Correlation of energy band alignment and turn-on voltage in organic light emitting diodes

The correlation of energy alignment and turn-on voltage of organic light emitting diodes (OLEDs) was investigated. With identical hole transport layers (HTLs) and electron transport layers (ETLs), the turn-on voltages of OLEDs are always the same, regardless of the cathode structures, such as Ca, Al, LiF/Al, and Cs2CO3/Al. For devices with various combinations of HTLs and ETLs, the turn-on voltages are equal to the energy difference between the lowest unoccupied molecular orbital of ETLs and the highest occupied molecular orbital of HTLs, taking into consideration of vacuum level shift at organic interfaces measured from ultraviolet photoemission spectroscopy.