Quasi-2D Ruddlesden-Popper phase induced preferentially oriented FA1-xMAxPbI3 for high-performance sequential perovskite solar cells

A multitude of methods have been developed to obtain highly crystallized and defect-free perovskite thin films. An excess of large-size cations within the bulk layer can detrimentally impact charge transport, resulting in relatively suboptimal solar cell performance. In this study, we substitute a portion of methylammonium chloride with an appropriate amount of long-chain n-butylammonium chloride in order to modulate the crystallization kinetics of two-step FAPbI3 polycrystalline films. We show that the addition of BA+ cation facilitates the in-situ formation of Ruddlesden–Popper quasi-2D phases that enables the growth control of 3D α-FAPbI3 perovskite films. Furthermore, the mixed alkylammonium system attenuates the elimination rate of dissociation of hydrochloride and alkylamine from deprotonated alkylammonium Cl-salts, yielding films with homogeneous morphology and negligible trap-mediated recombination. Remarkably, only traces of BA molecules are detected on the layer surface after complete induction of the oriented growth. The optimal device employing the mixed alkylammonium chloride achieves a power conversion efficiency of 23.23% and exhibit an exceptional stability