Effect of doping profile on the lifetime of green phosphorescent organic light-emitting diodes

Lee, Jun Yeob

doi:10.1063/1.2360223

Applied Physics Letters

2006

DOI: 10.1063/1.2360223

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Effect of doping profile on the lifetime of green phosphorescent organic light-emitting diodes

Jun Yeob Lee

Abstract: The lifetime of green phosphorescent light-emitting diodes was improved by using a graded doping structure in light-emitting layer. A green device with high doping concentration at the hole transport layer and light-emitting layer interface showed longer lifetime than a conventional device with uniform doping concentration for the whole light-emitting layer by more than 60%.

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Cited by 34 publications

(21 citation statements)

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“…The NPB emission in device I may be due to hole accumulation at the interface between NPB and PH1 and electron overflow from PH1 to NPB. In general, holes are strongly trapped by dopant materials in Ir͑ppy͒ 3 doped devices 7 and holes can be accumulated between NPB and PH1 because hole transport through Ir͑ppy͒ 3 is limited. The accumulated holes can recombine with some electrons injected from EML inside NPB, giving rise to NPB emission.…”

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“…2,6 A graded doping structure was studied by our group and it also gave a long lifetime as well as high power efficiency. 7 In this work, we studied the use of ͑4,4Ј-N , NЈ-dicarbazole͒biphenyl ͑CBP͒ and N , NЈ-dicarbazolyl-3,5-benzene ͑mCP͒ as an EBL to get high efficiency in green PHOLEDs. A detailed mechanism for exciton blocking of CBP and mCP was clarified and device performances were investigated.…”

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confidence: 99%

“…2 There have been many studies about light emission and high efficiency in PHOLEDs. [3][4][5][6][7] Many different device architectures were tried to improve the light-emitting efficiency of PHOLEDs. A hole blocking layer or exciton blocking layer ͑EBL͒ was introduced in PHOLEDs to block hole injection from light-emitting layer ͑EML͒ to electron transport layer and it was effective to get high efficiency in PHOLEDs.…”

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confidence: 99%

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High efficiency phosphorescent organic light-emitting diodes using carbazole-type triplet exciton blocking layer

Kim

Jang

Lee

2007

Applied Physics Letters

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107

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Device performances of green phosphorescent organic light-emitting diodes using ͑4, 5-benzene ͑mCP͒ as an exciton blocking layer were investigated. CBP and mCP were introduced between hole transport layer and emitting layer to block triplet exciton quenching and efficient hole transport to emitting layer. The efficiency of green devices could be improved by more than three times by using mCP exciton blocking layer. There have been many studies about light emission and high efficiency in PHOLEDs. [3][4][5][6][7] Many different device architectures were tried to improve the light-emitting efficiency of PHOLEDs. A hole blocking layer or exciton blocking layer ͑EBL͒ was introduced in PHOLEDs to block hole injection from light-emitting layer ͑EML͒ to electron transport layer and it was effective to get high efficiency in PHOLEDs. [3][4][5] An electron blocking layer was also used in blue PHOLEDs to block electron injection from EML to hole transport layer ͑HTL͒. 5 Fac-tris͑1-phenylpyrazolato-N , C2Ј͒ iridium ͓Ir͑ppz͒ 3 ͔ with its lowest unoccupied molecular orbital ͑LUMO͒ level of 1.7 eV was efficient as an electron blocking material. Other than these, a double EML structure was used and high quantum efficiency of 19.3% was reported. 2,6 A graded doping structure was studied by our group and it also gave a long lifetime as well as high power efficiency. 7 In this work, we studied the use of ͑4,4Ј-N , NЈ-dicarbazole͒biphenyl ͑CBP͒ and N , NЈ-dicarbazolyl-3,5-benzene ͑mCP͒ as an EBL to get high efficiency in green PHOLEDs. A detailed mechanism for exciton blocking of CBP and mCP was clarified and device performances were investigated.The standard device structure used in this experiment was indium tin oxidetris͑2-phenylpyridine͒ iridium ͓Ir͑ppy͒ 3 ͔ ͑30 nm, 5% doping͒/biphenoxy-bi͑8-hydroxy-3-methylquinoline͒ aluminum ͑5 nm͒/tris͑8-hydroxyquinoline͒ aluminum ͑20 nm͒ / LiF ͑1 nm͒ /Al ͑200 nm͒. Three devices with different HTL structures were fabricated to investigate the effect of CBP and mCP EBLs on device performances. Device I had NPB ͑30 nm͒ as a HTL and device II had both NPB ͑20 nm͒ and CBP ͑10 nm͒ as a double layer HTL, while device III had NPB ͑20 nm͒ and mCP ͑10 nm͒ as a double layer HTL. PH1 was supplied from Merck Co. and it has a spirobifluorene-type backbone structure with high electron transport properties because of spirobifluorene units. The triplet band gap of PH1 was 2.4 eV and the highest occupied molecular orbital ͑HOMO͒ and LUMO were 5.9 and 2.8 eV, respectively. The current density-voltageluminance characteristics of the devices were measured with Keithley 2400 source measurement unit and PR 650 spectrophotometer.It is important to confine excitons in EML to increase the recombination efficiency of holes and electrons in OLEDs by blocking charge carriers and exciton diffusion out of EML. 3 Hole and exciton blocking in PHOLED was effective in electron transport layer side and electron and exciton blocking in HTL side is expected to give high recombination efficiency through charge and exciton confin...

