Fyzikální ústav Akademie věd ČR

Nanostructured and organic semiconductors represent 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 transport nature constitutes the key knowledge for its improvements. We used time-resolved terahertz spectroscopy as a contact-free probe of ultrafast carrier transport. The technique allows us to access 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)].

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.

We investigated carrier injection and subsequent transport in dye-sensitized nanostructured ZnO and TiO₂. The generally accepted picture was that mobile electrons appear in the semiconductor conduction band in concert with their injection. Our results show that charge injection and formation of mobile charges are not necessarily connected, and that charge transport in the sensitized solar cell material can differ from that in bulk or nanocrystalline nonsensitized semiconductors. This is related to strong electrostatic interaction between injected electrons and dye cations at the surface of the semiconductor nanoparticle, see Fig. 2 [H. Němec et al., Phys. Rev. Lett. 104, 197401 (2010)]. (more...)

Fig. 2: Left panel. Scheme of a Grätzel photovoltaic cell. Incident radiation first excites dye molecules. Subsequently, the electron (e⁻) is injected into a semiconductor nanoparticle and it is transported to the anode. The oxidized dye cation (D⁺) is reduced by redox electrolyte. Right panel. In TiO₂, an electron is injected to the semiconductor nanoparticle in less then 1 ps after photoexcitation. After injection, the electron is free to diffuse through the nanoparticle network to the electrode. In contrast, injection into ZnO occurs via an intermediate electron-cation complex in which the electron and cation are strongly bound to each other. This state is formed within 5 ps and it breaks within 100 ps. After that, the electron is released, but it remains weakly attracted by the cation which makes its transport to the electrode much slower.