A\nNovel Crosslinked Hole Transport Layer with Enhanced\nCharge Injection Balance for Highly Efficient Inkjet-Printed Blue\nQuantum Dot-Based Light-Emitting Diodes

In\nthis work, an efficient and robust hole transport layer (HTL)\nbased on blended poly((9,9-dioctylfluorenyl-2,7-diyl)-<i>alt</i>-(9-(2-ethylhexyl)-carbazole-3,6-diyl)) (PF8Cz) and crosslinkable\n3,3′-(9,9-dimethyl-9<i>H</i>-fluorene-2,7-diyl)bis(9-(4-vinylphenyl)-9<i>H</i>-carbazole) (FLCZ-V) is introduced for high-performance\nand stable blue quantum dot-based light-emitting diodes (QLEDs), wherein\nFLCZ-V can in situ-crosslink to a continuous network polymer after\nthermal treatment and the linear polymer PF8CZ becomes intertwined\nand imprisoned. As a result, the blended HTL shows a high hole mobility\n(1.27 × 10<sup>–4</sup> cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>) and gradient HOMO levels (−5.4 eV\nof PF8CZ and −5.7 eV of FLCZ-V) that can facilitate hole injecting\nso as to ameliorate the charge balance and, at the same time, achieve\nbetter electron-blocking capability that can effectively attenuate\nHTL decomposition. Meanwhile, the crosslinked blended HTL showed excellent\nsolvent resistance and a high surface energy of 40.34 mN/m, which\nis favorable to enhance wettability for the deposition of a follow-up\nlayer and attain better interfacial contact. Based on the blended\nHTL, blue QLEDs were fabricated by both spin-coating and inkjet printing.\nFor the spin-coated blue QLED, a remarkable enhancement of external\nquantum efficiency (EQE) of 15.5% was achieved. Also, the EQE of the\ninkjet-printed blue QLED reached 9.2%, which is thus far the best\nresult for the inkjet-printed blue QLED.