Overcoming the electroluminescence efficiency limitations of perovskite light-emitting diodes.

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.This work was partially supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA-1402-07. A.S. was partially supported by the Engineering and Physical Sciences Research Council (UK).This is the author accepted manuscript. It is currently under an indefinite embargo pending publication by the American Association for the Advancement of Science

Efficient Ruddlesden-Popper Perovskite Light-Emitting Diodes with Randomly Oriented Nanocrystals

Ruddlesden-Popper phase (RP-phase) perovskites that consist of 2D perovskite slabs interleaved with bulky organic ammonium (OA) are favorable for light-emitting diodes (LEDs). The critical limitation of LED applications is that the insulating OA arranged in a preferred orientation limits charge transport. Therefore, the ideal solution is to achieve a randomly connected structure that can improve charge transport without hampering the confinement of the electron-hole pair. Here, a structurally modulated RP-phase metal halide perovskite (MHP), (PEA)(2)(CH3NH3)(m-1)PbmBr3m+1 is introduced to make the randomly oriented RP-phase unit and ensure good connection between them by applying modified nanocrystal pinning, which leads to an increase in the efficiency of perovskite LEDs (PeLEDs). The randomly connected RP-phase MHP forces contact between inorganic layers and thereby yields efficient charge transport and radiative recombination. Combined with an optimal dimensionality, (PEA)(2)(CH3NH3)(2)Pb3Br10, the structurally modulated RP-phase MHP exhibits increased photoluminescence quantum efficiency, from 0.35% to 30.3%, and their PeLEDs show a 2,018 times higher current efficiency (20.18 cd A(-1)) than in the 2D PeLED (0.01 cd A(-1)) and 673 times than in the 3D PeLED (0.03 cd A(-1)) using the same film formation process. This approach provides insight on how to solve the limitation of RP-phase MHP for efficient PeLEDs.

How to improve the structural stabilities of halide perovskite quantum dots: review of various strategies to enhance the structural stabilities of halide perovskite quantum dots

Abstract Halide perovskites have emerged as promising materials for various optoelectronic devices because of their excellent optical and electrical properties. In particular, halide perovskite quantum dots (PQDs) have garnered considerable attention as emissive materials for light-emitting diodes (LEDs) because of their higher color purities and photoluminescence quantum yields compared to conventional inorganic quantum dots (CdSe, ZnSe, ZnS, etc.). However, PQDs exhibit poor structural stabilities in response to external stimuli (moisture, heat, etc.) owing to their inherent ionic nature. This review presents recent research trends and insights into improving the structural stabilities of PQDs. In addition, the origins of the poor structural stabilities of PQDs and various methods to overcome this drawback are discussed. The structural degradation of PQDs is mainly caused by two mechanisms: (1) defect formation on the surface of the PQDs by ligand dissociation (i.e., detachment of weakly bound ligands from the surface of PQDs), and (2) vacancy formation by halide migration in the lattices of the PQDs due to the low migration energy of halide ions. The structural stabilities of PQDs can be improved through four methods: (1) ligand modification, (2) core–shell structure, (3) crosslinking, and (4) metal doping, all of which are presented in detail herein. This review provides a comprehensive understanding of the structural stabilities and opto-electrical properties of PQDs and is expected to contribute to future research on improving the device performance of perovskite quantum dot LEDs (PeLEDs)

Polymer Based Blue Electrophosphorescent Light Emitting Diode using a Bis-Orthometalated Ir(III) Complex as the Triplet Emitter

Polymer-based blue electrophosphorescent light-emitting diodes are reported by doping bisorthometalated (phosphine)cyanoiridium(III) complex Ir(ppy)2P(n-Bu)3CN (ppy = 2-phenylpyridine, P(n-Bu)3 = tri-n-butylphosphine) in poly(vinylcarbazole) (PVK) as the source of emission. The (phosphine)cyanoiridium(III) complex shows sky blue emission with two peaks at 467 and 496 nm originating from an admixture of triplet metal-to-ligand charge-transfer and ligand-centered states. The PL and EL spectra of an Ir(ppy)2P(n-Bu)3CN-doped PVK film and device exhibited no emission from PVK, indicating that the energy transfer from PVK to Ir(ppy)2P(n-Bu)3CN is efficient. A maximum external quantum efficiency (ηex) of 1.45% and power efficiency (ηp) of 0.99 lm/W were achieved at 13 and 6 mA/cm2 at a 10 wt % doping concentration, respectively.This work was financially supported by CRM-KOSEF, Korea University, and Dongwoo Fine-Chem Co., Ltd. R.R.D. is thankful to the government of Orissa, India, for granting a study leave

Piezochromic Fluorescence in Liquid Crystalline Conjugated Polymers

Liquid crystalline diphenylacetylene polymer derivatives showed piezochromic fluorescence via order-to-disorder phase transition