Thermally cross-linkable spirobifluorene-core-based hole transport layer with high solvent-resistivity for solution processible OLEDs

The fabrication of organic light-emitting diodes (OLEDs) composed of multi-layered structure through a solution-based process suffers from the dissolution of the preformed lower parts during the coating of upper layers. To prevent this problem during the solution process, a promising approach of introducing a cross-linkable layer with a high solvent-resistivity has been proposed. Herein, thermally cross-linkable spirobifluorene-core-incorporated hole transport layers (HTLs) with a cross-linking temperature of 180 °C are designed for solution-processible OLEDs composed of multi-layered structures. The enhanced morphology stability and solvent-resistive property of the synthesized HTLs are evaluated through a rinsing-test with an organic solvent used for the emitting layer. Considering the charge transport property of HTLs in OLEDs, the appropriate energy level and triplet energy values of the synthesized HTLs promote the efficient cascade hole migration. Consequently, the OLED composed of thermally cross-linkable HTL shows higher quantum efficiency (QE) of 16.5% and lower operation voltage of 5.1 V at 1000 cd/m², compared to that composed of a commercialized polymer, poly (9-vinylcarbazole) (PVK). Thus, in this study, the feasibility of the potential application of thermally cross-linkable HTL with the spirobifluorene-core-units as charge transport layers for solution-based optoelectronic devices composed of multi-layered structures has been verified.