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

A new small-molecular thermally cross-linkable material {[4-(9-phenyl-9 H -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 ( T ₁ ) 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.