaped-ZnO nanostructures (vertical nanorods, nanotubes, ball-like, urchin-like, and fine-tree) provide a higher interfacial area at the interface between the donor and the acceptor (polymer and ZnO, respectively), and therefore produce highly-efficient electron transport pathways. And then, light-scattering in the interfacial area give a choice to harvest more light. In the case of DSCs, the replacement of the nanoparticle electrode with vertically aligned nanostructures emerged as a possible way to obtain faster electron transport. Moreover, the urge to replace problematic liquid electrolytes in DSC by solid hole conductors with slower kinetics forces the incorporation of faster electron transport materials such as the vertically well aligned nanostructures as a electrode. In this presentation, we introduced a method to fabricate vertically aligned various shaped-ZnO nanostructures and reported the improved electrical and optical properties of the HSCs and the DSCs. We used laser interference lithography (LIL) to fabricate regularly arranged PR (Photo resist) nanoscale hole covering an area up to 2 inch2. This nano-scale hole PR structures were used as templates for the hydrothermal growth of vertically aligned various shaped-ZnO nanostructures.
9:00 PM - R3.4
Plasmonic Silver Nanoparticles Incorporated Within the Absorbing Layer of Organic Photovoltaics.
Thomas Reilly 1 , Anthony Morfa 1 2 , Jao van de Lagemaat 1
1 Chemical Sciences and Nanoscience, NREL, Golden, Colorado, United States, 2 Chemistry, University of Colorado, Boulder, Colorado, United States
Show AbstractOrganic photovoltaics show much promise as a means of producing clean electricity. While organic photovoltaics generally have high optical absorption coefficients compared to their inorganic counterparts, the poor electrical transport properties of OPV materials result in significant losses. At very thin film thicknesses, where electrical transport losses are minimized, optical absorption of the active layer is insufficient. In this study, surface plasmons were used as a means to enhance the optical absorption of thin organic solar cells. We have prepared plasmonic silver nanoparticles with a series of insulating layers and incorporated them in to the photo-active layer of organic solar cells. Results will be discussed in terms of how to optimize plasmonic effects within OPV materials given the additional loss mechanisms introduced by such nanomaterials.
9:00 PM - R3.40
Chemistry between Zn2+ and eosinY for self-assembly of ZnO/eosinY Hybrid Materials.
Shigeo Hori 1 , Tsukasa Yoshida 1
1 , Gifu university, Gifu, Gifu, Japan
Show Abstract Addition of surfactants or organic molecules into the bath of chemical bath deposition or electrochemical deposition of ZnO results in a formation of unique nanostructures. These morphological changes are caused by the adsorption of the additives. Thus, their adsorption behavior is significantly important factor to control the structure of ZnO films. In our previous work, we found that electrodeposition of ZnO from aqueous solutions of Zn salts in the presence of eosinY results in electrochemical self-assembly (ESA) of ZnO/eosinY hybrid thin films consisting of nanowire shaped ZnO crystals whose diameter is approximately 10-30nm hybridized with aggregates of eosinY filling the gaps between them. EosinY molecules loaded into the film can be easily extracted by dipping the films in dilute KOH aqueous solution to create interconnected nanopores within ZnO crystal grains. Such a “porous crystalline ZnO” performs as an efficient photoelectrode material for dye-sensitized solar cells and is applicable for plastic substrates as the process does not need high temperature heating. In the present study, adsorption behavior of eosinY on ZnO and its complex formation with Zn2+ were studied in detail, changing the parameters relevant to the ESA of ZnO/eosinY hybrid thin films, such as Zn2+ concentration, temperature, and pH. EosinY typically undergoes Lamgmuir type (monolayer) adsorption onto ZnO surface. However, in the presence of Zn2+, the amount of eosinY adsorbed onto ZnO particles clearly exceeded the saturation of a monolayer, indicating a formation multi layer of dyes. Such experiments were accompanied with the decrease of concentrations of not only eosinY but also Zn2+ in the solution, suggesting that the multilayer in fact is their mixed salt. Red precipitates were obtained when ZnCl2 was added to highly concentrated (30mM) eosinY solution. The chemical analysis of the precipitate indeed confirmed the presence of Zn2+ in approximately 0.8 molar equivalent to that of eosinY, irrespective of the composition of the mixed solutions. Aggregation of eosinY with Zn2+ was further enhanced when the temperature of their mixture was elevated. In the absence of Zn2+, increase of pH resulted in a decrease of adsorbed eosinY. In the presence of Zn2+, however, pH increase lead to an increase of eosinY. These aggregation behavior of eosinY in the presence of Zn2+ compares with the formation of the nanopores during the ESA of the hybrid film.
9:00 PM - R3.41
Environmental and Processing Sensitivity of Lead Chalcogenide Nanocrystal Solar Cells.
Octavi Semonin 1 2 , Matthew Beard 2 , Barbara Hughes 3 2 , Aaron Midgett 3 2 , Hugh Hillhouse 4 , Arthur Nozik 2
1 Physics, University of Colorado, Boulder, Colorado, United States, 2 Chemical Sciences and Nanoscience, National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Chemistry, University of Colorado, Boulder, Colorado, United States, 4 Chemical Engineering, Purdue University, West Lafayette, Indiana, United States
Show AbstractSolar cells developed from quantum dot (QD) films are promising both for their ease of manufacture and for their potential enhanced photon conversion efficiencies. The potential enhanced efficiency is based on multiple electron-hole pair generation from single photons, a process we term Multiple Exciton Generation (MEG); it has been observed in transient absorption, THz, and photoluminescence studies. Recently, the measurement of MEG has been subject to some controversy, which we believe is a result of varying sample preparations and consequential differences in surface chemistry; concerns about the effects of QD charging have also been reported recently. A demonstration of MEG effects in the photocurrent from QD devices would help resolve much of the controversy. In previous work we have shown that ethanedithiol-treated PbSe and PbS QD assemblies produce Schottky junction solar cells with very high photocurrents (25-35 ma/cm2) and with internal quantum efficiencies in excess of 80% in the visible and UV regions of the spectrum; but we have not yet observed a clear indication of MEG-enhanced photocurrent. Moreover, the development of these QD solar cells has been hindered by the large variation in the sample preparation environment and processing of the NC films. The structure and function of these devices can vary wildly with factors such as oxygen and humidity exposure, solvent impurities, surface chemistry, and the details of the film assembly. The impact and some proposed mechanisms of these environmental and processing effects will be discussed, along with progress on observing improved device efficiency and enhanced photocurrent generation via MEG.
9:00 PM - R3.42
Photonic Crystal Geometry for Organic Solar Cells.
Doo-Hyun Ko 1 , John Tumbleston 2 , Lei Zhang 1 , Stuart Williams 1 , Joseph DeSimone 1 , Rene Lopez 2 , Edward Samulski 1
1 Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States, 2 Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Show AbstractPhotonic crystal solar cells have the potential for addressing the disparate length scales in polymer photovoltaic materials, thereby confronting the major challenge in solar cell technology: efficiency. One must achieve simultaneously an efficient absorption of photons with effective carrier extraction. Unfortunately the two processes have opposing requirements. Efficient absorption of light calls for thicker PV active layers whereas carrier transport always benefits from thinner ones, and this dichotomy is at the heart of an efficiency/cost conundrum that has kept solar energy expensive relative to fossil fuels.This dichotomy persists over the entire solar spectrum but increasingly so near a semiconductor’s band edge where absorption is weak. We report a 2-D, photonic crystal morphology that enhances the efficiency of organic photovoltaic cells relative to conventional planar cells [1]. The morphology is developed by patterning an organic photoactive bulk heterojunction blend of Poly(3-(2-methyl-2-hexylcarboxylate) thiophene-co-thiophene) and PCBM via PRINT™, a nano-embossing method that lends itself to large area fabrication of nanostructures [2]. The photonic crystal cell morphology increases photocurrents generally, and particularly through the excitation of resonant modes near the band edge of the organic PV material. The device performance of the photonic crystal cell showed a nearly doubled increase in efficiency relative to conventional planar cell designs [1].[1] Ko, D.-H.; Tumbleston, J. R.; Zhang, L.; Williams, S.; DeSimone, J. M.; Rene, L.; Samulski, E. T. Nano Lett. in press[2] Hampton, M. J.; Williams, S. S.; Zhou, Z.; Nunes, J.; Ko, D.-H.; Templeton, J. L.; Samulski, E. T.; DeSimone, J. M. Adv. Mater. 2008, 20, 2667.
9:00 PM - R3.43
First Principles Calculations and Experimental Assessment of Complex Intermediate Band Materials for Photovoltaic Devices.
Perla Wahnon 1 , Irene Aguilera 1 , Pablo Palacios 1 , Kefren Sanchez 1 , Raquel Lucena 2 , Daniel Gamarra 2 , Jose Carlos Conesa 2
1 Instituto de Energía Solar & Tecnologias Especiales, Universidad Politecnica de Madrid, ETSI Telecomunicacion, Madrid, Madrid, Spain, 2 , Instituto de Catalisis y Petroleoquimica, CSIC, Cantoblanco, Madrid, Spain
Show AbstractThin-film chalcogenide semiconductors are spreadly used nowadays in photovoltaic technology as light absorbers or window or buffer layers. We have recently proposed that the CuGaS2 chalcopyrite and In2S3 or MgIn2S4 thio-spinels, semiconductors having band-gap width ≥ 2.0 eV and thus far from being optimal as absorbers themselves, could provide maximal efficiency if Ga or In are partially substituted by some light transition metals. In these cases a partially-filled intermediate band appears that allows electron excitation across the full band gap using two low energy photons. The theoretically predicted band structure and optical properties of such materials have received support from experimental data. In this work we explore other material candidates to achieve the same features. Other type of spinels as CdIn2S4, layered sulphides as ZnIn2S4, disulphides of tetravalent elements and Si clathrate, are thus considered. Density functional theory (DFT) calculations reveal that several of these materials can provide as well the desired intermediate band characteristics (partial occupancy, band character, separation in energy from valence and conduction bands, absorption spectra in extended wavelength ranges). The experimental preparation of some of these new materials in polycrystalline form has been carried out using solvothermal methods, and the UV-Vis-NIR absorption spectra confirm the presence of sub-bandgap light absorption features in agreement with DFT predictions.
9:00 PM - R3.44
Reduction of Power Loss Mechanisms in InAs/GaAs QD Concentrator Solar Cell Grid Design.
Stephen Polly 1 , Christopher Bailey 1 , Michael Harris 1 , David Forbes 1 , Ryne Raffaelle 1 , Seth Hubbard 1
1 NanoPower Research Labs, RIT, Rochester, New York, United States
Show AbstractIncreasing the performance of III-V solar cells has a significant importance for the concentrator photovoltaics community. Efficiency can be increased by spectral tuning with quantum wells or quantum dots (QD), to more effectively match the material bandgap to the efficiency peak of a detailed balance calculation. In order for these effects to be seen, however, other device parameters must be optimized to reduce power loss mechanisms. Current density increases with concentration, and as power loss increases as the square of current, resistive elements in the cell rapidly become major sources of loss. Reduction of power loss mechanisms can be accomplished in the design of both the cell itself and how it is fabricated. Material properties such as emitter doping and base thickness can be modified to determine an optimal operating point at a specific concentration level. The design of the front contact grid is also critical to optimal performance. Proper design allows the resistance encountered within the emitter as current travels laterally before being collected by a grid finger, and the resistance of the grid fingers themselves, to be mitigated. A balance must exist between the thickness of the grid, the resistivity of the metallization used, and the grid design itself so that no one element acts as a bottleneck to the rest of the device. As grid fingers are brought closer together, series resistance in the emitter is reduced, but shadowing loss is increased, reducing power output. In this study, various grid designs were created based on a mathematical compensation between lateral emitter resistance loss and shadowing loss. These findings were then used to apply to fabricated GaAs cells with varying grid finger thicknesses, using state of the art electroplating techniques. Cells were grown with and without multiple stacked layers of strain compensated QDs. Devices were characterized with a large area pulsed solar simulator, using geometrical factors to calculate concentration values. Additional characterization includes spectral responsivity showing the response and contribution of the QDs, as well as Isc-Voc measurements to determine lumped series resistance. The devices were also simulated using SPICE to determine the operating point of highest efficiency.
9:00 PM - R3.5
Plasmon-Enhanced Thin Film Si Solar Cells.
Rene de Waele 1 , Maarten Hebbink 1 , Fiona Beck 2 , Marc Verschuuren 3 , Kylie Catchpole 2 1 , Albert Polman 1
1 Photonic Materials, FOM Amolf, Amsterdam, Noord-Holland, Netherlands, 2 , Australian National University, Canberra, New South Wales, Australia, 3 , Philips Research, Eindhoven Netherlands
Show AbstractDue to its indirect bandgap, silicon is a poor absorber in the near-infrared. As a result, Si solar cells require a wafer thickness of several 100 μm to fully absorb sunlight in the 800-1100 nm spectral range, leading to high materials costs. We demonstrate how suitably engineered arrays of metal nanoparticles deposited on a thin-film Si solar cell can lead to strong light trapping in the cell, dramatically enhancing its red response. The results are applicable to any thin-film semiconductor solar cell and can lead to solar cells designs with a thickness well below 1 μm, which might also give rise to an enhancement of the efficiency due to the increased optical concentration in the cell.
We present the fundamental principles for light scattering from plasmon particle-enhanced solar cells. We show that light interacting with Ag nanoparticles on the surface of a Si wafer is preferentially scattered into the wafer, at a fraction as high as 96% for a point dipole. We calculate the scattering fraction for 10-200 nm metal nanoparticles at the plasmon resonance and calculate the corresponding path length enhancement in the cell. Simulations are all done for cells with finite thickness, representing the thin-film Si case. We take into account interparticle coupling, and grating diffraction as well as coupling to the Si waveguide modes.
We show that cylindrical and hemispherical particles give rise to much higher path length enhancements than spherical particles, due to enhanced near-field coupling. The scattering cross-section of the particles is very sensitive to the thickness of a spacer layer at the substrate, due to interference effect in the driving field and variations in the local density of states. Coupling of the nanoparticles also leads to strong modification of the effective albedo, enabling further tuning of the light trapping.
At frequencies above the plasmon resonance, Fano interference effects reduce the coupling into the thin cell, an effect that can be minimized by optimizing the effective nanoparticle scattering cross section. These interference effects are absent for solar cells with particles arranged at the backside, as this geometry allows for absorption of the higher-energy photons before sunlight reaches the metal particles. A systematic comparison between cells with particles on the front and back is presented.
Finally, we demonstrate a novel soft-stamp nanoimprint lithography technique. With Substrate Conformal Imprint Lithography (SCIL) large arrays of Ag nanoparticles can be patterned on wafer-scale areas. The soft stamp method also enables patterning on finished thin-films solar cells, demonstrating the full potential of metal particle scattering in thin-film photovoltaics.
9:00 PM - R3.6
Effect of Barrier Thickness on Interband Transition Energies of InAs QD / GaAs Solar Cells.
Christopher Bailey 1 , David Forbes 1 , Michael Harris 1 , Stephen Polly 1 , Seth Hubbard 1 , Ryne Raffaelle 1
1 , RIT - NanoPower Research Labs, Rochester, New York, United States
Show AbstractIn the last decade, quantum dots (QDs) have gained the attention of the solar cell community for possible device enhancement purposes. Inserting layers of vertically stacked QDs into the i-region of a p-i-n solar cell has the potential benefit of realizing the intermediate band solar cell (IBSC) by facilitating mini-band formation. For this to occur with a reasonable bandwidth and density of states, sufficient QD-QD vertical coupling must be present. To ensure QD coupling, the energy barrier must be thin enough for the wavefunctions of one QD to overlap with those of neighboring QDs. In this study, wavefunction overlap was modeled using the Silvaco ATLAS device simulator program. Because the model is 1D, QDs were modeled as InAs quantum wells within GaAs barriers. This is justified for the quantum dot samples used in this study because of their high aspect ratio (6 x 25 nm, height x diameter). Thus, the energy of a single quantum dot will be dominated by confinement along the growth direction. It is for this reason that a QW model was sufficient for qualitative determination of wavefunction overlap versus barrier thickness. This was used to estimate the threshold of significant electron wavefunction overlap. Epitaxially grown p-i-n devices were fabricated with barrier thicknesses within this threshold and photocurrent and electroluminescence measurements were taken and their results discussed. The Stranski-Krastanow mode of QD growth used here exploits the lattice mismatch and compressive strain (7.2%) inherent in the InAs/GaAs system. Layers of GaP (3.2% tensile strained to GaAs) are inserted between QD layers to compensate for the compressive strain. Layers of high-temperature (HT) and low-temperature (LT) GaAs were used as spacers and capping layers between the InAs and GaP. The HT GaAs was varied resulting in total barrier thicknesses, including GaP, of 14 to 75 ML, respectively. Cells studied have 5 and 10 repeating sets of these layers within the i-region. External quantum efficiency measurements and electroluminescence were obtained for all samples. The carrier transitions are clearly identified with a GaAs peak near 870 nm, a wetting layer at ~ 910 nm, and four QD peaks at various wavelengths indicating transition energy values. An overall red-shift is observed for all QD transitions with decreasing barrier thicknesses. This indicates decreased energy transition values for thinner barrier layers, possibly due to miniband formation. Future QD test structures with a wider distribution of barrier thicknesses will be grown and tested using photoluminescence and trends extracted. Strain-modified k.p theory will be used to verify experimentally determined transitions. High resolution x-ray diffraction will be shown for all samples to determine the overall strain in the material. Modification of the ATLAS model to improve its representation of real devices is warranted based upon this study and will be discussed.
9:00 PM - R3.7
Absorption Limited Performance of Quantum Confined p-i-n Intermediate Band Solar Cells.
Alex Freundlich 1 , Andenet Alemu 1 , Andrea Feltrin 1
1 Center for Advanced Materials, University of Houston, Houston, Texas, United States
Show AbstractIntermediate band solar cells (IBSC) hold the promise of overcoming the Shockley-Queisser efficiency limit for conventional solar cells, and detailed energy balance calculations predict efficiencies in excess of 63% under maximum sunlight concentration (>40% under natural 1 sun AM0) . The predicted efficiency improvement is obtained by introducing an intermediate band in the optical gap region of a material and exploiting two-step photo-absorption processes. Different approaches have been attempted to synthesize such a solar cell and semiconductor nanostructures (as quantum wells, wires or dots) have been recognized as potential candidates. Although experiment confirms the existence of two photon carrier photo-generation, recorded practical efficiencies in quantum-confined p-i-n diodes and in particular in so called quantum dot solar cells, deceivingly, fail to perform at projected levels.These efficiency projections are thus far calculated using Detailed Energy balance models that assume that all photons below the energy gaps (conventional or intermediate) are fully absorbed and participate in the photo-generation process. While this approximation is reasonable for bulk like solar cells with relatively thick absorber, here we show that for quantum-confined solar cells such assumptions results in a significant overestimation of efficiency limits. In this work realistic band structure and absorption property calculations are integrated with conventional IBSC model for a more pragmatic and accurate evaluation of quantum confined p-i-n solar cells. The model takes into account realistic design parameters such as actual device thickness, nanostructure dimensions and distributions, The model is examined for conventional Kane-like semiconductors heterostructures (i.e. the archetype In(Ga)As/GaAs system) as well as, for highly electronically mismatched semiconductor (i.e. dilute nitride alloys of III-V). The presentation elucidates the physical origin of rather disappointing experimental data on the much sought after quantum dot solar cells, which, at least as they are currently designed, seem to be thermodynamically condemned to not exceed the efficiency of their conventional single junction counterpart. The presentation also provides a clear set of minimal design rules and examples that may allow quantum-confined IBSC to significantly exceed the Shockley- Queisser efficiency limit.
9:00 PM - R3.8
Enhancement of Optical Absorption Efficiency in Thin-film Organic Photovoltaic Solar Cells through the Excitation of Plasmonic Modes in Metallic Gratings.
Changjun Min 1 , Jennifer Li 1 , Georgios Veronis 2 1 , Jung-Yong Lee 3 , Shanhui Fan 3 , Peter Peumans 3
1 Center for Computation and Technology, Louisiana State University, Baton Rouge, Louisiana, United States, 2 Electrical and Computer Engineering, Louisiana State University, Baton Rouge, Louisiana, United States, 3 Electrical Engineering, Stanford University, Stanford, California, United States
Show AbstractThin-film organic photovoltaic solar cells are a promising candidate for low-cost energy conversion. However, significant improvement in the efficiency of the cells is required to make them competitive with grid power. In thin-film organic photovoltaic cells the thickness of the active layer must be smaller than the excitonic diffusion length. This, however, limits the photon absorption efficiency. In this paper, we theoretically investigate the effect of introducing metallic gratings in thin-film organic photovoltaic solar cells on their optical absorption efficiency. More specifically, we consider organic solar cells in which the top transparent electrode is partially substituted by a periodic silver grating. We use a full-wave finite-difference frequency-domain electromagnetic simulation method to calculate the optical absorption in such structures, and the absorption enhancement with respect to solar cells without gratings. We find that for TM polarized light the metallic grating can result in broadband optical absorption enhancement. The increase in light absorption is due to the large field enhancement in the vicinity of the metallic grating, associated with the excitation of plasmonic modes. We provide a detailed analysis of the plasmonic modes which result in increased absorption for TM light. For TE polarized light the metallic grating results in slight suppression in optical absorption. The absorption enhancement for TM light is much larger than the absorption suppression for TE light, so that the overall optical absorption is greatly enhanced by the metallic grating. We find that, if the grating parameters are optimized, the overall optical absorption can be enhanced by ~40% under AM1.5 illumination. We also find that the absorption enhancement due to the metallic grating increases as the active layer thickness is decreased. Finally, we investigate the effect of the shape of the metallic grating on the absorption enhancement.
9:00 PM - R3.9
The Effect of Plasmonic Scattering in Photovoltaics: A Comparison Between Metallic Nanoparticle Scatterers and Antireflection Coatings.
Jeremy Munday 1 , Vivian Ferry 1 , Harry Atwater 1
1 Thomas J Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California, United States
Show AbstractIntense research efforts have been put in place to improve the performance of photovoltaic devices. Recently, light management schemes involving the addition of plasmonic scattering objects have been proposed as a way to reach this goal; however, traditional antireflection coatings are often neglected in such calculations and experiments. Here we present a detailed comparison between solar cells covered by antireflection coatings and ones covered by metallic nanoparticles through finite difference time domain (FDTD) simulations over the AM 1.5 spectrum and as a function of incident angle. We find significant absorption enhancements (factor of 5-10) for longer wavelengths over a wide range of incident angles.Unlike previous simulations, we include a 5-10 nm SiO2 passivation layer between the particles and an absorbing Si region and antireflection coatings, as would be necessary in a practical cell. For the antireflection coating, we use a silicon nitride layer chosen to provide maximum absorption within the cell when weighted over the solar spectrum. Absorption enhancements of a factor of 5-10 are found for longer wavelengths at both normal incidence and at 60 degrees from normal incidence. Simulations are carried out for both ultrathin (200nm) Si cells and optically thick solar cells. In ultrathin cells we find increased absorption by increased scattering and waveguiding; however, for the optically thick cells, there are only improvements related to plasmonic scattering. We further discuss cells that incorporate both plasmonic scatterers and antireflection coatings to produce larger absorption enhancements than either of these strategies would individually. As a proof of principle, we will also discuss experimental results for ultrathin GaAs cells that incorporate plasmonic scatterers and antireflection coatings. Devices that incorporate these principles allow one to engineer the absorption for particular wavelengths and tailor the cell for maximum performance under a variety of conditions and angles of incidence.
Symposium Organizers
Arthur J. Frank National Renewable Energy Laboratory
Nam-Gyu Park Sungkyunkwan University
Tsutomu Miyasaka Toin University of Yokohama
(and Peccell Technologies, Inc.)
Laurie Peter University of Bath
Songyuan Dai Chinese Academy of Sciences
R4: Nanostructured Sensitized Solar Cells I
Session Chairs
Laurie Peter
Sylvia Tulloch
Tuesday AM, December 01, 2009
Room 312 (Hynes)
9:30 AM - **R4.1
Optimizing Photon Harvesting and Carrier Collection in Mesoscopic Solar Energy Conversion Systems.
Michael Graetzel 1
1 ISIC LPI, EPFL Lausanne, Lausanne Switzerland
Show AbstractThe performance of solar energy conversion devices employing mesoscopic photoelectrodes depends critically on the nanostructure. This is evident for the dye sensitized solar cell (DSC) where charge percolation through the TiO2 to the transparent conductive (TCO) electrodes takes milliseconds. Slow charge extraction increases chances of electron-hole recombination at the mesoporous TiO2 - electrolyte interface, and limits DSCs to be used with only a handful of electrolytes that offer low recombination rates. In addition, hematite photoelectrodes used for solar water splitting have been limited by poor photon absorptivity and low carrier mobility. These limitations can be overcome with advanced nanostructuring techniques. Here we describe our latest advances in optimizing the photon harvesting and the charge transport in these photoelectrochemical energy conversion systems by applying novel nanostructures to the materials used in the DSC and solar water splitting as well as applying composite nanostructures with new materials.
10:00 AM - **R4.2
Seven Subjects Regarding DSSCs: Water Electrolytes, Iodine Binding to Dyes, Regeneration Efficiencies, Recombination Rates In Light and Dark, Diffusion Lengths, Activation Energies, and Surface Charge, and Some Suggestions For Improving Efficiency.
Brian O'Regan 1 , James Durrant 1 , Piers Barnes 1 , Xiaoe Li 1 , Andrea Listorit 1 , Assaf Anderson 1 , Chun Law 1 , Mindaugas Juozapavicius 1 , Florent Deledalle 1 , Tarek Ghaddar 2 , Emilio Palomares 3
1 Chemistry, Imperial College London, London United Kingdom, 2 Chemistry, American University of Beirut, Beirut Lebanon, 3 , Institute of Chemical Research of Catalonia, Tarragona Spain
Show Abstract A range of new results concerning Dye Sensitized Solar Cells (DSSCs) will be summarized. These results span from manufacturability issues to theoretical description. Results to be discussed include:- Water based electrolytes, or at least water insensitive electrolytes, for DSSCs could simplify manufacturing and improve electrolyte stability. Using new hydrophobic dyes we show it is possible to use up to 60% water in the electrolyte without decreasing the efficiency. Water decreases Jsc slightly, but increases the oxidation potential of iodide, increasing the Voc. Organic solvent at the 20% level is still required for cell function. - We present the binding constants of iodine to standard and new dyes. The results show that catalysis of the recombination depends on more than just the binding strength. A correlation between binding constants, recombination rates, and Voc is established for specific dyes with and without iodine binding sites. - To establish a baseline on the regeneration of the oxidized dye by iodide, we present the dependence of the regeneration rate (and cell Jsc) on the iodide concentration, the dye oxidation potential, and the electrolyte redox potential.- We have measured the charge (by charge extraction) across the JV curve in both light and dark. We show that visible illumination of DSSCs causes an ~2-fold increase in the recombination rate constant relative to the dark condition. This simple data set allows us to calculate the recombination losses at short circuit. The data also allow us to model simple and complex JV shapes (eg. S-shaped) and to evaluate mechanisms for the changes in JV with accelerated aging.- We show conclusively that the previous method of determining diffusion lengths in DSSCs, based on transient or impedance measurements, over estimates the diffusion length considerably. A method based on front and back illumination spectral response measurements gives diffusion lengths ~2.5 fold shorter. The shorter diffusion lengths are in agreement with photocurrent results from cells with varying thickness and iodine concentration.- We use temperature dependence to measure the activation energy of recombination as a function of Voc. The results give an estimate of the conduction band edge potential and changes therein with illumination and/or charging. Activation energies for recombination determined from the dark current and the dark electron density (as a function of voltage) are different from those determined at Voc under light. These results can help explain the ideality of DSSCs.- Almost all aspects of DSSCs (e.g. the injection rate, regeneration rate, and Voc) depend on the TiO2 surface charge and the associated electric field between the TiO2 and the electrolyte. Yet this surface charge is never measured. Streaming potential offers a method to measure the surface charge, and shifts therein, in situ. Custom apparatus and initial results will be shown.
10:30 AM - R4.3
A General Strategy for Preparing Transparent Oriented TiO2 Nanotube Arrays for Dye-sensitized Solar Cells.
Jin Young Kim 1 , Kai Zhu 1 , Adam Halverson 1 , Nathan Neale 1 , Arthur Frank 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractWe describe a general approach for preparing films of transparent TiO2 nanotube (NT) arrays for dye-sensitized solar cells (DSSCs or Grätzel cells). This approach addresses a major issue limiting the solar conversion efficiency of TiO2 NT-based DSSCs. Typically in such cells, the incident light enters the cell through the counter electrode side, which consists of conducting glass (FTO: fluorine doped SnO2) covered with a Pt catalytic layer. This illumination geometry leads to significant loss of solar conversion efficiency owing to light attenuation by the Pt layer and triiodide/iodide redox electrolyte as well as a decline of the electron collection efficiency.[1] For example, dye-sensitized TiO2 nanoparticle solar cells exhibit as much as 30% efficiency loss when the cells are illuminated from the counter electrode side instead of from the transparent electron collector side on which the nanoparticle films are deposited.[2] In the case of TiO2 NT based DSSCs, the necessity to illuminate the cell through the counter electrode is due to the presence of Ti metal on which the NTs are formed. Normally, the arrays are prepared by electrochemically anodizing Ti foil. The Ti foil serves as the electron collector. It has been recently shown that TiO2 NT arrays can be prepared by anodizing a thin Ti film deposited on FTO.[3] However, this approach has limitations. For example, if the anodization time were too short, the film would not be transparent owing to the presence of the remaining Ti layer. If the anodization time were too long, the anodization process would corrode the conducting glass. Also, the synthetic conditions used for these approaches impose constraints on the length of the NTs. In this presentation, we discuss a simple strategy for making transparent TiO2 NT films on FTO substrates. We show that by depositing a protective TiO2 layer on the FTO first, the Ti metal film can be anodized completely––without degrading the FTO substrate––to produce a transparent as-deposited NT film. Annealing of this film in air results in the formation of TiO2 NT arrays that are pure anatase. We describe the procedure for preparing transparent TiO2 NT films on FTO, the protection mechanism by which it works, and the dynamics of electron transport and recombination in and the properties of DSSCs incorporating such films. These and other results will be discussed. [1] J. van de Lagemaat and A. J. Frank, J. Phys. Chem. B 105, 11194 (2001). [2] S. Ito et al., Chem. Commun., 4004 (2006). [3] G. K. Mor et al., Nano Lett. 6, 215 (2006).
10:45 AM - R4.4
Enhanced Efficiency Dye Sensitized Solar Cells Through Acid Pre-treatment.
David Worsley 1 , Trystan Watson 1 , Daniel Bryant 1
1 Materials Research Centre, Swansea University, Swansea United Kingdom
Show AbstractA great deal of research is currently directed at improving the efficiency of dye sensitized solar cells. This work presents a simple pre-treatment using nitric acid that is shown to increase the efficiency of a standard dye sensitised solar cell mounted on a conducting glass substrate from 4.6% to 5.6%, an effective enhancement of 22%. The acid pre-treatment was achieved by immersing a conducting glass electrode coated with commercial Titania paste in nitric acid for 1-60 minutes prior to sintering at 450 degrees celsius. The nitric acid treatment alters the nature of the surface of the Titania layer deposited creating a scattering effect which increases efficiency without the need for any complex post treatment steps. The anion of the acid appears critical in that anions that can adsorb strongly on the Titania, that are not removed on sintering, block dye absorption and reduce cell efficiency.
11:30 AM - R4.5
Organo-metal Quantum Dot Sensitizers for Mesoscopic TiO2 Solar Cell.
Tsutomu Miyasaka 1 2 , Akihiro Kojima 2 , Masashi Ikegami 1
1 Graduate School of Engineering, Toin University of Yokohama, Yokohama, Kanagawa, Japan, 2 Graduate School of Arts and Science, The University of Tokyo, Tokyo Japan
Show AbstractPerovskite type nanocrystalline quantum dot, CH3NH3PbX3, achieved energy conversion efficiency of 3.8% as visible light sensitizers on mesoporous TiO2 photovvoltaic cells. The lead halide-based perovskite nanocrystals were prepared on mesoporous TiO2 by spin coating of precursor solutions followed by self organization for crystallization. Perovskite-sensitized photoelectrochemical cells were assembled with bromide or iodide containing organic electrolytes. IPCE exhibited action spectra covering visible wavelength regions up to 800 nm for lead iodide perovskite. Lead bromide perobskite-sensitized cell, with IPCE response up to 580 nm, gives high open-circuit voltage exceeding 0.95V. With a maximum of 65%, a plateau of IPCE indicates that incident photons are strongly absorbed by the thin film (8 μm) of perovskite, which is generally difficult to realize with conventional organic sensitizers. Organic electrolyte solution employed redox couple of halogen/halide. Work function analysis on photoelectron spectroscopy for spin-coated polycrystalline films showed valence the band levels of perovskite bromide and iodide being around 5.38 eV and 5.44 eV vs. vacuum level, respectively. Electrochemically, these valence band levels are considered to be more positive than oxidation potentials of corresponding halide in electrolyte, which, depending on halide concentration, are 5.1-5.6 eV for Br2/Br-, 4.5-5.0 eV for I2/I-.Photocurrent-voltage (I-V) performance showed a highest efficiency, 3.8%, for lead iodide perovskite sensitizer, which is significantly high as a photovoltaic cell using non-organic sensitizers. Use of inorganic sensitizers is also promising in exploring the method to earn high voltage with the TiO2 sensitization technology. Organic-inorganic perovskite materials are under investigation which exhibit different energy gaps depending on combination of metal and halogen.
11:45 AM - R4.6
Oriented TiO2 Nanotube Arrays for Dye-Sensitized Solar Cells: Effect of Post-Growth Annealing on Structure, Light Harvesting, and Charge Transport and Recombination.
Kai Zhu 1 , Nathan Neale 1 , Adam Halverson 1 , Jin Young Kim 1 , Arthur Frank 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractThere is growing interest in understanding the electrical and optical properties of dye-sensitized nanocrystalline solar cells (DSSCs) based on ordered nanostructured architectures, such as oriented TiO2 nanotube (NT) arrays that are aligned perpendicular to the charge-collecting substrate. In this presentation, we report on the effect of post-growth annealing condition on the relationship of the oriented TiO2 NT morphology to the electron dynamics (charge transport and recombination) and the photovoltaic properties of NT-based DSSCs. Nanotube arrays were prepared by electrochemically anodizing titanium foil in a fluoride-containing electrolyte. The as-deposited amorphous NT films were converted to polycrystalline material by annealing. Charge transport and recombination properties of the NT-based DSSCs were studied by frequency-resolved modulated photocurrent/photovoltage spectroscopies. The annealing condition is found to affect significantly the morphologies and crystal structures of the NT arrays. These structural changes have a strong influence on the electron dynamics and device properties (e.g., light absorption and charge collection). These results and others are discussed.
