Steering the crystallization of perovskites for high-performance solar cells in ambient air

The fabrication of perovskite solar cells in ambient air is of significant economic benefit. However, to date, there is no satisfactory solution to achieve this, and the reported results of solar cells fabricated in ambient air vary significantly. It has been observed through studies of the "turbid point", a time point at which the appearance of the precursor film changes from transparent to turbid, that the crystallization of methylammonium lead iodide (MAPbI₃) precursor films is strongly affected by humidity. A theoretical model based on the dependence of crystallization on evaporation was developed. We further designed a "humidity-insensitive antisolvent method" based on the turbid point and its tuning to control the fabrication of high-quality MAPbI₃ films and solar cells irrespective of the ambient humidity levels. Note that a champion device prepared in ambient air at 90% RH exhibits the efficiency of 19.5%, which is highest among those of the devices prepared in ambient air with over 30% RH. Our study may also impact other perovskite devices and inspire other researchers to perform innovative experiments.