Phosphorescent materials for application to organic light emitting devices

108

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%

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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

108

“…On the other hand, the excitonic states are essentially metal-to-ligand charge-transfer states in the Ir(ppy) 3 molecules. 36,37 The charge-transfer states are more sensitive to local electrical polarization due to their stronger ionic wavefunctions as compared to Frenkel excitons. Therefore, increasing the matrix dielectric constant can enhance the ionic properties of wavefunctions of chargetransfer states through electrical polarization.…”

mentioning

confidence: 99%

Changing inter-molecular spin-orbital coupling for generating magnetic field effects in phosphorescent organic semiconductors

Yan

Shao

Graeff

et al. 2012

Phosphorescent organic semiconductors normally show negligible magnetic field effects in electronic and optic responses. These phenomena have been generally attributed to strong spin-orbital coupling which can dominate internal spin-dephasing process as compared with applied magnetic field. This paper reports both positive and negative magnetocurrents from phosphorescent organic semiconductors through dissociation and charge-reaction channels when the intermolecular spin-orbital coupling is changed based on materials mixing. Our experimental results indicate that inter-molecular spin-orbital coupling is essentially responsible for the generation of magnetic field effects in phosphorescent organic semiconductors.

“…1 However, quantum efficiency of PHOLEDs tends to decrease at high current density because of triplet-triplet exciton quenching mainly originated from long triplet excited lifetime of phosphorescent emitting materials. 2,3 There have been many studies on the origin of efficiency roll-off in PHOLEDs. Triplet-triplet annihilation at high current density was the main reason for the efficiency roll-off and it was proportional to the square of triplet exciton density.…”

mentioning

confidence: 99%

Stable efficiency roll-off in phosphorescent organic light-emitting diodes

Kim

Jang

Yook

et al. 2008

102

Origin of efficiency roll-off in phosphorescent organic light-emitting diodes was investigated with triplet mixed host devices and stable devices with little efficiency roll-off was developed. Efficiency roll-off was significant in the device with narrow recombination zone ͑RZ͒ and charge leakage out of emitting layer at high luminance was critical to efficiency roll-off. Efficiency roll-off could be reduced in triplet mixed host device with broad RZ and little charge leakage at high driving voltage. Triplet mixed host devices with an exciton blocking layer showed a quantum efficiency over 90% of maximum quantum efficiency at a luminance of 20 000 cd/ m 2 . © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2836270͔ Phosphorescent organic light-emitting diodes ͑PHOLEDs͒ have a merit of high quantum efficiency and 100% theoretical maximum quantum efficiency was already reported. 1 However, quantum efficiency of PHOLEDs tends to decrease at high current density because of triplet-triplet exciton quenching mainly originated from long triplet excited lifetime of phosphorescent emitting materials. 2,3 There have been many studies on the origin of efficiency roll-off in PHOLEDs. Triplet-triplet annihilation at high current density was the main reason for the efficiency roll-off and it was proportional to the square of triplet exciton density. 4,5 Triplet-polaron quenching was reported to be responsible for triplet efficiency roll-off through triplet energy transfer to charged molecules. 3 The triplet-polaron quenching scales with triplet exciton density and its effect was not so significant as triplet-triplet quenching. Baldo et al. proposed that triplet-triplet annihilation is the main mechanism for efficiency decrease at high luminance. 3 In addition to triplet-triplet annihilation and triplet-polaron quenching, dissociation of excitons into free charge carriers contributes to efficiency decrease at high current density. 6 These triplet quenching processes mentioned above can be correlated with device structures and triplet devices with long triplet excited state lifetime showed serious triplet-triplet exciton quenching and narrow emission zone was found to have negative effect on triplet-triplet quenching because triplet exciton density is high in the exciton formation zone. [7][8][9] Even though there have been some experimental studies about efficiency roll-off of PHOLEDs, there has been no systematic study about the relationship between efficiency roll-off and device structures. In this work, triplet efficiency roll-off was studied using triplet mixed host devices and the origin of the triplet efficiency decrease at high luminance was revealed by monitoring recombination zone ͑RZ͒ of triplet mixed host devices according to driving voltage.Triplet mixed host device structures used in this work were indium tin oxide ͑150 nm͒ / N , NЈ-diphenyl-nm͔/light emitting layer ͓͑EML͒, 30 nm͔/2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline ͓͑BCP͒ 5 nm͔/tris͑8-hydroxyquinoline͒ aluminium ͑20 nm͒ / LiF ͑1 nm͒ / Al ͑200 nm͒...