Effects of Illumination Direction on the Surface Potential of CH3NH3PbI3 Perovskite Films Probed by Kelvin Probe Force Microscopy

The development of organicinorganic hybrid perovskite solar cells requires critical understanding in the charge-carrier behaviors in the perovskite light absorbers and devices. Kelvin probe force microscopy (KPFM) has been applied as a powerful tool to probe the electrical potential distribution of perovskite films and devices, providing fundamental insights into their charge-carrier properties. When measuring the material photoresponses, various approaches have been employed to illuminate the samples. Here, we measured the surface potential of the layer in the regular mesoporous structure (CH3NH3PbI3/m-TiO2/c-TiO2/FTO) and inverted planar structure (CH3NH3PbI3/NiO/FTO) devices via KPFM. Effects of two representative illumination methods are compared-illumination from top, and from underneath through the transparent glass substrate. By comparing the variation in surface potential under two illumination methods, the surface potential of the perovskite-absorbing layer in a regular structure is higher than that in the inverted structure. The potential difference in two structures implies that the photogenerated charge carriers are injected to the TiO2 electron-transport layer and NiO hole-transport layer, resulting in positive charges and negative charges accumulated in the perovskite-absorbing layer. We demonstrated that the illumination direction has an impact on the surface potential measurement. For the CH3NH3PbI3/TiO2 structure, illumination from underneath facilitates a larger potential change. While for the CH3NH3PbI3/NiO structure with insensitive photoresponse in potential change, the illumination direction has a minor effect.