Abstract Dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophenes (DTBDT) is a kind of prospective candidate for constructing donor-π-acceptor (D-π-A) copolymer donors applied in organic solar cells but is restricted due to its relatively poor photovoltaic performance compared with benzo[1,2-b;4,5-b′]dithiophenes (BDT)-based analog. Herein, three conjugated polymers (PE51, PE52 and PE53)-based DTBDT and benzo[d][1,2,3]triazole (BTA) bearing different lengths of alkyl side chain were designed and synthesized. The change in alkyl chain length can obviously affect the energy level distribution, molecular stacking, miscibility and morphology with the non-fullerene acceptor of Y6. Polymer PE52 with a moderate alkyl chain realized the highest short-current density (J _SC) and fill factor (FF) of 25.36 mA cm⁻² and 71.94%, respectively. Compared with BDT-based analog J52-Cl, the significantly enhanced crystallinity and intermolecular interaction of PE52 had effectively boosted the charge transport characteristic and optimized the surface morphology, thereby increasing the power conversion efficiency from 12.3% to an impressive 14.6%, which is the highest value among DTBDT-based and BTA-based polymers. Our results show that not only could high efficiency be achieved via using DTBDT as a D unit, but the length of the alkyl chain on BTA has a significant impact on the photovoltaic performance.