Overcoming the Challenges of Phase Formation and Band Gap Tuning in Solution-Processed Cu 2 BaSn(S,Se) 4 Thin Films with Favorable Band Alignment for H 2 Evolution

Cu 2 BaSnS 4 (CBTS) is considered a next-generation, sustainable absorber for solar energy conversion, exhibiting a crucial advantage over the traditional Cu 2 ZnSnS 4 (CZTS) in the form of reduced cationic disorder. However, its relatively larger band gap of ∼2.0 eV limits its application in single-junction photovoltaic devices. Anionic substitution via Se alloying serves as a potential strategy for band gap tailoring and enhancement of optoelectronic properties. However, the challenges in phase stability and microstructural homogeneity due to Se incorporation must be addressed first. Herein, we report a strategy to prepare Cu 2 BaSn(SSe) 4 (CBTSSe) thin films from nontoxic precursor solutions, emphasizing the realization of a single-phase material with a consistent microstructure. We find that a high Ba concentration in the precursor solution, and optimized dwelling time and the amount of Se during the selenization process are critical for the formation of single-phase CBTSSe by effectively suppressing detrimental secondary phases. The films with an eventual [Se]/([Se]+[S]) of 0.22 exhibited an optimal band gap of ∼1.55 eV and an electrical resistivity of ≈10 2 Ω·cm. Detailed electroimpedance analysis confirmed the p-type conductivity of the films, having a carrier concentration of ∼10 17 cm –3 . The estimated flat band potential and position of valence and conduction band edges reveal superior electrochemical behavior with favorable band alignment for the hydrogen evolution reaction.