All organic transport materials (TMs) based QLEDs: Revisiting molecular electron TMs with mapping hole TMs via cross-linking strategy

Compared to metal oxide nanoparticles, small-molecule electron transport materials (SM-ETMs) possess accurate molecular weight, definitive chemical structure, good environmental stability and excellent batch-to-batch consistency in mass synthesis. Employing SM-ETMs for QLEDs, as does in commercialized OLEDs, demonstrates huge potentials for compatibility with current OLED fabrication process. To explore the feasibility of all organic transport materials for QLEDs, two phosphine oxides were selected in pair with three classical polymer hole transport materials (HTMs). Initially, the charge imbalance between the SM-ETM and polymeric HTM obstructed the device to achieve its best performance. To overcome this issue, the cross-linking strategy was introduced for HTMs and four cross-linkers were synthesized. Among them, the X4 cross-linked with TFB as the HTM, in combination with the TP3PO as the ETM, provided the best performance, with an EQE of approaching 11% (11.4 lm W−1, 15.5 cd A−1). Detailed investigations revealed that the TP3PO and TFB:X4 based QLED had a more balanced charge transport as anticipated. The present work indicates the potential of all organic materials for constructing high-performance QLEDs, pointing a new direction to further boost QLED performance.