Nanoscale Imaging of Photocurrent in Perovskite Solar Cells using Near-field Scanning Photocurrent Microscopy

We study photocurrent generation and collection of methylammonium lead iodide perovskite solar cells with nanoscale resolution using a near-field scanning photocurrent microscopy (NSPM) technique. For NSPM measurements, we employ a non-contact mode atomic force microscopy probe with an attached optical fiber coated with Cr/Au metal. We observe an increased photocurrent at grain boundaries in samples annealed at moderate temperature (100 degrees C); however, the opposite spatial pattern is observed in samples annealed at higher temperature (130 degrees C). Combining the NSPM results with other characterization techniques such as electron microscopy, X-ray diffraction, and quantum efficiency measurements, we show that the cause of the photocurrent contrast is the material inhomogeneity and the dynamics of lead iodide. The NSPM technique is further used to establish the mechanism of the cell degradation under extended light illumination.