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Published on: October 13, 2017
Suppression of Excimer Formation via Conjugation-Driven Interactions in Crosslinked Hole Transport Layer for
Zhanpeng Qin1,2, Shirong Wang1,2, Yanliu Zhu1,2
1School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China.
Abstract:
Blue quantum dot light-emitting diodes (QLEDs), underperforming relative to red and green counterparts, shoulder the critical responsibility for full-color displays. Crosslinked 9,9'-([1,1'-biphenyl]-4,4'-diyl)bis(3-ethenyl-9H-carbazole) (CBP-V), owning a deep HOMO level (∼-5.9 eV), is a promising hole transport layer (HTL) candidate for blue QLEDs. However, π-π interactions and molecular rotatability in CBP-V promote parallel stacking and electron cloud overlap. This gives rise to excimer formation, which exacerbates the energy-level disorder in crosslinked CBP-V and thus degrades its hole mobility. Herein, 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) and 4,4',4″-tris(carbazol-9-yl)-triphenylamine (TCTA), featuring analogous phenylcarbazole structural fragments, are incorporated into CBP-V to suppress excimer formation via a conjugate-interaction-driven strategy. The asymmetric configuration of 26DCzPPy and the branched architecture of TCTA enable modulation of molecular stacking behavior and suppress parallel stacking of CBP-V during the crosslinking process. This realizes spatial segregation of CBP-V and thus eliminates the prerequisite for excimer formation. Resultantly, suppression of excimer allows HTL to achieve a tenfold enhancement of hole mobility from 5.37 × 10-4 to 5.97 × 10-3 cm2·V-1·s-1. Target QLEDs gain prominent enhancements in external quantum efficiencies for deep-blue emission (456 nm) from 11.48% to 22.92% and pure-blue emission (470 nm) from 13.39% to 24.12%, with maximum brightness values of 29735 and 91268 cd m-2.
