Photoexcited charge-carrier transport in monolayer and bulk bismuth oxyiodide: the impact of the polaronic effect and deep-level defects

Abstract

Bismuth oxyiodide (BiOI), in monolayer and bulk forms, is a lead-free semiconductor material that has sparked increased interest for applications in perovskite solar cells and x-ray detectors. It is vital to clarify the transport nature of photoexcited charge-carriers to improve device performance. However, the transport scattering mechanisms remain poorly understood, and a detailed explanation of the measured charge-carrier mobilities in this material system is still under scrutiny. Herein, we implement transport scattering models that include LO phonon scattering based on the polaronic effect and ionized impurity scattering due to deep-level defects to elucidate photoexcited charge mobilities. We found that large polarons produced by photoexcited charge carriers coupled with LO phonon modes of 86 cm−1 and 156 cm−1 play a key role in the transport process of the BiOI system. Large polaron mobility provides a good explanation for the measured mobilities in single crystal samples between 26–83 cm2V−1s−1 at 295 K. The estimated results from both transport scattering models agree with the temperature-dependent mobilities measured in thin-film samples, between 13 cm2V−1s−1 at 5 K and 3 cm2V−1s−1 at 295 K. This work provides important insights into a band-like transport feature in the BiOI system.