Density-functional theoretical study of fluorination effect on organic/metal interfaces Kenji Toyoda a,⇑ , Ikutaro Hamada b , Susumu Yanagisawa c , Yoshitada Morikawa c a Advanced Technology Research Laboratories, Panasonic Corporation, 3-4 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0237, Japan b WPI-Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan c Department of Precision Science and Technology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan article info Article history: Received 15 September 2010 Received in revised form 10 November 2010 Accepted 14 November 2010 Available online 25 November 2010 Keywords: Density-functional theory Organic/metal interface van der Waals interactions Vacuum level shift Fluorination Carrier injection abstract Density-functional theory with a semi-empirical dispersion correction was used to system- atically examine how the number of fluorine (F) atoms affects atomic and electronic struc- tures of fluorinated pentacene ðC 22 F n H 14n Þ adsorbed on Cu(1 1 1) surfaces. The fluorination effect on the carrier injection efficiency at organic/metal interfaces was investigated. We found that as the number of F atoms decreases, the electron affinity of isolated molecules decreases, suggesting that the molecule becomes less reactive. However, for adsorbed sys- tems, as the number of F atoms decreases, molecular orbitals of C 22 F n H 14n strongly hybrid- ize with the substrate states while retaining the n-type energy level alignment, resulting in lowering the barrier height of the carrier injection. Based on the calculation results, we pro- pose using C 22 F n H 14n ðn 6 8Þ with Cu electrodes for efficient electron injection. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Understanding the electronic structure at organic/metal interfaces is of great importance in developing organic de- vices [1,2]. The interface dipole at organic/metal interfaces induces vacuum level shift that modifies the barrier height of the carrier injection, which plays a decisive role in organic devices [1,2]. Therefore, accurate prediction and control of the interface dipole is crucial to designing the electrodes of organic devices. Pentacene (C 22 H 14 , Pen) and perfluoropentacene (C 22 F 14 , PFP) are prototypical p-type [3] and n-type [4] organic semiconductors, respectively. Accordingly, the interactions of Pen and PFP with metal substrate have been studied extensively both experimentally and theoretically [5–20]. In particular, elucidation of the electronic properties of PFP on metal is crucial, because n-type organic/metal interfaces are not as well understood as p-type organic/ metal interfaces [21,22]. Efficient n-type interfaces must be designed to fabricate complementary integrated circuits. PFP is expected to lower the electron injection barrier height, because larger electron affinity makes the lowest unoccupied molecular orbital (LUMO) level closer to the Fermi energy of the electrode than that of Pen [23]. However, experimental reports have shown that the high- est occupied molecular orbital (HOMO) state relative to the Fermi energy for PFP/Cu(1 1 1) is almost the same as that for Pen/Cu(1 1 1) [11], and moreover, that the PFP– substrate distance is larger than that for Pen/Cu(1 1 1) [16]. These indicate that the electron injection for PFP/ Cu(1 1 1) is less efficient than that for Pen/Cu(1 1 1), because the overlap between the molecular orbitals and the sub- strate for PFP/Cu(1 1 1) becomes smaller than that for Pen/ Cu(1 1 1). Our previous paper pointed out that the large PFP–Cu(1 1 1) distance originates from repulsion of F atoms by the substrate [10]. Therefore, it might be possible to modify the adsorption distance and electron injection barrier, by changing the number of F atoms in PFP. 1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.11.010 ⇑ Corresponding author. E-mail address: toyoda.kenji@jp.panasonic.com (K. Toyoda). Organic Electronics 12 (2011) 295–299 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel