Tin-Selenium Secondary Phase Etching of Cu₂ZnSnSe₄: A Selective Removal Route to Improve Solar Cell Efficiency

In this study, we removed type II Sn-Se secondary phases from Cu₂ZnSnSe₄ (CZTSe) absorber surface via selective chemical etching and then examined the influence of this etching on the performances of solar cell devices. Experiment results indicate that the morphology of Sn-Se secondary phases are determined by the Ar gas flow rate (Sn/Se ratio) of the selenization process in the CZTSe absorber layer. Round or semicircular structures (type II) of Sn-Se secondary phases formed in films with a higher Ar gas flow rate (Sn/Se ratio) selenization process. We found that HNO₃ with 3 HCl could remove type II Sn-Se secondary phases, thereby improving average device efficiency from 3.8% to 5.6%.