Solution-processable double-layered ionic p-i-n organic light-emitting diodes

We report on the efficient emission of light from double-layered ionic p-i-n phosphorescent organic light-emitting diodes (OLEDs), which are formed using a solution process. The OLEDs under investigation comprise an emitting material layer (EML) and a photo-crosslinkable hole-transporting layer (HTL), which is inserted between the anode and the EML. The EML was doped with ionic salt and the double layers were annealed using simultaneous thermal and electrical treatments. Because simultaneous treatments cause the ions from the decomposed ionic salt to accumulate at the electrode surfaces, the energy levels of the organic layers are altered by the accumulated ions. This annealing process results in the formation of a double-layered ionic p-i-n structure. As a result, an annealed double-layered ionic p-i-n OLED with a peak brightness of about 38,000 cd/m2 and a peak current efficiency of ∼35 cd/A was achieved. These results demonstrate that simultaneous treatments can lead to the increased and balanced injection of carriers in solution-processed double-layered OLEDs.