Fully stoichiometric Cu2BaSn(S1−xSex)4 solar cells via chemical solution deposition

Cu2BaSn(S1−xSex)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(S1−xSex)4. A fully-tunable ratio of S/Se is the key to broaden the bandgap of Cu2BaSn(S1−xSex)4. Here, we introduce a thionothiolic acid metathesis process to readily tune the stoichiometry of Cu2BaSn(S1−xSex)4 films for the first time. Different stoichiometric Se/(S + Se) of Cu2BaSn(S1−xSex)4 from zero to one can vary the bandgap range from 2 to 1.68 eV. The grain size of Cu2BaSn(S1−xSex)4 films can be grown more than 10 μm. The optimized bandgap and high-quality growth of Cu2BaSn(S1−xSex)4 films ensure the best power conversion efficiency of 2.01% for solution-processed Cu2BaSn(S1−xSex)4 solar cells. This method provides an alternative solution-processed way for the synthesis of fully stoichiometric Cu2BaSn(S1−xSex)4.