Influence of oxygen deficiency in indium tin oxide on the performance of polymer light-emitting diodes Ming-Chih Chen, Show-An Chen ⁎ Chemical Engineering Department, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC abstract article info Article history: Received 16 February 2008 Received in revised form 22 June 2008 Accepted 10 September 2008 Available online 2 October 2008 Keywords: Indium tin oxide Oxygen deficiency Polymer light-emitting diodes Leakage current We investigated effects of oxygen deficiency in the indium tin oxide (ITO) on the performance of poly(2- methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV)-based polymer light-emitting diodes, in which the ITO anode was deposited by radio frequency magnetron sputtering at different oxygen flow rates. We found that the degree of oxygen deficiency in the ITO films can affect the device performance significantly and is a source of current leakage. At the optimal oxygen flow rate, the leakage current of devices can be reduced and the balance between hole and electron fluxes can be promoted in the MEH-PPV layer to improve device efficiency. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Indium tin oxide (ITO) has been widely utilized as the anode in organic or polymer light-emitting diodes (OLEDs or PLEDs) [1,2] because of its high transmittance in visible region, low electrical resistivity, and high work function. ITO is a heavily doped n-type semiconductor with a free carrier concentration in the range 10 19 – 10 21 cm - 3 [3]. The low resistivity of ITO film is due to the doubly charged oxygen vacancies and substitutional four-valent Sn positioned on three-valent In sites. When using ITO transparent anodes, there is a well-known trade-off between the resistivity and the transmittance, which is dominated by the number of oxygen vacancies in the ITO films [4,5]. Oxygen vacancies act as donors in the ITO films; consequently, when ITO is oxygen-deficient, electrical transport can be realized. However, if the ITO films are too oxygen-deficient, the transmittance and crystallinity of ITO can be deleteriously affected [6–8]. Therefore, it is important to determine the optimal deposition condition for ITO in order to achieve high transmittance, stable crystalline structures and low resistivity. In general, the work function and surface roughness of ITO anode are important factors that affect the device performance of OLEDs or PLEDs because the work function of ITO determines the hole injection barrier at the ITO/organic layer interface [9–11] and the surface roughness of ITO is related to the leakage current [12,13]. Therefore, to adjust the work function and the surface roughness of ITO in order to obtain high device performance is the common practice. Although much works have been done on tuning the work function of ITO anode, less efforts have been focused on the origin of leakage current. Hence, investigation on the leakage current issue is necessary to further improve the performance of OLEDs or PLEDs. Here, we find that the oxygen deficiency in ITO films is an additional source of current leakage in devices, and that the device performance can be optimized by adjusting the level of oxygen deficiency. 2. Experimental details ITO films were deposited by 13.56 MHz radio frequency (RF) magnetron sputtering from an ITO target (90 wt.% of In 2 O 3 and 10 wt.% of Sn) using a commercial batch-type sputtering system with the magnetic field strength of 0.09 Wb/m 2 . The substrates were the alkaline-free glass. The vacuum chamber was evacuated down to pressure 2.0 × 10 - 6 Torr prior to the deposition and then a mixed gas of argon and oxygen was introduced into the chamber through independent mass flow controllers to build the required working pressure (2.0 ×10 - 3 Torr). The flow rate of argon was kept constant at 800 sccm while that of oxygen was varied from 0 to 10 sccm (with an interval of 2 sccm). ITO films deposited at oxygen flow rate of 0, 2, 4, 6, 8, and 10 sccm are designated as ITO(0), ITO(2), ITO(4), ITO(6), ITO(8), and ITO(10) films, respectively. The RF power and deposition temperature were 2000 W (power density 2.0 W cm - 2 ) and 210 °C, respectively. The distance between the substrate and the target was 70 mm. The thickness of the deposited ITO films was found to be 180 + 10 nm by a Tencor alpha-step profilometer. Transmittances between 300 nm to 2500 nm were checked with a Perkin-Elmer spectrometer. The sheet resistance was measured by four-point probe method. The carrier concentration and mobility of ITO films were measured at room Thin Solid Films 517 (2009) 2708–2711 ⁎ Corresponding author. Tel.: +886 3 5710733; fax: +886 3 5720906. E-mail address: sachen@che.nthu.edu.tw (S.-A. Chen). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.09.088 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf