Realizing 22.3% EQE and 7-Fold Lifetime Enhancement in QLED: via Blending Polymer TFB and Cross-linkable Small Molecule for Solvent-Resistant Hole Transport Layer.

Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt(4,4'-(N-(4-butylphenyl))] (TFB) has been widely used as a hole transport layer (HTL) material in cadmium-based quantum dots light-emitting diodes (QLEDs) due to its high hole mobility. However, as the highest occupied molecular orbital (HOMO) energy level of TFB is -5.4 eV, the hole injection from TFB to quantum dots (QDs) layer is higher than 1.5 eV. Such high oxidation potential at the QD/HTL interface may seriously degrade the device lifetime. In addition, TFB is not resistant to most solvents, which limits its application in inkjet-printed QLEDs display. In this study, blended HTL consisting of TFB and cross-linkable small molecular 4,4'-bis(3-vinyl-9H-carbazol-9-yl)1,1'-biphenyl (CBP-V) was introduced into red QLEDs, because of the deep HOMO energy level of CBP-V (-6.2eV). Compared with the TFB only devices, the external quantum efficiency (EQE) of devices with blended HTL improved from 15.9 % to 22.3 % without the increase of turn-on voltage for spin-coating fabricated device. Furthermore, the blended HTL prolonged the T90 and T70 lifetime from 5.4 h and 31.1 h to 39.4 h and 148.9 h，respectively. These enhancements in lifetime are attributed to the low hole-injection barrier at HTL/QD interface and high thermal stability of blended HTL after crosslinking. Moreover, the crosslinked blended HTL showed excellent solvent resistance after cross-linking and the EQE of the inkjet-printed red QLEDs reached 16.9 %.