Organic light-emitting diodes (OLEDs) have attracted much attention in research and industry thanks to their capability to emit light with high efficiency and to deliver high-quality white light that provides good color rendering. OLEDs feature homogeneous large area emission and can be produced on flexible substrates. In terms of efficiency, OLEDs can compete with highly efficient conventional light sources but their efficiency typically decreases at high brightness levels, an effect known as efficiency roll-off. In recent years, much effort has been undertaken to understand the underlying processes and to develop methods that improve the high-brightness performance of OLEDs. In this review, we summarize the current knowledge and provide a detailed description of the relevant principles, both for phosphorescent and fluorescent emitter molecules. In particular, we focus on exciton-quenching mechanisms, such as triplet-triplet annihilation, quenching by polarons, or field-induced quenching, but also discuss mechanisms such as changes in charge carrier balance. We further review methods that may reduce the roll-off and thus enable OLEDs to be used in high-brightness applications.

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Efficiency Roll-Off in Organic Light-Emitting Diodes

New benzoimidazoles (bi) have been synthesized. These compounds readily undergo cyclometalation with iridium trichloride, and bis-cyclometalated iridium complexes, (bi)2Ir(acac) (bi = cyclometalated benzoimidazole; acac = acetylacetonate), can be isolated. One of the complexes, (fbi)2Ir(acac) (fbi = 2-(9,9-diethyl-9H-fluoren-2-yl-1H-benzoimidazole), was also characterized by single-crystal X-ray structural determination. Some of the complexes, (bi)2Ir(acac), are highly phosphorescent at ambient condition. Light-emitting devices using these complexes as dopants were fabricated, and the emission colors range from green to red. Some green- and yellow-emitting devices exhibit very high efficiencies.

含benzoimidazole 配位基的銥金屬高效能磷光材料;Highly Phosphorescent Bis-Cyclometalated Iridium Complexes Containing Benzoimidazole-Based Ligands

Organic light-emitting diodes (OLEDs) are considered as an ideal in next generation of flat panel displays and solid state lighting source. Still, the stability and efficiency of OLEDs remain great challenges for its commercialization application. This article provides an overview on working principle of different kinds of luminescence, effective methods to improve quantum efficiency, recent progress of white emission OLEDs, novel types of transparency electrode for flexible OLEDs and the stability of OLEDs. A series of interesting and promising ideas to improve the performance of OLEDs are summarized from physical engineering based on the recent achievement of high brightness, high efficient and good stability of OLEDs.

Key issues and recent progress of high efficient organic light-emitting diodes

http://www.cell.com/article/S2589004222000748/pdf

Recent progress on organic light-emitting diodes with phosphorescent ultrathin (<1nm) light-emitting layers

http://jss.ecsdl.org/content/2/11/R258.full.pdf

Comprehensive Study on the Electron Transport Layer in Blue Flourescent Organic Light-Emitting Diodes

The electroluminescence (EL) characteristics of phosphorescent organic light-emitting diodes (OLEDs) with an undoped bis(1,2-dipheny1-1H-benzoimidazole) iridium (acetylacetonate) [(pbi) 2 Ir(acac)] emissive layer (EML) of various film thicknesses were studied. The results showed that the intensity of green light emission decreased rapidly with the increasing thickness of (pbi) 2 Ir(acac), which was relevant to the triplet excimer emission. It suggested that the concentration quenching of monomer emission in the undoped (pbi) 2 Ir(acac) film was mainly due to the formation of triplet excimer and partly due to the triplet–triplet annihilation (TTA) and triplet–polaron annihilation (TPA). A green OLED with a maximum luminance of 26,531 cd/m 2 , a current efficiency of 36.2 cd/A, and a power efficiency of 32.4 lm/W was obtained, when the triplet excimer emission was eliminated. Moreover, the white OLED with low efficiency roll-off was realized due to the broadened recombination zone and reduced quenching effects in the EML when no electron blocking layer was employed.

Electroluminescence of organic light-emitting diodes consisting of an undoped (pbi)2Ir(acac) phosphorescent layer

Influence of interlayer on the performance of phosphorescent white organic light-emitting devices

Comparison of electron transporting layer in white OLED with a double emissive layer structure

Influence of Doping Concentrations in White Organic Light-emitting Diodes based on Phosphorescent Iridium Complexes

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Electroluminescence of organic light-emitting diodes consisting of an undoped (pbi)2Ir(acac) phosphorescent layer

Influence of interlayer on the performance of phosphorescent white organic light-emitting devices

Comparison of electron transporting layer in white OLED with a double emissive layer structure

Influence of Doping Concentrations in White Organic Light-emitting Diodes based on Phosphorescent Iridium Complexes