GeSe-evoked synchronous strategy for electrodeposited CZGSe solar cells

Sn-free Cu₂ZnGeSe₄ (CZGSe) is emerging as a promising non-toxic and earth-abundant photovoltaic absorber material due to its attractive electrical and optical properties as well as its high theoretical conversion efficiency. Nevertheless, no photovoltaic device fabricated through the green electrodeposition process has yet been reported, likely due to the poor solubility of Ge-based salts and harsh electrodeposition conditions. Herein, we propose a GeSe-evoked synchronous strategy involving a Ge incorporation and selenization-regulated co-heating process of GeSe and Se, following electrodeposition of a Cu-Zn preformed layer. We experimentally found that the low-melting-point GeSe could promote the crystal growth and induce a high-quality bulk absorber layer and good back interface. In the GeSe-promoted sample, it was found that MoSe₂ could ensure a good back quasi-Ohmic contact, and the band bending at the grain boundaries (GBs) was favorably inverted. Moreover, the depletion region width was also prolonged, and the deleterious Cu_Zn near E_F was passivated, leading to an increased carrier separation. In turn, a surprising progress in device performance was found, achieving a ground-breaking efficiency of 3.69%, and it could fill the bank of green electrodeposited CZGSe-based solar cells.