1D TiO2 nanostructures probed by 2D transmission electron microscopy

Fakultät für Chemie und Pharmazie - Digitale Hochschulschriften der LMU - Teil 05/06

Hybrid solar cells based on nanoparticulate TiO2, dye and poly(3-hexylthiophene) are a common benchmark in the ?eld of solid-state dye-sensitized solar cells. One-dimensionally nanostructured titanium dioxide is expected to enhance power-conversion efficiency (PCE) due to a high surface area combined with a direct path for electrons from the active interface to the back electrode. However, current devices do not meet those expectations and cannot surpass their mesoporous counterparts. This work approaches the problem by detailed investigation of diverse nanostructures on a nanoscale by advanced transmission electron microscopy (TEM). Anodized TiO2 nanotubes are analyzed concerning their crystallinity. An unexpectedly large grain size is found, and its implication is shown by corresponding solar cell characteristics which feature an above-average fill factor. Quasi-single crystalline rutile nanowires are grown hydrothermally, and a peculiar defect structure consisting of free internal surfaces is revealed. A growth model based on Coulombic repulsion and steric hindrance is developed to explain the resulting V-shaped defect cascade. The in?uence of the defects on solar cell performance is investigated and interpreted by a combination of TEM, electronic device characterization and photoluminescence spectroscopy, including lifetime measurements. A speci?c annealing treatment is proposed to counter the defects, suppressing several loss mechanisms and resulting in an improvement of PCEs by 35 %. Simultaneously, a process is developed to streamline electron-tomographic reconstruction of complex nanoparticles. Its suitability is demonstrated by the reconstruction of a gold nanostar and a number of iron-based particles distributed on few-layered graphene.