Symposium Organizers
Paul Alivisatos University of California-Berkeley
Nathan S. Lewis California Institute of Technology
Arthur J. Nozik National Renewable Energy Laboratory
Michael R. Wasielewski Northwestern University
FF1: Organic Photovoltaics
Session Chairs
Tuesday PM, April 18, 2006
Room 2008 (Moscone West)
9:30 AM - **FF1.1
Solar Cells and Light-emitting Field effect Transistors: Recent Progress in Santa Barbara.
Alan Heeger 1
1 Physics, UCSB, Santa Barbara, California, United States
Show AbstractPostproduction annealing at 150oC yields polymer solar cells with power conversion efficiencies approaching 5%.[1] These devices exhibit remarkable thermal stability. The improved nanoscale morphology, the increased crystallinity of the semiconducting polymer, and the improved contact to the electron collecting electrode enhance the device efficiency by lowering the series resistance. By depositing a TiOx layer (using a sol-gel process) between the active layer and the electron collecting aluminum electrode, approximately 50% enhancement in power conversion efficiency is obtained.[2] The TiOx layer increases the efficiency by modifying the spatial distribution of the light intensity inside the device, thereby creating more photogenerated charge carriers in the bulk heterojunction layer.Ambipolar light emitting field effect transistors have been successfully fabricated; transport data show ambipolar behavior. Recombination of electrons and holes results in a narrow zone of light emission within the channel. The location of the emission zone is controlled by the gate bias.[3] [1]W. Ma, C. Yang, K. Lee, A.J. Heeger, Adv. Func. Mater. 15 1617 (2005).[2]J.Y. Kim, S.H. Kim, H.-H. Lee, K. Lee, W. Ma, X. Gong, A.J. Heeger Adv. Mater (in press)[3]James Swensen, Cesare Soci and Alan J. Heeger, Appl. Phys. Lett. (in press)
10:00 AM - **FF1.2
A Self-Consistent Model for Exciton Formation, Doping, and Carrier Transport in Excitonic Semiconductors.
Brian Gregg 1 , Si_Guang Chen 1 , Sophie Gledhill 1 , Brian Scott 1 , Pingrong Yu 1
1 , NREL, Golden, Colorado, United States
Show AbstractBased primarily on experimental studies of highly ordered liquid crystalline organic semiconductors, we propose the first self-consistent model for the three key processes in excitonic (organic) semiconductors, XSCs: exciton formation, doping, and carrier transport. Not surprisingly, our model disagrees in parts with the several existing models of the individual processes, which are themselves mutually inconsistent. N-type doping reveals that electrostatic attractions between the dopant electron and its conjugate cation cause the free carrier density to be much lower than the doping density. Such electrostatic interactions determine much of the behavior of XSCs. The activation energy of the current, EaJ, decreases with increasing field and with increasing dopant density, nd. The free electron density and the electron mobility are non-linearly coupled through their shared dependences on both field and temperature. The data are fit to an analytical, modified Poole-Frenkel-like model that is surprisingly accurate for what is actually a complex nonlinear problem. Since basic physics is independent of system, we extrapolate our model to less well-defined systems such as π-conjugated polymers. The first experimental result of this endeavor shows that the reported observation of space-charge-limited (SCL) currents in π-conjugated polymers may, in fact, be better interpreted as the observation of Poole-Frenkel (PF) currents. These two models lead to a very different interpretation of experimental results. If, as appears likely, there is a substantial density of mobile ionic charges, nion, in the amorphous π-conjugated polymer, even the PF model is an oversimplification.
10:30 AM - **FF1.3
Nanostructured Hybrid Organic-inorganic Photovoltaic Cells.
Michael McGehee 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show Abstract11:30 AM - FF1.4
Photophysical Studies of CuPc-acceptor Heterostructures and Steps Towards a Hybrid Carbon Nanotube-polymer Solar Cell.
Thomas Kempa 1 , James Durrant 1 , Tim Jones 1
1 , Imperial College London, London United Kingdom
Show AbstractWe report results elucidating the dynamics of charge recombination in copper phthalocyanine (CuPc) based organic donor-acceptor solar cells. We employ nanosecond transient absorption spectroscopy and single photon counting to probe the excited state dynamics leading to exciton formation in pristine CuPc films and both bilayer and blend films containing CuPc and either C60 (fullerene) or perylene-tetracarboxylic-dianhydride (PTCDA). For excitation of the pristine CuPc film at 532nm, we attribute a prominent transient species at 775nm to the long-lived triplet exciton of CuPc which gives rise to a charge transfer state. Similar studies on CuPc-fullerene heterostructures reveal that the fastest charge recombination occurs within the blend system. For CuPc-PTCDA heterostructures a transient species at 825nm appears and its decay characteristics are polaron-like. With detailed information as to the mechanism and dynamics of charge carrier generation in CuPc heterostructure systems, we design a hybrid carbon nanotube-CuPc solar cell.
11:45 AM - FF1.5
Structure Control in ZnO/Polymer Composite Solar Cells.
Thomas Sounart 1 , Erik Spoerke 1 , Dana Olson 2 , David Scrymgeour 1 , Nolanne Chang 1 , Terry Guilinger 1 , Sean Shaheen 2 , Julia Hsu 1 , David Ginley 2 , James Voigt 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractWith the potential for low cost manufacture and ease of deployment, hybrid polymer-inorganic solar cells (SCs) offer distinct advantages over conventional solar cell technologies. For this SC technology to become viable, however, significant improvements in cell efficiencies are necessary. Recently, researchers have begun to try to establish the relationship between nanoscale polymer/oxide structural order and SC performance. Here we describe our work on this issue for the case of SCs constructed via low-temperature, aqueous-phase growth of ZnO nanorod arrays infiltrated with P3HT polymer. Through modifications in nanorod nucleation site densities and subsequent growth conditions, array structures with different rod morphology, alignment, and spacing have been grown. Using this along with various rod surface treatments to influence polymer infiltration, we have produced a wide range ZnO/P3HT SC structures. Example relationships to be discussed include the effect of substrate roughness on ZnO nanorod array structure, and nanorod surface chemistry on the ZnO/P3HT interface. The importance of structural control will be discussed further by correlating cell properties, such as efficiency, with heterostructure morphology.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.
12:00 PM - FF1.6
Transparent Carbon Nanotube Sheets in Organic Solar Cells
Anvar Zakhidov 1 2 , Ross Ulbricht 1 , Sergey Lee 1 2 , Mei Zhang 2 , Ray Baughman 2
1 Physics, University of Texas at Dallas, Richardson, Texas, United States, 2 , NanoTech Institute, UTD, Richardson, TX 75083, Texas, United States
Show AbstractOrganic photovoltaic (OPV)solar cells present a low cost more versatile alternative to the current inorganic silicon based solar cells. In this research, carbon nanotubes have been used to replace the conventional transparent anode, indium tin oxide of OPV. Carbon nanotubes exhibit electronic, optical and mechanical properties desirable for polymeric based organic solar cells. In this study, an oriented multiwall carbon nanotube sheets, made by recently developed method [1] are used as transparent hole collecting three-dimensional anode of flexible photocells with regioregullar polyhexyl-thiophene (P3HT) as the donor material and fullerene derivative PCBM as the acceptormaterial. An open circuit voltage of 0.57V, a short circuit current of 5.53mA/cm2, a fill factor of 0.37, and an efficiency of 1.16% at AM 1.5 and 2.4% at 10 mW/cm2 has been obtained. Performance dependence on incident light intensity and spectral studies along with other various investigations are presented. The physics of charge generation and separation within a three-dimensional network of transparent carbon nanotube charge collector is discussed in terms of optimal charge collection length, enhanced light absorption, local field enhancement in vicinity of nanotubes and photoinduced charge transfer between nanotubes and polymer. The use of transparent nanotube sheets as charge recombination layers in flexible multijunction tandem solar cells is also demonstrated and the prospects of increasing the efficiency of OPVs with transparent nanotubes is discussed. [1] M. Zhang, S.Fang, A.Zakhidov, S.B.Lee, A.Aliev, K.Attkinson and Ray Baughman, Science, 309,(2005) 1215
12:15 PM - FF1.7
Polymer Photovoltaic Optimizations at Molecular Structure and Orbital Levels
Sam-Shajing Sun 1 , Cheng Zhang 1 , Soobum Choi 1 , James Haliburton 1 , Taina Cleveland 1 , Abram Ledbetter 1 , Carl Bonner 1
1 Center for Materials Research, Norfolk State University, Norfolk, Virginia, United States
Show AbstractFuture polymeric or ‘plastic’ photovoltaic materials and devices are attractive for solar energy conversion applications where large area, low cost, lightweight, and flexible shape are desired. However, the photoelectric power conversion efficiencies of currently reported organic/polymeric photovoltaic materials are still relatively low (typically less then 5%) compared to inorganic crystalline photovoltaic materials (typically over 15%). This low efficiency photoelectric conversion in organics can be attributed mainly to the three major losses including the “photon loss”, the “exciton loss” and “carrier loss” due to improper materials energy levels and poor spatial morphologies that are correlated to molecular structures and orbital levels. In this presentation, key development and types of polymeric photovoltaic materials/devices will be briefly reviewed first, then the optimization approaches of polymeric solar cells in both space and energy regimes will be described at molecular levels. For instance, in spatial regime, a ‘tertiary’ block copolymer supra-molecular nano structure has been designed, and a series of –DBAB- type of block copolymer, where D is a conjugated donor block, A is a conjugated acceptor block, and B is a non-conjugated and flexible bridge unit, have been synthesized, characterized, and preliminarily examined for photovoltaic applications. In comparison to simple donor/acceptor (D/A) blend system with same energy levels, -DBAB- block copolymer exhibited much better photoluminescence (PL) quenching, short circuit photo conductivity and open circuit voltage. These are mainly attributed to improvement in spatial domain for charge carrier generation and transportation. At energy and electron transfer dynamic regime, preliminary theoretical analysis revealed that, the photo induced charge separation as well as charge recombination appears most efficient only at a certain energy offsets. Implications of these findings and ways of optimizing energy levels/offsets are also discussed in order to develop inexpensive, high efficiency, flexible, and lightweight ‘plastic’ solar cells or photo detectors.
12:30 PM - FF1.8
Characterization of Materials and Multilayer Structures of Organic Solar Cells by Spectroscopic Ellipsometry.
Christophe Defranoux 1 , Jean-Jacques Simon 2 , Florent Monestier 2 , Philippe Torchio 2 , Ludovic Escoubas 2 , Jean Michel Nunzi 3 , Laurent Kitzinger 1
1 Application, SOPRA, BOIS COLOMBES France, 2 C.O.M., Institut Fresnel, MARSEILLE France, 3 Laboratoire POMA, Universite d'Angers, Angers France
Show AbstractSpectroscopic Ellipsometry (S.E) is a well adapted optical technique widely used for the characterisation of all types of thin films for thickness and optical indices on glass or plastic substrates. Flat Panel display manufacturers and research institutes are routinely using this method to monitor and control the production line by taking advantage of the high throughput, high accuracy, and the limited measurement probe size of this technique. Recently, S.E. is also being applied to the characterization of materials and multilayer structures of organic materials like organic light-emitting diodes (OLEDs) or Organic Solar Cells; all parameters of each layer being given in one single measurement. We present the determination of the refractive indices of Organic materials used in these Solar Cells like P3HT, PCBM, Pentacene, Perylene, and their blends. Complex organic materials can be analyzed accurately from their absorption bands in the visible and UV range. Transmission and absorption can be also simulated to be compared with spectrophotometer results. Using these refractive indices, measurements of real multi-layer stacks can be done easily. The refractive indices can be used afterwards to automatically optimise and balance the energy flow dissipation Q inside an organic solar cell composed of a thin film stack. We present example performed on a single cell and tandem bi-layer cell structure.Since these materials are sensitive to moisture and pollution, it can be necessary to measure the materials optical properties and thickness values through an encapsulated media. Only S.E. is able to measure these parameters when the layer is encapsulated. We will demonstrate how we can measure single layer, and multi-layer stacks, through encapsulated samples from the back side of the substrate. This technique can be applied to real Organic Solar Cell monitoring and control in a production line. As demonstrated by our results, “Backside” and “through the cap” measurements can also be applied to determine the ITO resistance, without contact, by using Near Infra-Red ellipsometry. We will study the effect of the change of the refractive index versus the resistance of the ITO layer. Correlation measurements with 4 Point Probe will also be presented.
FF2: Natural and Artificial Photosynthesis
Session Chairs
Tuesday PM, April 18, 2006
Room 2008 (Moscone West)
2:30 PM - **FF2.1
Design Principles and Regulation of Natural Photosynthetic Light Harvesting.
Graham Fleming 2 1 , Krishna Niyogi 3
2 Directorate, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 1 Chemistry, University of California, Berkeley, Berkeley, California, United States, 3 Plant and Microbial Biology, University of California, Berkeley, Berkeley, California, United States
Show AbstractNatural photosynthetic light harvesting is both astoundingly efficient and highly regulated. Recent developments in two-dimensional optical spectroscopy reveal the interactions and spatial/energetic pathways of energy flow in light harvesting complexes. The molecular mechanisms of the regulation of light harvesting have been very difficult to elucidate, and recent progress using ultrafast spectroscopy, genetics and electronic structure calculations will be described.
3:00 PM - **FF2.2
Oxygen Evolving Complex of Photosystem II
Gary Brudvig 1
1 Department of Chemistry, Yale University, New Haven, Connecticut, United States
Show AbstractBased on a consideration of biophysical studies and inorganic chemistry, we proposed that the O-O bond-forming step in photosystem II (PSII) involves nucleophilic attack by a calcium bound water molecule on an electron deficient Mn(V)=O species (1-2). In order to test this mechanism, we have determined the affinity of a series of metal ions for the Ca(II) binding site (3). Comparing the physical characteristics of the metal ions studied, we identify the pKa of the aqua-ion as the factor that determines functional competence of the metal ion. These results are consistent with a role of Ca(II) as a Lewis acid, binding a substrate water molecule and tuning its reactivity. In order to model this chemistry, we have synthesized and structurally characterized the complex [(H2O)(terpy)Mn(O)2Mn(terpy)(OH2)](NO3)3, terpy = 2,2':6,2''-terpyridine. This complex catalyzes the conversion of oxone or hypochlorite to O2 (4-5). 18O isotope labeling has shown that water is the source of the O-atoms in the O2 evolved. Kinetics studies are consistent with a mechanism involving a Mn(V)=O intermediate. The recently published 3.5 Å resolution X-ray crystal structure of a cyanobacterial PSII proposes a detailed architecture of the oxygen-evolving complex (OEC) and the surrounding amino-acids (6). The revealed geometry of the OEC lends weight to certain hypothesized mechanisms for water-splitting, including the one propounded by our group. The water-splitting mechanism has been reexamined in the light of the new crystallographic information (7). This enables detailed suggestions concerning the mechanistic functions (particularly the redox and proton-transfer roles) of calcium, chloride and certain amino acids in and around the OEC. Supported by the National Institutes of Health (GM32715).1. V. A. Szalai et al., J. Chem. Soc., Dalton Trans. (1999) 1353-1363.2. J. S. Vrettos et al., Biochim. Biophys. Acta (2001) 1503, 229-245.3. J. S. Vrettos et al., Biochemistry (2001) 40, 7937-7945.4. J. Limburg et al., Science (1999) 283, 1524-1527.5. J. Limburg et al., J. Am. Chem. Soc. (2001) 123, 423-430.6. K. N. Ferreira et al., Science (2004) 303, 1831-1838.7. J. P. McEvoy and G. W. Brudvig, Phys. Chem. Chem. Phys. (2004) 6, 4754-4763.
3:30 PM - **FF2.3
Protein Design for Stabilization of Electron and Proton Transfer Reactions.
Marilyn Gunner 1
1 Physics, City College of New York, New York, New York, United States
Show Abstract4:30 PM - **FF2.4
Chemical Approaches to Artificial Photosynthesis
Thomas Meyer 1
1 Chemistry, University of North Carolina, Chapel Hill, North Carolina, United States
Show AbstractIn artificial photosynthesis the goal is to use solar energy to make high energy chemicals for energy production. In one approach photochemical electron transfer is used to create oxidative and reductive equivalents to drive reactions such as water splitting into O2 and H2 in integrated mocular assemblies or on separate electrodes in photelectrochemical cells.
