Efficient non-doped blue electroluminescence based on phenanthroimidazole–benzoylfluorene hybrid molecules with high spin–orbit coupling and balanced charge mobilities

Abstract

Developing non-doped blue organic light-emitting diodes (OLEDs) with excellent photoluminescence quantum yield (PLQY), high exciton utilization efficiency (EUE) and balanced carrier mobility is an urgent challenge. Herein, two blue molecules based on hot excitons were reported, namely PPIFB and PPIBF. Using a plane-rigid fluorene as the main conjugated π bridge with a small torsion angle of PPIFB restricted the rotation and vibrational relaxation of molecules, which makes PPIFB achieve a PLQY as high as 90% in neat films. Moreover, fluorene participates in long-range π stacking, enriching weak intermolecular forces and the introduction of benzoyl fluorene as an acceptor ushers in hydrogen bonding interactions, which increase and aid in achieving balanced carrier mobilities, with hole and electron mobilities up to 2 × 10⁻⁶ and 7.71 × 10⁻⁷ cm² V⁻¹ s⁻¹ respectively. Also, the introduction of benzoyl fluorenes increased the contribution of the n-orbitals, resulting in a higher spin–orbit coupling (SOC) of up to 14.7 cm⁻¹, which is beneficial for achieving high exciton utilization. The non-doped OLED of PPIFB exhibits a maximum external quantum efficiency (EQE_max) of 9.1% at 459 nm, which is one of the best results for non-doped blue fluorescent OLEDs.