Solution-Processed Double-Layer Electron-Transport Layer for Conventional Blue Phosphorescent Organic Light-Emitting Diodes

Zinc oxide (ZnO) is a potential material for the electron-transport layer (ETL) in organic light-emitting diodes (OLEDs). However, ZnO may induce exciton dissociation at the emission layer (EML)/ZnO interface, which lowers the device efficiency. Here, polyethyleneimine (PEI)-doped lithium acetylacetonate (Li(acac)) is used as a hybrid interfacial layer (IL) between a ZnO nanoparticle (NP) ETL and a phosphorescent EML, with a thermally evaporated aluminum film as the cathode contact on the top. To avoid the IL being dissolved, ZnO NPs were re-dispersed in octane (an orthogonal solvent for most ILs) and doped with trans-polyisoprene (PI) to prevent aggregation on the film. The hybrid IL markedly improves electron injection and transport and suppresses triplet exciton quenching at the EML/ZnO interface, resulting in a lower driving voltage and doubled luminous efficiency compared with those of the control device without IL. ZnO NPs can serve as an ETL in conventional OLEDs cooperating with PEI-doped Li(acac) as an IL and this research provides an easy way to construct a stable double-layer ETL configuration in high-efficiency OLEDs.