Fyzikální ústav Akademie věd ČR

Nanostructured and organic semiconductors form the basis of a new generation of prospective materials for solar cell fabrication. The efficiency of solar cells crucially depends on the speed of long-range charge carrier transport and the understanding of the transport mechanisms in the mentioned materials is a key knowleged for its improvements. We used time-resolved terahertz spectroscopy as a contact-free probe of ultrafast carrier transport. The technique permits to access easily the complex conductivity in the terahertz spectral range with subpicosecond time resolution. The experiments were carried out in a concert with numerical simulations of the conductivity spectra and this allowed us to elucidate the transport mechanisms in a number of complex materials. We described the connection of the terahertz spectra with the inter- and intra-nanoparticle transport processes in thin films made of ZnO and TiO₂ nanoparticles [H. Němec et al., Phys. Rev. B 79, 115309 (2009)]. Subsequently, these findings allowed us to determine the role of small and large grains in the carrier transport in microcrystalline silicon [L. Fekete et al., Phys. Rev. B 79, 115306 (2009)]. We also studied a blend of polymer and electron acceptor with the conclusion that the motion of holes along polymer chains is significantly reduced by potential barriers which may be connected to the torsional disorder of the chains [H. Němec et al., Phys. Rev. B 79, 245326 (2009)]. (více...)

Fig. 1: Scheme of the principle of the charge carrier transport in a blend of polymer LBPP1 and a fulleren acceptor. We show at the top the model of potential barriers used for the calculations of the charge transport between individual segments of the polymer; at the bottom we plot a dramatic decrease of the conductivity due to the charge localization between potential barriers observed at sub-picosecond time scale.