Micro-mechanism study of the effect of Cd-free buffer layers ZnxO(x = Mg/Sn) on the performance of flexible Cu2ZnSn(S, Se)4 solar cell

The traditional CdS buffer layers in flexible CZTSSe solar cells lead to light absorption losses and environmental pollution problems. Therefore, the study of Cd-free buffer layer is very important for the realization of environmentally friendly and efficient CZTSSe solar cells. The Zn1-xMgxO(ZnMgO) and Zn1-xSnxO(ZnSnO) alternate buffer layers are studied in this study using the simulation package solar cell capacitance simulator(SCAPS-1D) numerical simulation model, and the theoretical analysis is further verified by the results of the experiments. We simulate the performance of CZTSSe/ZnXO(X = Mg/Sn) heterojunction devices with different Mg/(Zn+Mg) and Sn/(Zn+Sn) ratios and analyze the intrinsic mechanism of the effect of conduction band offsets(CBO) on the device performance. The simulation results show that the CZTSSe/ZnXO(X = Mg/Sn) devices achieve optimal performance with a small “spike” band or “flat” band at Mg and Sn doping concentrations of 0.1 and 0.2, respectively. To investigate the potential of Zn_(0.9)Mg_(0.1O) and Zn_(0.8)Sn_(0.2)O as alternative buffer layers, carrier concentrations and thicknesses are analyzed. The simulation demonstrates that the Zn0.9Mg0.1O device with low carrier concentration has a high resistivity, serious carrier recombination, and a greater impact on performance from thickness variation. Numerical simulations and experimental results show the potential of the ZnSnO buffer layer as an alternative to toxic CdS, and the ZnMgO layer has the limitation as a substitute buffer layer. This paper provides the theoretical basis and experimental proof for further searching for a suitable flexible CZTSSe Cd-free buffer layer.