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 siloxane-terminated 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 siloxane-terminated 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.