Karin Keis

A 5% efficient photoelectrochemical solar cell based on nanostructured ZnO electrodes

Nanoporous ZnO electrodes, dye-sensitized with a ruthenium bipyridyl complex, were used as photoanodes in photoelectrochemical solar cells. By improving the interfacial contact between dyes and ZnO particles in the film, overall solar-to-electric energy conversion efficiencies of up to 5% were obtained. The solar cell performance was studied as a function of film thickness, residence time of the electrodes in the dye solution, electrolyte composition and light intensity.

Nanostructured ZnO electrodes for dye-sensitized solar cell applications

Dye-sensitized photoelectrochem. solar cells constitute a promising candidate in the search for cost-effective and environment-friendly solar cells. The most extensively studied, and to date the most efficient systems are based on titanium dioxide. In this paper, the possibilities to use nanostructured ZnO electrodes in photoelectrochem. solar cells are investigated. Various exptl. techniques (e.g. IR, photoelectron, femtosecond and nanosecond laser spectroscopies, laser flash induced photocurrent transient measurements, two- and three-electrode photoelectrochem. measurements) show that the thermodn., kinetics and charge transport properties are comparable for ZnO and TiO2. The prepn. techniques of ZnO provide more possibilities of varying the particle size and shape compared to TiO2. However, the dye-sensitization process is more complex in case of ZnO and care needs to be taken to achieve an optimal performance of the solar cell.

Nanostructured ZnO electrodes for photovoltaic applications

Abstract Photoelectrochemical properties of nanostructured ZnO (wurtzite) electrodes have been investigated. Studies were carried out on various types of electrodes with controlled morphology, particle size and doping. The monochromatic photon-to-current conversion efficiencies were recorded in the UV spectral region on bare ZnO and in the visible region on ruthenium dye-sensitized cells. The results show relatively high efficiencies for such systems and demonstrate the potential of ZnO nanostructured cells as materials for photovoltaic applications.

Photoelectrochemical properties of nano- to microstructured ZnO electrodes

The photoelectrochem. properties of ZnO electrodes were studied in the near-UV region. Studies were carried out on electrodes with different morphologies, porosities, and film thicknesses to probe the influence of these parameters on the photoelectrochem. characteristics. The highest incident photon-to-current conversion efficiencies of 90 and 30% for illumination through the conducting substrate and directly on the ZnO, resp., were obtained with a 8 μm thick ZnO film consisting of 150 nm spherical particles. For this system, no significant decrease in the photocurrent due to addn. of iodine or oxygen to the electrolyte was found. The electron losses from the conduction band of the ZnO electrodes are restricted to the zone of charge-carrier generation.

Charge transport properties in the nanostructured ZnO thin film electrode – electrolyte system studied with time resolved photocurrents

The charge transport in nanoporous ZnO was studied by laser flash induced photocurrent transients. The results are discussed using a diffusion model and compared with previous results on TiO2. The charge transport was highly dependent on the potential giving apparent diffusion coeffs. for the electron ranging from 1×10-4 to 1×10-6 cm2/s with an applied bias of +100 and +300 mV vs. Ag/AgCl in ethanol, resp. The electrolyte was 0.5 M LiClO4 in ethanol. The potential dependence was much more pronounced for ZnO than for TiO2. The charge transport was also dependent on the electrolyte giving a linear dependence between the cond. of the electrolyte and the apparent electronic diffusion coeff. The dependence of the light intensity was also studied. Intensity-dependent losses were obsd.

A detailed analysis of ambipolar diffusion in nanostructured metal oxide films

In this paper a transport equation is derived which describes the behaviour of the nanostructured metal oxide films in a photoelectrochemical cell. It is shown that a detailed analysis of the charge compensation mechanism necessarily leads to a transport equation with characteristics similar to but logically distinct from the pure diffusion equation. The studied phenomenon was named ambipolar diffusion in the early 1950s. It takes into account the fact that the diffusion processes of ions and electrons occur at different speeds. A weak electric field therefore couples the processes together to preserve charge neutrality. The electric field in turn affects the transport resulting in a deviation from purely diffusive behaviour. However, this has not been widely recognised in the literature for nanostructured semiconductor films until very recently. In this paper a detailed analysis is presented. It is based on the assumption that the current density is solenoidal. It is shown that application of the ambipolar diffusion model to a photoelectrochemical cell based on a nanostructured metal oxide film leads to an additional term in the transport equation, rather than only a new diffusion coefficient as in earlier work. It is also shown that the boundary conditions interact closely with the equation to form a transport model.

Optical characterization of nanostructured ZnO and TiO2 films

Abstract The comparative studies of optical properties in the ultraviolet and visible regions for ZnO and TiO2 nanostructured films prepared by different methods are carried out in air and in the aqueous medium. Results show how the electrolyte surrounding the nanoparticles influence both the total transmittance and reflectance as well as light scattering. The measurements demonstrate also some difficulties and errors when interpreting the results for thin porous coatings. Great care has to be taken to avoid the losses caused by light coupling and scattering into the substrate.