Life extension of organic LED's by doping of a hole transport layer

We investigated the influence of a series of aromatic hydrocarbon dopants in the hole transport layer on the life of organic light emitting devices (OLED) with the following configuration: ITO/NPB+dopant/Alq3+fluorescent dye/Mg:Ag. The addition of selected dopants to NPB led to an increase in device lifetime by more than an order of magnitude. Half-life, t1/2, of the devices was measured at the current density of J=25 mA/cm2. Device life, at initial luminance of 100 cd/m2, was estimated by using the scaling law established by A. Van Slyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 69 (1996) 2160, (Linitial×t1/2=constant). Typically extrapolated device life exceeded 10 000 h, with the best devices exceeding 50 000 h. Life extension by use of dopants is consistent with the recently proposed mechanism that OLED degradation is primarily caused by holes injected into Alq3.