Ordered Donor–Acceptor Complex Formation and Electron Transfer in Co-deposited Films of Structurally Dissimilar Molecules
Andreas Opitz, Clea Peter, Berthold Wegner, H. S. S. Ramakrishna Matte, Adriana Röttger, Timo Florian, Xiaomin Xu, Paul Beyer, Lutz Grubert, Stefan Hecht, Valentina Belova, Alexander Hinderhofer, Frank Schreiber, Christian Kasper, Jens Pflaum, Yadong Zhang, Stephen Barlow, Seth R. Marder, Norbert Koch
Abstract
The electrical and optoelectronic properties of organic semiconductor thin films can be tailored by mixing two molecular materials, e.g., by co-deposition. Possible resulting morphologies include phase separation or mixed crystals, which can form either solid solutions or ordered complexes, but it is difficult to predict a priori the morphology that will result for a given material combination. Here, we study electron transfer between planar electron-donor molecules and a nonplanar electron acceptor in co-deposited films by analyzing morphological, vibrational, and optical properties. For the donor under study here that does not undergo ground-state electron transfer to the acceptor, we find phase separation in mixed film. If ground-state electron transfer is present, the balance between crystal binding energy of the single-component materials and the Coulomb attraction between ions formed in the co-deposited film drives the co-deposited films either into phase separation or mixed-crystal formation. To rationalize the resulting morphology of these co-deposited films within the laws of thermodynamics, it is necessary to consider structural incompatibility of the molecules as well as the Coulomb attraction between molecular ions when formed via ground-state electron transfer.