5:00 PM - **FF2.5
Concatenation of Antenna Function and Photoinduced Electron Transfer in Artificial Photosynthetic Molecules.
Devens Gust 1 , Thomas Moore 1 , Ana Moore 1
1 Chemistry and Biochemistry, Arizona State University, Tempe, Arizona, United States
Show AbstractA variety of structural motifs are found in natural photosynthetic antenna systems. In several organisms, antennas include rings of chlorophyll and carotenoid molecules. Light absorbed by these pigments generates excited states that can move among the ring chromophores by energy transfer processes, and ultimately move out of the ring to another antenna or to the reaction center. With such systems as inspiration, we are constructing model antenna systems based on hexaphenylbenzene as an organizing framework.To begin to investigate this approach, we synthesized a model triad that features two porphyrins, a free base (P2H) and a zinc (PZn) porphyrin, linked to the hexaphenylbenzene core. The free base porphyrin, with its lower-lying excited singlet state, acts as an excitation energy trap. In addition, it bears a fullerene (C60) as an electron acceptor. Spectroscopic studies in 2-methyltetrahydrofuran show that excitation of the zinc porphyrin antenna moiety to form 1PZn-P2H-C60 is followed by singlet-singlet energy transfer to the free base (τ = 59 ps), yielding PZn-1P2H-C60. The free base porphyrin first excited singlet state decays by photoinduced electron transfer to the fullerene (τ = 25 ps), producing a PZn-P2H●+-C60●- charge-separated state. Charge shift (τ = 167 ps) yields PZn●+-P2H-C60●-. This final charge-separated state is formed with quantum yields >90% following excitation of any of the three chromophores. Charge recombination in 2-methyltetrahydrofuran (τ = 50 ns) occurs by an apparently endergonic process to give triplet states of the chromophores, rather than the ground state. In benzonitrile, charge recombination yields the ground state (τ = 220 ns).Building on what was learned from the triad, photosynthetic antenna-reaction center complexes comprising five bis(phenylethynyl)anthracene antenna moieties and a porphyrin-fullerene dyad organized by a central hexaphenylbenzene core have been prepared and studied spectroscopically. The molecules successfully integrate singlet-singlet energy transfer and photoinduced electron transfer. Energy transfer from the five antennas to the porphyrin occurs on the ps time scale with a quantum yield of 1.0. Comparisons with model compounds and theory suggest that the Förster mechanism plays a major role in the extremely rapid energy transfer, which occurs at rates comparable to those seen in some photosynthetic antenna systems. The porphyrin first excited singlet state donates an electron to the attached fullerene to yield a P●+-C60●- charge-separated state, which has a lifetime of several ns. The quantum yield of charge separation based on light absorbed by the antenna chromophores is 80% for the free base molecule and 96% for the zinc analog.
5:30 PM - *FF2.6
Electron Transfer in Bio-inspired Supramolecular Donor-Acceptor Arrays.
Michael Wasielewski 1
1 Dept. of Chemistry, Northwestern University, Evanston, Illinois, United States
Show Abstract
Symposium Organizers
Paul Alivisatos University of California-Berkeley
Nathan S. Lewis California Institute of Technology
Arthur J. Nozik National Renewable Energy Laboratory
Michael R. Wasielewski Northwestern University
FF3: Nanostructures and Quantum Dots for Solar Cells I
Session Chairs
Wednesday AM, April 19, 2006
Room 2008 (Moscone West)
9:30 AM - **FF3.1
Coherent Superposition of Multi-Exciton Complexes in Semiconductor Nanocrystals.
Alexander Efros 1 , Andrew Shabaev 1
1 Center for Computational Material Science, Naval Reserach Laboratory, Washington DC, District of Columbia, United States
Show Abstract10:00 AM - FF3.2
Highly-Efficient Multiple Exciton Generation in Semiconductor Quantum Dots, and Implications for Solar Energy Conversion
Randy Ellingson 1 , Matthew Beard 1 , Justin Johnson 1 , James Murphy 1 2 , Pingrong Yu 1 , Olga Micic 1 , Andrew Shabaev 3 4 , Alexander Efros 3 , Arthur Nozik 1 2
1 Center for Basic Sciences, National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, United States, 3 , Naval Research Laboratory, Washington, District of Columbia, United States, 4 School of Computational Sciences, George Mason University, Fairfax, Virginia, United States
Show AbstractUnabated use of fossil fuels currently continues to drive our atmospheric concentration of CO2 toward unacceptable levels. Development of tremendous quantities of carbon-free energy can circumvent the potentially disastrous problems associated with anthropogenic climate change. Sunlight represents a carbon-free renewable source of fuels and electricity capable of far exceeding our power demands. The Sun’s radiative power at Earth’s surface provides a quantity of energy in one hour nearly equal to the total energy consumed on Earth in one year.To generate economically competitive solar-based electricity, photovoltaic cells need to be both inexpensive and significantly more efficient than the ~10-13% sunlight-to-electricity conversion efficiency achieved by traditional silicon-based cells. We have demonstrated that in semiconductor quantum dots, absorption of a single photon can produce three or more electron-hole pairs through a process of multiple exciton generation. Our studies show the production of an average of three excitons per absorbed photon for 5.4 nm diameter PbSe QDs at an incident photon energy of 4 times the effective bandgap. We observe similarly high efficiencies of multiple exciton generation in quantum dots of PbS and PbTe.Our results demonstrate the intrinsic possibility of largely bypassing the normal heat-generating electron-phonon scattering process, and provide a pathway toward substantially improved solar cell efficiency. In addition, highly efficient multiple exciton generation provides new motivation for understanding and controlling interfacial charge transfer in systems consisting of combinations of nanomaterials and/or molecular species.
10:15 AM - FF3.3
Coherent Dynamics of Multiple Exciton Generation in Colloidal PbSe Quantum Dots.
Justin Johnson 1 , Matt Beard 1 , James Murphy 1 , Randy Ellingson 1 , Arthur Nozik 1
1 , National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractThe recent demonstration of highly-efficient multiple exciton generation (MEG) in colloidal lead-salt quantum dots has led to the possibility of greatly enhanced efficiencies for third-generation solar cells. The development of a coherent superposition model for MEG has yielded several predictions about the process, including the energetic threshold for MEG to occur and how it is capable of competing with Auger and phonon-assisted relaxation. The dynamics at early times after photoexcitation appear crucial for MEG to occur with high efficiency; however, experimental studies that are capable of yielding detailed and definitive information about the process are sparse. Various forms of nonlinear coherent spectroscopy, including transient grating spectroscopy, are utilized for testing the coherent model’s predictions and for measuring the ultrafast exciton dynamics in colloidal PbSe quantum dots that exhibit MEG. Connections are made between the data and the practical application of MEG in real devices.
10:30 AM - FF3.4
Colloidal Lead Salt Nanocrystals For Photovoltaics: Synthesis And Characterization.
James Murphy 1 3 , Matthew Beard 1 , Andrew Norman 1 , S. Ahrenkiel 1 , Justin Johnson 1 , Pingrong Yu 1 , Olga Micic 1 , Randy Ellingson 1 , Arthur Nozik 1 3
1 Center For Basic Science, National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, Colorado, United States
Show AbstractTwo of the major processes limiting efficiencies in conventional photovoltaics include hot carrier cooling to the semiconductor band edges and not absorbing photons of energy lower than the band gap for conversion to electricity. Lead salt colloidal semiconductor nanocrystals have the potential to convert more of the highest and lowest energy portions of the solar spectrum that reach the Earth’s surface through the process of multiple exciton generation and by tuning the band gap of the material, respectively. An alternative method of synthesizing colloidal spherical and cubic PbTe and PbSe nanocrystals will be presented. We have synthesized spherical PbSe and PbTe NCs having small size distributions, ranging in diameter from about 2 - 9 nm, with first exciton transitions tuned from 1 µm to 2.4 µm. We report the first photoluminescence quantum yield measurement of PbTe NCs to be as high as 52 %. A comparison of structural and optical properties of lead salts will also be presented.
11:15 AM - **FF3.5
Quantum Efficiency of 700% in Conversion of Light Quanta into Charge Carriers Using Semiconductor Nanocrystals.
Victor Klimov 1
1 Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractThe efficiency with which photons are converted into charge carriers determines the ultimate efficiencies of various photo-induced physical and chemical processes including photo-generation of electricity and production of solar fuels. Normally, it is assumed that the absorption of a single photon results in a single electron-hole pair (an exciton), meaning that the quantum efficiency (QE) in generating charge carries is 100%. A potential approach to surpassing this limit is through carrier multiplication (CM), in which absorption of a single photon produces multiple excitons [1]. Recently, we discovered that CM is extremely efficient in ultrasmall semiconductor nanocrystals (NCs) [2]. By utilizing a significant difference in relaxation behaviors of single excitons and multiexcitons in NCs [3], we demonstrated that two and even three excitons could be produced in a PbSe NC per single absorbed photon resulting in QEs of up to 220%. These results were confirmed in Ref. 4, where QE up to 300% was observed using the same “dynamical” detection method. These first experimental observations raise the questions regarding the mechanism for high-efficiency CM in NCs, the generality of this phenomenon and the limits of exciton multiplicity that can be obtained via CM. To address these issues, here we perform a comparative study of CM in NCs of PbSe and CdSe. Despite significant differences in electronic structures and carrier relaxation behaviors, both compositions show comparable CM QEs, which is indicative of the generality of this phenomenon to quantum-confined, semiconductor NCs. We observe that CdSe NCs show a lower CM activation threshold than PbSe NCs (2.5 vs. 2.9 energy gaps), which can be explained using simple carrier effective-mass arguments. We also detect a monotonic increase in QE with increasing excess energy above the CM threshold up to ~700% in PbSe NCs and ~165% in CdSe NCs. Interestingly, the QE of 700%, which is measured for photon energy of 7.8 energy gaps corresponds to the ultimate limit of exciton multiplicity allowed by energy conservation. Finally, we observe that in both types of NCs, the populations of the lowest quantized states in the regime of CM develop on extremely short sub-200 femtosecond time scales, which strongly suggests that multiexcitons are generated instantaneously by a single absorbed photon. To explain this observation we propose a new model that describes CM in terms of direct generation of multiexcitons via virtual single-exciton states [5]. This process relies upon confinement-induced enhancement of Coulomb interactions in NCs and large spectral densities of high-energy single-exciton and multiexciton states. 1. A. J. Nozik, Physica E 14, 115 (2002).2. R. D. Schaller and V. I. Klimov, Phys. Rev. Lett. 92, 186601 (2004).3. V. I. Klimov et al., Science 287, 1011 (2000).4. R. Ellingson et al., Nano Lett. 5, 865 (2005).5. R. D. Schaller, V. M. Agranovich, and V. I. Klimov, Nature Phys. (December, 2005).
11:45 AM - FF3.6
Mechanism for High-Efficiency Carrier Multiplication in Semiconductor Nanocrystals: Direct Photo-generation of Multiexcitons via Virtual Single-Exciton States
Richard Schaller 1 , Vladimir Agranovich 2 3 , Milan Sykora 1 , Jeffrey Pietryga 1 , Victor Klimov 1
1 Chemistry Division, Los Alamos National Lab, Los Alamos, New Mexico, United States, 2 Institute of Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow Russian Federation, 3 NanoTech Institute, University of Texas at Dallas, Richardson, Texas, United States
Show AbstractRecently, we demonstrated that absorption of a single photon by a nanocrystal quantum dot can produce two or more excitons with up to 100% efficiency in a wavelength range that is relevant to solar energy conversion [Phys. Rev. Lett. 92, 186601 (2004)]. This effect, referred to in the most general terms as carrier multiplication, can significantly improve the performance of solar cells above the Shockley-Queisser Limit on power conversion efficiency [J. Appl. Phys. 32, 510 (1961)] via an enhanced photocurrent and can also lead to improved photoelectrochemical efficiency. To understand the mechanism of this phenomenon, we have performed detailed transient absorption studies (~50 fs pulsewidth) of ultrafast buildup dynamics of multiexciton populations in two quantum dot materials that are very distinct electronically: CdSe, a II-VI semiconductor, and PbSe, a IV-VI semiconductor. We observe that the generation of multiexcitons occurs on extremely short, sub-picosecond time scales in both materials, and appears to be an instantaneous event. To explain this result, we propose a new mechanism in which direct (instantaneous) photo-generation of multiexcitons occurs via virtual single-exciton states [Nat. Phys. in press]. This process relies on confinement-enhanced Coulomb coupling between single excitons and multiexcitons and also takes advantage of a large spectral density of high-energy single- and multiexciton resonances in nanosized semiconductor crystals. This effect is particularly attractive for Generation III photovoltaic devices because it can function in low-cost, single gap photovoltaic device structures that can be fabricated via simple wet-chemistry techniques.
12:00 PM - FF3.7
Photovoltaic Response of Solar Cells Based on Lead Selenide Quantum Dots and Conducting Polymers: Toward Carrier Multiplication Enhancement
Xiaomei Jiang 1 , Sergey Lee 1 , Anvar Zakhidov 1 , Richard Schaller 2 , Jeffrey Pietryga 2 , Victor Klimov 2
1 Nanotech Institute, University of Texas at Dallas, Richardson, Texas, United States, 2 Chemistry Division, Los Alamos National Lab, Los Alamos, New Mexico, United States
Show AbstractPhotovoltaic devices based on nanocrystal quantum dot (QD) – conjugated polymer composites have been actively studied during the past several years. Of particular interest are new types of infrared (IR) QD – conjugated polymer photocells because they provide the possibility of extending spectral sensitivity into the IR. However, so far these devices have primarily been studied in the photodiode regime. In this paper, we report novel types of solar cells based on nanocomposites of conjugated polymers (polythiophene and polyphenylenevinylene derivatives) and IR-sensitive PbSe QDs that have a size-tunable energy gap between ca. 0.3 and 1 eV. Thin film cells show very good diode characteristics and sizable photovoltaic response with an open circuit voltage of ~0.3-0.4 V and short-circuit current density of ~0.2mA/cm2. These values represent a significant improvement compared to those for previously reported devices based on PbS QDs. Photocurrents under reverse bias are up to ~ 1 mA/cm2, which indicates that the polythiophene/PbSe QD structures can also be used as effective infrared photodetectors. Analysis of spectrally resolved photocurrent measurements reveals some evidence of carrier multiplication-enhanced photocurrent in our devices. Specifically, we observe indications of a rapid increase in the photocurrent at spectral energies that correlate with ~3x the size-dependent energy gap of the QD component. The onset of this increase correlates with what is expected for the onset of carrier multiplication in PbSe QDs as observed in other studies [Phys. Rev. Lett. 92, 186601 (2004)]. The good performance of our devices in both photovoltaic and photodiode regimes indicates quite efficient charge separation between the polymer and QD components. To elucidate the mechanism for charge separation in these composite structures, we analyze conduction and valence-band energy offsets derived from cyclic voltammetry measurements. Furthermore, we study photoluminescence quenching of a polyphenylenevinylene derivative by PbSe QDs to gain a better understanding of energy and charge transfer dynamics in these systems.
12:15 PM - **FF3.8
Modeling of Nanomaterials Using Hybrid Density Functional Theory.
Richard Friesner 1 , Eric Knoll 1
1 Chemistry, Columbia University, New York, New York, United States
Show AbstractWe have used hybrid density functional theory to model a number of different nanostructured materials, including silicon nanoparticles, carbon nanotubes, and titanium dioxide clusters, that are potentially relevant to solar energy conversion. An overview of this work will be given. While DFT methods have had great success in many applications to materials, there are still serious deficiencies in all currently used DFT functionals. We will describe a new approach to improving the results of DFT calculations based on localized orbital corrections, which yields dramatic improvements in atomization energies, ionization potetentials, electron affinities, and other properties. For example, an average error of 0.8 kcal/mole is obtained for atomization energies of the 222 molecules in the G3 test set, a qualitative improvement as compared to the average error for the B3LYP hybrid functional, and in fact superior to the 1.0 kcal/mole average error obtained with G3 theory. Further development of the methodology will be required in order to treat key issues in the modeling of nanomaterials; prospects for such advances will be discussed.
