Alkyl fluorene-based cross-linkable hole transport materials with high triplet energy for high-efficiency solution-processed green PHOLEDs

Abstract

Hole transport materials (HTMs) with high triplet energies (E_T) are necessary for high-efficiency phosphorescent organic light-emitting diodes (PHOLEDs), as they can effectively block the reverse transport of triplet excitons in the emitting layer. Here, two cross-linkable HTMs based on an alkyl fluorene core, 4,4′-(9,9-dipropyl-9H-fluorene-2,7-diyl)bis(N-phenyl-N-(4-vinylphenyl)aniline) (V-PFPDPA) and 4,4′-(9,9-dihexyl-9H-fluorene-2,7-diyl)bis(N-phenyl-N-(4-vinylphenyl)aniline) (V-HFPDPA), were designed and synthesized. Due to the inherently high E_T of the fluorene core structure, V-PFPDPA and V-HFPDPA were found to have desirable E_T of 2.72 eV and 2.73 eV, respectively, thus favoring the suppression of exciton quenching. The cross-linked hole transport layers had high thermal stability and excellent resistance to solvation. The resultant solution-processed green PHOLEDs have a maximum current efficiency of 81.06 cd A⁻¹ and a maximum external quantum efficiency of 23.25%. This work demonstrates a feasible strategy to advance the performance of solution-processed PHOLEDs for display and lighting applications.