Structure modulation for bandgap engineered vacancy-ordered Cs₃Bi₂Br₉ perovskite structures through copper alloying

Lead-free, vacancy-ordered, Cs₃Bi₂Br₉ perovskite is considered promising inorganic, stable, and non-toxic halide perovskite for optoelectronic and photovoltaic applications. However, its wide bandgap limits its state-of-art applications. We notice an observable enhancement of light absorption of Cs₃Bi₂Br₉ perovskite crystals upon CuBr₂ addition to the perovskite precursor. X-ray diffraction, ¹³³Cs solid state NMR, Raman spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations were studied to find out the nature of the perovskite crystal formation mechanism. With the addition of up to 50% CuBr₂, the Cs₃Bi₂Br₉ perovskite retains its matrix structure with homogeneously distributed Cs₂CuBr₄ large domains. Drop-casting of perovskite/DMSO solutions over TiO₂ thin films reveals a reduction of the direct bandgap from 2.56 eV for pristine Cs₃Bi₂Br₉ to 1.77 eV for a 50% Cu²⁺ alloyed one. First-principles calculations reveal the possibility of Cs₂CuBr₄ phase formation with higher Cu alloying, which can be the main reason behind bandgap narrowing. For homogeneously Cu alloyed Cs₃Bi₂Br₉, reduction of the effective bandgap can occur due to the formation of compensating defect levels (V_Br, Cu_Br) above the pristine valence band maxima (VBM). These results highlight the importance of alloying for structure modulation for bandgap engineered vacancy-ordered perovskite structures.