Oriented Low‐n Ruddlesden‐Popper Formamidinium‐Based Perovskite for Efficient and Air Stable Solar Cells

The relatively lower crystallinity and random orientation of quantum well structures hinder carrier transport and limit the performance of formamidinium (FA) based low-n 2D perovskite devices. In this work, the crystallization and quantum well orientation are fine tuned to achieve efficient low-n FA based Ruddlesden-Popper perovskite solar cells. The effects of different ionic additives on the crystallization, orientation, and photovoltaic performance of FA based low-n 2D perovskites are comparatively investigated. It is found that NH4+ and SCN- can significantly retard the heterogeneous crystallization of low-n phases in the intermediate state, and drive the templated growth of quantum well structure with vertical orientation. The optimized photovoltaic device based on (BA)(2)(FA)(3)Pb4I13 achieved a power conversion efficiency (PCE) of 18.14%, setting the highest record for FA-based low-n 2D perovskite solar cells as far as it is known (average n = 4). The unencapsulated device exhibited excellent stability and maintained 93.3% of its original PCE at 85 degrees C, and 86.3% at 1 Sun illumination after 720 h in humid ambient condition.