Polymery pro elektroniku a fotoniku
The department consists of two groups, jointly working on the development of new polymer and composite materials for electronic and optoelectronic active components. It is focused especially on organic photovoltaic cells, light-emitting diodes, electrical and optical sensors and organic field-effect transistors (OFETs) for flexible printed electronics. The aim is to prepare these electronic components from soluble derivatives of dielectric, semiconductive or conductive polymers using inexpensive casting or printing techniques.
The basic research is focused on the phenomena of generation, transport, and recombination of free charge carriers, charge and energy transfer at the interfaces of organic materials or organic-inorganic heterojunctions, injection of charges from electrodes, photochromic transformation and on plasmonic phenomena in polymer composites with metal nanoparticles. Quantum chemical calculations and computer modeling are used to correlate the observed macroscopic properties with chemical structure and morphology of molecules. The transient optical absorption spectroscopy with time resolution of 30 fs is used to study the photophysical phenomena in early stage after photoexcitation. Such measurements are important for understanding and optimization of the solar energy conversion in organic photovoltaic solar cells. The laboratory is engaged in the development of the concept of molecular electronics, both on an experimental and theoretical level.
Significant advances were achieved in the development of the methods of preparation and understanding the properties of nanocomposites of plasmonic metal (silver and gold) nanoparticles embedded in matrices of conjugated polymers. A new research field has been opened on the nanocomposites with the p-conjugated polyelectrolyte (CPE) matrices. The main advantage of these systems is in the possibility of their solution processing from “green” solvents such as alcohols and water. It has been shown that the aggregation of nanoparticles in the environment of CPEs, as in highly regioregular cationic polythiophene polyelectrolyte can be controlled to obtain the nanocomposites containing "hot spots" with extreme local amplification of the optical field. The hot spots are formed within the fractal aggregates of nanoparticles and their presence was evidenced through their typical optical extinction and the strong Raman scattering enhancement. The results bring the possibility of preparing plasmonic nanocomposites of semiconducting polymers, in which various photophysical phenomena like the charge transfer and fluorescence can be enhanced or locally quenched. Plasmonic nanoparticles were successfully applied to enhance the conversion efficiency in bulk heterojunction solar cells (Fig.1).
Figure 1: The photon-to-electron conversion efficiency in bulk heterojunction organic photovoltaic cell can be enhanced by gold or silver nanoparticles.
The team recently performed femtosecond optical absorption studies of ω-bis(terpyridyl)oligothiophenes during their spontaneous assembling into conjugated constitutional dynamic polymers (dynamers), which might be utilized in optoelectronics. In another time resolved study the deactivation pathway of photoexcited state of a novel soluble nickel (II) phthalocyanine peripherally substituted with octabutoxy groups was observed. It was found that the photoexcitation evolves in two independent branches (Fig.2). In one of them, assigned to J-aggregates, the photoexcitation decayed to a triplet state with a very long lifetime. The finding is important for the applications that rely on singlet oxygen formation or fast non-radiative deactivation of the excited state, as for photoprotection or photodynamic therapy.
Figure 2: Relaxation pathway of the excitation energy in 2,3,9,10,16,17,23,24-octabutoxy nickel (II) phthalocyanine in solution.
The strong sensitivity of the electronic properties of organic materials to various analytes in the environment has been exploited to design printable organic sensors. A new electrochemical sensing cell for NO2 detection has been developed that combines the advantages of a solid polymer electrolyte (ionic-liquid plasticized PVDF) and a modified carbon indicator electrode. The system is currently used to evaluate organic catalysts to improve the accuracy and selectivity of gas-sensing electrodes. This “artificial nose” may be incorporated into multiple sensor arrays to monitor air quality in alarm systems for the detection of toxic gases. Good results were achieved with new soluble derivatives of metallophthalocyanines, particularly Zn-phthalocyanine (ZnPc) with sulfonamide side groups. ZnPc is suitable for thin-film preparation by printing, and its electrical conductivity and optical absorption are sensitive to various gases. ZnPc forms a charge transfer complex with NO2, which changes the absorption spectrum and increases electrical conductivity by two orders of magnitude (Fig. 3).
This result provides a foundation for the development of detectors for NO2, one of the most common environmentally polluting gases. ZnPc fulfils the demands for printed flexible electronics, which require cheap and reliable gas sensors composed of soluble organic sensing materials.
Figure 3: Electrochemical sensor printed on paper with responses to exposure of nitrogen dioxide
New model of the photogeneration of free charge carriers in polymers was developed with the assumption on the reversible resonant coupling of the localized excitation to the charge transfer states and, consequently, the dependence of internal parameters (permittivity, effective mass of the hole, rate constants) on electric field. The primary model was improved by the quantum chemical approach that incorporated the electron-hole potential and the charge delocalization effects of the charge transfer state, taking into account the discrete character of polymer transport sites, energetic disorder and reorganization effects in the polymer chain. This model of photogeneration allows a combination with our charge carrier mobility model for polymers.8 The calculated charge carrier concentration dependence of the mobility provides a unified description of the mobility for the low charge carrier concentration regime (as in photovoltaics) and accumulated concentration regime (OFET regime).
Figure. 4: Concentration dependence of the hole mobility in the crystalline and amorphous phases of poly(3-hexylthiophene-2,5-diyl) (P3HT) with a typical polycrystalline nanofibre structure calculated using the combined quantum-mechanical/semiclassical model developed in the department.
Wroclaw University of Technology, Poland (Prof. W. Bartkowiak, Prof. J. Sworakowski, Prof. A. Miniewicz)
Institute for Physical Chemistry, Johannes Kepler University, Linz, Austria (Prof. N. S. Sariciftci)
Institute of Microelectronics, NCSR “Demokritos”., Athens, Greece (Prof. N. Glezos)
Faculty of Chemistry, Lodz University of Technology, Poland (Prof. J. Ulanski)
Faculty of Chemistry, Warsaw University of Technology, Poland (Prof. M. Zagorska, Prof. A. Proň)
Faculty of Science, Charles University, Prague (Prof. J. Vohlídal, Prof. B. Vlčková)
Centre for Organic Chemistry Ltd., Pardubice, Czech Republic (Ing. Kubáč)
Faculty of Chemistry, Brno University of Technology (Prof. M. Weiter)
“Flexible printed electronics based on organic and hybrid materials FLEXPRINT”, Centrum of competence, TAČR, 2012 - 2019
“Polyelectrolytic conjugated dynamers: preparation, constitutional dynamics and functional properties of new materials”, GAČR, 2012 – 2015.
“ECNP Growth”, EU Coordinated action, 2012-2015.
“Inovative molecular-electronic elements with charge trapping in the metal nanoparticles assemblies”, MSMT, 2012-2014.
"Engineering of surface-modified optical processes in molecules and semiconductor quantum dots by plasmon resonances in metal nanoparticle assemblies", project GACR, 2010 - 2014.
EU Network of Excellence "FlexNet", 2010 – 2012.
“Electronic transport in III-N-V compounds nanostructures”, MŠMT, COST OC 1007, 2010-2012.
„Mechanism of energy a charge transfer in semiconductors“, MŠMT, KONTAKT LH II 121 86, 2012-2014.
„Electronic and optical properties of hybrid nanostructures“, MŠMT, LD14011 (HINT COST Action MP1202), 2014-2016.
“Photogeneration and charge transport in molecular semiconductors for organic photovoltaics”, GAČR, 2010–2013.
“Study of electronic processes in molecular systems for organic photonics”, GAČR, 2015–2017.
“Polymeric composite materials for optoelectronic applications”, AV ČR, 2012–2015.