12:45 PM - FF3.9
Ultrafast Spectroscopy of Single Wall Carbon Nanotubes Substitutionally Doped and Coupled to Molecules and Quantum Dots.
Kelly Knutsen 1 , Jeff Blackburn 1 , Chaiwat Engtrakul 1 , Matthew Beard 1 , Timothy McDonald 1 , Marcus Jones 1 , Garry Rumbles 1 , Randy Ellingson 1 , Michael Heben 1
1 , National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractNanoscale materials are of great interest for their unique optoelectronic properties. Tailoring charge and energy transfer on the nanoscale may permit new approaches to converting solar energy into electricity and fuels. Recently our group measured the dynamics of charge carriers within single walled carbon nanotubes (SWNTs) by performing femtosecond transient absorption (TA) measurements on polydisperse aqueous solutions of semiconducting and metallic HiPCO SWNTs. It was shown that the photoresponse of metallic tubes was mostly invariant to the excitation pump frequencies, whereas the semiconducting tubes showed a clear difference in the TA spectrum when exciting the sample above and below the first excitonic energy level (E11), indicative of the presence of a true bandgap in those materials. We have extended these efforts to explore heteroatom-doped SWNTs and nanoscale heterosystems in which molecules or quantum dots are adsorbed onto nanotube surfaces. The dynamic behavior and the possibilities for managing charge and energy transfer on the nanoscale for solar energy conversion will be presented.We will present ultrafast transient absorption data over the spectral range of 450 to 2000 nm, to delay times as long 1.6 ns. Preliminary data reveals differences in the optoelectronic behavior of boron-doped SWNTs as compared to neat carbon SWNTs. We study the ultrafast response of metallic and semiconducting nanotubes, comparing and contrasting B-SWNTs and neat C-SWNTs with regard to the dynamics of charge carrier relaxation and recombination, as well as the spectral response in the regions of excitonic transitions. The differences are evaluated by considering the effect of B-doping on the pi electron distribution, the SWNT Fermi level, and the oscillator strength of the suspended SWNTs. We also present results for SWNTs in contact with adsorbed quantum dots and aromatic hydrocarbons. In the former case we observe quantum dot photoluminescence quenching due to the presence of the nanotubes, while the latter case shows changes in the photoluminescence excitation spectrum for the contacted nanotubes. Preliminary TA data for both of these systems will be presented.
FF4: Report on DOE Office of Science/Office of Basic Energy Sciences Workshop on Basic Research Needs Solar Energy Utilization
Session Chairs
Harriet Kung
Nathan Lewis
Wednesday PM, April 19, 2006
Room 2008 (Moscone West)
2:30 PM - *FF4.1
Report on U.S. DOE Basic Energy Sciences Workshop on "Basic Research Needs for Solar Energy Utilization."
Paul Alivisatos 2 , Nathan Lewis 4 , Arthur J. Nozik 1 , Michael Wasielewski 3
2 Department of Chemistry, University of California, Berkeley, California, United States, 4 Department of Chemistry, California Institute of Technology, Pasadena, California, United States, 1 , National Renewable Energy Laboratory, Golden, CO, Colorado, United States, 3 Department of Chemistry, Northwestern University, Evanston, Illinois, United States
Show AbstractA workshop sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, was held in Washington on April 18-21, 2005; the workshop title was Basic Research Needs for Solar Energy Utilization. The purpose of the workshop was to identify the key scientific challenges and research directions that will enable efficient and economic use of the solar resource to provide a significant fraction of global primary energy by the mid 21st century. The workshop was attended by about 200 scientists from academia, national laboratories, and industry from the U.S. and abroad. This talk will summarize the results and recommendations of this workshop.
FF5: Nanostructures and Quantum Dots for Solar Cells II
Session Chairs
Wednesday PM, April 19, 2006
Room 2008 (Moscone West)
3:15 PM - **FF5.1
Nanostructured Silicon Quantum Dot Tandem and Hot-Carrier Solar Cells.
Martin Green 1
1 , UNSW, Sydney, Queensland, Australia
Show AbstractSilicon has substantial advantages over other photvoltaic materials in that it is abundant, non-toxic and, in crystalline phases, has demonstrated exceptional stability and durability in the field. It is therefore an ideal material for both present and future generations of solar cells.The above attributes makes it difficult to find comparably ideal materials as partners in tandem cells. The solution we are investigating is the use of quantum-confinement to allow control of silicon's bandgap to allow all-silicon tandem devices. Similar technology is being investigated for even more challenging hot-carrier solar cells.
3:45 PM - FF5.2
Dual Nanocrystal Solar Cells.
Ilan Gur 1 2 , Neil Fromer 2 , Michael Geier 1 , A Alivisatos 2 3 1
1 Materials Science and Engineering, University of California - Berkeley, Berkeley, California, United States, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Chemistry, University of California - Berkeley, Berkeley, California, United States
Show AbstractWe introduce a class of solar cells based exclusively on colloidal semiconductor nanocrystals. They are ultra-thin, solution-processed, and stable in ambient environments. Comprised of dense nanocrystal films that mirror the basic properties of semiconducting polymers, these cells appear to function by means of a diffusion assisted donor-acceptor heterojunction. Sintering is found to enhance the performance of these devices, allowing for efficient, air-stable power conversion efficiencies approaching 3%.
4:00 PM - FF5.3
Photoconductivity in Arrays of Semiconductor Nanoparticles.
Matt Beard 1 , James Murphy 1 2 , Kelly Knutsen 1 , Randy Ellingson 1 , Arthur Nozik 1 2
1 Basic Sciences, NREL, Golden, Colorado, United States, 2 Chemistry, University of Colorado, Boulder, Colorado, United States
Show Abstract4:45 PM - FF5.4
MOCVD Growth and Characterization of InGaAs/GaAsP Strain-Balanced Quantum Dot Superlattices for Third-Generation Solar Cells.
Andrew Norman 1 , Mark Hanna 1 , Pat Dippo 1 , Scott Ward 1 , Mowafak Al-Jassim 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractThe special properties of quantum dots make them increasingly attractive for next-generation photovoltaic devices, such as the quantum dot intermediate band solar cell proposed by Luque and Martí. In this device, an intermediate band, formed by the presence of a quantum dot array, is used to enable absorption of photons below the energy gap of the barrier material. This is predicted to cause an enhancement in the photogenerated current without a reduction in open circuit voltage and lead to a significant increase in the device efficiency in comparison to a single junction solar cell of the barrier material. Growth of a close-packed array of quantum dots, such that the discrete energy levels formed by quantum confinement in individual quantum dots overlap and form mini-bands, is one of the requirements for such a device to work. Quantum dot superlattices can be grown epitaxially by molecular beam epitaxy and metal organic chemical vapor deposition (MOCVD) using the strain-induced Stranski-Krastanov growth mechanism. A quantum dot superlattice containing a large number of periods to enable sufficient absorption, and a low defect density to minimize non-radiative recombination, is needed for high efficiency cell operation. In the previously studied InGaAs/GaAs system, growth of quantum dot superlattices with a high number of periods is limited due to the introduction of a high density of misfit dislocations caused by increasing lattice mismatch between the quantum dot superlattice and the GaAs susbstrate. Such misfit dislocations lead to increased non-radiative recombination in InGaAs/GaAs quantum dot solar cells and can result in a large decrease in open circuit voltage and efficiency. In this work, we report the successful MOCVD growth of low defect density, strain-balanced InGaAs/GaAsP quantum dot superlattice solar cells on {113}B GaAs substrates, with up to 50 layers of quantum dots. These devices exhibit open circuit voltages as much as 0.4 V higher than similar InGaAs/GaAs quantum dot superlattice solar cell device structures and photoresponses extended out to a longer wavelength (about 1100 nm) than GaAs control cells.
5:00 PM - FF5.5
Criticality of Electron and Hole Escape Sequence in Nano-Structured Photovoltaic Devices.
A. Alemu 1 , J.A.H. Coaquira 1 , Alex Freundlich 1 2
1 Texas Center for Advanced Materials, University of Houston, Houston, Texas, United States, 2 Physics Department, University of Houston, Houston, Texas, United States
Show Abstract In nanostructured solar cells, confined states could act as recombination centers when carriers are not rapidly extracted. Thus an efficient escape and collection of photo-generated carriers from the well potentials is of critical importance and a necessary path toward higher efficiency devices. In p-i-n type solar cells, such a condition is satisfied when a large built-in potential exists across the i-region of the cell (which contains the nanostructure). This imposes an upper limit on the thickness of the i-region. Experimentally best efficiency trade-off is often achieved in the vicinity of this critical thickness where the Voc degradation remains minimal and a higher photocurrent is afforded by the larger number of wells. But, even for devices that satisfy this condition, occasionally a severe Voc degradation occurs. To understand these experimental observations and to identify design strategies to prevent I_V characteristics degradation, in this work we have undertaken a detailed analysis of carrier escape mechanisms. The experimental work has been carried out on a large set of InAsP/InP multiquantum well solar cells with nearly identical i-region thickness (E-field and number of wells) and absorption threshold energies. Different electron and hole band discontinuities have been achieved by carefully choosing the As composition and the thickness of the InAsP well material. The escape sequence of the first confined electron-to-conduction band and heavy/light holes-to-valence band is extracted from the photoluminescence versus temperature analysis and by comparing the measured activation energies to calculated hole/electron well depths and theoretical carrier escape times. Light holes, as expected for most III-V nanostructure systems (e.g. GaAs/InGaAs, InP/InAsP, AlGaAs/GaAs), are found to be the fastest escaping carriers in all samples.The escape of electrons prior to heavy holes is shown to be a prerequisite to prevent severe open circuit voltage degradation. The origin of this effect is tentatively ascribed to the kinematical time-dependant evolution of the band-bending associated with the charge imbalance in the well during the sequential escape of carriers. Finally, in the light of these findings, the presentation discusses a set of design rules to maximize the efficiency of the quantum-confined solar cells.
5:15 PM - FF5.6
Carrier Injection From PbSe Quantum Dots to Organic Semiconductors and Mesoporous TiO2
Joseph Luther 1 2 , Kathrine Gerth 1 3 , Jorge Piris 1 , James Murphy 1 3 , Qing Song 1 , Donald Selmarten 1 , Mark Hanna 1 , Reuben Collins 2 , Sean Shaheen 1 , Arthur Nozik 1 3
1 , NREL, Golden , Colorado, United States, 2 Applied Physics, Colorado School of Mines, Golden, Colorado, United States, 3 Chemistry, University of Colorado, Boulder, Colorado, United States
Show AbstractLow bandgap lead salt (PbS, PbSe, and PbTe) nanocrystals offer the attractive properties of tunable optical characteristics, ease of fabrication, and ability to be incorporated in many different types of structures. A novel behavior of quantum-confined lead salt nanocrystals is multiple exciton generation (MEG) after absorption of one photon. This may be a key characteristic to achieve highly efficient, inexpensive systems for converting sunlight into electricity. In this system, nanoparticles can be used to absorb light and transfer photogenerated carriers to other materials with better charge transport properties. Recently, we have published articles discussing electron/hole transport from various nanoparticle systems including InP [1,3] and CdS [2]. Here we discuss carrier transfer from PbSe quantum dots to various organic semiconducting systems as well as colloidal wide bandgap semiconducting matrices. The efficiency of charge transfer is highly dependent on the length of the organic capping ligands. Here, we present results showing that a shorter capping ligand results in more efficient charge transfer. We compare oleic acid to octylamine capped PbSe and measure carrier injection using techniques such as photocurrent spectra, microwave reflectivity, and photoluminescence spectroscopy. Analysis of the photocurrent specta is presented with regard to observing possible MEG in the PbSe film.1 Blackburn, J. L.; Selmarten, D. C.; Ellingson, R. J.; Jones, M.; Micic, O.; Nozik, A. J. (2005). Electron and Hole Transfer from Indium Phosphide Quantum Dots. Journal of Physical Chemistry B. Vol. 109(7), 2005; pp. 2625-2631.2 Blackburn, J. L.; Selmarten, D. C.; Nozik, A. J. (2003). Electron Transfer Dynamics in Quantum Dot/Titanium Dioxide Composites Formed by in Situ Chemical Bath Deposition. Journal of Physical Chemistry B. Vol. 107(51), 2003; pp. 14154-14157.3 Selmarten, D; Jones, M; Rumbles, G; et al. Quenching of semiconductor quantum dot photoluminescence by a pi-conjugated polymer. Journal of Physical Chemistry B, 109 (33): 15927-15932 AUG 25 2005.
5:30 PM - FF5.7
Nanowire Dye-sensitized and Polymer-Inorganic Solar Cells; Improving Charge Transport with a 1-D Geometry
Lori Greene 1 , Matt Law 1 , Peidong Yang 1 2
1 Department of Chemistry, UC Berkeley, Berkeley, California, United States, 2 Materials Sciences Division, Lawerence Berkeley National Laboratory, Berkeley, California, United States
Show Abstract Non-silicon based solar cells – including polymer blend, polymer-inorganic and dye-sensitized cells (DSCs) – hold great promise as inexpensive, efficient designs for large-scale solar energy conversion. The DSC is currently the most efficient of these cells and usually has an inorganic framework made with 15-30 nm-diameter nanoparticles that form a three-dimensional network. The geometry of this nanoparticle network constrains the electron transport to occur by ambipolar diffusion, a slow mechanism that limits device efficiency, especially for cells with solid-state electrolytes. By replacing the nanoparticle film with a dense, oriented array of crystalline ZnO nanowires we have shown that with a direct electrical pathway more current is collected from the device. A full sun efficiency of 1.5% is demonstrated with a ZnO nanowire photoanode that has one-fifth the surface area of a traditional DSC.1 The nanowire geometry has been extended to include core-shell nanowire arrays. The addition of a shell provides an electrical barrier that decreases the recombination processes that occur at the nanowire/dye/electrolyte interface. Precise layering of TiO2 on the ZnO nanowire arrays is achieved using atomic layer deposition (ALD) leading to a doubling of the cell efficiency. Nanowire arrays can also improve the polymer-inorganic hybrid solar cell. Typically, blended polymer-inorganic films are produced by spin coating a solution containing a mixture of a conjugated polymer and either a fullerene derivative, chalcogenide nanorod, or oxide nanocrystal onto a transparent conducting substrate. Films made in this way can be sufficiently thick and intimately mixed to efficiently absorb light and separate charges, but they are poorly structured for efficient charge transport to the electrodes because the donor-acceptor interface is convoluted and discontinuous. In principle, a film designed to optimize charge collection would consist of a perfectly ordered array of continuous and crystalline inorganic nanorods oriented normal to the electrode surface and encased in a layer of the polymer. We have developed a method of growing vertical ZnO nanorod arrays from textured nucleation sites created by ZnO nanocrystals with their c axes normal to the substrate.2,3 The ZnO rod arrays are easily filled with hole-conducting polymers such as poly(3-hexylthiophene) to yield functioning solar cells. (1) Greene, L. E.; Law, M.; Johnson, J. C.; Saykally, R. J.; Yang, P. Nature Mater. 4, 455, 2005.(2) Greene, L. E.; Law, M.; Tan, D. H.; Montano, M.; Goldberger, J.; Somorjai, G.; Yang, P. Nano Lett. 5, 1231, 2005.(3) Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J.; Zhang, Y.; Saykally, R.; Yang, P. Agnew. Chem. Int. 42, 3031, 2003.
5:45 PM - FF5.8
Vertically Assembled Quantum-Wire p-i-n Solar Cells: Realistic Performance Predictions Beyond Detailed Energy Balance Calculations.
