Crystalline orientation dependent photoresponse and heterogeneous behaviors of grain boundaries in perovskite solar cells

Using photoconductive atomic force microscopy and Kelvin probe force microscopy, we characterize the local electrical properties of grains and grain boundaries of organic-inorganic hybrid perovskite (CH3NH3PbI3) thin films on top of a poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS)/ITO substrate. Three discrete photoconductivity levels are identified among perovskite grains, likely corresponding to the crystal orientation of each grain. Local J-V curves recorded on these grains further suggest an anti-correlation behavior between the short circuit current (JSC) and open circuit voltage (VOC). This phenomenon can be attributed to diffusion-limited surface recombination at the non-selective perovskite-tip contact, where a higher carrier mobility established in the perovskite grain results in an enhanced surface recombination and thus a lower VOC. In addition, the photoresponse of perovskite films displays a pronounced heterogeneity across the grain boundaries, with the boundaries formed between grains of the same photoconductivity level displaying even enhanced photocurrent and open circuit voltage compared to those of the adjacent grain interiors. These observations highlight the significance of controlling the microstructure of perovskite thin films, which will be a necessary route for further improving the efficiency of perovskite solar cells.