A Novel Crosslinked Hole Transport Layer with Enhanced Charge Injection Balance for Highly Efficient Inkjet-Printed Blue Quantum Dot-Based Light-Emitting Diodes

In this work, an efficient and robust hole transport layer (HTL) based on blended poly((9,9-dioctylfluorenyl-2,7-diyl)- alt -(9-(2-ethylhexyl)-carbazole-3,6-diyl)) (PF8Cz) and crosslinkable 3,3′-(9,9-dimethyl-9 H -fluorene-2,7-diyl)bis(9-(4-vinylphenyl)-9 H -carbazole) (FLCZ-V) is introduced for high-performance and stable blue quantum dot-based light-emitting diodes (QLEDs), wherein FLCZ-V can in situ-crosslink to a continuous network polymer after thermal treatment and the linear polymer PF8CZ becomes intertwined and imprisoned. As a result, the blended HTL shows a high hole mobility (1.27 × 10 –4 cm 2 V –1 s –1 ) and gradient HOMO levels (−5.4 eV of PF8CZ and −5.7 eV of FLCZ-V) that can facilitate hole injecting so as to ameliorate the charge balance and, at the same time, achieve better electron-blocking capability that can effectively attenuate HTL decomposition. Meanwhile, the crosslinked blended HTL showed excellent solvent resistance and a high surface energy of 40.34 mN/m, which is favorable to enhance wettability for the deposition of a follow-up layer and attain better interfacial contact. Based on the blended HTL, blue QLEDs were fabricated by both spin-coating and inkjet printing. For the spin-coated blue QLED, a remarkable enhancement of external quantum efficiency (EQE) of 15.5% was achieved. Also, the EQE of the inkjet-printed blue QLED reached 9.2%, which is thus far the best result for the inkjet-printed blue QLED.