Diverse siloxane-terminated side chain ameliorated simple polymer donors towards efficient and stable air-processed organic solar cells

Side chain engineering plays an important role on performance of an organic semiconducting material. Herein, three linear siloxane-terminated side chains of different siloxane lengths and a novel T-shaped siloxaneterminated side chain were attached on a simple quinoxaline-thiophene backbone to construct four polymer donors PMQ-L3Si, PMQ-L5Si, PMQ-L7Si, and PMQ-T4Si for organic solar cells (OSCs). The different siloxane terminals showed great impacts on photophysical and aggregation properties of polymers. When pairing with acceptor BTP-H2, the resulting binary OSCs showed power conversion efficiencies (PCEs) between 16.76% and 18.15%. The highest PCE and OSC stability were achieved with PMQ-L5Si. Fibrous blend morphology, high glass transition temperature, efficient exciton dissociation, high face-on ratio, and balanced charge transport were found for the PMQ-L5Si based binary active layers. All the polymer based active layers, being processed in high humidity air, could exhibit comparable efficiencies to nitrogen condition, confirming advantage of the siloxaneterminated side chains. Polymer PMQ-L5Si was selected to construct ternary active layer, giving higher PCE of 18.82%. This work initiates the linear and T-shaped siloxane-terminated side chains to decorate polymer donors for OSC application, and also provides a guidance for side chain engineering aiming on tunable morphology and optimal matching to non-fullerene acceptor.