Solution-Processed, Alkali Metal-Salt-Doped, Electron-Transport Layers for High-Performance Phosphorescent Organic Light-Emitting Diodes

High-performance, blue, phosphorescent organic light-emitting diodes (PhOLEDs) are achieved by orthogonal solution-processing of small-molecule electron-transport material doped with an alkali metal salt, including cesium carbonate (Cs2CO3) or lithium carbonate (Li2CO3). Blue PhOLEDs with solution-processed 4,7-diphenyl-1,10-phenanthroline (BPhen) electron-transport layer (ETL) doped with Cs2CO3 show a luminous efficiency (LE) of 35.1 cd A-1 with an external quantum efficiency (EQE) of 17.9%, which are two-fold higher efficiency than a BPhen ETL without a dopant. These solution-processed blue PhOLEDs are much superior compared to devices with vacuum-deposited BPhen ETL/alkali metal salt cathode interfacial layer. Blue PhOLEDs with solution-processed 1,3,5-tris(m-pyrid-3-yl-phenyl)benzene (TmPyPB) ETL doped with Cs2CO3 have a luminous efficiency of 37.7 cd A-1 with an EQE of 19.0%, which is the best performance observed to date in all-solution-processed blue PhOLEDs. The results show that a small-molecule ETL doped with alkali metal salt can be realized by solution-processing to enhance overall device performance. The solution-processed metal salt-doped ETLs exhibit a unique rough surface morphology that facilitates enhanced charge-injection and transport in the devices. These results demonstrate that orthogonal solution-processing of metal salt-doped electron-transport materials is a promising strategy for applications in various solution-processed multilayered organic electronic devices.