Low sublimation temperature cesium pivalate complex as an efficient electron injection material for organic light-emitting diode devices Ronggang Shangguan a , Guangyuan Mu a , Xianfeng Qiao a , Lei Wang a,⇑ , Kok-Wai. Cheah b , Xunjin Zhu b , Chin H. Chen b,⇑ a Wuhan National Laboratory for Optoelectronics, College of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China b Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Hong Kong, PR China article info Article history: Received 16 June 2011 Received in revised form 9 August 2011 Accepted 10 August 2011 Available online 31 August 2011 Keywords: Electron injection material Cesium pivalate Organic light-emitting devices abstract Cesium pivalate ((CH 3 ) 3 CCOOCs) has been synthesized and applied as an electron injection material for organic light-emitting diodes, which showed low sublimation temperature of 180 °C. Typical bilayer structure of ITO/NPB (60 nm)/Alq 3 (50 nm)/EIL/Al was used to eval- uate the electron injection efficacy of (CH 3 ) 3 CCOOCs, the results showed (CH 3 ) 3 CCOOCs/Al exhibits better electron injection than LiF/Al cathode and the power efficiency was improved by about 19% at current density of 50 mA/cm 2 . More interestingly, in the typical three layer OLED structure ITO/2-TNATA (60 nm)/NPB (10 nm)/Alq 3 :2% C545T (40 nm)/ MADN (15 nm)/(CH 3 ) 3 CCOOCs (2 nm)/Al, the maximum current efficiency is up to 20 cd/ A with Commission Internationale d’Eclairage (CIE x , y ) color coordinates of (x = 0.30, y = 0.65) at current density of 140 mA/cm 2 , which indicates that the non-aromatic alkali metal complex can also have good match with the chemically stable compound and exhibit good electron injection properties. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction In organic light-emitting diodes (OLEDs), aluminum metal has been widely used for the cathode material be- cause of its high stability in air and easy processability. However, the work function of Al (4.2 eV) as a cathode is not low enough to inject electrons into the organic layer at low driving voltages [1–2]. As we know, the balance of electrons and holes in emitting layer is of great importance for the device efficiency, the mobility of electrons is much lower than that of holes in organic materials, electron injection and transport ability must be improved to obtain charge balance [3–4]. In 1997, Hung et al. [5] reported a bi- layer structure of LiF/Al, the presence of a thin LiF layer at the organic–Al interface significantly enhances electron injection. After that, many other inorganic insulating mate- rials, such as CsF [6], MgF 2 [7], CsCl [8], alkali metal (Li + , Na + ,K + , Rb + , and Cs + ) acetates [9] and carbonates [10] in combination with Al, have also been reported and shown similar beneficial effects. But the sublimation temperature of most of inorganic salts is high, which is inconvenient for the fabrication of device. In these reported inorganic metal complexes, it has been demonstrated by quartz crystal microbalance (QCM) that they decomposed in the process of sublimation and the alkali metal formation is the most probable mechanism to enhance electron injection in or- ganic EL devices. Meanwhile, the electron transport layer is easily reactive and the liberated cations can dope the adjacent electron transport layer to form complexes lead- ing to improved electron injection from the cathode into the electron transport layer (ETL) [11–12]. Recently, a series of aromatic organic alkali metal complex/Al bilayer cathode were reported for the better thickness tolerance and lower evaporation temperature 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.08.005 ⇑ Corresponding authors. Tel./fax: +86 27 87793032 (L. Wang). E-mail addresses: wanglei@mail.hust.edu.cn (L. Wang), fredchen @mail.nctu.edu.tw (C.H. Chen). Organic Electronics 12 (2011) 1957–1962 Contents lists available at SciVerse ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel