Nanoscale investigation on large crystallites in TiO₂ nanotube arrays and implications for high-quality hybrid photodiodes

Abstract

Anodized TiO2 nanotube arrays fabricated on a TiO2 thin film on conducting glass substrates can be readily implemented in diverse applications like hybrid solar cells. In this study, we concentrate on morphologies with inner tube diameter being around 30 nm which is in dimension of the exciton diffusion length of common organic hole conductors. Cross-sectional preparation of the intact tube array in correlation with transmission electron microscopy has been performed to get local information on the TiO2 nanotubes and their arrangements, depending on anodization voltage. Crystallites have been found to be anatase and in size of several hundred nanometers along tube walls with increasing length for increasing anodization voltages. Inter-tube connections with similar crystal orientations of adjacent tubes are found. These give rise to large areas of uniform orientation. Thus, the number of grain boundaries within the film is low compared to the reported values for different TiO2-polymer material systems. Using the arrays, hybrid TiO2 solar cells were fabricated, which show high fill factors indicating good electron transport. The results suggest high electron mobility and are encouraging for a utilization of the nanotube arrays in next generation photovoltaics.