Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session B39: Focus Session: Materials and Applications for Solar Energy I |
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Sponsoring Units: FIAP DMP Chair: Mike McGehee, Stanford University Room: Colorado Convention Center 502 |
Monday, March 5, 2007 11:15AM - 11:51AM |
B39.00001: Multiple Exciton Generation for Highly Efficient Solar Cells Invited Speaker: In order to utilize solar power for the production of electricity and fuel on a massive scale, it will be necessary to develop solar photon conversion systems that have an appropriate combination of high efficiency and low capital cost ({\$}/m$^{2})$. One new potential approach to high solar cell efficiency is to utilize the unique properties of semiconductor quantum dot nanostructures to control the relaxation dynamics of photogenerated carriers to produce either enhanced photocurrent through efficient multiple exciton generation (MEG) or enhanced photopotential through hot electron transport and transfer processes. To achieve these desirable effects it is necessary to understand and control the dynamics of electron relaxation, cooling, multiple exciton generation , transport, and interfacial electron transfer of the photogenerated carriers with fs to ns time resolution. We have been studying these fundamental dynamics in bulk and nanoscale semiconductors (quantum dots, quantum wires, and quantum wells) using femtosecond transient absorption, photoluminescence, and THz spectroscopy. This work will be summarized and recent advances in creating multiple excitons from a single photon will be discussed, including a unique model to explain efficient MEG based on the coherent superposition of multiple excitonic states. Various possible configurations for quantum dot solar cells that could produce ultra-high conversion efficiencies for the production of electricity, as well as for producing solar fuels (for example, hydrogen from water splitting), will be discussed, along with associated thermodynamic calculations that show the increase in the maximum theoretical gain in solar photon conversion efficiency for both electricity and fuel production. [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:03PM |
B39.00002: Optical properties of II-VI structures for solar energy utilization Joshua Schrier, Denis Demchenko, Lin-Wang Wang Although II-VI semiconductor materials are abundant, stable, and have direct band gaps, the band gaps are too large for optimal photovoltaic efficiency. However, staggered band alignments of pairs of these materials, and also the formation of intermediate impurity levels in the band gap (which has been demonstrated to increase the efficiency as compared to both single-junction devices), could be utilized to improve the suitability of these materials for solar energy utilization. Previous theoretical studies of these materials are limited, due to the well-known band gap underestimation by density-functional theory. To calculate the absorption spectra, we utilize a band-corrected planewave pseudopotential approach, which gives agreements of within 0.1 eV of the bulk optical gaps values. In this talk, I will present our work on predicting the optical properties of ZnO/ZnS and ZnO/ZnTe heterostructures, nanostructures, and alloys. This work was supported by U.S. Department of Energy under Contract No.DE-AC02-05CH11231 and used the resources of the National Energy Research Scientific Computing Center. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B39.00003: Local Structures Around S in CdS:O Thin Films Photovoltaic Materials Probed by S K-edge X-ray Absorption Fine Structures Y. L. Soo, W. H. Sun, S. C. Weng, Y. S. Lin, S. L. Chang, L. Y. Jang, X. Wu, Y. Yan Local Structures around S in thin films of CdS:O have been investigated using EXAFS and NEXAFS techniques at the S K absorption edge. Our S K-edge EXAFS results clearly indicate the presence of S-O bonds that coexist with S-Cd bonds in the oxygen-containing samples. The S K-edge NEXAFS data further identify SO$_{3}$ and SO$_{4}$ complexes in the samples. As indicated by our previous results on Cd K-edge EXAFS, Cd atoms are predominantly bonded with S. These x-ray results demonstrate that the oxygen atoms actually combine with S to form SO$_{3}$ and SO$_{4}$ complexes instead of being incorporated into the CdS host. In combination with the evidence of nanoparticles revealed by TEM, our results suggest that oxygen-free CdS nanocrystals are formed in the films due to the O content. The bandgap of the samples is therefore found to increase with O concentration as opposed to the bandgap decrease for O doping expected in the band anticrossing model. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B39.00004: P-type InGaN alloys D.M. Yamaguchi, R.E. Jones, N.R. Miller, E.E. Haller, J.W. Ager, K.M. Yu, W. Walukiewicz, H. Lu, W.J. Schaff We have demonstrated via electrolyte-based capacitance-voltage (CV) measurements that a set of Mg-doped In$_{1-x}$Ga$_{x}$N thin films (x=.05,.30,.33,and .80) exhibit bulk p-type activity. There is a change in the slope of the Mott-Shockley plots of In$_{1-x}$Ga$_{x}$N with x $\le $ .33 which is consistent with p-type bulk material underneath an n-type surface inversion layer. In contrast, CV analysis of Mg-doped In$_{0.2}$Ga$_{0.8}$N indicates p-type activity throughout the film. These results are consistent with surface Fermi level pinning at --4.9 eV with respect to the vacuum level. Based on the known valence band offsets between GaN and InN, a surface inversion layer is predicted for In$_{1-x}$Ga$_{x}$N with x $\le $ .66 and a surface Schottky barrier for x $>$ .66. These results provide the first evidence of p-type doping of InGaN alloys in the whole composition range. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 1:03PM |
