Study of the feasibility of controlling the length scale of the phase separation of organic molecular mixtures on Si(111) substrates

We present investigations on the arrangements of phase separating small organic molecules in mixtures subsequent to deposition from liquid solution. This phenomenon may have technological applications in efficient organic solar cells. The spontaneous formation of a morphology during phase separation from a solvent-based, bimolecular solution onto a substrate depends on several parameters, including relative molecular concentrations, individual molecules/solvent/substrate interactions, solvent evaporation rate, and annealing conditions. Our molecular mixtures consist of tetranitro zinc-phthalocyanine (tn-ZnPc) and [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) in chloroform, deposited on native oxide-covered Si(111) substrates. We investigate the role that solvent evaporation rate during deposition, solvent vapor annealing (SVA) and thermal annealing play on the morphology formation of molecular domains. We find that slow enough rates of solvent evaporation, along with SVA action, lead to ordered PCBM structures. Most of tn-ZnPc molecules aggregate in rod-shaped clusters of size ~ 100 nm while in liquid solution, even at diluted concentrations, but some remain solvated, and form a wetting layer on the Si(111) surfaces, affecting the morphology of PCBM deposited through mixture solutions. We find that PCBM aggregates vertically diffuse upon heating in spin-coated samples, while tn-ZnPc clusters and wetting layer are not affected. Investigations of different relative concentrations of individual molecules in mixtures show that the concentration of tn-ZnPc strongly affects the crystallization process of PCBM: for a 1:1 ratio PCBM crystallization is suppressed, while changing the amount of tn-ZnPc, PCBM crystals form. While PCBM domains display several different morphologies, tn-ZnPc cluster morphology is not affected by changes in the deposition conditions or by post-deposition treatments. Overall we find that, under the conditions studied here, PCBM molecules undergo heterogeneous nucleation at the substrate, while tn-ZnPc molecules predominantly precipitate. This combination of donor/acceptor molecules deposited through the methods described here, show a slight tendency toward vertical growth of molecular domains for deposition onto a periodic array of cavities patterned onto a substrate. No significant effects are observed in presence of H-terminated surfaces. Finally, our studies on evaporation of resting droplets shed light on individual molecules/solvent interactions, leading to a better understanding of the variation in domain coverages across the substrate.