Nanoimaging of local photocurrent in hybrid perovskite solar cells via near-field scanning photocurrent microscopy
Dongheon Ha, Yohan Yoon, Ik Jae Park, Paul M. Haney, Nikolai B. Zhitenev
Photocurrent generation of methylammonium lead iodide (CH3NH3PbI3) hybrid perovskite solar cells is observed at the nanoscale using near-field scanning photocurrent microscopy (NSPM). We examine how the spatial map of photocurrent at individual grains or grain boundaries is affected either by sample post-annealing temperature or by extended light illumination. For NSPM measurements, we use a tapered fiber with an output opening of 200 nm in the Cr/Au cladded metal coating attached to a tuning fork-based atomic force microscopy (AFM) probe. Increased photocurrent is observed at grain boundaries of perovskite solar cells annealed at moderate temperature (100 °C); however, the opposite spatial pattern (i.e., increased photocurrent generation at grain interiors) is observed in samples annealed at higher temperature (130 °C). Combining NSPM results with other macro-/microscale characterization techniques including electron microscopy, x-ray diffraction, and other electrical property measurements, we suggest that such spatial patterns are caused by material inhomogeneity, dynamics of lead iodide segregation, and defect passivation. Finally, we discuss the degradation mechanism of perovskite solar cells under extended light illumination, which is related to further segregation of lead iodide.