Built-in voltage in organic light-emitting diodes with a use of Li2O/Al cathode

An effect of bilayer cathode Li2O/Al system was studied in Alq3 based organic light-emitting diodes with a variation of Li2O layer thickness from 0 to 10 nm. The device was made in a structure of ITO/(TPD)/Alq3/Li2O/Al. Current density-luminance-voltage (J-L-V) characteristics and a built-in voltage of the device were measured at ambient conditions. Built-in voltage in the device is generated due to a difference of work functions between the anode and cathode. From the J-L-V characteristics of the device, we observed an increase in luminance and current efficiency by more than 100 times and 2 times, respectively, for the device with 0.5 nm thick Li2O layer. The measured built-in voltage shows that the device with 0.5 nm thick Li2O layer has relatively higher built-in voltage compared to the others. Since the higher value of built-in voltage corresponds to the lower value of electron barrier height in cathode, the improvement in the efficiency for the device with 0.5 nm thick Li2O layer is thought to be due to a lowering of the electron barrier height.