Fyzikální ústav Akademie věd ČR

Dye-sensitized nanostructured semiconductors represent a new generation of prospective materials for solar cell fabrication. Their operation relies on a cascade of complex physical processes. The efficiency of solar cells crucially depends on the speed of long-range charge carrier transport and the character of this transport constitutes the key knowledge for its improvements. Here we are interested in processes occurring on the sub-nanosecond time scale which include namely the electron injection into the semiconductor and the initial phase of the electron transport towards the anode (see Fig. 1). We use time-resolved terahertz spectroscopy as a contact-free probe of ultrafast carrier transport complemented with numerical simulations. We investigated carrier injection and subsequent transport in dye-sensitized nanostructured ZnO and TiO₂. The generally accepted picture of the photoconductivity of these systems was that mobile electrons appear in the semiconductor conduction band in concert with their injection from the dye. 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. For ZnO an electron-cation complex is formed within 5 ps which causes fast charge recombination. Moreover, the electron mobility is significantly decreased even after the dissociation of the complex (100 ps) due to strong electrostatic interaction between injected electrons and dye cations. In contrast, sensitized TiO2 nanocrystals does not suffer from this problem due to their high permittivity efficiently screening the charges. We believe that the described processes are responsible for the different power conversion efficiencies of TiO2 and ZnO-based Grätzel cells. [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.