Halide-Alloying-Induced Structural Modulation and Enhanced Photodetection Performance in Lead-Free Cs ₃ Bi ₂ (X 0.5 :X′ 0.5 ) ₉ (X, X′ = Br, I, Cl) Perovskites

In this study, Pb-free mixed-halide perovskites of the type Cs 3 Bi 2 (X 0.5:X′ 0.5 ) 9 (X, X′ = Br, I, Cl) were synthesized using an antisolvent recrystallization method and applied as active layers in photodetector devices. X-ray diffraction confirmed the formation of single-phase solid solutions for all compositions, and the shift in diffraction peaks indicated the successful incorporation of different halide ions into the lattice. Raman spectroscopy further confirmed halide mixing by revealing changes in vibrational modes. X-ray photoelectron spectroscopy confirmed the presence of Cs, Bi, and halide species in their expected oxidation states, indicating high chemical purity. Field-emission scanning electron microscopy revealed well-defined polyhedral grains with uniform surface coverage, suggesting good crystallinity and film quality. Optical properties were studied using UV–Visible spectroscopy and Tauc plot analysis. The results showed that the band gap can be tuned by varying the halide composition, with values of 2.7 eV for Cs 3 Bi 2 (Cl 0.5:Br 0.5 ) 9, 2.30 eV for Cs 3 Bi 2 (Cl 0.5:I 0.5 ) 9, and 2.03 eV for Cs 3 Bi 2 (I 0.5:Br 0.5 ) 9 . Photoluminescence measurements supported this tunability, showing shifts in emission peaks and changes in intensity. Photodetectors were fabricated using an FTO/c-TiO 2 /Cs 3 Bi 2 (X 0.5:X′ 0.5 ) 9 /graphite structure. The devices showed good photoresponse under illumination, with a maximum responsivity of 1.13 mA/W, a detectivity of 27.2 × 10 8 Jones, a photosensitivity of 50.4, and an internal quantum efficiency of 0.24%. The devices also exhibited fast response and recovery times of 36 and 330 ms, respectively. Wavelength-dependent I–V and time-resolved I–t measurements showed enhanced photocurrent and fast switching behavior. Among all compositions, Cs 3 Bi 2 (I 0.5:Br 0.5 ) 9 showed a strong spectral response, indicating its suitability for broadband photodetection. Overall, halide engineering effectively tunes the structural, optical, and electrical properties, making these materials promising for next-generation lead-free photodetectors.