Engineering the electronic structure of the ZnPc/C60 heterojunction by temperature treatment

Using synchrotron induced photoelectron spectroscopy the temperature-dependent electronic properties 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 degrees C and in C-60 up to 0.4 eV at 205 degrees C. 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 degrees C the band bending could be reduced to 0.17 eV in ZnPc and 0.14 eV in Cop 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. (C) 2009 Elsevier B.V. All rights reserved.