Ti–S antibonding coupling enables enhanced bandgap tuning in Ti-substituted BaHfS₃ perovskite

Chalcogenide perovskites have been proposed as one of the most promising candidates for solar cells due to their suitable bandgap, nontoxicity, and environmental stability. As a typically distorted perovskite, although BaHfS₃ possesses extraordinarily strong light absorption near the band edge, the slightly large bandgap of ∼2.1 eV has limited its applications in photoelectronic. In this work, the Ba(Hf_1−xTi_x)S₃ (0 ≤ x ≤ 0.05) perovskites with good crystallinity were first successfully synthesized and exhibited a reduced bandgap from 2.01 to ∼1.4 eV. Moreover, it is striking that the bandgap sharply decreases to 1.52eV (24 % compared to undoped compound) with only 1 atom % Ti-alloying concentration. The Density functional theory calculation shows that the contribution of Ti near the conduction band minimum (CBM) increases through the partial substitution of Ti for Hf. It is also demonstrated that the significant decrease in the bandgap is due to the downshift of the CBM caused by Ti–S antibonding coupling accordingly. This study provides a promising Ti-substituted BaHfS₃ material with excellent ambient and thermal stability, which can promote the use of chalcogenide perovskites in photovoltaics.