Local Charge Carrier Dynamics in Lead Halide Perovskites by Nano Surface Photovoltage Spectroscopy

Understanding electron and ion dynamics is an important task for improving lead halide perovskites based solar cells and related devices. Perovskite materials usually have a delicate nano- and micro structure that influences the device parameters. Here, macroscopic measurement techniques are not sufficient. This study investigates the spatial defect distribution in the vicinity of grain boundaries (GB). To this end, we introduce Nano surface photovoltage spectroscopy (Nano-SPV) via time-resolved Kelvin probe force microscopy (tr-KPFM) [1,2]. By measuring the SPV decay on perovskite samples with small, large, and passivated grains, areas of increased charge carrier recombination, ion migration, and defects were locally detected. Using Nano-SPV, we revealed local SPV overshoots in the vicinity of grain boundaries following an illumination pulse. Furthermore, we introduce a new KPFM-based method to map the local light ideality factor within the perovskite films. The ideality factor is correlated to the dominant charge recombination processes within the absorber layer. Our results clearly show an improved uniformity of SPV and SPV decay distribution within the perovskite films upon passivation. Furthermore, the perovskite films with large grains show better recombination properties based on SPV decay and ideality factor values.<br/> <br/>[1] A. Axt, I. M. Hermes, V. W. Bergmann, N. Tausendpfund, and S. A. L. Weber, <i>Know Your Full Potential: Quantitative Kelvin Probe Force Microscopy on Nanoscale Electrical Devices</i>, Beilstein J. Nanotechnol. <b>9</b>, 1809 (2018).<br/>[2] S. A. L. Weber, I. M. Hermes, S.-H. Turren-Cruz, C. Gort, V. W. Bergmann, L. Gilson, A. Hagfeldt, M. Graetzel, W. Tress, and R. Berger, <i>How the Formation of Interfacial Charge Causes Hysteresis in Perovskite Solar Cells</i>, Energy Environ. Sci. <b>11</b>, 2404 (2018).