Improved device lifetime of organic light emitting diodes with an electrochemically stable π-conjugated liquid host in the liquid emitting layer

The device lifetimes of organic light emitting diodes with a liquid emitting layer (liquid OLEDs) were improved by proper combination of host and guest molecules in the liquid emitting layer. The device lifetime strongly depends on the electrochemical stability of radical cations in the liquid emitting layer. The electrochemical stability of the liquid host materials was achieved by the dimerization of the alkyl-substituted carbazole 9,9'-{2- [2-(2-methoxyethoxy)ethoxy]ethyl}-3,3'-bis(9H-carbazole) [(TEGCz) ₂]. The use of a guest compound with its highest occupied molecular orbital level higher in energy than that of (TEGCz) ₂ is a critical factor for realizing stable electroluminescence performance. A liquid OLED with proper combination of the guest and host materials showed an improved device lifetime of longer than 1 h, which is 100 times longer than that of our previous reports.