Degradation-induced energy level mismatch in cohost-dopant blue phosphorescent OLEDs after device operation

•Degradation of blue PhOLEDs consisting of a cohost-dopant type emissive layer (EML) is investigated.•XPS and UPS depth profile technique with Ar gas cluster reveals the changes in chemical states and energy level alignments inside the blue PhOLEDs.•Most molecules are chemically stable upon device operation, and only molecular aggregation of blue dopant is observed.•The energy level mismatch near the EML is observed after the degradation, regardless of the blue dopant doping.