Assembly of PCBM and tn-ZnPc Molecular Domains and Phase Separation of Molecular Mixtures from Liquid Solution on Si(111).

We present the results of investigations aimed at understanding the mechanisms by which small, mutually immiscible organic molecules self-assemble into domains during phase separation from liquid solutions in the presence of a solid substrate. As an example system, we investigated molecular mixtures of tetranitro zinc-phthalocyanine (tn-ZnPc), an electron donor, and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), an electron acceptor, in chloroform solution, deposited onto native oxide-covered Si(111) substrates. We found qualitatively different behavior in the formation of domains of the two types of molecules, seemingly because of a large difference in their solubilities in the solvent. The morphology of PCBM molecule domains varies widely depending on the solvent evaporation rate, the presence/absence of tn-ZnPc molecules in the solution, solvent annealing conditions, and seemingly the presence/absence of trace impurities. By contrast, we find evidence that the shape of the tn-ZnPc domains is insensitive to evaporation; these form while in liquid suspension, and the assembly of domains into clusters varies according to the rate and mode of solvent evaporation.