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. High‐quality self‐detached TiO2 nanotube array membranes by electrochemical anodization exhibit excellent structure integrity and crystal phase stability under high temperature crystallization. These membranes, with improved crystallinity and reduced electron trap states, possess superior electron transport for application in dye‐sensitized solar cells, leading to the significantly improved solar power conversion efficiency.