A binary solvent mixture-induced aggregation of a carbazole dendrimer host toward enhancing the performance of solution-processed blue electrophosphorescent devices

Abstract

The emissive layer morphology strongly correlates with the charge transport and light-emitting performance of solution-processed phosphor-doped organic light-emitting diodes (PhOLEDs). Herein, morphology manipulation of the solution-processed emissive layer comprising of carbazole dendrimer (H2) host:blue phosphor (FIrpic) guest is realized via processing of the solvent and its influence on charge transport and light-emitting properties is investigated. The formation of H2 aggregates within its amorphous matrix processed with the toluene:p-xylene solvent mixture distinctively improves the hole and electron transport within the emissive layer, helping to lower the driving voltages and improve the light-emitting efficiency. However, excess aggregation of H2 would result in non-uniform dispersion of the FIrpic guest within the H2 host, leading to non-complete host-to-guest energy transfer and decreased electroluminescence performance. Through manipulation of the aggregates within the H2 host by varying the solvent mixture ratio, the trade off between charge transport and energy transfer is realized. Finally, the solution-processed blue PhOLED with optimized emissive layer morphology processed with toluene : p-xylene (9 : 1) solvent mixture achieves a high light-emitting efficiency of 27.8 cd A⁻¹, corresponding to 25% enhancement compared to 22.2 cd A⁻¹ of the control device processed with commonly used toluene solvent.