Dye-Sensitized Tandem Solar Cells with Extremely High Open-Circuit Voltage Using Co(II)/Co(III) Electrolyte

For achieving a high open-circuit voltage (V-oc) in dye-sensitized tandem solar cells, series-connected tandem solar cells were fabricated. In order to optimize series-connected tandem solar cell systems, the current density of the top and bottom cells should be well matched to be identical, and the V-oc of each of the cells should also be as high as possible. Furthermore, the top cell should be transparent and the bottom cell should have the longer-wavelength absorption, for utilizing only the light passing through the top cell. This leads to a high V-oc. In this study, we report dye-sensitized tandem solar cells having an extremely high V-oc using the Co(bpy)(3)(2+/3+) (bpy=2,2'-bipyridine) redox couple. Dye-sensitized tandem solar cells employing JK303/HC-A1 with the Co(bpy)(3)(2+/3+) redox couple as the top cell and N749/HC-A4 with the I-/I-3(-) redox couple as the bottom cell were shown to have an extremely high V-oc of >1.66 V, the highest value for dye-sensitized tandem solar cells reported to date.