Mechanism of influence of solvent types and Sn valence states on performance of Cu₂ZnSn(S, Se)₄ solar cell

In the present work, two types of Cu₂ZnSn(S, Se)₄ (CZTSSe) solar cells, namely cell-MOE (MOE refers to 2-methoxyethanol) and cell-DMSO (DMSO refers to dimethyl sulfoxide), were fabricated using two Cu₂ZnSnS₄ (CZTS) precursor solutions: one employing MOE solvent and Sn⁴⁺ salt (MOE-Sn⁴⁺), and the other using DMSO solvent and Sn²⁺ (DMSO-Sn²⁺). Effects of the solvent types and the valence state of Sn on power conversion efficiency (PCE) of CZTSSe solar cell were investigated. It is found that the PCE of CZTSSe solar cells prepared with solution MOE-Sn⁴⁺ is higher than that of those prepared with solution DMSO-Sn²⁺. The highest PCE reaches 10.17% for cell-DMSO and 11.25% for cell-MOE. The advantage of MOE-Sn⁴⁺ solution is attributed to the fact that CZTS precursor film prepared with MOE-Sn⁴⁺ (CZTS-MOE) possesses a looser crystal structure and smaller grain size than that prepared with DMSO-Sn²⁺ (CZTS-DMSO). These structural and grain characteristics enable the CZTSSe derived from the selenization of CZTS-MOE (CZTSSe-MOE) to exhibit superior crystallinity compared with the CZTSSe obtained via the selenization of CZTS-DMSO (CZTSSe-DMSO). This enables CZTSSe-MOE to have a lower hole concentration, fewer band tail states, lower bulk and surface deep-level defect densities, and a reduced content of the Zn(S, Se) compared with CZTSSe-DMSO. These characteristics in turn endow cell-MOE with a wider depletion region width, stronger light absorption capacity, and less carrier recombination than cell-DMSO, thereby contributing to a higher photogenerated current density and a lower reverse saturation current density in cell-MOE relative to cell-DMSO.