12:00 PM - R4.7
Application of Doped-Materials in Dye-Sensitized Solar Cells.
Huajun Tian 1 , Changneng Zhang 1 , Linhua Hu 1 , Songyuan Dai 1
1 , Institute of Plasma Physics, Chinese Academy of Sciences, Hefei China
Show AbstractSolar cells based on dye-sensitized nanoporous films of TiO2 are low cost alternatives to commercial solar cells based on silicon. It is necessary to further improve the energy conversion efficiency and the lifetime of dye-sensitized solar cells (DSC), in order to commercialize DSC successfully. The nanoporous semiconductor electrode, as one of the more puzzling components of DSC, received various concentrations during the past decades. The modification of anodic materials, such as doped-titania, is supposed to be an important method to improve the performance of DSC. In our previous works, we introduced non-metal doped titania into DSC to stabilize the solar cells due to the replacement of oxygen deficient titania, and the results show that nitrogen, as the most effective dopant, improved the stability of the DSC obviously. It is also shown that nitrogen-doped titania shift the conduction band edge of TiO2 toward a negative potential and photovoltage is enhanced. At the same time, the lanthanide ions doped in TiO2 nanoparticles could suppress the combination of electron-hole pairs and prolong the lifetime of carriers, which is benefit to improve the efficiency of both the net charge transfer and the separation of carriers at the semiconductor / electrolyte interface. The application of other doped-materials including some transition metal ions-doped nanocrystalline titania photoelectrodes in DSC was discussed in this paper.
12:15 PM - R4.8
Fabrication of Highly Efficient 10cm-by-10cm DSC Sub-Modules and Their Long-Term Stability.
Hironori Arakawa 1 , Takeshi Yamaguchi 1 , Kyouhei Noguchi 1 , Yutaro Koishi 1
1 Department of Industrial Chemistry, Tokyo University of Science, Tokyo Japan
Show AbstractDye-sensitized solar cell (DSC) is recognized as one of next-generation solar cells in terms of their potentially low production costs and relatively high efficiencies. The best efficiency reported so far is 11 to 12 %. However, the reported solar cell efficiencies of DSC modules and sub-modules are around 6 to 7%. In order to commercialize DSC, improvement with both solar cell efficiency and long-term stability of DSC sub-module is essential. We have been conducting R&D on fabrication of efficient 10cm-by-10cm DSC sub-modules and their accelerated long-term stability tests. [1,2 ] We report here the fabrication of highly efficient 10cm-by-10cm DSC sub-module with 10.3% efficiency based on the active area of 78.20cm2. In addition, the advanced research result of accelerated long-term stability tests using DSC sub-modules is reported.The fabricated 10cm x 10cm sub-module has a current-collecting structure composed of Ag grids, protected by glass frit and polymer over-layers against iodine compounds in electrolyte solution. Multi-layered TiO2 photoelectrodes were prepared by screen printing of TiO2 pastes including light scattering TiO2 large particles with different ratio, followed by calcination. As photosensitizers, Black dye and N719 dye were used. The purification of dyes is very important to obtain a high efficiency of DSC. The electrolyte was composed of a mixture of 0.005M-I2, 0.1M-I2, 0.6M-DMPImI and 0.5M-TBP in AN or MPN solvent. As a counter electrode, Pt sputtered FTO/glass or Ti foil was used. The solar cell performance of DSC sub-module was measured by a super solar simulator (WACOM WXS). By the optimization of TiO2 photoelectrode, Ag grids, dye purification and cell fabrication atmosphere, the best efficiency such as ηac=10.3%, Jsc=21.6mA/cm2, Voc=0.69V and ff=0.69 was obtained. The verified solar cell efficiency of our 10cm-by-10cm DSC sub-module by PCRC, AIST, Japan was ηap =8.3% (aperture area=82.9cm2), Isc=1.54A, Voc=0.71 and ff=0.63. This efficiency is the world record as verified efficiency of DSC sub-modules with more than 10cm-by-10cm size. Environmental and endurance tests for a-Si solar cell modules, JIS C-8938, were applied to our DSC sub-modules. These include a dry heat cycle test (-40 C to 90 C, 200 cycles), a heat and humidity cycle test (-40 C to 90 C, 85 % humidity, 10 cycles) and a light soaking test (255 W/m2 (300 nm to 700 nm), 500 h). Good stability suggesting a promising future to industrialization of DSC was obtained for the 10cm-by-10cm DSC sub-module. Detailed results will be introduced. Refferences[1]H. Arakawa, T. Yamaguchi, S. Agatsuma, T. Sutou and Y. Koishi, Proc.of 23rd EU-PVSEC, 207 (2008). [2] H. Arakawa, T. Yamaguchi, S. Agatsuma, T. Sutou, Y. Koishi, N. Tobe, D. Matsumoto and T. Nagai., Abstract of MRS fall meeting 2008, QQ9.1, pp-1070 (2008).
12:30 PM - R4.9
Use of Anodic TiO2-nanotubes in Dye-sensitized Solar Cells.
Robert Hahn 1 , Doohun Kim 1 , Kiyoung Lee 1 , Poulomi Roy 1 , Patrik Schmuki 1
1 University Erlangen, Department of Material Science, Institute of Surface Science, Erlangen Germany
Show AbstractThe presentation deals with the use of a high aspect ratio titanium dioxide nanotube layers, grown by anodization of Ti metal in dye-sensitized solar cells [1]. The highly ordered geometry of the nanotubes represent a highly suitable architecture for solar energy conversion. In particular these structures can exhibit enhanced electronic properties in comparison with conventional nanoparticulate layers, such as a better electron transport and lower recombination rates [2-4].TiO2 nanotubes can be grown by self-organized anodization or alternatively by rapid breakdown anodization. The lengths of the nanotubes are in the range of several micrometers up to few dozens, with single tube diameters of several tens of nm [5-7].In the present work we discuss the electronic properties of different tube geometries and demonstrate that significant light-to-electricity conversion efficiencies can be achieved using these dye-sensitized nanotubes in solar cell architectures with an optimized morphology [2, 4, 8, 9].Literature:[1] B. ORegan and M. Grätzel, Nature 1991, 353, 737. [2] J. R. Jennings, A. Ghicov, L. M. Peter, P. Schmuki, A. B. Walker J. AM. CHEM. SOC. 2008, 130, 13364. [3] K. Zhu, N. R. Neale, A. Miedaner, A. J. Frank, Nano Letters 2007, 7, 69.[4] A. Ghicov et al. Chem. Asian J., 2009, 4, 520 – 525.[5] J. M Macak et al., Curr Opin Solid State Mater Sci 11 (2007)[6] A. Ghicov, P. Schmuki, Chem. Commun. 2791, doi:10.1039/b822726h (2009)[7] R. Hahn, J. M. Macak, P. Schmuki, Electrochem. Comm. 2007, 9, 947. [8] R. Hahn et al., Phys. Stat. Sol. (RRL) 2007, 1, 135.[9] P.Roy et al. submitted (2009)
12:45 PM - R4.10
Emulsion Templated Titania Coatings for Dye Sensitized Solar Cells.
Sarika Phadke 1 , Aurelien Du Pasquer 1 , Dunbar Birnie,III 1
1 Materials Science & Engineering, Rutgers University, Piscataway, New Jersey, United States
Show AbstractDye sensitized solar cells use titania coatings as photoanodes. To obtain effective dye adsorption and electrolyte permeation in the titania coating, meso- and macroporosity is required. In this work, TiO2 coatings with interpenetrating, meso- and macroporous structures are fabricated using oil-in-water emulsion as the templating material. Very uniform, defect free titania coatings have been obtained, with pores in the range of 30- 1000 nm, using the emulsion templating method. Electrochemical impedance spectroscopy is used to study the effect of templating on the impedance of the TiO2/dye/electrolyte interface during solar cell operation. The charge transfer resistance and chemical capacitance associated with the TiO2/dye/electrolyte interface show a typical behavior for the templated cells. The resistance decreases while the capacitance increases with templating, which results in more efficient diffusion of redox species in the electrolyte. The increase in the photocurrents of the emulsion templated solar cells prove that the higher porosity titania coatings provide better dye adsorption and electrolyte permeation exhibiting improved performance of the solar cell. SEM, mercury porosimetry and I-V characterization are used to correlate the impedance data.
R5: Nanostructured Architectures
Session Chairs
Songyuan Dai
Shozo Yanagida
Tuesday PM, December 01, 2009
Room 312 (Hynes)
2:30 PM - **R5.1
CdSe Quantum Dots Anchored on TiO2 and Carbon Nanotubes. 1-1D Architectures as Scaffolds to Improve the Efficiency of Solar Cells.
Prashant Kamat 1 2 , Blake Farrow 2
1 Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana, United States, 2 Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana, United States
Show AbstractCarbon nanostructures and vertically aligned TiO2 nanotube arrays are well suited as scaffolds to collect electrons from excited semiconductor nanocrystals (CdSe quantum dots) and transport them to the conducting electrode surface. These 1-D architectures provide the directionality for electron transport and reduce charge recombination pathways at the grain boundaries. When CdSe quantum dots linked to TiO2 nanotube arrays are excited with visible light they undergo charge separation. The capture and transport of charge carriers to the collecting electrode surface is an important step towards maximizing the efficiency of solar cells. For example, the maximum IPCE (photon-to-charge carrier generation efficiency) obtained with 3 nm size CdSe quantum were 35% for particulate TiO2 and 45% for tubular TiO2 morphology. Ways to maximize the efficiency of quantum dot senitized solar cells will be discussed.The charge separation between excited CdSe semiconductor quantum dots and stacked-cup carbon nanotubes (SCCNT) has also been successfully tapped to generate photocurrent in a quantum dot sensitized solar cell (QDSC). By employing an electrophoretic deposition technique we have cast SCCNT-CdSe composite films on optically transparent electrodes (OTE). The quenching of CdSe emission, as well as transient absorption measurements, confirms ultrafast electron transfer to SCCNT. The rate constant for electron transfer increases from 9.51 x 10^9 s-1 to 7.04 x 10^10 s-1 as we decrease the size of CdSe nanoparticles from 4.5 nm to 3 nm. The ability of SCCNT to collect and transport electrons from excited CdSe has been established from photocurrent measurements. Thin films of CdSe dots alone show limitations in transporting the photogenerated electrons towards the electrode surface as evidenced by a nearly 10-fold increase in observed photocurrent upon the addition of SCCNT to the photoanode. Excited state interaction between CdSe and SCCNT films is realized from the fast electron transfer between the two. Composites of CdSe quantum dots with stacked-cup carbon nanotubes provide a new and promising direction towards developing effective light energy harvesting strategies.
3:00 PM - **R5.2
Organic Solar Cell Based on Nanostructured Benzoporphyrin Crystals.
Yoshiharu Sato 1 , Iwao Soga 1 , Takaaki Niinomi 1 , Naoki Obata 1 , Hideyuki Tanaka 1 , Yoko Abe 2 , Yutaka Matsuo 1 2 , Eiichi Nakamura 1 2
1 ERATO Nakamura Project, Japan Science & Technology Agency, Tokyo Japan, 2 Department of Chemistry, The University of Tokyo, Tokyo Japan
Show AbstractWe have fabricated organic solar cell using tetrabenzoporhyrin (BP) as a donor material. Tetrabenzoporhyrin is formed by thermal conversion of the soluble precursor that has four bicyclo rings. Upon heating above 150°C, the precursor molecule is converted to semiconductive tetrabenzoporphyrin, which is insoluble against conventional organic solvents. Taking advantage of this insoluble character of BP, p-i-n bulk heterojunction solar cell is successfully fabricated from solution process, with BP/BP:fullerene/fullerene trilayer, where p-layer is crystal BP, i-layer consists of both crystal BP and fullerene, and fullerene acts as n-layer. The performance of the p-i-n solar was improved by introducing a new fullerene, 1,4-bis(dimethylphenylsilylmethyl)[60]fullerene (SIMEF), for the i- and n--layers ; Jsc=10.1 mA/cm2; Voc=0.79V; FF=0.68, PCE=5.4%.The p-i structure that was treated with toluene to remove fullerene in the i-layer revealed columnar growth of BP crystals in the i-layer by SEM. The diameter of each column was ca. 20nm, which is comparable to exciton diffusion length. We have also found that the morphology of BP films in the p-layer strongly affected the crystal growth of BP in the i-layer. X-ray diffraction measurements and TOF-SIMS depth profile results support the SEM observation. The dependence of nanostructdure of BP crystals on the fullerenes and process conditions will be described.
3:30 PM - R5.3
Optimization of Photoinduced Charge Transfer in Nanostructured Donor-acceptor Solar Cells.
Saif Haque 1
1 Chemistry, Imperial College London, London United Kingdom
Show AbstractSaif A. Haque, Henry Leventis and Simon KingPresenting author:
[email protected] displacement of CO2 emissions by renewable sources of energy critically depends upon the development of low-cost and widely accessible routes to clean energy generation. Photovoltaic device based upon nanostructured donor –acceptor heterojunctions are now attracting considerable interest for this purpose. The function of such devices is based upon a photoinduced charge separation reaction at the electron donor-acceptor interface. Efficient device operation critically depends on the ability to achieve a high yield and long-lived charge separation. In this presentation we will focus on two key issues that are important to the achievement of efficient solar cells: (i) control of donor acceptor film morphology at nanometre length scale (ii) extending light harvesting ability in the near infrared to device efficiency. More specifically this talk will address:1)The use of self-organizing block copolymers to control the donor-acceptor interface structure. Such materials display several attractive properties including phase separation of the blocks on length scales commensurate with exciton diffusion lengths in these materials, and an ability to self assemble into a range of different morphologies, including bicontinuous structures and hexagonally packed cylinders that bare a striking resemblance to ‘optimised’ nanostructures proposed for organic solar cells in the literature. Our studies have focussed on a number of promising semi-conducting block copolymers, especially with regard to photo-physical effects. Here we will present a transient spectroscopic study examining the influence of block copolymer crystallinity, ionization potential and domain size and nanomorphology upon the dynamics of charge separation and recombination. A aim of this study is to develop quantitative structure-function relationships that can be used to guide materials design and synthesis.2)The use of semiconducting quantum dots as light harvesting materials in dye sensitized solar cells. Such materials display a number of attractive properties that make them ideally suited as light harvesting materials including the ability to tune their band gap and the possibility of multiexciton generation. In this presentation we will focus on the influence of driving force (ΔG) on the yield of both the electron injection and hole regeneration processes occurring at the metal oxide / QD / HTM heterojunction within the DSSC by modulating the energetics of each component of the system (metal oxide, quantum dot and hole conductor) systematically.
3:45 PM - R5.4
Type II Heterojunction Photovoltaic Device Fabrication Based on an All Inorganic ZnO/ZnS Nanocable Array.
Kai Wang 1 , Jiajun Chen 1 , Zhongming Zeng 1 , Josh Tarr 1 , Weilie Zhou 1 , Yong Zhang 2 , Yanfa Yan 3 , Jiang Chunsheng 3 , John Pern 3 , Angelo Mascarenhas 3
1 AMRI/Chemistry, Advanced Materials Research Institute/UNO, New Orleans, Louisiana, United States, 2 Department of Electrical and Computer Engineering, University of North Carolina at Charlotte, Charlotte, North Carolina, United States, 3 , National Energy Renewable Laboratory, Golden, Colorado, United States
Show AbstractIn this presentation, we report a first time fabrication of all-inorganic type II photovoltaic device based on vertically aligned ZnO/ZnS heterojunction core/shell nanocable array. The quenching in photoluminescence but enhancement in photocurrent with faster response upon coating the ZnO core with the ZnS shell provides the evidence that the charge separation and collection in the type II nanocable is greatly improved. This demonstration brings much greater flexibility in designing next generation PV devices in terms of material selections and device operation mechanisms for achieving their ultimate energy conversion efficiencies at low cost and in an environment friendly manner.
4:30 PM - R5.5
Photocurrent Induced by Non-radiative Energy Transfer from Nanocrystal Quantum Dots to Adjacent Nanowire Conducting Channels: Towards a New Solar Cell Architecture.
Siyuan Lu 1 , Zachary Lingley 2 , Tetsuya Asano 2 , Tymon Barwicz 3 , Supratik Guha 3 , Anupam Madhukar 1 2
1 Department of Physics, University of Southern California, Los Angeles, California, United States, 2 Mork Family Department of Chemical Engineering and Materials Sciences, University of Southern California, Los Angeles, California, United States, 3 , IBM J. T. Waston Research Center, Yorktown Heights, New York, United States
Show Abstract In this talk, we discuss the potential of photovoltaic solar energy conversion employing our recently proposed [1] paradigm of nonradiative energy transfer (NRET) from the photon absorbing inorganic quantum dots (QDs) to adjacent high charge (electron and hole) carrier mobility inorganic transport channels for collection at the electrodes. This solar energy conversion paradigm may provide a viable trade-off with the bottlenecks of charge carrier generation and/or transport faced by excitonic solar cells given its following advantageous features: (1) high absorption efficiency enabled by QDs, (2) spontaneous exciton breakup at room tempereature in the inorganic channels where typical exciton binding energy is ~10meV, (3) efficient carrier transport in inorganic channels with high mobility (~100-1000 cm2/V-s). We present the results of two basic proof-of-concept measurements to support the feasibility of such NRET based solar energy conversion paradigm: (1) demonstration of efficient NRET between QDs and from QDs to adjacent carrier transport channels using time-resolved photoluminescence measurements, and (2) demonstration of photocurrent flow in silicon nanowire transport channels induced by NRET from the photon absorbing layer of QDs in proximal contact using time-resolved photocurrent measurements. Using the results of these proof-of-concept measurements as the inputs, the optimal design of the NRET based solar cell architecture and the achievable range of conversion efficiency will be discussed. This work is supported by AFOSR grant No. FA9550- 08-1-0146. [1] S. Lu., A. Madhukar, "Nonradiative Resonant Excitation Transfer from Nanocrystal Quantum Dots to Adjacent Quantum Channels," Nano Lett. 7, 3443-3451 (2007).
4:45 PM - R5.6
Assembly of 3D Nanoarchitectures for Quantum Dots Sensitized Solar Cells.
Xiangyang Kong 1
1 School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai China
Show Abstract Assembly of nanoscale building blocks into 3D nanoarchitectures is of key interest for high efficiency of energy coversion and storage. In this talk, we report a novel technique to fabricate a series of semiconducting oxides with grid-like nanostructures replicated from the bio-templates. These semiconducting oxides, including n-type TiO2 or p-type NiO nanogrids, were sensitized with the dye molecules, or semicondutor quantum dots, then assembled into 3-D stacked-grid arrays on a flexible substrate by means of Langmuir-Blodgett method for the photocatalytic electrodes. The quantum dots sensitized solar cells with 3-D stacked-grid arrays could achieve the conversion efficiencies around 7%. it is found that the stacked-grid array provide the direct path to improve the efficient electron diffusion and rapid transport through the network to the conductive substrate, resulting in the increase of the output voltage or the photogenerated current.
5:00 PM - R5.7
Tandem and Hybrid 3-D-Dye-Sensitized Solar Cells Aiming at High Efficiency Cells.
Shuzi Hayase 1 , Mitsuru Kono 2 , Yoshiriro Yamguchi 2 , Jun Usagawa 1 , Kenshiro Uzaki 1 , Takafumi Inoue 1
1 , Kyushu Institute of Technology, Kitakyushu Japan, 2 , Nippon Steel Chemical Co. Ltd, Kitakyushu Japan
Show AbstractEfficiency of dye sensitized solar cells (DSC) [1] has reached 11%. In order to increase the efficiency, we propose 3D-DSCs consisting of hybrid and tandem structures to cover wide ranges of wavelengths and collect electrons effectively. The following items are discussed.1)Proposals of molecular structures of dye molecules which have high photoconversion efficiency in the region of Near IR and IR regions. The relationship between the dye structure and the photoconversion efficiency is discussed in terms of HOMO-LUMO level, molecular orbital of LUMO, electron diffusion coefficient and electron life time in titania layer, surface passivation of titania surface states by the dye molecules, and dye dipoles. One of the directions is to introduce length alkyl chains to these dye molecules. Dye molecules bearing long alkyl groups increased the dye coverage rate of the titania surface and improved the electron diffusion coefficient and electron life time, leading to increase in open circuit current (Voc) and short circuit current (Jsc). It is discussed that surface potential of a dye/titania layer measured by a Kelvin Probe Force Microscopy has some relationship with Voc.2)A transparent conductive layer less structure (TCO-less structure) is discussed because the TCO layer absorbs light of Near IR and IR regions [2-3]. We report TCO-less-DSCs with all-metal electrodes. A porous 3D-Ti electrode was fabricated on a TiO2 layer by using nano ZnO crystals as the sacrifice materials [3]. 8% efficiency is reported for the all-metal-DSC. In addition, TCO-less-DSCs consisting of a floating electrode are reported [4].3)3D-structures consisting of the following structure are reported. 1. DSCs consisting of a dye double layer structure [4]. 2. DSCs consisting of a floating electrode. 3. DSCs consisting of a fiber structure aiming at “an infinite tandem structure” [5]. This enabled to harvest light with wide ranges of wavelengths. These general ideas, how to fabricate them and their photovoltaic performances are discussed. It was proved that these structures have a potential for high efficiency cells. References[1] B. O’Regan, M. Grätzel, Nature, 353, 737 (1991). [2] Y. Kashiwa, S. Hayase et al., Appl. Phys. Lett., 92, 033308 (2008), [3] T. Beppu, Y. Kashiwa, S. Hayase, M. Kono, and Y. Yamaguchi, Jpn. J. Appl. Phys., in press. [4] F. Inakazu, Y. Noma, Y. Ogomi and S. Hayase, Appl. Phys. Lett., 92, 093304 (2008). [5] J. Usagawa, S. S. Pandey, S. Hayase, M. Kono, and Y. Yamaguchi, APEX, in press.
5:15 PM - R5.8
3D-Nanostructured Templating for Heterojunction Organic Solar Devices.
Stefan Schumann 1 , Ross Hatton 1 , Tim Jones 1
1 Chemistry, University of Warwick, Coventry, West Midlands, United Kingdom
Show AbstractThere has been much recent interest in solution processed organic solar cells due to their potential for low cost manufacturing. One critical factor for the current low efficiencies of organic solar cells is the limited diffusion length of the excitons generated by light absorption. Initial improvements to increase exciton dissociation have included mixed layer and blended donor-acceptor device structures, resulting in high interface area heterojunctions and short exciton transport paths to a nearby interface. Unfortunately, these types of structure can also lead to significant charge trapping. A new approach in achieving efficiency improvement for solution processed organic solar cells is a three-dimensionally structured, highly interpenetrating donor/acceptor composite structure which has the advantage of short diffusion paths for increased exciton dissociation, but also provides defined charge pathways towards the electrodes.In this study we demonstrate a novel fabrication route for producing large area three-dimensionally ordered macroporous (3DOM) organic semiconductor thin films with a sub-100 nm open cellular interconnected structure in order to match the short exciton diffusion lengths of the organic semiconductors. The films are produced by vertical co-deposition of the organic semiconductors with ‘small’ polystyrene (PS) spheres and 3DOM arrays with very low crack densities were grown from a water-soluble small molecular semiconductor, copper(II)phthalocyanine-tetrasulfonic acid tetrasodium salt (TS-CuPc), as well as from a polymeric semiconductor, sodium poly[2-(3-thienyl)ethoxy-4-butylsulfonate] (PTEBS). After the removal of the colloids a second infiltration with an appropriate acceptor material, in this case fullerene (C60), completes the highly interpenetrating donor/acceptor (D/A) system. Sandwiched between appropriate electrodes complete solar cell device structures have been fabricated and characterised.For film and device characterisation complementary analysis techniques were used including electron microscopy, absorption spectroscopy and photovoltaic characterisation to develop a deeper understanding of the structure/function relationship of the donor-acceptor composite structures and also for the performance of the complete photovoltaic device.This work was supported by BP Solar and the Engineering and Physical Sciences Research Council (EPSRC), UK.
5:30 PM - R5.9
Improved Endohedral Metallofullerenes for Bulk Heterojuctions with Poly(3-hexylthiophene): Blend Morphology and Carrier Transport.
Jamie Adamson 1 2 , Russel Ross 3 , Edward Van Keuren 3 4 , Nikos Kopidakis 5 , Dana Olson 5 , Matthew Reese 5 , William Rance 1 5 , Claudia Cardona 4 , Francis Swain 4 , Steven Joslin 4 , Martin Drees 4 , Kenneth Walker 4 , David Ginley 1 2 , Reuben Collins 1 2
1 Physics, Colorado School of Mines, Golden, Colorado, United States, 2 , Renewable Energy Materials Research Science and Engineering Center, Golden, Colorado, United States, 3 Physics, Georgetown University, Washington, District of Columbia, United States, 4 , Luna Innovations, Inc., Danville, Virginia, United States, 5 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractOne of the fundamental limitations for the efficiency of organic photovoltaics (OPV) to date is the mismatch between the molecular orbitals of donor and acceptor materials. This mismatch leads to significant energy loss during the charge transfer from donor to acceptor and results in low open circuit voltages (Voc). To overcome this limitation, we have developed novel acceptor materials based on TRIMETASPHERES® carbon nanomaterials (TMS). TMS are endohedral metallofullerenes that consist of a trimetallic nitride cluster enclosed in a C80 cage. TMS derivatives display Lowest Unoccupied Molecular Orbitals (LUMO) (electron affinities) that are 280 meV closer to the LUMO levels of the donor materials compared to commonly used acceptors molecules such as [6,6]-phenyl-C61-butyric methyl ester (C60-PCBM). TMS acceptors share many properties with C60-PCBM with regard to processability, efficient and stable charge transfer and charge carrier mobility in donor/acceptor blends. The donor polymer poly(3-hexylthiophene) (P3HT) also exhibits desirable ordering in blends with TMS much like it does in blends with C60-PCBM.We have previously reported the first methano family of Lu3N@C80 derivatives notated as Lu3N@C80-PCBX (X= M [methyl], B [butyl], H [hexyl] or O [octyl]). P3HT:TMS OPV devices based on these electron acceptors showed similar current densities and fill factors to P3HT:C60-PCBM references but with Voc increased by up to 280 mV, resulting in increases in conversion efficiencies of 4.2% relative to 3.6% in the references. Here, we present the results from Lu3N@C80-PCBEH (ethyl hexyl), our most promising new derivative that shows further improvements with conversion efficiencies up to 4.7%. Carrier transport and blend morphology are characterized using time-resolved microwave conductivity and grazing incidence x-ray diffraction, respectively. Novel device structures with either Lu3N@C80-PCBEH and C60-PCBM as the electron accepting material are also discussed.
Symposium Organizers
Arthur J. Frank National Renewable Energy Laboratory
Nam-Gyu Park Sungkyunkwan University
Tsutomu Miyasaka Toin University of Yokohama
(and Peccell Technologies, Inc.)
Laurie Peter University of Bath
Songyuan Dai Chinese Academy of Sciences
R6: Nanostructured Sensitized Solar Cells II
Session Chairs
Wednesday AM, December 02, 2009
Room 312 (Hynes)
9:30 AM - **R6.1
Cost-benefit Photovoltaics by Hybridization of Dye-anchored Nano-crystalline TiO2 and Organic Hole Conductors.
Shozo Yanagida 1
1 Center for Advanced Science and Innovation, Osaka University, Suita Japan
Show AbstractNext generation photovoltaics (PV) must be high-throughput printable devices with respectable photocurrent conversion efficiency and reliability, which may cope with increasing demand of average consumers of the coming low-carbon-energy era. The superiority of dye-sensitized TiO2 solar cells (DSC) can be explained as due to three wonders of nano-crystalline anatase TiO2; one is the low conduction band potential, which enable nc-TiO2 to accept photoelectron from most of anchored dye molecules through bond and/or through space molecular-atom orbital, the second is that nc-TiO2 elongates the lifetime of accepted photo-electrons, third is that mesoscopic anatase nc-TiO2 layers have respectable diffusion coefficient through adsorption of cationic species like imidazolium cation from electrolytes. The long-lived electron and respectable diffusion coefficients give a μm-size diffusion length, which means that dye-anchored nc-TiO2 layers work n-type layers in n/p junction photovoltaics. On the other hand, the organic photovoltaics (OPV) exemplified by PCBM/P3HT, suggest that the fullerene derivatives should work as n-type layers and the polythiophene derivatives should work as p-type layers with respectable diffusion length. With these in mind, we have attempted a kind of hybridization of DSC with OPV. The fabrication of quasi-solid-state nc-TiO2/Dye/PEDOT or Poly (3-hexylthiophene) (P3HT) solar cells will be discussed, in which the dyes with oleophilic thienyl groups were employed and ionic liquid, 1-ethyl-3-methylimidazolium (EMIm) containing lithium bis(trifluromethanesulfone)imide (Li-TFSI) and 4-tert-butylpyridine (t-BP) were assembled with the dye-anchored TiO2 surfaces. One of the devices gave a high conversion efficiency of up to 2.70% under 1 sun illumination. The respectable performance is ascribed to successful molecular self-organization of thiophene groups of the dye molecules with ethylenedioxythiophene groups of the hole conductors at interface in the case of the nc-TiO2/HRS-1/PEDOT-structured DSC.Such OPV-DSC hybrid PV will be discussed in terms of diffusion length of charge transfer materials, because the diffusion length for efficient charge separation that is acquired by introduction of the ionic liquid (IL) coupled with Li-TFSI should contribute to the respectable efficiency.Reference: Ke-Jian Jiang, Kazuhiro Manseki, You-Hai Yu, Naruhiko Masaki, Kazuharu Suzuki, Yan-lin Song and Shozo Yanagida, Adv.Fun. Mat., in press.
10:00 AM - **R6.2
DSC Tool-Box - an Integrated Approach.
Anders Hagfeldt 1 2
1 Dept. of Physical & Analytical Chemistry, Uppsala University, Uppsala Sweden, 2 Royal Institute of Technology, Center of Molecular Devices, Stockholm Sweden
Show AbstractThe talk will summarize our research and development activities of dye-sensitized solar cells (DSC), focusing on the development of characterisation techniques of complete DSC devices - what we call the ‘tool-box’.The scientific challenge of DSC is to handle the complex molecular interactions and the inherent multi-scaling properties, both in time and length. We have approached this scientific challenge by developing ‘tool-box’ techniques. These techniques make it possible to extract information at the molecular level on complete DSC devices operating under normal working conditions. The methods provide information on for example energetics, charge transport and electron transfer processes. Some examples will be given at the meeting including Photoinduced Absorption Spectroscopy as a suitable method to study for example the quality of pore filling in case of solid hole conductors.Some of our materials research will be presented. We have developed series of organic dye molecules with the general structure donor – conjugated linker – acceptor. Best efficiencies, above 7%, were obtained with polyene-diphenylaniline type dyes, using an iodide/triodide based redox electrolyte. The influences of substitutions of the donor group (a diphenylaniline moiety), the length of the conjugated linker chain and different acceptor/anchoring groups are presently being studied and will be discussed at the meeting.The majority of studies in this field are based on the sensitization of a n-type semiconductor. However, DSCs in which the cathode is photoactive are also possible and the development of a photocathode can provide an entry to the preparation of a tandem solar cell, in which both electrodes are photoactive. Until now there have been few studies on the sensitization of p-type semiconductors. We will report on new organic dyes, designed for a p-type DSC, and its performance in a photovoltaic device based on NiO as the photocathode and a passive anode. The external quantum yield of this system has a maximum of above 60% which is among the highest values recorded so far for p-type DSCs.
10:30 AM - **R6.3
Electron Lifetime in Dye-Sensitized Solar Cells.
Juan Bisquert 1
1 Grup de Dispositius Fotovoltaics i Optoelectronics, Universitat Juame I, Castello Spain
Show AbstractImpedance spectroscopy (IS) is a major tool to determine the properties of dye-sensitized solar cells (DSC) and organic solar cells. We show how the measured parameters (resistances, capacitances) can be related to the electronic properties such as the electron conductivity, diffusion coefficient, and lifetime. The electron lifetime is a central quantity to determine the recombination dynamics in a solar cell.1 We summarize the main models that describe the lifetime dependence on bias voltage or carrier density, and find that there are two main approaches to clarify the structure of the lifetime. The first is to treat the lifetime as a product of the chemical capacitance and recombination resistance. This approach is important because the resistance largely determines steady state operation characteristics of the solar cell close to open-circuit voltage. The second approach is based on a kinetic model that describes in detail the different processes governing the decay of the carrier population in a measurement of . The results are presented for a variety of solar cells, including DSC with standard molecular sensitizers, with colloidal CdSe quantum dot sensitizers, full solid-state DSC with OMETAD hole conductor, and organic BHJ formed with (P3HT:PCBM) blends.1 J. Bisquert, F. Fabregat-Santiago, I. Mora-Seró, G. Garcia-Belmonte, S. Giménez, J. Phys. Chem. C, to be published, 2009.
11:45 AM - R6.5
Charge Transfer Enhanced Al-doped ZnO Transparent Conducting Films for ZnO Based Dye-Sensitized Solar Cells.
Hyun Suk Jung 1 , Sung-Hae Lee 1 , Jee-Hey Lee 1 , Hyunjung Shin 1 , Jung-Kun Lee 2
1 School of Advanced Materials, Kookmin Univ., Seoul Korea (the Republic of), 2 Department of Mechanical Engineering & Material Science, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show AbstractRecently, ZnO nanorod or nanowire based photoelectrodes were exploited to improve the electron transfer by virtue of eliminating grain boundaries. The single crystalline ZnO nanorod has similar energy band position with TiO2, which makes it suitable for high performance photoanodes. However, the efficiency (η) of ZnO nanorod-based DSSCs is still inferior to that of TiO2 nanoparticle-based DSSCs. ZnO nanorod-based DSSCs are still challenging and need to be studied further for improving theire photovoltaic properties. For example, Wu et al. recently reported that a mercurochrome (C20H8Br2HgNa2O) sensitizer is more suitable than the Ru(dcbpy)2(NCS)2 (N719) dye for ZnO nanorod-based DSSCs, which demonstrates that new type of materials containing sensitizers, electrolytes, TCOs need to be exploited for enhancing the energy conversion efficiencies of ZnO nanorod-based DSSCs. Usually, F-doped SnO2 (FTO) thin films have been used as TCO materials in ZnO NR-based solar cell. The large work function of FTO may induce a Schottky barrier with ZnO NR, which deteriorates the charge transfer properties. In the present study, we employed Al-doped ZnO (AZO) TCO materials, which yields a better junction property between TCO and ZnO NR array. The performance of ZnO NR based-solar cells, which contained AZO and FTO TCO materials were investigated using impedance analysis. The AZO based ZnO NR solar cell showed higher energy conversion efficiency than FTO based one, which is ascribed to the enhanced charge collection property of AZO film. This study demonstrates that the AZO thin film is a promising TCO material, which improves the charge transfer in ZnO NR-based solar cell.
