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In situ cyclized polyacrylonitrile (CPAN) is developed to replace n‐type metal oxide semiconductors (TiO2 or SnO2) as an electron selective layer (ESL) for highly efficient and stable n‐i‐p perovskite solar cells (PSCs). The CPAN layer is fabricated via facile in situ cyclization reaction of polyacrylonitrile (PAN) coated on a conducting glass substrate. The CPAN layer is robust and insoluble in common solvents, and possesses n‐type semiconductor properties with a high electron mobility of 4.13×10−3 cm2 V−1 s−1. With the CPAN as an ESL, the PSC affords a power conversion efficiency (PCE) of 23.12 %, which is the highest for the n‐i‐p PSCs with organic ESLs. Moreover, the device with the CPAN layer holds superior operational stability, maintaining over 90 % of their initial efficiency after 500 h continuous light soaking. These results confirm that the CPAN layer would be a desirable low‐cost and efficient ESL for n‐i‐p PSCs and other photoelectronic devices with high performance and stability. [ABSTRACT FROM AUTHOR]

• A single-crystal complex FA 4 Pb 3 I 6 Ac 4 is synthesized. • FAPbI 3 perovskite film has been fabricated from FA 4 Pb 3 I 6 Ac 4 via anion-exchange. • The FAPbI 3 film is dense and uniform with low trap-state density. • The PSC shows a PCE of 23.65% with enhanced stability. PbI 2 as precursor has been extensively employed for the fabrication of organic–inorganic hybrid perovskite films for high-efficiency perovskite solar cells (PSCs) by using two-step solution deposition method. However, the resultant perovskite film suffers from the incomplete reaction of PbI 2 and the volumetric expansion of the film, affecting both the performance and stability of the PSCs. Here, a single-crystal complex FA 4 Pb 3 I 6 Ac 4 (Ac−: CH 3 COO−) is synthesized, and utilized to replace PbI 2 for the fabrication of FAPbI 3 -based perovskite film in ambient conditions. The solution-processed crystal complex has good film-forming ability, and can be directly converted into stable black-phase FAPbI 3 perovskite at room temperature through anion exchange reaction between Ac− and I− upon coating with iodide salt solution. By the strategy, a dense and uniform perovskite film is obtained with reduced trap-state density and prolonged charge lifetime. The champion device delivers a power conversion efficiency (PCE) of 23.65% with high reproductivity. Importantly, the unencapsulated device exhibits a dramatically improved operational stability under temperature and humidity conditions. [ABSTRACT FROM AUTHOR]