Charge Transfer and Performance Enhancement of Dye-Sensitized Solar Cells by Utilization of a Tandem Structure

For increasing the light-harvesting ability of dye-sensitized solar cells (DSSCs), thick TiO2 films were always utilized. However, a thicker TiO2 film also triggers a higher charge-transfer resistance. To solve the contrary effects of fihn thickness on the light harvest and charge transfer, a tandem-structured cell constructed by two sub-DSSCs is proposed. By using a high transparent Pt counter electrode, the light unabsorbed by the top cell can transmit trough and be utilized by the bottom cell. The performances of the tandem cells are evaluated using N719-sensitized TiO2 photoelectrodes and the thickness effects of top and bottom cells are studied. The experimental results show that the tandem structure can significantly improve the conversion efficiency of DSSCs. For the single cells studied in this work, the highest efficiency is 9.3%, achieved by a cell with 12-mu m main layer and 4-mu m scattering layer. For the tandem cells, the best performance appears at the structure with 12-mu m top cell and 8-mu m bottom cell (12/8), and the efficiency achieved is 9.54% in the absence of scattering layer. Therefore, the effect of the 8-mu m bottom cell in the tandem structure is more significant, in comparison with that of the 4-mu m scattering layer, on enhancing the cell performance. Furthermore, if a scattering-layer effect is also applied on the bottom cell of the 12/8 tandem structure, the efficiency can further be improved to 10.16%. If the powers of top and bottom cells are outputted individually, the overall efficiency achieved by the tandem cell is 11.6%, which is more than 2% higher than that obtained by the single cell.