Bottom-up Growth of Hierarchical Electrodes for Highly Efficient Dye-Sensitized Solar Cells Youngshin Lee, Chang-Yeol Cho, Su-Jin Ha, Hye-Na Kim, and Jun Hyuk Moon* Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, South Korea * S Supporting Information ABSTRACT: The nonconventional bottom-up growth of TiO 2 was first demonstrated in the preparation of hierarchical TiO 2 electrodes for use in highly efficient dye-sensitized solar cells. The simple immersion of a substrate in a precursor solution enabled the growth of TiO 2 particulate films. Here, we have implemented a hierarchical growth strategy in which two stages of controlled growth yielded first macroscale TiO 2 particles, followed by mesoscale TiO 2 particles. We success- fully fabricated electrode films up to 20 μm thick via a growth rate of 0.3 μm/min. The specific area of the electrodes was controlled via the deposition of mesoscale TiO 2 particles. The deposited particles displayed a rutile phase with an average size of several tens of nanometers in diameter, as confirmed by XRD and high-resolution TEM imaging. After depositing the second layer of mesoscale TiO 2 particles, the photocurrent density increased by a factor of 3. A maximum efficiency of 6.84% was obtained for the hierarchically structured TiO 2 electrodes under 1 sun illumination. The hierarchical TiO 2 electrodes were compared with macroporous TiO 2 electrodes, revealing that the higher photocurrent density could be attributed to a longer electron recombination lifetime and a high specific area. The longer recombination lifetime was supported by the presence of fewer defective TiO 2 surfaces, as confirmed by the XPS spectrum. KEYWORDS: hierarchical structures, solution deposition, TiO 2 , macroporous, recombination lifetime, dye-sensitized solar cells ■ INTRODUCTION Dye-sensitized solar cells (DSSCs) based on oxide semi- conductors and organic dyes or metallo-organic complex dyes have attracted much attention because of their low production costs and their unique advantages for fabricating transparent cells over silicon or thin-film solar cells. 1 DSSCs employ a wide band gap semiconductor, such as a TiO 2 nanoparticle film, to provide a mesoporous structure with a large specific area for the adsorption of light-harvesting dye molecules. 2 Although recent work by Grä tzel et al. updated the record of photon-to-electric conversion efficiency to 12%, 3 an efficiency ceiling of 11% has persisted for nearly two decades. The limit in efficiency has been mainly attributed to the recombination of the photo- generated electrons during electron transport through the nanoparticulate electrode. 4 To address these issues, several efforts have focused on engineering microstructured electrodes. Electrode engineering was initially achieved by utilizing directional and/or ordered macroscale (>50 nm) morpholo- gies, such as nanotubes, 5,6 nanowires, 7 or 3D periodic inverse opals. 8 Many promising results have reported increases in electron transport by up to 4 times or in electron lifetimes by 2-4 times, 9,10 but the external conversion efficiencies have remained at 70-80% of the efficiency record. One issue is that the specific areas of these macroscale morphologies are low; therefore, they yield a low absorption of dye molecules and thereby low photocurrent density under illumination. For example, cells comprising nanotube array electrodes yield 70% of the dye adsorption and 50% of the short-circuit current density of conventional nanoparticle electrodes, resulting in a 30-50% lower conversion efficiency. 11-13 This limitation has led the utilization of hierarchical structures that combine macro and mesoscale morphologies. Macroscale morphologies enhance light absorption efficiency, electron transport, and facilitate infiltration of polymeric electrolytes. Mesoscale morphologies allow high specific areas for dye adsorption. Previously, the hierarchical structures have been fabricated using a simple strategy of sequential growth of mesoscale morphologies on macroscale structure, for example, nano- particles decoration on nanotubes, nanowires, or 3D inverse opal structures. 14-18 An additional layer of macroscale structures, such as nanotubes or inverse opals has been introduced on top of a conventional mesoporous electrode. 19,20 Occasionally, a growth in heated pressurized solution or physical deposition in vacuum has been employed to directly grow the hierarchical structures. 21-23 In this paper, we demonstrated a facile approach to preparing hierarchically structured electrodes via nonconventional, bottom-up growth approach. Briefly, the deposition of TiO 2 Received: April 15, 2012 Accepted: June 14, 2012 Published: June 27, 2012 Research Article www.acsami.org © 2012 American Chemical Society 3589 dx.doi.org/10.1021/am300664x | ACS Appl. Mater. Interfaces 2012, 4, 3589-3595