Selenium Inclusion in Cu2ZnSn(S,Se)4Solar Cell Absorber Precursors for Optimized Grain Growth

Abstract

Cu2ZnSn(S,Se)4 precursors are fabricated by compound cosputtering from metal sulfide and selenide targets, and annealed in mixed argon, sulfur, and selenium atmosphere at temperatures between 540 and 580 $^\circ$ C and at pressures between 24 and 47 kPa. We produce solar cell devices from these absorbers that range from 2.0% to 9.0% power conversion efficiency. We extensively characterize the morphology and elemental composition of the absorbers, and are able to closely relate the annealing conditions, precursor sulfur–selenium content, device performance, and absorber quality. We develop a qualitative model which relates the sulfur–selenium distribution in the precursor and the relative partial pressures of sulfur and selenium during the annealing process to the absorber properties. We show that selenium inclusion in the precursor allows more rapid recrystallization of the absorber at lower temperature. Alternating stacking of sulfur and selenium containing precursor material leads to differential rates of recrystallization, which allows some control over the morphology of the annealed absorber and Zn(S,Se) secondary phase segregation in that absorber. We further show that selenium containing precursors can be used to fabricate the superior devices relative to sulfur-only precursors, when the annealing phase space is subject to severe practical restrictions.