Solar cells based on dye-sensitized nanocrystalline TiO2 electrodes
This thesis presents a new type of photovoltaic solar cell based on dye-sensitized nanocrystalline titanium dioxide electrodes. In contrast to conventional solar cells, where light absorption is due to band gap excitation of the semiconductor itself, TiO2 with its wide band gap is transparent in the visible spectrum. The light is rather absorbed by a dye, e.g. a ruthenium polypyridine complex or a chlorophyll derivative, attached to the semiconductor surface. Charge separation occurs by electron injection from the excited dye molecule into the conduction band of TiO2, followed by rapid reduction of the oxidized dye by a redox electrolyte. Such a purely kinetic charge separation mechanism is also found in the reaction centers of natural photosynthetic systems. The analogy goes even further: Just as the chlorophyll containing thylakoid membranes of the chloroplast are folded to grana stacks for efficient light harvesting, a thin film of nanocrystalline TiO2 particles results in a several hundred times enlarged surface area for dye adsorption. While the efficiency of conventional polycrystalline solar cells is limited by charge carrier recombination at grain boundaries as well as bulk defects and impurities, the TiO2 serves only as conductor for majority carriers (injected electrons), so that recombination in the semiconductor is absent in this device. Thus high charge separation efficiencies are obtained even with thin film electrodes made from cheap TiO2 powder. We have investigated the theoretical principles, preparation and performance of the different components constituting the dye-sensitized nanocrystalline TiO2 solar cell. In the first chapter the current status of photovoltaics is briefly reviewed, followed by an introduction into the basic concepts of photoelectrochemical cells and dye sensitization. The second chapter deals with the nanocrystalline thin film photoelectrode. First the choice of the substrate and its pretreatment by deposition of an underlayer are discussed. Then different methods for the preparation of nanoporous TiO2 films are investigated. Three procedures were found to be suitable: in situ hydrolysis of titanium alkoxides, coating with a colloidal TiO2 dispersion and chemical vapor deposition of mixed TiO2 / B2O3 films. The colloid route is the most versatile, since the particle size distribution can be influenced during colloid preparation, e.g. by precipitation, peptization and hydrothermal treatment or by dispersion of a TiO2 powder made by flame hydrolysis or plasma oxygenolysis of TiCl4. Moreover, the colloid can be applied to the substrate by many different techniques, such as with a doctor blade, screen printing, spraying or electrodeposition. Finally the after-treatment of the photoelectrode by deposition of a TiO2 overlayer and the firing and handling before dye adsorption are addressed. The thus achieved improvements in electrode performance are rationalized by modeling the dye adsorption on the mostly exposed, dehydroxylated surface planes of the nanocrystalline anatase particles. In chapter three we investigate the sensitization by dyes, beginning with a list of the general requirements that have to be met by the sensitizer. An experimental set-up for the acquisition of photocurrent action spectra is described and the results for some of the most successful ruthenium polypyridine complexes are reported. The sensitization by chlorophyll derivatives and related natural porphyrins is investigated in some detail and the effect of different coadsorbates, such as cholanic acids and sugars, compared and rationalized by molecular modeling. The mechanism of photosensitization by chlorophyll derivatives is deduced from static and time resolved fluorescence quenching as well as laser flash photolysis and photocurrent/voltage transients in combination with cyclic voltammetry and spectroelectrochemistry. The importance of surface states on the nanocrystalline TiO2 particles in trapping of the injected electrons becomes obvious from these studies. Finally we present the results obtained with some other sensitizers, such as carotenoate and a merocyanine. Chapter four treats the redox-electrolyte, which serves to transfer the electron arriving from the external circuit at the counter electrode back to the oxidized dye. We list the general requirements for the redox couple and discuss the peculiarities of the iodide/triiodide couple. The current status of solid electrolytes and their application in the dye-sensitized nanocrystalline TiO2 solar ce11 are reviewed. In chapter five we briefly report on the preparation and properties of the counter electrode, which should reduce the redox couple at low overvoltage and may at the same time serve a mirror on the backside of the solar cell. In chapter six we draw some conclusions with regards to the further development of this new type of solar cell.