12:00 PM - R6.6
Porous Nanostructure Photoanodes and Their Application in Dye Sensitized Solar Cell.
Tao Yu 1 2 , Xiangyan Wang 3 , Zhipeng Tian 1 , Zhigang Zou 1 2 3
1 Ecomaterials and Renewable Energy Research Center, Department of Physics, Nanjing University, Nanjing, Jiangsu, China, 2 National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, Jiangsu, China, 3 Department of Materials Science and Engineering, Nanjing University, Nanjing, Jiangsu, China
Show AbstractA highly (110)-oriented ZnO porous nanosheet framework is designed as the photoanode in the photoelectrochemical systems, by virtue of its anisotropic electronic properties. It can be facilely prepared in large scale via a hydrothermal method. X-ray diffraction (XRD) analyses show that the orientation indexes of the (110) diffraction plane was 3.54, indicating the films possess (110) preferred orientation. Field-emission scanning electron microscope (FE-SEM) images exhibit that most of the nanosheets stand nearly perpendicularly on the substrate. The {002} lattice planes work just like conducting wires and induce the electrons to transport to the substrate. Their electron collection function can be effectively improved in chronoamperometry measurement. A conversion efficiency of 3.7% was obtained in dye-sensitized solar cell (DSSC) with an active area of 0.4 cm2 when the nanostructure photoanode was introduced.On the other hand, TiO2 multilayer structure was prepared in DSSC work electrode by enlarging the porosity in each layer being coated on the fluorine-doped tin oxide transparent conducting glass from bottom to top. The multilayer structure exhibits an improved light scattering character, which resulted in better light harvesting of the cell. An obvious improvement in short circuit current is obtained. I-V characteristic measurement indicates an improved efficiency by 13% as compared to homogeneous pore-size samples. Diffuse reflectance spectra, scanning electron microscope images, and porosity measurements demonstrate that larger porosity is the cause of enhanced light scattering.
12:15 PM - R6.7
A New Dye-sensitized Solar Cell Architecture Based on Vertically Aligned Carbon Nanofiber Array Coated with TiO2 Shells.
Jun Li 1 , Jianwei Liu 1 , Yen-Ting Kuo 1 , Kenneth Klabunde 1 , Caitlin Rochford 2 , Judy Wu 2
1 Chemistry, Kansas State University, Manhattan, Kansas, United States, 2 Physics, University of Kansas, Lawrence, Kansas, United States
Show AbstractDye sensitized solar cell (DSCs) is a promising low-cost technology to convert solar energy into electricity. Many recent studies have been focused on the new nanostructured materials and device architecture, particularly using vertically aligned TiO2 nanotube array and ZnO nanowires arrays. Here we report a novel dye sensitized solar cell architecture for solar energy conversion using the vertically aligned carbon nanofiber (VACNF) array as a structural template. VACNFs are bamboo-like multi-walled carbon nanotubes grown by plasma enhanced chemical vapor deposition (PECVD). Due to the effect of strong electric field during growth, VACNF arrays present superior vertical alignment and unique freestanding brush-like structure. The diameter can be tuned from 20 nm to 200 nm and the length can be controlled from 1 micron to 40 microns. Each carbon nanofiber (CNF) is fully separated from the neighbors by ~300-400 nm, leaving sufficient space for adding on other photoactive materials. We found that a nanoneedle-textured anatase TiO2 film can be coated uniformly around each CNF as a thin shell of ~100 nm thickness by metal-organic chemical vapor deposition (MOCVD). The work function of CNFs is ~4.5-5.0 eV, matching well with that of indium-tin-oxide (ITO) photonanode, which places its Fermi level ~200-700 meV below the conduction band edge of TiO2. This property enables the VACNF array as an ideal charge collection core and the thin TiO2 coating as a charge separation shell. This unique core-shell architecture separates the roles of charge separation and electron collection, making it possible to optimize the physical properties independently by tailoring each materials. The large surface area of the nanoneedle-textured TiO2 coating would be also ideal for large dye adsorption. Photoluminescence measurements of the VACNF\TiO2 core-shell structure show that the fluorescence emission is completely quenched even at low temperature down to 10 K. This is likely due to the efficient charge separation at the TiO2-CNF junction. A dye-sensitized solar cell has been constructed using this architecture by further coating the TiO2 surface with a ruthenium complex dye. An encouraging overall conversion efficiency of ~1.1% and a rather high open circuit voltage of ~0.64 V haven been obtained. The incident photon to charge carrier conversion efficiency (IPCE) measurement shows a maximum photoconversion efficiency of ~34% at 515 nm, correlating well with the peak absorption of the Ru(II) dye. The IPCE is comparable with DSCs made of TiO2 NT arrays. By doping TiO2 with carbon, the CNF\TiO2 core-shell structure has also been demonstrated as a photoanode which can initiate water splitting with visible light. Our results demonstrate efficient charge separation at the CNF-TiO2 junction and the potential advantages in combining the highly conductive VACNF core as the electron collector with the nanostructured TiO2 shell as the charge separation barrier.
12:30 PM - R6.8
Spherical TiO2 Nanostructures for Dye-sensitized Solar Cell.
Kyeong Ha Kim 1 , In Gyoung Yu 1 , Wan In Lee 1
1 Department of Chemistry, Inha University, Incheon Korea (the Republic of)
Show AbstractRecently, dye-sensitized solar cell (DSC) has attracted great attention with their low production cost of electricity and relatively high energy conversion efficiency. One of the key elements in DSC is the design of the porous photoelectrode, and typically TiO2 nanoparticles are utilized for this purpose to obtain high surface area and to generate nanopore structure. In this work, the nanoporous TiO2 microspheres and hollow TiO2 spheres with ultra-high surfaces were selectively synthesized and applied to the photoelectrode of DSC. First, several nanoporous TiO2 microspheres with the size of 200-500 nm were synthesized, and were applied to main electrode layer of DSC. The achieved conversion efficiency was considerably higher than that of the cells derived from the commercial TiO2 pastes. The highest conversion efficiency was 10.52%, and the advantages of the nanoporous TiO2 microspheres were fully analyzed in the work. Second, the hollow TiO2 microspheres with size of 1.5–4.0 micron were synthesized by solvothermal reaction of titanium isopropoxide in tetrabutylammonium hydroxide (TBAH) solution. The shell of hollow TiO2 microspheres, with a thickness of ~250 nm, was constructed by the self-assembly of highly crystallized TiO2 nanoparticles with the size of 18 nm. The synthesized hollow TiO2 microspheres were physically robust and thermally-stable. When they were applied as the scattering layer of DSC, it was found that they were more efficient than other materials such as rutile or zirconia particles.
R7: Modeling and Characterization Nanostructured Materials and Devices
Session Chairs
Tsutomu Miyasaka
Peng Wang
Wednesday PM, December 02, 2009
Room 312 (Hynes)
2:30 PM - **R7.1
Advanced Solar Cells with Nanowires and Nanocrystals.
Yi Cui 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractThe development of nanoscale science has produced a variety of bottom-up and top-down nanostructures such as nanowires and nanocrystals with controlled sizes and shapes, which afford novel materials for advanced solar cells. Here I will show several examples on how to exploit those nanostructures for effective photon management, fast charge collection, low temperature and low cost processing and clear fundamental material and interface characterization. This talk covers amorphous Si, CuInSe2 and CuInS2 nanostructured solar cells.
3:00 PM - R7.2
Ga-Assisted MBE Grown GaAs Nanowires for Photovoltaic Applications.
Carlo Colombo 1 , Martin Heiss 1 2 , Emanuele Uccelli 1 , Michael Graetzel 1 , Anna Fontcuberta i Morral 1
1 , École Polytechnique Fédérale de Lausanne, Lausanne Switzerland, 2 , Walter Schottky Institute, TUM, Munich Germany
Show AbstractSemiconductor nanowires are currently attracting high attention as new potential nanoscale building blocks for photovoltaic applications. In this study we report about GaAs nanowires grown by Molecular Beam Epitaxy (MBE) via a self-catalytic vapor-liquid-solid (VLS) process. The extremely clean conditions of MBE and the absence of using gold as catalytic seed give rise to high crystal quality nanowires. Core-shell p-i-n junctions have been created in the single nanowire in a two step procedure: first the nanowire p-core has been grown epitaxially on a GaAs substrate and then switching the growth conditions, an intrinsic layer followed by an n-doped GaAs shell have been grown around. This radial geometry offers the opportunity to achieve efficient carrier extraction since diffusion length is relatively short, while a long axial length allows high optical absorption. Other possible advantages like the reduction of material used and the decrease in the reflectivity will be discussed as well. In order to carefully investigate the properties of such device, single core-shell nanowires have been dispersed on silicon substrate covered by a silicon dioxide layer and they have been contacted by optical lithography. The contact to the p-core has been realized by a precisely calibrated etching step which allowed to remove only the n-doped shell. While the contact to the n-doped side have been done in a following lithography step. Measurements performed under one solar equivalent (1-sun) illumination yielded to a photocurrent of about 70pA, an open circuit voltage of 0.6V and a fill factor of 65%, meaning an efficiency of 4.5%. More information about the functioning of these nanowires p-i-n junctions were obtained by spatially resolved photocurrent measurements. The experiment has been performed scanning the nanowire with a laser spot of 1 µm and mapping the short-circuit current produced. The response is homogeneous along the wire, in agreement with the existence of a homogeneous deposition of the intrinsic and n-type layers on the nanowire facets. Finally the contacted devices have been operated in a LED mode and the spectrum emitted has been acquired. The emission peak at 1.42 eV is in good agreement with the room temperature band gap of GaAs and further corroborates the good quality of the nanowires.We believe that this kind of nanowire structures can open new ways for the realization of efficient nano-structured solar cells.References:[1] C. Colombo, M. Heiβ, M. Graetzel, A. Fontcuberta i Morral, Appl. Phys. Lett. 94, 173108 (2009)[2] A. Fontcuberta i Morral, C. Colombo, J. Arbiol, J.R. Morante, G. Abstreiter, Appl. Phys. Lett. 92 063112 (2008)[3] C. Colombo, D. Spirkoska, M. Frimmer, G. Abstreiter, A. Fontcuberta i Morral, Phys. Rev. B, 77, 155326 (2008)[4] A. Fontcuberta i Morral, D. Spirkoska, J. Arbiol, M. Heigoldt, J. Ramon Morante and G. Abstreiter, Small, 4, 899-903 (2008)
3:15 PM - R7.3
Novel Nanostructured Photoelectrodes - Electrodeposition of Metal Oxides onto Transparent Conducting Oxide Nanofibers.
Rainer Ostermann 1 , Melanie Rudolph 2 1 , Thomas Loewenstein 2 , Bernd Smarsly 1
1 Institute of Physical Chemistry, Justus Liebig University, Giessen Germany, 2 Department of Physics, Justus Liebig University, Giessen Germany
Show AbstractNanostructured metal oxides with high surface areas have been shown to be efficient photoelectrodes for light-to-energy conversion in (Graetzel-type) dye-sensitized solar cells and as photoanodes for water splitting. For these purposes the metal oxide layer needs to be deposited onto an electrode (serving as the current collector), typically a TCO (transparent conducting oxide) coated glass. Electrodeposition appears to be the most convenient and suitable path to generate these oxide layers, ensuring good electrical contact to the underlying electrode. Recently, we have developed methods to produce nanofibrous mats of transparent conducting oxides that are very promising as nanostructured electrodes for electrodeposition. The nanofibers have been obtained by electrospinning TCO materials. By controlling the process parameters, non-woven mats with more or less densely packed fibers (filling 5-40% of the volume) were collected on glass or TCO-coated glass. The fiber mats served as macro-porous electrode for the electrodeposition of non-porous and meso-porous TiO2 layers of 20-250nm in thickness on the fibers. Due to the high surface area of the nanofibers and their enhanced electron transport, the photocurrents were substantially higher (up to 5 mA/cm2) than on flat electrodes of comparable thickness and even if the overall efficiency of up to 2% should be still improvable, it clearly validates this novel approach of nanostructured dye-sensitized solar cells.A detailed study of the time- and frequency- resolved measurements of the photocurrent (IMPS) and photovoltage (IMVS) will be presented to elucidate the individual steps of photoelectrochemical reactions.Furthermore, preliminary, but promising results on water cleavage by TiO2 and Fe2O3 electrodeposited onto the TCO fibers will also be shown.
3:30 PM - R7.4
Electrical and Photochemical Properties of Titania Nanotube Arrays.
Ragen McAdoo 1 , Mohamed Abd Elmoula 1 , Nathan Israeloff 1 , Latika Menon 1
1 Physics, Northeastern University, Boston, Massachusetts, United States
Show AbstractTitania nanotubes have been fabricated by means of electrochemical anodization of titanium foils in an electrolyte. The nanotube dimensions (diameter, wall thickness and length) can be controlled in our fabrication approach by varying the electrolyte used and by adjusting the anodization voltage. We have also demonstrated deposition of gold nanoparticles on the tubes surface by means of a modified deposition-precipitation method. We show that by adjusting the time of deposition and concentration of the solution, we can obtain a high deposition density of the gold particles over the nanotube surface and also have good control over the size of the gold nanoparticles. All samples have been characterized by means of scanning and transmission electron microscopy. Our results on the electrical properties (conductivity, capacitance) of such nanotube arrays will be reported. We will also discuss our results on the photochemical properties of the blank and the Au-deposited titania nanotubes under simulated solar radiation.
3:45 PM - R7.5
Optical Properties of Free-Standing Titania Nanotube Arrays.
Mohamed Abd Elmoula 1 , Don Heiman 1 , Latika Menon 1
1 Physics, Northeastern University, Boston, Massachusetts, United States
Show AbstractTitania nanotube arrays have important applications in photoelectrochemical cells. For such applications, light conversion to electron-hole pairs at the surface of the nanotubes is expected to be significantly improved due to high aspect ratio and low recombination probability at the nanotube surface. In this work we report the optical properties of free standing titania nanotubes arrays, that is, arrays without a barrier layer and the underlying titanium surface. Such free standing nanotube arrays allow us to measure the optical properties of the nanotubes alone precisely. We investigate the transmission of light through the nanotubes as a function of tube length, to explore the penetration depth for various wavelengths in the nanotubes array. This will allow us to find the most effective tube length for maximum light absorption in the nanotubes arrays. In order to carry out such a systematic study we will investigate the optical properties of blank nanotubes arrays as a function of tube length and diameter. These results will be compared with the optical properties of dye-adsorbed titania nanotubes arrays and gold nanoparticle modified titania nanotube arrays. As a result of this work we will report the most effective nanotube parameters that maximize the solar spectrum absorption.
4:30 PM - R7.6
Point Defects in CdSe and CdTe Nanostructures– An Ab Initio Study.
Thomas Sadowski 1 , Ghanshyam Pilania 1 , Rampi Ramprasad 1
1 Institute of Material Science CMBE, University of Connecticut, Seymour, Connecticut, United States
Show AbstractThe properties of materials are often controlled by defects and impurities. Native point defects, in particular, control many aspects of a semiconductor’s behavior. Such defects may be charged, which can affect numerous properties such as the structure, thermal diffusion rates, and the trapping and recombination rates for electrons and holes. A detailed understanding of the behavior of point defects is therefore crucial to the application of any semiconductor. In this work, density functional theory (DFT) based computational techniques have been employed to investigate the atomic and electronic properties of neutral and charged cation and anion vacancies in bulk wurtzite CdSe and CdTe, as well as in the neighborhood of CdSe surfaces, CdTe surfaces, and CdSe-CdTe interfaces. To begin with, the electronic and elemental atomic chemical potential dependence of the vacancy formation energy has been determined for Cd, Se, and Te vacancies. Results show that under Cd rich conditions, the neutral and 2+ charged anion states are the most stable at low and high values of the electronic chemical potential, respectively. Under Cd poor conditions, the anion 2+ charge state is the most stable at low electronic chemical potentials whereas at high potentials it is the 2- Cd vacancy that is most stable. It thus appears that, regardless of the chemical environment, the 2+ anion (Se and Te) vacancies persist for a wide range of electronic chemical potentials, more than any other defect species. Structurally, in the case of the anion vacancies, the nearest neighbor Cd atoms relax inward toward the vacancy for the neutral charge state and radially outward for the 2+ charge state. In the latter case, nearest neighbor Cd atoms adopt a planar sp2 bonding configuration. With a comprehensive understanding the behavior of point defects in bulk CdSe and CdTe at hand, the tendency of Cd, Se and Te vacancies to segregate to various polar and non-polar CdSe and CdTe surfaces [1], and non-polar CdSe-CdTe interfaces have been evaluated. These results will be presented along with a discussion of the tendency of species from one component to occupy vacancies in the other component, thereby enabling mixing of the two components.[1] G. Pilania, T. Sadowski, R. Ramprasad, J. Phys. Chem. C 113, 1863 (2009).
4:45 PM - R7.7
Controlling Interfacial Electron Transfer Kinetics in Semiconductor Quantum Dots Sensitized Solar Cells.
Yasuhiro Tachibana 1 , Takahisa Higuchi 1 , Keisuke Yoshimura 1 , Kazuya Umekita 1 , Susumu Kuwabata 1
1 Applied Chemistry, Osaka University, Osaka Japan
Show AbstractSemiconductor quantum dots (QDs), or nanocrystals, have attracted significant attentions owing to their optical and electronic properties. One of the most interesting advantages of these materials is control of their band gap energy (Eg) by adjusting the QD size, i.e. quantum size effect. For example, the photoluminescence (PL) wavelength can be tuned from ultra-violet to infrared by controlling their size. This superior advantage is directly introduced to a wide range of advanced applications such as biological sensors, solar cells and electroluminescence devices.For photovoltaic applications, QDs sensitized solar cells are regarded as an alternative emerging source to dye sensitized solar cells. One attractive characteristic is that potential energy levels of QD conduction and valence bands can be adjusted by controlling the QD size,[1,2] and thus an appropriate size can be tuned to optimize efficient electron injection and slow charge recombination between a QD and a metal oxide. In contrast, fundamental understanding for the factors controlling the interfacial electron transfer reactions in QDs sensitized semiconductor electrodes is still limited. In this presentation, we attempt to elucidate relationship of the interfacial nanostructures with the electron transfer rates and the solar cell performance.[3,4]Charge recombination decays were compared for in-situ chemical bath deposited CdS/TiO2 films. Sub-microsecond to millisecond transient absorption studies were conducted by a home-built transient absorption spectrometer. Generally, the faster recombination rate was observed with the smaller CdS crystallinity size. The half lifetimes were analysed with respect to the distance between the electron in the TiO2 and the hole in the CdS. The details of the analysis and other related results will be discussed.This work was financially supported by Grant-in-Aid for Scientific Research, No. 21550133, from the Ministry of Education, Culture, Sports, Science and Technology, Japan. TEPCO Research Foundation, and the Venture Business Laboratory, Osaka University are also acknowledged for the financial support.References.[1] Y. Tachibana, et al., Chem. Lett., 36(1), 88-89 (2007).[2] Y. Tachibana, et al., Proc. SPIE (Invited), 6340 (Solar Hydrogen and Nanotechnology), 634014/1-634014/13 (2006).[3] Y. Tachibana, et al., J. Phys. Chem. C, 113 (16), 6852-6858 (2009).[4] Y. Tachibana, et al., J. Phys. D: Appl. Phys., 41(10), 102002/1-102002/5 (2008).
5:00 PM - R7.8
Electrochemically Prepared Metal-free Silicon Wire Arrays for Solar Cell Applications.
Hong-Seok Seo 1 , Han-Don Um 1 , Jin-Young Jung 1 , Sang-Won Jee 1 , Kwang-Tae Park 1 , Kye Jin Jeon 1 , Syed Abdul Moiz 1 , Jung-Ho Lee 1
1 Department of Materials and Chemical Engineering, Hanyang University, Ansan Korea (the Republic of)
Show AbstractSilicon wired solar cells provide a potential solution for high-efficiency with cost-effectiveness since a radial p-n junction can allow for high efficiencies in light absorption and carrier separation. Numerous approaches such as vapor-liquid-solid (VLS) method, reactive ion etching, and electroless etching have been suggested for preparing wires and aligned silicon wire arrays; however, these approaches have their own drawbacks for use in solar cell application. Most of silicon wires have been grown by the well known VLS method using metal catalyst, which requires high thermal budget to grow wires with relatively long process time. This feature critically requires post-doping for compensating the doping profiles devastated during wire growth. Moreover, the presence of metal residue in semiconducting regions due to metal catalyzed growth remains a potentially insoluble hurdle to boosting the conversion efficiency as high as possible. Given metal-contamination, metal-assisted-chemical etching of silicon recently suggested for making wire arrays at room-temperature shares the same issue because the catalyzing metal particles (usually silver) always remain at very narrow, confined corners between the wire bottoms after electroless etching. Here, we present a simple metal-free approach using electrochemical etching to fabricate vertical silicon wire arrays for solar cell application. A periodically patterned array of silicon wires has been made using electrochemical etching of a p-type <100> silicon wafer in an aqueous hydrofluoric (HF) solution. We demonstrate precise control over a rather large area (~2 cm2) in a wire diameter and interspacing between vertical wires by controlling the applied current. The wires that had lengths greater than 20 μm were easily formed while maintaining their diameters as small as 300 nm. We explored that high-quality single-crystal silicon wires with desirable diameter and interspacing can be readily prepared on single-crystal silicon substrates with identical doping characteristics. Then, spin on dopant (SOD) was used to make a conformal radial p-n junction of wires. In spite of high temperatures (900~1050 °C) doping, no morphological degradation has been observed in wire arrays having diameters of 300~500 nm, and a good controllability over the doping range from 1018 to 1020 cm-3 was obtained. For realizing highly efficient silicon wired solar cells, this approach provides a simple route for making waferscale, high quality, and low-cost wire arrays without the need of metal.
5:30 PM - R7.10
Nanoporous `Black Silicon' by a Nanocatalyzed Etch for Photovoltaic Anti-reflection.
Howard Branz 1 , Hao-Chih Yuan 1 , Vernon Yost 1 , Matthew Page 1 , Paul Stradins 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractWe fabricate graded-density nanoporous ‘black-silicon’ antireflective surfaces by a low-cost nanocatalyzed liquid etch, in a single step. We have measured the scaling law that governs reflectance from density-graded surfaces: there is an exponential decay of reflectance versus the wavelength-scaled density grade depth. Reflectance from Si falls below 5% when the grade depth reaches lambda/4, meaning a grade depth of about 500 nm provides excellent photovoltaic antireflection. Using this nanoporous surface in place of conventional anti-reflection coating, we have fabricated 14.8% multi-crystalline silicon cells and confirmed 16.8%-efficient single-crystal silicon solar cells.
5:45 PM - R7.11
Fabrication and Modeling of Three-Dimensionally Structured CdTe and CdTe/CsS Thin Film Photovoltaic Devices with Micrometer Scale Self-Aligned Back-Contacts.
Daniel Josell 1 , Suyong Jung 2 , Carlos Beauchamp 1 , Behrang Hamadani 2 , Lee Richter 3 , John Bonevich 1 , Nikolai Zhitenev 2 , Thomas Moffat 1 , Jon Guyer 1
1 Metallurgy Division, NIST, Gaithersburg, Maryland, United States, 2 CNST, NIST, Gaithersburg, Maryland, United States, 3 CSTL, NIST, Gaithersburg, Maryland, United States
Show AbstractWe have produced CdTe homojunction and CdTe/CdS heterojunction back contact thin film solar cells by an electrochemical deposition process onto 5 mm square interdigitated metal electrodes (i.e., electrode comb structures) with micrometer scale pitch. Growth of the devices is accomplished on the same electrodes that are used to extract current during operation under light. Independent control of deposition potential on the electrodes with/without change of electrolyte during fabrication enables deposition of n-type material around every other metal line (one side of the comb) with an overlayer of p-type material that makes contact with the other electrodes. Homojunction devices are fabricated in a single step in a single electrolyte; heterojunction devices are fabricated through two electrodeposition processes conducted in different electrolytes. Measurements of i-V response as well as External Quantum Efficiency over a broad spectral range are detailed. Results are described for devices with different geometries, including varying electrode pitch and deposit thickness. Results are interpreted through modeling of the 3-d device geometry including light absorption, electron and hole distributions, and carrier transport and recombination in the bulk and at interfaces. The results promise a versatile means of evaluating the interfacial and bulk factors controlling the performance of nanostructured photovoltaic devices.D. Josell, C. R. Beauchamp, S. Jung, B.H. Hamadani, A. Motayed, L.J. Richter, M. Williams, J.E. Bonevich, A. Shapiro, N. Zhitenev, T.P. Moffat, Three-Dimensionally Structured CdTe Thin Film Photovoltaic Devices with Self-Aligned Back-Contacts: Electrodeposition on Interdigitated Electrodes, J. Electrochem. Soc., in Press.
R8: Poster Session: Nanostructured Sensitized Solar Cells
Session Chairs
Thursday AM, December 03, 2009
Exhibit Hall D (Hynes)
9:00 PM - R8.1
Combined CdS/CdSe Quantum Dot-Sensitized Solar Cells.
Taro Toyoda 1 , Keita Oshikane 1 , Qing Shen 1
1 Department of Applied Physics and Chemistry, The University of Electro-Communications, Tokyo Japan
Show AbstractRecently,much attention has been devoted to semiconductor quantum dot-sensitized solar cells. The use of semiconductor quantum dot (QDs) as sensitizers has advantages in solar cell applications[1]. In this paper, we report high efficiency combined CdS/CdSe QD-sensitized solar cells. QDs were adsorbed onto nanostructured TiO2 electrodes using a chemical bath deposition technique for different periods[2,3]. For adsorbing of combined CdS/CdSe QDs, CdS adsorption onto TiO2 electrodes was carried out prior to CdSe adsorption. Then, the surfaces of the electrodes adsorved with semiconductor QDs were modified with ZnS coating[3]. Finally, a sandwich structure solar cells were prepared. The counter electrode was a Cu2S film on brass[4]. Mixing of the 1M S and 1M Na2S solution (polysulfide redox system) was used as the regenerate redox couple. Photovoltaic properties were studied by using a solar simulator (AM 1.5). We found that there was optimum adsorption period of CdS and CdSe QDs against the photovolataic conversion efficiency. The maximum photovoltaie conversion efficiency of 3.8% has been obtained for CdS/CdSe QD-sensitized solar cells. [1]A.Nozik, Physica E Vol.14,115(2002).[2]S.Gorer and G.Hodes, J.Phys.Chem. Vol.98,5338(1994).[3]Q.Shen,J.Kobayashi,L.J.Diguna,and T.Toyoda, J.Appl.Phys. Vol.103,084304(2008).[4]G.Hodes,J.Manassen,and D.Cahen, J.Electrochem.Soc. Vol.127,544(1980).
9:00 PM - R8.10
Effect of Interfacial Layer Between Transparent Conducting Oxide and TiO2 Layer in the Efficiency of Dye-sensitized Solar Cells.
Kyeong Ha Kim 1 , In Gyoung Yu 1 , Song Yi Han 1 , Wan In Lee 1
1 , Inha University, Incheon Korea (the Republic of)
Show Abstract A few nanometer-sized SnO2, TiO2 and ZrO2 nanoparticles were synthesized by a solvothermal reaction, and applied to the formation of interfacial layers between transparent conducting electrode (TCO) and nanocrystalline TiO2 layer in dye-sensitized solar cells (DSCs). The sizes of the synthesized nanoparticles were determined to 2.5-4.5nm by TEM and AFM analysis. The thickness of each interfacial layer was controlled layer-by-layer by the self-assembly of those nanoparticles. Then, the main TiO2 layer with a thickness of 10μm was formed by doctor-blade method. The introduction of ultra-thin interfacial layers enhanced the photovoltaic conversion efficiency of DSCs by 10-20%. This indicates that the interfacial layer blocks the back-transport of electrons from TCO to the electrolyte and promotes the electron transfer from the main TiO2 to TCO. SnO2/TiO2 system revealed the highest current density (Jsc), suggesting that the interfacial SnO2 layer with lower conduction band position than that of TiO2 expedites the electron transfer from the main TiO2 layer to TCO.
9:00 PM - R8.11
Electronic Structures of Raw Materials of Dye-Sensitized Solar Cell Estimated with ``Photo-electron Spectroscopy in the Air (PESA)".
Yoshiyuki Nakajima 1 , Daisuke Yamashita 1 , Atsushi Ishizaki 1 , Yuko Tsutsui 2 , Satoshi Uchida 2 , Hiroshi Segawa 2
1 , RIKEN KEIKI Co.,Ltd., Tokyo Japan, 2 RCAST, The University of Tokyo, Tokyo Japan
Show AbstractThe electronic structures of organic materials and conductive oxides used for semiconductor devices can be analyzed with the aid of “Photo-electron Spectroscopy in the Air (PESA)” by estimating the work function (WF), the ionization potential (IP) and the density of states (DOS). For this purpose the “open counter” [1]-[3] must be employed as a detector, because it can detect and count small numbers of low energy photoelectrons, one by one, in the air under an atmospheric pressure. A PESA measurement was carried out as follows. UV photons emitted from a deuterium lamp were monochromatized by a grating spectrometer and focused on a sample. Photoelectrons emitted from the sample were counted by the open counter. Here, the energies of monochromatized UV photons were shifted at 0.05 eV intervals up to 6.20 eV, if necessary up to 7.00eV. When photons with higher energies than 6.20eV are used, the air in the monochromator must be replaced by N2 gas because the photons with higher energies than 6.20eV are absorbed in the air. PESA has, if compared with such photoelectron spectroscopies as XPS and UPS, the advantage of performance in a non-vacuum measurement, a high energy-resolution and low photo-excitation energies. A non-vacuum measurement is very much useful for ones engaged in investigations on organic semiconductors in powdered and liquid states. Photoelectron excitation efficiency is high when an electron, localized on the highest occupied molecular orbital (HOMO) or nearby of a condensed matter, is ionized with low energy photons. In addition, radiation damage can be ignored when organic materials are lightly irradiated with photons with low energies. IPs of dyes and conductive oxides which has been used as a raw material for a Dye-Sensitized Solar Cell was derived from an observed photoemission threshold energy.[1] M. Uda and H. Kirihata, Japanese Patent S55-179922 (1980), 1447157 (1988)[2] H. Kirihata, and M. Uda, Rev. Sci. Instr. 52, 68 (1981).[3] M. Uda, Jpn. J. Appl. Phys. 24, 284 (1985)
9:00 PM - R8.12
Controlling Electron Diffusion Length Through Molecular Variation of Outer-sphere Redox Couples in Dye-sensitized Solar Cells.
Jesse Ondersma 1 , Thomas Hamann 1
1 Department of Chemistry, Michigan State University, East Lansing, Michigan, United States
Show AbstractA series of one-electron outer-sphere redox couples were investigated as redox shuttles in dye-sensitized solar cells, DSSCs. Electrochemical impedance spectroscopy, EIS, was employed to probe the electron dynamics of nanoparticle-based TiO2 DSSCs in contact with the various redox shuttles. Analysis of the impedance spectra allowed calculation of electron lifetimes and diffusion lengths. Charge recombination was found to be highly dependent on the physical properties of the redox shuttle employed, with electron lifetimes spanning several orders of magnitude. These results demonstrate the ability to control the electron diffusion length through molecular variation of the redox shuttle. The EIS results were found to correlate well with the photovoltaic characteristics of the DSSCs. Atomic layer deposition was used to deposit an ultra-thin alumina shell on nanoparticle TiO2 photoanodes. The effect of the alumina layer was characterized via EIS and was found to significantly reduce charge recombination, improving diffusion lengths and external quantum yields for all DSSCs containing outer-sphere redox couples.
9:00 PM - R8.13
Layer-by-Layer Deposition of Engineered M13 Bacteriophage for the Construction of Dye-sensitized Solar Cells with Novel Titania Nanowire Architectures.
Rebekah Miller 1 2 , Rebecca Ladewski 1 2 , Paula Hammond 2 , Angela Belcher 1 3
1 Bioengineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractHigh performance dye-sensitized solar cells (DSSCs) rely on a network of titania nanoparticles to conduct the electrons injected by a photosensitizer to the electrode. The titania architecture is crucial to the performance of the cell, determining the surface area available for dye adsorption, the degree of interaction with the electrolyte, and the efficiency of electron transport. Here we report the construction of DSSCs incorporating a novel nanowire architecture templated by the M13 bacteriophage. Taking advantage of the molecular recognition and self assembly behavior demonstrated by viruses, M13 bacteriophage can be engineered to nucleate and assemble a wide variety of inorganic materials.For solar cell applications, the assembly of the phage on a conducting substrate is achieved using layer-by-layer (LbL) deposition, in which electrostatic interactions promote the accumulation of oppositely charged polyelectrolytes from solution. By substituting a negatively charged variant of M13 for the negatively charged polymer during the assembly process, phage is incorporated into the film, resulting in a hybrid material with an architecture that reflects both its biological and polymeric components. By adjusting the assembly and post-assembly pH, the thickness and porosity of the film can be finely tuned to maximize the film’s surface area for dye adsorption and its porosity for electrolyte interpenetration. Similarly, the electrostatic interactions between phage particles within polymeric films can be adjusted to control the packing density and distribution of phage. After the desired morphology is attained, amines groups on the polymer and phage are protonated, and liquid phase infiltration is used to nucleate the growth of titania.To increase the density of titania nanowires, spray LbL was employed to construct films from both positively and negatively charged M13 phage. A negatively charged phage was constructed by mutating the major coat protein to display three glutamic acid residues, and a positively charged phage was constructed by appending a series of polyamines to these residues. The materials characterization and device performance of the resulting structures are described.
9:00 PM - R8.14
A Novel Approach for Fabrication of Dye-sensitized Solar Cell.
Tingli Ma 1 , Mingxing Wu 1 , Li Yang 1 , Gang Xin 1 , Liqiong Wu 1
1 State Key Laboratory of Fine Chemicals, Dalian Univ. of Tech., Dalian, Liaoning, China
Show AbstractSince Graetzel and co-workers developed a new type of solar cells based on the nanocrystalline porous TiO2 electrode, dye-sensitized solar cells have attracted considerable attention as a low-cost alternative to conventional silicon solar cells. The use of flexible substrates in dye sensitized solar cells is desirable because it enables lightweight and flexibility of shape. It also considerably reduces production costs due to roll-to-roll coating production lines. In general, the photoanodes and Pt counter electrode can be fabricated by screen printing and sputting methods. In this paper, in order to achieve binderless low-temperature annealing and reduction of production cost, an all-spray approach is developed for preparing DSCs. As a result, a highly efficient dye-sensitized solar cell (DSC) is obtained; the overall conversion efficiency of the DSCs reaches 8.5% and 4.0% at 1 sun on the conductive glass substrate and polymer electrode substrate respectively. Additionally, the photoelectrochemical properties and EIS of the photoanodes and DSCs fabricated are investigated, the results will be presented.
