Effect of Admixture on Domain Morphology Transitions in Phase Separation of PCBM:tn-ZnPc from CCl3 Solution

We present results of investigations aimed at understanding how small mutually immiscible organic molecules self-assemble into domains during phase separation from liquid solutions. As a prototypical system we investigated molecular mixtures consisting of tetranitro zinc-phthalocyanine (tn-ZnPc), an electron donor, and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), an electron acceptor, in chloroform and deposited on native oxide-covered Si(111) substrates. Previously we showed that for a 1:1 mixture PCBM:tn-ZnPc which has much higher solubility, precipitates onto the silicon substrate with domain morphologies which vary widely over the range of supersaturation rates investigated. For the highest rates the domains are isotropic, and apparently amorphous – with a mesoscale near-periodicity. Slowing the supersaturation rate causes an abrupt change in the domain size and shape, to micron-scale faceted domains; we speculated that either (1) a transition from spontaneous decomposition kinetics to nucleation and growth of crystalline domains or (2) a “cascade” series of transitions through increasingly stable structures was responsible for this. Here we present results in which the admixture of PCBM:tn-ZnPc is varied systematically to distinguish between these two mechanisms.