Alex Freundlich 1 2 , Andrea Feltrin 1 , Andenet Alemu 1
1 Texas Center for Advanced Materials, University of Houston, Houston, Texas, United States, 2 Physics Department, University of Houston, Houston, Texas, United States
Show AbstractWith staggering efficiency projections, quantum confined solar cells have been suggested to overcome the fundamental efficiency limitation of single junction solar cells. The basic idea is to introduce nanoscopic dots/wells of a narrow bandgap semiconductor material within the intrinsic (i) region of a wide bandgap p-i-n diode. One major practical shortcoming of quantum well/ dot devices resides in the difficulty of extracting photo-generated carriers from the quantum-confined region, which leads to important recombination losses and marginal (if any) efficiency enhancement. Thus the remedy to achieve a successful quantum-confined solar cell is to develop a design of the intrinsic region that will allow a rapid transfer of the photo-converted carriers to the conventional p- and n- conductivity regions. This work discusses our recent work on a new solar cell design, where a vertical array of nanometric wires (or vertical closely coupled dots) is inserted between the p and n conductivity regions of a conventional solar cell. In this approach, a rapid and efficient collection of the photo-generated carriers is afforded by the alignment of the wire (well) axis with the direction of the electric field. One thus would expect the device to fully benefit from improved absorption properties associated with the presence of quantum-confined states. We will first present the device performance calculated in the "detailed energy balance " framework. We will then introduce a more accurate (less optimistic) deterministic model for the simulation of the quantum wire p-i-n solar cell that takes into account realistic design parameters including actual optical matrix elements and density of states of the confined and continuum levels. The source code of the latest comprises a detailed calculation of the quantum wire absorption spectrum including excitonic effects and determines the wavelength-dependant quantum efficiency (spectral response) and the I-V characteristics of the quantum confined p-i-n solar cell. The model is then applied to different set of quantum wire test materials (InGaAs, InGaP material system including dilute nitrides) and the evolution of photovoltaic characteristics as a function of viable design and material combinations is analyzed with a special emphasis on identifying optimal candidates for highest practical efficiencies.
FF6: Poster Session
Session Chairs
Thursday AM, April 20, 2006
Salons 8-15 (Marriott)
9:00 PM - FF6.1
Optical Properties of Crystalline Silicon Surfaces Textured by Spontaneous Dry Etching with Chlorine Trifluoride Gas.
Yoji Saito 1 2 , Hidemasa Yamazaki 2 , Takeshi Kosuge 2
1 Electrical and Mechanical Eng., Seikei University, Musashino, Tokyo, Japan, 2 Electrical Eng. and Electronics, Seikei University, Musashino, Tokyo, Japan
Show AbstractMajor methods to reduce surface reflection are surface texturization and anti-reflection coating. The anti-reflection coating greatly reduces the reflectance at specific wavelength. On the other hand, texturization of the surfaces decreases totally reflectance from short wavelength to long wavelength by the multiple reflection.Single crystalline silicon surfaces are usually textured to pyramid-like structures using an alkaline solution. However, the texturization using an alkaline solution is not applicable to polycrystalline silicon wafers. In this research, instead of wet etching solution, we used chlorine trifluride (ClF3) gas as a dry etchant . The advantage of plasmaless etching using ClF3 gas is isotopic property independent of the crystal orientation without damage to the substrates. We have found that random and microscopic structures would be formed on the substrate by the treatment. In this study, we investigated surface structures and optical properties of the textured surfaces. Single crystalline substrates with (100) orientation and polycrystalline silicon substrates were used in this study. ClF3 as an etching gas and Ar as a diluting gas were introduced into the etching chamber, and total pressure was maintained to 240 Pa typically by evacuating with a dry pump. The surfaces of the substrates were slightly etched and textured by the ClF3 treatment. Optical reflection measurements were performed at wavelengths between 300 nm and 800 nm using a double beam spectrophotometer with an integrating sphere and an angle spacer to prevent specular reflection. In addition, we observed the textured surfaces with the scanning electron microscopy (SEM).When the substrate were treated at the ClF3 partial pressure of 9 Pa for 8 minutes near room temperature, the reflectance of the textured surface was obtained to be below 10% at the wavelength over 500nm and 6% at the wavelength of 800nm without anti-reflection coating. Moreover, the treated surfaces do not have anisotropic structures like random pyramids from SEM observation. The etched surface structures hardly depend on the crystal orientation. The treatment with ClF3 gas is effective to reduce the reflection loss of polycrystalline silicon substrates for solar cells.
9:00 PM - FF6.10
Non-contact Metrology for Electrical Characterization of Photo-Voltaic Materials
Michael Current 1 , Vladimir Faifer 1 , Tim Wong 1 , Tan Nguyen 1
1 , Frontier Semiconductor, San Jose, California, United States
Show AbstractA non-contact metrology for electrical characterization of p-n junctions has been developed for use on photo-voltaic materials. By analysis of junction photo-voltage (JPV) measurements at multiple light beam modulation frequencies and multiple light penetration depths, a comprehensive electrical analysis of photo-voltaic materials is provided for junction sheet resistance, leakage current, capacitance and bulk carrier diffusion length. The JPV analysis can be made with screen and native oxides on the surface, so no pre-measurement chemical etching is required. Since the probe does not contact the p-n junction surface, measurements can be made in a continuous fashion with the junction moving under the probe, providing a method for efficient high-resolution mapping of local electrical properties. The non-contact probing also allows for non-damaging measurements in both "hard" (Si) and "soft" (organic) photo-voltaic materials. Examples of p-n junction properties in polished, crystalline-Si and cast poly-Si will be discussed.
9:00 PM - FF6.11
Metalorganic Vapor Phase Epitaxy of InGaAsN Using Dilute Nitrogen Trifluoride and Tertiarybutylarsine.
Siu Cheng 1 , Robyn Woo 1 , Robert Hicks 1
1 Chemical Engineering, UCLA, Los Angeles, California, United States
Show AbstractThe incorporation of nitrogen into InGaAsN grown by metalorganic vapor phase epitaxy using helium-diluted nitrogen trifluoride and tertiarybutylarsine is reported. Nitrogen contents up to 2% were readily achieved while contents beyond 2% were observed at reduced growth rate. The growth rate of InGaAsN was found to decrease as the nitrogen trifluoride fraction in the gas increased. Additionally, the RMS roughness of lattice-matched InGaAsN films increased with nitrogen trifluoride fraction in the gas. Atomic force microscopy of a lattice-matched InGaAsN film annealed to 510 deg C in ultra high vacuum with 1x10^-6 Torr of nitrogen trifluoride reveals the formation of etch pits on the surface. These trends point to an etching effect that competes with deposition as the nitrogen trifluoride fraction in the gas increase.
9:00 PM - FF6.12
Carbon Nanotube Sheets as Counter Electrodes for Gratzel Solar Cells
Hasan Shodiev 1 , Ali Aliev 1 , Mei Zhang 1 , Sergey Lee 1 , Ray Baughman 1 , Anvar Zakhidov 1
1 Nanotech Institute, The Univ. of Texas at Dallas, Richardson, Texas, United States
Show AbstractDye sensitized solar cells (DSSC) are of great interest due to combination of of their high efficiency and relatively low cost. An effective counter electrode with high electrochemical activity is an important component of DSSC to enhance its practical utility. Presently used Pt coated ITO counter electrode can not be applied in flexibleDSSC architectures, while there is a growing need for flexible anodes which are transparent. In this work in order to search for an alternative counter electrode in dye sensitized solar cells, newly developed strong and transparent carbon nanotube sheets [1] are tested. To increase the electrochemical activity of the anode the CNT sheets are coated with highly conductive SWCNT and compared with pure multiwall CNT sheets. Compared with an unmodified MWCNT sheet anode, it is observed that the short circuit photocurrent (Isc) of the SWCNT modified sheet is increased, while the transparency is nearly same. We showed that the transparent sheets of SWCNT/MWCNT perform as a flexible anode and as electrochemical catalyst and also can be used in tandems of dye sensitized solar cells as transparent charge recombination or interconnect layers.[1] M. Zhang, S.Fang, A.Zakhidov, S.B.Lee, A.Aliev et.al., Science, 309,(2005) 1215
9:00 PM - FF6.13
Water Splitting to Hydrogen by RuO2-dispersed AInGeO4(A=Li,Na ) Photocatalyst with d10-d10 Configuration
Haruhiko Kadowaki 1 , Junya Sato 1 , Hisayoshi Kobayashi 2 , Nobuo Saito 1 , Hiroshi Nishiyama 1 , Yasunobu Inoue 1
1 Department of Chemistry, Nagaoka University of Technology, Nagaoka Japan, 2 Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Kyoto Japan
Show Abstract The overall splitting of water by a solid photocatalyst is an important issue for hydrogen production. Most of the photocatalysts so far developed were the transition metal oxides involving d0 metal ions. In aiming at establishing a new photocatalyst group, we have successfully employed p-block metal oxides having the metal ions with d10 configuration. 1-4 In the present study, the photocatalytic activities of complex p-block metal oxides,AInGeO4 (A=Li,Na), with d10-d10 configuration were studied. In water splitting on 1wt% RuO2-loaded AInGeO4 (A=Li,Na), both H2 and O2 were stably produced under Xe-Hg lamp irradiation. The photocatalytic activity of AInGeO4 (A=Li,Na) was remarkably larger than that of A2GeO3 and AInO2 (A=Li,Na). LiInGeO4 has heavily distorted metal-oxygen octahedral InO6 and tetrahedral GeO4 units with dipole moment inside the units, and an electric field due to dipole moment promotes the photoexcited charge separation. The band structures calculated by a DFT method showed that the combination of In3+ and Ge4+ produces the dispersed conduction bands of hybridized In 5s5p + Ge4s4p orbitals, indicating that the excited electrons have large mobility and contribute high photocatalytic performance. The combination of two p-block metal ions with d10 configuration is concluded to be promising in the design of efficient photocatalysts for water splitting. This work was supported by CREST and SORST, JST.1) J. Sato, N. Saito, H. Nishiyama, and Y. Inoue, J. Phys. Chem., B 105, 6061 (2001). 2) J. Sato, N. Saito, H. Nishiyama, and Y. Inoue, J. Phys. Chem. B 107, 7965 (2003). 3) J. Sato, H. Kobayashi, and Y. Inoue, J. Phys. Chem. B 107, 7970 (2003). 4) K. Ikarashi, J. Sato, H. Kobayashi, N. Saito, H. Nishiyama, and Y. Inoue, J. Phys. Chem. B 106, 9048 (2002).
9:00 PM - FF6.14
Photocatalysis for Water splitting by RuO2 -Loaded MInO3 (M=Sc,Y,La) with d10 Configuration.
Naoki Arai 1 , Hisayoshi Kobayashi 2 , Nobuo Saito 1 , Hiroshi Nishiyama 1 , Kazunori Sato 1 , Yasunobu Inoue 1
1 Department of Chemistry, Nagaoka University of Technology, nagaoka Japan, 2 Department of Chemistry and Materials Technology , Kyoto Institute of Technology, kyoto Japan
Show AbstractFor the decomposition of water to produce hydrogen by photocatalysts, we have developed p-block metal oxide photocatalysts with d10 configuration1-4 as opposed to conventional transition metal oxide photocatalysts with d d0 configuration. Most of the photocatalysts we have examined are composed of the combination of p-block metal ions (with d10 configuration) and monovalent alkaline metal or divalent alkaline earth metal ions. In the present study, we have paid attention to the role of trivalent metal ions in photocatalysis and investigated the photocatalytic activity of indium metal oxides involving rare earth metal ions and lantanides. The compounds of MInO3 (M=Sc,Y, La) were synthesized in air by solid reactions at high calcination temperatures. Under UV irradiation, 1 wt % RuO2 -loaded ScInO3 and YInO3 stably produced hydrogen and oxygen from water, whereas RuO2 - loaded LaInO3 showed a little production. The influences of calcination temperature in the preparation of YInO3 and the amount of RuO2 loaded showed that high dispersion of fine RuO3 particles on well-crystallized YInO3 caused high photocatalytic activity. The density function theory (DFT) calculation showed that the conduction band is composed of hybridized In 5s+5p orbitals with large dispersion. This indicates that the electrons with large mobility are generated on the conduction bands which are responsible for good photocatalytic performance of YInO3This work was supported by CREST and SORST, JST.1.J. Sato, S. Saito, H. Nishiyama, and Y. Inoue, J. Phys. Chem., B 105, 6061 (2001). 2.J. Sato, N. Saito, H. Nishiyama, and Y. Inoue, J. Phys. Chem. B 107, 7965 (2003). 3.J. Sato, H. Kobayashi, and Y. Inoue, J. Phys. Chem. B 107, 7970 (2003). 4. K. Ikarashi, J. Sato, H. Kobayashi, N. Saito, H. Nishiyama, and Y. Inoue, J. Phys. Chem. B 106, 9048 (2002). .
9:00 PM - FF6.15
Enhancing Energy Conversion Efficiency of Dye-Sensitized Solar Cells by Employing Nanoporous MgO-Coated TiO2 Nanoparticles
Hyun Suk Jung 1 , Jung-Kun Lee 1 , Michael Nastasi 1 , Sang-wook Lee 2 , Jin-Young Kim 2 , Kug Sun Hong 2 , Hyunho Shin 3
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 , Seoul National University, Seoul Korea (the Republic of), 3 , Kangnung National University, Kangnung Korea (the Republic of)
Show AbstractSol-gel derived Mg(OH)2 gel was coated on TiO2 nanoparticles, followed by thermal topotactic decomposition of the gel to form a highly nanoporous MgO crystalline coating. The specific surface area of the core-shell based electrode was significantly improved as compared to the electrode with uncoated TiO2 particles, which was attributed to the topotactic reaction-based highly nanoporous MgO layer. Such feature promoted the dye adsorption and subsequent cell performance. When coated by optimum amount of MgO, energy conversion efficiency increased as much as 45 % compared to the case of the uncoated TiO2 electrode. It was emphasized that increasing the specific surface area by coating nanoporous MgO layer (extrinsic factor) is one of the very effective ways to improve the dye adsorption and conversion efficiency of a TiO2-based solar cell, in addition to intrinsic factors.
9:00 PM - FF6.16
Hybrid Grätzel Solid Solar Cells Modified by Atomic Layer Deposition
Tingying Zeng 1 , Cassie Norris 1
1 Chemistry, Western Kentucky University, Bowling Green, Kentucky, United States
Show AbstractSensitized organic-inorganic hybrid Grätzel Solid Solar Cells (GSSC) is a very promising new generation of photovoltaics. However, to achieve cell overall efficiency over 10% in full sunlight spectrum and to develop low cost and high performance of novel nanostructural photoactive layers so as to construct the desired GSSCs are critical challenges both in materials choice and in manufacturing methods. To overcome the large interfacial charge recombination problem in this type of solar cells, we investigated the feasibility to modify the TiO2 mesoporous film by an ultrathin insulating layer of Al2O3 before self-assemble the photosensitizer monolayer into the mesoporous TiO2 film through atomic layer deposition (ALD), which was confirmed not to affect the electron collection efficiency due to the tunneling effect, while it blocks the connection between the electrons injected in TiO2 layer and the holes in the hole transport materials layer. The atomic layer deposition (ALD) method plays a very important role in today’s art of modifying the nanostructural materials to achieve desired optoelectronic properties. This method allows the building up of ultrathin films and controlling the film thickness at the atomic level, typically in a few angstroms.
9:00 PM - FF6.17
Modeling of the Optical Properties of Capped Porous Silicon in Reflecting Multilayer Stacks.
Ahmed Abouelsaood 1 , Moustafa Ghannam 1 , Izabela Kuzma Filipek 2 , Filip Duerinckx 2 , Jef Poortmans 2
1 Physics, American University, Cairo Egypt, 2 , IMEC, Leuven, ., Belgium
Show AbstractPorous silicon stacks consisting of layers of alternate (high/low) porosity are possible candidates for use as back reflectors in thin-film silicon solar cells on cheap substrates. During the epitaxial growth process of the silicon film on the stack, the pore morphology changes, and the pores take the form of relatively large closed cavities as evidenced by SEM images. In this work a model for the optical properties of closed-porosity silicon is developed. The new model takes into account the fact that the topology of capped porous silicon is the topology of the Maxwell-Garnett model where the closed pores are completely surrounded by silicon material rather than the symmetric topology assumed in the widely-used Bruggeman's model where silicon and air particles would exist in a symmetric mixture. Thus both the silicon material and the cavities as considered embedded in an effective medium such that the value of whose refractive index is the one that leads to a vanishing coherent component of the scattered optical field. The building units of the effective medium are taken as silicon spherical shells of random locations, making use of a generalization of the Mie theory. This takes into account retardation and interference effects which play an important role in our case where the sizes of the cavities are comparable to the wavelength in the silicon material, but are totally neglected in both the Bruggeman and the Maxwell-Garnett models, treating the building blocks of the composite medium as having very small sizes. To verify the validity of the model, the front-surface spectral reflectance of structures consisting of a silicon epitaxial layer with or without a plasma-textured front surface grown over a reflecting porous silicon stack is calculated and compared with the experimental measurements done at IMEC for such structures. In this calculation light rays are traced through the structure, taking into account the wavelength and angle dependence of the reflectivity of the porous stack calculated using the optical properties of porous silicon determined by the new model, and allowing for a combination of specular and diffuse transmission at the textured front surface. The comparison shows good agreement between theory and experiment.