9:00 PM - R8.15
High Performance Dye-sensitized Solar Cell with a Multiple Dye System.
Reiko Ogura 1 , Masahiro Morooka 1 , Masaki Orihashi 1 , Tetsuhiro Yamada 1 , Kazuhiro Noda 1
1 , Sony Corporation, Atsugi, Kanagawa, Japan
Show AbstractA Dye-sensitized Solar Cell (DSC) using two dyes achieved high external quantum efficiency as sensitizers. We confirmed that terpyridine complex (black dye, solaronix) and an indoline dye (D131, Mitsubishi Paper Mill) were adsorbed by the TiO2 electrode without either dye interfering with the electron transfer of the other dye to the electrode. The high performance of the new arrangement is made possible by the dissociation function of these two particular dyes. The multiple dye system achieved a power conversion efficiency of 11.0%. Moreover, the fabrication of the multiple dye system only mixed the two reagents in one pot.
9:00 PM - R8.16
Highly-Ordered TiO2 Nanotube Arrays for Enhancing Charge Collection Characteristics of DSSC.
Sangwook Lee 1 , Ik Jae Park 1 , Jin Young Kim 2 , Hyun Suk Jung 3 , Kug Sun Hong 1
1 , Seoul National Univ., Seoul Korea (the Republic of), 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 , Kookmin University, Seoul Korea (the Republic of)
Show Abstracta highly oriented TiO2 nanotube array was grown on Ti substrate by using anodizing procedure and the anodized titanium oxide (ATO) was used for a photoelectrode of a dye-sensitized solar cell (DSSC) in order to compare the photovoltaic characteristics with a TiO2 nanoparticle (NP)-based DSSC. The anodized titanium oxide (ATO) arrays exhibited higher electron diffusion and slower recombination characteristics compared to TiO2 NP film. Finally, the electron diffusion length of ATO was much longer than that of NP films, and the energy conversion efficiency of ATO was higher than that of NP films. Through observation of nanostructures for the nanotubes by using a high-resolution transmission electron microscopy, it was found that the ATO is composed of about 100 nm-sized c-axis direction oriented grains with a number of 20 nm-sized primary nanocrystallines. The enhanced charge transport properties were discussed on a view point of the well-aligned crystal structure of the ATO.
9:00 PM - R8.17
Fabrication of Quasi Solid-State Dye-Sensitized Solar CellUsing Heteropolyacid (HPA) - Polyethylene Oxide (PEO) Gel Composite Electrolyte.
M. Shaheer Akhtar 1 , Jung Geun Park Park 2 , Ui-Yeon Kim Kim 3 , O-Bong Yang Yang 4
1 School of Semiconductors and Chemical Engineering, Chonbuk National University, Jeonju, Chonbuk, Korea (the Republic of), 2 School of Semiconductors and Chemical Engineering, Chonbuk National University, Jeonju, Chonbuk, Korea (the Republic of), 3 School of Semiconductors and Chemical Engineering, Chonbuk National University, Jeonju, Chonbuk, Korea (the Republic of), 4 School of Semiconductors and Chemical Engineering, Chonbuk National University, Jeonju, Chonbuk, Korea (the Republic of)
Show AbstractIn order to overcome leakage and evaporation of liquid electrolyte, efforts have been made to replace the liquid electrolyte using gel and solid electrolyte and improved the long term life. Recently, solid polymer electrolytes (SPEs) have received considerable attention in order to improve the ionic conductivity, thermal stability and mechanical properties of polymer. Polymer based electrolytes have displayed a low ionic conductivity at room temperature which is responsible for conduction of ion or charge transport. In this work, novel inorganic (heteropolyacid, HPA)-polymer (polyethylene oxide, PEO) gel composite electrolytes were prepared and characterized for the quasi solid-state dye sensitized solar cell (DSSC). The composites were prepared by simple mechanical method with acetonitrile and mixed solvent of chloroform and methanol (Ch-Me). Mixed solvent of chloroform and methanol (Ch-Me) was found to be the most effective solvent for the preparation of HPA-PEO composite electrolytes with the advanced morphological, cross-linking and conductivity properties. DSSCs fabricated with HPA-PEO/Ch-Me electrolyte showed significantly high conversion efficiency 3.1% with open circuit voltage of 0.524 V and a short circuit current of 9.7 mA/cm2. HPA in the composite gel electrolytes may act as an electron acceptor to prohibit photo-reduction of I-, resulting in the advanced photocurrent density and stability without any significant decline in the PV performance for 7 days.
9:00 PM - R8.18
Polyanionic Overlayers in Dye-Sensitized Nanocrystalline Titanium Dioxide Films.
Paul Hoertz 1 2 , Anna Goldstein 2 , Kyle Brennaman 2 , Jonah Jurss 2 , Javier Concepcion 2 , Carrie Donley 3 , Thomas Meyer 2
1 Center for Aerosol Technology, RTI International, Research Triangle Park, North Carolina, United States, 2 Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States, 3 Chapel Hill Analytical and Nanofabrication Laboratory, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Show Abstract We are exploring the dye-sensitized solar cell (DSSC) approach for solar fuel applications and the issue of forming long-term, stable, surface-bound linkages in aqueous environments is a paramount issue. There have been varying reports in the literature about the extent of loading and stability towards hydrolysis of phosphonate surface links. In our experience, true long term hydrolytic stability is difficult to achieve especially on the longer timescales required for useful applications. We report here results on surface binding and stability and the investigation of strategies for achieving long-term surface stabilization on nanocrystalline titanium dioxide. In one strategy, stabilization of pre-formed monolayer sensitizer structures is accomplished with polyanionic overlayers. In addition to surface stabilization, the polyanionic overlayers were also used to adsorb cationic complexes to produce two-component molecular assemblies adjacent to semiconductor surfaces. These molecular assemblies were studied with time-resolved and steady-state spectroscopic techniques along with photoelectrochemical measurements and the results from these experiments will be presented.
9:00 PM - R8.19
Enhancement of TIO2 Interconnection Using Sol-Gel Processed Nanoglue for Plastic Dye-sensitized Solar Cells.
Yuelong Li 1 2 , Doh-Kwon Lee 1 , Kyungkon Kim 1 , Nam-Gyu Park 1 , Min Jae Ko 1
1 Solar Cell Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 International R&D Academy, University of Science and Technology, Daejeon Korea (the Republic of)
Show AbstractThe formation of well-necked TiO2 electrode at low temperature is critical for plastic dye-sensitized solar cells (DSSCs). However, low temperature process cannot provide good interparticle connection as conventional high temperature process does. Therefore, enhancement of interconnection at low temperature is required for high efficiency plastic DSSCs.A TiO2 colloidal sol as a “glue” agent was developed by hydrolysis and polycondensation of titanium alkoxides to enhance interconnection. The crystalline phase of this TiO2 glue is pure anatase with average particles size of less than 10 nm, which was characterized by powder X-ray diffraction (XRD) and high revolution transmission electron microscopy (HRTEM). The viscous alcoholic paste without organic binder was prepared with commercial P25 powder and this TiO2 glue. The photovoltaic performance was optimized with variation of paste composition and process parameters. The conversion efficiency of 5.8% and 4.3% were achieved on FTO/glass and ITO/PEN substrate, respectively. Further improvement of cell performance with this TiO2 glue system is in progress.
9:00 PM - R8.2
CdSe Quantum Dot-Sensitized Solar Cells Based on TiO2 Nanotube Electrodes.
Taro Toyoda 1 , Satoru Tamura 1 , Qing Shen 1
1 Department of Applied Physics and Chemistry, The University of Electro-Communications, Tokyo Japan
Show AbstractA great deal of attention has been devoted to dye-sensitized solar cells (DSSCs) made from nanostructured TiO2 electrodes. The morphologies of TiO2 electrodes and sensitizers are important factors for improving the photovoltaic conversion efficiency of DSSCs. TiO2 electrodes with a higher degree of order than those made from a disordered assembly of nanoparticles are desirable for improvements in electron transfer rate and photovoltaic conversion efficiency. On the other hand, semiconductor quantum dots (QDs) have also been the subject of considerable interest as light harvesters in DSSCs as an alternative to organic dyes[1-3]. The use of semiconductor QDs as sensitizers has some advantages in solar cell applications[4]. In this study, TiO2 nanotube electrodes are prepared, following the electrochemical anodization of Ti metal[5]. Electrochemical experiments were carried out using high voltage potensiostat. Electrolytes were 0.5wt% NH4F in glycerol at 40 centigrade. Counter electrode is a Pt gauze and applied voltage of 20 V with a sweep rate of 0.1 V/s, after which the applied voltage was held at 20 V for 12 hours. The lengh of the nanotube is ~6 micrometers. CdSe QDs were prepared by chemical bath deposition technique. Na2SeSO3, CdSO4, and [N(CH2COONa)3] were mixed and TiO2 nanotube electrodes were innnersed in the mixed solution at 10 centigrade for various times (11~48 hours).Then, the surfaces of the electrodes were modified with ZnS coating[3]. Finally sandwich structure solar cells were prepared with a counterelectrode of Cu2S film on brass[6]. Polysulfide redox system was used as the regenerate redox couple. Photovoltaic properties were studied by using solar simulator (AM 1.5). We have found that there was an optimum adsorption time of CdSe QDs (~32 hours) against the photovoltaic conversion efficiency. The maximum photovoltaic conversion efficiency of 1.9% has been obtained for CdSe QD-sensitized solar cells based on TiO2 nanotube electrodes.This vale is higher than that made from a disordered assembly of TiO2 nanopartcles on Ti metal substrates. [1]Q.Shen,M.Yanai,K.Katayama,T.Sawada,and T.Toyoda, Chem. Phys.Lett. Vol.442,89(2007).[2]L.J.Diguna,Q.Shen,J.Kobayashi,and T.Toyoda, Appl.Phys.Lett. Vol.91,023116(2007).[3]Q.Shen,J.Kobayashi,L.J.Diguna,and T.Toyoda, J.Appl.Phys. Vol.103,084304(2008)[4]A.Nozik, Physuica E Vol.14,115(2002).[5]M.Macak,H.Tsuchiya,L.Taveira,S.Aldabergerova,and P.Schmuki, Angew.Chem.Int.Ed. Vol.44,7463(2005).[6]G.Hodes,J.Manassen,and D.Cahen, J.Electrochem.Soc. Vol.127,544(1980).
9:00 PM - R8.20
CdSe QDs/Ruthenium Dye Co-sensitized Solar Cells.
Yan-Fen Li 1 , Norris Jordan 1 , Tzu-Fan Chen 1 , Tingying Zeng 1
1 Laboratory for Nanostructures, Department of Chmeistry, Western Kentucky University, Bowling Green, Kentucky, United States
Show Abstract*Corresponding author‘s current address: Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307. Email:
[email protected] quantum dots (QDs) are attracting great attention for nanostructured solar cells to efficiently use sunlight from visible to IR range. The optical property of quantum dot depends on its particle size and surface physics and chemistry. There are considerable research ongoing for QDs-sensitized titanium dioxide solar cells. However, the reported efficiency is relatively low compared to dye-sensitized solar cells. Technically, large amount of QDs mass-loading on TiO2 nanocrystals is a critical challenge to achieve high light conversion efficiency, especially to obtain high photocurrent from the devices. Here, we report a co-sensitization strategy using CdSe QDs and a ruthenium dye, which presents improved photocurrent and a much higher fill factor, giving a promising direction for the development of high performance QDs/dye nanostructured solar cells.
9:00 PM - R8.21
Influence of Particle Size on Charge Dynamics and Performance of Solid-State Dye-Sensitized TiO2 Solar Cells.
Song-Rim Jang 1 , Kai Zhu 1 , Nathan Neale 1 , Arthur Frank 1
1 Chemical and Biosciences Center, National Renewable Energy Laboratory (NREL), Golden, Colorado, United States
Show Abstract Dye-sensitized solar cells (DSSCs) have attracted widespread interest for the conversion of sunlight into electricity because of their low cost and promising efficiency levels. During the past 10 years, research on replacing the traditional iodide-containing liquid electrolyte with a solid-state hole-conducting phase has aimed at improving the durability and extending the operating temperature of cells. One example of such solid-state dye-sensitized solar cells consists of dye-sensitized mesoporous TiO2 nanoparticle films infiltrated with spiro-OMeTAD [2,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9’-spirobifluorene] as an organic hole-transport material. Until now, research efforts on such solid-state DSSCs have centered mainly on (1) developing methodologies to fill the nanopores of TiO2 films completely with spiro-OMeTAD electrolyte and (2) studying the effect of the electrolyte composition (e.g., spiro-OMeTAD concentration, doping, and additives) on cell performance. However, little is known about the influence of TiO2 film morphology (e.g., particle size and film porosity) on the electron dynamics (charge transport and recombination) and cell characteristics. In this presentation, we discuss the results of our studies on the effect of particle and pore size on the electron dynamics and performance of solid-state DSSCs. We report the synthesis and physical characteristics of TiO2 nanoparticles ranging in size from 15 to 40 nm and films made from these particles with pore sizes ranging from 16 to 37 nm. The pore-filling parameters and performance of spiro-OMeTAD-based solid-state DSSCs will be presented. Also, the TiO2 film morphology is shown to affect strongly the charge transport and recombination properties. These results and others will be discussed.
9:00 PM - R8.22
Comparison of Titanium Dioxide Blocking Underlayers Prepared by Different Techniques for Dye-sensitized Solar Cells.
Kazuteru Nonomura 1 , Tannia Marinado 1 , Jarl Nissfolk 1 , Gerrit Boschloo 2 , Anders Hagfeldt 2
1 Department of Chemistry, Inorganic Chemistry, Royal Institute of Technology (KTH), Stockholm Sweden, 2 Dept. of Physical and Analytical Chemistry, Uppsala University, Uppsala Sweden
Show AbstractDye-sensitized solar cells have been investigated intensively due to its environment friendly properties. Up to now, the highest efficiency of 11.3 % has been reported.(i) Relatively long lifetime of electrons compared to the electron transport time in TiO2 porous thin films is one of the reasons for this high efficiency. One of the ways to improve the efficiency further is using a redox electrolyte which has more positive redox potential than I-/I3- and which needs less driving force to regenerate the oxidized dyes following the electron injection to TiO2. Another way to improve the efficiency is to extend the electron lifetime more. One of the recombination pathways is a recombination via FTO substrate. The blocking underlayer is important to block such recombination. Moreover, for solid state dye sensitized solar cells, it is important to block the FTO surface to avoid short circuit with hole conductors. TiO2 compact layer is deposited on the surface of FTO mostly either by spray pyrolysis or immersing the substrate into TiCl4 aqueous solution at 70 degrees. The aim of this study is to compare the blocking layers prepared by different techniques. Also the case having no blocking layer is tested as reference. Electrochemical and photoelectrochemical characterization have been done to see the effect of these under layers in dye-sensitized solar cells. Thin and compact TiO2 layers deposited on FTO substrate are characterized by cyclic voltammetry and electrochemical impedance spectroscopy. Dye-sensitized solar cells are also fabricated with these TiO2 compact layers and their properties are characterized by measuring current-voltage curve, incident photon to current conversion efficiency (IPCE), electron transport time, electron lifetime and the charge extraction measurement. (i) Gao. F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Grätzel, M. J. Am. Chem. Soc. 2008, 130, 10720
9:00 PM - R8.24
Investigation of the Processes Involved in Preparation of Dye Sensitized Photovoltaic Cells.
Carsten Maedler 1 , Harald Graaf 1 , Mirko Kehr 1 , Torsten Oekermann 2
1 Physics, Chemnitz University of Technology, Chemnitz Germany, 2 Physical Chemistry and Electrochemistry, Leibniz University Hannover, Hannover Germany
Show AbstractDye-sensitized photovoltaic cells with zinc oxide (ZnO) as the inorganic semiconductor and organic dye molecules as the sensitizer are well known devices with high efficiency. Such cells are prepared by electrochemical deposition of an aqueous zinc salt solution including dye molecules. After deposition the dye is desorbed to obtain a porous ZnO network followed by re-adsorption of the dye as sensitizer. Up to now studies concerning the influence of different processing steps on the structure of the ZnO are sparse. We discuss the growth mechanism during film deposition and the crystal structure changes of the ZnO accompanying the desorption process, which is performed in an alkaline aqueous solution. X-ray investigation shows the influence of the dye on the structure of the formed ZnO/dye hybrid film. AFM topography and Kelvin Probe Force Microscopy investigations suggest the following deposition process: at first dye molecules are adsorbed on the electrode followed by ZnO formation within the pores of the organic network. This ZnO, which shows high crystallinity, seems to be oriented with the zinc face up.
9:00 PM - R8.25
Cyclometalated Ru Chromophores in the Dye-Sensitized Solar Cell.
Bryan Koivisto 1 , Kiyoshi Robson 1 , Paolo Bomben 1 , Terry Gordon 1 , Pavel Sedach 1 , Curtis Berlinguette 1
1 Chemistry, University of Calgary, Calgary, Alberta, Canada
Show AbstractThe highest power output achieved to date in the dye-sensitized solar cell (DSSC) utilizes light-harvesting units based on derivatives of [Ru(bpy)3]2+ (e.g., [Ru(bpy-COOH)2(NCS)2]). Consequently, the vast majority of dyes reported in the literature have been based on related Ru-polypyridyl complexes. Departing from this line of inquiry, our program is exploring the viability of a various cyclometalated metal complexes to accomplish the necessary light absorption and charge-injection events. This approach should not only help to improve the thermal and photostability of the dye complex, it also serves to extend the absorption profile into the NIR region of the solar spectrum (Figure 1). This presentation will provide an assessment of the performance of these organometallic Ru complexes in the DSSC.
9:00 PM - R8.27
High-Performance Dye-Sensitized Solar Cell Based On Large Surface Area Aqueous Synthesized Titania Nanoparticles.
George Demopoulos 1 , Cecile Charbonneau 1 , Keeeun Lee 1 , Guobin Shan 1 , Raynald Gauvin 1
1 Materials Engineering, McGill University, Montreal, Quebec, Canada
Show AbstractAqueous synthesized titania nanoparticles based on controlled forced hydrolysis of aqueous Ti(IV) chloride solution have been successfully developed. The as-synthesized titania nanoparticles possess high specific surface area (160 m2/g), enlarged bandgap, and enhanced surface hydroxylation endowed by the aqueous synthesis route. The plentiful hydroxyl groups on the titania surface appear to give rise to a relatively strong interaction between the dye molecules and the titania that facilitates electron injection and transport within the dye-sensitized titania electrode. The aqueous-synthesized titania nanoparticles were used to prepare electrodes by screen printing on FTO, annealing at 450 oC and sensitization with the N719 dye and then assemble dye-sensitized solar cells (DSSCs) with iodide electrolyte. Performance parameters of DSSCs based on the aqueous-synthesized titania were compared to those of cells built with benchmark sol-gel and commercial paste materials and found to reach and exceed the best among them. The enhanced performance of the aqueous titania-based DSSC is discussed on the basis of extensive characterization work that includes HRTEM, Raman/FTIR spectroscopy, and Electrochemical Impedance Spectroscopy.
9:00 PM - R8.28
Dye Sensitized Solar Cells based on Hydrothermally Synthesized TiO2-Nanoleaves.
Vivek Dhas 1 , Subas Muduli 2 , Wonjoo Lee 3 , Shankar Patil 1 , Sung-Hwan Han 3 , Satishchandra Ogale 2
1 Department of Physics, University of Pune, Pune, Maharashtra, India, 2 Physical and Materials Chemistry Division, National Chemical Laboratory, Pune, Maharashtra, India, 3 Department of Chemistry, Hanyang University, Seoul Korea (the Republic of)
Show AbstractMesoporous films made of elongated n-type metal oxide nanostructures are desirable for dye-sensitized solar cells because they provide direct conduction pathways for photogenerated electrons. In this work elongated TiO2-nanoleaves are synthesized by hydrothermal route using mixed precursors and controlled conditions. The crystal structure, morphology, and mechanism of formation of titanium dioxide nanoleaves produced by the alkaline hydrothermal method are critically analyzed. For comparison, TiO2 nanoparticles are also synthesized by hydrothermal route by controlling the growth conditions. Solar cells are prepared with these two powders as well as P25 (Degussa) as reference. It is observed that dye sensitized solar cells made using doctor bladed film of TiO2-Nanoleaves yield conversion efficiency of 5.8% which is considerably higher than that of TiO2 nanoparticles (~4.9%) and P25 sample (~4.7%). This can be attributed to an improved charge carriers transport afforded by elongated structure i.e. direct pathway for the transportation of charge carriers without hopping and thereby possibly lower effective carrier recombination rate as well. Detailed characterizations involving X-ray diffraction, Optical absorption, Transmission Electron Microscopy (TEM), FT-IR and J-V characteristics measurements are performed and will be presented /discussed.
9:00 PM - R8.29
Highly Efficient Nano-platinum Counter Electrode for Dye-sensitized Solar Cell.
Tzu Chien Wei 1 , Jo-Lin Lan 1 , Wen-Chi Hsu 1 , Ya-Huei Chang 1 , Hai-Peng Cheng 1 , Wen-Hsiang Chen 1
1 Hsinchu Laboratory, Tripod-technology, Hsinchu City Taiwan
Show AbstractWe developed a unique process to fabricate nano-sized Pt on TCO substrate for DSC, Pt size, consumption and catalytic performance of tri-iodide reduction can be precisely controlled via a 2-step dip coating process in water-based solution. Data revealed that an ultra-thin layer of Pt sizing about 20-30 nm was deposited on FTO glass at the Pt usage below 10 μg/cm2. After a post heat treatment at 270oC for 5 min, the catalytic performance of tri-iodide reduction can improve to <1ohm-cm in acetonitrile-based media. DSC employed with nano-Pt/FTO counter electrode reaches 9% at 1 sun AM1.5G test environment. This nano-Pt/FTO electrode also showed excellent thermal stability test (60oC, 1000hours) under simple sealed cell. This process will be soon commercialized as a fast and cost-effective process for counter electrode production.
9:00 PM - R8.31
Synthesis of TiO2 Nanowires and Application to Bi-functional Light Scattering Layers in Dye-Sensitized Solar Cells.
JiHye Lee 1 , Hyunsuk Jung 1
1 School of Advanced Materials Engineering, Kookmin university, Seoul Korea (the Republic of)
Show AbstractDye-sensitized solar cells (DSSCs) have received a vast amount of interest as a promising alternative to conventional photovoltaic devices due to their high efficiency and low production cost. The advanced solar cell performance has been achieved by extensive studies on optimization of dyes, semiconductor films, and redox electrolytes.One of the promising methods for improving DSSC efficiency is to utilize incident light more efficiently. For example, the energy conversion efficiency of the cell has been improved by employing a light scattering layer that contains the several hundreds nanometer scaled TiO2 on the anatase active layer surface. If we can improve the surface area of bulky TiO2 without changing the dimension, the light scattering layer can exhibit another function as an active layer. In the present study, we synthesized TiO2 nanowires using a glycolate process and tried to investigate their feasibilities as light scattering layers. The TiO2 nanowires that possess an aspect ratio of 16 were approximately 1x16 um characterized using field emission electron scanning microscopy (FESEM). The combination of high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) revealed that the TiO2 nanowire consisted of approximately 20 nm anatase nanocrystals. Also, Brunauer Emmett Teller (BET) measurement revealed that the TiO2 nanowire possess a high specific surface area, 50.431 m2/g. The TiO2 nanowires were coated onto TiO2 active layers and characterized as scattering materials in DSSCs. The energy conversion efficiencies of DSSCs that contained the TiO2 nanowires scattering layers were comparable to those of DSSCs utilizing commercial scattering materials (CCIC) with similar film thickness. Given the proper optical properties and the large amount of dye adsorption, the TiO2 nanowire layers were considered another alternative for functional light scattering layer.
9:00 PM - R8.32
Highly Crystalline TiO2 Nanoparticle Based Inverse Opal: A New Light Scattering Layer in Dye-Sensitized Solar Cell.
SeHoon Han 1 , HyunSuk Jung 1
1 , kookmin university, Seoul Korea (the Republic of)
Show AbstractPhotoelectrochemical solar cells such as dye-sensitized cells (DSSCs), which exhibit high performance and are cost-effective, provide an alternative to conventional p-n junction photovoltaic devices. However, the efficiency of such cells plateaus at 11-12%, in contrast to their theoretical value of 33%. The majority of research has focused on improving energy conversion efficiency of DSSC. For example, the employment of light scattering layers in DSSCs has been reported to enhance efficiencies because of efficient utilization of incident solar light. In the present study, we successfully fabricated nanostructured light scattering layers that possessed an inverse opal structure. By using highly crystalline TiO2 colloids and monodispersed carboxylated polystyrene (PS) particles, the nanostructured inverse opal layers containing highly-crystalline anatase nanoparticles were deposited on the anatase active layers and the resultant DSSC performance was characterized in comparison with DSSCs containing commercial light scattering layers. The photocurrent density of DSSCs that employed the nanostructured inverse opal layer was significantly improved up to approximately 20 %. The optical characterization of photoelectrode showed that the enhanced DSSC performance was ascribed to the efficient light scattering effect. Also, our inverse opal scattering layers were found to work as active layers because the nanostructured scattering layers consist of highly crystalline anatase nanoparticles, which facilitate dye adsorption. Therefore, it was concluded that the bi-functions of nanostructured inverse opal layers, light scattering and dye adsorption, were responsible for the improved DSSC performance.
9:00 PM - R8.33
Nanoparticle Layers Transformed from Ordered TiO2 Nanotube Arrays and Based Dye-Sensitized Solar Cells.
Yahya Alivov 1 , Xuan Pan 1 , Zhaoyang Fan 1
1 Electrical and Computer Engineering, Texas Tech University, Lubbock, Texas, United States
Show AbstractTransformation of nanotubes to nanoparticles has been observed in titanium dioxide (TiO2) ordered nanotube (NT) arrays after thermal annealing. The formed nanoparticles have truncated bipyramid-like shape with high percentage of {001} facets. The original amorphous TiO2 nanotube arrays were grown by electrochemical anodic oxidation of titanium sheets in ethylene glycol + NH4F electrolyte. From a series of experiments it was found that the nanotube-nanoparticle transformation results from catalytic reaction of fluorine ions (F-) from the electrolyte residues in long nanotubes. This transformation occurs at high ramping rate of annealing temperature and with the opening end of nanotubes “semi-sealed” by in contact with a supporting plate. These conditions ensure that evaporation of electrolyte residues in nanotubes is slow enough to remain until 500 oC when this transition starts. Such a transition can be also achieved by annealing of nanotube arrays in a sealed container with droplets of NH4F aqueous solution present. Size of the formed TiO2 nanoparticles can be controlled within 20-500 nm range by varying fluorine concentration. The crystal and optical properties of nanoparticle layers are much superior compared to those of nanotube arrays. Using nanoparticle layers transformed from nanotube array films dye-sensitized solar cells (DSSC) were fabricated and their performance studied as a function of nanoparticle size. It was found the efficiency of DSSCs under illumination using AM 1.5 solar simulator (100 mW/cm2) greatly depended on NP size (d) varying in the range 1.45% - 9.05%. It had maximum value 9.05% for DSSCs with nanoparticle size 65 nm, and decreased with d to 1.45% corresponding to d=350 nm. Open-circuit voltage was about 0.65 V until d=90 nm, and then dropped to 0.51 V for d=350 nm. Short-circuit current first increased from 17.7 mA to 22.7 mA when d increased from 35 nm to 90 nm, and then decreased with d to 7.8 mA corresponding to DSSC with NP size 350 nm. Unlike ISC, fill factor monotonically decreased from 0.72 to 0.37 when d increased from 35 nm to 350 nm. The behavior of solar cell parameters is discussed in view of competition between recombination losses and photo-electron generation. This work demonstrates a simple method for producing anatase TiO2 nanoparticles layers and high efficiency DSSCs.
9:00 PM - R8.34
Titania Nanofibers Produced by Electrospinning as a Promising Alternative for DSSC.
Jan Macak 1 , Radek Polej 1 , Jiri Hillebrand 1 , Jiri Duchoslav 1 , Lukas Rubacek 1
1 Research and development, Elmarco, Ltd., Liberec Czechia
Show AbstractThe presentation deals with the exploitation of the 2D titanium dioxide nanofibers in the dye-sensitized solar cell. The nanofibers are produced by electrospinning from the polymer-based solution using the Elmarco Nanospider technology allowing for the production of large material quantities. By using various experimental conditions the nanofiber dimensions can be varied significantly. In general, the as-prepared nanofibers are several micrometers long and have diameter in the range of few hundred nanometers. Our new concept is based on the substitution of conventionally used nanoparticles by nanofibers in the classical cell architecture with the aim to improve the conversion effieciency of the cell. We show that significant efficiencies can be achieved using the TiO2 nanofiber layers. We will discuss the synthesis and properties of the nanofibers as well as critical issues of their use in the DSSC.
9:00 PM - R8.35
Triarylamine-Functionalized Ruthenium Dyes for Efficient Dye-sensitized Solar Cells.
Hideki Masuda 1 , Zhengzhe Jin 1 , Noriyo Yamanaka 2 , Masaki Minami 2 , Yoshinori Nishikitani 2
1 Department of Frontier Materials science, Nagoya Institute of Technology, Nagoya, Aichi, Japan, 2 Central Technical Research Laboratory, Research & Development Division, Nippon Oil Corporation, Yokohama, Kanagawa, Japan
Show AbstractDSSCs based on mesoporous nanocrystalline TiO2 films have attracted intensive interest for scientific and industrial applications due to their high photo-to-electricity conversion efficiency and low production cost. The best energy conversion efficiency of up to 11% was achieved only by using ruthenium dyes, such as N3, N719, and black dye in standard global air mass 1.5 sunlight. To further increase the efficiency of these cells, much effort has been directed toward the development of highly efficient solar sells based on ruthenium dyes. A recent strategy for identification of new ruthenium dyes is to replace one of the dcbpy ligands of N3 with a highly conjugated ancillary ligand. The main disadvantage of this strategy arises due to a very small electronic excitation contributed by the highly conjugated ancillary ligand, which is not directly connected to TiO2 particles. This electronic excitation affects the conversion efficiency of the DSSCs and causes relatively low adsorption of molecules onto the TiO2 particles. Based on the above consideration, we have designed novel ruthenium dyes J series consisting of dcbpy (which acts as the electron acceptor and anchoring ligand), thiocyanato ligands, Ru metal acting as the electron donor, and a triarylamine ligand as the electron donor, where organic dyes including triarylamine moieties which act as an electron donors are known to exhibit highly efficient conversion of solar energy into electricity. Under standard global AM 1.5 solar conditions, the J6-sensitized solar cell produces a short circuit current density (Jsc) of 17.9 mA cm-2, Voc of 630 mV, and a fill factor (FF) of 0.70, corresponding to an overall conversion efficiency (η) of 7.9%. These values are close to those of the N719-sensitized cell. In contrast, the J5-sensitized solar cell has a Jsc value of 14.1 mA cm-2, Voc of 580 mV, and FF of 0.71, corresponding to η of 5.8%. The higher efficiency of the J6 relative to the J5-sensitized solar cell reveals the important role of the substituted methoxy group in the ancillary ligand of the ruthenium complex. The substituted methoxy groups provide directionality to the excited state and prevent the triiodide in the electrolyte from recombining with injected electrons in the TiO2 conduction band. This effect leads to increased open-circuit potentials relative to the J5-sensitized solar cell. A similar influence is also observed for the J9-sensitized solar cell, which has a Jsc value of 16.5 mA cm-2, Voc of 620 mV, and FF of 0.69, corresponding to η of 7.1%. This value of overall efficiency is 22.4% higher than that of the J5-sensitized solar cell. The disadvantage of J9 may be its relatively large molecular size compared to J5 and J6, which may decrease the uptake of sensitizer onto the TiO2 surface. Thus, the J9 type sensitizers would be more effective for a dye-sensitized solar cell with larger TiO2 particles.
9:00 PM - R8.36
Fast Electron Collection in Dye-Sensitized Solar Cell employing Zn/ZnO Core-Shelled Nanoarchitecture as Anode.
Zhenzhen Yang 1 , Tao Xu 1
1 Chemistry , Northern Illinois University, Dekalb, Illinois, United States
Show AbstractSolar cells based on dye-sensitized nanostructured semiconductors are promising for low cost solar energy conversion. Fast electron collection from the charge generation interface to collecting electrode is critical to reduce the chance of charge recombination, thus to improve the conversion efficiency of the device. We report here Zn/ZnO core-shelled nanoarchitectures as anode in a dye-sensitized solar cell for faster electron collection. The self-assembled core-shelled structure is composed of internal Zn micropyramids, which are wrapped by densely self-assembled ZnO nanohairy structures. The device exhibit much higher open-circuit voltage (by about 100-150mV) under 1.5 AM solar radiation in comparison to typical values obtained with ZnO-based nanostructured anode including ZnO nanocrystalline and nanowires. In addition, charge collection time constant is measured to be faster than conventional ZnO-based dye sensitized solar cells.
9:00 PM - R8.37
Dual Functional Mesoporous TiO2 Particles for Light Scattering Layer in High Performance Dye-sensitized Solar Cell.
Sung Kim 1 , Mingshi Jin 1 , Yoon Lee 1 , Seong Hwang 1 , Kiyoung Moon 1 , Byong Shin 2 , Ji Lee 2 , Ji Kim 1
1 Chemistry, Sungkyunkwan University, Suwon Korea (the Republic of), 2 Energy Lab, Samsung SDI, Youngin Korea (the Republic of)
Show AbstractDye-sensitized solar cell (DSSC) is regarded as a regenerative low-cost alternative to conventional devices. To increase photo-current density, bi- or tri-layer structure film using various size and type TiO2 particles is fabricated as a light scattering layer such as sphere, hollow and inverse opal. However, these materials is too restriction of dye adsorption for enhance photo-conversion efficiency. A dual functional material with performed enough adsorption amounts of dye and light scattering role is required to overcome limitation of pre-existence materials and go for high performance DSSC. Here, we modify the application of mesoporous TiO2 particles which is synthesized by nanocasting method from mesoporous silica template as a light scattering layer in dye-sensitized solar cells. The light scattering layer using mesoporous TiO2 particles shows that it has much adsorption amount of dye by high surface area which is the representative characteristic of mesoporous material and also light scattering effect by its particle size around 200 nm spheres. This dual functionality of mesoporous TiO2 particles suggests new advance for high photo-conversion DSSC.
9:00 PM - R8.38
Hydrothermal Synthesis of Single Crystalline TiO2 Nanowire Arrays on Any Arbitrary Substrate and its Application in Dye-Sensitized Solar Cells.
