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1165-M04-03

Phase stability and electronic structure of In-free photovoltaic semiconductors, Cu2ZnSnSe4 and Cu2ZnSnS4 by first-principles calculation T. Maeda, S. Nakamura and T. Wada Department of Materials Chemistry, Ryukoku University, Seta, Otsu 520-2194, Japan ABSTRACT We have theoretically evaluated the phase stability and electronic structure of Cu2ZnSnSe4 (CZTSe) and Cu2ZnSnS4 (CZTS). The enthalpies of formation for kesterite, stannite and wurtz-stannite phases of CZTSe and CZTS were calculated using a plane-wave pseudopotential method within the density functional formalism. For CZTSe, the calculated formation enthalpy (∆H) of the kesterite phase (-312.7 kJ/mol) is a little smaller than that of the stannite phase (-311.3 kJ/mol) and much smaller than that of the wurtz-stannite phase (-305.7 kJ/mol). For CZTS, the ∆H of the kesterite phase (-361.9 kJ/mol) is smaller than that of the stannite phase (-359.9 kJ/mol) and much smaller than that of the wurtz-stannite phase (-354.6 kJ/mol). The difference of ∆H between the kesterite and stannite phases for CZTS is greater than that for CZTSe. This indicates the kesterite phase is more stable than the stannite phase in CZTS compared with CZTSe. The valence band maximums (VBMs) of both the kesterite- and stannite-type CZTSe(CZTS) are antibonding orbitals of Cu 3d and Se 4p (S 3p). The conduction band minimums (CBMs) are antibonding orbitals of Sn 5s and Se 4p (S 3p). The Zn atom does not affect the VBM or the CBM in either CZTSe(CZTS). The theoretical band gap of the kesterite phase calculated with sX-LDA in both CZTSe and CZTS is a little wider than that of the wurtz-stannite phase and much wider than that of the stannite phase. INTRODUCTION Cu(In,Ga)Se2 (CIGS) has been anticipated as one of the most promising materials for thin film solar cells. Recently, the substitution of indium has become an important issue because it is scarce and expensive. Cu2ZnSnSe4 (CZTSe) and Cu2ZnSnS4 (CZTS) are among the indium-free absorber materials. The preparation and characterization of CZTSe and CZTS have been investigated by some groups [1]. CZTSe is a I2-II-IV-VI4 semiconductor with a stannite-type structure. CZTSe has a direct band gap of 1.5 eV. On the other hand, CZTS has a kesterite-type structure. The experimental band gap of CZTS is reported to be around 1.5 eV. The electronic structure of CZTS has not been studied theoretically due to the complexity of the quaternary compound. Three kinds of crystal structures have been reported for Cu2-Zn-IV-VI4 compounds. Cu2ZnSnS4 has a kesterite-type, Cu2ZnSnSe4 has a stannite-type, and Cu2ZnGeSe4 and Cu2ZnSiSe4 have wurtz-stannite-type structures. Some groups studied electronic structures of CuInSe2 (CIS) and the related chalcopyrite compounds by first principles calculations [2, 3]. For CIS, the valence band maximum (VBM) consists of Cu 3d and Se 4p orbitals, while the conduction band minimum (CBM) consists of In 5s and Se 4p orbitals. Recently, we reported the electronic structure of stannite-type CZTSe [4]. The VBM is an antibond

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