Investigating Lead Halide Perovskites by Synchrotron X-Ray Scanning Tunneling Microscopy

Despite the remarkable success of lead halide perovskites, it was recognized early on that external factors, e.g. light exposure, elevated temperatures, or electric fields can disrupt the perovskite lattice, resulting in non-photoactive phases or even in film decomposition. This phenomenon can be mainly attributed to the ionic bonding character of the lead-halide sub-lattice: both anions and cations can migrate through the perovskite film enabled by bulk and surface defects such as vacancies, interstitials or antisite substitution. Thus, to improve the stability and performance of lead halide perovskites a fundamental understanding of the impact of external factors on the local ionic chemical environment is required, which enables the development of mitigation strategies for applications in photovoltaics and light-emitting diodes.<br/>In this contribution, we present a novel approach to measure the change in the local halide environment in perovskite thin films under applied external electric fields by synchrotron x-ray scanning tunneling microscopy (SX-STM). We investigate the influence of various applied bias polarities on the structural change of the halide ions at the perovskite interface by probing the iodide M_4,5 absorption edge by x-ray absorption spectroscopy (XAS) in the far field by SX-STM. By performing time-dependent XAS studies, we further track the change in the halide environment under continuous cycling of the applied bias. Additionally, to investigate the impact of the electric field on the spatial variation on the surface, we perform SX-STM experiments in the near field. In this geometry, the STM tip is brought into tunneling range which results in a strong field enhancement, thus local information can be obtained with nanometer resolution. Complemented by theoretical calculations, our results provide an in-depth understanding of crystal structure changes under electric fields investigated by STM.