Fabrication of earth abundant Cu2ZnSnSSe4 (CZTSSe) thin film solar cells with cadmium free zinc sulfide (ZnS) buffer layers

Cadmium free ZnS buffer layers were deposited on Cu2ZnSn(SSe)(4) (CZTSSe) absorber layer by chemical bath deposition method. The thickness of ZnS buffer layers was tuned by varying deposition time and its effect on CZTSSe thin film solar cells were investigated. The thickness of the buffer layers increased linearly with the deposition time. The films showed lower optical transmittance with increasing ZnS buffer layer thickness. The optical band gaps of the films were found to be in the range from 3.52 to 3.68 eV. The best solar cell with conversion efficiency of 3.8% was obtained with ZnS buffer layer deposited with 56 nm thickness obtained at 40 min. External quantum efficiency (EQE) of the CZTSSe solar cell showed higher transmittance in the short wavelength region indicating less absorption. The performance of the solar cells increased with increasing deposition time initially and then decreased for a long deposition times. The variation in short circuit current density of the solar cells was attributed to increase in transmittance of the ZnS buffer layers. However, the cell exhibited poor collection efficiency at mid and long wavelengths suggesting defects at the interface and in the bulk of the CZTSSe absorber. Further, fine tuning the buffer layer and controlling the defects at the interface between ZnS and CZTSSe absorbers may lead to improved conversion efficiencies. (C) 2016 Elsevier B.V. All rights reserved.