Bi-layer photoanode films of hierarchical carbon-doped brookite-rutile TiO<inf>2</inf> composite and anatase TiO<inf>2</inf> beads for efficient dye-sensitized solar cells
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© 2016 Elsevier LtdDye-sensitized solar cell (DSSC) is one of the most promising alternatives to the conventional p-n junction photovoltaic device. Here, we have explored the morphology and structure variation and the accompanying solar cell performance change of two different TiO2 beads-based photoanodes made from the same precursor. The first was synthesized hydrothermally at acidic condition whereas the second was synthesized solvothermally at slightly basic condition. The first material, referred as Bead-A with an average particle size of ~2.0 µm consists of rutile and brookite TiO2 phases while the second material, Bead-B, with smaller diameter of ~500 nm consists solely of anatase TiO2 phase. The pertaining formation mechanisms for both materials are explored. The performances of DSSCs based on Bead-A-P25 and Bead-B-P25 bi-layer photoanodes are higher than that based on pure P25. Cell based on Bead-B showed slightly better DSSC performance than Bead-A. It becomes apparent that Bead-A is advantageous in terms of fast electron transport due to its larger particle size and the presence of 1D rutile nanorods in addition to the carbon doping. Bead-B, on the other hand, gives advantages of superior light scattering properties and high surface area for dye loading. We further improved the cell performance by combining the advantages from both materials via optimized layer arrangement. By placing larger Bead-A particles on top of the smaller Bead-B particles, the top layer suppresses carrier recombination and confines the incident light within the photoanode, essentially enhancing charge harvest and collection efficiency. The performance of DSSC based on bi-layer Bead-A and Bead-B photoanode is superior to their counterpart based on bead-P25 bi-layer films. The cell operated using this bi-layer photoanode demonstrated short-circuit current density (Jsc) of 15.67 mA cm-2, open circuit voltage(Voc) of 721 mV and power conversion efficiency of 7.24%.
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