High temperature crystallization of free-standing anatase TiO₂ nanotube membranes for high efficiency dye-sensitized solar cells

Despite the one-dimensional ordering of anodic TiO₂ nanotube arrays (TNAs), the electron diffusion towards the substrate in TNA-based dye-sensitized solar cells (DSSCs) is comparably slow. The improvement of electron mobility by enhancing TNA crystallinity under high-temperature annealing, however, is infeasible with the existence of Ti metal substrate. Herein, it is shown that, by high temperature (up to 700°C) crystallization of high-quality free-standing TNA membranes, the TNAs can maintain their structure integrity and phase (anatase) stability as a result of the absence of the nucleation sites and the high quality of the membrane obtained by a self-detachment method. The electron transport is much faster (≈4 times) in the 700°C-annealed TNA membranes than that in the 400°C-treated ones for 20 μm-length nanotubes, which is mainly attributed to the improved crystallinity and reduced electron trap states. In spite of slightly reduced dye loading capacity (decreased by ≈30%) in the 700°C-annealed membranes, the superior electron transport leads to a significantly improved efficiency of 7.81% (enhanced by ≈50%). The strategy of manipulating the electron transport dynamics by high temperature treatment on high-quality TNA membranes may open new route for further improvement in the performances of TNA-based DSSCs.