The reduction of driving voltage in organic light-emitting devices by inserting step barrier layer
The electron injection and transport in OLEDs have been improved by using a tris-[8-hydroxyquinoline] gallium (Gaq) layer as step barrier between tris-[8-hydroxyquinoline]aluminum (Alq3) (or 4,7-diphyenyl-1,10-phenanthroline (Bphen)) and 2-t-butyl-9,10-di-(2-naphthyl)anthracene (TBADN). Since the LUMO (lowest unoccupied molecular orbital) of Gaq (2.9eV) lies in between that of Alq3 (3.1eV) (or Bphen (3.0eV)) and TBADN (2.8eV), step barrier from Alq3 (or BPhen) though Gaq to TBADN can be formed. The experimental results indicate that the JV characteristics of both the electron-only and the complete devices show the increase of the current density in devices with step barrier compared with the devices without step barrier. For electron-only devices, the driving voltage at the current density of 20mA/cm2 is reduced from 7.9V to 4.9V for devices with Alq3, and from 4.2V to 3.1V for devices with BPhen, respectively, owing to the introduction of step barrier. For the complete devices, when Gaq step barrier is introduced, at 20mA/cm2, the driving voltage is reduced from 7V to 5.8V for devices with Alq3 and from 6.2V to 5.1V for devices with BPhen. It has also been observed that for devices with step barrier layer, the luminance at 200mA/cm2 is increased from 1992cd/m2 to 3281cd/m2 for device with Alq3, and from 1745cd/m2 to 2876cd/m2 for devices with BPhen, respectively. The highest luminance reaches 3420cd/m2 in devices with Alq3 as ETL and 3176cd/m2 in devices with BPhen as ETL after the introduction of step barrier. The phenomena are explained by using tunnel theory.
step barrier oleds tunnel theory low voltage