Modeling the Photoelectrochemical Evolution of Lead-Based, Mixed-Halide Perovskites Due to Photosegregation

Abstract

Lead-based, mixed-halide perovskites such as methylammonium lead iodide-bromide [MAPb­(I1–x Br x )3] undergo anion photosegregation under illumination. This is observed as low-band-gap photoluminescence from photogenerated iodine-rich domains due to favorable band offsets that induce carrier funneling into them. Unfortunately, theoretical rationalizations of mixed-halide photosegregation are complicated by biases inherent in photoluminescence-based observations. Recent compositionally weighted X-ray diffraction (XRD) measurements now reveal broad distributions of photosegregated stoichiometries not captured by existing photosegregation models. To better bridge experiment and theory, we perform kinetic Monte Carlo (KMC) simulations of photosegregation within the context of a band-gap-based thermodynamic model, which has previously accounted for numerous experimental observations. Our KMC simulations are modified to consider high carrier density Fermi–Dirac statistics that result from carrier funneling and accumulation within photosegregated I-rich domains. Obtained KMC results reproduce broad terminal halide (x terminal) distributions seen experimentally and illustrate how they are characterized by a central, heavily I-enriched stoichiometry. I-rich domain “drifting” during photosegregation rationalizes the long photosegregation time scales seen experimentally with drifting simultaneously, producing a wake of variable stoichiometry I-rich inclusions that form the lion’s share of stoichiometric heterogeneities seen in compositionally weighted XRD measurements. These simulations and accompanying rationalizations further reveal a general criterion for realizing favorable free energies to induce demixing. Central to the criterion is the statistical occupation of low gap inclusions in the parent alloy by excitations. The resulting model thus provides a general framework for conceptualizing mixed-halide perovskite light and temperature sensitivities mediated by photocarriers.

Fast ultraviolet detection response achieved in high-quality Cs3Bi2Br9 single crystals grown by an improved anti-solvent method

“…Such structural disorder is likely to reduce the carrier mobility, thereby impeding charge transport. 13 Consequently, it can lead to diminished photocurrent and lower efficiency than lead-based perovskite photodetectors. 31…”

“…12 The outstanding optoelectronic properties of Pb-based perovskite materials are largely attributed to the unique atomic electronic configuration of Pb, characterized by a lone pair of Pb 6s2 electrons and vacant Pb 6p orbitals, fostering strong spin–orbit coupling. 13 Therefore, ideal non-toxic substitutes for Pb should exhibit a similar atomic electronic configuration to maintain comparable properties. Among various alternatives, trivalent bismuth cations (Bi 3+ ) have emerged as a promising replacement due to their electronic configuration, which closely resembles that of Pb 2+ .…”