The reduction of driving voltage in organic light-emitting devices by inserting step barrier layer

Abstract

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 (Alq 3) (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 (2.9 eV) lies in between that of Alq 3 (3.1 eV) (or Bphen (3.0 eV)) and TBADN (2.8 eV), step barrier from Alq 3 (or BPhen) though Gaq to TBADN can be formed. The experimental results indicate that the J– V 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 20 mA/cm 2 is reduced from 7.9 V to 4.9 V for devices with Alq 3, and from 4.2 V to 3.1 V for devices with BPhen, respectively, owing to the introduction of step barrier. For the complete devices, when Gaq step barrier is introduced, at 20 mA/cm 2, the driving voltage is reduced from 7 V to 5.8 V for devices with Alq 3 and from 6.2 V to 5.1 V for devices with BPhen. It has also been observed that for devices with step barrier layer, the luminance at 200 mA/cm 2 is increased from 1992 cd/m 2 to 3281 cd/m 2 for device with Alq 3, and from 1745 cd/m 2 to 2876 cd/m 2 for devices with BPhen, respectively. The highest luminance reaches 3420 cd/m 2 in devices with Alq 3 as ETL and 3176 cd/m 2 in devices with BPhen as ETL after the introduction of step barrier. The phenomena are explained by using tunnel theory.