Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session D29: Physics and Materials for Inorganic Photovoltaics: II |
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Sponsoring Units: DMP GERA Chair: Jeffrey Neaton, Lawrence Berkeley National Laboratory Room: C123 |
Monday, March 15, 2010 2:30PM - 2:42PM |
D29.00001: Graphene as a transparent conducting electrode for photovoltaics Razvan Nistor, Marcelo Kuroda, Ahmed Maarouf, Dennis Newns, Glenn Martyna Current photovoltaic techonologies rely on expensive oxide films as transparent conducting electrodes (TCEs). With less than two percent absorption per layer and a high carrier mobility, graphene stands as a prominent candidate for TCEs. In this work, we investigate the structural and electronic properties of several doped graphene layers using density functional theory. Of particular interest is the interface between graphene and the semiconducting material of the solar cell, and the effect that various dopants have on the electronic properties of the system. Our aim is to control and quantify the amount of charge transfer to the graphene layers from the dopant molecules. These results can aid the design of carbon based TCEs that have minimum contact barriers with the semiconducting surface and low overall sheet resistances. [Preview Abstract] |
Monday, March 15, 2010 2:42PM - 2:54PM |
D29.00002: Searching for new transparent conducting oxides( TCO) for energy applications within the A$_2$BO$_4$ family using defect calculations for A=Co, Rh, Ir and B=Mg, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr and Cd Tula R. Paudel , Stephan Lany, Alex Zunger TCO's manifest the unlikely coexistence of optical transparency with electric conductivity and require that electron or hole dopants will not instigate the spontaneous production of defect centres that quench electrons (e.g cation vacancy) or holes ( e.g anion vacancy ), respectively. We use first principle based methods to screen and design effective hole-TCO's in A$_2$BO$_4$ by predicting the concentration of free holes as well as hole-compensating centres as a function of chemical potential and temperature. We search for the ``design principles'' that control hole production and survival in such materials. This involves calculation of the energy of formation for large number of the defects and dopant as well as the transition levels, which in turn allows us to calculate the career concentration as a function of Fermi energy for a given material. Provided that the data for the group of materials, we would be able to suggest the few best candidates for the p-type TCO in this class of materials. [Preview Abstract] |
Monday, March 15, 2010 2:54PM - 3:06PM |
D29.00003: Surface dependent band alignments for chalcopyrite-ZnO interface Christian Pettenkofer, Andreas Hofmann Chalcopyrites are promising absorber materials for thin film solar cells. A Buffer of CdS is technologically used between Absorber and TCO window layer. To study the junction properties investigations on well defined model systems were performed. CuInS$_{\mbox{e2}}$ (112) and (001) surfaces were prepared by MBE and investigated by UPS, XPS, LEED and PEEM with respect to junction properties to ZnO. ZnO as TCO is deposited by MOMBE in situ without breaking UHV conditions between preparation, transfer and analysis. Instead of an abrupt CuInS$_{\mbox{e2}}$-ZnO interface a intermediate ultra thin buffer layer of epitaxial ZnSe is formed on the chalcopyrite substrate despite of the admittance of the oxidizing agent in the MOMBE process. On top of ZnSe a ZnO film growth is observed. ZnO grows in registry with the ZnSe with its own lattice parameters. Annealing to temperatures above the TCO deposition temperature show In diffusion into the ZnO layer forming an oxide as derived from the Auger-parameter. Energy filtered PEEM reveals an inhomogenity of the annealed interface with local In enrichment in the ZnO film. Band alignments determined from our data for the CuInSe2-ZnSe-ZnO junction are beneficial for an application in solar cells. [Preview Abstract] |
Monday, March 15, 2010 3:06PM - 3:42PM |
D29.00004: Exploring beneficial properties of defects in Earth-abundant solar cell materials Invited Speaker: |
Monday, March 15, 2010 3:42PM - 3:54PM |
D29.00005: Density Functional Theory Calculations of the Role of Defects in Amorphous Silicon Solar Cells Eric Johlin, Lucas Wagner, Tonio Buonassisi, Jeffrey C. Grossman Amorphous silicon holds promise as a cheap and efficient material for thin-film photovoltaic devices. However, current device efficiencies are severely limited by the low mobility of holes in the bulk amorphous silicon material, the cause of which is not yet fully understood. This work employs a statistical analysis of density functional theory calculations to uncover the implications of a range of defects (including internal strain and substitution impurities) on the trapping and mobility of holes, and thereby also on the total conversion efficiency. We investigate the root causes of this low mobility and attempt to provide suggestions for simple methods of improving this property. [Preview Abstract] |
Monday, March 15, 2010 3:54PM - 4:06PM |
D29.00006: Probing the Electronic Structure of Chalcogen Ultra-doped Silicon Joseph T. Sullivan, Bonna Newman, Tonio Buonassisi Chalcogen impurity atoms in silicon have been shown to introduce deep energy levels within the bandgap. Absorption coupled to these defect levels could account for the enhanced sub-bandgap absorption. We will report on the methods and preliminary results of measuring the absorption coefficient in the isolated S-doped layer. Additionally, we use x-ray spectroscopy to probe the electronic properties of the material, and to understand the specific role that dopant states play in enhanced absorption. [Preview Abstract] |
