Alloy-induced phase transition and enhanced photovoltaic performance: the case of Cs3Bi2I9−xBrx perovskite solar cells

Abstract

Cs₃Bi₂I₉ is a promising inorganic, nontoxic, and stable perovskite material for solar cell applications, but the power conversion efficiency of Cs₃Bi₂I₉-based solar cells is relatively low because its band gap is too large and indirect. Here we show that the band gap and optoelectronic properties of Cs₃Bi₂I₉ can be improved by alloying Br into Cs₃Bi₂I₉, despite the common expectation that the band gap of the Cs₃Bi₂I_9−xBr_x alloy will increase with Br concentration. By systematically growing and characterizing Cs₃Bi₂I_9−xBr_x perovskite materials, we discover that at x = 3 Cs₃Bi₂I₆Br₃ shows the lowest bandgap of 2.03 eV, while that of Cs₃Bi₂I₉ is 2.20 eV. Our photophysical characterization and density functional theory calculations show that this unusual band gap reduction and better optoelectronic properties at x ∼ 3 are mainly caused by a phase transition from the P6₃/mmc phase to the Pm phase. Moreover, we found that the composition engineering also leads to compact Cs₃Bi₂I₆Br₃ films with high crystallinity and orientation, which results in a champion device with a PCE of 1.15%. This novel discovery will definitely deepen our understanding of Cs₃Bi₂I_9−xBr_x materials and afford more promising strategies for highly stable lead-free perovskite optoelectronic devices.