Electron injection and transport for high-performance inverted organic light-emitting diodes

Flexible organic light-emitting diodes (OLEDs) are considered the next-generation displays. To realize stable and high-performance flexible OLEDs, inverted OLEDs (iOLEDs) integrated into oxide thin-film transistors are desired for active-matrix OLEDs. Unfortunately, however, the efficiency of iOLEDs has not been as good as that of the conventional OLEDs up to recent years, due to the difficulties in electron injection from the bottom cathodes to the organic layers in iOLEDs. In this paper, electron injection and transport are described to explain how to enhance the efficiency of iOLEDs. Various methods of improving the electron injection from the bottom cathodes to the organic layers are reviewed, especially focusing on the n-doping technology. The importance of the n-electron-transporting layer (ETL)/undoped ETL homo-and hetero-junction for efficient iOLEDs is also explained. Finally, the p-n junction is introduced as a new kind of electron injection layer that works very well independently of the cathode materials.