Akshay Kumar 1 , Chongwu Zhou 2
1 Materials Science, University of Southern California, Los Angeles, California, United States, 2 Electrical Engineering, University of Southern California, Los Angeles, California, United States
Show AbstractPhotovoltaic devices have become one of the most important research directions in view of the ever increasing demand of energy. Among various technologies, excitonic solar cells including organic, hybrid organic-inorganic and dye sensitized cells (DSSCs) are the prime candidates for low-cost, highly-efficient solar cells. Dye solar cells were first proposed by Gratzel. et. al in which they used TiO2 film sensitized with organic dye molecules which form type II band alignment. Upon exposure to light, excitons are created in dye and due to the offset in the conduction band energies of dye and TiO2, photogenerated electron migrates to TiO2. These electrons, after traveling through the TiO2 layer, are subsequently collected at the anode. Recent efforts have been focused on replacing thin film of TiO2 with zero dimensional nanoparticles and one dimensional nanowires to improve upon the charge collection efficiencies. Because of several orders of magnitude difference in electron diffusion coefficient between the nanparticles and their single crystal bulk counterpart, nanowire arrays can significantly enhance the performance of dye soalr cell if used with optimized configuration. Towards this objective, use of TiO2 nanotubes and ZnO nanowires have been reported recently as a conduit for electrons. In this work, we report on the synthesis of vertical array of TiO2 nanowires directly on the transparent conductive electrode and its use in dye-sensitized solar cell. A mild hydrothermal synthesis protocol was developed to grow high quality TiO2 nanowires on virtually any substrate including FTO, ITO, Glass and Si (100). XRD and TEM analysis indicated that as synthesized nanowires are of rutile phase. Effect of various titanium precursors on the growth rate and nanowire morphology was studied. Careful examination indicated that the alignment of nanowires is critically dependent on the angle and position of the substrate inside the autoclave. For increased growth time, a self-standing film of vertical TiO2 nanowires was obtained. The versatility of growth conditions were demonstrated by growing nanowires on glass rod and glass pipes. Finally, vertical TiO2 nanowires on FTO were sensitized with standard dye molecules to obtain a solar cell with a power conversion efficiency of 1.8%. Incorporation of a thin TiO2 barrier layer and TiCl4 treatment improved the efficiency. Research on further improving the efficiency by employing solid electrolyte is under progress.
9:00 PM - R8.39
Modification of Anodized TiO2 Nanotubes for Efficient Dye-sensitized Solar Cells.
Seok-Soon Kim 1
1 , Kunsan National University, Kunsan Korea (the Republic of)
Show AbstractInterest in the fabrication of 1-dimensional inorganic nanostructures, in particular, Titania (TiO2) nanotubes, which is promising semiconductor material for gas sensors, hydrogen generators, and dye-sensitized solar cells(DSSCs), have been considerably increased. Among various methods including hydrothermal treatment, template-assisted deposition, and electrospinning, fabrication of self-organized TiO2 nanotube arrays by anodizing of Ti substrate has been extensively investigated due to the defined nanoscale morphology of a high ordering and controllability of morphology by electrochemical conditions. Although several groups are studying on the anodized TiO2 nanotube based dye-sensitized solar cells that can provide high surface area and minimized grain boundaries, the efficiency is still low compared to the conventional TiO2 nanocrystalline based system. Here, we report on the fabrication and modification of self-organized TiO2 nanotubes with secondary inorganic nanostructures to improve the dye absorption and injection of electrons from excited dye to conduction band of TiO2. The structural and kinetic properties, even the conversion efficiency and IPCE(Incident photon to current conversion efficiency) of solar cells, will be discussed.
9:00 PM - R8.4
Significant Influence of Surface Treatment of Mesoporous SnO2 Photoelectrode by Bi or/and Zr Derivatives on its DSC Performance.
Takahiko Ono 1 , Takeshi Yamaguchi 1 , Hironori Arakawa 1
1 Department of Industrial Chemistry, Tokyo University of Science, Tokyo Japan
Show AbstractDye-sensitized solar cell (DSC) is receiving much attention because of its high efficiency and estimated low cost. In order to increase efficiency of DSC, improvement of Voc without any decrease of Jsc is important. The surface treatment of photoelectrode by different oxide thin layer is reported as one of way to realize it. [1] However, systematic studies with the relationship between various oxide species and improvement of Voc are scarce. We report here the detailed study with the significant improvement of DSC efficiency by double surface treatments using Bi2O3 and ZrO2. Mesoporous SnO2 photoelectrodes were prepared by screen printing of SnO2 paste onto FTO/glass. SnO2 pastes included nano SnO2 particles with 10-12 nm in diameter. Surface treatment by various oxide species were conducted by dipping SnO2/FTO/glass into the ethanol solution of oxide precursors, followed by calcination at 500 C for 1 hour. N719 dye was used as photosensitizer. The electrolyte was composed of the mixture of 0.05mol/L-I2, 0.1mol/L-LiI, 0.6mol/L-DMPImI and 0.5mol/L-TBP in acetonitrile solvent. Solar cell performance of DSC was measured under the irradiation of AM1.5, 100mW/cm2. XRD, BET surface area measurement, SEM, XPS, EIS, IPCE and open circuit voltage decay (OCVD) method were used for characterization. The efficiency of DSC with non-treated SnO2 photoelectrode was η=1.7% (Jsc=11.6mA/cm2, Voc=0.33V, ff=0.45). Among nine oxide species used here, only Bi2O3 and ZrO2 improved the efficiency more than 3%. The efficiency of DSC treated by Bi2O3 was η=3.2% (Jsc=11.7mA/cm2, Voc=0.50V, ff=0.54) because of Voc improvement. It was clarified by the OCVD method [2] that there was a good relationship between the electron life-time (τ) in SnO2 photolectrode and Voc. Therefore, The improvement of Voc by the surface treatment was ascribed to the increase of electron life-time by suppression of back-electron transfer reaction due to blocking oxide species over SnO2. Furthermore, double treatment of SnO2 by Bi2O3 and ZrO2 improved the efficiency of DSC up to η=4.2% (Jsc=11.6mA/cm2, Voc=0.57V, ff=0.64). The fil factor (ff) was also improved by the synergy effect of two oxide species. Surface composition of SnO2 photoelctrode was investigated by XPS measurement. The atomic ratio of Sn to oxygen of non-treated SnO2 photoelectrode was 1 to 1.8. On the other hand, it of treated SnO2 photoelectrode by Bi2O3 and ZrO2 was 1 to 1.9-2.0. That is, oxygen defciencies were observed in non-treated SnO2 photoelectrode. It is speculated that this oxygen deficiencies on the surface of non-treated SnO2 photoelectrode act as electron recombination sites and this leads to the decrease of Voc of DSC using non-treated SnO2 photoelectrode. The details will be explained by the poster.References[1] Yishay Diamant, Shlomit Chappel, S. G. Chen, Ophira Melamed, Arie Zaban, Coord. Chem. Rev. 248 1271 (2004)[2] Arie Zaban, Miri Greenshtein, Juan Bisquert ChemPhyschem. 4 859 (2003)
9:00 PM - R8.40
Hierarchical Rutile TiO2 and Their Surface Modified Nanostructures: Synthesis, Characterization and Evaluation of Dye-sensitized Solar Cells Performance.
In-Sun Cho 1 2 , Seong Sik Shin 2 , Sangwook Lee 2 , Dong Wook Kim 2 , Hyun Suk Jung 3 , Kug Sun Hong 2
1 Research Institute of Advanced Materials, Seoul National University, Seoul Korea (the Republic of), 2 Department of Materials Science & Engineering, Seoul National University, Seoul, Shillim-dong, San 56-1, Gwanak, Korea (the Republic of), 3 School of Advanced Materials Engineering, Kookmin University, Seoul, Jeongneung-dong, Seongbuk-gu, Korea (the Republic of)
Show AbstractThree-dimensionally organized rutile TiO2 and their surface modified heterogeneous nanostructures were prepared by a simple two-step hydrothermal method. The resulting nanostructured TiO2 powders were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high-resolution TEM (HR-TEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and UV-Vis spectroscopy. The prepared rutile TiO2 powders exhibited a bur-like morphology of self-assembled nanorods and their morphology was also maintained after surface modification process by TiO2 (anatase) and/or MgO coating. Using these 3D hierarchical TiO2 powders, the dye-sensitized solar cell (DSSCs) performance was evaluated and the effects of surface modifications on the dye adsorption and solar cell performance were also investigated.
9:00 PM - R8.41
Supercritical CO2 Drying of Vertically Aligned TiO2 Nanotube Arrays and their Use in Dye-Sensitized Solar Cells.
Tae-Sik Kang 1 2 , Xiaoyin Xiao 1 , James Deneault 1 2 , Barney Taylor 1 2 , Michael Durstock 1
1 , Air Force Research Laboratory, Wright Patterson AFB, Ohio, United States, 2 , Universal Technology Corporation, Beavercreek, Ohio, United States
Show AbstractVertically oriented TiO2 nanotube arrays provide an optimal material architecture for solar cell devices because of the potentially lower recombination losses and improved charge transport along the nanotube axis as compared to standard mesoporous titania films. In the sol-gel fabrication of TiO2 nanotube arrays using porous anodic alumina templates, agglomeration of the nanotubes is one of the major problems. After removal of the alumina template, solution processing methodologies and attractive interactions between tubes result in a non-uniform distribution of the aligned nanotubes on the substrate surface. In this study, we employ supercritical CO2 drying of the aligned nanotube films in order to avoid such behavior. We demonstrate the successful fabrication of non-agglomerated, vertically aligned TiO2 nanotube arrays of controllable geometry (length, diameter, wall thickness) using porous anodic alumina. A combination of morphology and structure analysis has shown that the nanotubes have the desired stoichiometric chemical composition, anatase crystal structure, and are aligned vertically on the substrate after drying. The correlation of these physical characteristics to device performance parameters of ruthenium dye-sensitized solar cells will be discussed.
9:00 PM - R8.42
Black Colloidal Graphenes as Light Absorbers for Photovoltaics.
Liang-shi Li 1 , Xin Yan 1
1 Chemistry Department, Indiana University , Bloomington, Indiana, United States
Show AbstractDeveloping organic dyes that can absorb light in the long-wave range of the solar spectrum is an important step toward plastic solar cells with high energy-conversion efficiency. Comparing with the most often used dyes that contain metals, organic dyes typically have larger extinction coefficient, and are truly sustainable.In this talk we will present a large colloidal graphene that can absorb solar light with wavelength shorter than 900 nm. The graphene colloids are highly soluble in common solvents, and were synthesized through stepwise solution chemistry we developed recently. As a result, the graphenes have identical size, shape, and chemical identity. More interestingly, they are large enough to show electronic band structures that are characteristic of solids, with a band gap of ~ 1.4 eV. The graphene has a molar extinction coefficient up to 100,000, an order of magnitude higher than Ruthenium complexes currently used in dye-sensitized solar cells. Our preliminary results on dye-sensitized solar cells with the graphene colloids as sensitizers will also be discussed.
9:00 PM - R8.43
Dependence of Growth Parameters of Titanium Dioxide Nanotube on the Open Circuit Voltage and Short Circuit Current Density in DSC.
Mukul Dubey 1 , Hongshan He 1
1 , South Dakota State University, Brookings, South Dakota, United States
Show AbstractVertically aligned titanium dioxide nanotube arrays (TiNT) has been considered as a promising alternative to the nanoparticle based dye sensitized solar cell (DSC). TiNT has high charge collection efficiency [2] as compared to NP based photoelectrode. However, the maximum efficiency achieved till now is ~7% [3] which is far below the NP based DSC. It was reported that electrolyte system, electrodes used as well as the anodization voltage have significant effect on the photovoltaic performance of TiNTs. This paper focuses on the optimization of aspect ratio and the insulating TiO2 layer formed at the bottom of the tube and investigates how the growth parameters affect the open circuit voltage and short circuit current in dye-sensitized solar cells. We have found that at constant voltage the diameter, length and effective surface area of nanotube increases with anodization time. It was also observed that even though the short circuit current increases but the open circuit voltage decreases leading to decreased fill factor. A trade off between these two properties led us to study further which shows that the nonuniformity in diameter distribution profile of nanotube increases with increase in anodization time, which may lead to the reduction in light trapping capability of nanotubes and also may provide alternate recombination paths. Also, it is suggestive that with increased time the thickness of insulating TiO2 layer formed at the tube bottom may lead to poor electron transport and hence reduced open voltage and fill factor. We are currently investigating the change in tube properties due to anodization voltage and trying to optimize the aspect ratio suitable for optimal DSC performance. The detailed results will be presented on the conference.This research was supported by the National Science Foundation/ EPSCoR, Grant No. 0554609
9:00 PM - R8.44
Realization of Front Electrode Illumination in Titanium Dioxide Nanotube Based Dye-sensitized Solar Cells.
Mukul Dubey 1 , Yihan Zhong 1 , Hongshan He 1
1 , South Dakota State University, Brookings, South Dakota, United States
Show AbstractTitanium dioxide nanotube (TiNT) based dye sensitized solar cells (DSC) has emerged as a promising alternative to nanoparticle (NP) based system. The TiNT is usually fabricated by anodization of 0.25 mm thick titanium foil. One of the major challenges for low performance of TiNT based DSC is the back illumination of device due to the direct application of TiNT on Ti foil in the cell. This leads to significant loss in photon flux due to the absorption of light in platinum and electrolyte system. Several reports are available on peeling technique of NT sheet and front electrode device, but substrate adhesion of NT sheet on TCO has still remained a challenge. Excellent device performance can be achieved by electronically binding the NT sheet over TCO and not just the physical binding. Moreover, it is also important to develop a reproducible technique to prevent curling of NT sheet while sintering. In this presentation, we will report the novel methods for the realization of front electron illumination of DSC by implantation of NT sheet on fluorine-doped tin oxide (FTO) glass. NT sheets were peeled off from Ti substrate right after anodization by selective etching process. It was found that the NT sheets can be peeled off from substrate within two minutes, which is easy to handle. Scanning electron microscopic images showed that peeled sheet suffered minimal damage and has retained the tube morphology. We have also observed that curling of the NT sheet when dried or sintered depends largely over the shape of the NT film grown during anodization as different shape shows different strain behavior during thermal stress. It was also observed that adhesion of NT sheet to FTO also depends on the shape, adhesion technique and pre treatment of NT sheet before it was attached to the FTO glass. Also, sintering process of the sample largely governs the curling of sample during heat treatment.This research was supported by the National Science Foundation/ EPSCoR, Grant No. 0554609
9:00 PM - R8.45
The Effect of Ag+ ions on Morphologies and Photoelectrochemical Properties of Electrochemically Prepared Cuprous Oxide Electrodes.
Ho Seong Jang 1 , Colleen McShane 1 , Kyoung-Shin Choi 1
1 Department of Chemistry, Purdue university, West Lafayette, Indiana, United States
Show AbstractCuprous oxide (Cu2O) is a direct bandgap semiconductor highly desirable for use in solar energy conversion due to its narrow bandgap (Eg = 1.9-2.2 eV) and relatively high absorption coefficient in the visible region. For the past few years, our group has explored the electrochemical construction of Cu2O photoelectrodes of various morphologies. We specifically focused on controlling shape, branching pattern, nucleation density, and crystal domain size in order to investigate the effect of these morphological variations on the photoelectrochemical properties of Cu2O. In this study, we report the effect of Ag+ ions present in the plating solution on the morphologies of Cu2O electrodes and their photoelectrochemical properties. Our original intention of adding Ag+ ions to the plating solution for Cu2O deposition was to probe the possibility of incorporating silver ions or atoms into the Cu2O lattice to alter the energetics (e.g. band positions, bandgap energy) or charge transport properties. However, we observed that the incorporation of silver into the Cu2O lattice did not occur. Instead, the use of Ag+ ions provided a systematic way to alter the nucleation density of the Cu2O crystals on the substrate. This was achieved by placing Ag seed crystals on the substrate during the early stage of deposition on which Cu2O crystals can grow. In addition, the presence of Ag+ ions in the plating solution modified the crystal habit of Cu2O by stabilizing (111) planes. The most interesting morphology obtained through the addition of Ag+ ions was observed when the plating solution was buffered with acetate (pH 4.9). In this case, anisotropic plate-like Cu2O crystals (thickness ~ 150 nm) with dendritic patterns creating nano-size holes within the plates were formed. This presentation will offer detailed discussions on how Ag+ ions can cause various morphological changes of Cu2O electrodes and how these changes in turn affect the photoelectrochemical properties.
9:00 PM - R8.46
Photoelectrochemical Properties of Nanocrystalline α-Fe2O3 Electrodes Prepared Electrochemically from Non aqueous Media And Their Modification with Various Oxygen Evolution Catalysts.
Kenneth McDonald 1 , Kyoung-Shin Choi 1
1 Chemistry, Purdue University, West Lafayette, Indiana, United States
Show Abstractα-Fe2O3 is an n-type semiconductor with many desirable properties for use as a photoanode in photoelectrochemical cells. It can utilize a significant portion of visible light (Eg ~2.1 eV) while remaining chemically and photochemically stable in neutral and basic media. This presentation reports a new condition to prepare transparent films of nanocrystalline α-Fe2O3 via a two-step process. Films of α-Fe2O3 were prepared by first cathodically electrodepositing nanocrystalline films of Fe metal from a dimethylsulfoxide (DMSO) solution of Fe2+ ions. The Fe metal films were then annealed at 500°C and converted to α-Fe2O3. In general, cathodic electrodeposition of Fe film in aqueous media does not result in uniform and well-adherent films due to hydrogen evolution interfering with film deposition on the working electrode. Employing DMSO plating media allows for using larger deposition potential windows without decomposing the medium, providing more freedom to tailor the quality of the films and the size of Fe metal particles. The x-ray diffraction and the scanning electron microscopy (SEM) study showed that the as-deposited Fe film was composed of 7-10 nm nanoparticles. During thermal oxidation, aggregation of the nanoparticles occurred but the uniformity and the smoothness of the films remained intact, resulting in the formation of transparent α-Fe2O3 films. The photoelectrochemical properties of α-Fe2O3 films were characterized using 1M NaOH solution as the electrolyte. In order to lower the overpotential of O2 evolution and minimize undesired effect of slow water oxidation kinetics of α-Fe2O3, various conditions to place Co- and Ni-based catalysts on the α-Fe2O3 films were investigated. The compositions of these catalysts and their effects on photoelectrochemical properties of α-Fe2O3 as well as their long-term stabilities will be discussed in detail in this presentation.
9:00 PM - R8.5
Development of Highly Durable Dye-sensitized Solar Cells / Modules.
Hiroshi Matsui 1 , Kenichi Okada 1 , Hiroki Usui 1 , Nobuo Tanabe 1
1 Environment and Energy Laboratory, Fujikura Ltd., Chiba Japan
Show AbstractLarge-area and highly durable dye-sensitized solar cells (DSCs) and their modules with current collecting metal grid were fabricated. Almost components of a photoelectrode were formed on a transparent conductive glass substrate by screen-printing technique, and ionic liquids were used as the nonvolatile solvent for electrolytes instead of conventional organic solvents. Several endurance tests using the device were carried out to investigate reliability of DSC. At first, leakage of inner materials was checked, and no obvious leakage of them such as ionic liquid itself, iodine and iodide salt, was observed by storing in a hermetically closed container at 85 °C. It was inferred that the leakage of the inner materials is not the critical problem for thermal stability at least on the ionic liquid type DSC. On the other hand, it was clarified that moisture intrusion into the cell deteriorated thermal stability. It was supposed that moisture intrusion into the cell enhanced desorption of sensitizing dye, dissolution of platinum counter electrode, deterioration of protective layer of current collecting grid. Therefore, we developed a novel cell package with multi-sealing structure to prevent the cell from moisture intrusion during long-term operation. Our devices with the multi-sealing were successfully operated with negligible degradation of the performance under the heat and humidity environment (85 °C, 85% RH) for more than 1000 hours. In the initial stage of the test, both gradual increase of current and decrease of voltage were observed. Resulting photoelectric conversion efficiency was almost constant from first to last. Test cells were also operated without obvious degradation under the heat and cool cycle stress test condition (-40 °C and 90 °C, 200 cycles), and under simulated solar light irradiated condition (1 sun, 500 hours). Moreover, outdoor operation test of 80 x 80 cm DSC module panels has been started to survey PV performance and durability under practical operating conditions. Performance of the module panels with volatile solvent-based electrolyte was deteriorated in the case of operation without UV-cut filter and improved by using of the one. In contrast, ionic liquid-based electrolyte system has been relatively stable with UV exposure. In addition, it was clarified that influence of strain by rapid thermal stress become serious due to an enlargement of panels, so that structure of the module panels was modified to absorb the stress. Now, our module panels are operating stably for more than 5 months.
9:00 PM - R8.6
The Use of Near Infra Red as a Rapid Heat Treatment Process in the Manufacture of Metal-based Dye-sensitized Solar Cells.
Trystan Watson 1 , David Worsley 1 , Ian Mabbett 1
1 Materials Research Centre, Swansea University, Swansea United Kingdom
Show AbstractOne of the main disadvantages of dye sensitized solar cells in terms of their large scale manufacture is the long heat treatment time required in the processing of the titanium dioxide layer (TiO2). In this work we show that near infra red (NIR) is a considerably faster alternative to convection heat treatment for binder removal on metal based dye sensitized solar cells. The NIR equipment operates by moving the TiO2 coated metal substrate at a set speed in front of the NIR lamp. The TiO2 paste is deposited directly onto the metal surface (either titanium or stainless steel) that, on exposure to NIR radiation, heats up very rapidly.By altering both the platform speed and the NIR lamp power output, a range of temperature profiles can be constructed. Consequently the heat can be delivered to the organic binder at a closer proximity and in a more organised fashion. This allows a significant reduction in binder removal time from typically 30 minutes using a convection oven to under 30 seconds using NIR
9:00 PM - R8.7
The Effect of Nanoporous TiO2 in Dye-sensitized Solar Cell.
Byong Cheol Shin 1 , Jiwon Lee 1 , ByungHak Lee 1 , SungSoo Kim 2 , JiMan Kim 2
1 Energy lab., SamSung SDI, Yongin-si, Gyeonggi-do, Korea (the Republic of), 2 Department of Chemistry, SungKyunKwan University, Suwon, Gyeonggi-do, Korea (the Republic of)
Show Abstract We report on the nanoporous TiO2 particles synthesized by nanocasting from the nanoporous silica template, which is applicable to the scattering layer for the high performance dye-sensitized solar cells (DSSC). The TiO2 nanoparticles have spherical shape with the size of around 200 nm and enlarged surface area. The scattering layer by as-synthesized TiO2 nanoparticles showed a large amount of dye adsorption, and enhanced light scattering effect as well. The dual functionality of the nanoporous TiO2 particles by nanocasting method suggests a new pathway for the high photo-conversion efficiency in DSSC.
9:00 PM - R8.8
Efficiency Enhancement in Dye-sensitized Solar Cell with Titanium Substrate by Simple Acid Processing.
Ho-Gyeong Yun 1 2 , Hun-Kyun Pak 1 , Mi-Hee Jung 1 , Zin-Sig Kim 1 , Byeong-Soo Bae 2 , Man Gu Kang 1
1 Advanced Solar Technology Research Team, ETRI, Daejeon Korea (the Republic of), 2 Department of Materials Science and Engineering, Kaist, Daejeon Korea (the Republic of)
Show AbstractDye-sensitized solar cell (DSSC) has been nominated as a promising solution to the future energy and environmental problems since its origin by O’Regan and Grätzel [1] due to its low fabrication cost, eco-friendly characteristics. Furthermore, DSSC realized on a flexible substrate can create innovative paradigm on mobile IT tools by virtue of its superior portability. However, relatively low conversion efficiency of the DSSC restricts its further applications so far. In order to improve the conversion efficiency of the DSSC, continuous attempts have been made. Researchers have concentrated their attention on the electrode materials, optimal dye, nano-structures for enhanced light scattering and so on. Recently, Hayase demonstrated improved conversion efficiency by only a mechanical treatment of the substrate. [2] However, Hayase’s report is a case of rigid FTO glass and the increase ratio of the conversion efficiency is not so much. In the case of flexible metal substrates, there have been few reports on the importance of the TiO2/substrates interface. Our team once announced the 4.2 % DSSC with flexible stainless steel (StSt) substrate [3] and the importance of the interface between TiO2 and StSt substrate. [4]In this communication, we report highly improved conversion efficiency in the case of the DSSC with titanium (Ti) substrates by a simple acid treatment. Considering optical properties and sintering behaviors at the interfaces of TiO2 nano-particles and Ti substrates, the surfaces of the Ti substrates have been treated with acid solutions and the modified optical reflectance and electrochemical impedance have been endowed as a result. For a comparison, we have also prepared DSSC having non-treated Ti substrates. The microstructures of these non-treated and treated Ti substrates have been analyzed using STEM. In addition, the electrical characteristics of the DSSCs have been examined with electrochemical impedance spectroscopy and its efficiency was measured with J-V measurement under air mass (AM) 1.5 condition. For the identification of the relation between optical reflectance and light-to-current conversion efficiency, optical characteristics have also been measured with UV-VIS spectrophotometer. The performances of the DSSCs with Ti substrates have been highly improved by this acid processing of the Ti surface. In detail, the J-V measurement demonstrates the conversion efficiency over 9%; open circuit voltage (Voc) and short circuit current density (Jsc) exceeding 850 mV and 15mA/cm2 respectively.References[1] B. O’Regan, M. Graetzel, Nature, 1991, 353, 737.[2] Y. Yoshidaa, S. Hayasea,_et al., Solar Energy Materials & Solar Cells, 2008, 92, 646[3] M. G. Kang, et al., Solar Energy Materials and Solar Cells, 2006, 90, 574[4] H. G. Yun, et al., Applied Physics letters, 2008, 93, 133311
9:00 PM - R8.9
A Density Functional Theory Study of Additives in Electrolytes of Dye Sensitized Solar Cell.
Maeng-Eun Lee 1 , Won-Hee Jeong 1 , Seung-Bum Suh 1 , Moon-Sung Kang 1
1 R&D center, Samsung SDI, Yongin-si, Gyeonggi-do, Korea (the Republic of)
Show AbstractThe effect of additives on the conversion efficiency of dye sensitized solar cell was investigated. A density functional theory (DFT) method was used to examine the adsorption of nitrogen-containing heterocyclic additives on a TiO2 surface.Two observations were discussed. 1. Negative shifts of the TiO2 Fermi level caused by the adsorption of additives were observed. This shift causes the enhancement of the open-circuit photovoltage (Voc). 2. Additives which were specifically designed to prevent energy loss from TiO2 to electrolytes increased the short circuit photocurrent density (Jsc).We have carried out a theoretical study of a TiO2 Fermi level shift caused by the adsorption of nitrogen-containing heterocyclic additives, and have related this shift to the effect of an additive in the electrolyte on the performance of dye-sensitized solar cell. Our results were in good agreement with experimental data.
Symposium Organizers
Arthur J. Frank National Renewable Energy Laboratory
Nam-Gyu Park Sungkyunkwan University
Tsutomu Miyasaka Toin University of Yokohama
(and Peccell Technologies, Inc.)
Laurie Peter University of Bath
Songyuan Dai Chinese Academy of Sciences
R9: Nanostructured Sensitized Solar Cells III
Session Chairs
Juan Bisquert
Hiroshi Matsui
Thursday AM, December 03, 2009
Room 312 (Hynes)
9:30 AM - **R9.1
Hybrid Solar Cells with Water-soluble Conducting Polymers and SWCNTs.
Sung-Hwan Han 1 , W. Lee 1 , S. Min 1
1 , Center for Creative Chemists Hanyang University School, Seoul Korea (the Republic of)
Show AbstractInorganic-organic hybrid solar cells were prepared by manipulating the interfacial contact of layers. The polyacetylene photosensitizers with quaternary pyridinium salts were layered on n-type materials by in situ polymerization followed by anion exchange. The presence of single-walled carbon nanotubes (SWCNTs) on ITO substrate controls the interface enhancing charge collection efficiency with reduced recombination reaction, and overall efficiency increased significantly.
10:00 AM - **R9.2
Fast Track Towards Reliable Results for Academic and Industrial Dye Solar Cell Development.
Hans Disilvestro 1 , Michael Bertoz 1 , Ravi Hariksun 1 , Gavin Tulloch 1 , Sylvia Tulloch 1 , Damion Milliken 1
1 , Dyesol, Queanbeyan, New South Wales, Australia
Show AbstractWith increasing interest in Dye Solar Cell (DSC) development from academia and increasingly from industry, reliable and easy-to-implement ‘fast-track’ methods are needed for cost-effective work towards further improvement of performance and product reliability. There are many variables in the design and manufacture of DSC – both in the materials and the processing methods. The ‘learning curve’ for newcomers to the DSC area can be reduced with the use of consistent materials, components and methods, as well as proven processing equipment. This paper deals with some of the key variables – the titanium dioxide material for processing into screen-printable pastes and efficient electrodes, dyes, electrocatalysts, electrolyte systems and sealing materials – and how their characteristics and processing methods influence performance and stability of cells and entire modules. Such standard materials and components allow to quickly establish a number of baseline configurations, e.g. for highest performance, for maximum long-term stability under light soaking or heat stress situations or for an industrially most meaningful basis in terms of cell geometry and materials employed. Once a sound baseline is established specific improvements to one or more components can be studied systematically. Since a change in one parameter may have a significant impact on requirements for other components in a DSC a thorough understanding of the key interactions is important and careful readjustment and re-optimisation of experimental parameters may be required. Some representative examples will be highlighted.*) This paper is dedicated to family and friends of Michael Bertoz who tragically deceased in June 2009
10:30 AM - **R9.3
Photovoltaic Properties and Ultrafast Carrier Dynamics of Semiconductor Quantum Dot-Sensitized Solar Cells.
Taro Toyoda 1 , Qing Shen 1
1 Department of Applied Physics and Chemistry, The University of Electro-Communications, Tokyo Japan
Show AbstractDye-sensitized solar cells (DSSCs) have been the focus of attension as a promising alternative to conventional Si solar cells. However, the expensive and using special organic dyes has encoureged the pursuit of alternative light harvesters. Semiconductor quantum dots (QDs) could provide the solutions due to its tunable energy gaps (HOMO-LUMO transition) large intrinsic dipole moments leading to rapid charge separation, and large extinction coefficient. Alos, the QD-sensitized solar cells have capability of producing multiple electron-hole pairs (excitons) per photon with impact ionization[1]. We demonstrate a nobel approach to sensitized solar cells, based on different kinds of TiO2 electrodes (nanoparticle assembly, inverse opal, and nanotube structures) and the use of semiconductor QDs as sensitizers. The photovoltaic performance is depend on the morpholy of the electrode and significantly improved with surface passivation of semiconductor QDs [2,3] and use of Cu2S counterelectrode. A photovoltaic conversion efficiency of 3.1% has been attained and this value is relatively high for semiconductro quantum dot-sensitized solaar cells. Ultrafast carrier dynamics of measeured with transient grating (TG) technique [4] show a fast (hole) and slow (electron) decay processses with lifetimes of a few picosecond and a few tens to hundred picoseconds, respectively. There is acorrelation between photovoltaic properties and the lifetimes of the photoexcited carriers. [1]A.J.Nozik, Physica E Vol.14,115(2002). [2]L.J.Diguna,Q.Shen,J.Kobayashi,and T.Toyoda, Appl.Phys.Lett. Vol.91,023116(2007). [3]Q.Shen,J.Kobayashi,L.J.Diguna,and T.Toyda, J.Appl.Phys. Vol.103,084304(2008). [4]K.Katayama,M.Yamaguchi,and T.Sawada,Appl.Phys.Lett. Vol.82,2775(2003).
11:30 AM - R9.4
High Performance Quasi-Solid Dye-Sensitized Solar Cells with Nano Clay Electrolyte.
Satoshi Uchida 1 , Tomoyuki Inoue 1 , Takaya Kubo 1 , Hiroshi Segawa 1
1 Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Tokyo Japan
Show AbstractFor the improvement of durability of dye-sensitized solar cells (DSSC), the solid state electrolytes have been studied. It is well known that the conventional solidification methods have problems such as low ionic conductivity and poor electrolyte/electrode interface contact. In this study we successfully prepared a quasi-solid electrolyte using artificial nano clay mineral with high performance. Further chemical and physical properties of these clay electrolyte were also discussed in related with photo-current energy conversion efficiency.
11:45 AM - R9.5
Transmittance Modulated Nb-doped TiO2 / Transparent Conducting Oxide Multilayered Thin Film for the Dye-sensitized Solar Cells.
Dong Hoe Kim 1 , Sangwook Lee 1 , Jun Hong Noh 1 , Jong Hoon Park 1 , Hyun Suk Jung 2 , Kug Sun Hong 1
1 Department of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 , Kookmin Universtiy, Seoul Korea (the Republic of)
Show AbstractRecently, in our group, the dye-sensitized solar cells (DSSCs) incorporating the Nb-doped TiO2 (NTO)/Transparent conducting Oxide (TCO) (such as F doped SnO2 (FTO) or Sn doped In2O3 (ITO)) multilayered thin films were investigated. The NTO overlayer has many benefits such as 1) blocking layer to prevent back electron transfer, 2) to reduce a Schottky barrier and 3) to conserve a conductivity of cation doped-TCO (i.e., ITO in this study). The NTO deposited by pulsed laser deposition (PLD) deteriorated the transmittance of underlayer. The transmittance plays an important role in the current density (Jsc) of the DSSCs. So, we suggest that the method to improve the transmittance of the NTO/TCO multilayer. We improve the transmittance using sol-gel spin-coated TiO2 thin film. The thickness of TiO2 overlayer influence the transmittance. We found the optimum thickness to get the best transmittance. As a result, the transmittance of the NTO/FTO is increased from 69% to 79% by sol-gel TiO2 layer. Similarly, in the NTO/ITO, the transmittance increased 30%. The photovoltaic characteristics of DSSCs incorporating the optimum TiO2 layer was investigated through current-voltage (I-V) characteristics, electrochemical impedance spectra (EIS), the dark current, and open circuit voltage (Voc) decay. These results demonstrate that a sol-gel spin-coated TiO2 layer on the NTO/TCO is suitable to improve the transmittance of the NTO/TCO and for achieving highly efficient photo-energy conversion devices.
12:00 PM - R9.6
Charge Separation and Collection Losses in Dye Sensitized Cells: Evaluated at Short Circuit and Across the Current-Voltage Curve.
Piers Barnes 1 , Assaf Anderson 1 , Lingxuan Liu 1 , Xiaoe Li 1 , Andrea Listorti 1 , Mindaugas Juozapavicius 1 , Hawraa Kisserwan 2 , Tarek Ghaddar 2 , James Durrant 1 , Brian O'Regan 1
1 Chemistry, Imperial College London, London United Kingdom, 2 Chemistry, American University of Beirut, Beirut Lebanon
Show AbstractTo maximise photocurrent in dye cells, a long electron diffusion length (L) relative to film thickness is essential for efficient collection of electrons. Photocurrents generated by thick, strongly absorbing, dye sensitized cells were reduced when electrolyte iodine concentration was increased. Diffusion lengths derived by Ln = (τn Dn)1/2, the standard equation relating the dynamic electron recombination lifetime (τn) and diffusion coefficient (Dn),[1,2] were at least two times higher than diffusion lengths evaluated from incident photon-to-electron-conversion efficiency measurements (LIPCE).[3,4] In this study we show conclusively that charge collection efficiency calculated using Ln seriously over predicted photocurrent, while LIPCE correctly predicted photocurrent.[5] This has implications for optimizing cell design since L is shorter than previously assumed.
