Chang-Lyoul Lee

Overcoming the electroluminescence efficiency limitations of perovskite light-emitting diodes

Brighter perovskite LEDs Organic-inorganic hybrid perovskites such as methyl ammonium lead halides are attractive as low-cost light-emitting diode (LED) emitters. This is because, unlike many inorganic nanomaterials, they have very high color purity. Cho et al. made two modifications to address the main drawback of these materials, their low luminescent efficiency. They created nanograin materials lacking free metallic lead, which helped to confine excitons and avoid their quenching. The perovskite LEDs had a current efficiency similar to that of phosphorescent organic LEDs. Science, this issue p. 1222 Efficient organic-inorganic perovskite light-emitting diodes were made with nanograin crystals that lack metallic lead. 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.

Multicolored Organic/Inorganic Hybrid Perovskite Light‐Emitting Diodes

Bright organic/inorganic hybrid perov-skite light-emitting diodes (PrLEDs) are realized by using CH3 NH3 PbBr3 as an emitting layer and self-organized buffer hole-injection layer (Buf-HIL). The PrLEDs show high luminance, current efficiency, and EQE of 417 cd m(-2) , 0.577 cd A(-1) , and 0.125%, respectively. Buf-HIL can facilitate hole injection into CH3 NH3 PbBr3 as well as block exciton quenching.

Polymer phosphorescent light-emitting devices doped with tris(2-phenylpyridine) iridium as a triplet emitter

We have fabricated phosphorescent polymer light-emitting devices with tris(2-phenylpyridine) iridium [Ir(ppy)3] as a triplet emissive dopant in poly(vinylcarbazole) (PVK) host. The device with 8% doping concentration of [Ir(ppy)3] in PVK showed the external quantum efficiency of 1.9% and the peak luminance of 2,500 cd/m2. The emission spectrum of the device exhibited no emission from PVK, indicating that the energy transfer from PVK to [Ir(ppy)3] is efficient. This work demonstrates that efficient electrophosphorescent light-emitting devices can be realized with polymers.

Energy transfer and device performance in phosphorescent dye doped polymer light emitting diodes

Singlet and triplet–triplet energy transfer in phosphorescent dye doped polymer light emitting devices were investigated. Poly(N-vinylcarbazol) and poly[9,9′-di-n-hexyl-2,7-fluorene-alt- 1,4-(2,5-di-n-hexyloxy)phenylene] (PFHP) were selected as the host polymer for the phosphorescent dopants fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)3] and 2,3,7,8,12,13, 17,18-octaethyl-21H,23H-porphyrin platinum(II) (PtOEP) because of their high triplet energy levels and long phosphorescence lifetimes. In case of PVK film, efficient triplet energy transfers to both PtOEP and Ir(ppy)3 were observed. In contrast, the triplet energy transfer did not occur or was very weak from PFHP to both PtOEP and Ir(ppy)3 although usual requirements for triplet energy transfer were satisfied. Furthermore, the singlet–singlet energy transfer did not take place from PFHP to Ir(ppy)3 in doped films even though the Forster radius is more than 30 A. However, the blended film of Ir(ppy)3 with PFHP and PMMA showed the green emission from ...

Electroluminescence from Graphene Quantum Dots Prepared by Amidative Cutting of Tattered Graphite

Size-controlled graphene quantum dots (GQDs) are prepared via amidative cutting of tattered graphite. The power of this method is that the size of the GQDs could be varied from 2 to over 10 nm by simply regulating the amine concentration. The energy gaps in such GQDs are narrowed down with increasing their size, showing colorful photoluminescence from blue to brown. We also reveal the roles of defect sites in photoluminescence, developing long-wavelength emission and reducing exciton lifetime. To assess the viability of the present method, organic light-emitting diodes employing our GQDs as a dopant are first demonstrated with the thorough studies in their energy levels. This is to our best knowledge the first meaningful report on the electroluminescence of GQDs, successfully rendering white light with the external quantum efficiency of ca. 0.1%.

Highly efficient polymer light-emitting diodes using graphene oxide as a hole transport layer.

We present an investigation of polymer light-emitting diodes (PLEDs) with a solution-processable graphene oxide (GO) interlayer. The GO layer with a wide band gap blocks electron transport from an emissive polymer to an ITO anode while reducing the exciton quenching between the GO and the active layer in place of poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS). This GO interlayer maximizes hole-electron recombinations within the emissive layer, finally enhancing device performance and efficiency levels in PLEDs. It was found that the thickness of the GO layer is an important factor in device performance. PLEDs with a 4.3 nm thick GO interlayer are superior to both those with PEDOT:PSS layers as well as those with rGO, showing maximum luminance of 39 000 Cd/m(2), maximum luminous efficiencies of 19.1 Cd/A (at 6.8 V), and maximum power efficiency as high as 11.0 lm/W (at 4.4 V). This indicates that PLEDs with a GO layer show a 220% increase in their luminous efficiency and 280% increase in their power conversion efficiency compared to PLEDs with PEDOT:PSS.

