High efficiency perovskite light-emitting diodes of ligand-engineered colloidal formamidinium lead bromide nanoparticles

Formamidinium (FA, CH(NH2)(2)) lead bromide perovskite (FAPbBr(3)) nanoparticles (NPs) are promising emitters due to their high stability and ability to emit pure green color in both film and solution states. Even though various types of metal halide NP emitters in solution have shown high photoluminescence quantum efficiencies (PLQEs), electroluminescence efficiencies of the light-emitting diodes (LEDs) using the NP films are still much poorer, possibly due to the insulating ligands which can impede the charge injection and transport in films. Therefore, the organic ligand of NPs should be designed to facilitate charge injection and transport in LEDs. Here, we synthesize ligand-engineered colloidal FAPbBr(3) NPs at RT and demonstrate high efficiency perovskite NP LEDs based on the FAPbBr(3) NPs. Control of ligand length reduces trap-assisted recombination of carriers at the surface traps, and thus maximizes the PLQE of FAPbBr(3) NPs. Ligand engineering can also improve the charge injection and transport capability in FAPbBr(3) NP films. With this ligand engineering method, we achieve maximum current efficiency of 9.16 cd/A in LEDs based on FAPbBr(3) NPs, which is the highest efficiency in FAPbBr(3) NP-LEDs to date. The ligand engineering method reported here can be a simple way to improve the luminescence efficiency of optoelectronic devices based on perovskite NP LEDs.