Studying the defect-photocarrier interaction in halide perovskites

Abstract

Organometal halide perovskites have emerged as exceptional materials for use in photovoltaics (PVs), light-emitting diodes, and detector devices due to their outstanding optoelectronic properties. These materials show a high absorption coefficient, a tunable band gap, and a long carrier diffusion length. Because of these outstanding properties, halide perovskite PVs have shown a rapid improvement in power conversion efficiency in recent years, with a current record exceeding 26%. However, a major obstacle to the commercialization of these perovskite PVs is their inherent instability, which makes them less reliable devices. These materials intrinsically have a high concentration of point defects that migrate and interact with electronic charges, resulting in poor photostability under various illumination conditions. In this thesis, we investigate defect-photocarrier interactions in halide perovskites. We fabricated samples by intentionally enhancing or suppressing the amount of iodide vacancies (VI). Our finding indicates that samples with more VI exhibit a large and reversible Fermi level shift (~0.7 eV) when samples are exposed to light illumination. Moreover, we observe that VI can interact with photoelectrons to form prolonged deep electron traps, which can lead to photo-induced instabilities. These observations can be attributed to the capturing of photocarriers by VI, leading to the formation of Pb-Pb dimer that shifts the energy level of the defect state. This process resembles the polaron formation at a defect site. To mitigate these polaronic interactions with point defects, we partially replaced the MA+ cation with cations of different ionic radii. We observed that light-induced deep electron traps and the associated Fermi level shifts are suppressed by replacing MA+ with the smaller cesium (Cs+) ions but are enhanced by replacing MA+ with the larger formamidinium (FA+) ions. Point defects can also migrate easily, leading to defect segregation within the perovskite film. The segregation of defects affects electron transport and results in instability in the performance of halide perovskite devices. Conventional ultrafast spectroscopies used to study electron dynamics in halide perovskites commonly illuminate the sample continuously with millions of laser pulses, resulting in unavoidable disruption of steady-state conditions, such as the steady-state concentration of photocarriers and the extent of ion segregation, in the perovskite film. Hence, it is crucial to develop an experimental technique to unveil the correlation between charged defect segregation and fast electron transport. Here, an experimental approach is developed to study how the charged defect segregation induced by light can influence the electron transport time. In this approach, we combine single-pulse femtosecond (fs) excitations with electrical measurements on a perovskite/graphene heterostructure to probe nanosecond scale electron transport times under different continuous light illumination conditions. We observed that light-induced ion segregation shortens electron and hole transport times in perovskite film. This enhancement in the transport rate can be explained by the formation of an internal p-n junction induced by the ion segregation.