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

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)3Pb4I13-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 degrees C, and 3000 h exposure in the air at 85 degrees 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. This work presents a novel 4APP(FA)3Pb4I13-based quasi-DJ-2D perovskite, where 4APP is 4-aminopiperidinium. The remarkable rigidity of 4APP prolongs the perovskite intrinsic stability. It also establishes tri-solvent engineering to achieve pure-phase DJ-2D perovskites that improve solar cell efficiency by 48%. It is concluded that controlling rigidity and phase purity simultaneously in 2D perovskites will lead to new directions for fundamental research.image