Influence mechanism of Cu+/(Cu++Cu2+) ratio in Cu-Zn-Sn-S precursor solution on performance of Cu2ZnSn(S,Se)4 solar cells

Abstract

Five types of Cu2ZnSn(S,Se)4 (CZTSSe) films were fabricated by selenization of Cu2ZnSnS4 (CZTS) precursor films preparing through Cu-Zn-Sn-S precursor solutions with nominal Cu+/(Cu++Cu2+) ratio of 0, 0.2, 0.4, 0.8 and 1. Five types of CZTSSe solar cells were prepared by using the five types of CZTSSe films as absorbers, respectively. The crystal quality and phase composition of the CZTS and CZTSSe films were characterized by XRD, Raman and SEM, and the photovoltaic performance for the CZTSSe solar cells were measured by J-V and EQE. It is found that the CZTS film consists of kesterite CZTS when the ratio is 0 and of kesterite CZTS together with a small amount of SnS when the ratio is 0.2 to 1. All of the CZTSSe films are comprised of kesterite CZTSSe and a small amount of Zn(S,Se). The crystal quality and lattice constants of the kesterite CZTSSe increase with increasing the ratio, while the amount of Zn(S,Se) decreases with increasing the ratio and almost to zero as the ratio is 1. The power conversion efficiency (PCE) of the CZTSSe solar cells increases from 5.47% to 8.43% as the ratio increases from 0 to 1, which is due to improvement in short-circuit current density (Jsc) and filling factor (FF) and decrease in open circuit voltage deficit (Voc,def). The influence mechanism of the ratio on the PCE is suggested by analyzing effect of the ratio on electrical parameters of the CZTSSe solar cells.