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Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Current density-voltage (I-V) curves of DSSCs based on closed-ended freestanding TiO2 nanotube arrays: (a) Without carbon materials and large TiO2 NPs; (b) With carbon materials; (c) With large TiO2 NPs; and (d) With carbon materials and large TiO2 NPs.