Nanostructured ZnO electrodes for solar cell applications

Abstract: In this thesis the possibilities of using nanostructured dye-sensitized ZnO electrodes for solar energy conversion in photoelectrochemical solar cells are investigated. In order to characterize the photoelectrochemical properties of unsensitized ZnO films, measurements were performed using a short laser pulse (laser flash induced current transients) and under continuous illumination (steady-state). These methods give information about charge separation processes as well as charge transport mechanisms in the film. Studies were carried out on various types of electrodes with controlled morphology, particle size, film thickness and doping. For an 8 μm thick ZnO film consisting of 150 nm sized spherical particles, incident photon-to-current efficiencies (IPCE) of 80-90 % were obtained when illuminating through the conducting glass substrate. For the ZnO system, no significant decrease in the photocurrent due to the addition of electron scavengers, such as tri-iodide and oxygen, to the electrolyte was found.The studies of dye-sensitized ZnO electrodes using IR- and Raman spectroscopies, together with traditional two-electrode measurements using a sandwich-type of cell, gave evidence of dye aggregation in the nanostructured ZnO film. The studies revealed that the aggregation is due to the dissolution of Zn surface atoms in which protons from the carboxylic groups of the dye are involved. It is shown that dye aggregation can be avoided by varying the pH of the solution or dye composition. The dye-sensitizing time and concentration of the solution are also of importance. By improving the interfacial contact between dye and ZnO particles and avoiding aggregation, overall solar-to-electric energy conversion efficiencies of up to 5 % were obtained under 100 W/m2 illumination using a solar simulator.