LAUSR

The in situ fabricated perovskite nanocrystals (PNCs) obtained through spin‐coating a precursor solution are suitable candidates to achieve efficient perovskite light emitting diodes (PeLEDs). In this work, the solvent effects of on‐chip crystallization are investigated by correlating the nucleation and growth process of in situ fabricated formamidinium lead iodide (FAPbI3) nanocrystals with their optical and electronic properties. The FAPbI3 nanocrystals obtained from a precursor solution in γ‐butyrolactone (GBL) are smaller than those obtained from N,N‐dimethylformamide and dimethyl sulfoxide, and the relatively weak coordination between GBL and the precursor molecules enables reduced defect states in the resulted PNCs with enhanced photoluminescence properties. A modified LaMer model is proposed to describe the solvent effects in the on‐chip crystallization process. Based on these understandings, red emissive FA0.87Cs0.13PbI3 nanocrystal films with absolute photoluminescence quantum yields up to 70% are realized. Finally, an efficient PeLED with maximum luminance of 218 cd m−2 and peak external quantum efficiency of 15.8% is achieved with good reproducibility.