Comprehensive Strategies to Increase Electroluminescence Efficiency in Colloidal Metal-Halide Perovskite Nanocrystals

As the technology has advanced and the amount of information grows in an information society, display technology needs to show information more vividly and efficiently. In this sense of line, display technology has found a high color purity light emitter, metal halide perovskite, and tried to demonstrate high efficiency light-emitting diodes (LEDs). However, device efficiency of perovskite LEDs is limited by lack of comprehensive materials design strategies both to suppress formation of defects and to enhance charge carrier confinement inside perovskite nanocrystals (PNCs). Here, we report comprehensive strategies that generate smaller, monodisperse colloidal particles (confining electrons and holes and boosting radiative recombination) with fewer surface defects (reducing nonradiative recombination) to demonstrate high efficiency perovskite LEDs.<br/>Firstly, we performed ligand engineerings during the PNC synthesis. We synthesized methylammonium lead bromide (MAPbBr₃) and formamidinium lead bromide (FAPbBr₃) PNCs with a dimension larger than exciton Bohr diameter (regime beyond quantum size), which have emitting light with size-insensitively high color-purity and constant wavelength, by controlling ligand concentration and ligand length, respectively. Ligand-engineered PNCs can increase photoluminescence quantum efficiency (PLQE) upto 70% by preventing non-radiative recombination of charge carriers and also improve the charge injection/transport capability in PNC films, resulting in external quantum efficiency (EQE) of 5.5% in PNC-LEDs.<br/>Secondly, we conducted crystal engineering on the PNCs. We used substitutional doping of guanidinium (GA) into FAPbBr₃ PNCs to incorporate an optimal proportion of GA cations into the structure. The GA cations can reside in the bulk of the PNC in low concentrations (~10%). The surplus GA then accumulates on the surface of the PNC. Guanidinium incorporation provides bulk entropy stabilization, surface stabilization (by additional hydrogen bonding contributed by their extra amino group), and better electron–hole confinement, which results in high PLQE over 90%. A GA to FA ratio of 0.1 was shown to maximize the electroluminescence efficiency. Moreover, a 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene (TBTB) overcoat was introduced to heal the residual halide vacancy defects in FA_1–xGA_xPbBr₃ PNCs and to improve the charge balance in PeLEDs. The result is highly efficient PNC-based LEDs that have current efficiency of 108 cd/A (EQE of 23.4%), which rises to 205 cd/A (EQE of 45.5%) with a hemispherical lens.<br/>Furthermore, we report that highly efficient large-area perovskite LEDs with high uniformity can be realized through the use of colloidal PNCs which decouple the crystallization of perovskites from film formation. PNCs are precrystallized and surrounded by organic ligands, and thus they are not affected by the film formation process, in which a simple modified bar-coating method facilitates the evaporation of residual solvent to provide uniform large-area films. PeLEDs incorporating the uniform bar-coated PNC films achieve an external quantum efficiency (EQE) of 23.26% for a pixel size of 4mm² and EQE of 22.5% for a large pixel area of 102mm² with high reproducibility and EQE of 21.46% in 900mm².