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 find qualitatively different behavior for formation of domains of the two types of molecules, seemingly due to a large difference in their solubilities in the solvent. The morphology of PCBM molecule domains varies widely depending on 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 tn-ZnPc domains is insensitive to evaporati...