Influence of controlled-charge anodization processes on the morphology of TiO2 nanotubes and their efficiency in dye-sensitized solar cells

The effect of the electrochemical anodization growth process on the development of self-organized TiO2 nanotube (NT) films and their efficiency as photoelectrodes in dye sensitized solar cells (DSCs) has been comparatively investigated, by keeping constant the total anodization charge. Slow and rapid potentiostatic anodization processes were accordingly compared to the galvanostatic one, while a two step potentiostatic-galvanostatic technique was applied for the first time for the growth of TiO2 NT arrays, as a step forward in relation to the existing potentiostatic-potentiostatic (P-P) technique. Scanning electron microscopy and Raman spectroscopy verified the wide diversity in the morphological and structural characteristics of the TiO2 NTs obtained by the different anodization modes. The novel approach of galvanostatic tube growth on a potentiostatically patterned Ti foil provided the most uniform TiO2 nanotubular films with clean top surface exempt of nanograss or cracks over extended areas. Evaluation of the TiO2 NTs performance as photoelectrodes in DSC devices showed distinct differences of their electrical parameters that reflected finely the underlying structure/morphology variations of the different anodic oxidation conditions. Galvanostatic TiO2 NT films presented the most favorable (open-ordered) structure for DSC photoelectrodes with superior electrical performance, essentially impaired by a relatively low fill factor that requires improvement by appropriate post-treatment. Furthermore, despite the marked differences in morphology, the TiO2 NT photoelectrodes exhibited comparable overall performance (of the order of 4%), with only exception the P-P samples which presented slightly lower (about 25%) photovoltaic efficiency. These results indicate that the anodization charge is a critical factor that effectively controls the nanotubes behavior when they are used as photoelectrodes in DSCs.