Open Access Version

Abstract:
The efficiency of the quasi-solid-state dye-sensitized solar cell developed by Junghänel and Tributsch, the so-called Nano Surface Conductivity Solar Cell (NSCSC), was improved from 2% to 3.5% introducing a compact TiO2 underlayer, modifying the surface of the mesoporous TiO2 electrode, optimizing the deposition process of the electrolyte film, and replacing the platinum counter electrode by a carbon layer. Space-resolved photocurrent images revealed the importance of a homogeneous distribution of the electrolyte film. An uneven dispersion led to localized areas of high and low photocurrents, whereas the latter were attributed to an insufficient concentration of the redox couple. Impedance spectroscopy was performed on cells containing different concentrations of the redox couple. By modeling the spectra, the characteristic parameters of electron transport in the TiO2 were obtained. The measurements indicated that the transport of the positive charge to the counter electrode is the main process limiting the efficiency of the cells. Excess charge carrier decay in functioning devices was analyzed by contactless transient photoconductance measurements in the microwave frequency range (TRMC). The lifetime of the photogenerated charge carriers was observed to decrease with increasing applied potential, reaching its maximum close to the open-circuit potential of the cell, where the photocurrent density was minimal, i.e. the potential dependent decay observed was limited by the injection of electrons into the front contact. The functioning of this NSCSC indicated that the transport of the positive charge occurs by solid-state diffusion at the surface of the TiO2 particles. TRMC measurements on subset devices in the form of sensitized TiO2 layers revealed charge carrier kinetics strongly dependent on the concentration of the redox species in the electrolyte film, having the fastest decay at the lowest concentration of the redox couple. This was due to the regeneration of the oxidized dye by iodide,screening the positive charge from recombination with injected electrons. The replacement of the iodide/iodine redox couple by the kinetically fast ferrocene/ferrocenium (Fc/Fc+) system caused a dramatic increase of the decay rates of photo-generated charge carriers in subset devices. This increase was attributed to a large availability of recombination sites introduced by the presence of Fc/Fc+. Thus, showing the importance of the kinetically slow reduction rates of the iodide/iodine couple, leading to an increase of the electron lifetime by the reduction of the dye cation. For the Fc/Fc+ system the dominant decay channel was ascribed to the fast recombination of injected electrons with Fc+. The analysis of charge carrier kinetics in TiO2 powders and films displayed a decrease of the decay rate upon dye-sensitization after band-to-band excitation at 355 nm. This was attributed to a reduced recombination rate due to the trapping of excess holes by adsorbed dye molecules. In the case of ZnO films, the presence of the dye induced a significantly accelerated decay after excitation at 355 nm, which was assigned to a higher electron-hole recombination rate upon dye adsorption and the introduction of additional recombination states through dissolution of the ZnO. However, after excitation at 532 nm the prolonged decay observed is explained by the separation of injected electrons in the ZnO and a positive charge in the form of the dye cation with a relatively low cross section for the recombination between electrons and dye cations. In contrast to the ZnO films, ZnO nanorods displayed no such destructive influence of the dye adsorption. Furthermore, after exciting the sample at 355 nm, the decay was found to be independent of the dye and mainly depending on the recombination of electron-hole pairs and electrons with the dye cation at 355 nm and 532 nm, respectively.