Optimizing kesterite solar cells from Cu₂ZnSnS₄to Cu₂CdGe(S,Se)₄

Kesterite solar cells, based on the prototypical absorber material Cu₂ZnSnS₄(CZTS), are cheap, nontoxic, and chemically stable, thus rendering them promising, beyond-Si photovoltaic technologies. Their efficiencies, however, are limited by the formation of defects that decrease the short-circuit current by creating deep traps where nonradiative recombination of photoexcited charge carriers occursviathe Shockley-Read-Hall mechanism. To suppress the formation of these defects, specifically the most deleterious 2Cu_Zn+ Sn_Znantisite cluster, we devised an ion substitution strategy involving complete Cd- and Ge-substitution and partial selenization, ultimately arriving at the optimal composition, Cu₂CdGeS₃Se (CCdGSSe). Using density functional theory andab initiothermodynamics, we predict that complete Cd- and Ge-substitution leads to a 125% increase in the formation energy of the deep-trap-inducing 2Cu_Cd+ Ge_Cd. Additionally, 25% selenization optimizes the predicted band gap (1.43-1.47 eV, as calculated from a hybrid functional) with respect to the Shockley-Queisser limit. In addition to providing a practical and novel ion substitution strategy, we also elucidate the mechanisms of defect suppression and promotion by Ge and Se, highlighting the key role of the inert pair effect and metal-chalcogen bond covalency, respectively. Due to its ideal thermodynamic and electronic characteristics, CCdGSSe should reinvigorate research on kesterite-based solar cells, optimizing the rich materials space afforded by ion substitution and post-quinary compositions.