Halogen free solvent processed light-emitting diodes achieving EQE nearly 25% for imidazole-based host materials synthesized by ball milling

Abstract

Although lab-scale organic light-emitting diode (OLED) efficiencies continue to increase, most systems use halogenated solvents to dissolve the emitting layer. Halogenated solvents achieve better film morphologies and efficiencies due to their better solubility for organic semiconductors, but they often have negative environmental impacts. Therefore, we have designed and synthesized three highly efficient new host materials for solution processable green phosphorescent OLEDs (PHOLEDs) using non-halogenated solvents. We utilized environmentally friendly high energy ball milling, with strong mechanical forces producing desired host materials with high reactivity. Impaction and friction followed by mechanical forces also heat the reactants during synthesis. All three materials exhibited excellent photoluminescence quantum yields and high triplet energy for efficient energy transfer to guest molecules. Optimized PHOLEDs by solution processing using halogenated solvent exhibited highest external quantum efficiency (EQE) and current efficiency (CE) of 25.20% and 96.85 cd A(-1) with highest illumination of 65931 cd m(-)(2), correspondingly. PHOLEDs utilizing non-halogenated solvents also achieved high EQE and CE (24.70% and 93.07 cd A(-1), respectively), currently the highest reported amongst solution processed PHOLEDs utilizing both halogenated and non-halogenated solvents.