Phase distribution regulation of formamidinium-based quasi-2D perovskites through solution engineering.

Quasi-2D perovskites have attracted attention as potential solar energy absorber materials due to their balanced efficiency and stability and their unique quantum-well structures. In order to facilitate directional excitons and charge carrier transport and preferential energy transfer landscape in photovoltaic thin films, the phase distribution formed by different types of microstructural domains should be regulated. In this work, the Dion-Jacobson-type spacer 1,4-phenylenedimethanammonium (PDMA) was used, and different strategies were pursued to control the phase distribution in formamidinium-based (FA) quasi-2D perovskites based on the composition of (PDMA)FAPbI. In general, doping with FACl modulated the crystallization kinetics, forming 2D low- crystals on the top surface or a reversed-gradient phase distribution, depending on whether excess or substitutional doping was employed. Alternatively, mixing with a Ruddlesden-Popper spacer helped bridging to adjacent octahedra in pure PDMA-based perovskites and improved crystallization, while regulating the quantum-well structures to give a normal-gradient phase distribution, where 2D domains resided on the bottom side. By combining FACl doping and spacer mixing, the film showed both a reversed-gradient phase distribution and larger vertically aligned grains. This work contributes to the knowledge of how to manipulate and regulate the phase distribution in FA-based quasi-2D perovskites and further paves the way for fabricating corresponding devices with high efficiency and stability.