Dye-sensitized solar cells (DSSCs) were intensively studied in the last two decays as alternatives to conventional siliconbased solar cells due to its low cost and relatively high power conversion efficiency.1 TiO 2 is used as a semiconductor layer in DSSCs and the structure influences on the power conversion efficiency significantly. One of the causes which lower the power conversion efficiency is recombination; generally occur on the surface of TiO 2 nanoparticles. Many strategies have been reported to reduce the recombination. One is passivation of TiO 2 surface by coating with metal oxides as a barrier, such as Nb 2 O 5 , 2 metal oxide,
3MgO, 4 and another is preparation of 1-dimensional (1D) semiconductor structures, such as nanorods, nanotubes, for the fast electron transfer.5-8 However, when 1D semiconductor structures were used as working electrodes, low power conversion efficiencies were obtained because their specific surface area was small, resulting in less adsorption of dye.
9,10In this study, we report a double-layer structure for DSSCs consisting of TiO 2 nanorods array (TRA) and echinoid-like particles. This double-layer structure satisfies both fast electron transfer rate and large surface area which are important factors for the high power conversion efficiency.TRA film was prepared by hydrothermal method, reported in the literature.7 In brief, 15 mL D. I. water, 15 mL HCl, and 0.5 mL titanium(IV) isopropoxide (TTIP) were mixed and stirred. FTO glass was placed in the teflon-lined stainless steel autoclave and the prepared solution was added. The hydrothermal synthesis was conducted at 150°C for 10 h, and then, the sample was washed with D. I. water and dried at 70°C.TiO 2 echinoid-like TiO 2 particles were synthesized as a same process reported earlier by our group. 11 0.6 g poly (vinyl alcohol) (PVA) was dissolved in 18 mL dimethyl sulfoxide (DMSO) and TTIP was added to 30 mL 2-propanol. The PVA/DMSO solution and TTIP/2-propanol solution were added dropwise to 600 mL acetic acid under vigorous stirring and the resultant solution was refluxed at 100°C. After 4 days, the synthesized particles were centrifuged and washed with acetone several times and dried at 70°C.For the formation of the double-layer film, the TiO 2 echinoid paste was prepared by using α-terpineol, dibutyl phthalate, lauric acid, and ethyl cellulose. The paste was deposited on the TiO 2 array film by doctor blade method and the resultant was annealed at 450°C for 30 min. The prepared working electrode was immersed into N719 dye in the mixture of acetonitrile and tert-butanol for 18 h. Pt-coated FTO glass was used as a counter electrode, and the electrolyte was prepared with 1-propyl-3-methylimidazolium iodide (PMII), I 2 , LiI, tert-butylpyridine, and acetonitrile. The active area was 0.25 cm 2 . The XRD patterns of TRA film and echinoid-like TiO 2 particles are shown in Figure 1. According to the XRD patterns, the crystal structures of TRA film and echinoid-like TiO 2 particles are rutile and anatase, respectively. As shown i...