FAI-Induced Defect Passivation for Enhanced Performance in Spray-Deposited Cs ₃ Bi ₂ I ₉ Broadband Self-Powered Photodetectors

Trap-mediated non-radiative recombination constitutes a critical bottleneck in achieving high responsivity, detectivity and low-noise operation in solution-processed perovskite photodetectors. In this work, spray-coated Cs 3 Bi 2 I 9 (CBI) perovskite is employed as the active layer and systematically passivated using formamidinium iodide (FAI) organic molecules dissolved in low-toxicity isopropanol (IPA), applied over 4, 6, and 8 spray cycles. Structural and vibrational analyses using X-ray diffraction and Raman spectroscopy confirm the interaction of FAI with the CBI crystal lattice, as evidenced by systematic peak shifts toward lower 2θ values and enhanced diffraction intensities with increasing passivation cycles. Notably, the FAI-6 cycles treatment exhibits the highest crystallinity. Field-emission scanning electron microscopy reveals a pronounced increase in grain size, well-defined grain boundaries and reduced void density, collectively indicating suppressed recombination and improved carrier transport at the optimal passivation condition. Optical characterization demonstrates a substantial enhancement in absorption intensity, while photoluminescence spectra further corroborate effective defect passivation induced by FAI treatment. X-ray photoelectron spectroscopy shows systematic shifts toward lower binding energies for all constituent elements, indicating strong interaction between FAI and the CBI material at both surface and grain-boundary levels. Consequently, the optimized self-powered photodetector with FAI-6-cycles passivated CBI in the device architecture FTO/TiO 2 /CBI/carbon delivers a responsivity of 4.14 × 10 –4 A W –1 and a detectivity of 1.73 × 10 10 Jones, with rise and decay times of 100 and 90 ms, respectively, under white-light illumination. Under ultra-violet illumination of 372 nm wavelength, the device achieves a peak responsivity of 5.0 × 10 –2 A W –1 and a detectivity of 3.71 × 10 12 Jones. These results demonstrate that molecular passivation via FAI is an effective strategy for mitigating trap-assisted losses and significantly enhancing the optoelectronic performance of lead-free Cs 3 Bi 2 I 9 -based perovskite photodetectors.