9:00 PM - FF6.18
Surface Molecularly Imprinted TiO2 Nanoparticle for Photoreduction of Viologen
Takashi Sagawa 1 , Mayu Kudo 1 , Joachim Steinke 2 , Takashi Morii 1 , Katsutoshi Ohkubo 1
1 Institute of Advanced Energy, Kyoto University, Uji, Kyoto, Japan, 2 Department of Chemistry, Imperial College London, London United Kingdom
Show AbstractWhen small organic molecules (e.g. amino acids, nucleotides, and so on) are incorporated in metal oxide films formed from metal alkoxide in the sol-gel process, the organic moieties can be removed readily by solvent washing or by degradative oxidation. Under suitable conditions, nano-sized cavities corresponding to the shape of the individual organic moiety are produced within the metal oxide network, a molecularly imprinted material such as molecularly surface-imprinted TiO2. We attempted the fabrication of nano-sized particles of TiO2 to increase the effective contact area for the reactants, and further additional function for molecular recognition was intended to be introduced through surface modification of the imprinting method in order to enhance the photoreduction of viologen. In this context, we prepared viologen imprinted TiO2 as an active and stable photocatalyst for photoreduction of viologen as a well-defined, readily identifiable and reversible electron carrier with appropriate redox potential for photoactivated TiO2. The basic concept of molecularly imprinting in this system is as follows. (i) Oxidized viologen is generally twisted and a dicationic molecule, while (ii) reduced viologen is planar and a monocationic radical. Once TiO2 has recognition site for the latter on its surface, it would be distinguishable through shape or charge differences and facilitates the inclusion of the target molecule results in enhancement of the reduction efficiency. After the preparation of the template, sol-gel imprinting was performed. Monitoring changes by dynamic light scattering the average particle diameter was stabilized ca 120 nm after 1 week stirring. X-ray diffraction pattern of the TiO2 powder after drying but prior to calcination indicates a dominant peak for anatase. Almost 50% of the template could be removed after alkaline rinsing monitoring the supernatant with UV-vis. A buffered solution of Tris including viologen was irradiated through a xenon lamp under stirring in a quartz cuvette under Ar with and without the TiO2 with cut-off filter at room temperature. The formation of the viologen monocation radical, whose color is blue, was easily recognized only in the reaction with TiO2. The formation of the viologen monocation radicals from three types of viologens (methyl, hexyl, and pentacarboxyl) was monitored spectrophotometrically with time in the presence of the imprinted TiO2 and also using a blank. Aliphatic C6 side chains of bipyridinium and terminal carboxyl groups were not effective in enhancing photoreduction, however the less bulky methyl viologen produced the highest yield among the three types of methyl, hexyl, and pentacarboxyl viologens. The most important result is that higher conversion was observed in every case of imprinted TiO2 compared with the blank indicating that recognition is not an ill-defined surface effect but involves the bipyridinium part of the viologen.
9:00 PM - FF6.2
Photo-Catalytic Decomposition of Volatile Organic Compounds with Mesoporous Silica.
Masanari Takahashi 1 , Yuichi Hayashi 2 , Gohei Yoshida 2 , Seiji Watase 1
1 , Osaka Municipal Technical Research Institute, Osaka Japan, 2 , Honjyo Chemical Corporation, Neyagawa Japan
Show Abstract9:00 PM - FF6.4
Poly(methylphenylsilane)(PMPS)-SWNT Photoelectric Composites: Covalent Graft and Non-covalent Wrapping.
Zhang Zhenghua 1 2 , Xu Weijian 1 2 , Lu Yanbing 1 , Xiong Yuanqin 1 , Yu Jiang 1 2
1 , Hunan Universiity, Changsha China, 2 , State Key Laboratory of Chemo/Biosensing and Chemometrics, Changsha China
Show Abstract9:00 PM - FF6.5
Study of PV Module Methylmetacrylate Encapsulation
Sergey Karabanov 1 , Yuri Kukhmistrov 1
1 , Ryazan Metal Ceramics Instrumentation Plant JSC, Ryazan Russian Federation
Show AbstractThe paper deals with the study of PV module encapsulation using liquid methylmetacrylate composition hardened by UV radiation. The solar cells encapsulation method by methylmetacrylate composition in the glass-glass and glass PET-foil constructions of PV modules is investigated. The UV hardening modes of methylmetacrylate composition are investigated and developed. The studied method allows to eliminate heating during encapsulation and reduce the PV module production cost. The new method makes it possible to encapsulate solar cells on any substrates, including easily melted polymeric ones, and also in the glass-glass construction.The structure and technological modes of liquid methylmetacrylate composition hardening during encapsulation have been investigated and selected. PV modules investigation for the resistance to external actions according to IEC61215 standard procedure provided positive results.
9:00 PM - FF6.6
AFM Studies on the Morphological Evolution of Chemically Deposited In2S3 Thin Films as a Function of Deposition Time.
Merida Sotelo-Lerma 1 , Omar A. Castelo-Gonzalez 1 , Francisco Javier Espinoza-Beltran 2 , Rafael Ramirez-Bon 2
1 Departamento de Investigacion en Polimeros y Materiales, Universidad de Sonora, Hermosillo, Sonora, Mexico, 2 Centro de Investigacion y Estudios Avanzados del IPN. Unidad Queretaro, CINVESTAV, Queretaro, Queretaro, Mexico
Show Abstract9:00 PM - FF6.7
Non-Reflecting Silicon Surfaces for Application in Solar Cells
Svetoslav Koynov 1 , Martin Brandt 1 , Martin Stutzmann 1
1 E 25, Walter Schotky Institut - Techncal University Munchen , Garching b. Munchen Germany
Show AbstractRecently we proposed a new process for the modification of Si surfaces, which results in an almost complete suppression of the reflectivity in a broad spectral range leading to black Si surfaces. The process results in a nano-scale texturing of the topmost 150-200 nm of the material independent of the surface orientation and doping. Thus, it can be applied to various structural forms of bulk silicon (single-, poly- or multi-crystalline) as well as to thin Si films (amorphous or microcrystalline).In this presentation, we discuss different aspects of the black silicon surfaces, which are critical for their application in solar cells. The purity and surface defects after completion of the surface treatment are investigated on single crystalline Si samples by using X-ray Photoemission Spectroscopy (XPS) and by Electron Paramagnetic Resonance (EPR). It is shown that the optical properties of such surfaces are not affected by the common chemical treatments used in solar cell technology such as degreasing, HF dipping, chemical oxidation by boiling in H2NO3 or piranha etch. The high-temperature stability of the surface texture is tested by treatments such as annealing in the 1000oC range, wet oxidation and doping from liquid sources. A test photovoltaic structure is fabricated by striping the antireflection coating from a peace of commercial multi-crystalline solar cell and subsequent texturing of the surface at the emitter side by the new process. Comparative measurements of the solar cell characteristics before and after the treatment will be presented.
9:00 PM - FF6.8
Nano-Carbon Counter Electrodes for Dye Sensitized Solar Cells.
Wonjae Lee 1 , Easwaramoorthi Ramasamy 1 2 , Dongyoon Lee 1 , Jaesung Song 1
1 Electric & Magnetic device research group, Korea Electrotechnology Research Institute, Changwon Korea (the Republic of), 2 , University of Science and Technology, Daejeon Korea (the Republic of)
Show Abstract9:00 PM - FF6.9
Improved Performance of the Nanoporous TiO2 Electrode Sensitized with a Ruthenium Complex Using Thiophene Polymer
Seung Jae Roh 1 , KyungHee Hyung 1 , Kuk Hee Park 1 , Sung-Hwan Han 1
1 Chemistry, Hanyang University, Seoul Korea (the Republic of)
Show AbstractIt was widely known that photoelectrochemical cells consist of ruthenium complex on nanoporous TiO2. The TiO2 film was fabricated by spin coating onto conducting glass substrate (tin-doped indium oxide, ITO). The TiO2 films were immersed in solution of RuL2(NCS)2 (L = 2,2’-bypyridine-4,4’-dicarboxylic acid) for 20 hours. And then the thiophene polymer layers were added to the photoelectrochemical cell in various ways. The polymer layers were characterized by UV-vis spectroscopy, cyclic voltammetry and SEM(scanning electron microscopy). It enhanced the energy conversion efficiency upto 20% by adding a thiophene layers on ITO/TiO2/Ru complexes. Photocurrent measurements were performed in a two electrodes system under the illumination intensity of 100mW/cm2.
Symposium Organizers
Paul Alivisatos University of California-Berkeley
Nathan S. Lewis California Institute of Technology
Arthur J. Nozik National Renewable Energy Laboratory
Michael R. Wasielewski Northwestern University
FF7: Thermoelectrics and Thermophotovoltaics
Session Chairs
Thursday AM, April 20, 2006
Room 2008 (Moscone West)
9:00 AM - FF7.1
Edge Illuminated Vertical Multi-Junction Thermo-photovoltaic Cells Based on GaSb and GaxIn1-xSb
Sujatha Sridaran 1 2 , Alex Tran 2 , Partha Dutta 1 2 , Omkaram Nalamasu 2
1 Electrical, Computer and System Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, New York, United States
Show Abstract9:15 AM - FF7.2
Triple and Quadruple Junctions Thermophotovoltaic Devices Lattice Matched to InP.
Lekhnath Bhusal 1 2 , Alex Freundlich 1 2
1 Texas Center for Advanced Materials, University of Houston, Houston, Texas, United States, 2 Physics Department, University of Houston, Houston, Texas, United States
Show AbstractThemophotovoltaic (TPV) conversion of IR radiation emanating from a radioisotope heat source is under consideration for deep space exploration. Ideally, for radiator temperatures of interest, the TPV cell must convert efficiently photons in the 0.4–0.6 eV spectral range. Best experimental data for single junction cells are obtained for lattice-mismatched 0.55 eV InGaAs based devices. It was suggested, that a tandem InGaAs based TPV cell made by monolithically combining two or more lattice mismatched InGaAs subcells on InP would result in a sizeable efficiency improvement. However, from a practical standpoint the implementation of more than two subcells with lattice mismatch systems will require extremely thick graded layers (defect filtering systems) to accommodate the lattice mismatch between the sub-cells and could detrimentally affect the recycling of the unused IR energy to the emitter.Recently we have shown that by adjusting the thickness of individual sublayers and the nitrogen composition, strain balanced GaAs1-xNx/InAs1-yNy superlattice can be designed to be both lattice matched InP and have an effective bandgap in the desirable 0.4-0.7eV range. The insertion of such a superlattice-like alloy within the intrinsic region of a 0.74 eV GaInAs p-i-n diode was previously evaluated and it was shown that such a single junction device exhibits a photovoltaic response comparable to its lattice-mismatched 0.55 eV-InGaAs counterpart.In this work we have extended the approach to multi-junctions devices. Here three or more subcells with different effective bandgaps for the superlattice region are monolithically series connected. The external quantum efficiency and the evolution of the I-V characteristics of triple and quadruple junction TPV cells are evaluated as a function of the superlattice/cell design and black body emitter temperatures. The study stresses the potential of the proposed approach for a significant enhancement of TPV converter efficiencies.
9:30 AM - FF7.3
Overview of Thermoelectric Materials Research.
Terry Tritt 1
1 Physics & Astronomy, Clemson University, Clemson, South Carolina, United States
Show AbstractThermoelectric materials and devices may prove quite beneficial in the arena of solar energy conversion and storage. Thermoelectric materials allow the conversion of waste or stored heat to be converted into useful electrical energy. The focus in this talk will be on some recent directions in bulk thermoelectric materials research that have been pursued. The requirements for a potential thermoelectric material will be discussed and how these relate into giving a favorable figure of merit, ZT, defined as ZT = a^2T/rk; where a is the Seebeck coefficient, r is the electrical resistivity, k, is the thermal conductivity and T is the temperature in Kelvin. Thermoelectric materials are inherently difficult to characterize and a short discussion of these difficulties will be part of the presentation. These difficulties are magnified at high temperatures and many challenges remain. Specific materials will be discussed, and especially those results in bulk materials that exhibit favorable properties for potential mid to high temperature power generation capabilities. A discussion of some of the future directions in our materials research will be highlighted, including some bulk materials, which are based on nano-scaled composites.Thermoelectric materials are inherently difficult to characterize and a short discussion of these difficulties will be part of the presentation. These difficulties are magnified at high temperatures and many challenges remain. Specific materials will be discussed, and especially those results in bulk materials that exhibit favorable properties for potential mid to high temperature power generation capabilities. A discussion of some of the future directions in our materials research will be highlighted, including some bulk materials, which are based on nano-scaled composites.
9:45 AM - **FF7.4
Bismuth Nanowire Thermoelectrics.
Jim Heath 1 , Akram Boukai 1 , Ke Xu 1
1 Chemistry, Caltech, Pasadena, California, United States
Show AbstractThermoelectric materials convert a temperature difference into electricity and vice versa. Currently thermoelectrics find only limited use because of their poor efficiency. The thermoelectric figure of merit has been predicted to be significantly enhanced for Bismuth nanowires (NWs) due to quantum confinement effects.[1] However, quantitative measurements on Bismuth NWs are challenging because of the instability of Bi towards device fabrication methods[2]. We have overcome several of these limitations by coupling a device architecture used by Small et. al.[3] and Llaguno et. al.[4] for nanotube studies with nanofabrication steps that are compatible with the material limitations of bismuth and that allow for the fabrication of very small (<10 nm) Bi NWs.[5] This resulting devices provide for reliable four-point electrical conductivity measurements, allows for accurate measurements of thermoelectric power, and permits quantitation of electric and magnetic field effects on the transport of individual Bi NWs. In this talk I will present such measurements on individual Bi NWs, as a function of NW size. [1]. L. D. Hicks, M. S. Dresselhaus, Phys. Rev. B 1993, 47, 16631.[2]. R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O'Quinn, Nature 2001, 413, 597.[3]. J. Small, K. Perez, P. Kim, Phys. Rev. Lett. 2003, 91, 256801.[4]M. Llaguno, et al., Nano Lett. 2004, 4, 45.[5]. N. Melosh, et al., Science 2003, 300, 112.
10:15 AM - FF7.5
Can Doping Improve the Thermoelectric Performance of β-Zn4Sb3?
Birgitte Pedersen 1 2 , Eiji Nishibori 3 , Henrik Birkedal 1 2 , Anders Bentien 1 2 , Makato Sakata 3 , Matts Nygren 4 , Poul Frederiksen 5 , Bo Iversen 1 2
1 Department of Chemistry, University of Aarhus, Aarhus Denmark, 2 Interdisciplinary Nanoscience Center, University of Aarhus, Aarhus Denmark, 3 Department of Applied Physics, Nagoya University, Nagoya Japan, 4 Department of Chemistry, Stockholm University, Stockholm Sweden, 5 , Grundfos A/S, Bjerringbro Denmark
Show AbstractFF8: Inorganic Photovoltaics
Session Chairs
David Ginley
Martin Green
Thursday PM, April 20, 2006
Room 2008 (Moscone West)
10:45 AM - **FF8.1
Electronic Structure of PbSe Quantum Dots and Mechanisms of Carrier Multiplication .
