Nanoscale imaging of photocurrent generation of perovskite solar cells

The efficiency of perovskite solar cells is steadily improving, however, the fundamental operational principles and the mechanisms behind the degradation are insufficiently known, causing the delay of commercialization. We study photocurrent generation of methylammonium lead iodide perovskite solar cells using a near-field scanning photocurrent microscopy (NSPM). We observe that the spatial pattern of photocurrent is dependent on the sample annealing temperature: photocurrent is high at grain boundaries in samples annealed at moderate temperature (100 °C), while the opposite pattern (photocurrent is high at grain interiors and low at grain boundaries) is observed in samples annealed at higher temperature (130 °C). Correlating NSPM results with other characterization techniques (electron microscopy, x-ray diffraction, current-voltage and external quantum efficiency), we show that the spatial pattern of photocurrent is caused by material inhomogeneity and dynamics of segregation of lead iodide. Next, we examined the nanoscale signatures of aging caused by light exposure under normal operation. It is found that the extended light exposure drives further structural and compositional changes of materials revealed by the nanoscale photocurrent imaging.