### Session A29: Physics and Materials for Inorganic Photovoltaics: I

 Monday, March 15, 2010 8:00AM - 8:12AM A29.00001: Optoelectronic characterization of single PbS nanowire field effect transistors Dong Yu , Christopher Miller , Rion Graham , Matthew Caulfield Nanostructures composed of narrow bandgap semiconductors have the potential for highly efficient solar energy conversion. Here we investigated the optoelectronic properties of field effect transistors (FETs) incorporating single PbS nanowires grown by vapor-liquid-solid approach. At high bias voltage, the dark current saturates and the gate voltage can tune its magnitude. A zero-bias current occurs when the metal-nanowire contact is illuminated. The polarity and the bias dependence of the photocurrent are consistent with a downward band bending at the contact. The photocurrent spot is elongated into the nanowire if the energy barriers at the contacts are reduced. A physics model based on the charge drift/diffusion and band bending is proposed to account for these behaviors. Monday, March 15, 2010 8:12AM - 8:24AM A29.00002: Searching for new solar absorber materials for energy applications within the Fe2(IV)VI4 family using electronic structure theory Liping Yu , Stephan Lany , Alex Zunger Not all direct gap semiconductors have sufficiently strong absorption at threshold. New semiconducting absorbers, free of expensive, toxic or rare elements are examined to maximize the absorption intensity at E$_g$+delta for small delta and the band gap E$_g$ in solar range. We search for the design principles'' that control the absorption intensity in the space of chemical composition of Fe$_2$-(IV)-(VI)$_4$ (where IV=Si,Ge,Sn,Ti and VI=S,Se,Te) as well as structural and configuration degrees of freedom (e.g. crystal distortion). We use the tools of electronic structure theory to identify the chemical and structural motifs that enhance absorption. We investigate the chemical trends for the different elemental compositions within this compound family, focusing on the band gap, effective mass, absorption spectra as well as the interplay between magnetic structure and the optical properties. Monday, March 15, 2010 8:24AM - 8:36AM A29.00003: Monitoring Photo-excitation and Electron-Hole Separation in Photovoltaic Materials Yoshiyuki Miyamoto Efficiency of photovoltaic phenomena is governed not only by optical transition but also by separation of excited electrons and holes. However, these two events are in a tradeoff relation. For example, increasing optical transition rate by confining carriers in a quantum dot sacrifices carrier mobility and thus lowers electron-hole separation rate. For optimizing an efficiency of photovoltaic phenomena, theoretical analysis which can simultaneously treat photo-excitation and electron- hole separation are needed, while a conventional theory can merely treat these two events individually. In this presentation, I will introduce an approach of using the time- dependent density functional theory (TDDFT) for treating real- time propagation of electrons [1] under illumination of light which is mimicked by an alternating electric field. Then, photo- induced electron-hole creation and subsequent separation in polar crystallographic direction of 3C-SiC [2] will be demonstrated. In this simulation, the numerical stability was checked by the energy conservation rule [3] throughout the TDDFT simulation. \\[4pt] [1] O. Sugino and Y. Miyamoto, PRB{\bf 59}, 2579 (1999); PRB {\bf 66}, 89901(E) (2002).\\[0pt] [2] Y. Miyamoto, {\it submitted}.\\[0pt] [3] Y. Miyamoto and H. Zhang, PRB{\bf 77}, 165123 (2008). Monday, March 15, 2010 8:36AM - 8:48AM A29.00004: Photovoltaic Properties of TiO2/Cu2O Heterostructure Dongdong Li , Pai-chun Chang , Sheng Chu , C.J. Chien , Jia G. Lu TiO$_{2}$ is an $n$-type semiconductor with a wide band gap energy of 3.2 eV. It has been known for its photo catalytic effect and widely used in commercial products. Particularly in the growing photovoltaic industry, dye-sensitized solar cells (DSC) made by TiO$_{2}$ nanotube electrode have demonstrated to have conversion efficiency up to 6.9 {\%}. However, the TiO$_{2}$ nanotube based DSC is humbled by the nature of its electrolyte environment. Hence, an all solid-state core shell $p-n$ junction utilizing the TiO$_{2}$ nanostructure solar cell is of great potential to provide another solution for the rising photovoltaic industry. In order to fabricate heterostructures, cuprous oxide (Cu$_{2}$O), a $p$-type semiconductor with a direct band gap of 2.0 eV, is a promising candidate to form $p-n$ heterojunction with TiO$_{2}$. Here we present a method to achieve Cu$_{2}$O/TiO$_{2}$ $p-n$ junction through electrochemical approaches. A self-doping method is addressed on crystallized TiO$_{2}$ nanotubes to further improve the contact and device performance. The