Alberto Franceschetti 1 , Alex Zunger 1
1 , NREL, golden, Colorado, United States
Show Abstract11:15 AM - **FF8.2
Design Approaches and Materials Processes for Ultrahigh Efficiency Lattice Mismatched Multijunction Solar Cells
Harry Atwater 1
1 Applied Physics, California Institute of Technology, Pasadena, California, United States
Show AbstractFuture ultrahigh efficiency multijunction solar cells will employ designs that feature 3 or 4 or more subcells utilizing lattice-mismatched structures to achieve an optimal bandgap sequence for solar energy conversion. While lattice-mismatched multijunction cells have been fabricated recently using metamorphic growth approaches, use of direct wafer bonding techniques to enable lattice mismatch accommodation at the subcell interfaces enables considerably more design freedom and inherently higher quality, defect-free active regions. Our work has focused on a four junction InGaP/GaAs/GaInAsP/InGaAs cell featuring two optimized, internally lattice-matched two junction subcells. The top two junction cell is grown on a template lattice-matched to GaAs, whereas the bottom two junction cell is grown on a template lattice-matched to InP. Subcells are integrated via a direct bond interconnect between the GaAs and GaInAsP We report here on epitaxial growth of GaAs and GaInP on single crystal Ge thin film templates transferred onto low cost substrates and on recent progress on a direct bond interconnect between GaAs and InGaAs subcells. Finally, we describe a nonimaging microconcentrator receiver that can enable cost-effective utilization of these heterostructure multijunction cells in superficially flat terrestrial photovoltaic modules.
11:45 AM - FF8.3
p-n Junction Heterostructure Device Physics Modeling of Four Junction Solar Cell Efficiency
Melissa Griggs 1 , Brendan Kayes 1 , Harry Atwater 1
1 , California Institute of Technology, Pasadena, California, United States
Show AbstractWe present results from a p-n junction device physics model for ultra-high efficiency multi-junction solar cells, specifically for GaInP/GaAs/GaInAsP/GaInAs four junction solar cells. The model employs subcell thickness upper bounds of 5μm and lower bounds of 200nm which is just above the fully depleted case for the assumed doping of NA=1e18cm-2 and ND=1e17cm-2. Real behavior of solar cells is accounted for rather than assuming idealized operation as in detailed balance calculations. For example, free carrier absorption, temperature and doping effects on carrier mobility are included as well as two recombination pathways: Shockley–Read–Hall recombination from a single midgap trap level and surface recombination. Upper bounds set by detailed balance calculations can be approached by letting the parameters approach ideal conditions. By accounting for non-idealities, we can probe the diminishing return from adding subcells to multi-junction solar cells. Whereas detailed balance calculations always benefit from added subcells, current matching requirements in series connected p-n multi-junctions indicate a minimum performance required from added subcells for net contribution to the overall device. For instance, a subcell can lower the power production by shifting the current matching position further from the optimal operating position instead of adding to power generation. In the four junction cell examined here, if the diffusion length of the bottom cell (GaInAs) drops below a critical value, it degrades the overall device and higher efficiencies are achieved by omitting it. Optimizing the subcell thickness is a major part of calculating the efficiency for these multi-junction solar cells. The series resistance limitations for concentrator applications can be easily explored for a given set of subcells. For example, a critical interfacial resistance of 0.1 Ωcm2 is obtained for concentrator cells operating at 300 Suns. Moreover, the current matching limitation imposed by series connection reduces the efficiency as can be seen by comparison to similar calculations for independently-connected cells. The overall trend is toward an approximately 5% drop in efficiency for series connected cells relative to analogous independently connected cells, but the biggest difference is the sensitivity of the series connected device to spectral changes and individual subcell performance. If any single cell within the series connected device degrades in any way, the entire device is severely hindered whereas independently connected devices are more resilient to poor material quality and spectral variation since current matching is not required. This model allows novel solar cell structures to be evaluated by providing realistic predictions of the performance limitations of these multijunction devices.
12:00 PM - FF8.4
GaNxAs1-x-yPy Quaternary Alloys: a III-V Multiband System for High Efficiency Intermediate Band Solar Cells
KinMan Yu 1 , Wladek Walukiewicz 1 , Joel Ager 1 , David Bour 3 , Rouin Farshchi 2 1 , Oscar Dubon 2 1 , Ian Sharp 2 1 , Eugene Haller 2 1
1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 , Agilent Laboratories, Palo Alto, California, United States, 2 Department of Materials Science and Engineering, University of California, Berkeley, California, United States
Show AbstractThe concept of utilizing an intermediate band in a semiconductor to serve as a “stepping stone” for low energy photons to excite electrons from the valence to the conduction band has long been suggested as a possible way to realize high efficiency solar cells. These intermediate band solar cells (IBSCs) have the important advantage that they can realize high power conversion efficiencies with only a single p-n junction, which is a crucial simplification in comparison to multi-junction solar cells. We have previously reported the successful synthesis of multi-band ZnMnOTe alloys with up to 3% of Te replaced with O atoms [1]. This new material has three absorption edges that cover the entire solar spectrum, making it a good candidate for the envisioned high efficiency IBSCs. Using the band anticrossing (BAC) model, we have identified and practically realized another multi-band semiconductor alloy in the technologically more important III-V alloy system. This is the GaNxAs1-x-yPy quaternary alloy with a P content y>0.3 where the impurity N level lies below the conduction band of the GaAs1-yPy substrate. GaNxAs1-x-yPy layers with y=0 to 0.4 were synthesized using N+-implantation followed by pulsed laser melting and rapid thermal annealing techniques. With an implanted N concentration of 2%, the active N incorporated in the group V sublattice is about 1% and 0.3% for films with y<0.12 and y>0.12, respectively. Strong optical transitions from GaNxAs1-x-yPy are clearly observed corresponding to both the lower (E-) and upper (E+) conduction subbands that arise from the N induced splitting of the conduction band (EM) of the GaAs1-yPy host. As the P content y increases, the localized N level EN gets closer to EM and consequently we observed that the strength of the E+ transition increases and eventually becomes stronger than E- for y > 0.3. Hence GaNxAs1-x-yPy alloys with y>0.3 represent another three band semiconductor alloy that is potentially important for the fabrication of highly-efficient intermediate band solar cells. BAC calculations show that for a GaN0.02As0.58P0.4 multiband semiconductor a N derived narrow band of extended states (E-) is separated from the upper subband E+ by about 0.8 eV. A theoretical maximum efficiency of between 55% and 60% can be achieved for IBSCs fabricated using this alloy.[1].K. M.Yu, , W. Walukiewicz, J. Wu, W. Shan, J. W. Beeman, M. A. Scarpulla, O. D. Dubon, and P. Becla, Phys. Rev. Lett. 91, 246403 (2003).
12:15 PM - FF8.5
Direct-Bond Interconnected Multijunction GaAs/InGaAs Solar Cell.
Katsuaki Tanabe 1 , Daniel Aiken 2 , Mark Wanlass 3 , Anna Fontcuberta i Morral 1 , Harry Atwater 1
1 Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California, United States, 2 , Emcore Photovoltaics, Albuquerque, New Mexico, United States, 3 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractDirect bonded interconnect between subcells of a lattice-mismatched III-V compound multijunction cell would enable dislocation-free active regions by confining the defect network needed for lattice mismatch accommodation to tunnel junction interfaces, while metamorphic growth inevitably results in less design flexibility and lower material quality than is desirable. We report here the first direct-bond interconnected multijunction solar cell, a two-terminal monolithic GaAs/InGaAs two-junction solar cell, to demonstrate a proof-of-principle for the viability of direct wafer bonding for solar cell applications. The GaAs subcell and the InGaAs subcell lattice-matched to InP were epitaxially grown on a (001) GaAs and a (001) InP substrate, respectively, by metallorganic vapor phase epitaxy. After bonding of the two subcells, the GaAs substrate was removed to complete a GaAs/InGaAs/InP (substrate) heterostructure. To obtain low interfacial resistance at the bonded interface, a Se-doped GaAs layer with 1x 1019 cm-3 carrier concentration was grown on the GaAs subcell and a S-doped InP layer with carrier concentration of 2x 1019 cm-3 was growth on the InGaAs subcell. These structures were bonded at 0.5 MPa, 380 oC in atmosphere for 10 hours with the (011) edges aligned after degreasing and oxide-removal treatment followed by annealing in 10%-H2/N2 at 350 oC for 30 min after metallization with evaporated Au. Photovoltaic I-V characteristics of the bonded GaAs/InGaAs two-junction cell were measured with 0.337 cm2 active illumination area under AM1.5 Global solar spectrum with 1-sun total intensity (100 mW cm-2). The device parameters were Jsc = 15.0 mA cm-2, Voc = 1.22 V, FF = 0.58, and η = 10.6 %, where Jsc, Voc, FF and η are short-circuit current, open-circuit voltage, fill factor and energy conversion efficiency, respectively. The low fill factor can be accounted for by series resistance in the contacts, which will be lowered by contact redesign. Significantly the Voc of the bonded GaAs/InGaAs two-junction cell was 1.22 V, thus equal to the sum of those of the Voc s for the GaAs and InGaAs subcells (1.0 V for GaAs cell and 0.2 V for InGaAs, respectively). This Voc result indicates that the bonding process does not degrade the cell material quality since any generated crystal defects that act as recombination centers reducing the carrier lifetime would reduce Voc. Devising such as anti-reflective front surface and surface passivation, and optimization of cell assembly parameters including current matching would give us further improvement of the cell efficiency. This achievement shows direct bonding enables us to construct lattice-mismatched III-V multijunction solar cells and is extensible to ultrahigh efficiency InGaP/GaAs/InGaAsP/InGaAs four-junction cells by bonding a GaAs-based lattice-matched InGaP/GaAs subcell and an InP-based lattice-matched InGaAsP/InGaAs subcell.
12:45 PM - FF8.7
Hole Photocarrier Drift Mobility Measurements in CuI0.7Ga0.3Se2 Solar Cells.
Steluta Dinca 1 , Eric Schiff 1 , Brian Egaas 2 , David Young 2 , Rommel Noufi 2
1 Dept.of Physics, Syracuse University, Syracuse, New York, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractWe have measured hole photocarrier drift-mobilities on copper-indium-gallium-diselenide (CIGS) solar cells prepared at the National Renewable Energy Laboratory (NREL). We used transient photocurrent measurements to determine the hole time-of-flight for varying voltage bias. The typical value for the hole drift-mobility inCuI0.7Ga0.3Se2 cells was 0.2 cm2/Vs. This value is far below that obtained from Hall measurements on epitaxial films (1). We measured the temperature-dependence of the hole drift-mobility over the range 100-300 K. The drift-mobility has little temperature dependence, which suggests that the drift-mobility is not limited by carrier trapping processes. We did find evidence for hole "deep-trapping" at the lower-temperatures.A hole drift-mobility of this low a magnitude would be important for solar cell performance. Frequency-dependent capacitance measurements on polycrystalline CIGS solar cells prepared at University of Delaware with similar Ga/(In+Ga) ratio yielded a typical hole mobility of 10 cm2/Vs (2). Some of the differences in these mobility measurements are probably attributable to differing material microstructures. Additionally, the present measurements are sensitive to the top 500 nm of the sample thickness; the actual CIGS films were about 2500 nm thick. We speculate that vertical inhomogeneities in polycrystalline CIGS films may also be important, and thus that differing techniques may be probing differing portions of the films.This research has been supported by the NREL Thin Film Photovoltaics Partnership.(1) D. J. Schroeder, J. L. Hernandez, G. D. Berry, A. A. Rockett, J. App. Phy. 83, 1519 (1998).(2) J. D. Cohen and J. Heath, unpublished NREL report.
FF9: Dye-Sensitized Solar Cells
Session Chairs
Thursday PM, April 20, 2006
Room 2008 (Moscone West)
2:30 PM - **FF9.1
Mesoscopic Injection Solar Cells.
Michael Graetzel 1
1 , ecole polytechnique de lausanne, lausanne Switzerland
Show Abstract3:00 PM - **FF9.2
Characterization and Modelling of Transport and Interfacial Transfer of Electrons in Dye-Sensitized Solar Cells.
Laurence Peter 1 , Halina Dunn 1 , Joanna Harries 1 , Wendy Howie 1 , James Jennings 1 , Killian Lobato 1 , Diego Martinez 2 , Oanh Nguyen 1 , Alison Walker 2
1 Department of Chemistry, University of Bath, Bath United Kingdom, 2 Department of Physics, University of Bath, Bath United Kingdom
Show AbstractThe performance of dye-sensitized solar cells (DSCs) is ultimately determined by the behavior of the electrons injected from the photoexcited sensitizer dye into the conduction band of the nanocrystalline oxide. Under short circuit conditions, transport of electrons to the contact competes with loss by electron transfer to the adjacent ‘hole-conducting’ phase, which can be a redox electrolyte, an organic hole conductor or a p-type semiconductor. Under open circuit conditions, which correspond to a photostationary state, the photovoltage depends on the total rate at which photoinjected electrons cross back to the oxidized dye and the ‘hole’. Electron transport in DSCs appears to be driven by the entropic component of the free energy gradient. It is common practice to characterize electron transport in terms of an ‘effective’ diffusion coefficient that depends on illumination intensity as a consequence of electron trapping. However, this approach can be misleading since it is only electrons in the conduction band that carry the current. The problem is that the diffusion coefficient of these electrons is not readily accessible to measurement. The apparent diffusion is much lower than the true conduction band value because the observed relaxation time for electron transport is influenced by changes in trap occupancy. The same is true for the apparent electron lifetime, which is derived from the time constant for the relaxation of the conduction band density. The key parameter that determines whether the collection of photoinjected electrons will be efficient is the electron diffusion length. It can be shown that the electron diffusion length Ln is correctly given by the square root of the product of the apparent diffusion coefficient and the apparent lifetime because the effects of electron trapping exactly cancel out. It is therefore possible to measure Ln using a range of large and small amplitude methods that will be discussed using recent examples of cells based on iodide/tri-iodide redox electrolytes and on organic hole conductors. The implications of the experimental results for cell design will be discussed.In addition to back reaction of electrons at the interface between the nanocrystalline oxide and the hole conductor, it is also necessary to consider back reaction via the substrate. The driving force for this is zero at short circuit, but considerable at the maximum power point or at open circuit. This process can be characterized using thin layer cells that are analogs of the DSC, but which exclude certain components such as the dye. Examples of this approach will be given.Analytical models have been developed to describe electron transport and back reaction in DSCS. In addition, the Monte Carlo approach has been used to explore trapping. These calculations have been compared with experimental results for a range of different types of DSCs. The potential of this approach will be illustrated with examples.
3:30 PM - FF9.3
Photocarrier Trapping in Dye-sensitized Solar Cells: Evidence Against Exponential Conduction Bandtail Trapping.
Nikos Kopidakis 1 , Kurt Benkstein 1 , Jao van de Lagemaat 1 , Arthur Frank 1 , Quan Yuan 2 , Eric Schiff 2
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Physics, Syracuse University, Syracuse, New York, United States
Show AbstractMesoporous titania (TiO2) films are the basis of dye-sensitized solar cells, and are formed by sintering an aggregate of titania nanocrystallites. Understanding charge carrier transport in such mesoporous materials is generally important for solar cells based on nanocrystallite aggregates, and is part of the larger subject of disorder effects on transport. For mesoporous TiO2, several experiments are consistent with trap-limited electron transport, and workers have typically concluded that the distribution of level energies for these traps forms an exponential conduction bandtail. Exponential bandtail trapping has also been reported in non-crystalline inorganic materials such as hydrogenated amorphous silicon. In this work, we conclude that the distribution of trap properties in our mesoporous TiO2 samples cannot be mapped onto an exponential distribution of level energies, and we speculate more generally on the nature of charge carrier traps in porous semiconductors.In particular, we measured the temperature and photoexcitation density dependences of the electron transport dynamics in electrolyte-filled mesoporous TiO2 nanoparticle films. The thermal activation energy of the diffusion coefficient of photogenerated electrons ranged from 0.19–0.27 eV, depending on the specific sample studied. The diffusion coefficient also depends strongly on the photoexcitation density; however, the activation energy has little, if any, dependence on the photoexcitation density. The light intensity dependence can be used to infer temperature-independent dispersion parameters in the range 0.3–0.5. These results are inconsistent with the widely used transport model that assumes multiple trapping of electrons in an exponential conduction bandtail. In particular, exponential bandtail trapping predicts that the dispersion parameter should be proportional to the absolute temperature. Interestingly, temperature-independent dispersion has also been reported in porous silicon and in a number of polymeric semiconductors, which suggests that this behavior may be associated with porosity-associated disorder.
