Structural and Optical Properties of In-Free Cu2ZnSn(S,Se)4 Solar Cell Materials

We prepared a Cu2ZnSn(SxSe1-x)(4) (CZTSSe) solid solution from elemental powders. The CZTSSe solid solutions were synthesized by heating the elemental mixtures at 550 degrees C for 5 h in an N-2 gas. All CZTSSe materials were analyzed by Rietveld analysis using the kesterite structure with a space group of I (4) over bar (No. 82). Rietveld analysis showed that the lattice parameters, a and c, monotonically decreased with increasing S content. Moreover, the local structures of the Cu and S atoms were investigated by X-ray absorption near edge structure (XANES). Although the local structure of the S atom in CZTSSe hardly changed in relation to the S/Se ratio, we found that the surface of CZTSSe powders became slightly oxidized. On the other hand, Cu K edge XANES showed that the S/Se ratio could be easily determined from the XANES spectra. The band gap energies of the CZTSSe materials were determined by transmittance and diffuse-reflectance spectra. The transmittance spectra were recorded using CZTSSe films fabricated by a printing and high-pressure sintering (PHS) process. The band gap energy, E-g, monotonically increased from 1.05 eV for CZTSe to 1.51 eV for CZTS. (C) 2012 The Japan Society of Applied Physics