NH4Br‐Assisted Two‐Step‐Processing of Guanidinium‐Rich Perovskite Films for Extremely Stable Carbon‐Based Perovskite Solar Cells in Ambient Air

The substitution of a portion of methylammonium (MA) for guanidinium (GA) has been verified to be able to enhance the stability of MA‐based devices. However, high‐dose guanidinium cation will introduce localized distortions to the perovskite lattice structure and destroy the microstructure of the perovskite films, impairing the stability and reproducibility of perovskite solar cells (PSCs) eventually. Herein, for the first time, the NH4Br‐assisted all‐atmospheric two‐step process is adopted to fabricate GA‐rich (20%) perovskite films. The NH4Br induces the formation of the intermediate phase NH4PbI3 and alleviates the disorder of the octahedron caused by the big GA. Consequently, the modified perovskite film shows increased tolerance for the roughness fluctuation and reduced risk of forming voids and pinholes. The fabricated compact GA‐rich perovskite films behave extremely well in photovoltaic performance when assembled as carbon‐based perovskite solar cells, delivering a high power conversion efficiency (PCE) of 16.19% and stability against moisture and sunlight. Especially, the unencapsulated devices in ambient air sustain 95.1%, 91.8%, and 95.7% of their initial PCEs after 2400 h of storage, after 1000 h of 65 °C heat environment, and after 800 h of sunlight illumination, respectively. Guanidinium (GA)‐rich perovskite films are prepared via an NH4Br‐assisted two‐step process in air conditions, in which NH4Br is introduced to facilitate the transient phase (NH4PbI3) formation and reduce voids and pinholes in films. The fabricated carbon‐based perovskite solar cells (C‐PSCs) in ambient air deliver the highest efficiency of 16.19% and exhibit excellent stability against moisture, heat, and sunlight.