In-operando Investigation of Purcell Effect on Efficiency Roll-off in Top-emitting Phosphorescent Organic Light Emitting Diodes
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Purcell effect
Blue phosphorescence and white phosphorescence from organic light-emitting devices (OLEDs) based on phosphorescent iridium complexes are discussed. To improve emission efficiency, 4,4'-Bis(9-carbazolyl)-2,2'-Dimethyl-biphenyl (CDBP), which has a high triplet energy, was used as the carrier-transporting host for the emissive layer. The blue phosphorescent OLED exhibited a maximum external quantum efficiency of 10.4%, which corresponds to a current efficiency of 20.4 cd/A. This result can be explained as due to the efficient confinement of triplet energy on blue phosphorescent molecules, which is consistent with the results of transient photoluminescence experiments. The white phosphorescent OLED with greenish-blue and red emissive layers exhibited a maximum external quantum efficiency of 12% and a luminous efficiency of 18 cd/A. This is primarily attributed to improved greenish-blue emission efficiency as well as the emission efficiency of the blue phosphorescent OLED.
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Abstract Much effort has been devoted to developing highly efficient organic light‐emitting diodes (OLEDs) that function through phosphorescence or thermally activated delayed fluorescence (TADF). However, efficient host materials for blue TADF and phosphorescent guest emitters are limited because of their requirement of high triplet energy levels. Herein, we report the rigid acceptor unit benzimidazobenzothiazole (BID‐BT), which is suitable for use in bipolar hosts in blue OLEDs. The designed host materials, based on BID‐BT, possess high triplet energy and bipolar carrier transport ability. Both blue TADF and phosphorescent OLEDs containing BID‐BT‐based derivatives exhibit external quantum efficiencies as high as 20 %, indicating that these hosts allow efficient triplet exciton confinement appropriate for blue TADF and phosphorescent guest emitters.
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Organic light-emitting diodes (OLED) materials have been widely applied in many fields, among which phosphorescent OLED materials have more and more attention due to their luminescence efficiency and performance. At present, the luminescence layer of many OLED devices adopts phosphorescent materials as the main body to achieve a better visual experience for users. The research and development of blue electrophosphorescent materials are not mature enough. The two big aspects including color purity and the service life are major problems, and many researchers are now working on research methods of conquering the blue phosphorescent OLED materials shortage. In this article, fluorescent and phosphorescent OLED materials have been mentioned. The applications and branches of phosphorescent OLED materials are described. The article also analyzes the shortcomings of phosphorescent OLED and explained the reasons, mainly thermal activation delay fluorescence (TADF). Its purpose is to reduce the expensiveness of phosphorescent OLED materials. Meanwhile, the luminescence efficiency of fluorescent materials can be greatly improved. Additionally, the basic principles of luminescent OLED materials and the applications of phosphorescent OLED materials are also illustrated, including the prospect of blue phosphorescent OLED materials.
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Abstract Although the remarkable growth of the organic light‐emitting diode (OLED) industry has occurred via continuous, extensive efforts toward the utilization of various organometallic luminophores as phosphorescent emitters, the development of blue phosphorescent emitters with improved efficiencies and high electrochemical stabilities is essential. To this end, herein the preparation of two novel tetradentate Pt(II) complexes, Pt1 and Pt2 , and their application as blue phosphorescent emitters in OLED devices is described. Both complexes exhibit intense bluish emission in the solution and solid states. In addition, these complexes display very high phosphorescent quantum efficiencies (>89%) with host materials and thermal stabilities (>390 °C). Multilayer phosphorescent OLEDs containing Pt1 or Pt2 as emitters with mCBP/CNmCBP‐CN mixed‐host systems are fabricated. The devices exhibit outstanding performances, including high current, power, external efficiencies, and potential device lifetime in addition to sky‐blue ( Pt1 ) or blue ( Pt2 ) electroluminescence. These results clearly suggest that these tetradentate Pt(II) dopants are promising candidates as highly efficient, stable blue phosphorescent emitters in OLEDs.
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Abstract Organic light-emitting diodes (OLEDs) have been intensively studied as a key technology for next-generation displays and lighting. The efficiency of OLEDs has improved markedly in the last 15 years by employing phosphorescent emitters. However, there are two main issues in the practical application of phosphorescent OLEDs (PHOLEDs): the relatively short operational lifetime and the relatively high cost owing to the costly emitter with a concentration of about 10% in the emitting layer. Here, we report on our success in resolving these issues by the utilization of thermally activated delayed fluorescent materials, which have been developed in the past few years, as the host material for the phosphorescent emitter. Our newly developed PHOLED employing only 1 wt% phosphorescent emitter exhibits an external quantum efficiency of over 20% and a long operational lifetime of about 20 times that of an OLED consisting of a conventional host material and 1 wt% phosphorescent emitter.
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By incorporating a methoxy group at the 6- and 7-sites of the quinazoline ring of phenylquinazoline cyclometalating ligands, the iridium complexes realize pure red phosphorescence. The PHOLED based on Ir2 achieves CIE of (0.65, 0.34) and a higher external quantum efficiency of 26.22%.
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Some new symmetric and asymmetric platinum(II) Schiff base complexes with bulky substituents such as tert-butyl and triphenylamino groups have been synthesized which effectively reduced the aggregation or excimer formation. Using selected complexes as phosphorescent emitting materials, yellow light-emitting devices were fabricated with improved efficiency compared with the previously reported analogues. In addition, the phosphorescent white organic light-emitting device (WOLED) was fabricated using a single emissive layer composed of yellow- and blue-emitting materials.
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