Two novel wide‐bandgap copolymers, PBDT‐TDZ and PBDTS‐TDZ, are developed based on 1,3,4‐thiadiazole (TDZ) and benzo[1,2‐b:4,5‐b′]dithiophene (BDT) building blocks. These copolymers exhibit wide bandgaps over 2.07 eV and low‐lying highest occupied molecular orbital (HOMO) levels below −5.35 eV, which match well with the typical low‐bandgap acceptor of ITIC, resulting in a good complementary absorption from 300 to 900 nm and a low HOMO level offset (≤0.13 eV). Compared to PBDT‐TDZ, PBDTS‐TDZ with alkylthio side chains exhibits the stronger optical absorption, lower‐lying HOMO level, and higher crystallinity. By using a single green solvent of o‐xylene, PBDTS‐TDZ:ITIC devices exhibit a large open‐circuit voltage (V_oc) up to 1.10 eV and an extremely low energy loss (E_loss) of 0.48 eV. At the same time, the desirable high short‐circuit current density (J_sc) of 17.78 mA cm⁻² and fill factor of 65.4% are also obtained, giving rise to a high power conversion efficiency (PCE) of 12.80% without any additive and post‐treatment. When adopting a homotandem device architecture, the PCE is further improved to 13.35% (certified as 13.19%) with a much larger V_oc of 2.13 V, which is the best value for any type of homotandem organic solar cells reported so far.