Su-Hun Jeong

Organic-inorganic hybrid perovskites are emerging low-cost emitters with very high color purity, but their low luminescent efficiency is a critical drawback. We boosted the current efficiency (CE) of perovskite light-emitting diodes with a simple bilayer structure to 42.9 candela per ampere, similar to the CE of phosphorescent organic light-emitting diodes, with two modifications: We prevented the formation of metallic lead (Pb) atoms that cause strong exciton quenching through a small increase in methylammonium bromide (MABr) molar proportion, and we spatially confined the exciton in uniform MAPbBr3 nanograins (average diameter = 99.7 nanometers) formed by a nanocrystal pinning process and concomitant reduction of exciton diffusion length to 67 nanometers. These changes caused substantial increases in steady-state photoluminescence intensity and efficiency of MAPbBr3 nanograin layers.

Perovskite light-emitting diodes (PeLEDs) based on three-dimensional (3D) polycrystalline perovskites suffer from ion migration, which causes overshoot of luminance over time during operation and reduces its operational lifetime. Here, we demonstrate 3D/2D hybrid PeLEDs with extremely reduced luminance overshoot and 21 times longer operational lifetime than 3D PeLEDs. The luminance overshoot ratio of 3D/2D hybrid PeLED is only 7.4% which is greatly lower than that of 3D PeLED (150.4%). The 3D/2D hybrid perovskite is obtained by adding a small amount of neutral benzylamine to methylammonium lead bromide, which induces a proton transfer from methylammonium to benzylamine and enables crystallization of 2D perovskite without destroying the 3D phase. Benzylammonium in the perovskite lattice suppresses formation of deep-trap states and ion migration, thereby enhances both operating stability and luminous efficiency based on its retardation effect in reorientation.

We have achieved high-efficiency polycrystalline perovskite light-emitting diodes (PeLEDs) based on formamidinium (FA) and cesium (Cs) mixed cations without quantum dot synthesis. Uniform single-phase FA1–xCsxPbBr3 polycrystalline films were fabricated by one-step formation with various FA:Cs molar proportions; then the influences of chemical composition on film morphology, crystal structure, photoluminescence (PL), and electroluminescence (EL) were systematically investigated. Incorporation of Cs+ cations in FAPbBr3 significantly reduced the average grain size (to 199 nm for FA:Cs = 90:10) and trap density; these changes consequently increased PL quantum efficiency (PLQE) and PL lifetime of FA1–xCsxPbBr3 films and current efficiency (CE) of PeLEDs. Further increase in Cs molar proportion from 10 mol % decreased crystallinity and purity, increased trap density, and correspondingly decreased PLQE, PL lifetime, and CE. Incorporation of Cs also increased photostability of FA1–xCsxPbBr3 films, possibly due to suppressed formation of light-induced metastable states. FA1–xCsxPbBr3 PeLEDs show the maximum CE = 14.5 cd A–1 at FA:Cs = 90:10 with very narrow EL spectral width (21–24 nm); this is the highest CE among FA-Cs-based PeLEDs reported to date. This work provides an understanding of the influences of Cs incorporation on the chemical, structural, and luminescent properties of FAPbBr3 polycrystalline films and a breakthrough to increase the efficiency of FA1–xCsxPbBr3 PeLEDs.

We introduce a simple, inexpensive, and large-area flexible transparent lamination encapsulation method that uses graphene films with polydimethylsiloxane (PDMS) buffer on polyethylene terephthalate (PET) substrate. The number of stacked graphene layers (nG) was increased from 2 to 6, and 6-layered graphene-encapsulation showed high impermeability to moisture and air. The graphene-encapsulated polymer light emitting diodes (PLEDs) had stable operating characteristics, and the operational lifetime of encapsulated PLEDs increased as nG increased. Calcium oxidation test data confirmed the improved impermeability of graphene-encapsulation with increased nG. As a practical application, we demonstrated large-area flexible organic light emitting diodes (FOLEDs) and transparent FOLEDs that were encapsulated by our polymer/graphene encapsulant.