Engineering Colloidal Perovskite Nanocrystals and Devices for Efficient and Large-Area Light-Emitting Diodes

Conspectus Colloidal metal halide perovskite nanocrystals(PNCs) have highcolor purity, solution processability, high luminescence efficiency,and facile color tunability in visible wavelengths and therefore showpromise as light emitters in next-generation displays. The externalquantum efficiency (EQE) of PNC light-emitting diodes (LEDs) has beenrapidly increased to reach 24.96% by using colloidal PNCs and 28.9%using on-substrate in situ synthesized PNCs. However,high operating stability and a further increase of EQE in PNC-LEDshave been impeded for three reasons: (1) Colloidal PNCs consist ofionic crystal structures in which ligands bind dynamically and thereforeeasily agglomerate in colloidal solution and films; (2) Long-alkyl-chainorganic ligands that adhere to the PNC surface improve the photoluminescencequantum efficiency and colloidal stability of PNCs in solution butimpede charge transport in PNC films and limit their electroluminescenceefficiency in LEDs; (3) Unoptimized device structure and nonuniformPNC films limit the charge balance and reduce the device efficiencyin PNC-LEDs. In this Account, we summarize strategies to solvethe limitationsin PNCs and PNC-LEDs as consequences of photoluminescence quantumefficiency in PNCs and the charge-balance factor and out-couplingfactor in LEDs, which together determine the EQE of PNC-LEDs. We introducethe fundamental photophysical properties of colloidal PNCs relatedto effective mass of charge carriers and surface stoichiometry, requirementsfor PNC surface stabilization, and subsequent research strategiesto demonstrate highly efficient colloidal PNCs and PNC-LEDs with highoperating stability. First, we present various ligand-engineeringstrategies that havebeen used to achieve both efficient carrier injection and radiativerecombination in PNC films. In situ ligand engineeringreduces ligand length and concentration during synthesis of colloidalPNCs, and it can achieve size-independent high color purity and highluminescent efficiency in PNCs. Postsynthesis ligand engineering suchas optimized purification, replacement of organic ligands with inorganicligands or strongly bound ligands can increase charge transport andcoupling between PNC dots in films. The luminescence efficiency ofPNCs and PNC-LEDs can be further increased by various postsynthesisligand-engineering methods or by sequential treatment with differentligands. Second, we present methods to modify the crystal structurein PNCs to have alloy- or core/shell-like structure. Such crystalengineering is performed by the correlation between entropy and enthalpyin PNCs and result in increased carrier confinement (increased radiativerecombination) and reduced defects (decreased nonradiative recombination).Third, we present strategies to boost the charge-balance factor andout-coupling factor in PNC-LEDs such as modification of thicknessof each layer and insertion of additional interlayers, and out-couplinghemispherical lens are discussed. Finally, we present the advantages,potential, and remaining challenges to be solved to enable use ofcolloidal PNCs in commercialized industrial displays and solid-statelighting. We hope this Account will help its readers to grasp theprogresses and perspectives of colloidal PNCs and PNC-LEDs, and thatour insights will guide future research to achieve efficient PNC-LEDsthat have high stability and low toxicity.