High‐Quality Pure‐Phase MA‐Free Formamdinium Dion‐Jacobson 2D Perovskites for Stable Unencapsulated Photovoltaics

Abstract It is generally difficult to achieve high quality in low‐n‐value (MA‐free) FA‐based Dion‐Jacobson (DJ) 2D perovskites due to their low crystallinity, random well‐width distribution of multiple quantum wells (QWs) with disordered orientation, which has seriously hindered further advances in photovoltaics. Meanwhile, pure‐phase DJ 2D perovskites with uniform QWs width and fixed low‐n values are highly desirable for high efficiency and stability. Herein a novel 4APP(FA) 3 Pb 4 I 13 ‐based quasi‐DJ‐2D perovskite is presented first, where 4APP is 4‐aminopiperidinium. The remarkable rigidity of 4APP and its strong hydrogen bonding with the perovskites prolong the perovskite intrinsic stability. the tri‐solvent engineering approach is further established by matching solvent properties to achieve pure‐phase DJ‐2D perovskites with uniform QWs width structure and finely oriented crystal grains, which enhance the power conversion efficiency of unencapsulated solar cells by 48%, representing one of the highest values in MA‐free DJ 2D perovskite with n≤4. Remarkably, the unencapsulated devices retain 95% of their initial PCE after 2000 h of operation under 1‐sun illumination at 40 °C, and 3000 h exposure in the air at 85 °C and 60–90% relative humidity (RH). The simultaneous control of rigidity and phase purity in DJ‐2D perovskites will open up new directions for fundamental research and application exploration.