A thermally cross-linkable hole-transporting small-molecule for efficient solution-processed organic light emitting diodes

Abstract

Fabrication of multilayered organic light-emitting diodes (OLEDs) through solution process involves several challenges, especially in preventing dissolution of prior layers during subsequent coating. To overcome, extensive efforts had been made in developing cross-linkable materials. In this work, a thermally cross-linkable hole-transporting material, (9,9′-bis(4-vinylbenzyl)-9H,9′H-3,3′-bicarbazole) (VyPyMCz), is synthesized, characterized and successfully applied to multilayered OLEDs via solution-process. After cross-linking, the hole-transporting material forms robust, smooth and solvent-resistant network, enabling a subsequent spin-coating without deteriorating its film integrity. The measured energy level suggests that VyPyMCz facilitates the injection of hole and effectively blocks electron to realize high efficiency, especially at high luminance. At 1000 cd m−2 for example, the power efficiency of a studied red device is increased from 7.5 to 11.9 lm W−1, an increment of 58%, and the maximum brightness improved from 7724 to 13,560 cd m−2, an increment of 75%, as this electron confining, hole transporting material is incorporated. Remarkably, VyPyMCz also works for a high band gap (2.90 eV) with a high triplet energy (2.80 eV) blue emitter containing OLED device, the power efficiency is increased from 6.6 to 11.8 lm W−1, an increment of 78%, and the maximum luminance enhanced from 5260 to 6857 cd m−2, an increment of 30%, because of its higher triplet energy (2.87 eV).