First-principles theoretical study of organic/metal interfaces: Vacuum level shifts and interface dipoles Yoshitada Morikawa a, b, * , Kenji Toyoda c , Ikutaro Hamada d , Susumu Yanagisawa e , Kyuho Lee f a Department of Precision Science and Technology, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan b Research Center for Ultra-Precision Science and Technology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan c Advanced Technology Research Laboratories, Panasonic Corporation, 3-4 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0237, Japan d WPI-Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan e Department of Physics and Earth Sciences, Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213, Japan f Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854-8019, United States article info Article history: Received 22 March 2012 Received in revised form 25 June 2012 Accepted 25 June 2012 Available online 20 July 2012 Keywords: Organic devices Interfaces DFT Level alignment Van der Waals interaction abstract In this article, we discuss recent progress in theoretical studies on the electronic properties of organic/ metal interfaces, especially on the origin of the interface dipoles. We first discuss the effect of the interface dipole on the charge injection barriers at organic/metal interfaces. Then, we observe the importance of the interface structure, especially of the organicemetal distances in physisorption systems. The experimentally observed substrate dependence of the interface dipole can be attributed mainly to the difference in the organicemetal distance. In the case of chemisorption systems, the induced density of interface states tends to pin the Fermi level relative to the HOMO and LUMO levels of molecules. We also point out an important role of van der Waals (vdW) interaction between organic molecules and metal substrates. Density functional theory (DFT) within a generalised gradient approx- imation (GGA) plus a recently proposed semi-empirical vdW correction can describe atomic geometries of organic/metal interfaces reasonably well. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction First-principles simulations based on fundamental laws of physics, such as quantum mechanics, electromagnetism, and statistical physics, have been significantly developed and play important roles in clarifying various phenomena in the basic materials science. They are also gaining more and more importance in various application fields like semiconductor devices, catalysis, and so on as tools to predict properties of new materials and to design valuable materials on the basis of computer simulations. In this article, we discuss recent developments in first-principles theoretical studies on organic/metal interfaces. 1.1. Interface level alignment and interface dipole Organic devices such as organic light-emitting diodes (OLEDs) [1,2], organic field-effect transistors (OFETs) [3,4], and organic photovoltaic cells [5,6] are attracting enormous amounts of atten- tion because of their promising properties such as low-cost pro- cessing and flexibility [7]. The energy level alignment at the organic/metal interfaces in organic devices governs the efficiency of the carrier injection, which ultimately determines the perfor- mance of the organic devices [8,9]. The barrier heights of electron ðF n B Þ and hole ðF p B Þ injection is related to the substrate metal work function (F m ) by, F n B ¼ F m  A þ D; (1) F p B ¼F m þ I  D; (2) where A and I are the electron affinity and the ionisation energy of the organic molecule, respectively, and D is the vacuum level shift (VLS) (see Fig. 1). The VLS is induced by a dipole layer formed at an organic/metal interface, which changes the barrier height of the carrier injection as large as 1 eV. It is therefore essential to clarify the factors that determine the formation of the interface dipole layer in order to control the alignment of the Fermi level of a metal electrode with the molecular levels. * Corresponding author. Department of Precision Science and Technology, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan. Tel.: þ81 6 6879 7288; fax: þ81 6 6879 7290. E-mail address: morikawa@prec.eng.osaka-u.ac.jp (Y. Morikawa). Contents lists available at SciVerse ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2012.06.021 Current Applied Physics 12 (2012) S2eS9