Single-step sulfo-selenization method for achieving low open circuit voltage deficit with band gap front-graded Cu₂ZnSn(S,Se)₄ thin films

In this study, we investigate the electrical, structural, and optical properties of band gap front-graded Cu₂ZnSn(S,Se)₄ (CZTSSe) thin films grown by a modified single-step sulfo-selenization process from copper-poor and zinc-rich precursor metallic stacks prepared by co-evaporation. To investigate how the bandgap was graded in connection with the compositional distribution, we calculated the bandgap energy distribution along the film thickness, based on the transmission electron microscopy and energy-dispersive X-ray spectroscopy composition profile. The band gap of the CZTSSe phase with high S content near the surface layer is determined to be 1.161 eV. From the surface to the bottom, there is a decrease in the S content of the CZTSSe phase, and the band gap subsequently decreases to, 1.029 eV, close to the value of CZTSe. From the results of dimpling-Raman and scanning transmission electron microscopy line scanning, we confirm that the S content drastically increases from the bottom to the top surface of the CZTSSe thin film. The CZTSSe thin-film solar cell exhibits a power conversion efficiency (PCE) of 10.33%, with an open-circuit voltage (V_oc) of 0.505 V, short-circuit current density (J_sc) of 31.61 mA/cm², fill factor (FF) of 64.6%, and V_oc deficit of 525 mV. Compared with the performance of the CZTSe solar cell, which had PCE of 7.23%, V_oc of 0.424 V, J_sc of 32.83 mA cm⁻², FF of 51.9%, and V_oc deficit of 576 mV, the V_oc and V_oc deficit of the CZTSSe cell improved considerably. The high V_oc, low V_oc deficit, and less loss of J_sc are attributed to the effect of band gap front-grading induced by S grading into the CZTSSe thin film.