Highly Efficient and Stable Dion−Jacobson Perovskite Solar Cells Enabled by Extended π‐Conjugation of Organic Spacer

2D Dion-Jacobson (DJ) perovskites have become an emerging photovoltaic material with excellent structure and environmental stability due to their lacking van der Waals gaps relative to 2D Ruddlesden-Popper perovskites. Here, a fused-thiophene-based spacer, namely TTDMAI, is successfully developed for 2D DJ perovskite solar cells. It is found that the DJ perovskite using TTDMA spacer with extended π-conjugation length exhibits high film quality, large crystal size and preferred crystal vertical orientation induced by the large crystal nuclei in precursor solution, resulting in lower trap density, reduced exciton binding energy and oriented charge transport. As a result, the optimized 2D DJ perovskite device based on TTDMA (nominal n = 4) delivers a champion PCE up to 18.82%. Importantly, the unencapsulated device based on TTDMA can sustain average 99% of their original efficiency after being stored in N2 for 4400 h (over 6 months). Moreover, light, thermal, environmental and operational stabilities are also significantly improved in comparison with their 3D counterparts.