Efficient Light-Emitting Diodes Based on <i>in Situ</i> Fabricated FAPbBr₃ Nanocrystals: The Enhancing Role of the Ligand-Assisted Reprecipitation Process

In this paper, we reported the in situ fabrication of highly luminescent formamidinium lead bromide (FAPbBr₃) nanocrystal thin films by dropping toluene as an anti-solvent during the spin-coating with a perovskite precursor solution using 3,3-diphenylpropylamine bromide (DPPA-Br) as a ligand. The resulting films are uniform and composed of 5-20 nm FAPbBr₃ perovskite nanocrystals. By monitoring the solvent mixing of anti-solvent and precursor solution on the substrates, we illustrated the difference between the ligand-assisted reprecipitation (LARP) process and the nanocrystal-pinning (NCP) process. This understanding provides a guideline for film optimization, and the optimized films obtained through the in situ LARP process exhibit strong photoluminescence emission at 528 nm, with quantum yields up to 78% and an average photoluminescence lifetime of 12.7 ns. In addition, an exciton binding energy of 57.5 meV was derived from the temperature-dependent photoluminescence measurement. More importantly, we achieved highly efficient pure green perovskite based light-emitting diode (PeLEDs) devices with an average external quantum efficiency (EQE) of 7.3% (maximum EQE is 16.3%) and an average current efficiency (CE) of 29.5 cd A^-1 (maximum CE is 66.3 cd A^-1) by adapting a conventional device structure of ITO/PEDOT:PSS/TFB/perovskite film/TPBi/LiF/Al. It is expected that the in situ LARP process provides an effective methodology for the improvement of the performance of PeLEDs.