High-Performance Inkjet-Printed Blue QLED Enabled by Crosslinked and Intertwined Hole Transport Layer

A novel crosslinkable 4,4 '-(9,9-dimethyl-9H-fluorene-2,7-diyl)bis(N-phenyl-N-(4-vinylphenyl)aniline) (FLTA-V) based on TFB repeat unit is designed and synthesized. A blended film with optimized 2:1 weight ratio of TFB and FLTA-V is employed to act as the effective hole transport layer (HTL) for quantum dot light-emitting diode (QLED). The resulted HTL is crosslinked and exhibits excellent solvent-resistant property without any initiators. It also forms a robust network structure and the TFB molecules are intertwined and imprisoned in the network. The HTL then becomes denser, which enhances both intermolecular interactions and pi-pi stacking to further promote efficient charge transportation. Moreover, the closeness of highest occupied molecular orbital levels between TFB (-5.4 eV) and FLTA-V (-5.6 eV) is beneficial for hole injection from HTL to QD layer. The increased surface energy of blended HTL ensures better electrical contact between HTL and QDs, which reduces the leakage current of the device. The combination of these favorable factors has led to the fabricated blue QLED showing a remarkable enhancement of external quantum efficiency (EQE) from 7.23% for the control TFB-based device to 10.20% for blended HTL-based device. In addition, a maximum of 6.82% of EQE for inkjet-printed blue QLED has been achieved, which is the best-reported high-efficiency inkjet-printed blue QLED to date.