Side chain engineering of copolymers based on benzotriazole (BTA) and dithieno[2,3-d;2',3'-d']benzo[1,2-b;4,5-b']dithiophenes (DTBDT) enables a high PCE of 14.6.

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 (OSCs), but restricted for its relatively poor photovoltaic performance compared with benzo[1,2-b;4,5-b']dithiophenes (BDT)-based analogue. Herein, three conjugated polymers (PE51, PE52 and PE53) based DTBDT and benzo[d][1,2,3]triazole (BTA) bearing different length of side alkyl chain unit were synthesized. The change of alkyl chain length can effectively affect energy level distribution, molecular stacking, miscibility and morphology with the non-fullerene acceptor of Y6, resulting in gradually increased open-circuit voltage (VOC) from 0.79 to 0.83 V, improved short-current density (JSC) and fill-factor (FF) reached up to 25.36 mA cm-2 and 71.94% in PE52-based device (with moderate alkyl chain). Impressively, compared with BDT-based analogue 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 (PCE) from 12.3% to 14.6%, which is the highest PCE among DTBDT-based polymers and BTA-based polymers. Our results show that not only high efficiency could be achieved via using DTBDT as D unit, but also the length of the alkyl chain on BTA has a significant impact on photovoltaic performance for DTBDT-based polymers.