Monday, March 15, 2010 4:06PM - 4:18PM |
D29.00007: Photonic crystals for enhanced absorption in thin film solar materials Rana Biswas, Benjamin Curtin, Weijun Zhao, Vikram Dalal Photonic crystals are designed as back reflectors for thin film amorphous silicon cells. Optimum absorption of red and near infrared photons are found using rigorous scattering matrix simulation. Using photo-lithography and reactive ion etching these photonic-plamsonic crystal substrates have been fabricated and solar cells have been grown on top of them. Experimental solar devices demonstrate 7-8{\%} improved absorption with superior harvesting of red and near infrared photons. Comparison with random roughened back reflectors and plasmonic schemes using metallic nanoparticles will be made. [Preview Abstract] |
Monday, March 15, 2010 4:18PM - 4:54PM |
D29.00008: Inorganic Alloys and Nanostructures for Photovoltaics Invited Speaker: Semiconductor alloys and nanostructures are often used as photovoltaics materials and building blocks. In this talk, I will present results using large scale ab initio calculations to study the electronic structures and optical properties of semiconductor alloys and nanosystems. First, I will discuss the results of GaN:ZnO alloy, its atomic structures and electronic properties. GaN:ZnO is an unconventional locally nonstoichiometric alloy. A special model Hamiltonian is developed to describe its configuration total energy and to study its atomic structure. We found that GaN:ZnO forms a homongeneous alloy with strong short range order at high temperature. Such short range order has big effect on its band structure and band edge electron states. The second system to be discussed is ZnTe:O. In this alloy, the O atoms induce an intermediate band within the ZnTe band gap. This can be used for intermediate band solar cell. The change of this intermediate band as a function of the O centration will be discussed. The theoretical solar cell efficiency calculated based on the optical absorption spectra between different bands is 63\%. The other aspects of the material in order to reach such high efficiency will be discussed. Finally, I will present results of exciton binding energies in heterojunction nanostructures (e.g., nanorods or wires), especially for CdSe/CdTe and ZnO/ZnS nanorods. We found large exciton binding energies in such nanostructures at the interface. The implication of such large exciton binding energy and its effect on solar cell efficiency will be discussed. [Preview Abstract] |
Monday, March 15, 2010 4:54PM - 5:06PM |
D29.00009: Electronic Level Alignment in Multicomponent~CdSe/CdTe Nanostructures from First Principles Shenyuan Yang, David Prendergast, Jeffrey Neaton Inorganic CdSe/CdTe nanorod heterojunctions, with type-II level alignment and band gaps in the solar spectrum, comprise ideal model components of nanostructure-based solar cells. Here we perform density functional theory calculations on CdSe/CdTe nanowire heterojunctions, exploring how the electronic properties of their nanoscale interfaces are affected by quantum confinement, and mechanical and electrical boundary conditions. Many-body perturbation theory within the GW approximation is used to predict quantitative bulk band gaps and infer bulk level alignment. We find that band offsets at bulk epitaxial interfaces are quite sensitive to biaxial strain due to lattice mismatch. The computed band gaps of small linear nanorod heterojunctions increase with decreasing diameter due to quantum confinement, but band offsets are seen to be largely unaffected. In the core/shell nanorod heterojunctions, band offsets are strongly dependent on strain and confinement, both of which can be tuned by the core size and shell thickness. [Preview Abstract] |
Monday, March 15, 2010 5:06PM - 5:18PM |
D29.00010: Band structure investigations of SnZrCh$_{3}$ (Ch=S and Se) by DFT and XPS Annette Richard, Daniel Harada, Andriy Zakutayev, Robert Kykyneshi, Janet Tate, Andreas Klein SnZrS$_{3}$, a p-type semiconductor with a measured band gap of 1.5 eV, is a possible material for a solar cell absorber layer. We report the theoretical investigations into this layered compound structure. Preliminary results show that SnZrS$_{3}$ is p-type semiconductor with an estimated indirect band gap of 0.73eV. The valence band is constructed from sulfur p and tin s-orbitals. Theoretical results are compared to x-ray photoelectron spectroscopy measurements. Correlations to the binary compounds ZrCh$_{2}$ and SnCh will be made and the effects of $p$-type and $n-$type doping in SnZrCh$_{3}$ (Ch= S, Se) will be discussed. [Preview Abstract] |
Monday, March 15, 2010 5:18PM - 5:30PM |
D29.00011: Electronic structure and energy level alignment of zinc and free-base tetraphenylporphyrin derivatives adsorbed on ZnO(11-20) Senia Katalinic, Keyur Chitre, Sylvie Rangan, Elena Galoppini, Robert Bartynski Tetraphenylporphyrin dye molecules are studied extensively as possible candidates for the active layer in dye-sensitized solar cells. Still, many fundamental properties of the dye/metal oxide interface are not known and need careful consideration. Using direct and inverse photoemission we have measured the occupied and unoccupied electronic states as well as the energy level alignment of several zinc and free-base tetraphenylporphyrin derivatives adsorbed on ZnO (11-20). For a full interpretation of the electronic structure, we have compared these measurements to ab-initio calculations. UV-visible absorption properties of these dyes were also obtained. In addition to the spectroscopic surface averaged probes, we have used scanning tunneling microscopy to study local bonding geometries at the surface. Recent results will be presented. [Preview Abstract] |
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