Improved performance and stability by an Al/Ni bilayer cathode in organic light-emitting diodes Shengwei Shi, Dongge Ma * State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Changchun 130022, PR China Received 2 September 2005; received in revised form 10 February 2006; accepted 20 February 2006 Available online 2 May 2006 Abstract Al/Ni bilayer cathode was used to improve the electroluminescent (EL) efficiency and stability in N,N 0 -bis(1-naphthyl)-N,N 0 -diphenyl-1,1 0 biphenyl 4,4 0 -dimaine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq 3 )-based organic light-emitting diodes. The device with LiF/Al/Ni cathode achieved a maximum power efficiency of 2.8 lm/W at current density of 1.2 mA/cm 2 , which is 1.4 times the efficiency of device with the state-of- the-art LiF/Al cathode. Importantly, the device stability was significantly enhanced due to the utilization of LiF/Al/Ni cathode. The lifetime at 30% decay in luminance for LiF/Al/Ni cathode was extrapolated to 400 h at an initial luminance of 100 cd/m 2 , which is 10 times better than the LiF/Al cathode. # 2006 Elsevier B.V. All rights reserved. PACS: 85.60.Jb; 85.60.Ha Keywords: Al/Ni cathode; Organic light emitting diode; Stability 1. Introduction In organic light-emitting diodes (OLEDs), cathodes play an important role on the performance of OLEDs [1,2]. A proper selection of the cathodes may not only enhance the electroluminesceent (EL) efficiency due to the improvement of electron injection ability, but also improve the stability of OLEDs. A bilayer cathode consisting of a reactive low work function metal or an insulating buffer layer covered by a high work function metal, such as Ca/Al, Li/Al, LiF/Al, CsCl/Al, Ca(acac) 2 /Al, LiF/Ag, etc., has widely used in OLEDs [3–10]. Furthermore, some metal alloys were also used as the cathode in OLEDs, such as Mg:Ag, Li:Al, Ca:Al, Cu:Al, Ni:Al [11–15]. It can be seen that the application of metal alloy did prolong device lifetime, in some case, also significantly enhanced the efficiency and brightness of OLEDs [13,14]. However, it is worth noting that the utilization of metal alloy cathode increases the fabrication difficulty of OLEDs because the device performance depends strongly on the mixture ratio of the two metals in the alloy, as a result, the ratio of two metals has to be exactly controlled to guarantee the performance of devices. In this paper, we present an alternative way to keep the simple processing, simultaneously improve the EL efficiency and stability. A highly stable Ni was used to cover the state-of- the-art LiF/Al to form a composition of LiF/Al/Ni as the cathode. It was found that the green OLEDs with LiF/Al/Ni cathode showed both a higher EL power efficiency and a longer lifetime than that of LiF/Al. The OLEDs with a LiF/Al/Ni cathode achieved a maximum power efficiency of 2.8 lm/W at current density of 1.2 mA/cm 2 , which is 1.4 times the efficiency of device with the state-of-the-art LiF/Al cathode. The device with LiF/Al/Ni cathode had a 30% decay lifetime of 400 h at an initial luminance of 100 cd/m 2 , which is 10 times longer than the LiF/Al cathode-based device. 2. Experimental Patterned indium–tin–oxide (ITO) (80 V/&) glasses were used as substrates. Prior to film deposition, the substrates were cleaned with detergent (TFD 7 of Franklab S.A.), rinsed in de- ionized water, then dried in an oven, and finally positioned in a thermal evaporation chamber. Devices with a configuration of www.elsevier.com/locate/apsusc Applied Surface Science 253 (2006) 1551–1554 * Corresponding author. Tel.: +86 431 5262357; fax: +86 431 5262873. E-mail address: mdg1014@ciac.jl.cn (D. Ma). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.038