Enhancement of hole-injection and power efficiency of organic light emitting devices using an ultra-thin ZnO buffer layer

• Published in: Journal of alloys and compounds 2009-06, Vol.479 (1), p.520-524
• Main Authors: Huang, Hsin-Hsuan, Chu, Sheng-Yuan, Kao, Po-Ching, Chen, Yung-Chen, Yang, Ming-Ru, Tseng, Zong-Liang
• Format: Article
• Language: English
• Subjects: Applied sciences; Electronics; Exact sciences and technology; Hole-injection; OLED; Optoelectronic devices; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; X-ray and ultra-violet photoelectron spectroscopies; ZnO
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2008.12.122

The advantages of using an anode buffer layer of ZnO on the electro–optical properties of organic light emitting devices (OLEDs) are reported. ZnO powders were thermal-evaporated and then treated with ultra-violet (UV) ozone exposure to make the ZnO layers. The turn-on voltage of OLEDs decreased from 4 V (4.2 cd/m 2) to 3 V (3.4 cd/m 2) and the power efficiency increased from 2.7 lm/W to 4.7 lm/W when a 1-nm-thick ZnO layer was inserted between indium tin oxide (ITO) anodes and α-naphthylphenylbiphenyl diamine (NPB) hole-transporting layers. X-ray and ultra-violet photoelectron spectroscopy (XPS and UPS) results revealed the formation of the ZnO layer and showed that the work function increased by 0.59 eV when the ZnO/ITO layer was treated by UV-ozone for 20 min. The surface of the ZnO/ITO film became smoother than that of bare ITO film after the UV-ozone treatment. Thus, the hole-injection energy barrier was lowered by inserting an ZnO buffer layer, resulting in a decrease of the turn-on voltage and an increase of the power efficiency of OLEDs.