Construction of a gradient-type 2D/3D perovskite structure for subsurface passivation and energy-level alignment of an MAPbI3 film

Abstract

Passivating defects on the surface and at the grain boundaries of perovskite films has been recognized as a rational approach to improve the performances of perovskite solar cells (PSCs). However, less attention has been paid to the subsurface region in which defect states are also spread, and there is lack of strategies available for the passivation of subsurface defects. Here, by regulating the amount of PbI₂ on the surface of the MAPbI₃ film, we constructed a gradient-type 2D/3D perovskite structure in which the 2D perovskite transition layer forms in the subsurface region of the 3D perovskite film, quite different from the widely studied bilayered 2D/3D perovskite structure. Compared to the latter, which mainly passivates the surface defects, the gradient-type 2D/3D structure can heal both the surface and subsurface defects, more efficiently suppressing the charge recombination. Moreover, only small n value (such as n = 1–3) 2D perovskites are found in the bilayered 2D/3D film, but larger n value 2D species are also revealed in the gradient-type film, leading to a gradient energy-level alignment that can promote the hole transport/extraction. Thus, the gradient-type 2D/3D structure performs better in PSCs than its bilayered counterpart, demonstrating the significance of the subsurface defect passivation and the energy-level gradient for the performance of PSCs.