High‐Quality Additive‐Free α‐FAPbI3 Film Fabricated by Alkane/Nanocrystals Method and Surface Chemistry Modulation for Efficient Perovskite Solar Cell

Abstract It's still a scientific issue to synthesize exotic amines‐free and pure‐phase FAPbI 3 film because most FAPbI 3 films are synthesized by adding MACl‐like exotic amines salts in their precursor solution to reduce the phase transition barrier. In this work, high‐quality and additive‐free phase‐pure α‐FAPbI 3 films are successfully fabricated via the sustainable Alkane/Nanocrystals method. FAPbI 3 nanocrystals (NCs) are performed as the heterogeneous nuclei to promote the nucleation and crystallization of FAPbI 3 films. Cryogenic‐TEM and in situ GIWAXS evidence the seed function and interesting phase evolution of FAPbI 3 NCs. The whole phase transition course of pure FAPbI 3 film is systematically studied. IR‐AFM and ToF‐SIMS reveal ligands capped on NCs are extruded to the film surface. PbAc‐IPA solution treating the film surface can effectively wash off the surplus ligands at the surface and modulate the surface chemistry and energy levels. As a result, an average power conversion efficiency (PCE) of 21.45% is achieved for MA‐free pure α‐FAPbI 3 perovskite solar cells (PSCs), as well as the best light‐soaking stability record of retaining 95% of the initial PCE after 800 h. This work paves a new way to fabricate MA‐free pure α‐FAPbI 3 PSCs.