Curing temperature reduction and performance improvement of solution-processable hole-transporting materials for phosphorescent OLEDs by manipulation of cross-linking functionalities and core structures

Abstract

Three new thermally cross-linkable hole-transporting materials (HTMs) with a bicarbazole (BCz) core and vinylbenzyl or alkoxystyrene-based cross-linking functionalities viz. BCzMS, BCzBOS and MeO-BCzBOS have been synthesized, characterized and applied to solution-processed green phosphorescent organic light-emitting diodes (PhOLEDs) by achieving cross-linking at different temperatures. The thermal curing behavior and structure–property relationships of our new thermally cross-linkable HTMs were systematically investigated by changing styrene-based cross-linking functionalities and a record low thermal curing temperature (150 °C, solvent resistance > 95%) for thermally cross-linkable HTMs for solution-processed PhOLEDs was achieved by introducing flexible butoxystyrene functionalities to the HTMs. First, the effect of the curing temperature on the performance of devices was studied by fabricating green PhOLEDs using the new materials as HTMs cured at different temperatures and it was found that lower temperature cured devices performed better than those processed at higher temperatures. Taking 150 °C as the best curing temperature, the devices employing BCzBOS (having butoxystyrene functionalities) showed an increase in the current and power efficiencies, respectively of 136% and 137% (at a luminance of 1000 cd m⁻²) as compared to the control device with no HTM. Furthermore, modification of the HTM core structure by introducing methoxy groups at the 6,6′-position of the bicarbazole core of BCzBOS (MeO-BCzBOS) resulted in further increases in the current and power efficiencies of 157% and 176% (at a luminance of 1000 cd m⁻²), respectively, as compared to the control device incorporating no HTM.