Management of Triplet Energy and Charge-Transport Properties of Hosts by CN Position Engineering

2020

Chemistry A European J

Self Cite

N‐type hosts for long lifetime in sky‐blue thermally‐activated delayed fluorescence (TADF) organic light‐emitting diodes (OLEDs) were investigated by synthesizing four hosts with zig‐zag‐type backbone structure for high triplet energy. The four hosts had two CN units at different positions of the zig‐zag‐type backbone structure and two dibenzofuran units through either the 2 or 4‐position of dibenzofuran. The position of the CN unit was controlled at the meta and para‐positions in the zig‐zag‐type backbone to study the relationship between material parameters and lifetime of the TADF OLEDs. It was revealed that the meta‐orientation of the CN units in the backbone was advantageous to extend device lifetime of the sky‐blue TADF OLEDs.

Section: Resultsmentioning

confidence: 99%

Zig‐Zag Type Molecular Design Strategy of N‐Type Hosts for Sky‐Blue Thermally‐Activated Delayed Fluorescence Organic Light‐Emitting Diodes

Yang

2020

Chemistry A European J

Self Cite

“…Ideal device performances of the blue PhOLEDs were achieved by the weakly bound exciplex host harvesting triplet excitons and stabilizing the device operation. The device data of the deep blue PhOLEDs are in Table and they were compared with other data reported in other papers . In addition, the device data of the mCBP:DBFTrz mixed host device were also added in Figure S7 and Table S3 (Supporting Information).…”

Section: Methodsmentioning

confidence: 99%

Simultaneous Achievement of High Efficiency and Long Lifetime in Deep Blue Phosphorescent Organic Light‐Emitting Diodes

Choi

Advanced Optical Materials

Kim

2019

Self Cite

phosphorescent emitters and stable high triplet energy hosts. [7][8][9][10][11][12] The only lifetime results of the deep blue PhOLEDs was described in the work by Li et al., which tested the lifetime of the deep blue Pt emitters. [10] However, the high efficiency and good lifetime could not be simultaneously achieved in the deep blue PhOLEDs because high triplet energy hosts could not be properly chosen. For example, the deep blue device in Li's work with a y coordinate of 0.18 showed only 800 h lifetime at 100 cd m -2 and low external quantum efficiency of 3.2%. Even the sky blue device with y coordinate of 0.21 exhibited only 9.8% EQE although the lifetime at 100 cd m -2 was about 17 000 h. [10] In order to realize high efficiency and long lifetime at the same time, a stable high triplet energy host as well as a stable deep blue emitter is essential. In general, high triplet energy hosts for deep blue PhOLEDs were either single bipolar hosts such as 3,6-bis(diphenylphosphoryl)-9-phenyl-9H-carbazole, 9-(3-(9H-carbazole-9-yl)phenyl)-3-(dibromophenylphosphoryl)-9H-carbazole or single unipolar hosts such as diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide). [13][14][15][16][17] However, their chemical instability was an issue for application in the lifetime study of the deep blue PhOLEDs. In order to overcome the challenging stability issues of the singlet hosts, mixed hosts of p-type host and n-type host were developed, but lifetime results were not described. [18][19][20][21] Recently, we demonstrated an exciplex host and an electroplex host for extended lifetime in sky blue PhOLEDs. [22,23] Although those two hosts were effective to elongate the lifetime of the sky blue PhOLEDs, intrinsic low triplet energy of each host comprising the exciplex and electroplex hosts limited the application of them in the deep blue PhOLEDs. Therefore, it is necessary to design a high triplet energy host system for deep blue PhOLEDs.In order to develop the high triplet energy host, we focused on the weakly bound exciplex host because the emission energy of the strongly bound exciplex host is largely decreased by strong binding of the p-type and n-type host. The weak binding of the p-type and n-type hosts is advantageous to secure high emission energy for efficient energy transfer to the deep blueemitting phosphorescent emitter. In this work, we developed a high emission energy exciplex host for application in high efficiency and long lifetime deep blue PhOLEDs. Two n-type High triplet energy exciplex hosts are developed using new electron transport type hosts with triphenylsilyl blocking groups to obtain high solid state triplet energy and exciplex triplet energy. Two triphenyltriazine derived hosts, 2-phenyl-4,6-bis(4-(triphenylsilyl)phenyl)-1,3,5-triazine (pSiTrz) and 2-phenyl-4,6-bis(3-(triphenylsilyl)phenyl)-1,3,5-triazine (mSiTrz), are synthesized as the high triplet energy hosts with electron transport properties. The two hosts show high triplet energy over 2.90 eV both in solution and solid film, and afford high triplet e...

