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Highly Efficient Solution-Processed Organic Light-Emitting Diodes Containing a New Cross-linkable Hole Transport Material Blended with Commercial Hole Transport Materials

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journal contribution
posted on 29.04.2021, 08:44 by Hyein Ha, Young Jae Shim, Da Hwan Lee, Eun Young Park, In-Ho Lee, Seok-Keun Yoon, Min Chul Suh
A new small-molecular thermally cross-linkable material {[4-(9-phenyl-9H-carbazol-4-yl)­phenyl]-bis-(4′-vinylbiphenyl-4-yl)-amine} (PCP-bis-VBPA, PbV) containing the styrene moiety was synthesized for hole transport layers in wet processed organic light-emitting diodes (OLEDs). It was found that PbV exhibited relatively high glass temperatures above 154 °C and a triplet energy (T1) greater than 2.81 eV. This new synthetic hole transport material (HTM) forms very uniform films after cross-linking reaction with little pin-holes, although it was small-molecule-based cross-linkable HTM. However, to solve the certain minor non-uniformity caused by pinholes with various sizes, a semi-interpenetrating network was formed with well-known polymeric HTM with high mobility [e.g., poly­(9,9-dioctylfluorene-co-N-(4-butylphenyl)­diphenyl amine), TFB, or poly­(N,N′-bis-4-butylphenyl-N,N′-bisphenyl)­benzidine, poly-TPD]. As a result, we successfully fabricated red phosphorescent OLED showing an efficiency of about 16.7 cd/A and 12.4% (external quantum efficiency) if we applied PbV blended with 20% of TFB or poly-TPD. In particular, the efficiency and lifetime are significantly improved by 1.5 and 4.5 times, respectively, compared to those of the control device without using blended HTM.

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