A Novel Crosslinked Hole Transport Layer with Enhanced Charge Injection Balance for Highly Efficient Inkjet-Printed Blue Quantum Dot-Based Light-Emitting Diodes
Liming Xie, Jinrong Shi, Ting Wang, Qing Li, Yuan‐Qiu‐Qiang Yi, Qing Zhang, Yang Liu, Wenming Su, Byung Seong Bae, Damian C. Onwudiwe, Lei Wei, Zheng Cui, Christine K. Luscombe
Abstract
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.