Organic light-emitting diodes with hydrogenated In-doped ZnO thin films as transparent conductive electrodes
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Young Sung Kima) Advanced Material Process of Information Technology, Sungkyunkwan University, Suwon 440-746, Korea (Received 19 July 2007; accepted 28 February 2008)
Hydrogenated In-doped ZnO (ZIO:H) films grown at different ratios, R, of hydrogen to argon were deposited at a substrate temperature of 100 °C for the organic light-emitting diodes (OLEDs). The OLEDs with the ZIO:H (R ⳱ 0.08) anode achieved a maximum luminance efficiency of 3.4 cd/A and a power efficiency of 0.6 lm/W, which are as good as the values of the control device fabricated on a tin-doped indium oxide (ITO) anode. This indicates that the efficiency of the OLEDs is critically affected by the ratio of injected hydrogen gas during the deposition of the ZIO and that the ZIO:H developed herein is promising as an alternative to conventional ITO.
I. INTRODUCTION
Recently, transparent conducting oxide films have been extensively studied because they exhibit high optical transparency and electrical conductivity, and are often used in photoelectronic devices such as organic lightemitting diodes (OLEDs), solar cells, and flat-panel displays (FPDs).1,2 Among the various components of OLEDs, transparent conducting oxide films have become particularly attractive for the anode component.3–5 Tindoped indium oxides (ITOs) have been extensively used as anodes in OLEDs because of their high conductivity and transparency over the visible range, and their high work function. However, the chemical instability, toxic nature,6,7 high cost, and scarcity of indium have led researchers to seek an alternative to ITO.8 Among the alternative candidate thin films, ZnO thin films have been considered as suitable anodes because ZnO is more abundant and less expensive than ITO.9–11 It is also chemically stable under exposure to the hydrogen plasma that is commonly used for the fabrication of solar cells.12 Because ZnO thin films are large-band gap oxide semiconductors with a large excitonic binding energy,13,14 their physical properties have stimulated applications in many promising optoelectronic devices, such as FPDs. In the last decade, moreover, ZnO films doped with impurities, such as Al, Ga, In, and Zr, have been actively studied.10,14–17 Because the utilization possibility of Ina)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0207 1674 J. Mater. Res., Vol. 23, No. 6, Jun 2008 http://journals.cambridge.org Downloaded: 18 Mar 2015
doped ZnO (ZIO) thin films grown on glass substrates as anodes in OLEDs is strongly affected by the electrical, optical, and electronic properties, it is very important to study the transparent conducting properties of ZIO films to improve the efficiencies of OLEDs. In our work, to investigate the effect of hydrogen on the physical properties of the anodic films, hydrogenated In-doped zinc oxide (ZIO:H) films were deposited by pulsed direct current (dc) magnetron sputtering with various flow ratios of each hydrogen and argon gas. Also, the performance characteristics of OLEDs with ZIO:H anodes wer
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