“…[ 4–6 ] However, the stable host materials for blue PhOLEDs with high E T over 3.0 eV and suitable frontier molecular orbital energy levels (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) are scarcely reported, because it is challenging to achieve all these parameters together. [ 24–35 ] In addition, the device lifetime of blue PhOLEDs is far inferior to that of red and green counterparts and thus the further attention is highly needed to promote the lifetime of blue PhOLEDs to replace the blue fluorescent emitters in commercial applications. [ 29–41 ] In general, many factors affect the lifetime of the blue PhOLEDs and one of the key factors is the stability of the host material.…”

Section: Introductionmentioning

confidence: 99%

“…[ 24–35 ] In addition, the device lifetime of blue PhOLEDs is far inferior to that of red and green counterparts and thus the further attention is highly needed to promote the lifetime of blue PhOLEDs to replace the blue fluorescent emitters in commercial applications. [ 29–41 ] In general, many factors affect the lifetime of the blue PhOLEDs and one of the key factors is the stability of the host material. [ 23,29–41 ] On the other hand, it has been demonstrated that the mixed host systems are advantageous in improving the performance of PhOLEDs compared to single host systems due to balanced carrier density, broad recombination zone, and little charge leakage.…”

Section: Introductionmentioning

confidence: 99%

“…[ 29–41 ] In general, many factors affect the lifetime of the blue PhOLEDs and one of the key factors is the stability of the host material. [ 23,29–41 ] On the other hand, it has been demonstrated that the mixed host systems are advantageous in improving the performance of PhOLEDs compared to single host systems due to balanced carrier density, broad recombination zone, and little charge leakage. [ 29,31,33,42–45 ] Although several mixed host systems were reported for blue PhOLEDs, but many of them were mainly focused on improving the EQE rather than the lifetime because the high triplet energy electron transporting type host material design is difficult.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Molecular Engineering of Cyano‐Substituted Carbazole‐Based Host Materials for Simultaneous Achievement of High Efficiency and Long Lifetime in Blue Phosphorescent Organic Light‐Emitting Diodes

Konidena

Chung

2020

Adv Elect Materials

Self Cite

quenching, triplet-triplet annihilation and exciton diffusion. [4][5][6] As the major component of the emitting layer (EML) is the host material, it plays a pivotal role in managing the carrier mobility, charge recombination zone and operational lifetime of the device. [11][12][13][14][15][16][17][18][19][20][21][22][23] Therefore, the host materials are equal important as the dopant materials in dictating the final device performance. Currently, efficient host materials with superior external quantum efficiency (EQE) and long operational lifetime for red and green PhOLEDs have been developed and the phosphors replaced the fluorescent emitters in commercial application. [4][5][6] However, the stable host materials for blue PhOLEDs with high E T over 3.0 eV and suitable frontier molecular orbital energy levels (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) are scarcely reported, because it is challenging to achieve all these parameters together. [24][25][26][27][28][29][30][31][32][33][34][35] In addition, the device lifetime of blue PhOLEDs is far inferior to that of red and green counterparts and thus the further attention is highly needed to promote the lifetime of blue PhOLEDs to replace the blue fluorescent emitters in commercial applications. [29][30][31][32][33][34][35][36][37][38][39][40][41] In general, many factors affect the lifetime of the blue PhOLEDs and one of the key factors is the stability of the host material. [23,[29][30][31][32][33][34][35][36][37][38][39][40][41] On the other hand, it has been demonstrated that the mixed host systems are advantageous in improving the performance of PhOLEDs compared to single host systems due to balanced carrier density, broad recombination zone, and little charge leakage. [29,31,33,[42][43][44][45] Although several mixed host systems were reported for blue PhOLEDs, but many of them were mainly focused on improving the EQE rather than the lifetime because the high triplet energy electron transporting type host material design is difficult. [29,31,[41][42][43] Therefore, the development of electron transporting type hosts with high E T and long lifetime for blue PhOLEDs is highly desirable.Till date, several molecular design approaches have been reported for the development of stable and high E T hosts for blue PhOLEDs, in which the design featuring chemically stable and high E T building units with nonconjugative mode energy host materials named 1CNCzBN, 2CNCzBN, 3CNCzBN, and 4CNCzBN containing a cyanocarbazole and a benzonitrile for blue phosphorescent organic light-emitting diodes (PhOLEDs) is presented. The effect of the substitution position of the cyano unit in the carbazole core on physicochemical and electroluminescence characteristics is described. All the compounds show high triplet energy (E T ) of above 2.85 eV and the 3CNCzBN compound containing the cyano unit at C3 position of carbazole exhibits superior E T of 3.04 eV. Except for 1CNCzBN, all the compounds reveal good electron-transporting properties, ...