Tuning Charge Balance in Solution-Processable Bipolar Triphenylamine-diazafluorene Host Materials for Phosphorescent Devices

Abstract

Three bipolar hosts, namely TPA-DAF, TPA-DAF2, and TPA-DAF3, comprising an electron-donating triphenylamine (TPA) group and electron-accepting 4,5-diazafluorene (DAF) units are investigated for phosphorescent organic light-emitting diodes (PhOLEDs). Given the nonplanar structure of the sp(3)-hybridized C9 atom in DAF unit, these molecules have a highly nonplanar configuration, good film-forming property, and high triplet energy (ET) of 2.88-2.89 eV. Among them, TPA-DAF shows more balanced carrier injecting/transporting ability, suitable highest occupied molecular orbital (MO) energy level and higher current density, and therefore TPA-DAF-based devices exhibit the best performances, having an extremely slight efficiency roll-off with current efficiency of 20.0 cd/A at 973 cd/m(2), 19.5 cd/A at 5586 cd/m(2), and 17.6 cd/A at 9310 cd/m(2) for blue PhOLEDs; 23.5 cd/A at 1059 cd/m(2) and 15.3 cd/A at 8850 cd/m(2) for green PhOLEDs; and 12.2 cd/A at 1526 cd/m(2), 10.5 cd/A at 5995 cd/m(2), and 9.2 cd/A at 8882 cd/m(2) for red PhOLEDs, respectively. The results also provide a direct proof for the influence of charge balance on the device performance.

Insights into the spectral property and electronic structure of di-triphenylaniline-modified monothiophene, dithiophene and thienothiophene

“…Meanwhile, its near planar structure would allow more π conjugations compared with a single benzene ring and lead to light absorption and emission red shift [31]. Therefore, triphenylamine derivatives have been widely used in hole transport materials and donor acceptor systems of diodes [32][33][34][35],…”

A thermally activated delayed fluorescence exciplex to achieve highly efficient and stable blue and green phosphorescent organic light-emitting diodes

“…Thus, the design of new aza-SBFs has become an interesting improvement by which to develop new materials systems with better charge mobility, higher T g , and suitable T 1 and to then achieve highly efficient OLEDs with low roll-off. [26][27][28][29][30] Indeed, the creation of aza-SBFs with substitutions at 4-position of the uorene moiety leads to efficient p-conjugation breaking down due to strong steric congestion. It is known that a decrease in the LUMO energy levels through the incorporation of electron accepting moieties such as pyridine, 14,31,32 triazine, 10, [33][34][35][36][37] or diphenylphosphine oxide, 38 and an increase in the HOMO energy levels via the introduction of electron-donating groups such as arylamines, 39,40 have remarkable consequences on the device performances.…”

Synthesis and Applications of Excimer Host Materials Based on 2,4,6-Triphenyl-1,3,5-triazine and Fluorene Moieties

“…[19] In general, bipolar molecules are obtained by chemically bonding electron-donating and electron-withdrawing moieties within a single molecule. [20] Electron donor moieties used usually in bipolar host materials are carbazole, [21,22] triphenylamine, [23,24] fluorene [25,26] and phenoxazine, [27] while electron acceptor moieties are pyrimidine, [28,29] cyano, [30,31] 2,4,6-triphenyl-1,3,5-triazine [32～34] and pyridine. [25] Especially, fluorene and 2,4,6triphenyl-1,3,5-triazine moieties have been used widely to synthesize suitable bipolar host materials due to their favorable hole and electron-transporting capabilities.…”