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

Abstract

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, ...

High Triplet Energy Hosts for Blue Organic Light‐Emitting Diodes

“…Four isomeric n‐type hosts, 1CNCzBN, 2CNCzBN, 3CNCzBN, and 4CNCzBN, were synthesized by altering the position of CN group in carbazole. [ 139 ] All four hosts showed high triplet energy above 2.85 eV and the 3CNCzBN exhibited the highest triplet energy (3.04 eV) among the series. Based on the availability, the four hosts were mixed with mCBP and doped with Ir(CNpi) 3 .…”

Through Positional Isomerism: Impact of Molecular Composition on Enhanced Triplet Harvest for Solution-Processed OLED Efficiency Improvement

“…33 Lee's group also depicts the substitutional effect of cyano on the electroluminescence properties of the organic host materials. 34 Lee et al show how switching the substitutional position is responsible for voltage reduction in the fabricated device. 35 Microscopic-level studies prove that the electronic factors in the organic materials are responsible for tuning the efficiency of the fabricated devices.…”

“…43 The combination of cyano, carbazole, and imidazole units can result in promising hosts for producing high-efficiency devices. 43,44 Positional change of the cyano unit can lead to unprecedented alteration in electronic properties, which can enhance the device efficiency drastically. 43 E T has always been in the light of discussion for host materials, but even some high-E T host compounds are not able to deliver high-efficiency devices.…”

“…43,44 Positional change of the cyano unit can lead to unprecedented alteration in electronic properties, which can enhance the device efficiency drastically. 43 E T has always been in the light of discussion for host materials, but even some high-E T host compounds are not able to deliver high-efficiency devices. 45 High E T must be backed up by proper structural constitution to have high-performance devices.…”

Phenanthroimidazole‐based chromophores for organic light‐emitting diodes: synthesis, photophysical, and theoretical study

“…In addition, there have been numerous published reports on new host materials for OLEDs (benzocarbazole and quinazoline, pyrimidine, diphenylamine/triazine, phthalonitrile, pure hydrocarbon, and cyano-substituted carbazole). [28][29][30][31][32][33] In recent years, series of bipolar host materials have been synthesized and utilized for OLEDs.…”