Influence of Na2S treatment on CZTS/Mo interface in Cu2ZnSnS4 Solar Cells annealed in sulfur-free atmosphere

Abstract The Na2S-treated CZTS/Mo interface in back contact of Cu2ZnSnS4 (CZTS) thin films solar cell was reported firstly. The CZTS thin films were deposited on Mo/Glass substrates by radio frequency magnetron sputtering using a copper-zinc-tin-sulfur composite target and annealed in a sulfur-free atmosphere. The element composition of excess-sulfur CZTS target was modified to Cu:Zn:Sn:S=4.12:1.61:1:17.65. The microstructure, morphology, optical and electrical properties of Cu2ZnSnS4 were analyzed. It was found that the sulfur-free annealing suppressed the formation of resistance MoS2 layer at CZTS/Mo interface. The CZTS/Mo interface was treated in a Na2S solution which promoted a better crystallization of CZTS absorbing layer.The optical band gap (Eg) value were found to be approximately ~1.47 eV and ~1.50 eV for CZTS and Na:CZTS samples. Finally, the photovoltaic properties of ZnO/ITO/CdS/CZTS/Mo solar cells were investigated. The open circuit voltage increased from 510.28 to 622.16 mV which ascribes to the recombination center diminishing through Na passivation. The conversion efficiency was improved from 2.37% to 3.26% after Na2S-treated method.