Bistriazoles with a Biphenyl Core Derivative as an Electron-Favorable Bipolar Host of Efficient Blue Phosphorescent Organic Light-Emitting Diodes

Abstract

High-quality host materials are indispensable for the construction in the emitting layer of efficient organic light-emitting diodes (OLEDs), especially in a guest and host system. The good carrier transport and energy transfer between the host and emitters are out of necessity. In this work, a wide bandgap and bipolar organic compound, 2,2′-bis(4,5-diphenyl-(1,2,4)-triazol-3-yl)biphenyl ( BTBP ), conjugating two electron-transporting triazole moieties on a hole-transporting biphenyl core, was synthesized and characterized. The wide bandgap of 4.0 eV makes the promise in efficient energy transfer between the host and various color emitters to apply as the universal host, especially for blue emitters. The close electron and hole mobilities perform the same order of 10 –5 cm 2 ·V –1 ·s –1 , identified as bipolar behavior and benefited for carrier balance at low bias. Although carrier transportation belongs to bipolar behavior at a low electrical field, the electron mobility is much faster than the hole one at a high electrical field and belongs to electron-transporting behavior. Employing the BTBP as the host matrix mixed with a phosphor dopant, iridium(III)bis[4,6-di-fluorophenyl-pyridinato-N,C 2 ]picolinate, a high-efficiency sky-blue phosphorescent organic light-emitting diode (OLED) was achieved with a maximum current efficiency of 65.9 cd/A, maximum power efficiency of 62.8 lm/W, and maximum external quantum efficiency of 30.2%.

Bipolar 1,8-naphthalimides showing high electron mobility and red AIE-active TADF for OLED applications

“…Such charge-transporting properties of bipolar OLED functional materials were shown to be useful for getting low roll-off efficiencies of blue phosphorescent OLEDs. 61 It should be mentioned that both the hole and electron mobilities of compound 2 were lower than those of 1 and 3 apparently because of the relatively bulky sulphur atom of phenothiazine leading to bigger intramolecular distances and less HOMO–HOMO and LUMO–LUMO overlapping between neighbouring molecules.…”

Acridone‐Based Host Materials for Green Phosphorescent and Thermally Activated Delayed Fluorescent OLEDs with Low‐Efficiency Roll‐Offs

“…Host-guest doped EML with long lifetime emitters homogeneously dispersed in suitable host materials to reduce intermolecular interaction was proposed to address this issue. [10][11][12] Unipolar hosts such as 1,3-bis(9H-carbazol-9yl)benzene (mCP) [13] or 4,4'-bis(9H-carbazol-9-yl)-biphenyl (CBP) [14] were firstly used in doped OLEDs. However, the unipolar (hole) carrier-transport property of these hosts tends to narrow the recombination zone and gets close to the EML/ electron-transport layer (ETL), which leads to moderate device performance and severe efficiency roll-off at high luminance.…”

“…Such a quenching process limits the electroluminescent (EL) performance of the OLEDs at high brightness, thus limits the practical application of OLEDs in commercial display products. Host‐guest doped EML with long lifetime emitters homogeneously dispersed in suitable host materials to reduce intermolecular interaction was proposed to address this issue [10–12] . Unipolar hosts such as 1,3‐bis(9 H ‐carbazol‐9‐yl)benzene (mCP) [13] or 4,4′‐bis(9 H ‐carbazol‐9‐yl)‐biphenyl (CBP) [14] were firstly used in doped OLEDs.…”

Does Through‐Space Charge Transfer in Bipolar Hosts Affect the Efficiency of Blue OLEDs?

“…[20] When appropriate host was selected for bis [2-(4,6difluorophenyl)pyridinato-C 2 ,N](picolinato)iridium(III) (FIrpic), OLEDs with maximum EQEs above 30 % were fabricated. [21,22] For instance, pyridoindole derivatives 9-(3′-(9 H -carbazol-9yl)-[1,1′-biphenyl]-3-yl)-9 H -pyrido [2,3-b ]indole (CzBPCb) and 3,3′-bis(9 H -pyrido [2,3-b ]indol-9-yl)-1,1 ′ -biphenyl (CbBPCb) were reported as efficient hosts for blue PhOLEDs. [21] Despite higher charge mobilities, more appropriate HOMO and LUMO energies for charge injection and the similar triplet energy level observed for the host CzBPCb containing one electron donating carbazole moiety and electron accepting pyridoindole unit, higher maximum EQE of 30.1% was observed for the device based on the host CbBPCb containing two pyridoindole moieties.…”