Nanoscale photocurrent mapping in perovskite solar cells

In this work, we study spatially-resolved generation of photocurrent of methylammonium lead iodide (CH₃NH₃PbI₃) perovskite solar cells to reveal the microscopic effects of annealing temperature and material degradation under light exposure. Correlating a novel nanoscale near-field scanning photocurrent microscopy (NSPM) technique with X-ray diffraction and electron microscopy data, we found that the segregation of lead iodide (PbI₂) driven either by a temperature treatment or by extended light exposure can impact the photocurrent at grain boundaries. In samples annealed at a moderate temperature (100 °C), a small amount of expelled PbI₂ passivates the grain boundaries and improves photocurrent generation. A higher annealing temperature (130 °C) causes further segregation of PbI₂ at grain boundaries, decreasing the photocurrent. Extended light illumination drives further material segregation, decreasing photocurrent both at grain boundaries and grain interiors.