Fabrication of TiO 2 Nanobelt Network for Dye-Sensitized Solar Cells
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Fabrication of TiO2 Nanobelt Network for Dye-Sensitized Solar Cells Haiyan Li and Jun Jiao* Department of Physics, Portland State University, Portland, OR 97207, U.S.A. *Corresponding author. E-mail: [email protected] ABSTRACT Interconnected TiO2 nanobelt networks were prepared to serve as anode materials. The aim is to enhance the electron transport through the anode of dye-sensitized solar cells. Using an alkaline hydrothermal procedure and by controlling the reaction time two kinds of nanostructures were synthesized. One is TiO2 nanobelts and another is TiO2 nanobelts protruding from TiO2 nanoparticles. The investigation suggests that TiO2 nanobelts resulted from the rearrangement of the adjacent [Ti(OH)6]2- monomers formed during the erosion process of TiO2 nanoparticles. The nanostructures of as-synthesized nanobelts were woven and interconnected, forming networks after an annealing process. Raman analysis indicated that both kinds of nanostructures were pure anatase. Electrical characterization suggests that the conductivities of these TiO2 nanobelt networks were higher than those of the TiO2 nanoparticle films. Under simulated sunlight with an intensity of AM 1.5 G, the solar cells made of TiO2 nanobelt networks show exceptional photocurrent in comparison to those made of TiO2 nanoparticles. INTRODUCTION The photosensitization of wide band gap semiconductor, such as TiO2 (3.4 eV), is a promising and low cost method (reel-to-reel mode for mass production) to fabricate dye sensitized solar cell (DSSC), in which the sensitizer generates electron-hole pairs after harvesting photons and then injects the excited electrons into TiO2 conduction band. Subsequently, the electrons and holes are collected by the opposing electrodes correspondingly.1-3 Using a ruthenium complex sensitized TiO2 nanoparticle (NP) based photoanode, the DSSC is capable of absorbing most of visible sunlight and a high light-electricity power conversion efficiency (ηPCE) of 11 % was obtained.4 After photogenerated excitons were split, the efficiency of charge collection is related to the competition between charge transport and electron-hole recombination.5-6 Since the diffusion of oxidizing species is exceedingly slow, the enhancement of electron transport will reduce the charge recombination. Because of many surface defects and crystal boundaries in the TiO2 NP layer, the electron transport from the excited dye molecules to the electrode is slow and follows a trap-mediated diffusion mechanism.7 Using TiO2 nanorods (NR), nanowires (NW) or nanobelts (NB) instead of TiO2 NPs can significantly increase the electron diffusion length up to over tens of micrometers by removing the boundaries between TiO2 NPs.8, 9 ZnO NWs (wide band gap of 3.3 eV, similar to that of TiO2) are also used as the dye carriers and electron transport materials showing one or two order of magnitude enhancement of the electron transport relative to that of NP based cell.10 Nevertheless, the reported one dimensional materials based DSSCs have very low conversion effi
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