3:45 PM - FF9.4
Transparent and Conducting Carbon Nanotube Films for the Dye-sensitized Solar Cell
Jessika Trancik 1 , Adam Hurst 2 , Bhupesh Chandra 2 , Yuhao Sun 3 , James Hone 2 , Scott Calabrese Barton 3
1 Earth Institute, Columbia University, New York, New York, United States, 2 Mechanical Engineering, Columbia University, New York, New York, United States, 3 Chemical Engineering, Columbia University, New York, New York, United States
Show AbstractWe investigate the possibility of using single walled carbon-nanotubes (SWCNT) as catalysts and transparent conducting films, to replace the transparent conducting oxide and thin platinum film commonly used for the cathode of the dye-sensitized solar cell (DSSC). Carbon nanotube films offer the potential for high optical transparency, low sheet resistances, and high surface areas for catalysis. Carbon nanotube films are also less brittle than commonly used transparent conducting oxides (TCO). These properties may be particularly important for increasing the efficiency of cells on flexible substrates, and of multi-junction cells. Films of long tubes were grown at 900°C by chemical vapor deposition (CVD), using Fe nanoparticles as catalysts. A simple transfer procedure can be used to move films onto the desired substrate after growth. Light transmittance measurements of SWCNT films on a transparent substrate show higher transmittance in the visible and infrared range (85-90%), as compared to the light transmittance for platinum (3-5 nm) and a transparent conducting oxide (TCO) on a transparent substrate. Suitably low sheet resistances are also achievable (on the order of 102 Ω/square). We investigate the catalytic properties of the SWCNT films using electrochemical impedance spectroscopy (EIS), in comparison to the thin platinum layer currently used in the DSSC. Our results suggest that SWCNT films may increase efficiencies of the DSSC (improved reaction kinetics at the counter-electrode, increase light transmittance) and facilitate the use of flexible substrates (better mechanical properties). Increased efficiencies and the use of flexible substrates could reduce costs of solar-generated electricity.
4:45 PM - FF9.6
Use of Highly-Ordered TiO2 Nanotube-Arrays in Dye-Sensitized Solar Cells.
Gopal Mor 1 , Karthik Shankar 1 , Maggie Paulose 1 , Oomman Varghese 1 , Craig Grimes 1
1 Electrical Engineering, Penn State University, University Park, Pennsylvania, United States
Show AbstractWe describe the use of highly-ordered transparent TiO2 nanotube arrays in dye-sensitized solar cells (DSCs). Highly-ordered nanotube-arrays of 22 nm pore diameter and 200 nm length were grown perpendicular to a fluorine doped tin oxide coated glass substrate by anodic oxidation of a titanium thin film. After crystallization by an oxygen anneal, the nanotube arrays are treated with TiCl4 to enhance the photogenerated current, then integrated into the DSC structure using a commercially available ruthenium based dye. Although the negative electrode is only 200 nm thick, i.e. a thickness just 2% of the current DSC gold standard of a 10 µm layer of titania nanoparticles, under AM 1.5 illumination the generated photocurrent is 7.87 mA/cm2, with a photocurrent efficiency of 2.9 %. Voltage decay measurements indicate that the highly-ordered TiO2 nanotube-arrays, in comparison to nanoparticulate systems, have superior electron lifetimes and provide excellent pathways for electron percolation. Our results indicate that remarkable solar photoconversion efficiencies may be obtained, possibly much beyond that of the titania nanoparticulate based cells, with an increase of nanotube-array length to several microns.
5:00 PM - FF9.7
Numerical Simulation of Light Propagation Through Highly-Ordered Titania Nanotube Arrays: Dimension Optimization for Improved Photoproperties.
Keat Ong 1 , Oomman Varghese 1 , Gopal Mor 1 , Craig Grimes 1
1 Electrical Engineering, Penn State University, University Park, Pennsylvania, United States
Show AbstractPropagation of electromagnetic waves in the ultraviolet-visible range (300 to 600 nm) through a unique highly-ordered titania nanotube array structure is studied using the electromagnetic computational technique of Finite Difference Time Domain (FDTD). The nanotube arrays are of considerable interest for application to the solar generation of hydrogen by water photolysis, and in the fabrication of highly-efficient dye solar cells when used on the negative electrode. For example, 6.2 µm thick TiO2 nanotube-arrays, made by anodization of a thick-film titanium foil generate hydrogen by water photoelectrolysis with a photoconversion efficiency of over 12.25% under 320 nm – 400 nm illumination (O. K. Varghese, et al., J. Nanosci. Nanotech. 2005, 5, 1158-1165). The highly-ordered architecture allows for improved charge separation and charge transport, with a calculated quantum efficiency of over 80% for incident photons with energies larger than the titania bandgap. Through numerical simulation the transmittance, reflectance and absorbance of the nanotube-arrays are obtained as a function of tube length and diameter. The nanotube-arrays are found to completely absorb light having wavelengths less than approximately 330 nm. For wavelengths above 380 nm absorption increases as a function of nanotube length, while above 435 nm absorption increases with decreasing pore size. The computational simulations, which could be readily extended to other ordered geometries, closely match experimental measurements, indicating the suitability of the computational technique for helping to understand experimental results as well as guide material optimization.
Symposium Organizers
Paul Alivisatos University of California-Berkeley
Nathan S. Lewis California Institute of Technology
Arthur J. Nozik National Renewable Energy Laboratory
Michael R. Wasielewski Northwestern University
FF10: Photoelectrolysis and Photocatalysis
Session Chairs
Friday AM, April 21, 2006
Room 2008 (Moscone West)
9:15 AM - FF10.2
A Novel Route to Synthesize Rutile TiO2 Nanorods with High Photocatalytic Activity.
Edson Leite 1 , Rafael Libanori 1 , Cristiane Vila 1 , Caue Ribeiro 1 , Paulo Bueno 2 , Elson Longo 2
1 Chemistry, Universidade Federal Sao Carlos, Sao Carlos, SP, Brazil, 2 Chemisrty, Universidade Estadual Paulista, Araraquara, SP, Brazil
Show AbstractTitania, or titanium oxide (TiO2), is a functional material for which there are several technological applications strongly related to its crystalline structure and nanocrystal size and morphology. Each phase of TiO2 displays different physical properties having different functionalities. Thus, from the technological and scientific standpoint, it is important to develop new routes of synthesis whereby the crystalline structure and the size and shape of TiO2 nanocrystals can be tailored. In this work, we described an alternative route to process rutile TiO2 nanorods with very high photocatalytic activity. Colloidal dispersion of rutile TiO2 nanorods showed an improved photo-degradation of organic dye (Rhodamine B) when compared with nanocrystals of anatase and anatase+rutile TiO2(P-25 Degusa commercial powder). The synthetic route used was based on the decomposition of a peroxo complex of titanium under hydrothermal condition. The high-resolution transmission electron microscopy results presented in this work clearly demonstrate that the anisotropic morphology reported for rutile TiO2 nanocrystals is related to the oriented attachment mechanism. The high photocatalytic activity found for the rutile TiO2 nanorods could be an indicative this material can be suitable for the development of solar cell devices based on the Graetzel cell.
9:30 AM - **FF10.3
Molecular Chemistry of Consequence to Renewable Energy
Daniel Nocera 1
1 Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractEnergy conversion cycles are aimed at driving unfavorable, small-molecule activation reactions with a photon harnessed directly by a transition metal catalyst or indirectly by a transition metal catalyst at the surface of a photovoltaic cell. The construction of such cycles confronts daunting challenges because they rely on chemical transformations not understood at the most basic levels. These transformations include multielectron transfer/reactivity, proton-coupled electron transfer and bond-breaking and -making reactions of energy poor substrates. We have begun to explore these poorly understood areas of molecular science with transition metal complexes that promote hydrogen and oxygen production from the water splitting reaction. Both hydrogen and oxygen-producing reactions will be presented in this seminar.
10:00 AM - **FF10.4
A Combinatorial Approach to the Discovery of Metal Oxides that Can Photoelectrolyze Water.
Bruce Parkinson 1 , Micheal Woodhouse 1 , Michelle Romanishan 1
1 Department of Chemistry, Colorado State University, Fort Collins, Colorado, United States
Show AbstractA new simple and high throughput combinatorial method to search for materials capable of the photoelectrolysis of water is presented. Ink jet printing is used to pattern metal oxide precursors (simple nitrate salts, sol gel solutions, metal oxide nanoparticles etc.) onto conductive glass substrates. Subsequent pyrolysis yields electrodes with patterns of metal oxide compositions that, when immersed in an electrolyte, are easily screened for photoelectrolysis activity using a simple scanning laser system. Some promising metal oxide compositions were identified using this approach.
10:45 AM - FF10.6
Photocatalytic Water Splitting on RuO2-dispersed p-type GaN with d10- Electronic Configuration.
nobuo saito 1 , Naoki Arai 1 , Hiroshi Nishiyama 1 , Hisayoshi Kobayashi 2 , Kazunari Domen 3 , Yasunobu Inoue 1
1 Department of Chemistry, Nagaoka University of Technology, Nagaoka Japan, 2 Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Kyoto Japan, 3 Department of Chemical System Engineering, School of Engnering, The University of Tokyo, Tokyo Japan
Show AbstractIn view of current interest of solar energy conversion to hydrogen, the development of new kinds of the photocatalysts for overall water splitting is one of the important issues. For conventional (in the past three decades) transition metal oxides involving Ti4+, Zr4+, Ta5+ and Nb5+ ions with d0 electronic configuration, we have newly employed typical metal oxides involving Ga3+, In3+, Ge4+, Sn4+ and Sb5+ ions with d10 electronic configuration and found that typical metal oxides, MGa2O4 (M=Sr,Ba,Zn), MIn2O4(M=Ca,Sr), Zn2GeO4, M2SnO4 (M=Ca,Sr), NaSbO3 and M2Sb2O7 (M=Ca,Sr), became strong photocatalysts for water splitting when RuO2 was added as a co-catalyst. From a viewpoint of electronic structure, the DFT calculation for the p-block metal oxides showed that the top of valence band (HOMO) was composed of O2p orbitals, whereas the bottom of conduction band (LUMO) consisted of hybridized sp orbitals with large dispersion. It was demonstrated that the largely-dispersed conduction band in the p-block metal oxides with d10 configuration produced a high mobility of photoexcited electrons and hence was responsible for high photocatalytic performance. In an attempt to design a photocatalyst working in a visible light regime, the band narrowing is achieved by raising the valence band level, for which p-block metal nitride is promising. According to this thought, the photocatalytic activity of GaN for water splitting was investigated. Zn, Mg or Be doped p-type GaN powder was prepared by an ammonolysis method. The doped GaN powder was impregnated up to incipient wetness with a ruthenium carbonyl complex, Ru3 (CO) 12, in THF, and oxidized at moderate temperatures in air to convert the complex to RuO2. A little photocatalytic activity was observed for non-doped GaN under irradiation by a 200W He-Xe or 450W high pressure mercury lamp, but we have discovered that the metal ion-doped p-GaN became strong photocatalysts that were able to decompose water to hydrogen and oxygen with a stoichiometric ratio. The first runs showed a lower production of oxygen, compared to that of hydrogen, but with repeated run, the photocatlaytic performance was improved, and after 4th run, the production of hydrogen and oxygen reached the highest level. The activity was quite stable. The absorption occurred at around 400 nm, increased with decreasing wavelength, and leveled off at around 380 nm. There were no significant differences in absorption characteristics between non-doped and doped p-GaN. By excitaiton at 330 nm light, strong photoluminescence was observed at 450 nm for Mg-doped GaN. The present results indicate that typical metal nitrides with d10 electronic configuration become a new series of the photocatalysts for water splitting.This work was supported by CREST and SORST, JST.
11:00 AM - FF10.7
Photoelectrochemical Studies of Semiconducting Potassium Tantalate Photoanodes for Hydrogen Production via Water Dissociation.
Irene Paulauskas 1 , Jordan Katz 3 , Lynn Boatner 2 , Nathan Lewis 3 , Raymond Buchanan 1
1 Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States, 3 Chemistry, California Institute of Technology, Pasadena, California, United States, 2 Condensed Matter Sciences Division, Oak Ridge National Laboratory, Knoxville, Tennessee, United States
Show AbstractHydrogen and oxygen production from the photo-assisted dissociation of water provides a mechanism for the direct generation of hydrogen fuel by utilizing solar energy. Although this photoelectrochemical process has been intensively studied in the past, serious fundamental problems still exist that are associated with the cell photoanodes. These problems include: finding materials with an appropriate band gap that matches the solar spectrum, materials with band edges located at potentials that allow for the evolution of H2 and O2, and materials that exhibit a high level of chemical stability against photoanodic dissolution. These combined problems need to be resolved in order to obtain a practical and efficient photoelectrochemical device for hydrogen generation. Pure potassium tantalate (KTaO3) is a highly resistive (>1016 ohm-cm), cubic perovskite crystal with a band gap of ~3.8 eV. When doped with impurities such as calcium or barium the KTaO3 resistivity can be decreased to ~0.05 ohm-cm. This increase in conductivity is generally attributed to the replacement of K+ sites by Ca2+ (or Ba2+) thereby producing an n-type semiconducting oxide. In the present study, doped KTaO3 single crystals were grown using a technique of solidification from a nonstoichiometric melt. Predetermined levels of CaCO3 or BaCO3 were added to stoichiometric ratios of Ta2O5 and K2CO3 in the crystal growth charge in order to produce the semiconducting material. The photoelectrochemical measurements included obtaining the action spectra of the doped electrodes, current-voltage (IV) curves in the dark and under two different sources of illumination (including a solar simulator), as well as electrochemical impedance spectroscopy (used for Mott-Schottky analysis). The electrolyte used was 8.5M NaOH at room temperature, and Hall Effect measurements were also performed. The results show that Ca- and Ba-doped KTaO3 crystals absorb light only in the UV range with a peak absorption at ~260 nm. While a strong correlation between the dopant and the semiconducting properties of KTaO3 was not observed, the IV-curve measurements showed that, under simulated solar illumination, a photocurrent was observed for all of the doped crystals. Additionally, the tantalates were shown to be chemically stable and thermodynamically capable of H2 and O2 evolution. Current work is focused on finding a proper dopant (or combination of impurities) such as TiO2, V2O5, etc; and/or heat treatments under reducing environments in order to alter the bandgap and sensitize KTaO3 in order to produce an efficient response to solar light.
11:15 AM - FF10.8
Understanding the Microstructural Parameters Responsible the High Performance of Novel Sol-Gel Mesoporous Tungsten Trioxide Films for the Photoelectrocatalytic Splitting of Water
Vittorio Luca 1 , Bin Yang 1
1 Institute of Materials and Engineering Sciences, ANSTO, Lucas Heights, New South Wales, Australia
Show AbstractFF11: Photovoltaic and Nano Materials I
Session Chairs
Friday PM, April 21, 2006
Room 2008 (Moscone West)
11:30 AM - **FF11.1
Optical Studies of Semiconductor Nanoparticles and Nanotubes
Tony Heinz 1
1 Depts. of Physics and Electrical Engineering, Columbia University, New York, New York, United States
Show AbstractOptical spectroscopy permits the electronic properties of nanostructures to be examined without the complication of contacts. In this paper, we review recent progress in applying optical spectroscopy based on ultrafast laser techniques to probe the electron dynamics and transport properties of semiconductor nanostructures. In addition to conventional pump-probe techniques, we will describe recent experiments in which THz time-domain techniques have been applied to examine the conductivity of semiconductor nanoparticles and carbon nanotubes. New optical methods with sufficient sensitivity to probe individual nanostructures, such as single-wall carbon nanotubes, will also be described and applications of these approaches presented. Such studies provide fundamental information needed to assess the potential of novel nanostructures for applications in solar energy conversion.
12:00 PM - FF11.2
Characterization of Cubic Mesoporous TiO2 Thin Films by Spectroscopic Ellipsometric Porosimetry.
