Solubility Limit and Optoelectronic Evolution of Heterovalent Bi3+ Doping in MAPbBr3 Perovskite Powders: A Study from Electrochemical Impedance Spectroscopy (EIS)

This paper reports a comprehensive physicochemical and electrical investigation of MAPb1−xBixBr3 perovskite powders (nominal x = 0 to 0.16) synthesized by antisolvent (acetone) precipitation. Structural (XRD/Rietveld) and chemical (EDX/EDS) analyses reveal a dual-regime doping mechanism governed by a thermodynamic solubility limit, with the true bismuth content saturating at approximately 1.3–1.4 at.% (nominal x ≈ 0.04). Below this limit, bismuth insertion causes anomalous lattice expansion through the creation of lead vacancies (V″Pb) and leads to a marked increase in the absolute intensity of the main diffraction peak, despite a slight broadening of the FWHM driven by local lattice microstrains. Above the limit, excess bismuth is expelled toward the grain boundaries as a highly disordered, amorphous-like, or short-range segregated secondary phase, degrading crystallinity and narrowing the effective optical band gap by up to 35 meV (from 2.278 eV to 2.243 eV) following an initial stabilization at low introduction levels (1–2% Bi), driven by the tentative formation of localized sub-gap defect states (Urbach-type tails) rather than intrinsic band-structure reorganization. Electrochemical impedance spectroscopy (EIS) confirms this dual regime: the absence of an electrode-polarization peak in ε″ indicates a Maxwell–Wagner–Sillars-type response, allowing the bulk conductivity to be extracted directly from the dielectric loss. At low temperatures, two distinct conduction regions coexist—a high-frequency electronic (hole-like) process and a low-frequency ionic process linked to bismuth-related defects, with ionic transport becoming dominant at high temperature; correspondingly, the dc-conductivity activation energy decreases from 66.3 kJ/mol (x = 0) to 30.7 kJ/mol (x = 0.16). Finally, the high-frequency limiting permittivity and geometric capacitance increase significantly with bismuth content (from ~15 pF to ~30 pF), reflecting enhanced dielectric performance driven by defect engineering.