Additionally we show that the analysis of differential IPCE spectra can be used to find the electron injection efficiency [6] or charge separation efficiency and LIPCE not just at short circuit but at different light intensities and operating voltages, even going beyond open circuit voltages. By combining these observations with electron concentrations measured by charge extraction we can infer the factors limiting cell performance under operating conditions. For example LIPCE is typically observed to increase at high charge concentrations such that transport of the electrons to the collection electrode generally does not limit photocurrent at working potentials. However as the cell potential and thus electron concentration increases a different loss mechanism is observed which cannot be attributed to a change in injection efficiency. We speculate that this may be related to electron recombination with dye cations.
1.Bisquert, J.; Vikhrenko, V. S. J. Phys. Chem. B 2004, 108, (7), 2313-2322.
2.Jennings, J. R.; Peter, L. M. J. Phys. Chem. C 2007, 111, (44), 16100-16104.
3.Halme, J.; Boschloo, G.; Hagfeldt, A.; Lund, P. J. Phys. Chem. C 2008, 112, (14), 5623-5637.
4.Barnes, P. R. F.; Anderson, A. Y.; Koops, S. E.; Durrant, J. R.; O'Regan, B. C. J. Phys. Chem. C 2009, 113, (3), 1126-1136.
5.Barnes, P. R. F.; Liu, L.; Li, X.; Anderson, A. Y.; Kisserwan, H.; T. H. Ghaddar; Durrant, J. R. and O’Regan, B. C. submitted Nano Lett.
6.Koops, S. E.; O'Regan, B. C.; Barnes, P. R. F.; Durrant, J. R. J. Am. Chem. Soc. 2009, 131, (13), 4808-4818.
12:15 PM - R9.7
Coupling of Titania Inverse Opals to Nanocrystalline Titania Layers in Dye-Sensitized Solar Cells.
Seung-Hyun Lee 1 , Neal Abrams 1 , Greg Barber 2 , Tom Mallouk 1
1 Chemistry, The Pennsylvania State University, University Park, Pennsylvania, United States, 2 Materials Research Institute, The Pennsylvania State Univesity, University Park, Pennsylvania, United States
Show AbstractAlthough dye-sensitized solar cells (DSSCs) have a promising future due to low cost production, the energy conversion efficiency of the cells has not yet reached the level of solid state thin film solar cells. There are intensive on-going investigations to improve the conversion efficiency. The widely used N719 dye absorbs light weakly in the 550 – 700 nm region. If non-absorbed photons in the red region can be utilized in DSSC, it will be possible to achieve a much higher efficiency. While progress has been made by designing full-spectrum dyes, an alternative way to enhance the efficiency is to design a DSSC to increase light scattering in TiO2. Recently, we reported a quantitative comparison of the photoaction spectra, short circuit current densities, and power conversion efficiencies of dye-sensitized solar cells (DSSCs) that contain bilayers of nanocrystalline TiO2 (nc-TiO2) and titania inverse opal photonic crystals (PCs). Cells were fabricated with PC/nc-TiO2 and nc-TiO2/PC bilayer films on glass/tin oxide anode of the cell, as well as in a split configuration in which the nc-TiO2 and PC layers were deposited on the anode and cathode sides of the cell, respectively. The results showed the enhancement of the response of the DSSC in the red region of the spectrum. We postulated that this enhancement factor strongly depends on the proximity of PC layer to the nc-TiO2. We are currently investigating an improved fabrication technique to eliminate the physical gap in the bilayers and a novel application of PCs to DSSC-Si tandem cells.
12:30 PM - R9.8
Optimizing the Photocurrent Efficiency of Dye-Sensitized Solar Cells through the Controlled Aggregation of Chalcogenoxanthylium Dyes on Nanocrystalline TiO2 Films.
Jonathan Mann 1 , Anthony Smith 1 , Brandon Calitree 1 , Michael Gannon 1 , Thomas Fitzgibbons 1 , Justin Onyeji 1 , Michael Detty 1 , David Watson 1
1 Chemistry, University at Buffalo, Buffalo, New York, United States
Show AbstractThis presentation will focus on dye-sensitized solar cells (DSSCs) incorporating chalcogenorhodamine dyes that undergo controlled aggregation on surfaces. To date, four series of dyes have been characterized: 2,7-bis(dimethylamino)-9-(2-thienyl-5-carboxy)chalcogenoxanthylium dyes (1-E, where E = O, S, Se); 2,7-bis(dimethylamino)-9-(3-thienyl-2-carboxy)chalcogenoxanthylium dyes (2-E, where E = S, Se); a 2,7-bis(amino)-9-(phenyl)thioxanthylium dye (3-S); and 4-Se, a constrained analog of 1-Se. The orientation and aggregation state of dyes were controlled systematically by varying the position of the surface-attachment group relative to the xanthylium core. Series 1 dyes and 4-Se underwent H-aggregation on TiO2 surfaces; series 2 dyes adsorbed in amorphous monolayers; and 3-S, which adsorbed through the amino groups, underwent J-aggregation on TiO2 surfaces. H-aggregated dyes exhibited broader absorption bands and increased light-harvesting efficiencies relative to dyes that adsorbed in amorphous monolayers. Remarkably, H-aggregated dyes also exhibited dramatically increased electron injection yields and/or charge-collection efficiencies, leading to greatly improved photoelectrochemical performance relative to non-aggregating dyes.1 J-aggregated 3-S exhibited increased light-harvesting efficiency but poorer photoelectrochemical performance. To date, the maximum incident photon-to-current efficiencies (IPCEs) of series 1 dyes range from 70% to 84%, whereas those of 2-S and 2-Se are 11% and 20%. Our findings reveal that the light-harvesting efficiency, IPCE, and absorbed photon-to-current efficiency (APCE) of DSSCs with organic dyes can be optimized by systematically varying the structure of the dyes and their orientation and aggregation state on the surface.1. Jonathan R. Mann, Michael K. Gannon, Thomas C. Fitzgibbons, Michael R. Detty, and David F. Watson J. Phys. Chem. C 2008, 112, 13057-13061.
12:45 PM - R9.9
Atomic Layer Deposition-Grown TiO2 Nanotube Arrays for Efficient Dye-Sensitized Solar Cells.
Hyunjun Yoo 1 , Changdeuck Bae 2 , Jooho Moon 2 , Hyun Suk Jung 3 , Hyunjung Shin 1
1 National Research Lab. for Nanotubular Structures of Oxides, Center for Materials and Processes of Self-Assembly, and School of Advanced Materials Engineering, Kookmin University, Seoul Korea (the Republic of), 2 Department of Materials Science and Engineering, Yonsei University, Seoul Korea (the Republic of), 3 School of Advanced Materials Engineering, Kookmin University, Seoul Korea (the Republic of)
Show AbstractNanotubular photoanode for dye-sensitized solar cells (DSSCs) can allow for improved surface area as well as provide direct pathway for photo-generated and injected electrons. Here we report the newly designed photoanodes of precisely dimension-controlled TiO2 nanotubes (NTs) with elongated crystalline anatase grains by template-directed atomic layer deposition (ALD). Using ALD techinique, we were able to fabricate high aspect-ratio TiO2 NTs with precise control of the wall thickness. Upon heating the grain structure evolutions of TiO2 NTs with a variety of different wall thicknesses has been investigated. The grain growth of NTs was found to be boosted as the wall thickness is increased. Smaller grains with the diameter of few nanometer which are the case of NTs with thinner wall thicknesses (< 10 nm) have been observed, while elongated grain growth along the axial direction of NTs in the case of thicker wall NTs (> 10 nm). The elongated anatase single grains are mostly lager than 500 nm in length. As a result, we can design TiO2 NTs with elongated single grains by simple control of the wall thickness as optimized photo-anodes. Electrical conductivity of the NTs was measured using an Ohmic-Ohmic device configuration. The measured conductivity of NTs is more than three orders of magnitude higher than the one of mesoporous anatase TiO2 particulate films (~10-5-10-6 S/cm) used as photo-electrodes in conventional dye-sensitized solar cells (DSSCs). The higher conductivity in NTs is also comparable with that of the bulk or thin film anatase (~10-1 -10-2 S/cm). With our highly conductive photoanodes of TiO2 NTs, we’ve been obtaining reproducible photon-to-electricity conversion experimental results with DSSCs of our TiO2 NTs. For example, under AM 1.5 illumination, the generated photovoltage was ~ 0.7 V and photocurrent was ~10 mA/cm2 with a photon energy conversion efficiency of ~ 4 %. Cell design optimization of our DSSCs and surface treatment of TiO2 NTs provide even higher efficient cells.
R10: Light Harvesting Materials and Charge Injection, Transport and Recombination Process
Session Chairs
Anders Hagfeldt
Taro Toyoda
Thursday PM, December 03, 2009
Room 312 (Hynes)
2:30 PM - **R10.1
Improving Efficiency of Dye-Sensitized Solar Cells (DSCs) through Long Range Forster Energy Transfer and Increased Spiro-OMeTAD Pore Filling.
Michael McGehee 1 , I-Kang Ding 1 , Brian Hardin 1 , Jean M. J. Frechet 2 , Md. Nazeeruddin 3 , Michael Graetzel 3
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Chemistry, University of California at Berkeley, Berkeley, California, United States, 3 Laboratory of Photonic and Interfaces, Ecole Polytechnique Fédérale de Lausanne, Lausanne Switzerland
Show AbstractConventional dye-sensitized solar cells (DSCs) have excellent charge collection efficiencies, high open circuit voltages, and good fill factors. However, DSCs do not completely absorb all of the photons from the visible and near infrared domain and consequently have lower short circuit photocurrent densities compared to inorganic photovoltaic devices. Here we present a new design where high energy photons are absorbed by highly photoluminescent chromophores unattached to the titania and undergo Förster resonant energy transfer (FRET) to the sensitizing dye. This novel architecture allows for broader spectral absorption, an increase in dye loading, and relaxes the design requirements for the sensitizing dye. We demonstrate a 26% increase in power conversion efficiency when using an energy relay dye (PTCDI) with an organic sensitizing dye (TT1). We estimate the average excitation transfer efficiency in this system to be at least 47%. This system offers a viable pathway to develop more efficient DSCs. #####Pore filling in solid-state dye-sensitized solar cells is an important topic, yet it has not been studied in detail before. In this study, the pore filling of spiro-MeOTAD (2,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenylamine)9,9’-spirobifluorene) in mesoporous TiO2 films is quantified for the first time using XPS depth profiling and UV-Vis absorption spectroscopy. We show that spiro-OMeTAD can penetrate the entire depth of the film, and its concentration is constant throughout the film. We determine that in a 2.5-μm-thick film, the volume of the pores is 60-65% filled. The pores become less filled when thicker films are used. Such filling fraction is much higher than the solution concentration because the excess solution on top of the film can act as a reservoir during the spin coating process. Lastly, we demonstrate that by using a lower spin coating speed and higher spiro-OMeTAD solution concentration, we can increase the filling fraction and consequently the efficiency of the device.
3:00 PM - **R10.2
Visible to Near IR Photon Harvesting in Polymer Based Hybrid Solar Cells Using Dye-sensitized TiO2 Nanotube Arrays.
Gopal Mor 1 , Sanghoon Kim 1 , Oomman Varghese 1 , Maggie Paulose 1 , Karthik Shankar 1 , James Basham 1 , Craig Grimes 1
1 , Penn State University, University Park, Pennsylvania, United States
Show AbstractPhotovoltaic devices employing an intimate blend of semiconducting p-type polymer such as Poly(3-hexylthiophene) (P3HT) and n-type methanofullerene (PCBM) have demonstrated efficiencies approaching 5 %. P3HT-PCBM blend based bulk heterojunction solar cells exhibit excellent charge transfer characteristics but absorb very little light of wavelength longer than 650 nm. The poor harvesting of red and near-infrared photons is the primary factor limiting efficiencies in these devices. In view of this, several research groups are pursuing the synthesis and application of low bandgap polymers to replace P3HT. We use a different route, namely the infiltration of the organic semiconductor blend into a red and near IR absorbing dye sensitized TiO2 nanotube array to extend photon harvesting. Our devices are also different from conventional dye sensitized solid state solar cells1,2 which are characterized by the nonparticipation of the hole conducting p-type inorganic or n-type organic semiconductor in light absorption and photogeneration. Herein, we successfully demonstrate an “organic dye - polymer” based hybrid solar cell of tandem configuration where dye sensitized TiO2 nanotube arrays absorb red and near IR portions of the solar spectrum and the polymer absorbs visible and near UV light. Organic dyes, including the well known unsymmetrical squaraine dye3 and styrylindolium – a new organic dye (SK1 dye) we have synthesized, are used due to their adjustable spectral response and high extinction coefficient. The squaraine dye absorbs light from 500 to 700 nm with an extinction coefficient of 159,700 M-1cm-1 at λmax of 637 nm. The SK1 dye absorbs light from 450 to 720 nm with an extinction coefficient of 44,700 M-1cm-1 at λmax of 580 nm. We use 500 nm to 1 μm long vertically oriented TiO2 nanotube arrays, on FTO coated glass, that are sensitized with the organic dyes, with the tube pores then filled with hole transporting and light absorbing p-type polymer/ blend of p-type polymer and n-type molecules. These hybrid solar cells demonstrate a significant photocurrent in the wavelength range 350 to 720 nm. Without dye sensitization generated photocurrent appears only up to 650 nm. The photovoltage of the hybrid solar cell can be varied from 0.45 V to 0.78 V by surface treatment of the TiO2, and choosing different types of polymer blends (such as P3HT/PCBM and PV2000 photoactive ink - a proprietary item of Plextronics); the extent of polymer penetration into the nanotube array pores is evaluated by FESEM studies. We also discuss the effect of PCBM in the polymer blend and its effect on photovoltaic cell performance. 1 Nature395 (1998) 583-585.2 Inorganica Chimica Acta 361 (2008) 581–5883 Chem Commun. (2007) 4680–4682
3:30 PM - **R10.3
Development of New Materials and Structures for Efficient Organic Solar Cells Fabricated with Ionically and Electrically Conducting Polymers.
Yoshinori Nishikitani 1 , Hideki Masuda 2
1 Central Technical Research Laboratory, Nippon Oil Corporation, Yokohama Japan, 2 Applied Chemistry, Nagoya Institute of Technology, Nagoya Japan
Show AbstractOrganic solar cells have attracted much attention due to their great potential for low-cost, flexible and light-weight photovoltaic devices. Organic solar cells are broadly classified into two types: so-called Graetzel type dye-sensitized solar cells (DSC); and organic thin-film solar cells (OPV). DSCs are photoelectrochemical cells consisting of mesoporous TiO2 photoelectrodes with Ru dyes. In contrast, OPVs are solid-state photovoltaic devices using organic semiconductors. Great progress has been made in increasing the conversion efficiencies of these solar cells, but more effort will be required to further increase conversion efficiency and durability before they can be used outdoors. Recently, we developed quasi-solid state DSCs using gel-type, ionically conducting polymers (ICPs) based on poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) to reduce leakage of the electrolyte solution, which is one of the main factors which decreases the durability of DSCs. We achieved a balance between high ionic conductivity and mechanical strength by optimizing the film thicknesses and ingredients of the ICPs. Aimed at increasing DSC conversion efficiency, we have developed two key materials: (1) ultrahigh-aspect-ratio TiO2 nanotubes made by anodic oxidation of Ti metals, to establish good carrier passages; and (2) novel triarylamine-functionalized Ru dyes, to harvest the sunlight efficiently. DSCs with the TiO2 nanotubes were found to give higher conversion efficiencies, compared to conventional DSCs with TiO2 nanoparticle photoanodes1), 2). We synthesized several different triarylamine-functionalized Ru dyes to achieve efficient electron injection from the dye to the TiO2 as well as faster regeneration of the dye. DSCs fabricated with triarylamine-functionalized Ru dyes were confirmed to give conversion efficiencies as high as those of N719 dye-based DSCs3), 4).We also proposed donor/acceptor-type (D/A) block copolymers for bulk heterojunction organic thin-film solar cells, aimed at establishing self-organized well-ordered donor/acceptor nanophase separation, which provides good carrier passages for increased carrier collection efficiency. Our numerical simulation showed that the HOMO and LUMO energy levels and segment lengths of the donor and acceptor blocks were key parameters, and also indicated that the conversion efficiency would be higher if each segment length were designed to be shorter than the respective exciton diffusion length. In this talk, strategies for increasing the conversion efficiency of DSCs and organic thin-film solar cells will be discussed in more detail.References1) K. Zhu, N. R. Neale, A. Miedaner, and A. J. Frank: Nano Lett. 7 (2007) 69. 2) K. Nakayama, T. Kubo, and Y. Nishikitani: Appl. Phys. Exp. 1 (2008) 112301. 3) M. Graetzel: Bull. Jpn. Soc. Coord. Chem. 51 (2008) 3.4) Z. Jin, H. Masuda, N. Yamanaka, M. Minami, T. Nakamura, and Y. Nishikitani: J. Phys. Chem. C 113 (2009) 2618.
4:30 PM - R10.4
Infrared Light Trapping in Nanocrystalline Inverse Opal Photoelectrodes.
Nathan Neale 1 , Arthur Nozik 1 , Arthur Frank 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractWe have been investigating the effects that ordered porous nanocrystalline film morphologies have on various photoconversion processes, such as light harvesting and charge-collection efficiencies. In this presentation, we describe our development of electrochemical deposition protocols for preparing ordered photonic nanocrystalline semiconducting films. Typically, inverse opals display a strong photonic bandgap (PBG) owing to the alternating pattern of high and low dielectric constant phases that focuses narrow bands of light near the PBG in one of the two media. Unlike the expected optical effect resulting from a well-defined PBG, this architecture promotes absorption of light at energies below the bandgap, namely, in the infrared. We show that our inverse opal films trap infrared light within the material (spanning from >1.5 eV to <0.9 eV), a phenomenon that could potentially be used to enhance the infrared absorption of low bandgap quantum dots or molecular sensitizers.
4:45 PM - R10.5
Q-Dots for Efficient Light Harvesting in Solar Cells.
Jason Riley 1
1 Materials, Imperial College London, London United Kingdom
Show AbstractThree-dimensional quantum confinement of semiconductor nanocrystals leads to dramatic changes in their optical and electronic properties. In the 1-10 nm size regime II-VI compounds exhibit size-dependent optical properties and as such their absorption spectrum may be tuned by simply changing the dimension of the nanocrystals. The most striking effect observed due to quantum confinement is a discrete, blue-shifted absorption and emission profile. Nanocrystals also have high surface area to volume ratio as a result of their nanoscale. These unique properties are opening the doors to many potential applications, one of which is as a light harvesting material in solar cells.Nanocrystals of II-VI materials have been prepared via the Hot Injection Technique in which high energy surfaces are stabilised by trioctylphosphine oxide (TOPO). Post preparation surface groups exchange to yield particles optimised for deposition on to conducting substrates has been investigated.The as-prepared and modified nanocrystals have been characterised by absorption, fluorescence, transmission electron microscopy and X-ray diffraction. They have been shown to be crystalline, nearly monodisperse and quantum confined; with a variety of sizes and therefore absorption profiles accessible. Deposition of the nanoparticles on to flat and high surface area electrodes has been investigated. The optical and electronic properties of the as-deposited layers have been investigated using; absorption, fluorescence and photocurrent spectroscopies. Size dependent injection of electrons from the nanoparticles to a TiO2 substrate has been demonstrated.
5:00 PM - R10.6
Deep-Level Transient Spectra and Absorption Spectra of Prototype Intermediate-Band Solar Cells.
M. Levy 1 , A. Marti 1 , D. Fuertes Marrón 1 , E. Cánovas 1 , E. Antolín 1 , P. Linares 1 , C. Farmer 2 , C. Stanley 2 , A. Luque 1
1 Instituto de Energía Solar, Universidad Politécnica de Madrid, Madrid Spain, 2 Department of Electronics and Electrical Engineering, University of Glasgow, University of Glasgow, Glasgow United Kingdom
Show AbstractOngoing development of intermediate-band solar cells (IBSC) at the Instituto de Energía Solar is focused both on nanostructures and bulk semiconductors with high concentrations of deep levels. The authors characterize prototype IBSC using both deep-level transient spectroscopy (DLTS) and electroluminescent spectroscopy.The performance of a solar cell with multiple photo-induced electronic transitions may surpass that of a single-junction solar cell. The upper efficiency limit of a solar cell with electronic states that form a single intermediate band is 63.2%. The IBSC is of interest because its limiting efficiency nearly reaches that of a tandem stack of three single-junction solar cells (63.8%). The successful implementation of a high-efficiency IBSC depends on, among other things, (1) the energetic locations of the electronic states intermediate between those of the conduction band and valence band; and (2) the absorption coefficients describing (i) photo-induced transitions from the valence band to the intermediate states and (ii) photo-induced transitions from the intermediate states to the conduction band. At present, experimental studies of several IBSC prototypes are ongoing. With respect nanostructured prototypes, research and development concentrates on (In,Ga)As/GaAs quantum dot structures grown by molecular-beam epitaxy. With respect to semiconductor prototypes with high concentrations of deep levels, research and development concentrates on transition metals incorporated into Cu(In,Ga)S2 based solar cells.DLTS spectra are obtained using a Boonton 7200 capacitance meter and a liquid-nitrogen cooled cryostat. Preliminary DLTS studies of (In,Ga)As/GaAs quantum-dot solar cell indicate the presence of two peaks. One small peak is found at 190 K and a second broader and larger peak occurs above room temperature. The authors propose to present DLTS spectra and Laplace-transform DLTS analysis of prototype IBSC to ascertain the activation energies of the deep states and their emission rates. Absorption coefficients are derived from electroluminescent spectra, the latter obtained using a cooled photo-multiplier tube and a photon counter. Preliminary studies utilize single-pass multisection GaAs/AlGaAs waveguide structures incorporating InAs/GaAs quantum dots stacks in the waveguide. Each section has an electrically isolated metal contact that may independently generate photon current. Thus each section in the multisection may be used to emit photons or absorb them. Preliminary results, at room temperature, indicate that the onset of absorption occurs at 900 meV. Further, the peak absorption coefficient is 30 cm-1 and occurs at 1000 meV. The authors explain that this absorption coefficient may be related to photo-induced transitions from the valence band to the intermediate states. The authors propose to present the absorption coefficients of multi-section quantum-dot stacks at low temperature using a closed-cycle He cryostat.
5:15 PM - R10.7
Charge Photogeneration and Recombination in Dye Sensitized and Polymer/Fullerene Solar Cells.
James Durrant 1 , Brian O'Regan 1
1 Chemistry, Imperial College London, London United Kingdom
Show AbstractMy lecture will focus on charge separation and recombination in excitonic solar cells - comparing and contrasting dye sensitized solar cells with polymer / fullerene bulk heterojunctions. I will start of considering the energetics and kinetics of charge photogeneration. Concerning charge photogeneration, I will focus on the energetic cost of charge photogeneration, and its impact upon device efficiency. Topics I intend to address include triplet versus singlet injection, overcoming the coulomb attraction of photogenerated charges, the minimization of kinetic redundancy and a comparison of charge separation limitations upon device performance between different sensitiser dyes and polymers. I will then go on to consider recombination losses in such devices - including quantification of bimolecular recombination losses, transport issues and interface engineering approaches to reduce recombination losses.
5:30 PM - R10.8
The Effect of Nanoparticle Shape on Photoinduced Electron Transfer from Poly(3-Hexyl Thiophene) to CdSe Nanoparticles.
Nikos Kopidakis 1 , Smita Dayal 1 , Dana Olson 1 , David Ginley 1 , Garry Rumbles 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractDuring the past few years there has been substantial progress toward realizing photovoltaic devices based on nanoscale composites of a conjugated polymer and a quantum-confined inorganic semiconductor. In particular, CdSe quantum dots, nanorods and branched particles (tetrapods) have been used and the effect of their size and shape on device efficiency has been demonstrated. In this work we combine the contactless Time-Resolved Microwave Conductivity (TRMC) technique with photovoltaic device efficiency measurements using the same active layers to elucidate the origin of the dependence of device parameters like the short-circuit current and fill factor on the size and shape of CdSe nanoparticles blended with poly(3-hexylthiophene) (P3HT). The advantage of the contactless photoconductivity measurements is that it allows us to probe directly free electron and hole generation under illumination in the CdSe/P3HT active layer and to directly observe charge separation and long-lived free carrier generation. We show that in devices utilizing higher aspect-ratio particles (rods and tetrapods), the availability of a clear pathway for carrier separation at the CdSe/polymer interface enhances the charge separation process improving device performance, which is not the case in devices with CdSe quantum dots. Finally, we present certified efficiencies of CdSe/polymer devices that allow direct comparison with similar “bulk heterojunction” technologies.
5:45 PM - R10.9
Ultrafast Photoconductivity Dynamics in Diblock Copolymer Structured TiO2 Morphologies using Optical-Pump THz-Probe Spectroscopy.
Priti Tiwana 1 , Patrick Parkinson 1 , Michael Johnston 1 , Henry Snaith 1 , Laura Herz 1
1 Condensed Matter Physics, University of Oxford, Oxford United Kingdom
Show AbstractEfficiencies of currently available state-of-the-art dye-sensitized solar cells (DSCs) are still significantly lower than those achieved with silicon based photovoltaics. Prominent factors responsible for this poor performance include incomplete light absorption by the sensitizer dye monolayer and inefficient electron transport through the TiO2 matrix. Using a mesoporous metal-oxide electrode fabricated from sol-gel processed sintered TiO2 nanoparticles has the advantage of high surface area, and thus high dye-loading [1]. However, this disorganized randomly-interconnected network presents a very tortuous path for the electrons diffusing towards the anode. One promising way of improving charge transport through the TiO2 network involves the use of a sacrificial block copolymer which acts as a template for the TiO2 nanoparticles, assisting them in self-assembling into an ordered and well-interconnected matrix [2]. In this study, we use a hydrolytic sol-gel synthesis route, and by systematically tuning the ratio between the diblock copolymer, polyisoprene-block-ethyleneoxide (PI-b-PEO), and the TiO2 precursor, we have fabricated TiO2 films with a range of porosities and crystal sizes.
We use optical-pump terahertz-probe (OPTP) spectroscopy to measure photoconductivity in these TiO2 films in the sub-picosecond to nanosecond time range. The technique is particularly sensitive to short-scale carrier transport [3], such as intra-particle and interfacial electron dynamics in dye-sensitized systems. We excite the TiO2 films at 400 nm, near the TiO2 band-gap, in order to probe the affect of nano- and micro-scale film morphology on electron mobility and electron diffusion length in TiO2. Additionally, the electron trapping and recombination time is measured over a nanosecond time-range. These measurements can contribute towards optimizing the TiO2 film morphology for high electron mobility and transport time, before the electrons get trapped in sub band-gap energy states or surface trap-sites.
The TiO2 films are then dye-sensitized and photoexcited at wavelengths ranging between 475 and 700 nm in order to probe the rate of charge injection from the dye excited state into the TiO2 conduction band. OPTP measurements show biphasic charge injection – with a fast (sub-500 fs) and a slow (70-200 ps) component. This is followed by a decay in photoconductivity over 5-10 nanoseconds, due to electron trapping and recombination.
References:
- HJ Snaith, L Schmidt-Mende, Adv. Mater. 19, 3187 (2007)
- M Nedelcu et al, Soft Matter 5, 134 (2009)
- P Parkinson et al, Nano Lett. 7, 2162 (2007)
R11: Poster Session: Nanostructured Architectures, Light-Harvesting, Modeling, Charge Injection, Transport, and Recombination Processes
Session Chairs
Friday AM, December 04, 2009
Exhibit Hall D (Hynes)
9:00 PM - R11.1
Series Connections of Organic Photovoltaic Cells Based on Low-Molecular-Weight Semiconductors.
Tetsuya Taima 1 , Toshihiro Yamanari 1 , Jun Sakai 2 , Masahiro Ichihara 3 , Eiichi Matsumoto 3 , Atsushi Masuda 1 , Yuji Yoshida 1
1 Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan, 2 Advanced Technologies Development Laboratory, Panasonic Electric Works, kadoma, Osaka, Japan, 3 R&D Center, Tokki Corporation, Mitsuke, Niigata, Japan
Show AbstractRecently, power conversion efficiency (PCE)of organic photovoltaic cell (OPV) has been upgraded due to high photo current density. To increase OPV performance furthermore, open-circuit voltage (Voc) also should be improved. It is well-known that series connections of OPV cells produce higher Voc. Series connections are formed by stacking of OPV cells in vertical and modularization in plane. Here, we studied stacking technique and modularization techniques for OPV cells based on low-molecular-weight semiconductors. We developed new intermediate electrode (LiF/Au/MoO) to connect the front OPV cell and back OPV cell. Rubrene and fullerene (C60) were used as p- and n-type semiconductors, respectively. Original device structure (single OPV cell) was ITO/50nm rubrene/50nm C60/LiF/Al. Obtained Voc was perfectly in proportion to number of stacked OPV cells. The PCEs of dual-OPV cell and triple-cell were almost 30% and 50% higher than that of single OPV cell due to high Voc. The maximum of obtained Voc was 2.49 V under the condition of triple-OPV cells. Modularization of OPV cell was done by laser scribing method. Cupper phthalocyanine (CuPc) and fullerene (C60) were used as p- and n-type semiconductors, respectively. Original device structure (single OPV cell) was ITO/20nm CuPc/50nm C60/LiF/Al. Each layer was patterned by ultra-short fiber laser in nitrogen atmosphere. Finally, module of 18 OPV cells in series connection in plane showed high Voc of 4.7 V. The part of this work was supported by the NEDO under METI. The work of modularization was done in collaboration with AIST, Tokki Corporation and Mitsubishi Corporation.
9:00 PM - R11.10
Creation of Electrically Conductive, Hole-selective Contacts on ITO Using Polymer Electrodeposition and Control of Electrical Doping: Optimization of Efficiencies for Planar Heterojunction Solar Cells.
Erin Ratcliff 1 , Brian Zacher 1 , Neal Armstrong 1 2
1 Chemistry, University of Arizona, Tucson, Arizona, United States, 2 College of Optical Sciences, University of Arizona, Tucson, Arizona, United States
Show AbstractWe have developed a unique electrodeposition method for the formation of hole-selective poly(thiophene)-based contacts on ITO for efficient OPVs. Electrodeposition permits thickness control via the monitoring of charge passed during polymer deposition and creates a contact that is electrically wired directly into the substrate, improving homogeneity of the contact; selective contacts that are homogeneous over large device areas are an essential component of all organic photovoltaic devices (OPVs). Indium-tin oxide (ITO) electrodes have been shown to be heterogeneous in both surface roughness and electronic properties, with device performance being highly dependent on surface pretreatments. Most attention has been previously focused on creation of hole-selectivity on ITO using dispersions of conducting polymers with poly-anions (e.g. PEDOT:PSS). The presented electrodeposition approach results in the deposition of a conductive polymer that can be doped (or undoped to varying degrees) after film growth, facilitating control of both hole mobility and effective work function of the polymer through inclusion of electrolyte, without the use of an acidic counter ion. The addition of the polymer layer as a selective hole contact in both titanyl phthalocyanine (TiOPc)/C60 and pentacene/C60 vacuum deposited planar heterojunction organic photovoltaic devices demonstrates changes in both series and shunt resistances with respect to electrochemical doping, with the partially doped poly(thiophene) film yielding the lowest series and highest shunt resistances. The impact of electronic work function, hole mobilities, and hole injection efficiencies versus PEDOT:PSS films will be discussed for both small and large area (>0.1 cm2) devices.
9:00 PM - R11.12
Nanostructured C60 Fullerene Films Using Glancing Angle Deposition.
Michael Thomas 1 , David Rider 2 3 , Jillian Buriak 2 3 , Michael Brett 1 2
1 Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta, Canada, 2 , NRC National Institute for Nanotechnology, Edmonton, Alberta, Canada, 3 Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada
Show AbstractTo increase efficiencies in organic photovoltaic cells (OPVs), nanostructuring of photoactive layers is desirable. Several concepts such as polymer blends 1 and self-organized polymer blends 2 have been used to fabricate bulk heterojunctions by controlling phase-separated morphologies on the nanoscale. However, exciton dissociation and charge transport of free charge carriers is low due to a low degree of order in these blends. To improve device efficiencies, concepts to grow highly-ordered nanostructured organic films are investigated. Glancing angle deposition (GLAD) is a potential approach to achieve ordered nanostructured posts of organic semiconductors and was demonstrated on materials such as pentacene or C60 fullerene 3.GLAD structured vertical and slanted C60 posts were grown on indium-tin-oxide (ITO). Shadowing conditions for the fabrication of isolated posts were created in an advanced physical vapor deposition system with substrate rotation and an oblique angle of flux incidence near 80°. Post diameters range from about 50 to 100 nm. By adjusting the deposition angle α, the spacing in-between posts can be adjusted and optimized with respect to the exciton diffusion length of 5 to 8.5 nm in a poly(3-hexylthiophene) “P3HT” donor polymer. Compared with depositions at a constant deposition angle α and constant substrate rotation speed, nanostructuring of C60 posts was improved by applying a φ-sweep deposition method of reducing shadowing anisotropy 4. Inverted 2-layer GLAD heterojunctions were fabricated by filling the nanostructured C60 films with P3HT and evaporating silver on top as an anode. Current experiments will be presented. The morphology of the C60 films with and without P3HT filling is observed with secondary electron microscopy. The structure of the layers is analyzed using x-ray diffraction. Further, the optical transmission through C60 films which were simultaneously deposited on fused silica substrates is characterized for different deposition angles α using UV/vis spectroscopy. The potential for nanostructured C60 films grown by GLAD to produce large, highly-ordered photoactive surface areas and to improve exciton harvesting compared to unordered polymer blends will be discussed. GLAD can fabricate nanostructured films without the need for complex patterning methods such as photolithography and has the potential to be transferred to roll-to-roll manufacturing as a promising route to achieve inexpensive OPV fabrication.References:1 W. L. Ma, C. Y. Yang, X. Gong, K. Lee and A. J. Heeger, Advanced Functional Materials 15 (10), 1617-1622 (2005). 2 G. Li, V. Shrotriya, J. S. Huang, Y. Yao, T. Moriarty, K. Emery and Y. Yang, Nature Materials 4 (11), 864-868 (2005). 3 J. Zhang, I. Salzmann, S. Rogaschewski, J. P. Rabe, N. Koch, F. J. Zhang and Z. Xu, Applied Physics Letters 90, 193117 (2007). 4 M. O. Jensen and M. J. Brett, Applied Physics a-Materials Science & Processing 80 (4), 763-768 (2005).
