Efficiency improvement of dye-sensitized tandem solar cell by increasing the photovoltage of the back sub-cell

In tandem-structured dye-sensitized solar cell (DSSC) composed of two parallel-connected sub-cells, the photovoltage (Voc) generated by the back sub-cell is usually rather low, resulting in low Voc and conversion efficiency (η) of the tandem cell. To solve this issue, two simple but very efficient strategies, namely, the filling of Li+-absent electrolyte and/or coating Al2O3 on TiO2 electrode surface in the back sub-cell, were explored to enhance the Voc of the back sub-cell and hence that of the tandem cell. The former strategy was expected to heighten the energy level of TiO2 conduction band, and the latter one to retard the charge recombination. The photovoltaic performance measurements reveal that in the both cases, although there was a slight decrease in the photocurrent (Jsc), an obvious rise in the Voc was achieved for the tandem cells, leading to significant improvements in η of the tandem cells. Compared to the individual organic dye-sensitized solar cell (the highest η is 7.58%), the tandem cell with two organic dyes having complementary absorption spectra demonstrates an improved efficiency of up to 8.33% by a combinational application of Li+-absent electrolyte and Al2O3 overcoat. The results presented in this study highlight that the efficiency of a parallel-connected tandem-structured DSSC can be improved significantly through enhancing the photovoltage of the back sub-cell, which is first time reported.