Insights into efficiency deviation from current-mismatch for tandem photovoltaics

For tandem solar cells (TSCs), the highest efficiency is generally believed to occur when the top and bottom subcells obtain an identical photocurrent, i.e., the current-match condition. However, the real situation is that there is a slight deviation from the matching point, which is an interesting phenomenon, but lacks a clear explanation. Here, we report a coupled photoelectric investigation on the intrinsic mechanisms within TSCs under various current configurations. Taking an all-perovskite TSC as an example, we find that the efficiency deviation originates from the current reduction and fill factor (FF) compensation; moreover, the optimal efficiency depends primarily on the sub-cells with higher FF. Our analysis further reveals that the higher FF and efficiency are achieved under current-mismatch condition due to the differentiated voltage assignments among the sub-cells, i. e., the sub-cells with higher currents require larger voltages in order to effectively recombine the surplus currents. We also find that the impact of sub-cell leakage on TSC performance can be partially alleviated by increasing the current of the affected sub-cell. Such an optoelectronic insight offers a valuable guidance for designing high-efficiency TSCs.