Synthetic Metals 158 (2008) 539–543 Contents lists available at ScienceDirect Synthetic Metals journal homepage: www.elsevier.com/locate/synmet Energy level alignment in N,N ′ -bis(1-naphthyl)-N,N ′ -diphenyl-1,1 ′ -biphenyl- 4,4 ′ -diamine (NPB)/hexadecafluoro copper phthalocyanine (F 16 CuPc)/Au and NPB/CuPc/Au heterojunction Sang Wan Cho a , Yeonjin Yi b , Myungkeun Noh c,∗∗ , Mann-Ho Cho a , Kyung-Hwa Yoo a , Kwangho Jeong a , Chung-Nam Whang a,∗ a Institute of Physics and Applied Physics, Yonsei University, 134 Shinchon-dong Seodaemoon-gu, Seoul, 120-749, Republic of Korea b Division of Advanced Technology, Korea Research Institute of Standards and Science, Deajon, 305-340, Republic of Korea c LOT Vacuum Co., Ltd., Shingeonji-dong, Anseong-si, Gyeonggi-do, 456-370, Republic of Korea article info Article history: Received 12 February 2008 Received in revised form 17 March 2008 Accepted 24 March 2008 Available online 19 May 2008 PACS: 72.80.Le 73.20.-r 73.20.At 85.30.Tv Keywords: NPB CuPc F16CuPc Photoemission Interface dipole Band diagram abstract The interfacial electronic structures of N,N ′ -bis(1-naphthyl)-N,N ′ -diphenyl-1,1 ′ -biphenyl-4,4 ′ -diamine (NPB)/hexadecafluoro copper phthalocyanine (F 16 CuPc)/Au and NPB/copper phthalocyanine (CuPc)/Au were investigated by in situ X-ray and ultraviolet photoelectron spectroscopy to study the hole-injection barrier depending on the ionization energy of hole-injection layer materials. Although the measured ion- ization energy of F 16 CuPc (6.30eV) was much higher than that of CuPc (5.15eV), the difference in the barrier heights of the two different films was relatively marginal (0.27eV) due to the formation of inter- face dipole caused by the charge redistribution. We confirmed that the interface dipole of the buffer layer (CuPc and F 16 CuPc) pushed down the core levels as well as the valence levels of the top organic layer (NPB) in the NPB/F 16 CuPc/Au and NPB/CuPc/Au heterojunction. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Over the past two decades, metal/organic semiconductor heterojunctions have attracted increasing attention due to their potential applications in electronic and optoelectronic devices including organic photovoltaic cells [1] and organic light-emitting diodes (OLEDs) [2,3]. Much of the effort in organic electronics has been focused on understanding and modifying the electronic properties of the metal/organic interfaces [4,5]. At such interfaces, the magnitude of the hole-injection barrier depends on the energy level alignment between the Fermi level of the electrode and the highest occupied molecular orbital (HOMO) of the organic layer. ∗ Corresponding author. Tel.: +82 2 2123 2613; fax: +82 2 392 1592. ∗∗ Corresponding author. Tel.: +82 2 2123 3870; fax: +82 2 392 1592. E-mail addresses: nohmk@yonsei.ac.kr (M. Noh), cnwhang@yonsei.ac.kr (C.-N. Whang). According to the traditional Schottky–Mott model, the benefit of a small hole-injection barrier is expected when the ionization energy of the organic material is similar to the work function of anode metal in OLEDs. However, charge transfer across the interface results in the formation of an interface dipole when the Fermi level of the metal layer approaches HOMO level [6,7]. As a result, the benefit of a good energy level match between the work function of electrode and the ionization energy of a hole-injection layer (HIL) may disappear [6]. OLEDs using a high ionization energy material as an HIL instead of a low ionization energy material on the contrary have been reported recently [8]. Nevertheless, it was not clearly known how an interface dipole affects the energy level arrange- ment between the ionization energy of organic materials and the work function of metals. In this letter, we report the detailed inter- facial electronic structures as well as the energy level alignments for the devices with a high ionization energy HIL-hexadecafluoro copper phthalocyanine (F 16 CuPc) and compare the results with a low ionization energy HIL-copper phthalocyanine (CuPc). We 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.03.024