Construction of a fully conjugated cross-linked hole-transport film based on ethynyl to enable high mobility for efficient solution-processed OLEDs

Abstract

The utilization of cross-linked hole transport layers is deemed as an effective strategy to solve the interlayer miscibility problem in solution-processed organic light-emitting diodes (OLEDs). However, the general incomplete conjugated structure after cross-linking makes it incapable for high hole mobility. In this study, a novel hole transport material N,N′-bis(4-ethynylphenyl)-[1,1′-biphenyl]-4,4′-diamine (E-TPD) with cross-linkable ethynyl groups was designed and synthesized to construct a fully conjugated cross-linked hole transport film. The thermal properties, cross-linking characteristics, photophysical properties, surface morphology, and hole transport ability were studied in detail. The cross-linked E-TPD (p(E-TPD)) film demonstrated a suitable highest occupied molecular orbital (HOMO) energy level, excellent solvent resistance and uniform film morphology. Particularly, its hole mobility reached a high value of up to 8.2 × 10⁻⁴ cm² V⁻¹ s⁻¹, which is over three times than that of N,N′-di-p-tolyl-N,N′-bis(4-vinylphenyl)-[1,1′-biphenyl]-4,4′-diamine (V-p-TPD) (2.5 × 10⁻⁴ cm² V⁻¹ s⁻¹) with the same core structure based on cross-linkable vinyl groups. The corresponding solution-processed green phosphorescent OLED gained excellent performance with a turn-on voltage (V_on) of 3.3 V, maximum current efficiency (CE_max) of 60.74 cd A⁻¹ and maximum external quantum efficiency (EQE_max) of 17.33%. This study provides a novel tactic to achieve a high-mobility cross-linked hole transport layer by constructing a fully conjugated structure for solution-processed OLEDs.