Tuning the structural and optoelectronic properties of Cs3Bi2I9 through MA+/FA+ incorporation

Abstract

Bismuth-based halide perovskites have emerged as promising lead-free alternatives to conventional Pb-based optoelectronic materials because of their low toxicity, enhanced environmental compatibility, and superior chemical stability. Growing concerns regarding the toxicity and long-term instability of lead halide perovskites have accelerated research into environmentally benign perovskite materials. Amongst these, the vacancy-ordered inorganic perovskite Cs₃Bi₂I₉ has attracted considerable attention owing to its excellent moisture resistance, thermal stability, and non-toxic nature. In this work, pristine and MA⁺/FA⁺ simultaneously co-doped Cs₃Bi₂I₉ perovskites were successfully synthesized and systematically investigated. Structural analysis confirmed the formation of the crystalline perovskite phase, revealed slight structural modifications upon doping, and examined the surface morphology and grain distribution. Optical studies show tuneable absorption behaviour, with bandgap and Urbach energy reductions from ~2.09 to ~1.95 eV and ~0.180 to ~0.094 eV with increasing organic cation concentration. Raman spectra indicate changes in vibrational modes associated with structural interactions between organic cations and the inorganic structure. Thermal stability indicates the robustness of the perovskite, and I-V characteristics and space-charge-limited current (SCLC) measurements revealed increased mobility from ~6.95 × 10^–5 to ~1.94 × 10^–4 cm² V⁻¹ S⁻¹, reduced trap density, and enhanced conductivity after doping. These results demonstrate that MA⁺/FA⁺ simultaneous co-doping effectively tailors the structural, optical, thermal, and electrical properties of Cs₃Bi₂I₉, providing an effective strategy for enhancing the performance of lead-free bismuth-based perovskites for future optoelectronic applications.