B39.00005: Seven Excitons per Single Photon Using Semiconductor Nanocrystals Invited Speaker: The efficient conversion of photon energy into electrical charges is a central goal of much research in physics, chemistry, and biology. A usual assumption is that absorption of a single photon by a material produces a single electron-hole pair (exciton), while the photon energy in excess of the energy gap is dissipated as heat. In 2004, we reported for the first time that nanocrystals (NCs) of PbSe could respond to absorption of a single photon by producing two or more excitons with the unity probability (Phys. Rev. Lett. 92, 186601, 2004). Our more recent findings indicate that this carrier multiplication process can generate multiple charges with quantum efficiencies that correspond to the ultimate limit dictated by energy conservation. For example, for photon energy of 7.8 energy gaps, a maximal possible number of photogenerated excitons based on energy considerations is 7, which is exactly the number measured in our experiments (Nano Lett. 6, 424, 2006). Another unexpected feature of carrier multiplication is that it results in unusual distributions of carrier populations that cannot be described by Poisson statistics. Specifically, by selecting certain photon energies, we obtain photoexcited NC ensembles with nearly pure single multiplicities (i.e., all excited NCs contain the same number of excitons) that can be tuned in the controlled way from 1 to 7 (Phys. Rev. Lett. 96, 097402, 2006). While the exact mechanism for carrier multiplication in NCs is still under debate, one factor, which likely contributes to high efficiencies of this process, is a unique property of the NCs to produce significant carrier-carrier interactions as indicated, e.g., by our previous Auger recombination studies (Science 287, 1011, 2000). This confinement-enhanced Coulomb coupling can lead to the unusual mechanism for direct photogeneration of multiexcitons via virtual single-exciton states, which can explain our observations of very short, sub-200 femtosecond buildup times of multiexciton populations in the regime of carrier multiplication (Nature Phys. 1, 189, 2005). [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B39.00006: Theoretical investigation of vacuum thermionic energy conversion devices for efficient conversion of solar to electrical energy Joshua Smith, Robert Nemanich, Griff Bilbro A vacuum thermionic energy conversion device (TEC) would offer the potential of efficiently converting solar energy directly to electrical work. These devices consist of a heated emitter electrode and a collector electrode separated by an evacuated interelectrode space. Models for such conceptual devices are developed, and efficiency is calculated by considering electron transport across the device as well as Stefan Boltzmann radiation. A device operating with an emitter and collector temperature of $775K$ and $375K$, respectively is considered. The conceptual TEC features diamond materials having low emission barrier heights as electrodes. Hydrogen terminated diamond is known to have a negative electron affinity (NEA) and nitrogen or phosphorus doping introduces donor levels at $~1.7eV$ and $~0.6eV$, respectively, below the conduction band minimum. For the devices considered, the barrier heights are $1.1eV$ and $0.5eV$ for the emitter and collector, respectively. The Richardson constant is $10A/cm^{2}K^{2}$, consistant with experimental results. Assuming an emissivity of 0.5, the device has a Carnot efficiency of 0.52, and a calculated absolute efficiency of 0.17 at a maximum power of $0.25W/cm^{2}$. The theory is extended to include the negative space charge effect, and the NEA properties of the materials are shown to mitigate the space charge effect and increase output power. