Efficiency improvement of fluorescent OLEDs by tuning the working function of PEDOT:PSS using UV-ozone exposure

The efficiency improvement of fluorescent organic light-emitting diodes (OLEDs) is achieved by tuning the work function of a hole-injection layer (HIL) of poly(3,4-ethylene-dioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). This tuning is achieved by treating PEDOT: PSS with ultraviolet light (UV)-ozone plasma exposure. In order to demonstrate the effect of the UV-ozone treatment, two simple green-emitting device OLED architectures were used; Device I utilized tris(8-quinolinolato) aluminum (Alq(3)) as an emitter while Device II employed Alq(3) doped with 2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10-(2-benzothiazolyl)quinolizino[9,9a,1gh]-coumarin (C545T) emissive layer. The study showed that the OLEDs efficiency depends on the type of device configuration and the duration of the UV-ozone exposure. Here, a short time exposure of UV-ozone treatment of 4 min and 8 min was found optimal for the best device characteristics. The maximum power efficiency in Device I increased from 1.05 lm/W (untreated) to 2.70 lm/W, while Device II, doped with emitter increased from 4.10 lm/W to 8.24 lm/W. Consequently, Device I and Device II obtained the 100% increase in the power efficiency. The improvements in device efficiency is attributed to the efficient injection and balance in charge carriers resulting from the tuned work function of the PEDOT: PSS. (c) 2010 Elsevier B.V. All rights reserved.