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

ConspectusColloidal metal halide perovskite nanocrystals (PNCs) have high color purity, solution processability, high luminescence efficiency, and facile color tunability in visible wavelengths and therefore show promise as light emitters in next-generation displays. The external quantum efficiency (EQE) of PNC light-emitting diodes (LEDs) has been rapidly 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-LEDs have been impeded for three reasons: (1) Colloidal PNCs consist of ionic crystal structures in which ligands bind dynamically and therefore easily agglomerate in colloidal solution and films; (2) Long-alkyl-chain organic ligands that adhere to the PNC surface improve the photoluminescence quantum efficiency and colloidal stability of PNCs in solution but impede charge transport in PNC films and limit their electroluminescence efficiency in LEDs; (3) Unoptimized device structure and nonuniform PNC films limit the charge balance and reduce the device efficiency in PNC-LEDs.In this Account, we summarize strategies to solve the limitations in PNCs and PNC-LEDs as consequences of photoluminescence quantum efficiency in PNCs and the charge-balance factor and out-coupling factor in LEDs, which together determine the EQE of PNC-LEDs. We introduce the fundamental photophysical properties of colloidal PNCs related to effective mass of charge carriers and surface stoichiometry, requirements for PNC surface stabilization, and subsequent research strategies to demonstrate highly efficient colloidal PNCs and PNC-LEDs with high operating stability.First, we present various ligand-engineering strategies that have been used to achieve both efficient carrier injection and radiative recombination in PNC films. In situ ligand engineering reduces ligand length and concentration during synthesis of colloidal PNCs, and it can achieve size-independent high color purity and high luminescent efficiency in PNCs. Postsynthesis ligand engineering such as optimized purification, replacement of organic ligands with inorganic ligands or strongly bound ligands can increase charge transport and coupling between PNC dots in films. The luminescence efficiency of PNCs and PNC-LEDs can be further increased by various postsynthesis ligand-engineering methods or by sequential treatment with different ligands. Second, we present methods to modify the crystal structure in PNCs to have alloy- or core/shell-like structure. Such crystal engineering is performed by the correlation between entropy and enthalpy in PNCs and result in increased carrier confinement (increased radiative recombination) and reduced defects (decreased nonradiative recombination). Third, we present strategies to boost the charge-balance factor and out-coupling factor in PNC-LEDs such as modification of thickness of each layer and insertion of additional interlayers, and out-coupling hemispherical lens are discussed. Finally, we present the advantages, potential, and remaining challenges to be solved to enable use of colloidal PNCs in commercialized industrial displays and solid-state lighting. We hope this Account will help its readers to grasp the progresses and perspectives of colloidal PNCs and PNC-LEDs, and that our insights will guide future research to achieve efficient PNC-LEDs that have high stability and low toxicity.