9:00 PM - R11.13
Spinodal Decomposition in Polythiophene:Methanofullerene Blend Thin Films for Bulk Heterojunction Solar Cell Active Layers.
Jennifer Segui 1 , Yantian Wang 1 , Charles Black 2 , Miriam Rafailovich 1
1 Materials Science and Engineering, SUNY - Stony Brook , Stony Brook, New York, United States, 2 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York, United States
Show AbstractWe control the morphology of blends of poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) by incorporating the polymer polystyrene into the device active layer. Addition of a second polymer into the blend promotes lateral phase separation in the film via spinodal decomposition, with PCBM nanoparticles subsequently confined to the polymer-polymer interface. We compare the morphology of polymer blend thin films prepared with and without PCBM, with particular emphasis on localization of the PCBM nanoparticles, and the effect of PCBM on the resulting phase separation behavior in the polymer blends. We investigate the resulting thin film morphology by using transmission electron microscopy, tapping and contact mode AFM, and scanning transmission x-ray microscopy (STXM). We will discuss the relevance of our thin film morphology for improving the organic semiconductor bulk heterojunction solar cell architecture by introducing self-assembled ordered columnar phases in the device active layer.
9:00 PM - R11.14
Enhanced Light Harvesting in the High Energy Range of Inverse Opal Based Solar Cells.
Agustin Mihi 1 , Kevin Arpin 1 , Paul Braun 1
1 , Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show AbstractDye Sensitized Solar Cells (DSSC) are one of the most promising third generation photovoltaic technologies. A DSSC is composed of a nanocrystalline titania layer deposited onto a transparent conductive oxide. This nanostructured film is sensitized with an organic dye which serves as the light absorber in the system. The electrode is placed against an anode and the interstitial space is filled with a hole conductor, typically an iodine containing electrolyte. The maximum reported efficiency for a DSSC is around 11%.[1] A major limitation is the low absorption of the organic dye in the red part of the visible spectrum. One strategy to improve the quantum efficiency of the cell at longer wavelengths is to add a scattering layer. A more recent possibility is to use a three-dimensional photonic crystal. Photonic crystals have already been utilized to enhance the photocurrent produced by a DSSC for wavelengths laying within the photonic band gap of the structure.[2] In this work, we investigate how the high energy response of three dimensional photonic crystals, where the photonic band structure predicts modes with high density of states (that is, low group velocities), can be employed to enhance the absorption of a certain material placed within the walls of the photonic structure. To do so, three dimensional inverse titania photonic crystals with different lattice parameters have been fabricated onto transparent conductive oxides by means of self assembly and atomic layer deposition. These porous networks have been used as DSSC electrodes when sensitized with a ruthenium based dye and assembled into an operating cell. The photocurrent measurements obtained in these cells have been compared with a reference cell made of nanocrystalline titania. Also, theoretical analysis of the optical properties of these cells has been performed using FDTD based simulations, and the spectral position and relative absorptance increment compared with those achieved in the real cells. [1]B. Oregan and M. Gratzel, Nature 353 (1991) 737.[2]A. Mihi, M. E. Calvo, J. A. Anta and H. Miguez, Journal of Physical Chemistry C 112 (2008) 13.
9:00 PM - R11.15
Amorphous Silicon Nanodome Solar Cells with Efficient Light Harvesting.
Jia Zhu 1 , Ching-Mei Hsu 2 , Yi Cui 2
1 Electrical Engineering, Stanford University, Stanford, California, United States, 2 Material Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractRecently intensive effort has been focused on the development of efficient light harvesting, which could minimize the surface reflection and enhance light trapping over a broad range of wavelength. Here we demonstrate an easily fabricated novel nanodome solar cells of hydrogenated amorphous Si (a-Si:H) with efficient antireflection and absorption enhancement over a broad band of spectra. Nanodome solar cells can absorb 46% more sunlight than flat film devices with the same thickness. We demonstrate nanodome devices with a power efficiency of 5.9%, which is 25% higher than flat film one. The large short circuit current of 17.5 mA/cm2 in nanodome devices exceeds that in world record single junction a-Si:H solar cells. Excitingly, those light management effects remain efficient in a wide range of angles of incidence, favorable in the real environment with significant diffuse sunlight.
9:00 PM - R11.16
Optimizing Nanoparticle Height For Light Trapping Inside a Solar Cell Using Localized Surface Plasmons.
Sudha Mokkapati 1 , Fiona Beck 1 , Kylie Catchpole 1
1 CSES, Australian National University, Canberra, Australian Capital Territory, Australia
Show AbstractA drive to reduce the cost of energy generation using photovoltaics has motivated research in the field of high efficiency solar cells with thin active regions. However, solar cells with thin active regions require efficient light trapping in order to increase the optical thickness of the active region. Light trapping is especially important at longer wavelengths (~1000 nm), where Si, most commonly used material for solar cells, is an indirect band gap material. Charge density oscillations in isolated metal nanoparticles coupled to external light (localised surface plasmons) have been successfully used to demonstrate effective light trapping in solar cells with thin active regions. For effective light trapping using localised surface plasmons supported by metal nanoparticles, the scattering cross section of the nanoparticle has to be maximised. It is also important to maximise the fraction of scattered light coupled into the substrate. The scattering cross section and the fraction of scattered light coupled into the substrate depend critically on the particle size, shape and its dielectric environment. It is essential to optimize the particle properties to obtain the maximum possible absorption enhancement inside the solar cell. We undertake a systematic study on the effect of particle height on its scattering cross section and the fraction of scattered light coupled into the substrate.Results presented in this paper were obtained by performing numerical simulations using Lumerical finite dimension time domain (FDTD) software. Semi-infinite Si substrates with a single cylindrical Ag nanoparticle (cross sectional diameter of 200 nm) on the rear surface are simulated for this study. We find that an increase in the nanoparticle height is accompanied by only a small increase in the magnitude of the scattering cross section of the particles, while the fraction of scattered light coupled into the substrate is reduced from 85 % to 50 % at 1000 nm by increasing the particle height from 50 nm to 400 nm. An increase in the particle height results in ‘organ-pipe’ charge oscillation modes in the nanoparticles that give rise to additional peaks in the scattering cross section data in the short wavelength region. These modes contribute to either an increase or decrease in the fraction of scattered light coupled into the substrate in narrow wavelength regions that support these modes. However, in spite of these local maxima/minima, the net fraction of scattered light coupled into the substrate is reduced as the particle height increases, indicating that organ-pipe modes are less effective at directing light into a high index substrate than dipole modes. These results show that for efficient light trapping applications, nanoparticles of a small height should be employed on the rear of the substrate.
9:00 PM - R11.17
Energy Level Alignment of Zinc Tetraphenylporphyrins Derivatives Adsorbed on Wide Band Gap Semiconductor Oxides.
Sylvie Rangan 1 , Senia Katalinic 1 , Ryan Thorpe 1 , Robert Bartynski 1 , Jonathan Rochford 2 , Elena Galoppini 2
1 Physics and Astronomy, Rutgers University, Piscataway, New Jersey, United States, 2 Chemistry, Rutgers University, Newark, New Jersey, United States
Show AbstractMetalloporphyrins play an essential role in photosynthetic mechanisms and therefore are natural candidates for electron transfer mediator in dye sensitized solar cells (DSSCs). Among the possible metalloprophyrins, the Zn(II) tetraphenylporphyrins (ZnTPP) derivatives have been found to have similar electron injection and charge recombination properties as the important standard ruthenium dye N3 for DSSCs, as well as reasonable performances using TiO2 or ZnO as substrates.We have performed electron spectroscopic measurements on ZnTPP derivatives adsorbed on single crystal and nanostructured TiO2 and ZnO. The Ti 2p and Zn 2p core levels as well as the valence band have been probed using x-ray and ultra-violet photoemission spectroscopies (XPS and UPS), while the conduction band has been obtained from inverse photoemission spectroscopy (IPS) in the same UHV system. Complementary insights on the adsorption geometries of ZnTPP derivatives have also been obtained using scanning tunnel microscopy as a local probe on a TiO2(110) rutile surface.The electronic structure of single crystals (TiO2(110) rutile and ZnO(11-20)) will be compared to the one of nanostructured materials(TiO2 anatase nanoparticles and ZnO nanorods). We have also determined the electronic structure of ZnTPP derivatives on these different substrates, as a function of the selective functionalization with carboxylic anchoring groups of the meso-phenyl, by means of density functional theory (DFT) and electron spectroscopy methods in an ultra-high vacuum environment.The experimentally determined electronic structure compares well to the calculated density of states for the free molecules, allowing both a simple understanding of the adsorbate electronic properties, and a direct determination of the ZnTPP derivatives frontier orbitals with respect to the substrates band edges. These results are compared to the available literature on devices using ZnTPP derivatives as sensitizers.
9:00 PM - R11.18
Charge Transport in ZnO Nanowire Dye Sensitized Solar Cells.
Hasti Majidi 1 , Jason Baxter 1
1 Chemical and Biological Engineering, Drexel University, Philadelphia, Pennsylvania, United States
Show AbstractDye sensitized solar cells (DSSCs) are one of the most promising designs for low-cost solar energy conversion. DSSCs comprise a mesoporous, wide band gap semiconductor that is coated with a monolayer of sensitizing dye and infiltrated with a liquid electrolyte. Photons are absorbed by the dye molecules, which inject photoexcited electrons into the semiconductor conduction band where they are then transported to the conducting substrate. DSSCs require high surface area for dye adsorption to maximize light harvesting (LHE) as well as electron diffusion times that are much shorter than recombination times to achieve high collection efficiencies. These constraints are typically satisfied using a TiO2 nanoparticle (NP) film. However, replacing the NP film with an array of single crystal nanowires (NWs) can improve transport by providing electrons with a direct path for diffusion, rather than having electrons hop many times between particles. Faster electron transport can increase Voc by enabling the use of redox couples with more favorable redox potentials. These redox couples cannot be used with NP cells because they induce very fast recombination. Additionally, the aligned pores of a NW array allow facile conformal filling with solid state hole transport materials for improved robustness.In this paper, we study the effect of ZnO morphology on charge transport and cell performance in NP and NW DSSCs. NWs were grown by chemical bath deposition at 90 C. Short circuit current (Jsc), LHE, and incident photon to current conversion efficiency (IPCE) were higher in NP DSSCs than NW DSSCs because of their larger surface areas. However, transport time in NWs is insensitive to photon flux over the range of interest, while transport in NP cells is slower at low photon flux. Both cells show faster recombination at high photon flux. Characteristic times were measured by photocurrent and photovoltage decay and electrochemical impedance spectroscopy. Transport times are 2-3 orders of magnitude faster than recombination times in NW DSSCs, indicating favorable charge collection, particularly at low light intensities. The influence of nanowire length (1-5 µm) on charge transport was also investigated. As expected, Jsc and IPCE increase with NW length due to their larger surface area. Transport time is independent of NW length for these cells, indicating that transport is limited by some other material or interface within the cell. Annealed NWs showed transport times that are 2-3 times faster than those of the as-grown NWs. While NW DSSCs exhibit improved charge collection compared to their NP counterparts, they are limited by low surface area and LHE when sensitized with conventional dyes. Therefore we have begun exploring CdSe coatings for use in robust, all-inorganic, extremely thin absorber NW solar cells.
9:00 PM - R11.19
Charge Transport in Si / SiO2 Multilayers for Photovoltaic Cells.
Salvatore Lombardo 1 2 , Rosaria Puglisi 1 , Carmelo Vecchio 1 , Simona Lorenti 2 , Marco Camalleri 2 , Sebastiano Ravesi 2
1 IMM, CNR , Catania Italy, 2 , STMicroelectronics, Catania Italy
Show AbstractThird generation photovoltaics using abundant and non toxic elements such as Si and O, N, C, etc. is a promising route toward the achievement of a low-cost, high-performance PV modules. Si nanostructures are a viable approach to control the semiconductor gap and the light absorption, but the quantum confinement of the free carriers has to be traded off for a sufficient photocarrier mobility. We have investigated the charge transport of Si nanostructures realized by a multi-layer of Silicon Rich Oxide and ultra-thin SiO2 thin films deposited by PECVD. The Si nanostructures are formed by Si precipitation induced by high temperature annealing.The dark current and the photocurrent induced by monocromatic laser light and AM1.5G solar spectrum irradiation are studied in detail in a large range of voltages and temperatures. A detailed quantitative model containing as parameters only the effective band offsets limiting transport is proposed and compared to data, showing satisfactory agreement. The charge transport in structures containing Si nanocrystals is compared to the case of unannealed PECVD samples, where significant differences in dark- and photo-conductivity are found.Conditions in terms of materials parameters providing sufficient carrier mobility and light absorption are described.
9:00 PM - R11.20
Mapping Local Performance and Charge Trapping in Nanostructured Organic Solar Cells with trEFM.
Obadiah Reid 1 , David Ginger 1
1 Chemistry, University of Washington, Seattle, Washington, United States
Show AbstractWe use time-resolved electrostatic force microscopy (trEFM) to map the local distribution of traps in bulk heterojunction solar cells. Using locally photo-oxidized PFB/F8BT blend films as a test case, we compare images obtained using both trEFM and scanning Kelvin-probe microscopy (SKPM). Films that were exposed to a relatively low photon dose (in air) showed no alteration in the surface photovoltage, but tr-EFM measurements show a distinct drop in photo-induced charging rate, consistent with trap formation, and device measurements show a similar decline in external quantum efficiency. With higher photon doses, the overall surface potential, and the surface photovoltage both became more negative in the photooxidized region, while the bulk performance and trEFM charging rate continued to decline. We conclude that trEFM is superior to SKPM for studying BHJ solar cells. However, a combined approach using SKPM and trEFM in tandem shows promise as a method for locally mapping the presence of local charge traps in organic photovoltaic devices.
9:00 PM - R11.22
Efficient Si Wire Solar Cells.
Michael Kelzenberg 1 , Morgan Putnam 1 , Claire Baek 1 , Shannon Boettcher 1 , Daniel Turner-Evans 1 , Emily Warren 1 , Josh Spurgeon 1 , Nathan Lewis 1 , Harry Atwater 1
1 , California Institute of Technology, Pasadena, California, United States
Show AbstractPhotovoltaic devices based on arrays of VLS-grown Si nano- or micro-wires are being investigated as potential low-cost alternatives to wafer-based Si solar cells. [1] Single-wire solar cells have been demonstrated with efficiencies of 3.4%, showing promise as nano-electronic power sources. [2] However, to date the reported efficiencies of macroscopic, VLS-grown Si wire array solar cells remain below 1%. [3] We have developed a process to produce high-fidelity arrays of VLS-grown Si wires, over areas exceeding 1 cm2, each of which has a radial p-n junction. Wires in this study had diameters of ~2 µm and lengths of ~100 µm. Single wires from these arrays have been measured; exhibiting photovoltaic efficiencies up to 5.2%, Voc of up to 450 mV, and fill factors of up to 75% under AM 1.5G illumination. We will present a full characterization of these single-wire solar cells, including J-V, spectral response, and scanning photocurrent microscopy. From these measurements we have determined the doping levels, diffusion lengths, and surface recombination velocities within our wires. We will present these results, and discuss ongoing efforts to produce an efficient, solid-state, Si wire array photovoltaic cell.[1] Kayes, B. M., Atwater, H. A. & Lewis, N. S. Comparison of the device physics principles of planar and radial p-n junction nanorod solar cells. J. Appl. Phys. 97, 114302-114311 (2005).[2] Tian, B. et al. Coaxial silicon nanowires as solar cells and nanoelectronic power sources. Nature 449, 885-889 (2007).[3] Tsakalakos, L. et al. Silicon nanowire solar cells. Appl. Phys. Lett. 91, 233117-233113 (2007).
9:00 PM - R11.23
Nanostructured WO3 Films for Photovoltaic Application.
Haidong Zheng 1 , Abu Sadek 1 , Kourosh Kalantar-zadeh 1
1 School of Electrical and Computer Engineering, RMIT University, Melbourne, Victoria, Australia
Show AbstractTungsten trioxide (WO3) has been extensively used in photovoltaic and electro-chromic devices, gas sensors and batteries. For the fabrication of many of such devices and increasing their efficiencies, nanostructured WO3 is required to be thick and porous enough to provide maximum surface area for interaction. Recently methods of nanostructuring WO3 in liquid medium such as anodization and acid bathing in high temperature have drawn considerable attentions because they are cost effective while having high controllability on the nanostructures. However, neither of these methods can produce a nanostructured WO3 layer of more than 500 nm.In this study, we have investigated anodization together with high temperature acid bathing obtaining relatively thicker nanostructured WO3 films. The tungsten samples were placed in a reaction flask containing 200 mL of 1.5 mole nitric acid (HNO3) solution and the flask was kept at various temperatures (50 - 90 oC) during the process. Anodization was carried out in the heated solution using a conventional anode (target sample) – cathode (platinum plate) system, where different DC voltages were applied.The morphology of the films is made of nano-size plates, which are typically square-shaped, with the length of 100 nm to 1 μm. Their widths are in the order of 20 to 60 nm. It was found that neither voltage nor anodization time could change the surface morphology significantly. As a result of this joint etching on the tungsten sample, the nanostructured WO3 layer was significantly thickened to 1.66 μm after 4 hours of process. Photocurrent measurements were conducted for such films after annealing in 0.1M Na2SO4 electrolyte. Under the irradiation of a UV-Vis light of 12mW cm-2, the WO3 film produced a current of 25 μA cm-2, and the response95% time was found to be less than 1s. In addition to this, dye-sensitized solar cells were constructed using such WO3 films, and 0.45% efficiency was achieved.It is proved that this novel high temperature anodization method can synthesize more functional WO3 nanostructured films. Such a method can also be expanded for the anodization of W thin films (in addition to W foil which was studied in this work), which are deposited using various vacuum deposition methods. This nanostructured WO3 film could be a potential candidate for replacing the TiO2 nanoparticles in conventional dye-sensitized solar cells.
9:00 PM - R11.24
Single Nanowire CIS Solar Cells.
David Schoen 1 , Hailin Peng 1 , Yi Cui 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractNanostructured photovoltaics are drawing an increasing amount of interest: they are the application of one of the most exciting and quickly moving fields of study in materials science, nanoscience, with one of the most pressing technological requirements of society, renewable energy. However, in order to be effective, the particular nanostructuring strategy chosen for investigation must address some of the key limitations of the chosen photovoltaic system. Finding the right structures for each system to address their individual limitations is a challenge that can only be addressed with improved fundamental understanding of the physical processes underlying the energy conversion. We here report a strategy for fabricating single nanowire devices as a route to unambiguously correlate structural features such as interface quality and chemical homogenaity with actual solar cell performance. Our chosen system for investigation is CIGS, which holds the record efficiency for thin film photovoltaics, but which remains poorly understood. Since their small total size permits a variety of experimental techniques to be used, including TEM and NSOM, the investigation of single nanowire solar cells offers a unique potential to shed light on many of the persistant mysteries present in many solar material systems, especially CIGS.
9:00 PM - R11.25
Synthesis and Properties of WO3 Heternanostructures for Efficient Solar Energy Conversions.
Yongjing Lin 1 , Stafford Sheehan 1 , Xiaohua Liu 1 , Sa Zhou 1 , Dunwei Wang 1
1 Chemistry, Boston College, Chestnut hill, Massachusetts, United States
Show AbstractTo compete with that from fossil fuels, solar energy needs to be converted into useful forms, e.g., electricity or chemical energy, efficiently and at low cost. High efficiency is afforded when various steps involved in a conversion process are balanced: light absorption, charge generation, charge separation, charge transport, and charge collection. It has been a formidable challenge to optimize them simultaneously as each poses a unique requirement. For example, a material absorbs more light to generate more charges when it is thicker, whereas a thinner material is more effective in separating and transporting these charges. Detailed balance of various aspects to meet apparently conflicting requirements like this has been proven onerous, keeping efficient solar energy conversion devices expensive; conversely, efforts to lower the cost produce devices with inferior performance. Thus we are presented with an efficiency-cost dilemma.Answers to the dilemma lie in new materials. Here we reported our success in improving H2 generation from water with solar energy as the input. Our design consisted of a complex nanoscale core/shell WO3/TiSi2 structure. WO3 was synthesized by atomic layer deposition, which is highly reproducible and versatile. By incorporating TiSi2 nanonets structure, we showed the heteronanostructures were advantageous in photosplitting H2O as various components worked concertedly to carry out the reactions. The heterogeneous design opens up a new door in tackling the cost-efficiency challenges of solar energy harvesting.
9:00 PM - R11.26
Nanostructured Fe2O3 and FeS2 for Solar Energy Conversion.
Kevin Sivula 1 , Jeremie Brillet 1 , Michael Graetzel 1
1 Chemistry and Chemical Engineering, Swiss Federal Institute of Technology, Lausanne, Vaud, Switzerland
Show AbstractDue to its abundance, environmental stability, and favorable bandgap of 2.0 eV, iron (III) oxide (hematite) is a promising semiconducting oxide for solar energy conversion. We have already reported nanostructured Fe2O3 photoanodes for solar water splitting with state of the art performance corresponding to 3.3 % solar-to-hydrogen efficiency. Here we will investigate using these materials in two electrode photovoltaic devices to evaluate the limiting factors of the nanostructured hematite. In addition, we introduce a strategy to overcome the previously reported limitations of poor photon absorptivity and charge carrier transport in hematite by the facile conversion of Fe2O3 into FeS2 by sulfurization. Iron pyrite (FeS2) is also a very promising material for solar energy conversion due to its bandgap of 0.95 eV and an ultra-high absorption coefficient (α = 6 × 10^5 cm^−1). However, the successful implementation of this material for solar energy conversion has been inhibited by poor semiconducting properties. We examine morphological approaches to overcome these limitations.
9:00 PM - R11.27
Control of Structural and Optical Properties of Silicon Quantum Dot Embedded in SiC Matrix Superlattice.
Jihyun Moon 1 2 , Hyun Jong Kim 1 2 , Shin Ho Kim 1 3 , Jun Sik Cho 1 , Sang Hyun Park 1 , Byungsung Oo 2 , Kyung Hoon Yoon 1 , Jinsoo Song 1 , Jeong Chul Lee 1
1 Photovoltaic Research Center, Korea Institute of Energy Research, Daejeon Korea (the Republic of), 2 Department of Physics, Chungnam National University, Daejeon Korea (the Republic of), 3 Department of Material Science and Engineering, Pusan National University, Pusan Korea (the Republic of)
Show AbstractSilicon quantum dots by controlled size and spacing uniformity have been proposed for high efficient all-silicon tandem solar cell. First of all, we study the formation of silicon quantum dots in Si1-xCx precursor layers. The composition of as-depositied Si1-xCx precursor layers can be controlled by changing rf power of Si and C targets by co-sputtering system. Si QDs or silicon nanocrystal structures (about 3~10 nm) appear after annealing Si1-xCx precursor layers by high resolution transmission electron microscopy (HRTEM) image. Si-rich SiC/stoichiometric SiC superlattices is fabricated to be based on Si1-xCx precursor layers. Si QDs size is controlled by the thickness of Si-rich SiC layer for operating absorption range. The Si-rich SiC layer thickness is investigated from 1nm to 10nm under the fixed about 3nm of stoichiometric SiC layer thickness. According to HRTEM image, Si QDs size is equal or small in comparison with layer thickness. The crystal volume fraction of Si crystalline is estimated around 40% by Raman spectrum, it expect to equal Si QDs spacing uniformity in SiC matrix. According to previous research results, Si QDs in Oxide or Nitride matrix are known to controlled optical band gap by quantum confinement effect and quantum size effect. In comparison, Si QDs in Carbide matrix are not many studied those results of the optical properties. In this paper, the structure and optical properties of Si QDs in Si1-xCx thin film are studied to control Si QDs size. These results are expected to understand clearly the characterization of Si QDs in SiC matrix for all-silicon tandem solar cells application.
9:00 PM - R11.28
Core-Shell Nanowire Mesh Inverse Opal Photoelectrodes.
Soon Hyung Kang 0 , Nathan R Neale 0 , Arthur J Nozik 0 , Arthur J Frank 0
0 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractRecently, a third generation device concept featuring two tandem nanocrystalline photosystems was proposed [M. C. Hanna and A. J. Nozik, J. Appl. Phys. 100, 074510 (2006)]. Here, we present progress on developing the photocathode component of the tandem device. We show that electrodeposition of various metal oxides and metals within polystyrene bead templates affords nanowire mesh inverse opal photoelectrodes. The preparation and optical characterization of p-type semiconductors both with and without metal cores will be discussed. Comparison of the electronic properties among the various photoelectrodes will also be presented.
9:00 PM - R11.29
Photocurrent Generation in PbSe Quantum Dot-TiO2 Nanorod Structures Fabricated by a Laser Assisted Spray Process.
Jessica Kowzan 1 , Michael McConnell 1 , Jason Rejman 1 , Dino Ferizovic 1 , Pritish Mukherjee 1 , Martin Munoz 1 , Sarath Witanachchi 1
1 Physics, University of South Florida, Tampa, Florida, United States
Show AbstractPbSe quantum dots in the size range of 4-10 nm have been shown to generate multiple excitons when exposed to the uv spectrum of the sunlight. Generation of current in an excitonic solar devices rely on the dissociation dynamics of these excitons. It is well known that the dissociation is aided by the band-bending at the interface between the QD and the surrounding material. Efficient transfer of electrons from the QD to TiO2 nanoparticles has been observed for QD/TiO2 nanoparticle composite structures. In this paper we present the fabrication and characterization of an excitonic solar cell structure that utilizes the large surface area made available by vertically aligned TiO2 nanorod structures to form high density of TiO2-PbSe QD interfaces for effective charge transport. The cell structure is fabricated on a glass substrate that is coated with a conductive fluorine-doped tin oxide film as the bottom transparent contact. The vertically aligned TiO2 with nanotubular morphology were grown by a hydrothermal process. Nanorods have an average diameter of about 225 nm and a length of about 2 μm. Nanoparticles of PbSe with average diameter of 6 nm were grown by a solvothermal method and were dispersed in hexane. PbSe suspension was used as the precursor in the Laser Assisted Spray (LAS) process where aerosol of the precursor was laser heated to evaporate the surfactants. The surfactant-free PbSe QDs were deposited directly on the TiO2 nanorod template allowing QDs to fill the gap between adjacent nanorods. Finally, a layer of the p-type polymer P3HT was deposited over the PbSe particle coating using the LAS process. Al pads were evaporated on the polymer film as the top electrode. Comparison of the photocurrent generated by the two structures, one where surfactant-free PbSe QD were directly deposited on the TiO2 nanorod template by LAS process and the other where the caped PbSe QDs were spin-coated onto the TiO2 nanorod template, will be presented.
9:00 PM - R11.3
Directed Synthesis of Lead Selenide – Titania Core-shell Nanowire Heterostructures for High-efficiency Low-cost Solar Cells.
Evan Wujcik 1 , Arijit Bose 1
1 Chemical Engineering, University of Rhode Island, Kingston, Rhode Island, United States
Show AbstractThe need for new sources of energy has been of great concern to the scientific community over the past few decades. Due to this rise in interest, new and emerging technologies in efficient, clean energy conversion have been developed. Among these developments lie innovative and non-quintessential concepts of photovoltaic materials, which include photovoltaic nanocrystalline devices that have recently grown in popularity due to their highly efficient multi-exciton processes. We present a novel, high-efficiency, low-cost, all inorganic Lead selenide-Titania (PbSe /TiO2) nanowire heterostructure material synthesis for photovoltaic applications. PbSe Quantum dot (QD) nanocrystals that have been uniformly distributed within a TiO2 support with high connectivity for highly efficient charge carrier flow and electron-hole pair separation. We have characterized this material by High-resolution Transmission Electron Microscopy (HR-TEM), Electron Diffraction (ED), Energy Dispersive X-ray Spectroscopy (EDX), and UV-Visible Spectroscopy (UV-Vis).
9:00 PM - R11.30
Depositing Stable and Efficient Copper Oxide Nanowires for PEC Application from Corroded Copper Oxide Based Films.
Le Chen 1 , Kwang-Soon Ahn 1 , Fude Liu 1 , Sudhakar Shet 1 , Yanfa Yan 1 , Mowafak Al-Jassim 1 , John Turner 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractThe goal of the study is to develop p-type copper oxide based materials with high photoelectrochemical(PEC) efficiency and stability. Amorphous copper tungsten oxide thin films over FTO glass substrates were sputtered and tested for PEC performances. The films experienced corrosion under applied positive bias in the basic electrolyte (NaOH, pH 11) and regrowth under applied negative bias. As characterized by SEM and TEM, Cu2O nanowires were the main species electrochemically deposited over the corroded film surfaces, and tungsten based species were almost excluded from the films and merely existed at the grain boundaries of Cu2O nanowires in trace. The grown Cu2O nanowires (p-type) exhibited high stability in the basic electrolyte and improved photocurrent under illumination compared to the original copper tungsten oxide films. The optimization of the electrochemically deposited Cu2O nanowires yield by studying and adjusting applied bias, electrolytes and starting materials (i.e. different copper based oxide films) is under way.
9:00 PM - R11.31
Opportunities for Nanometer-sized Si Nanowires for PV Applications.
Rufi Kurstjens 1 2 , Frederic Dross 1 , Emmanuel Van Kerschaver 1 , Jef Poortmans 1 2 , Robert Mertens 1 2
1 , IMEC, Heverlee Belgium, 2 , KULeuven, Heverlee Belgium
Show AbstractIn recent years nanostructures have received increasing interest from people in the field of photovoltaics. Even though the research is still in early development, there are already quite some novel concepts for the beneficial application of these nanostructures in solar cells. The focus of this review paper is on silicon nanowires (NWs), since Si is the most ubiquitous material in photovoltaics, and since these structures can provide some unique combination of assets. We will discuss using quantum confinement in Si nanowires to form a high-bandgap crystalline Si material for the top junction in an all-Si tandem solar cell setup akin to the Si quantum dots on bulk c-Si concept. The small dimension (< 5 nm) of the NWs induces quantum confinement of excitons and increased bandgap, while the 1D structure maintains acceptable carrier transport. To support the feasibility of this concept of small diameter Si NWs, we will investigate the specific obstacles on the route towards the application of nanowires in a solar cell and discuss the most relevant results obtained in literature. The approach of IMEC to produce a proof of concept will also be explained.
9:00 PM - R11.36
Semiconductor Nanowires-embedding Photovoltaics.
Joondong Kim 1 , Ju-Hyung Yun 1 , Yong Jae Cho 2 , Jeunghee Park 2 , Yun Chang Park 3 , Chang-Soo Han 1
1 Nano-Mechanical Systems Research Division, Korea Institute of Machinery and Materials, Daejeon Korea (the Republic of), 2 Chemistry, Korea University , Seoul Korea (the Republic of), 3 , National Nanofab Center , Daejeon Korea (the Republic of)
Show AbstractOne-dimensional nanostructure, such as nanotubes and nanowires have proved high potential features. Carbon nanotubes having a large surface area at a fixed volume showed a sensitive performance to a NO2 gas [1]. Tiny size nanowire has been applied in microscopy tip [2] and nanoscale interconnect [3]. One of the most promising area of nanomaterial application would be the solar harvest. Nanostructure has a large photon-active region. But the promising of nanostructured solar cell has not been much fulfilled yet due mainly to the difficulty in nanoscale-architecture. Germanium nanowire (GeNW)-embedding Schottky solar cell was fabricated by dielectrophoresis. GeNW containing solution was dropped onto the metal electrodes under ac electric field and SiNWs were positioned. One metal formed an ohmic contact and the other metal gave a Schottky contact to the GeNW resulting in rectifying current flow. We discuss the design scheme of a rectifying junction formation by work function matching between metals and nanoscale semiconductors. It covers electrical characteristics of nanowire-embedding Schottky solar cells. References [1] J. Kim, J–H Yun, J. –W. Song and C.-S. Han Sens. Actuators B 2009, 135, 587-591. [2] J. Kim, Y. –H. Shin, J. –H Yun and C. S. Han Nanotechnology 2008, 19, 485713. [3] J. Kim and W. A. Anderson, Nano Lett. 2006, 6, 1356-1359.
9:00 PM - R11.37
Characterization of Ag Nanocrystals for use in Solar Cell Applications.
Annett Thoegersen 1 , Jack Bonsak 1 , Jeyanthinath Mayandi 1 , Erik Marstein 1 , Tom Johansen 2 , M. Umadevi 3
1 Solar Energy, Institute for Energy Research (IFE), Kjeller Norway, 2 Physics, University of Oslo, Oslo, Oslo, Norway, 3 Physics, Mother Teresa Women’s University, Kodaikanal India
Show AbstractSurface plasmon resonance in metal nanocrystals may be used to enhance the absorption in solar cells. Pillai et al [1] showed that Ag nanocrystals with an average Ag nanocrystal diameter of 16 nm contributed up to 16-fold enhancement in the photocurrent at certain wavelengths and a 33% increase of the total current of the device. The optical properties are strongly dependent on the nanocrystal density, size, lattice defects, shape, spatial arrangement and configuration of the nanocrystals [2]. It is therefore very important to investigate these properties in detail in order to make the most efficient solar cell.The Ag nanocrystals are made by chemical synthesis. By employing this method one can scale the process to industrial usage due to its economical cost and simplicity. The nanocrystals were made by mixing AgNO3 with the surfactant NaBH4 in an aqueous solution. With increasing ratio between the AgNO3 and NaBH4 a smaller particle size is to be expected. The nanocrystal size, distribution, composition and atomic structure of samples with different chemical ratios were analyzed by High Resolution Transmission Electron Microscopy (HRTEM) using a 200 keV JEOL 2010F microscope equipped with a Gatan imaging filter and detector. Energy Dispersive Spectroscopy (EDS) was carried out with a NORAN Vantage DI+ EDS system. Most of the Ag nanocrystals had a spherical shape, but some of the larger nanocrystals were elongated or hexagonal. Solutions with different ratios between the AgNO3 and NaBH4 were found to have a small difference in nanocrystal size. A sample with a 4-25 ratio had an average nanocrystal size of 10 ± 11 nm, while a sample with a 25-25 ratio had 16 ± 51 nm. The large standard deviation is due to a larger size distribution. HRTEM results showed that the samples contained both pure Ag nanocrystal as well as some Ag2O nanocrystals. Defects in the Ag nanocrystals have been reported to occur when the nanocrystals are embedded in an oxide layer or exposed to pressure [2, 4]. Similar defects were observed in the Ag nanocrystals studied in this work. Nanocrystals with six crystallographic areas separated by twin boundaries and a so-called twin pentajunction were observed, as were several cyclic twin boundaries, regular twins and stacking faults. These defects resulted in large facets at the nanocrystal surface and have been studied in detail. When the nanocrystals were exposed to a high energy electron beam, the nanocrystals started to nucleate. The electron beam induced heating in the sample also seems to remove some of the defects within the nanocrystals.References:[1]: S. Pillai, K. R. Catchpole, T. Trupke, and M. A. Green, J. Appl. Phys. 101, 093105 (2007)[2]: H. Hofmeister, M. Dubiel, G.L. Tan, and K.D. Schicke, Phys. Stat. Sol. 202 (12), 2321 (2005)[3]: K. J. Koski, N.M. Kamp, R.K.Smith, M. Kunz, J.K.Knight, and A.P. Alivisatos, Phys. Rev. B 78 165410 (2008)
9:00 PM - R11.38
TiO2 Nanotube Films via Laser Ablation for Solar Cells.
