Stabilizing 2D perovskite passivation layer with mixed spacer cations for efficient and stable perovskite solar cells

Post-treating perovskite films with phenylethylammonium iodide (PEAI) is a commonly used method to passivate surface defects for high performance perovskite solar cells (PSCs). However, this post-treatment usually leads to a rapid performance decay in the initial stage during the stability measurements, which are frequently observed in literature. In this work, we first disclose the degradation mechanism of the PEAI induced 2D perovskites, in which (PEA)2PbI4 (n =1 phase) degrades gradually over time instead of (PEA)2FAPb2I7 (n = 2 phase). To address this problem, we substitute parts of PEA+ with NMA+ in the n =1 phase, which consequently stabilizes in air over time with respect to the structures and optoelectronic properties. Moreover, the obtained 2D perovskites can harvest hot charge carriers generated in the 3D counterparts. As a result, the related PSCs deliver a champion power conversion efficiency (PCE) of 25.7 % with a VOC of 1.187 V. Moreover, the alleviated performance degradation is observed in the initial stage of the stability test, which is manifest in only 6 % power conversion efficiency (PCE) loss after being aged in air (relative humidity of 30 %-40 %) for 100 h, compared to 20 % decay for the PEAI-treated device.