photovoltaic property of Cu$_{2}$O/TiO$_{2}$ hetero-structure is measured, giving an open circuit voltage $\sim$0.25 V, a short circuit current $\sim$0.33mA/cm$^{2}$, and filling factor $\sim$27{\%}. Although the efficiency is still low, it demonstrates promising potential to achieve low cost flexible photovoltaic device. Monday, March 15, 2010 8:48AM - 9:00AM A29.00005: Characterization of Colloidal Silicon Nanoparticle Films Gregory Herman , David Neiman , Stephen Golledge Colloidal nanoparticle materials are under consideration for advanced manufacturing methods for inorganic photovoltaic systems. Silicon nanoparticles are a leading candidate for this application; however the propensity for silicon to form a native oxide prevents the efficient transport of electronic charge through the nanoparticle network. We have used x-ray photoelectron spectroscopy and secondary ion mass spectroscopy to evaluate the effect of various surface treatments and thermal processing on the oxidation of the nanoparticles and have correlated these with electrical measurements. We have found that HF treatments are most effective at removing the oxide and assist the formation of an interconnected nanoparticle network. Monday, March 15, 2010 9:00AM - 9:12AM A29.00006: Analysis of the effects of surface chemistry on the XAS spectra of CdSe nanomaterials Heather Whitley , David Prendergast , Tadashi Ogitsu , Eric Schwegler X-ray absorption spectroscopy (XAS) is an element-specific probe of local electronic structure, and is an ideal method to analyze chemical bonding. We investigate the consistency of theoretically predicted structures of CdSe nanomaterials with recently measured XAS via \textit{ab initio} calculations. Using plane-wave DFT, the x-ray absorption cross-section for the Cd L$_{3}$-edge of small CdSe clusters with a variety of surface ligands is calculated. We also highlight the importance of including excitonic effects in our simulations of core excitation spectra. We compare our simulations to existing experimental data on the ligand dependence of XAS for ligated quantum dots up to $\sim$3nm in diameter. Based on the favorable comparison of our theoretical spectra with experimental measurements, we infer the validity of our DFT-derived structure and surface passivation for these quantum dots and its relevance to understanding optoelectronic properties of solution-synthesized CdSe nanocrystals. Prepared by LLNL under Contract DE-AC52-07NA27344. Monday, March 15, 2010 9:12AM - 9:48AM A29.00007: Si Wire-Array Solar Cells Invited Speaker: Shannon Boettcher Micron-scale Si wire arrays are three-dimensional photovoltaic absorbers that enable orthogonalization of light absorption and carrier collection and hence allow for the utilization of relatively impure Si in efficient solar cell designs. The wire arrays are grown by a vapor-liquid-solid-catalyzed process on a crystalline (111) Si wafer lithographically patterned with an array of metal catalyst particles. Following growth, such arrays can be embedded in polymethyldisiloxane (PDMS) and then peeled from the template growth substrate. The result is an unusual photovoltaic material: a flexible, bendable, wafer-thickness crystalline Si absorber. In this paper I will describe: 1. the growth of high-quality Si wires with controllable doping and the evaluation of their photovoltaic energy-conversion performance using a test electrolyte that forms a rectifying conformal semiconductor-liquid contact 2. the observation of enhanced absorption in wire arrays exceeding the conventional light trapping limits for planar Si cells of equivalent material thickness and 3. single-wire and large-area solid-state Si wire-array solar cell results obtained to date with directions for future cell designs based on optical and device physics. In collaboration with Michael Kelzenberg, Morgan Putnam, Joshua Spurgeon, Daniel Turner-Evans, Emily Warren, Nathan Lewis, and Harry Atwater, California Institute of Technology. Monday, March 15, 2010 9:48AM - 10:00AM A29.00008: A Combined TDLDA/GW/CI Methodology for Multi-Exciton Processes Mark Lusk , Zhibin Lin , Alberto Franceschetti A computational methodology is introduced to facilitate the analysis of multi-exciton processes in photo-voltaic systems. Time-domain density functional theory within the local density approximation (TDLDA) is used to estimate the dynamic polarization response of molecules and small quantum dots. The response function is then used to estimate screening in a real-space implementation of screened Green function (GW) theory that is employed to construct quasi-particle wave functions. A linear combination of singly- or doubly-excited Slater determinants is constructed from these states and subjected to a variational argument in order to obtain the weighting coefficients. Significantly, a screened Coulomb