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confidence: 99%

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confidence: 99%

mentioning

confidence: 99%

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High efficiency phosphorescent organic light-emitting diodes using carbazole-type triplet exciton blocking layer

Kim

Jang

Lee

2007

Applied Physics Letters

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107

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“…Our group studied the lifetime of green PHOLEDs with a graded doping structure and reported that a graded doping structure was beneficial to get a long lifetime of PHOLEDs. 5 The lifetime of green PHOLEDs could be enhanced by more than three times by using a graded doping structure which has a high doping concentration near the hole transport layer.…”

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Lifetime improvement of green phosphorescent organic light-emitting diodes by charge confining device structure

Kim

Jang

Lee

2007

Applied Physics Letters

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Lifetime improvement of green phosphorescent organic light-emitting diodes by charge confinement inside an emitting layer was investigated. Excitons were confined within the emitting layer by using a charge confining structure with a high doping concentration at the center of the emitting layer. The lifetime of green devices could be improved by more than five times by confining the excitons at the center of the emitting layer. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2740587͔ Phosphorescent organic light-emitting diodes ͑PHOLEDs͒ have been actively studied for the past ten years due to their merit of high quantum efficiency. 100% internal quantum efficiency can be realized in PHOLEDs because both singlet and triplet excitons can contribute to light emission.1 However, the lifetime of PHOLEDs needs to be improved further even though red PHOLEDs are already commercialized.Lifetime is one of the critical issues of PHOLEDs, but there have not been many studies to improve the lifetime of PHOLEDs. One approach to get a long lifetime of PHOLEDs was to develop, stable host, dopant and hole blocking materials.2-4 Kwong et al. compared the lifetime of several hole blocking materials and reported that biphenoxybi͑8-hydroxy-3-methylquinoline͒ aluminum ͑Balq͒ is better than other hole blocking materials in terms of lifetime. Electron-transport-type host material was also effective to improve the lifetime of PHOLEDs and the long lifetime could be obtained by using thermally stable host materials. 4 The other approach to get a stable lifetime of PHOLEDs was to apply a device architecture which can give a stable lifetime. Our group studied the lifetime of green PHOLEDs with a graded doping structure and reported that a graded doping structure was beneficial to get a long lifetime of PHOLEDs. 5The lifetime of green PHOLEDs could be enhanced by more than three times by using a graded doping structure which has a high doping concentration near the hole transport layer.In this work, the lifetime of green PHOLEDs was investigated by using a charge confining structure with a high doping concentration at the center of the emitting layer. The lifetime of charge confining devices was compared with that of standard devices.The device configuration used in this experiment was indium tin oxide ͑ITO, 150 nm͒/ N , NЈ-diphenyl-150͑5 nm͒/ tris͑8-hydroxyquinoline͒ aluminum ͑Alq 3 ,25 nm͒ / LiF͑1 nm͒ /Al͑200 nm͒. Three devices were fabricated to investigate the effect of the device structure on lifetime. Two standard devices with single EML and one device with three EMLs were prepared. The total thickness of the EML was constant for all devices. Standard devices which have ͑4,4 ' -N , N'-dicarbazole͒biphenyl ͑CBP͒ and PH1 as hosts for the EML and a charge confining device with PH1 EML sandwiched between CBP EMLs were fabricated. A dopant material was tris͑2-phenylpyridine͒ iridium ͓Ir͑ppy͒ 3 ͔ and doping concentrations were 5% and 9% for CBP and PH1. PH1 was supplied from Merck Co. and it has a spirobifluorene-type backbone struct...

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“…[1][2][3][4][5] N,N 0 -bis(naphthalen-1-yl)-N,N 0 -bis(phenyl)-benzidine (NPB) has been extensively used as hole transporting layer (HTL), further studies on OLEDs with NPB used as HTL and 4,4 0 -N,N 0 -dicarbazole-biphenyl (CBP): Ir(ppy) 3 as an emission layer (EML) showed that the recombination zone was close to the HTL/EML interface. 6,7 However, such OLEDs were inefficient because of the lower triplet energy of NPB (2.3 eV) as compared to that of Ir(ppy) 3 (2.4 eV). 8,9 Therefore, material with higher triplet energy had to be used at the EML interface to confine the excitons within the EML.…”

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On the triplet distribution and its effect on an improved phosphorescent organic light-emitting diode

et al. 2012

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Cataloged from PDF version of article.We reported phosphorescent organic light-emitting diodes with internal quantum efficiency near 100% with significantly reduced efficiency roll-off. It was found that the use of different hole transporting layer (HTL) affects the exciton distribution in the emission region significantly. Our best device reaches external quantum efficiency (EQE), current, and power efficiency of 22.8% +/- 0.1%, 78.6 +/- 0.2 cd/A, 85 +/- 2 lm/W, respectively, with half current of 158.2 mA/cm(2). This considerably outperforms the control device with N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB) (HTL) and 4,4'-N,N'-dicarbazole-biphenyl (host) with maximum EQE, current and power efficiency of 19.1% +/- 0.1%, 65.6 +/- 0.3 cd/A, 67 +/- 2 lm/W, respectively, with half current of only 8.1 mA/cm(2). (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4749278

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Effect of doping profile on the lifetime of green phosphorescent organic light-emitting diodes

Cited by 34 publications

References 13 publications

High efficiency phosphorescent organic light-emitting diodes using carbazole-type triplet exciton blocking layer

High efficiency phosphorescent organic light-emitting diodes using carbazole-type triplet exciton blocking layer

Lifetime improvement of green phosphorescent organic light-emitting diodes by charge confining device structure

On the triplet distribution and its effect on an improved phosphorescent organic light-emitting diode

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