Improved Surface Morphology of Crosslinked Hole Transport Films by a Mixture of Polymer for OLEDs

Solution process for multilayer organic light-emitting diodes (OLEDs) brings in the dissolution problems between two stacked layers. It can be tackled using crosslinkable organic materials, especially for a hole transport layer (HTL). Similar to noncrosslinked HTL films based on small molecules, however, there appears the aggregation phenomenon in crosslinked HTL films fabricated by spin coating. We have demonstrated that aggregation can be suppressed through a mixture of noncrosslinkable polymer and crosslinkable small molecule HTL materials. Although such a mixed HTL film results in a slight degradation in the solvent resistance and device performance, it ensures the film uniformity and thus OLED stability. When cured under ambient air, however, the mixed HTL films exhibit a number of microaggregates that increase in proportion to the concentration of the polymer chains. It is attributed that the moisture adsorption near the hybrid interfaces causes lateral phase separation. We have found that they disappear by carrying out the thermal curing under inert gas atmosphere or increasing the polymer concentration in ambient air.