Dr. Jiri Pfleger
Institute of Macromolecular Chemistry AS CR, Prague, Czech Republic
Proposer: Prof. Ilaria Cacciotti
Downscaling metal fillers to nanometer scale in semiconducting polymer composites brings new phenomena, which can be exploited in advanced optoelectronic applications. Besides the increased electrical conductivity due to the presence of metallic filler such nanocomposites were suggested to work as photochemical catalysts, optical sensors, SERS probes or nonvolatile rewritable memories. Phenomena based on resonance excitations of surface plasmons – collective oscillations of free electrons excited in noble metal (such as Ag and Au) nanoparticles (NPs) by incident light can be used e.g. for localized fluorescence quenching or for better harvesting of absorbed light in photovoltaic energy conversion applications by the enhancement of charge transfer at the donor-acceptor interface located near the NP surface.
Various procedures of plasmonic composites preparation will be shown on the example of poly(3-alkylthiophene), P3AT: (i) simple mixing a polymer solution with an organosol of Au or Ag NPs, (ii) in-situ reduction of metal salts in the polymer solution using a phase transfer agent, (iii) preparation of P3AT derivatives with cationic groups, which are soluble in water miscible solvents, and hence can be mixed with NPs hydrosols, and (iv) laser ablation of Au or Ag target in a proper solvent and subsequent mixing of the sol with a polymer solution. The electrical and optical properties of these composites will be discussed. Also a more defined system consisting of a metal NPs layer prepared by physical vapor deposition onto surface of very thin spin-cast films of poly(3-hexylthiophene) was prepared for a more detailed study of underlying phenomena that accompany the plasmonic effects involved in the optoelectronic behavior of the composites.
Transient optical absorption studies showed, besides the laser pulse induced heating of electrons in conduction band of a metal nanoparticle resulting in spectral broadening of the plasmonic resonance peak, also several excited states relaxation processes in P3AT. Exciton absorption decayed faster, showing increased probability of excitons dissociation due to their interaction with NPs. As seen from the long-lived photobleaching signal the dissociation of excitons resulted in long living species. These observations could be explained by an enhanced photogeneration of long-living polarons with optical absorption in longer wavelength. In similar structures with admixed fullerene forming bulk heterojunction solar cell and Au NPs the power conversion efficiency under white light illumination was improved by about 25%.
Dr. Jiri Pfleger works currently as a head of the Department of Polymers for optoelectronics and photonics in the Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic in Prague. He obtained his Master in biophysics in the Charles University and PhD in physical chemistry in the Czech Academy of Sciences for studies of electrical phenomena in conjugated polymers. His research is now focused on optical and electrical properties of conjugated organic materials and nanocomposites with plasmonic and high-k dielectric nanoparticles for flexible electronics. In 1987-88 he stayed at SUNY at Buffalo studying nonlinear optical properties of polymers, later he worked also in Hahn-Maitner Institute in Berlin, City University in Hong Kong and Technical University in Lodz, Poland. He is a lecturer in the Charles University and in the Technical University in Prague and member of the scientific boards of these universities. He also worked as a project technical advisor for European projects and consultant for Lallemand SAS in France.