Amongst all types of photoanodes in sensitized solar cells, nanotube arrays (NTs) have become a suitable choice that can balance the dye absorption, electron transport, and effective thickness. However, the maximum performance they have shown now is still not comparable with porous films. In this work, we have established a simple yet effective method towards high performance solar cell based on NTs. Significantly increased carrier generation by better dye absorption can be realized by the

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... cle-like secondary structures grown on TiO 2 tubes. Nevertheless, this effect was often followed by a sacrifice of the electron transport properties of the NTs, which may greatly weaken the total net increase of efficiency by the secondary structures. With a subsequent TiCl 4 treatment and slightly modulated outer conditions (anodic voltage and stirring rate), we are able to adequately resolve this paradox. The short circuit current can be raised to over 110% higher, while the fill factor and open circuit voltage remain unharmed. As a result, an efficiency of 4.35% can be achieved in the back-side illuminated dye-sensitized solar cells, which is $114% higher than the basic efficiency of the plain cell. The formation of the secondary structures can be explained by the reversible dissolution-reprecipitation of TiO 2 with the reaction-diffusion process of F À and other fluoride species during the treatment.