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

Abstract

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.