A polymer tandem solar cell with 10.6% power conversion efficiency (PCE) has been developed. This achievement is the first certified PCE over 10% for polymer solar cells. The key to this success lies in the design of a low bandgap polymer, PDTP-DFBT, with a bandgap of 1.38 eV, high hole mobility, and a deep highest occupied molecular orbital (HOMO) level. This polymer enables a solution-processed tandem solar cell with a PCE of 10.6% under standard conditions (25°C, 1000 W/m², IEC 60904-3 global). The tandem structure consists of a high bandgap polymer (P3HT:ICBA) as the front cell and the low bandgap polymer (PDTP-DFBT:PCBM) as the rear cell. The front cell absorbs high-energy photons, while the rear cell captures lower-energy photons, maximizing the open circuit voltage (VOC) and overall PCE. The tandem structure reduces thermalization loss and optimizes the current balance between subcells. The low bandgap polymer PDTP-DFBT has a high external quantum efficiency (EQE) and a wide spectral response, extending to 900 nm. The polymer's design involved introducing fluorine atoms to lower the HOMO level and oxygen atoms to reduce the bandgap, resulting in a polymer with enhanced photovoltaic performance. The tandem solar cell's performance was validated through detailed characterization, including electrochemical cyclic voltammetry, energy-filtered transmission electron microscopy (EFTEM), and photo-induced charge carrier extraction measurements. The results demonstrate the effectiveness of the tandem structure in achieving high PCE and highlight the importance of optimizing the bandgap and energy levels in polymer solar cells. The study also discusses the potential of tandem solar cells to achieve higher efficiencies in the future, with the current design paving the way for further improvements in performance and stability.A polymer tandem solar cell with 10.6% power conversion efficiency (PCE) has been developed. This achievement is the first certified PCE over 10% for polymer solar cells. The key to this success lies in the design of a low bandgap polymer, PDTP-DFBT, with a bandgap of 1.38 eV, high hole mobility, and a deep highest occupied molecular orbital (HOMO) level. This polymer enables a solution-processed tandem solar cell with a PCE of 10.6% under standard conditions (25°C, 1000 W/m², IEC 60904-3 global). The tandem structure consists of a high bandgap polymer (P3HT:ICBA) as the front cell and the low bandgap polymer (PDTP-DFBT:PCBM) as the rear cell. The front cell absorbs high-energy photons, while the rear cell captures lower-energy photons, maximizing the open circuit voltage (VOC) and overall PCE. The tandem structure reduces thermalization loss and optimizes the current balance between subcells. The low bandgap polymer PDTP-DFBT has a high external quantum efficiency (EQE) and a wide spectral response, extending to 900 nm. The polymer's design involved introducing fluorine atoms to lower the HOMO level and oxygen atoms to reduce the bandgap, resulting in a polymer with enhanced photovoltaic performance. The tandem solar cell's performance was validated through detailed characterization, including electrochemical cyclic voltammetry, energy-filtered transmission electron microscopy (EFTEM), and photo-induced charge carrier extraction measurements. The results demonstrate the effectiveness of the tandem structure in achieving high PCE and highlight the importance of optimizing the bandgap and energy levels in polymer solar cells. The study also discusses the potential of tandem solar cells to achieve higher efficiencies in the future, with the current design paving the way for further improvements in performance and stability.