DP, PV, GG, MQ, GB, and JMB guided the experiment’s progress and manuscript writing and participated in mechanism discussions. SA, NPB, VM, and YC helped measure and collect the experimental data. All authors read and approved the final manuscript.”
“Background Dye-sensitized solar cells (DSCs) have received much attention since Grätzel and O’Regan achieved a remarkable level of efficiency through their use of mesoporous TiO2 films as a photoanode for DSCs in 1991 [1]. DSCs have several advantages compared to Si or copper indium gallium selenide (CIGS) solar cells as follows: (a) DSCs can be fabricated with non-vacuum processes, as opposed to Si or

CIGS solar DZNeP cells. The use of non-vacuum equipment offers the possibility to reduce costs. (b) Wet etching processes such as saw damage etching and texturing, PU-H71 which are widely used in Si solar cells, are not required

during the fabrication of DSCs. The fabrication of DSCs is thus simplified without a wet etching process. (c) Colorful DSCs can be easily fabricated because dyes have various colors according to their light absorption characteristics. Although DSCs have these merits, the relatively low power conversion efficiency has become the main cause which limits the commercialization of DSCs. Several attempts to enhance the performance levels of dyes [2–12], photoelectrodes [13–30], counter cathodes [31–36], Progesterone and electrolytes [3, 31, 37–41] have been attempted in an effort to obtain improved efficiency in DSCs. Among these efforts, increasing the surface area of the photoelectrodes and reducing the degree of charge recombination between the photoelectrodes and electrolytes have been shown to be critical factors when seeking to improve the power conversion efficiency

of DSCs. The TiO2 nanoparticle structure has shown the best performance in DSCs [3]. However, structural disorder, which exists at the contact point of TiO2 FG 4592 nanocrystalline particles, reportedly prohibits charge transport, resulting in limited photocurrents [27–29]. The effort to find alternative TiO2 nanostructures has been an important issue to researchers who attempt to increase the power conversion efficiency of DSCs. Various types of nanotechnologies have been applied to alternative TiO2 nanostructures such as nanorods [13], nanowires [14, 15], nanotubes [16, 18, 19, 22, 23, 25, 27–30, 42], [43], nanohemispheres [21, 24], and nanoforests [17, 20]. These structures were used to increase the surface area for dye adsorption and to facilitate charge transport through TiO2 films. Of these nanostructures, the TiO2 nanotube structure has the best potential to overcome the limitations of the TiO2 nanoparticle structure. A previous report showed that the electronic lifetimes of TiO2 nanotube-based DSCs were longer than those of TiO2 nanoparticle-based DSCs [30].