Molecularly Controlled Interfacial Layer Strategy Toward Highly Efficient Simple-Structured Organic Light-Emitting Diodes

A highly efficient simplified organic light-emitting diode (OLED) with a molecularly controlled strategy to form near-perfect interfacial layer on top of the anode is demonstrated. A self-organized polymeric hole injection layer (HIL) is exploited increasing hole injection, electron blocking, and reducing exciton quenching near the electrode or conducting polymers; this HIL allows simplified OLED comprised a single small-molecule fluorescent layer to exhibits a high current efficiency (∼20 cd/A).

An order/disorder/water junction system for highly efficient co-catalyst-free photocatalytic hydrogen generation

Surface engineering of TiO2 is faced with the challenge of high solar-to-hydrogen conversion efficiency. Recently, surface-disordered TiO2, referred to as black TiO2, which can absorb both visible and near-infrared solar light, has triggered an explosion of interest in many important applications. Unfortunately, the mechanism underlying the improved photocatalytic effect from an amorphous surface layer remains unclear and seems to contradict conventional wisdom. Here, we demonstrate selectively “disorder engineered” Degussa P-25 TiO2 nanoparticles using simple room-temperature solution processing, which maintain the unique three-phase interfaces composed of ordered white-anatase and disordered black-rutile with open structures for easy electrolyte access. The strong reducing agent in a superbase, which consists of lithium in ethylenediamine (Li-EDA), can disorder only the white-rutile phase of P-25, leaving behind blue coloured TiO2 nanoparticles. The order/disorder/water junction created by the blue P-25 can not only efficiently internally separate electrons/holes through type-II bandgap alignment but can also induce a strong hydrogen (H2) evolution surface reaction in the sacrificial agent containing electrolyte. As a result, the blue P-25 exhibited outstanding H2 production rates of 13.89 mmol h−1 g−1 using 0.5 wt% Pt (co-catalyst) and 3.46 mmol h−1 g−1 without using any co-catalyst.

CdSSe layer-sensitized TiO2 nanowire arrays as efficient photoelectrodes

Complete composition-tuned CdSxSe1−x alloy layers (avg. thickness = 50 nm) were deposited on pre-grown TiO2 nanowires by the thermal vapor transport of CdS/CdSe powders, producing core–shell nanocable arrays. CdSxSe1−x alloy nanowires were also synthesized with full composition tuning by the same method for comparison. The CdSSe nanowires consisted of Se-rich and S-rich pseudo binary phases, while the nanocable shell consisted of more complex multinary phases including CdSe and CdS. Remarkably, unique CdS–CdSSe–CdSe multishell structures were produced in the Se-rich composition range. The photoelectrochemical (PEC) cells fabricated using the as-grown nanocable arrays show higher solar photocurrents and hydrogen generation rates for the Se-rich shelled TiO2 nanocable arrays. This suggests that the CdS–CdSSe–CdSe multishell structures increase greatly the PEC performance by producing novel band alignment for efficient electron–hole separation following enhanced visible-range photon absorption.

Polymer electrophosphorescent device: comparison of phosphorescent dye doped and coordinated systems

Abstract We have synthesized a new polymer, (poly(Ir(ppy)2(2-(4-vinylphenyl)pyridine))-co-vinylcarbazole)) [Ir complex copolymer], containing both carbazole and iridium(III) complex as side groups and fabricated electrophosphorescent polymer light emitting devices. The 1MLCT transition of Ir complex copolymer is almost same position and absorption intensity of 8% Ir(ppy)3 doped PVK. At a lower molar concentration of the Ir complex copolymer, the polymer exhibited emission only from the carbazole units. However, with gradual increase in the concentration of the Ir complex copolymer, emission from the Ir complex became dominant. Present results indicated the energy transfer takes place by the intermolecular energy transfer not the intramolecular energy transfer. From the PL and EL of film state, the emission maximum was observed at 512 nm, due to the radiative decay from the 3MLCT state to the ground state, confirming almost a complete energy transfer from carbazole to Ir complex. The device showed 4.4% maximum external quantum efficiency and 5.0 lm/W power efficiency and peak luminance of 12,900 cd/m2.