  Organic-inorganic perovskite solar cells have attracted tremendous attention
because of their remarkably high power conversion efficiencies (PCEs). To
further improve the device performance, however, it is imperative to obtain
fundamental understandings on the photo-response and long-term stability down
to the microscopic level. Here, we report the first quantitative nanoscale
photoconductivity imaging on two methylammonium lead triiodide (MAPbI3) thin
films with different PCEs by light-stimulated microwave impedance microscopy.
The intrinsic photo-response is largely uniform across grains and grain
boundaries, which is direct evidence on the inherently benign nature of
microstructures in the perovskite thin films. In contrast, the carrier mobility
and lifetime are strongly affected by bulk properties such as the sample
crystallinity. As visualized by the spatial evolution of local
photoconductivity, the degradation due to water diffusion through the capping
layer begins with the disintegration of large grains rather than the nucleation
and propagation from grain boundaries. Our findings provide new insights to
improve the electro-optical properties of MAPbI3 thin films towards large-scale
commercialization.
