Optical absorption spectra of Cu2ZnSn(S,Se)4 thin-film solar cells by Fourier transform photocurrent spectroscopy

We demonstrated the potential of Fourier transform photocurrent spectroscopy (FTPS) in investigating the optical absorption and quality of a Cu2ZnSn(S,Se)(4) (CZTSSe) absorber within the cell structure. We fabricated thin-film solar cell structures with a CZTSSe absorber, which was prepared by a nanoparticle coating technique and a postannealing process. The optical absorption spectra of CZTSSe absorbers prepared at different annealing temperatures were measured by FTPS. We successfully evaluated the optical band-gap energy from the spectra. An exponential tail was found in the spectra. Urbach energy increased with increasing annealing temperature. The deterioration of photovoltaic performance with increasing Urbach energy might be due to potential fluctuation rather than to structural randomness. (C) 2018 The Japan Society of Applied Physics