Exploring a fused 2-(thiophen-2-yl)thieno[3,2-b]thiophene (T-TT) building block to construct n-type polymer toward high performance all-polymer solar cells

摘要： In the field of all-polymer solar cells (all-PSCs), 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 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 donor polymer J71. Finally, PBDB-T:PNDI-T-TT device obtains a power conversion efficiency (PCE) 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 improved photovoltaic performance in comparison with classic TVT unit.