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 (T 1 ) greater than 2.81 eV. This new synthetic hole transport material (HTM) forms very uniform films after crosslinking reaction with little pin-holes, although it was smallmolecule-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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In order to improve mobility, we can blend linear HTM (hole transfer material) with high mobility polymer to crosslinked polymer prepared as a semi-IPN (inter-penetrating network) form to realize highly efficient solution processed OLEDs. However, we cannot ensure a reproducibility of the devices because a noncrosslinked linear polymer, poly(9,9-dioctylfluorene-coN -(4butylphenyl)di-phenylamine) (TFB) or poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine (poly-TPD) could be still miscible with emitting layer (EML) at their successive wet processes. To overcome this problem, we attempted to eliminate intermixing by crosslinking organic soluble linear HTM polymers by adding a nitrene type photo-crosslinking agent inside the HTM solution.