Tandem dye-sensitized solar cells with efficiencies surpassing 33% under dim-light conditions

To fabricate dye-sensitized solar cells (DSSCs), the variation of TiO₂ film thickness elicits contrary effects on the light-harvesting and charge transfer, thereby affecting cell performance. To compromise the TiO₂ thickness on the two effects, tandem DSSCs constructed by stacking two cells in series were proposed herein. Cells with high transparent photoelectrodes and counter electrodes were employed as the top cells; thus, the light unabsorbed by the top cells can be transmitted through, and harvested by the bottom cells. Furthermore, cobalt redox couple was adopted to decrease the light loss caused by absorption of the electrolyte. By regulating the film thicknesses of the top and bottom cells, both light harvest and charge transfer performance can be improved simultaneously, thereby, achieving a conversion efficiency as high as 33.98% under 2000 lx fluorescent lighting. This efficiency is much higher than the value (26.57%) obtained by the single cell. The tandem structure can also be applied under sunlight conditions, which can increase the efficiency from 9.27% to 11.31%.