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 degrees C), a small amount of expelled PbI2 passivates the grain boundaries and improves photocurrent generation. A higher annealing temperature (130 degrees 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.