Fully stoichiometric Cu₂BaSn(S_1-x Se _x )₄ solar cells via chemical solution deposition

Cu₂BaSn(S_1-x Se _x )₄ has shown great prospects in the photoelectric field due to Earth-abundance, low toxicity, cost efficiency, direct bandgap, high absorption coefficient (>10⁴ cm^-1) and reduced anti-site disorder relative to Cu₂ZnSn(S_1-x Se _x )₄. A fully-tunable ratio of S/Se is the key to broaden the bandgap of Cu₂BaSn(S_1-x Se _x )₄. Here, we introduce a thionothiolic acid metathesis process to readily tune the stoichiometry of Cu₂BaSn(S_1-x Se _x )₄ films for the first time. Different stoichiometric Se/(S + Se) of Cu₂BaSn(S_1-x Se _x )₄ from zero to one can vary the bandgap range from 2 to 1.68 eV. The grain size of Cu₂BaSn(S_1-x Se _x )₄ films can be grown more than 10 μm. The optimized bandgap and high-quality growth of Cu₂BaSn(S_1-x Se _x )₄ films ensure the best power conversion efficiency of 2.01% for solution-processed Cu₂BaSn(S_1-x Se _x )₄ solar cells. This method provides an alternative solution-processed way for the synthesis of fully stoichiometric Cu₂BaSn(S_1-x Se _x )₄.