Fabrication of efficient formamidinium perovskite solar cells under ambient air via intermediate-modulated crystallization

Developing simple methods to synthesize perovskite layers under ambient air can facilitate the industrial production of perovskite solar cells (PSCs). While limited progress has been made for the ambient-air fabrication of formamidinium lead triiodide perovskite (α-FAPbI₃) layers due to the coexisting non-perovskite polymorph (δ-FAPbI₃). Here, a facile N-methyl pyrrolidone (NMP) additive-based method is proposed to produce FA-perovskite (FAPbI₃) layers in ambient air and smooth, highly-crystallized and δ-phase free perovskite films are produced under humidity of ∼40%. The resultant PSC delivers a power conversion efficiency (PCE) of 17.29%, ∼20% enhancement from that of PSC produced from traditional dimethyl sulfoxide (DSMO) additive. It is found that a distinct FAI·PbI₂·NMP intermediate phase forms during the film growth, providing an alternative crystallization process different to that of traditional δ-FAPbI₃ intermediate route. The fine nucleation of FAI·PbI₂·NMP intermediate contributes a smooth and dense morphology to the final film. The rapid detachment of NMP molecule from the intermediate enables a direct formation of α-FAPbI₃ film, averting the incomplete δ- to α- phase conversion under ambient air. Due to the high film quality, a PCE of 13.55% is still remained after 30-day storage for unencapsulated device, demonstrating the reliability of our method for FA-perovskite ambient-air fabrication.