interaction based on the TDLDA polarization response is used in the associated Hamiltonian. The methodology is used to quantify the rate of impact ionization, as compared with competing relaxation processes, in small CdSe quantum dots. Monday, March 15, 2010 10:00AM - 10:12AM A29.00009: Multivalency of Sn in Cu$_{2}$ZnSnS$_{4}$ Koushik Biswas , Stephan Lany , Alex Zunger The highly efficient ternary chalcopyrites such as 2(CuInSe$_{2})$ = Cu$_{2}$In$_{2}$Se$_{4}$ can be replaced as absorber materials in photovoltaic cells by Cu$_{2}$ZnSnS$_{4}$ (band gap $\sim$ 1.5 eV) in order to avoid the costly element In. Yet, the question remains whether a Fermi-level pinning defect can form spontaneously in these quaternary materials just as the In$_{Cu}$ intrinsic \textit{DX} centers in CuInSe$_{2}$ [1]. Here we study theoretically the deep gap levels introduced by the Sn$_{Cu}$ and Sn$_{Zn}$ defects in Cu$_{2}$ZnSnS$_{4}$. We find that these originate from the multi-valency of Sn, which can change into a +II oxidation state instead of the normal +IV state. Such a transition can even occur for Sn on its native site. Thus, we compare to the respective defect behavior in the kesterite Cu$_{2}$ZnGeSe$_{4}$, which has a similar band gap, but a less pronounced multi-valency of the respective IV-valent element Ge. \\[4pt] [1] S. Lany and A. Zunger, Phys. Rev. Lett. 100, 016401 (2008). Monday, March 15, 2010 10:12AM - 10:24AM A29.00010: Inspecting $\sim$700 A$_{2}$BX$_{4}$ compounds for energy applications: sorting their $\sim$40 crystal structures by diagramatic orbital radii maps without energy minimization Xiuwen Zhang , Alex Zunger The A$_{2}$BX$_{4}$ family of compounds covers $\sim$44 different crystal structure types and manifest a wide range of physical properties, including ferromagnetism, ferroelectricity, transparent conductivity, as well as superconductivity. We describe here a diagrammatic separation of the different crystal structures of $\sim$688 A$_{2}$BX$_{4}$ compounds by plotting a $R_{A}=R_{s}(A)+R_{p}(A)$ \textit{vs} $R_{B}=R_{s}(B)+R_{p}(B)$ map, where $R_{s}$ and $R_{p}$ are the $s$ and $p$ orbital radii'' of the neutral, free atoms, previously determined from first-principles pseudopotential theory. We find a 98\% successful separation of 688 A$_{2}$BX$_{4}$ compounds into 44 structure types. Applying this approach to separate Normal from Inverse spinel structures, we find a 96\% successful separation for 230 spinels known. These success rate using first-principles orbital radii uniformly exceed the success rates using classic radii (e.g Shannon's crystal radii; Pauling's covalent radii) or Pettifor's numbers. Once the separation maps was constructed, the crystal structure of a new chemical compound can be predicted by placing its $R_{A}$ and $R_{B}$ values in the map. Monday, March 15, 2010 10:24AM - 10:36AM A29.00011: Band alignment and interdiffusion at the BaCuSeF/ZnTe interface Andriy Zakutayev , Janet Tate , Heather Platt , Douglas Keszler , Alireza Barati , Wolfram Jaegermann , Andreas Klein In-situ ultraviolet- and x-ray photoemission spectroscopy experiments (XPS/UPS) on the wide-bandgap p-type semiconductor BaCuSeF indicate that it forms an ideal p-type contact to ZnTe. The interface is interdiffused, and there is no valence band offset. The transitivity rule for the valence band offsets and the similar chemistry of ZnTe and CdTe suggest that BaCuSeF is a promising p-type window layer for CdTe p-i-n double-heterojunction solar cells. BaCuSeF anode may also improve collection of charge carriers in thin film inorganic photovoltaic devices based on Cu(InGa)Se2 (CIGS) and Cu2ZnSnS4 (CZTS) absorbers. Monday, March 15, 2010 10:36AM - 10:48AM A29.00012: First-principles Study of Cu2ZnSnS4 Photovoltaic Absorber Ryoji Asahi , Akihiro Nagoya , Georg Kresse The quaternary semiconductor Cu2ZnSnS4 (CZTS) is a relatively new photovoltaic material whose constituents are non-toxic and abundant in the earth's crust. The highest conversion efficiency reported so far is 6.7{\%}, demanding further improvement. The key computational issue is to predict optical properties as accurate as possible, which is not possible using conventional semi-local density functionals. To this end, we have employed the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional as implemented in the Vienna Ab-initio Simulation Packages (VASP) code, which allows us to obtain results in good agreement with the experimental data for the lattice constants and the band gap of CZTS [1]. We then predicted optical properties and determined the dominant optical transitions for solar-light absorption. The defect formation energies of CZTS were also calculated in the allowed range of the chemical potentials bound by the precipitation conditions of the metal-sulfides. \\[4pt] [1] J. Paier et al., Phys. Rev. B 79 115126 (2009).