Tandem Organic Solar Cells with 18.7% Efficiency Enabled by Suppressing the Charge Recombination in Front Sub-Cell

The maximum photocurrent in tandem organic solar cells (TOSCs) is often obtained by increasing the thicknesses of sub-cells, which leads to recombination enhancement of such devices and compromises their power conversion efficiency (PCE). In this work, an efficient interconnecting layer (ICL) is developed, with the structure ZnO NPs:PEI/PEI/PEDOT:PSS, which enables TOSCs with very good reproducibility. Then, it is discovered that the optimal thickness of the front sub-cell in such TOSCs can be reduced by increasing the proportion of a non-fullerene acceptor in the active layer. The non-fullerene acceptor used in this work has a much larger absorption coefficient than the donor in the front sub-cell, and the absorption reduction of donor can be well complemented by that of the acceptor when increasing the acceptor proportion, thus leading to a significant overall absorption enhancement even with a thinner film. As a result, the optimal thickness of the front sub-cell is reduced and its charge recombination is suppressed. Ultimately, the use of this ICL combined with fine-turning of the composition in the front sub-cell enables an efficient TOSC with a very high fill factor of 78% and an excellent PCE of 18.71% (certified by an accredited institute to be 18.09%) to be obtained.