Optoelectronic Phenomena and Materials
Research activities are focused on studies of photo- and electro-active polymers and their low-molecular-weight analogues. Physical phenomena, such as electrical conductivity, photoconductivity, photochromism, photorefractivity, electroluminescence, as well as the injection, generation, recombination, and transport of charge carriers are investigated with respect to their dependence on the chemical structure of the materials. Both p - and s -conjugated polymers are studied. Understanding the processes associated with photon absorption and emission, like photoexcitation, excited state dynamics, photoinduced electron transfer, changes in molecular conformations, dissociation and recombination of ion-pairs enabled us to determine the basic principles of designing organic solar cells. The transport of free charge carriers and polarons are studied on polymer photoconductors and structures of organic FET transistors.
The electronic structure and conformation of molecules were studied also theoretically by ab initio and semi-empirical quantum chemical methods. Attention was paid to the formation of quasiparticle conformational defects (polarons, bipolarons and excitons) upon photoexcitation and charge injection. Photostabilization studies were focused on various polymer systems with photoactive impurities like the transformation products of stabilizers or sensitizing dopands and/or photoactive built-in functional moieties. Details of photochemical and photophysical activity mechanisms for various classes of light stabilizers were elucidated to minimize the depleting effects of the photoactive polymeric matrix. Experimental work was supported by theoretical studies and quantum chemical calculations of exciton-phonon coupling and vibrationally-induced energy dissipation.
Recently, new methods for the synthesis of p-conjugated polymers, such as the Stile coupling, were tested with positive results. Further, new types of polysilanes were synthesized and their electronic properties, mainly the influence of polar side-groups on the charge carrier transport, were studied. Light excitation induces several photophysical and photochemical changes on the surface of polymer films, e.g., Si-Si bond cleavage, formation of siloxane structures, formation of ion-pairs with long lifetime and of new local electronic states, which were studied by means of the orientation of liquid-crystalline structures deposited on the polymer surface, thermostimulated luminescence and photoluminescence.
Degradation fragments influence the visible photoluminescence. This effect was studied in detail on model poly[methyl(phenyl)silanediyl] (PMPSi), especially in connection with the metastability and reversibility of bond and electronic structure. Photoluminescence in visible region was found to be influenced also by the dimensionality of the Si skeletons. Two- and three-dimensional structures increased the photoluminescence in the spectral region of 480-600 nm, as was shown on materials prepared by plasma polymerization. In this context the possibility of the long-term stabilization of polymers sensitive to photodegradation (polymers with increased contents of chromophores) with light stabilizers from the group of phenolic UV absorbers was studied. In the light, excitation of polysilanes, photoinduced electron transfer from the main Si chain to p-conjugated side-groups and the formation of positive polarons was detected. The conformation and electronic structure of these polarons were studied by quantum chemical calculations. Experimentally, the photoinduced charge transfer was modelled using a double layer-structured phthalocyanine - tetracyano-1,4-quinodimethane. Phthalocyanine was further used for the construction of FET transistors; a possible use for the space-charge-limited-current method for the study of charge injection, which is important for the construction of electroluminescence diodes, was examined on phthalocyanine thin films. During the injection of a hole to the Pc molecule, a positive polaron was formed. It was found that on NiPc a singlet polaron was formed, whereas on CoPc a triplet polaron arose. The changes of IR spectra associated with the oxidation were found to be independent of the polaron multiplicity.