Advanced Computational Design of Intermediate-Band Photovoltaic Material V-substituted MgIn 2 S 4
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1218-Z04-02
Advanced Computational Design of Intermediate-Band Photovoltaic Material Vsubstituted MgIn2S4.
I. Aguilera, P. Palacios, K. Sánchez, P. Wahnón Instituto de Energía Solar & Dept. Tecnologías Especiales, Universidad Politécnica de Madrid, Ciudad Universitaria s/n, Madrid, 28040, Spain.
ABSTRACT An intermediate-band material based on thiospinel semiconductor MgIn2S4 is presented. This material is proposed as high efficiency photovoltaic material for intermediate-band solar cells. We analyze V substitution for In in the parent compound MgIn2S4 and the formation of the V d-states intermediate band. For the proper characterization of the width and position of this band inside the band gap, the standard one-shot GW method within the plasmon-pole approximation is applied. The electronic properties thus obtained are discussed and compared to those studied with Density Functional Theory (DFT), and the advantages and the limitations of the two methods are discussed. In addition, DFT electronic-charge density analysis is shown.
INTRODUCTION Thin-film technology for solar cells is emerging as one of the most promising proposals for higher-efficiency, lower-cost photovoltaic solar cells. The combination of the thin-film technology with the intermediate-band (IB) concept [1] could be the answer to the present efficiency-cost compromise. A partially-filled narrow band isolated from the valence and conduction bands of a host semiconductor would allow the absorption of sub-band-gap energy photons. For a solar cell, this would result, in the creation of additional electron-hole pairs and, in principle, in an increase in photocurrent without a decrease in open-circuit voltage. A cell based on such an approach could reach theoretical efficiencies up to 63.2% [1]. In this context, the first materials proposed were derivatives of chalcopyrites. Studies of intermediate-band materials based on CuGaS2 have already been presented [2-4], showing a potential suitability for enhanced photovoltaic applications. In these materials, Ga atoms were replaced by Ti or Cr at tetrahedral sites. For the thiospinel family of compounds, such as the one studied in this work, the groupIII atoms (In) occupy octahedral sites. The transition metal will thus be octahedrally coordinated in contrast with the case of chalcopyrite-based compounds. This situation for the transition metal is, in principle, more stable. The compound MgIn2S4 was chosen as host semiconductor because its band gap ranges from 2.1 to 2.28 eV [5,6], which are close to the optimum gaps for the implementation of an IB
material. The structure of MgIn2S4 is a cubic spinel structure, where sulfurs form a FCC lattice with Mg occupying tetrahedral sites and In octahedral sites. Substitution of V for In atoms at tetrahedral sites allows the formation of an IB with the desired characteristics [7]. THEORY Ground-state density functional theory (DFT) [8] calculations were performed with the plane-wave codes ABINIT (AB-INITio) [9] and VASP (Vienna Ab-initio Simulation Package) [10], in
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