Paper SS-TuP23
Crystallization and Phase Separation Kinetics of Organic Molecules from Solution on Si(111) Substrates

Tuesday, November 11, 2014, 6:30 pm, Room Hall D

Understanding the underlying science behind the arrangements of small organic molecules in mixtures is important for numerous technological applications, among which organic solar cells (OSCs) are especially noteworthy, given the drive toward alternative energy sources. An important process on which organic solar cells depend is the phase separation of organic small molecules. The spontaneous formation of a particular morphology during phase separation from a solvent-based, bimolecular solution onto a substrate depends on several parameters, including relative molecular concentrations, solubilities of each type of molecule in the solvent, mutual interaction between molecules of the same species, interaction of individual molecules with the substrate surface, solvent evaporation rate, and annealing conditions. In this work, we carry out a study of molecular mixtures consisting of tetranitro zinc-phthalocyanine (tn-ZnPc) and [6,6]-phenyl-C₆₁-butyric acid methyl ester ( PCBM) using chloroform as a solvent, and native oxide-covered Si(111) substrates. We investigate the role that the solvent evaporation rate during deposition, followed by solvent vapor annealing (SVA), plays on the formation of phase separated mixtures and their crystallization and phase transformation. We also investigated the relative concentration of individual molecules in the mixtures. We found that the PCBM molecules alone undergo several phase transformations as the evaporation rate of the solvent decreases and upon SVA, while the tn-ZnPc molecules alone are very stable. Moreover, the concentration of the phthalocyanine molecules in the mixture with the PCBM highly affects the crystallization process of the latter.