Which is a superior material for scattering layer in dye-sensitized solar cells— electrospun rice grain- or nanofiber-shaped TiO 2 ? Peining Zhu, a A. Sreekumaran Nair, * b Shengyuan Yang, c Shengjie Peng b and Seeram Ramakrishna * bde Received 3rd May 2011, Accepted 27th June 2011 DOI: 10.1039/c1jm11939g A thin film of rice grain- and nanofiber-shaped titanium dioxide (TiO 2 ) nanostructures was fabricated as an effective scattering layer on the nanoparticle electrodes in dye-sensitized solar cells by elec- trospinning. The rice grain-like nanostructures are found to be superior to the nanofibers in scattering light with a 15.7% enhancement in efficiency compared to 9.63% for nanofibers. Since the first report of dye-sensitized solar cells (DSCs) by O’Regan and Gr€ atzel in 1991, 1 the DSCs have attracted a lot of research attention as simple, cheap and effective means to convert solar energy. In the last decade, plenty of research has been done to improve the performance of DSCs by various methods such as TiCl 4 post-treatment, 2 incorporation of hole-blocking layers, 3 use of a mixture of dyes with different spectral responses, 4 etc. Doping of TiO 2 with non-metals 5 and transition metals 6 has been tried to enhance the DSC performance. The morphology dependence of TiO 2 has also been explored (such as nanofibers 7 and nanotubes 8 ), both as photoanodes and scattering layers. 9–11 By using an additional scat- tering layer, the capability of the photoanode to harvest light would be increased and hence the efficiency. However, normal scattering layers made of large spherical particles will unavoidably decrease the total surface area of the electrodes which in turn will decrease the dye- loading. 12 Recently, TiO 2 nanostructures with large diameters and high surface areas such as mesoporous microspheres and hemispheres have been used as scattering layers for both increasing the light harvesting efficiency as well as the dye-loading. 13–15 However, for the large dimensions of such TiO 2 nanostructures, the packing density is very low, which is not beneficial to the charge transport and the light scattering phenomena. 16 Shao et al. 16 recently reported a scattering layer made of hierarchical TiO 2 nanoplates with high surface area and large packing density to overcome the limitations mentioned above. In the present paper, we report the fabrication of scattering layers using alternative materials with the same advantages by a simpler method of electrospinning. This is a straightforward method of fabricating a scattering layer 9 as cumbersome routes conventionally employed such as material synthesis, paste prepara- tion (by mixing with a polymer and subsequent milling), and screen printing could be eliminated. In our previous papers, we have reported the fabrication of single crystalline and high surface area rice grain-shaped TiO 2 nano/meso- structures showing good photovoltaic parameters by electro- spinning. 17,18 In this paper, we have utilized the same material as a scattering layer on the TiO 2 nanoparticle-electrodes by direct elec- trospinning and a comparison of the performance of this material with that of the TiO 2 nanofibers (also directly electropsun) is attempted. The TiO 2 nanoparticle electrodes were fabricated with the commercial paste (T/SP, Solaronix, Switzerland) by the method of doctor blading on the FTO (fluorine-doped tin oxide) substrates (Solaronix, Switzerland, sheet resistance <10 U/ ,). The scattering layer was made by direct electrospinning of the rice grain-like TiO 2 nanostructures onto the as-fabricated TiO 2 electrodes (applied voltage of 30 kV and the distance between the needle and the collector (the nanoparticle electrodes) was 12 cm). The electrospinning solution was made of 1.2 g polyvinyl acetate (PVAc, M n ¼ 500 000, Aldrich), 10 mL N,N-dimethyl acetamide (DMAc, 99.8%, Aldrich), 2 mL acetic acid (99.7%, Aldrich), and 1 mL titanium(IV) isoprop- oxide (TIP, 97%, Aldrich). The TiO 2 electrodes with the TiO 2 –PVAc composite fibers deposited on the surface was subjected to sintering at 450  C for 1 h to evaporate the polymer and to convert the composite fibers into rice grain-shaped TiO 2 nanostructures. 17,18 The electrodes were then sensitized by immersing in a 1 : 1 volume mixture of acetonitrile and tert-butanol solution of a ruthenium-based dye [RuL 2 (NCS) 2 $2H 2 O; L ¼ 2,2 0 -bipyridyl-4,4 0 -dicarboxylic acid (0.5 mM, N3 Solaronix)] overnight, subsequently washed with acetonitrile and dried in vacuum. A scattering layer made of elec- trospun TiO 2 nanofibers was also fabricated by direct electrospinning from a solution of 3.5 mL ethanol (absolute, Fischer scientific), 1 mL acetic acid, 0.3 g polyvinylpyrrolidone (PVP, M n ¼ 1  10 6 , Aldrich), and 0.5 g TIP for a comparison. Fig. 1(a) shows the SEM image of the electrospun rice grain-sha- ped TiO 2 nanostructures, revealing their well-connected network and high packing density. The average dimensions of the nanostructures were 400 nm in length and 150 nm in diameter. TEM image of a single nanostructure (Fig. 1(c)) shows that the rice grain-like a Department of Mechanical Engineering, National University of Singapore, Singapore, 117574, Singapore b Healthcare and Energy Materials Laboratory, Nanoscience and Nanotechnology Initiative, National University of Singapore, Singapore, 117581, Singapore. E-mail: nniansn@nus.edu.sg; seeram@nus.edu.sg c NUS Graduate School for Integrative Sciences and Engineering, Singapore, 117456, Singapore d Institute of Materials Research and Engineering, Singapore, 117602, Singapore e King Saud University, Riyadh, 11451, Kingdom of Saudi Arabia 12210 | J. Mater. Chem., 2011, 21, 12210–12212 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. 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