The article reports the development of a high-performance low bandgap polymer (bandgap <1.4 eV) with a bandgap of 1.38 eV, high mobility, and deep highest occupied molecular orbital. This polymer enables a solution-processed tandem solar cell with a certified power conversion efficiency (PCE) of 10.6% under standard reporting conditions (25 °C, 1,000 W/m², IEC 60904-3 global), marking the first certified PCE over 10% for polymer solar cells. The single-junction device based on this polymer shows an external quantum efficiency (EQE) of >60% and spectral response extending to 900 nm, with a PCE of 7.9%. The tandem solar cell's performance is attributed to the complementary absorption spectra of the front and rear cells, which maximize the open-circuit voltage (VOC) and reduce thermalization loss. The study also discusses the importance of current balancing in subcells for achieving high efficiency in tandem solar cells.The article reports the development of a high-performance low bandgap polymer (bandgap <1.4 eV) with a bandgap of 1.38 eV, high mobility, and deep highest occupied molecular orbital. This polymer enables a solution-processed tandem solar cell with a certified power conversion efficiency (PCE) of 10.6% under standard reporting conditions (25 °C, 1,000 W/m², IEC 60904-3 global), marking the first certified PCE over 10% for polymer solar cells. The single-junction device based on this polymer shows an external quantum efficiency (EQE) of >60% and spectral response extending to 900 nm, with a PCE of 7.9%. The tandem solar cell's performance is attributed to the complementary absorption spectra of the front and rear cells, which maximize the open-circuit voltage (VOC) and reduce thermalization loss. The study also discusses the importance of current balancing in subcells for achieving high efficiency in tandem solar cells.