Multilayer cathode for organic light-emitting devices Xiufang Li a , Zhenbo Deng a, * , Zheng Chen a , Yumeng Shi a , Denghui Xu a,b a Institute of Optoelectronic Technology, Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044, PR China b Department of Mathematics and Physics, Beijing Technology and Business University, Beijing 100037, PR China Received 26 September 2006; received in revised form 19 August 2007; accepted 28 September 2007 Available online 12 October 2007 Abstract A thin layer of samarium (Sm) was introduced in the cathode fabrication of the organic light-emitting devices (OLEDs). An efficient cathode Sm/LiF/Al used to improve the performance of OLEDs was reported. Standard N,N 0 -bis(1-naphthyl)-N,N 0 -diphenyl-1,1 0 -biphe- nyl 4,4 0 -diamine (NPB)/tris-(8-hydroxyquinoline) aluminum (AlQ) devices with Sm/LiF/Al cathode showed dramatically enhanced elec- troluminescent (EL) brightness and efficiency. The optimized device with the 0.5 nm layer of Sm with the cathode structure of Sm/LiF/Al showed the improved power efficiency of 40% than the control device of the conventional LiF/Al cathode at 100 mA/cm 2 . The drive voltage of the device with the Sm/LiF/Al cathode was decreased about 2 V at 500 mA/cm 2 compared with the conventional LiF/Al cath- ode device. The enhanced properties of the device with such a multilayer cathode are considered to the improved balance of electron/hole injection in the emitting layer. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Organic light-emitting devices (OLEDs); Electroluminescence (EL); Exciton 1. Introduction Organic light-emitting diodes (OLEDs) based on organic small molecule and polymers have been extensively studied for potential applications, especially in the field of flat-panel displays [1–7]. In general, OLEDs are fabricated with multilayer structures consisting of active luminescent layers, hole transporting layers (HTLs) and electron trans- porting layers (ETLs) to achieve a balanced charge-carrier injection and enhanced luminescence efficiency [1–3]. Due to the work function differences between the cathode metal and ETL, an effective cathode structure for efficient elec- tron injection is critical to performances of OLEDs. There is a potential energy barrier that limits electron injection at the cathode-organic layer interface. Therefore, metals hav- ing a low work function such as Li, Ca, and Mg are used to reduce the energy barrier height [2]. However, such metals are very reactive and susceptible to oxidation in air without an appropriate passivation. Therefore, more environmen- tally stable electrodes such as Al are needed. However, the high work function of Al results in lower luminous effi- ciency and higher operating voltage in OLEDs with an Al cathode. Recently, a thin insulating layer such as LiF, CsF, MgF 2 , etc. deposited between an organic layer and Al cath- ode has been shown to greatly enhance the electron injec- tion and the EL efficiency [4–7]. Since such a scheme usually involves particular chemical interaction of metal/ EIL/ETL, its effectiveness is sensitive to the metal used and very often limits the choice of cathode metals to Al only. Although a lot of researches have carried out to understand the mechanism of the thin insulating layer for the enhanced electron injection, it is still not clearly under- stood. Recently, Cannon Inc. and other groups reported that cesium carbonate (Cs 2 CO 3 ), either vacuum deposited as an individual layer over the organic ETL or codeposited with ETL, effectively facilitates electron injection from a wide range of metal electrodes [8,9]. While device perfor- mances using such cathode structures are encouraging, mechanisms of this injection scheme are not understood. 0141-9382/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.displa.2007.09.015 * Corresponding author. Tel.: +86 10 51688675; fax: +86 10 51683933. E-mail address: zbdeng@bjtu.edu.cn (Z. Deng). www.elsevier.com/locate/displa Available online at www.sciencedirect.com Displays 29 (2008) 323–326