Exploring a Fused 2-(Thiophen-2-yl)thieno[3,2-b]thiophene (T-TT) Building Block to Construct n-Type Polymer for High-Performance All-Polymer Solar Cells

In the field of all-polymer solar cells, exploring new electron-donating units (D) to match with electron-accepting units (A) is an important subject to promote the performance of D–A-type polymer acceptors. Herein, we developed a fused D unit 2-(thiophen-2-yl)thieno[3,2-b]thiophene (T-TT) derivated from the famous 2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) unit. With classical naphthalene diimide (NDI) as A unit, the new D–A polymer PNDI–T-TT exhibits enhanced absorption coefficient, electron mobility, and miscibility with donor polymer in comparison with the analogous PNDI–TVT polymer. These advantages can be attributed to the enlarged conjugation and reduced rotamers due to the fused T-TT unit, leading to a stronger intermolecular interaction. When blending with the donor polymer PBDB-T, both NDI-based polymers can form better interpenetrating nanostructures than the corresponding blend films with the donor polymer J71. Finally, PBDB-T/PNDI–T-TT device achieves a power conversion efficiency of 6.1%, which is much higher than that of PBDB-T/PNDI–TVT device (4.24%). These results demonstrate that n-type polymer based on fused T-TT unit can ameliorate the absorption coefficient, molecular aggregation, and charge-carrier mobility and consequently achieve an improved photovoltaic performance in comparison with the classic TVT unit.