Adrien Darragon 1 , Jean Philippe Piel 1 , Yann Turcant 1 , Olivier Condat 1 , Patrice Heinrich 1
1 , SOPRA, Bois-colombes France
Show AbstractCrystalline TiO2 (Anatase configuration) thin films is widely used in the photocatalysis and photovoltaic industries (self cleaning surface and renewable energies for example) . The synthesis of these films is obtained from a dispersed solution of primary particles mixed with a solvent using Sol-Gel processes. It allows the introduction of organic molecules inside inorganic network to create what is called hybrid materials. Spectroscopic ellipsometry is the technique of choice to characterise thickness and indices of these thin layers. The adsorption of vapor of organic solvents at room temperature by these porous materials is used to modify the refractive index of the layer. The change in refractive index induced by the introduction of solvent is measured by ellipsometry and calculated following the Lorentz Lorenz effective medium model is used to calculated the volume of solvent adsorbed by the pores at different pressures. Ellipsometric porosimetry (EP) measurements become an effective method for characterization of porosity, pore size distribution (PSD) and Young modulus of porous films. EP gives accurately the pore size distribution and the porosity of the layer.In this paper, a detailed description of the ellipsometer will be exposed and several examples will be presented for micro-porous layers (with pore size lower than 1nm in radius) using the Dubinin Radushkevitch formalism and meso-porous layers (with pore size larger than 1nm) using the Kelvin formalism. The porosity of the layer ranges from few percent up to 40%
12:15 PM - FF11.3
GaAs Nanocrystal Surface Chemistry.
Matthew Traub 1 , Nathan Lewis 1
1 Chemistry, Caltech, Pasadena, California, United States
Show AbstractAlthough GaAs has been used as a material in some of the most efficient solar cells fabricated, its cost has prevented it from competing with Si in photovoltaic devices. GaAs nanocrystals provide a potential opportunity for great improvements in efficiency and cost, but to take advantage of them, effective methods are needed for both passivating nanocrystal surfaces and electronically coupling them to other device components. Developing these methods requires a careful study of the fundamental surface chemistry of GaAs surfaces.Using an established chemical synthesis, we have synthesized GaAs nanocrystals capped with an overlayer of oxide. The nanocrystals were then treated with a 1:1 solution of concentrated aqueous HCl and water, an etching solution known to produce flat, well-ordered, Cl-terminated surfaces on GaAs(111)A single crystals. X-ray photoelectron spectroscopy (XPS) confirmed the presence of Cl on the nanocrystals after treatment, as well the removal of surface oxides. Powder x-ray diffraction showed that the average particle size was reduced by the acid solution, confirming that the HCl solution etched the particles as well as the oxide overlayer.Cl-terminated GaAs nanocrystals provide a platform for reactions with functional groups containing donor moieties such as thiols. The reactions of several different functional groups with the Cl-terminated nanocrystals will be discussed. The chemistry of the functionalized nanocrystal surfaces is characterized by XPS. Data on the luminescent properties of the Cl-terminated and functionalized GaAs nanocrystals will also be presented.
12:30 PM - FF11.4
Structure and Optical Property of Modified Titanate Nanotubes
Yuanyuan Jia 1 , Alfred Kleinhammes 1 , Harsha Kulkarni 1 , Yue Wu 1
1 , University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Show AbstractTitanate nanotubes are tested as substrates for solar cells. In comparison with TiO2 nano-crystals the titanate nanotubes offer large surface area and open structure well suited to surface modification with small organic molecules like catechol. The ensuing compound shows optical adsorption in the red and appears stable in water. X-ray, Raman and NMR results will be presented as well as photoelectrical measurements.
FF12: Photovoltaic and Nano Materials II
Session Chairs
Friday PM, April 21, 2006
Room 2008 (Moscone West)
2:30 PM - FF12.1
High Yield Synthesis of Boron-Doped Single-Walled Nanotubes by Pulsed Laser Vaporization and Their Optical and Vibrational Properties
Jeff Blackburn 1 , Timothy McDonald 1 , Thomas Gennett 2 1 , Yanfa Yan 1 , Kim Jones 1 , Chaiwat Engtrakul 1 , Kelly Knutsen 1 , Randy Ellingson 1 , Anne Dillon 1 , Michael Heben 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , Rochester Institute of Technology, Rochester, New York, United States
Show AbstractControl of the electronic properties of carbon single-wall nanotubes (SWNTs) is highly desirable for a variety of applications. Specifically, the ability to produce large quantities of highly pure n- and p-type nanotubes should allow the fabrication of sensors, hydrogen storage media, and optoelectronic devices such as solar cells. SWNTs doped with boron are conceptually appealing since boron should substitute in the carbon lattice without the generation of bonding defects. To date, however, attempts to produce high quality boron-doped SWNTs (B-SWNTs) by laser vaporization have been unsuccessful. Also lacking in the literature are comprehensive reports on the effects that boron doping has on the optical and vibrational properties of SWNTs. To this end, we have recently developed the first synthetic procedure for the high yield synthesis of B-SWNTs by pulsed laser vaporization. Transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS) confirm boron incorporation as sp2 bonded species at doping levels of 1 – 2% in the laser-produced B-SWNTs. Boron doping leads to enhanced solubility relative to pure carbon SWNTs in colloidal solutions, allowing for a detailed analysis of solvated B-SWNTs with steady state and time-resolved spectroscopies. We find that boron incorporation into the SWNT lattice modifies the Raman vibrational modes, especially for metallic nanotubes, inducing a Breit-Wigner-Fanno lineshape below the G-band even for de-bundled SWNTs.1 The absorption cross-section and photoluminescence quantum yield of the semiconducting B-SWNTs are also modified. These results shed light on how boron controls the SWNT electronic structure and modifies the electron-phonon coupling, and highlight the great potential of these materials for a variety of adsorption and optoelectronic applications.(1)Paillet, M.; Poncharal, P.; Zahab, A.; Sauvajol, J. L.; Meyer, J. C.; Roth, S. Physical Review Letters 2005, 94, 237401.
2:45 PM - FF12.2
Novel Light Absorbers for Thin Film Photovoltaics Containing only Abundant and Inexpensive Elements.
Joel Haber 1 , Nathan Gerein 1 , Timothy Hatchard 1 , Matthieu Versavel 1
1 Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada
Show Abstract3:00 PM - FF12.3
Effects of Mg-doping on the Electrical and Optical Properties of InN.
Rebecca Jones 1 2 , Kin Yu 1 , Sonny Li 1 2 , Wladek Walukiewicz 1 , Joel Ager 1 , Eugene Haller 1 2 , Hai Lu 3 , William Schaff 3
1 Materials Sciences, Lawrence Berkeley National Lab, Berkeley, California, United States, 2 Materials Science and Engineering, UC-Berkeley, Berkeley, California, United States, 3 , Cornell University, Ithaca, New York, United States
Show AbstractIn order to attain high efficiency, “third-generation” photovoltaic devices, new materials need to be developed. The InGaN alloy system has been hailed as a semiconductor for high-efficiency, multijunction solar cells, because the bandgap range of the alloy system (0.7-3.4 eV) is a near match to the energies of the solar spectrum. Thus, the active layers of a multijunction cell with the optimal bandgaps could be formed entirely from different compositions of the ternary alloy, while simplifying the deposition process and minimizing the problem of interdiffusion and doping between the individual layers.One major barrier to the realization of InGaN solar cells is the difficulty in doping In-rich InGaN p-type. Undoped, In-rich InGaN films are n-type, most likely due to a high concentration of native donor defects [1]. This tendency increases with increasing In content; InN is the most extreme case. In addition, all In1-xGaxN films with x < ~0.67 exhibit an accumulation of electrons at the surface, due to pinning of the surface Fermi level high in the conduction band. In this work, we show evidence of p-type doping in Mg-doped InN by isolating the effects of the n-type surface layer and manipulating the bulk conductivity through heavy ion irradiation. Electrolyte-based capacitance-voltage measurements strongly suggest a concentration of ~1019 cm-3 ionized acceptors in Mg-doped InN beneath the n-type surface layer. In contrast, Hall effect measurements indicate only n-type conductivity with low free electron concentrations (~1018 cm-3) and low mobility (< 100 cm2/V s). Theoretical analysis of the electron concentration and mobility in Mg-doped films subjected to varying doses of 2 MeV He+-irradiation, which has been shown previously to introduce donor defects [1], is consistent with the conjecture of a two-layer parallel transport model: a thin surface n-type layer and a p-type bulk layer that converts to n-type with irradiation. This two-layer model is invoked to successfully explain the unusual Hall effect measurements in as-grown InN:Mg, as well as the recovery of photoluminescence in irradiated films.These results demonstrate that Mg atoms are present as acceptors in InN. However, the acceptor binding energy and the concentration of free holes, which ultimately will determine the effectiveness of Mg-doping, still need to be established. This will be a significant challenge if techniques cannot be developed to passivate the n-type surface layer.[1] S.X. Li et al., Phys. Rev. B 71, 161201(R), (2005).
3:15 PM - FF12.4
Synthesis and Characterization of Bulk and Thin Film Silicon and Germanium Based Clathrate Materials.
Matt Beekman 1 , R. Hyde 1 , H. Nagaraja 1 , P. Mukherjee 1 , S. Witanachchi 1 , G. Nolas 1
1 Department of Physics, University of South Florida, Tampa, Florida, United States
Show AbstractClathrates comprise a class of materials in which a guest species can be encapsulated inside of the crystallographic “cages” formed by a host framework. As a result of the unique interaction between the guest and framework atoms, clathrates show a wide variation in physical properties, from superconductors to semiconductors, from insulators to metals. These materials are currently being investigated by the authors in the search for novel thermoelectric and photovoltaic materials. The intrinsically low thermal conductivity in these crystalline materials combined with good semiconducting electrical properties has promoted some clathrate materials to the forefront of thermoelectric materials research. Other clathrate structures formed by silicon possess wide optical band gaps, of interest for optoelectronic and photovoltaic applications. This is of particular interest for integration into current silicon based technologies. We present recent results in the growth and characterization of type I and type II clathrates in bulk and thin film form. The electrical and thermal transport properties of NaxSi136 (0 < x < 24) clathrates show a clear dependence on Na content, and include resistivities that span several orders of magnitude together with very low thermal conductivities. Using laser ablation, we have also successfully produced clathrate films for the first time, indicating these materials could find wider application in technologies such as photovoltaics. The prospects clathrate materials have for thermoelectric and photovoltaic applications will also be considered.
3:30 PM - FF12.5
Epitaxial Growth of Highly Mismatched III-V Compound Semiconductors using Liquid Phase Epitaxy
Anika Kumar 1 , Ashutosh Dighe 2 , Partha Dutta 1
1 ECSE, Rensselaer Polytechnic Institute, Troy , New York, United States, 2 , Micron Technology, Inc. , Boise, Idaho, United States
Show Abstract3:45 PM - FF12.6
Optical Properties of Nanoporous Semiconductor Metamaterials
Iris Nandhakumar 1 , Jeremy Baumberg 1 , David Smith 1 , Matthew Markham 1 , Xiahong Li 1 , Timothy Gabriel 1 , George Attard 1
1 School of Chemistry, University of Southampton, Southampton United Kingdom
Show AbstractThree dimensional nanostructuring of materials is a clear route to produce metamaterials with novel and optimised functional properties. In particular nanostructured semiconducting materials with one or more critical dimension in the nanometer range possess novel electronic and optical properties in comparison to their non-mesoporous counterparts. Such materials are of considerable fundamental and technological interest for understanding the fundamental physics of low-dimensionality structures and to applications in sensors, optical devices and solar cells because they are entirely composed of surface atoms. As a consequence these materials should exhibit unusual charge carrier dynamics, improved collection of the photogenerated carriers and hence solar conversion efficiencies. The semiconducting metamaterials presented in this paper were fabricated by direct liquid crystal templating from non-ionic polyoxyethylene surfactants which were utilized as nanoscale moulds around which a solid thin film of the material was formed by electrochemical deposition. This resulted in well-defined birefringent films of nanostructured cadmium telluride1,2, lead telluride3 and zinc oxide which displayed good optical properties as evidenced by UV-VIS reflectance spectroscopy. Experimental and theoretical investigation of the optical and optoelectronic properties of these nanoporous metamaterials are presented and in particular a theorertical model which fully explains the nanostructuring induced birefringence in these films will be presented. We will also present the results of photocurrent spectroscopy and electrical impedance measurements which allow us to quantify charge transport within the films and the density and nature of surface states.(1)I. Nandhakumar et al. Chem. Comm. 12 (2004) 1374 (2)T.Gabriel et al., Appl. Phys.Lett. 86, 011912 (2005)(3)I. Nandhakumar et. al., Chem. Mat. (submitted)
4:30 PM - FF12.7
Properties of Aluminum Doped ZnO Grown by Atmospheric Plasma Enhanced Chemical Vapor Deposition
Michael Barankin 1 , Robert Hicks 1
1 Chemical and Biomolecular Engineering, University of California, Los Angeles, Los Angeles, California, United States
Show AbstractZinc Oxide is a promising material for the transparent conducting oxide (TCO) layer of thin-film photovoltaic cells, owing to its high conductivity and low optical absorption coefficient for wavelengths in the visible range (400 – 800 nm). Recent progress in photovoltaic technology, however, dictates a strong preference for low temperature growth processes (Ts < 200oC). In this report we will demonstrate the feasibility of high-speed (ca. 100 nm/min), low temperature (Ts = 100oC) growth of Al/ZnO using Atmospheric PECVD. Undoped ZnO films, deposited thus far, have exhibited resistivity values on the order of 10-2 Ω-cm with an average value of 90% transparency to light in the visible range. The growth rate and material properties (light absorption coefficient, % transmission, sheet resistance, film resistivity) are investigated as a function of the various process parameters. It is demonstrated that the resistance of the material may be manually tuned by modifying the film thickness and dopant concentration, so as to maximize conductivity while maintaining a high short-circuit current value (upon implementation into a photovoltaic device).
4:45 PM - FF12.8
Mayer-Neldel Rule and the Influence of Entropy on Capture Cross-section Determination in Cu(In,Ga)Se2.
Jehad AbuShama 1 , Steve Johnston 2 , Richard Crandell 2 , Rommel Noufi 2
1 , DayStar Technologies, HalfMoon, New York, United States, 2 , NREL, Golden, Colorado, United States
Show AbstractWe use deep-level transient spectroscopy (DLTS) to invetigate the electronic properties of ZnO/CdS/CuxIn1-yGaySe2/Mo/soda-lime glass thin-film solar cells. We deposited films with different x and y values using elemental evaporation. Devices made from these films exhibit more than 40 hole and electron traps, with activation energies ranging from 0.07 to 1.17 eV. Standard DLTS analysis (which assumes that entropy changes can be neglected) gives apparant capture cross sections that vary by 18 orders of magnitude over this energy range. All our data show that the charge-carrier emission rate obeys the Mayer-Neldel rule (MNR) with an isokinatic temperature of 340+-30K. By including the MNR relation in detailed balance, we show that there is a single cross section for all traps.
5:00 PM - FF12.9
High Resistivity and Transparency GaSb Substrates
Rajanish Bhat 1 , Partha Dutta 1
1 Electrical,Computer and System, Rensselaer Polytechnic Institute, Troy, New York, United States
Show Abstract5:15 PM - FF12.10
Materials Requirements for Efficient Thermally Assisted Luminescence Upconversion in Photovoltaic Applications.
Bauke Heeg 1 , Benjamin Buckner 1
1 , MetroLaser Inc., Irvine, California, United States
Show AbstractIt has been suggested that either electro-luminescence or photo-luminescence upconversion may provide an efficient means to convert (solar) heat into electricity. The basic concept is to use an intermediate active emitter between the heat source and a photovoltaic (PV) cell. The active emitter is pumped by an injection current or external light source, such that the emitted photon energy is larger than the energy required to create an emitting state. In that case, the emitter can act as a heat pump, converting broadband into narrowband radiation that can be efficiently converted into a photocurrent using a single stage PV with a matching bandgap energy. In order to make such a process efficient, however, several materials challenges will need to be addressed and overcome. Here, we analyze the efficiency of the upconversion process with realistic material input parameters over a wide temperature range. These include amongst others radiative and non-radiative rates, sub-bandgap spectral absorption coefficients, bandgap energy, escape efficiency and spectral luminescence profile. Our main focus is on GaAs, while the potential of several other selective emitters will be discussed as well.
5:30 PM - FF12.11
Chemical Bath Deposited CdS as Passivating Layer for GaSb.
Ashonita Chavan 1 , Partha Dutta 1 2
1 Electrical, Computer and Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, New York, United States
Show Abstract