The reactive nature of formamidinium: Amine passivation induces heterostructure formation in inverted perovskite cells

Abstract

Surface passivation via binding molecules is a strategy for eliminating electronic defects that arise from the abrupt termination of semiconductor lattices. Passivation is used in many systems, such as silanes on silicon surfaces, ligands on nanocrystals, and at interfaces. However, in halide perovskites, such strategies lead to complicated effects, such as the disruption of the crystal structure via molecular intercalation or change of dimensionality. Here, we demonstrate and detail that short diamines undergo a chemical reaction with the existing A-site molecule rather than attaching, forming a ringed molecule and thus, in turn, creating a well-defined and self-limited, lower-dimensional-oriented perovskite-perovskite heterostructure at the surface. Furthermore, we detail how the solvent polarity and the molecule of choice impact the final reaction composition and their effect on device performance. Our findings offer new insights into the perovskite surface chemistry of high-efficiency solar cells.