Bicarbazole/nitrogen heterocycle based bipolar host materials for efficient green phosphorescent organic light-emitting diodes

Abstract

In this paper, three novel bipolar host materials, 9,9′-Diphenyl-6-(4-pyridinyl)-3,3′-bicarbazole(BCz4Py), 9,9′-Diphenyl-6-(3-pyridinyl)-3,3′-bicarbazole(BCz3Py) and 9,9′-Diphenyl-6-(5-pyrimidinyl)-3,3′-bicarbazole(BCz5Pm), were synthesized by incorporation of bicarbazole as donor with pyridine/pyridimide based N-hexahydroxy nitric heterocycle as acceptor units. The photophysical, thermal and electrochemical properties of these three materials were fully investigated. These compounds all exhibited superior thermal stability with high glass-transition temperature (Tg > 130 °C) and decomposition temperature (Td > 400 °C). Appropriate HOMO (highest occupied molecular orbitial) -LUMO (lowest unoccupied molecular orbital) levels and triplet energy (2.74–2.75 eV) of these compounds make them suitable hosts for green iridium pohosphor Ir(ppy)3 in phosphorescent organic light emitting devices (PhOLEDs). Among them, the BCz3Py hosted device exhibited outstanding performance with maximum external quantum efficiency (EQE) of 14.9%, maximum current efficiency (CE) of 49.9 cd/A, and maximum power efficiency (PE) of 29.1 lm/W, suggesting that BCz3Py is a promising bipolar material for high-efficiency green PhOLEDs.