Dulip Perera 1 , Chalita Ratanatawanate 1 , Kenneth Balkus 1
1 chemistry, University of Texas at Dallas, Richardson, Texas, United States
Show AbstractTitanium dioxide (TiO2) nanostructures are widely employed in photoconversion processes. In particular titanium dioxide nanotubes (TNTs) have shown promise in many applications including solar cells. While there are methods for making TNT films the preparation of high surface area small diameter (<10nm) TNT films is a challenge. We have now prepared TNT films by pulsed laser deposition (PLD) of P25 and P90 TiO2 nanoparticles onto stainless steel foils followed by a hydrothermal treatment. The best results were obtained when the TiO2 films were deposited at 500°C. Subsequent hydrothermal treatment at 150°C results in a dense mat of TNTs that are ~10 nm in diameter with a pore size of ~4 nm. The TNTs were further functionalized with quantum dots including PbS with controlled particle size and location to access a greater portion of the solar spectrum. The titanium oxide nanotube/ quantum dot films were characterized by SEM, TEM, XRD UV-Vis and Raman spectroscopy. Preliminary results for the formation and characterization of solar cells using the TNT films will be described.
9:00 PM - R11.39
ZnO/CdSe Core/Shell Nanowire Arrays for Photovoltaic Applications.
Haojun Zhu 1 , Quan Li 1
1 Department of Physics, the Chinese University of Hong Kong, Shatin, New Territories, Hong Kong China
Show AbstractNanostructure based photovoltaic applications have aroused much interests in recent years. Among various possible configurations, conducting oxide nanostructure networks loaded with light absorbing molecules and/or particles serve as one of the most promising candidates for novel type of photovoltaic devices. In this study, we demonstrate ZnO/CdSe core/shell nanowire arrays on ITO substrate. The ZnO nanowire arrays are fabricated using two different methods, i.e., thermal evaporation and thermal decomposition of Zn(Ac)2. The polycrystalline CdSe layer are deposited on the ZnO nanowire surface via thermal chemistry method, by which we can control the both the grain size and the thickness of the CdSe. The optimum CdSe grain size and its layer thickness have been identified in order to maintain the balance between the light absorption and the carrier transport. The optical and photoelectrical properties of the dry ZnO/CdSe (on ITO) photoelectrode have been measured, and the photovoltaic cells have been assembled based on these nanostructured hierarchy architectures. The conversion efficiencies of the cells with different material parameters are compared and the limitations to the efficiency are discussed.
9:00 PM - R11.4
The Efficiency Enhancement of Organic Solar Cells using TCO Nano Rods Electrode.
Gwan Ho Jung 1 2 , Hak ki Yu 1 2 , Kihyon Hong 1 2 , Jong-Lam Lee 1 2
1 Graduate Institute of Advanced Materials Science, POSTECH, Pohnag, Gyungbuk, Korea (the Republic of), 2 Materials Science and Engineering, POSTECH, Pohang, Gyungbuk, Korea (the Republic of)
Show AbstractOrganic solar cells have attracted attention as a means to achieve low-cost solar-energy conversion owing to their ease of manufacture 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. However, the smooth ITO surface has only a small interface area between electrodes and electron donor materials. Incorporating both optical and electrical properties, we study the influence of the transparent conducting oxide (TCO) 3-D nanostructure (e.g., planar vs 3-D nano electrode) on donor-acceptor organic solar cell efficiencies based on the copper phthalocyanine (CuPc) and C60. Here we introduce a small molecule solar cell in which the traditional ITO film substrate is replaced by a dense array of TCO nano rods. The nano rods anode was grown by electron beam deposition method without catalyst. The various thickness of TCO nano rods (0, 20, 40, 100 nm) were used to change morphologies of ITO substrate. We measured the transmittance and absorption of TCO nano rods using tungsten-halogen lamp and monochromator. Solar-cell performance of the J–V characteristics and power-conversion efficiencies of the devices were measured under simulated AM 1.5G solar illumination at 1 sun intensity and keithley 2400 source-meter. The surface morphology and micro-structure of TCO nano rods were investigated using x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and secondary electron microscopy (SEM).The direct electrical pathways provided by the nanowires ensure the rapid collection of carriers generated throughout the device and a full Sun efficiency of 0.82% is demonstrated. Under the illumination of 100 mW/cm2, the VOC and the JSC are 0.42 V and 4.1 mA/cm2, respectively. The FF, which is defined as (VMJM) / (VOCJSC), was 48%, where VM and JM are the voltage and the current density at the maximum power output, respectively. This gives a power conversion efficiency defined as PCE=VOCJSCFF/ Iph, here Iph is the incident photon flux of 0.82%. An enhancement in JSC to 4.1 mA/cm2 (nano structured) from 2.5 mA/cm2 (planar) was obtained by increase of interface between electrode and electron donor. From these, we discussed about the effects of nanostructured TCO on the improvement of power conversion efficiency and proposed the methodology in enhancing the organic solar cell efficiency.
9:00 PM - R11.41
Fabrication of Si Wire Arrays by Noble Metal-Assisted Chemical Etching for Radial p-n Junction Solar Cells.
Jae Hyun Kim 1 , Seong Ho Baek 1 , Ho-Jin Choi 1
1 Division of Nano & BioTechnology, Daegu Gyeongbuk Institute of Science and Technology, Daegu Korea (the Republic of)
Show Abstract A photovoltaic device consisting of arrays of radial p-n junction wires enables a decoupling of the requirements for light absorption and carrier extraction into orthogonal spatial directions. Each individual p-n junction wire in the cell is long in the direction of incident light, allowing for effective light absorption, but thin in orthogonal direction, allowing for effective carrier collection. To fabricate radial p-n junction solar cells, p or n-type vertical Si wire cores need to be produced. The majority of Si wires are produced by the vapor-liquid-solid (VLS) method. But contamination of the Si wires by metallic impurities such as Au, which is used for metal catalyst in the VLS technique, results in reduction of conversion efficiency of solar cells. To overcome impurity issue, top-down methods like noble metal catalytic etching is an excellent candidate. We used two kinds of chemical etching methods to make Si wire arrays. One is electroless deposition of Ag and etching in various AgNO3 and HF solutions after masking Si surface by photolithography. The other is Au deposition on photolithographycally defined Si surface by sputtering and then etching in various HF and H2O2 solutions. The Si substrates were p-type (10 ~ 20 ohmcm). The areas that Ag was not deposited due to photo resist covering almost were not etched in Ag catalytic chemical etching. The Si wires of several tens of nm in size were formed in uncovered areas by photo resist. The solution composition observed in the highest etching rate was HF/AgNO3 (3.5 M / 0.015 M). In Au assisted chemical etching method, Si column arrays were formed and also thicker Si wire arrays than them in HF/AgNO3 etching appeared in between Si columns. We could control diameters of Si columns and remove the Si nanowires between Si columns by one more etching treatment in KOH solutions. Finally, Process conditions for fabricating various morphologies and sizes of vertical Si wire arrays could be established Radial p-n junction wire arrays were fabricated by spin on doping (phosphor), starting from chemical etched p-Si wire arrays. In/Ga eutectic metal was used for contact metal. The effect of Si column diameter and Si nanowires between Si column arrays on solar cell efficiency was studied. The existence of an additional Si nanowires lead to a increase of the efficiency of solar cells by 2 ~ 3 % compared with that without them.
9:00 PM - R11.42
Photoelectrochemical Cells of Ultra Thin Film Hematite: Towards Nano-Rust Solar Cells.
Benjamin Klahr 1 , Alex Martinson 2 , Thomas Hamann 1
1 Chemistry, Michigan State University, East Lansing, Michigan, United States, 2 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractHematite (Fe2O3) is an attractive material for the conversion of light to electricity. The 2.2 eV band gap allows for the direct absorption of a significant portion of the solar spectrum, however the exceptionally small hole diffusion length (< 10 nm) has been implicated in the very poor absorbed photon-to-current efficiencies reported to date. In principle this limitation can be overcome through development of nanostructured photoelectrodes. To this end, we have investigated thin films of hematite employed in photoelectrochemical cells, PECs, as models systems. The hematite was deposited via Atomic Layer Deposition (ALD), which allows for extremely precise control of film thickness. We report investigations of thickness dependence of photovoltaic performance characteristics, as well as the composition of the electroactive contacting phase. Initial attempts of surface modification will also be presented.
9:00 PM - R11.43
Growth of Si Nanowires on Commercial SnO2:F Coated Glass Substrates.
Somilkumar Rathi 1 , Joseph Beach 1 , Reuben Collins 1
1 , Colorado School of Mines, Golden, Colorado, United States
Show AbstractThere is presently considerable interest in incorporating silicon nanostructures into thin film photovoltaic cells. Silicon nanowires arrays, in particular, may have advantages in thin film photovoltaic cells. If efficient nanowires with diameters less than 10 nm can be developed it may be possible to take advantage of quantum confinement effects to tune device properties while the wire geometry allows for more efficient collection of photoexcited carriers than, for example, quantum dots. However, growing high quality silicon nanowires in a solar cell presents significant challenges. From a materials standpoint, common methods for growing nanowires such as metal seeded vapor-liquid-solid (VLS) growth, have focused on metals like Au which produce deep electronic levels in the silicon. From a device architecture point of view, synthesis must facilitate electrical connection of the wire array to the front and/or back cell electrode. One method for doing this is to grow silicon nanowires on a transparent conductor by metal nucleated VLS growth using seeds from columns III or IV which do not create deep levels. Previous work in the literature has demonstrated that hydrogen plasma treatments of SnO2 can cause the formation of Sn islands approximately 100 nm in diameter, and that those islands can seed silicon nanowire growth. The present study extends that in the published literature through a systematic study of Sn island formation, size, and properties on commercial SnO2:F as a function of hydrogen plasma condition. Pilkington Tec-15 substrates (soda lime glass with a 13 ohm/square SnO2:F film on one side) were cleaned with Micro-90 detergent, rinsed with deionized water, blown dry, and exposed to hydrogen plasmas. Variables in the plasma exposure included RF power density, hydrogen pressure, duration of exposure, and substrate temperature. Sn island formation was observable and characterized in scanning electron microscopy and atomic force microscopy. Size and spacing were correlated with process parameters. Si nanowire arrays were grown from Sn seeds produced in this manner. Correlation of nanowire properties with seed characteristics is also discussed along with production of metal seeds from alternative transparent conducting oxides on glass. Support of the NSF Renewable Energy Materials Research Science and Engineering Center and Center for Revolutionary Solar Photoconversion is gratefully acknowledged.
9:00 PM - R11.44
Fabrication of Silver Nanoparticles on Thin Film Electrodes for Photovoltaic Cell.
Huiqiong Zhou 1 , Chi Wah Leung 2 , Paddy Kwok Leung Chan 1
1 Department of Mechanical Engineering, Hong Kong Polytechnic University, HongKong China, 2 Department of Applied Physics, Hong Kong Polytechnic University, HongKong China
Show AbstractIn the last twenty year, photovoltaic cell has become a very popular research topic among different research groups all over the world. But the universally high cost and low efficiency hinder practical applications of this type of devices. Indium tin oxide (ITO) electrode, which is a kind of widely used electrodes in organic photovoltaic cell, has significant shortcoming: expansive cost of indium. Metal thin film electrodes have been proved to perform comparably to ITO electrodes and have the potential to decrease the cost of photovoltaic cell1. Light harvesting is particularly critical in such photovoltaic cell in order to increase light absorption and enhance cell efficiency. In the current work we fabricated nanoparticles on silver thin film electrodes and investigated optimal electrode structure for enhancing the efficiency of photovoltaic cell. Silver thin film electrodes were made by thermal evaporation, and silver nanoparticles/islands with different size are formed by varying the thickness of the silver films from 5 nm to 50 nm and the vacuum annealing conditions. After annealing at 400°C, thicker silver thin films produced bigger silver nanoparticles/islands, while the color of films changed from purple to yellow. At the same time, the nanoparticles/islands coverage area decreased 12% while the film thickness increased from 10 nm to 30 nm. Beneficial aspects of these transparent and conductive silver thin film electrodes with nanoparticles were also discussed. Reference:1. O. C. Brendan, H. Chelsea, A. Kwang-Hyup, P. P. Kevin, S. Max, Appl. Phys. Lett. 2008, 93, 223304.
9:00 PM - R11.45
Towards Nanostructured Photovoltaic Devices Based on Ordered Valve Metal Oxide-Nanotubes.
Dominik Koll 1 , Alexander Birkel 1 , Stefan Frank 1 , Martin Panthoefer 1 , Wolfgang Tremel 1
1 Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg University Mainz, Mainz, Rhineland Palatinate, Germany
Show AbstractThe design and improvement of nanostructured photovoltaic devices has received enormous attention over the last two decades after the introduction of dye sensitized solar cells (DSSCs) based on TiO2-nanoparticles by Graetzel and co-workers [1]. A promising approach towards a nanostructured photovoltaic device is the use of an ordered array of valve metal oxide nanotubes or nanopores as semiconducting material, instead of nanoparticles[2]. Anisotropic nanosystems such as tubes, wires or rods offer the advantage of a high surface area, combined with good electron conduction towards the electrode. Most valve metal oxides, e.g. TiO2, do not absorb light in the visible region of the sun’s spectrum, i.e. they need to be sensitized. In DSSCs a dye is used for this purpose, yet an inorganic p-type semiconductor can be employed as well. In our approach the first step is the synthesis of an ordered array of metal oxide nanotubes or nanopores. This is achieved via a two-step anodization of a valve metal foil. The procedure is based on the work of Lee et. al[3], but it is widened from titanium to other valve metals, like Tin and Vanadium, respectively. Depending on the experimental conditions during anodization, either free standing, highly ordered nanotubes or ordered nanopores are obtained. Other characteristics of these nanostructures, like diameter or wall thickness, can also be varied. In a second step p-type CuInS2 is electrodeposited into void spaces of the nanotubular network, as described by Frank et.al for the case of TiO2 and CuInSe2[4]. [1] Brian O'Regan and Michael Grätzel, Nature, 1991, 353, 737. [2] Bang-Ying Yu, Ating Tsai, Shu-Ping Tsai, Ken-Tsung Wong, Yang Yang, Chih-Wei Chu and Jing-Jong Shyue, Nanotechnology, 2008, 19, 255202 (5pp) [3] Yeonmi Shin and Seonghoon Lee, Nanoletters, 2008, 8, (10), 3171 [4] Qing Wang, Kai Zhu, Nathan R. Neale and Arthur J. Frank, Nanoletters, 2009, 9, (2), 806
9:00 PM - R11.46
Synthesis and Photoelectrochemical Studies of Vertically Aligned Si Nanowire Arrays.
Guangbi Yuan 1 , Huaizhou Zhao 2 , Xiaohua Liu 1 , Zainul Hasanali 1 , Yan Zou 1 , Andrew Levine 1 , Dunwei Wang 1
1 Department of Chemistry, Boston College, Chestnut Hill, Massachusetts, United States, 2 Department of Physics , Boston College, Chestnut Hill, Massachusetts, United States
Show AbstractPhoton absorption, charge separation, and charge collection are key steps involved in photovoltaics. Optimizing and balancing these steps is crucial to maximizing the overall power conversion efficiencies, but has been proven difficult because each aspect poses unique and often conflicting requirements. One example is found in optimizing material’s dimensions: a thick material is desired for better light absorption, whereas a thin material with short charge diffusion length is favored for charge collection. Dilemmas like this present great challenges in material syntheses and device fabrications, keeping the cost of efficient devices high. From a material and device structure standpoint, high aspect ratio nanowires (NWs) are a promising candidate to break these dilemmas. Absorbing light in the longitude direction and separating and collecting charges in the transverse direction, a NW easily circumvents the conflicting requirements described above. In addition, the development of synthetic chemistry has made it possible to produce NWs in bulk quantity inexpensively. For these reasons, immense research efforts have been devoted to realizing efficient power conversions using NWs. For NW-based solar cells, several important parameters, including lengths, diameters and doping levels of NWs, remain to be experimentally identified, answers to which are expected to lie in and consequently shed new light on the nanoscale photophysical processes. Initial efforts have been exerted primarily on individual NWs. Although highly revealing, the statistical significance of these results when extended to a large number of NWs have yet to be evaluated. Equally important is to conduct these studies on nanoscale structures. In this presentation, we show our studies toward this goal. We first show that vertical SiNWs are readily synthesized with excellent alignment under mild conditions. Enabled by the synthetic controls, we next systematically vary different parameters and study their influence on the photovoltaic processes. Our results reveal that NWs with diameters close to the space charge region width perform better than larger ones. These results are expected to pave the way for NW based devices that harvest solar energy with unprecedented cost-efficiency combinations.
9:00 PM - R11.47
Influence of the Nature of Nanometric TiO2 Particles on Photovoltaic Devices.
Sophie Cassaignon 1 2 , Magali Koelsch 1 2 , Jean-Pierre Jolivet 1 2
1 Lab. Chimie de la Matiere Condensee de Paris, UPMC, PARIS Cedex 05 France, 2 Lab Chimie de la Matiere Condensee de Paris, CNRS, UMR 7574, PARIS France
Show AbstractTitanium oxide TiO2 has found extensive use in a great variety of applications among which electrode materials for dye-sensitized solar cells. The polymorphs of TiO2, rutile, anatase and brookite exhibit specific physical properties, band gap, surface states... For many applications the size of particles is an important parameter because it determines the surface to volume ratio, which greatly influences many properties. TiO2 anatase is the most used phase for photovoltaic applications and brookite and small particles of rutile seem potentially interesting.Nanometric particles of the three polymorphs, were synthesized by thermohydrolysis of TiCl4 or TiCl3 in aqueous medium. The control of the precipitation conditions (acidity, nature of anions, ionic strength, titanium concentration…) allows the control of crystalline structure, size and morphology of particles. Spheroidal anatase with nanometric size was synthesized in the range 4 to 10 nm, rod like particle were also obtained. Nanoparticles of brookite with different size and morphology were also synthesized. At last, rutile with various shapes (needle, rod or spherical) was obtained.In order to have a better knowledge of the mechanisms involved in Dye Sensitized Solar Cells (DSSC), electrochemical studies were conducted on these various materials in aqueous medium. The influence of the nature of particles on the bandgap value was firstly studied. Spectroelectrochemical measurements revealed the formation of Ti(III) on the surface of TiO2. Finally, TiO2 phototensions developed in water and acetonitrile were also studied to understand the influence of parameters such as the crystalline structure, size and morphology of the nanoparticles.
9:00 PM - R11.48
Electrochemical Synthesis of Branched Titania Nanotube Thin Films.
G. Butail 1 2 , P. Ganesan 2 , R. Mahima 2 , N. Ravishankar 1 2 , G. Ramanath 1 2
1 Department of Material Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Center for Future Enregy Systems, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractTitania nanotubes are attractive for use in many emerging applications related to energy generation, storage and delivery, gas sensing, and water purification. Here, we report the first-time synthesis of branched architectures of titania nanotube array thin films by manipulating the voltage during potentiostatic anodization of Ti films or foils. While branching in alumina nanotubes has been studied extensively, controlled branching in titania nanotubes is new. The realization of branched titania films provides a means to tune the electronic properties of titania and fill with multi-sized quantum dots and branched metallic/semiconducting nanostructures for applications. In particular, we quantitatively demonstrate the correlation between the potentiostatic anodization voltage and nanotube diameter, and exploit the relationship to create branches or merge tubes with diameters in the 30-110 nm range, by downscaling or increasing the voltage, respectively, in both stand-alone Ti foils and Ti films on substrates. Although the qualitative features of titania tube branching are similar to that in alumina, the empirical relationships are completely different—alumina branching is described by a linear relationship between voltage and diameter, while titania branching is described by a parabolic relationship. Besides discussing the origin and implications of this mechanistic difference, we will discuss the roles of the anodization chemistry and the electrolyte. We will then present the optical properties of branched titania nanotubes and compare light-harvesting efficiency of CdS-sensitized solar cells fabricated with un branched and branched titania nanotubes. We will finally illustrate the formation of graded bandgap structures using branching, which opens up possibility for realizing rainbow solar cells to maximally harness sunlight for energy conversion.
9:00 PM - R11.5
Electrospinning Approach to Build Charge Transport Pathway of Organic Photovoltaic Cell.
Taehoon Kim 1 , Sang Won Kim 1 , Seung Jae Yang 1 , Chong Rae Park 1
1 Department of Material Science and Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractExciton dissociation and charge transport are major factors that determine the power conversion efficiency of organic photovoltaic (OPV) cells. Although bulk heterojucntion (BHJ) structure of active layer enables to accomplish almost 100% exciton dissociation, the random morphology can prohibit the successful charge transportation. Thus the effective formation of charge transport pathway is an important issue to achieve higher power conversion efficiency.Electrospinning is a useful method to produce polymer or metal oxide nanofibers. The length of electro-spun nanofibers is up to few centimeters, and the aspect ratio of them is much higher than that of nanowires fabricated by other methods such as vapor-liquid-solid (VLS) or self-assembly. This high aspect ratio guarantees the charge transport pathway to be formed effectively to each electrode. Furthermore, the high surface area of electro-spun fibers provides an essential characteristic of OPV active materials to the efficient exciton dissociation. In this work, we fabricated OPV cells based on the electrospinning method by using poly(3-hexylthiophene) (P3HT) as an electron donor and TiO2 as an electron acceptor. And their performances are compared to that of OPV cells fabricated based on the spin coating method.
9:00 PM - R11.50
Broadband Photovoltaic Effects in Axially Composition-graded Si1-xGex Nanowires.
Cheol-Joo Kim 1 , Hyun-Seung Lee 1 , Won-Mo Lee 1 , Jee-Eun Yang 1 , Moon-Ho Jo 1 2
1 Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyungbuk, Korea (the Republic of), 2 Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang, Gyungbuk, Korea (the Republic of)
Show AbstractNanowire (NW) based solar cells have attracted much interests due to the low temperature processing and enhanced charge collection efficiency. Single-crystalline Si1-xGex(0≤x≤1) nanowires can serve as an attractive model system for the broadband NW solar cells because of their continuously varying energy-band gaps from visible to near infrared (NIR) at the nanometer scale. Here, we report the investigations on photovoltaic properties of axially composition-graded Si1-xGex NWs (Si:Ge NWs), where their relative composition vary within individual NWs. Axially graded heteroepitaxy of Si:Ge NWs were achieved by the kinetic controls of the Au-catalytic decomposition of SiH4 and GeH4 precursors during chemical vapor syntheses. We have then conducted spatially resolved photocurrent measurements on the Si:Ge NW field effect transistors under the 532 nm and 1313 nm laser sources. We found a polarization sensitive light response, which is graded over the axial composition profile, in a dissimilar fashion in NIR to visible ranges. The observed photoconduction and photovoltaic mechanism is discussed around the roles of the surface trap states and the energy band-gap offset, which continuously vary along their local compositions.
9:00 PM - R11.52
Photovoltaic Performance and Stability of Flexible Large Area Roll-to-Roll Processed Polymer Solar Modules: Lessons with Real Operational Conditions Under the Natural Sunlight.
Eugene Katz 1 , Suren Gevorgyan 2 , Fredrik Krebs 2
1 Dept. of Solar Energy & Env. Physcics, J. Blaustein Inst. for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion Israel, 2 Riso National Laboratory for Sustainable Energy, Technical University of Denmark, Roskilde Denmark
Show AbstractIn spite of high potential of polymer photovoltaic (PV) cells considerable improvement of their stability under operational conditions needs to be achieved. This prompt the need for experimental tests of long-term evaluation of PV performance of polymer cells and modules under real out-door operational conditions. We report preliminary results of such experiments, with flexible Roll-to-Roll (RTR) proceed solar modules, at Sede Boqer (the desert Negev, Israel) using the documented closeness of the noontime clear sky spectrum at that site to the standard AM1.5G spectrum. The RTR modules with 8 serially connected ‘PET-ITO-ZnO-P3HT:PCBM-PEDOT:PSS-Ag paste’ cells (with active area of 120 sq.cm) were encapsulated with commercially available barrier material from Alcan Packaging. The barrier foil is based on PET and has a UV-filter incorporated.The long-term stability of the tested modules allowed studying the variation of the PV performance across a day that reflects simultaneous effects of changes in sunlight intensity and spectrum and ambient temperature. In order to study a beneficial effect of by-facial architecture of these thin transparent modules on their PV performance we conducted experiments with illumination the modules from front and back sides.
9:00 PM - R11.53
Mapping Spatial Heterogeneities of Optical and Electronic Properties in Thin Film Photovoltaic Materials.
David Ostrowski 1 2 , Micah Glaz 1 2 , David Vanden Bout 1 2
1 Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas, United States, 2 Center for Nano and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas, United States
Show AbstractScanning microscopy is used to better understand spatial heterogeneities of optical and electronic properties in polymer and nanoparticle thin films made for photovoltaic applications. Image maps of variations in optical and electronic properties are constructed pixel by pixel through local measurements made when only a small area of the film is illuminated. In order to obtain these images, a variety of measurements are coupled with scanning microscopy techniques that include confocal and near-field scanning optical microscopy (NSOM), which locally illuminates areas of around 250 nm and 75 nm, respectively. One electrical measurement used is light beam induced current microscopy (LBIC), which measures the variations of photogenerated current in the device upon local illumination. An extension of this measurement is to sweep the bias voltage and obtain local current to voltage characteristics. Along with measuring the electrical properties, numerous optical properties may be simultaneously studied, such as transmission and total fluorescence. Through these optical measurements, an understanding of variation in local chemical composition and its effect on exciton dissociation is obtained. The goal of this work is to correlate changes in electronic and optical properties and hence gain a more complete understanding the effect that compositional and morphological variations have on device processes in thin film photovoltaics. We will present results from account on two different photovoltaic systems. The first analysis focuses on the morphological effects of polymer blend films using the model system of poly(9,9’-di-n-octylfluorene-alt-bis-N-N’-(4-butylphenyl)-bis-N,N’-phenyl-1,4-phenylaminediamine) [PFB] and poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) [F8BT]. The second study characterizes local variations in photovoltaic properties of thin films made from novel inorganic nanoparticles composed of Copper Indium Gallium Selenide, Cu(InxGa1-x)Se2 [CIGS].
9:00 PM - R11.54
Thermal Deposition of Large Organic Molecules for Photovoltaic Applications.
Peter Kovacik 1 , Andrew Watt 1 , Hazel Assender 1
1 Department of Materials, University of Oxford, Oxford United Kingdom
Show AbstractOrganic photovoltaic cells have recently attracted considerable research and commercial interest due to their mechanical flexibility and low production costs. Organic semiconductors used for thin film production fall into two main groups - low molecular weight which are usually vacuum deposited (eg phthalocyanine) and large molecular weight which are solution processed (eg polythiophene). Solution processed conjugated polymers have demonstrated enhanced properties in terms of hole-transport and in combination with fullerene electron-acceptors the highest power conversion efficiency solar cells. Although some large molecular weight polymer materials have been successfully deposited by physical vapour deposition techniques, there have not been any attempts to deposit conjugated polymers in the same way. In this paper we present the fabrication of solar cells via the vacuum evaporation of P3HT (poly-3-hexylthiophene) and C60 in bilayer heterojunction device geometry.Different P3HT:C60 thickness-ratio studies showed that the performance of the device increases with the thickness of the fullerene layer. While VOC of 200 nm bilayers remained approximately the same (~0.55 V), the magnitude of JSC increased with the acceptor (C60) thickness (-0.35 mA/cm2 for 1:0.3 vs. -0.69 mA/cm2 for 1:3). Thermal heating of polymers to high temperatures is usually associated either with degradation or decomposition of the chains. Structural characteristics of the polymer before and after evaporation was performed using GPC, UV-vis absorption spectroscopy, NMR and FTIR. It was found from GPC that molecular weight decreased from 9500 g/mol to about 700 g/mol during the evaporation. While, UV-Vis spectroscopy measurements showed a blue-shift of the absorption peaks with decreasing molecular weight. FTIR and NMR spectra of the materials' chemical structure were very similar and we conclude that that the polymer is subject to molecular weight rather than a chemical change. Overall this new processing approach opens up a fabrication method with considerable potential to enhance the efficiency of large scale organic electronic device production. Further work will investigate the deposition of bulk heterojunctions with adjustable vertical composition and multilayer structures.
9:00 PM - R11.6
Nanostructured ZnO-Cu2O Heterojunction Solar Cells.
Kevin Musselman 1 , Lukas Schmidt-Mende 2 , Judith MacManus-Driscoll 1
1 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge United Kingdom, 2 Department of Physics and Centre for Nanoscience, Ludwig-Maximilians University, Muenchen Germany
Show AbstractElectrochemically deposited ZnO-Cu2O solar cells are attractive due to low material costs, absence of toxic compounds, and the low temperature, scalable fabrication method. In this work, charge transport limitations preventing higher efficiencies in bilayer ZnO-Cu2O cells are demonstrated and discussed. Enhanced charge collection is demonstrated by incorporating ZnO nanowire architectures in the heterojunctions. The performance of bilayer and nanowire devices is compared, and fundamental differences arising from nanoscale effects are highlighted. The potential for further improvements in nanostructured ZnO-Cu2O devices is discussed, including the potential to incorporate dyes and thin absorbing layers.
9:00 PM - R11.7
Nanostructured Networks of Transparent Oxides as Photoanodes in Excitonic Solar Cells.
Alberto Vomiero 1 2 , Isabella Concina 1 2 , Marta Natile 3 , Camilla Baratto 1 2 , Elisabetta Comini 1 2 , Guido Faglia 1 2 , Matteo Ferroni 1 2 , Giselle Jimenez 1 2 , Iskandar Kholmanov 1 2 , Nicola Poli 1 2 , Andrea Ponzoni 1 2 , Silvia Todros 1 2 , Giorgio Sberveglieri 1 2
1 , INFM-CNR, Brescia Italy, 2 Physics & Chemistry, Brescia University, Brescia Italy, 3 Dipartimento di Scienze Chimiche and INSTM Padova, Padova University, Padova Italy
Show AbstractAn intense development of nanostructured photoanodes is ongoing to enhance photoconversion efficiency and charge collection in excitonic solar cells (either dye- or quantum dot-sensitized). Among the very recent advances, remarkable interest has been paid towards integration of single crystal nanowires of transparent conducting oxides to obtain a photoelectrochemical system in which electronic transport takes place along the single crystalline backbone of 1-dimensional nanostructures. Thanks to the high electron mobility in single crystal nanowires (about 100 times higher than in polycrystalline network) this solution eliminates the detrimental drawback of polycrystalline photoanodes in which a single electron has to pass thousands of grain boundaries before reaching the anode, with high recombination probability. In principle this benefit could result in unprecedented cell efficiency, but only limited results have been obtained for nanowire-based cells up to now.A very critical point is the limited loading of the sensitizers (both dye molecules and quantum dots) on the nanowire bundle, which affects the optical density of the active layer. Engineered networks of mixed nanocrystals and single crystalline nanowires can merge the beneficial properties of both the systems, allowing high optical density of the active layer, which results in nearly complete light absorption, while maintaining direct electron path, which minimizes recombination processes. Such systems can be profitably applied in both dye- and quantum dot-based solar cells. Two nanonetworks have been prepared via physical methods, composed of either ZnO nanowires and TiO2 nanocrystals or SnO2 nanowires and SnO2 nanocrystals. In the first system the almost similar electronic band structure of ZnO and TiO2 guarantees high compatibility of such oxides from the point of view of the electron transport, with negligible detrimental electric fields which could affect electron mobility. The nanonetworks have been applied as photoanodes in dye- and quantum dot-sensitized solar cells. Commercial dye molecules have been applied in DSC, while CdSe quantum dots stabilized by a suitable bi-linker were synthesized in a single step process, thus avoiding the ligand exchange approach commonly used for this kind of solar cells. Improved efficiency has been proven in these kinds of cells with respect to the polycrystalline traditional counterparts.The beneficial effect of the composite network will be analyzed and discussed from the viewpoint of the functional features of the cells, and compared with traditional benchmarking cells based on polycrystalline photoanodes.
9:00 PM - R11.9
Characterization of Strained Layer Superlattice Solar Cells by X-ray Diffraction and Current-Voltage Measurements.
Joshua Samberg 2 , Nadia El-Masry 2 , Conrad Carlin 1 , Geoffrey Bradshaw 1 , Peter Colter 1 , Jeffrey Harmon 1 , John Hauser 1 , Salah Bedair 1
2 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 1 Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractInGaAs can be used to enhance the response of solar cells past the 1.43 eV cutoff of GaAs. InGaAs has a larger lattice constant than GaAs, causing a compressive strain. To counteract this compressive strain GaAsP, which has a smaller lattice constant than GaAs, is used to create a strain balanced superlattice matched to the lattice constant of GaAs. This strained layer superlattice is inserted as the intrinsic layer of a p-i-n GaAs solar cell (Bedair et. al., Solar Cells 12, 413 (1987)). SLS solar cells with high indium and phosphorus compositions (up to 30% and 85% respectively) have been successfully grown. The strain balance is first calculated with the use of a zero balance in-plane strain equation. Following growth, x-ray diffraction is used to monitor the interfaces within the SLS as well as measure the strain balance, in order to tune the growth conditions of the InGaAs and GaAsP layers. XRD 2θ-ω curves show that the SLS interfaces are quite abrupt, as indicated by sharp peaks. It has been observed from XRD that the In composition of GaInAs within the SLS could reach as high as 35%. The spectral response has been extended to as low as 1.27 eV. This enhancement is also shown by an increase in short circuit current (up to 16%) with a small reduction in short circuit voltage (5%) compared to a standard GaAs p-n junction for AM1.5 and one sun. We will report on the effect of growth conditions of the SLS on the value of dark current through temperature dependence. Dark current curves show the extent of recombination in the superlattice. The devices were split into two groups, one with high levels of recombination current and others with low recombination current. The reverse saturation current in the recombination region (0.2-0.8 V) was found using a non linear least squares fitting routine. Arrhenius plots were generated by finding the reverse saturation current over a temperature range of 300-370 K. The low recombination devices show non-ideality constants of 1.7 with activation energies of 1.3-1.4 eV. High recombination devices have higher non-ideality constants (~2.3) and lower activation energies (1.1 eV). High recombination was found in devices with 40 period superlattices and low recombination was found in devices with 20 period superlattices. For the 40 period superlattices, it is likely that the transport through the undepleted portion of the intrinsic region inhibited transport through the superlattice, which increased recombination in the low band gap InGaAs wells.