Effect of Zn(O,S) Buffer Layer on Cu2ZnSnS4 Solar Cell Performance from Numerical Simulation

ABSTRACT Zn(O,S) is a promising alternative buffer layer to CdS in Cu2ZnSnS4 (CZTS) based solar cell due to its large bandgap and nontoxic elements. In this study the performance of CZTS/Zn(O,S)/ Al:ZnO solar cell was numerically simulated by Solar Cell Capacitance Simulator (SCAPS). And the sulfur content of Zn(O,S) buffer layer was varied to investigate how the chemical composition of Zn(O,S) influences the conduction band offsets at absorber/buffer and buffer/window interfaces and then affects the cell performance. It was found that the conduction-band offset (CBO) of the CZTS/Zn(O,S) heterojunction played a significant role in the performance of the solar cell. The electron affinity and bandgap of Zn(O,S) are controlled by the sulfur-to-oxygen ratios, and the resulting offset span is from +0.7 eV in the “spike” direction to -0.1 eV in the “cliff” direction if the electron affinity of CZTS is considered as 4.5 eV. When S/(S+O) atomic ratio of Zn(O,S) is about 0.3, the cell has a high conversion efficiency of 14.90% with CBO of 0.2 eV. The simulation results will provide some important guidelines for fabricating high efficient CZTS solar cells.