Engineering the electronic structure of the ZnPc/C 60 heterojunction by temperature treatment Corinna Hein n , Eric Mankel, Thomas Mayer, Wolfram Jaegermann Darmstadt University of Technology, Materials Science Institute, Surface Science Division, Petersenstraße 23, 64287 Darmstadt, Germany article info Article history: Received 15 June 2009 Received in revised form 28 October 2009 Accepted 30 October 2009 Available online 26 November 2009 Keywords: Organic semiconductors photoemission spectroscopy ZnPc C 60 Interface band bending Electronic structure abstract Using synchrotron induced photoelectron spectroscopy the temperature-dependent electronic proper- ties of ZnPc and C 60 and of the ZnPc/C 60 interface have been investigated as a possible donor/acceptor system of an organic heterojunction solar cell. ZnPc and C 60 were deposited in-situ by PVD onto transparent conductive ITO substrates at increasing substrate temperature. Compared to room temperature, the Fermi level position E F in ZnPc is shifted towards the LUMO level up to 0.5 eV at 155 1C and in C 60 up to 0.4 eV at 205 1C. Different temperature conditions for deposition of C 60 onto ZnPc are used to vary the band alignment and the potential distribution at the interface. Deposition of C 60 at RT onto ZnPc that was deposited at RT induces band bending of 0.3 eV in C 60 and 0.2 eV in ZnPc hindering charge transport away from the interface. Depositing the bilayer at 155 1C the band bending could be reduced to 0.17 eV in ZnPc and 0.14 eV in C 60 due to formation of an interface dipole. In addition, this dipole increases the donor HOMO–acceptor LUMO interface gap, possibly enlarging the open circuit voltage. & 2009 Elsevier B.V. All rights reserved. 1. Introduction Developing organic semiconductor devices for energy conver- sion is a promising approach to cheap photovoltaics. The donor/ acceptor pair ZnPc/C 60 is used in organic photovoltaic cells both in bilayer heterojunction and bulk heterojunction devices [1–3]. High absorption coefficients are provided at reasonable hole and electron conductivities. The structures of the molecules are shown in Fig. 1. While ZnPc is a two-dimensional planar molecule [4], C 60 is spherical. At moderate temperatures ZnPc crystallizes in a metastable a-structure and at elevated temperatures in the stable b-structure. The alignment of HOMO and LUMO levels and the electronic potential distribution at the interface are crucial parameters for the efficiency of organic heterojunction solar cells. Exciton dissociation and charge separation may be optimized by en- gineering these parameters. In this contribution temperature treatment under ultra high vacuum conditions is used to modify the electronic structure of the single materials and the electronic structure of the donor/acceptor interface. Band diagrams of ZnPc/ C 60 interfaces are derived from synchrotron induced photoemis- sion spectroscopy at in situ formed hetero-junctions. In thermo- dynamic equilibrium the Fermi level has to be constant throughout the contacting phases. When the contact is formed the work function difference of the separate phases is overcome by induced band bending and formation of an interface dipole. Band bending is directly monitored in shifts of photoemission lines. The development of the work function is monitored by the secondary electron onset. Formation of interface dipoles is indicated as differences between induced band bending and work function changes. In this contribution the variation of the work function and ionization potential of the single materials ZnPc and C 60 with temperature treatments and the effect on the induced band bending and interface dipole at the ZnPc/C60 heterojunction are analysed. 2. Experimental Thin organic layers were produced by physical vapour deposition at a deposition rate of approximately 10 ˚ A/min in an integrated ultra high vacuum chamber with a base pressure of 10 À9 mbar. The deposition rate was deduced from damping of XPS emissions of substrate orbitals with deposition time. Commercial 120-nm-thick ITO films on glass were used as substrate after cleaning by ultrasonic treatment in isopropanol. ZnPc and C 60 cleaned by gradient sublimation were provided by the project partner from BASF SE. To investigate the temperature- dependent variation of work function and Fermi level position, ZnPc and C 60 were deposited for increasing substrate tempera- ture. ZnPc layers were deposited at room temperature and annealed for 20 min at 124 and 155 1C. C 60 layers were deposited ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.10.022 n Corresponding author: Tel.: + 49 6151 1669665, Fax.: + 49 6151 166308. E-mail address: chein@surface.tu-darmstadt.de (C. Hein). Solar Energy Materials & Solar Cells 94 (2010) 662–667