Electronegativity and charge-injection barrier at organic/metal interfaces J.X. Tang a , C.S. Lee a, * , S.T. Lee a , Y.B. Xu b a Department of Physics and Materials Science, Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong SAR, China b Department of Physics, Zhejiang University, Hangzhou 310027, China Received 11 February 2004; in final form 4 August 2004 Abstract Despite a large scatter in experimental data, charge-injection barrier at organic/metal interfaces has been traditionally described as a function of metal work function. By studying the interface between Alq 3 and various metals with photoelectron spectroscopy, we show that the charge-injection barrier can be better described as a linear function of the metal electronegativity. This is consistent with our theoretical analysis in terms of a simple model involving metal-induced gap states in organic layer. The interface parameter S is calculated to be 0.8, in good accordance with the experimental value of 0.81. The present work presents a method for predicting the injection barrier at organic/metal interfaces and should be useful for designing various organic-based electronic and optoelec- tronic devices. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Recently there has been increasing interest in the field of electronically functional organic materials with vari- ous applications, such as organic light-emitting diode (OLED) [1,2] organic thin film transistor (OTFT), and organic photovoltaic devices. In particular, the organ- ic/metal interfaces have attracted considerable attention due to their essential role in the charge-injection process, which significantly influences the device performance [2]. The electron and hole injection barriers (U e and U h ) de- pend, respectively, on the energy differences of the low- est unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of the or- ganic film with respect to the metal Fermi level (E F ). Where the charge-injection barriers are considered by device designers, they are often estimated by making an assumption that the metal and the organic have a com- mon vacuum level at the interface. This assumption is often referred as the Schottky–Mott limit. According to this limit, U e or U h would simply be the differences be- tween metal work function (U m ) and, respectively, the electron affinity (EA) or ionization potential (IP) of the organic film. While this model has clearly turned out to be invalid [3–8]. and can lead to significant errors [6], it has been extensively used. This wide popularity is partly due to its simplicity and partly due to fact that more accurate method for estimating the charge-injec- tion barrier at organic/metal interfaces is not yet available. This need for a better model has triggered extensive works on the studies of organic/metal interfaces which were summarized in several excellent reviews [9], which provided important insights to the problem. Ishii et al. [4] pointed out the invalidity of the common vacuum le- vel assumption is due to an interfacial dipole (D) of var- ious origins (i.e., charge transfer, chemical bonding, surface rearrangement, interface states, image effect, 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.08.021 * Corresponding author. Fax: +852 2788 7830. E-mail address: apcslee@cityu.edu.hk (C.S. Lee). www.elsevier.com/locate/cplett Chemical Physics Letters 396 (2004) 92–96