First-principles Calculations on the Zn 1-x Mg x O Window Layer Material for CIS Thin Film Solar Cells
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1012-Y03-14
First-Principles Calculations on the Zn1-xMgxO Window Layer Material for CIS Thin Film Solar Cells Tsuyoshi Maeda, Akio Shigemi, and Takahiro Wada Department of Materials chemistry, Ryukoku University, Seta, Otsu, 520-2194, Japan ABSTRACT We have theoretically evaluated the phase stability and electronic structure of a Zn1-xMgxO solid solution. The enthalpies of formation for both the wurtzite and rocksalt phases of Zn1-xMgxO were calculated using a plane-wave pseudopotential method within the density functional formalism. For 0 ≤ x < 0.5, the calculated enthalpies of formation (∆H) for the wurtzite phases were lower than those for the rocksalt phases. On the other hand, for x ≥ 0.75, the ∆H values for the wurtzite phase were larger than those for the rocksalt phases. This indicates that the wurtzite phase is more stable for a Zn1-xMgxO solid solution with 0 ≤ x < 0.5, while the rocksalt phase is more stable for a solid solution with x ≥ 0.75. The band gaps of a wurtzite Zn1-xMgxO solid solution increase with increasing Mg content. MgO substitution on Zn1-xMgxO largely affects the conduction band leaving the valence band nearly unchanged. The conduction band minimum (CBM) shifted to higher energy with increasing Mg content. These theoretical results qualitatively agree with the experimental results for Zn1-xMgxO thin films fabricated by RF magnetron co-sputtering. INTRODUCTION Cu(In,Ga)Se2 (CIGS) has long been expect to be one of the most promising materials for thin films solar cell fabrication. The conventional CIS solar cell device structure is ZnO/CdS/CIGS/Mo/soda-lime glass. Recently, the NREL group reported fabrication of CIGS solar cells with a conversion efficiency of 19.5% [1]. These high efficiency CIGS solar cells included a CdS buffer layer. Cd, however, is harmful from an environmental viewpoint. Thus, it is necessary to find an alternative material to CdS layer. Various research groups have proposed Cd-free window layers [2-4]. Zn1-xMgxO is a candidate for a Cd-free window layer material. The band gap of ZnO with the wurtzite structure is 3.24 eV [5]. MgO has a band gap of 7.7 eV, a value about 4.46 eV larger than that of ZnO implying that the bandgap of a Zn1-xMgxO solid solution may be controlled by changing the Mg content. One of the most important factors in obtaining high efficiency CIGS solar cells is an appropriate conduction band offset between the CdS buffer layer and CIGS absorber layer. The conduction band minimum (CBM) of CdS is about 0.2-0.3 eV higher than that of CIGS [5]. Minemoto et al. investigated CIGS solar cells with a Zn1-xMgxO buffer layer. They deposited Zn1-xMgxO thin films by radio frequency (RF) magnetron co-sputtering. They then investigated the valence band and conduction band offsets between the Zn1-xMgxO/CIGS layers [5-6]. They found that the performance of CIGS based solar cells improved when the CBM of Zn1-xMgxO window layer was higher than CBM of CIGS. In this study, first-principles calculations on a Zn1-xMgxO solid solution were performed using a plane-wa
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