As one of the methods for improving the efficiency of a dye-sensitized solar cell (DSC), we investigated series-connected tandem DSCs. In this system, the top cell is made up of a transparent cell and the bottom cell utilizes only the light passing through the top cell. We investigated several combinations of dyes in tandem-type DSCs. The best efficiency obtained in our study is 10.4% (J(sc) = 10.8 mA/cm(2), V(oc) = 1.45 V, and FF = 0.67) for a series-connected tandem DSC consisting of an N719 top cell and a black-dye bottom cell. (C) 2008 Elsevier B.V. All rights reserved.