In this study, we have demonstrated a simple way to tune the polarity of the photoconductivity in methylammonium bismuth iodide (MABI) thin films. Upon variation of precursor stoichiometry, photoconductivity of MABI thin films shows a transition from negative to positive polarity. The formation of light induced trap states, or local trapping of photogenerated excitons caused by the free rotation of the MA dipole in stoichiometrically balanced MABI (MA3Bi2I9) thin films could explain the observed unusual negative photoconductivity (NPC) signal. The increasing bismuth content in the precursor suppresses NPC by forming mixed phases of pure BiI3 and MA3Bi2I9 in stoichiometrically unbalanced MABI thin films. Additionally, we observed improved photoconductivity and yield mobility product upon replacement of MA cation by larger organic cations. The two-orders-of-magnitude increase in signal is possibly enabled through the suppression of cation/dipole movement through improved lattice structures with higher carrier mobilities, along with defect passivation induced by larger organic cations. Our findings provide a versatile platform to tune the optical-pump microwave-probe photoconductive response properties of Bi-based halide perovskites for information storage, logic memory, and next generation optoelectronic applications.
Paul et al. (Thu,) studied this question.