Research
Unlike the name of the laboratory might suggest, our research is focused on "small molecules", namely, liquid crystalline semiconductors for organic electronics application. Various organic semiconductors have been receiving a great deal of attention in "plastic" electronic devices such as organic photovoltaic cells, light-emitting diodes (OLED) and field effect transistors (OFET). The important physical parameters of microcrystalline films are strongly affected by dimensions of domains and domain boundaries, while large defect-free single crystals are difficult to fabricate and inappropriate for practical applications. At the same time, liquid crystals have been recognized as a new type of organic semiconductors, as they are capable to self-healing of structural defects and to self-organization in large structurally homogeneous domains. Influence of domain boundaries, if any, on carrier transport in liquid crystalline phases is very small.
Discotic liquid crystals
Synthesis and studies of discotic liquid crystals is our major area of interests. Disk-like molecules, typically comprising a flat rigid aromatic core and flexible peripheral substituents, self-organizes into one-dimensional "supramolecular wires". Due to high charge carrier mobility along the columns, discotic mesogens are considered as realistic candidates for application in organic electronic devices. In the recent years, we reported liquid crystalline hexaazatriphenylenes (HAT) and hexaazatrinaphthylenes (HATNA) for the charge transport and highly fluorescent columnar phases of pyrene derivatives. Currently we are focusing our efforts on metal-free derivatives of phthalocyanines. Variation of peripheral substituents allows for the tuning of the electron affinity, phase behavior and thermotropic properties of these molecules.
For application in devices, adequate alignment of molecular stacks between electrodes is a must. In particular, homeotropic or face-on alignment with columns perpendicular to electrodes is required for OLED and solar cells fabrication. However, spontaneous homeotropic alignment of discotic molecules is a relatively rare and well understood phenomenon. We obtained homeotropically aligned films of some phthalocyanines by slow cooling of isotropic melt confined between two substrates. Interestingly, alignment is independent on the nature or roughness of substrates, but confinement between TWO substrates is crucial!
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For the fabrication of non-confined homeotropically aligned thin films, we have developed sacrificial layer method. After the thin film of discotic material is deposited on the substrate, a layer of polyvinylphenol (PVP) is deposited on top of it, then homeotropic alignment is induced by heating-cooling cycle, and finally sacrificial layer is removed by washing with methanol.
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Calamitic liquid crystals
Another area of our research is synthesis and studies of rod-like or calamitic mesogens, in particular, oligothiophenes. Dialkyloligothiophenes are known to possess excellent charge transporting properties in smectic liquid crystalline phases. Our efforts aim to the design of the thermotropic behavior of liquid crystalline oligothiophenes for organic electronic applications as well as for the better understanding of the relationship between their molecular structure and phase order. The major principle in the design of new mesogens consists in a compromise between the order caused by the pi-pi stacking of flat aromatic cores and the disorder induced by flexible peripheral substituents.
Further, we designed non-symmetric oligothiophenes comprising three "incompatible" parts: a rigid aromatic core, a perfluoroalkyl chain and an alkyl chain. We expect these molecules to form thin films with different morphologies depending on deposition conditions and pack into highly ordered smectic mesophases promoting fast ambipolar charge transport.