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B39.00007: Novel photophysics and tandem device designs for solar hydrogen production Justin Johnson, Matt Law, Nathan Neale, Arthur Frank, Josef Michl, Arthur Nozik Solar hydrogen production by water photolysis could provide a means for generating large quantities of clean, transportable fuel cheaply and efficiently for a wide variety of energy uses. Previous schemes of solar hydrogen production have not resulted in a suitable combination of high efficiency, low cost, and good long-term stability to meet requirements for their practical utilization in large-scale energy production. Revolutionary technologies and application of novel photophysical concepts represent a pathway toward overcoming current barriers and achieving an entirely practical method for producing solar fuels. One such concept is the utilization of tandem device designs, which allow for the incorporation of visible/near-IR absorbing materials into the device, thus increasing solar flux harvesting. Moreover, including molecules capable of charge multiplication (multiple electrons/holes per single photon) holds the prospect for additional gains in solar-to-hydrogen efficiencies. Current progress as well as future challenges for developing such devices will be discussed, including simulations, fundamental spectroscopic experiments, and device design and construction. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B39.00008: Optimization of Nanostructured ZnO / Conjugated Polymer Photovoltaic Devices Dana Olson, Yun-Ju Lee, Erik Spoerke, Darren Dunphy, James Voigt, Julia Hsu, Matthew White, Sean Shaheen, David Ginley Nanostructured oxide semiconductor / conjugated polymer composites are promising systems for low cost photovoltaic devices. The use of nanostructures increases the heterojunction areas, resulting in more effective capturing of photogenerated charges. We have fabricated arrays of ZnO nanorods by low-temperature solution growth on patterned ITO substrates. The dense ZnO nanorod arrays are subsequently infiltrated with poly(3-hexylthiophene) (P3HT), and the devices are completed by depositing Ag top electrodes. Depending on the seeding conditions, we can control the alignment of ZnO nanorods on ITO: ordered (aligned perpendicular to the substrate) versus disordered. We will study the effects of nanorod array morphology and growth chemistry, as well as processing conditions used to infiltrate P3HT into the ZnO nanorod arrays. We will also examine surface treatment and modification of ZnO prior to polymer infiltration to enhance electron transfer efficiency at the ZnO/P3HT heterojunctions. Finally, these results are correlated with the device data to observe the effects of ZnO nanorod ordering, interfacial treatment, and the infiltration process on the device performance. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B39.00009: Theoretical Insights on Interfacial Charge Transfer across the P3HT/Fullerene Photovoltaic Heterojunction from ab Initio Calculations Yosuke Kanai, Jeffrey C. Grossman Within the current effort to develop more efficient and less expensive solar cell devices, the polymer/fullerene photovoltaic (PV) structure is considered to be very promising. The crucial component of such a PV structure is the nano-scale heterojunction interface of the polymer and the fullerene. This interface must facilitate the dissociation of the exciton which is formed in the polymer, so that separated charges can be generated across the interface. Our current understanding of the charge separation mechanism at an atomistic level is rather limited, slowing the progress in the structural design of the heterojunction interface. We employ ab initio calculations to investigate and characterize the charge transfer state which is responsible for the charge separation process. Our results elucidate several important phenomena regarding this mechanism, which lies at the heart of higher power conversion efficiency in polymeric solar cell devices. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B39.00010: Hybrid Tandem Solar Cells: CIGS/DSC with Carbon Nanotube Interlayer anvar Zakhidov, William Shafarman, Mei Zhang, Shaoli Fang, Ray Baughman Multi-junction solar cells enable harvesting of wider regions of the solar radiation spectrum leading thereby to increased overall efficiencies. We present here a first study of a hybrid monolithic structure composed of\textit{ dye sensitized solar cells (}DSCs) with thin film inorganic CIGS$.$ We have created several architectures of monolithic multi-junction cells and address fundamental connectivity issues by using sheets of strong, transparent carbon nanotubes (T-CNTs) recently produced at UTD [1] as a uniform interlayer platform. Free-standing T-CNT networks can be laminated onto any surface and their advanteges as transparent interlayers in tandems is shown here for a tandem in which a un-finished CIGS ( top ITO is absent) is coated by T-CNTs. Such CIGS with T-CNT shows Voc=0.6 V and Isc $\sim $ 10 mA/cm2. It has been combined with DSC playing role of a photoactive counter-electrode, with iodine based electrolyte and Ru-dye on TiO2 mesoscopic electrode. The tandem demonstrated Voc= 0.82 V, which is higher than Voc of our sole DSC-CNT and Isc= 1mA/cm2, smaller than photocurrent of single DSC due to unbalanced current. The physics of processes of charge recombination in hybrid tandems is discussed . [1] M. Zhang, S. Fang, A. Zakhidov, S. B. Lee, A. Aliev, R.H. Baughman, \textbf{\textit{Science,}} 309,(2005) 1215 [Preview Abstract] |
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