Solution processed alkali-metal and alkaline-earth-metal compounds as the efficient electron injection layer in organic light-emitting diodes

To enable the use of high work-function metals as the cathode in organic light emitting diodes (OLEDs), different types of alkali-metal and alkaline-earth-metal based compounds are examined as the electron injection layer (EIL). All the studied compounds can improve the device performance for aluminum, gold, and silver cathodes. Potassium hydroxide (KOH) and potassium carbonate (K2CO3) are employed as EIL for the first time in OLEDs. The performance of the device with sodium hydroxide (NaOH)/silver cathode almost doubles the conventional barium/aluminum device’s efficiency. By examining the work-function of EIL-modified metals through Kelvin Probe, a general correlation between the device performance and cathode work-function is revealed. The image-charge effect is found to be a contributor to the reduction of the metal work-function.