Ordered Donor–Acceptor Complex Formation and Electron Transfer in Co-deposited Films of Structurally Dissimilar Molecules

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.