Effect of Bis-diazirine-Mediated Photo-Crosslinking on Polyvinylcarbazole and Solution-Processed Polymer LEDs
Kaustav Dey, S. Roy Chowdhury, Erik Dykstra, H. Peter Lu, Ruth Shinar, J. Shinar, Pavel Anzenbacher
Abstract
The problems with fabrication of solution-processed organic light-emitting diodes (OLEDs) stem largely from the defects associated with sequential deposition of the layers, solvent-induced surface erosion, and layer mixing. Herein, we demonstrate that a photopolymerizable bis-diazirine molecule can easily convert soluble polymers into cross-linked insoluble materials, alleviating the problems associated with inter-layer mixing. Upon 5–20 min irradiation with long wavelength/low power UV (1.8 mW/cm2) bis-diazirines results in the formation of carbenes that can react via carbon–hydrogen bond insertion with polymers or small molecules yielding cross-linked networks. The effectiveness of the bisdiazirine-mediated photo-cross-linking has been studied by investigating the surface morphology of the hole-transporting material polyvinylcarbazole (PVK), which exhibited 45% decrease in surface roughness after 15 min of the UV irradiation. The effect of this cross-linking procedure on device performance was studied in OLEDs with the configuration of indium tin oxide/PEDOT:PSS/PVK:(0–10%) cross-linker/PFO:2% F8BT/TPBI/CsF/Al. After the photolysis of the diazirines, the experimental devices exhibited a 42% enhancement in the maximum external quantum efficiency (EQEmax), from 1.2 to 1.7%, and a maximum luminous efficiency improvement from 3.9 to 5.4 cd/A. Overall, this observation suggests that 3-trifluoromethyl(aryl)diazirine-based cross-linking processes is a promising method for fabrication of polymer LEDs.