Spiro[fluorene-9,9′-xanthene]-based hole transporting materials modulated by mono- and bis- benzodioxino[2,3-b]pyrazine pendant groups for perovskite QLEDs

Abstract

In this work, we designed and synthesized two new hole-transporting materials with a nonplanar three-dimensional (3D) conformation. They were achieved by incorporating spiro[fluorene-9,9′-xanthene] as a cross-shaped configuration scaffold and adding either mono- (H1) or bis- benzodioxino[2,3-b]pyrazine (H2) pyrazine as pendant groups. Both compounds exhibit remarkable thermal stability, with thermal decomposition temperature (T_d) of 462 (H1) and 504 °C (H2), respectively. Moreover, the structural substitution with benzodioxino[2,3-b]pyrazine units successfully aligned the energy levels of both materials with the perovskite quantum dot luminescence layer. Hereby, the fabricated perovskite QLEDs using H1 as hole-transporting materials (HTMs) featured an excellent average external quantum efficiency (EQE) of up to 9.5% with a maximum luminance of 22368 cd m⁻², which is much higher than that of the H2-based devices with an EQE of 6.6% under the same conditions. The excellent device performance from H1 can be attributed to its asymmetric structure by the introduction of monosubstituted benzodioxino[2,3-b]pyrazine groups, as evidenced by its high hole mobility of 1.90 × 10⁻⁴ cm² V⁻¹ s⁻¹ and improved interface interaction with adjunct layers. Thus, this design approach may bring a fresh perspective to the utilization of solution-processable small-molecule HTMs in high-performance Pe-QLEDs and other optoelectronics in the future.