Exploring a Fused 2‑(Thiophen-2-yl)thieno[3,2‑<i>b</i>]thiophene (T-TT) Building Block to Construct n‑Type Polymer for High-Performance All-Polymer Solar Cells AnNing RanHuijuan GengYanfang ZengQingdao HuJianyong YangJing SunYanming WangXiaochen ZhouErjun 2019 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-<i>b</i>]­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 <b>PNDI–T-TT</b> exhibits enhanced absorption coefficient, electron mobility, and miscibility with donor polymer in comparison with the analogous <b>PNDI–TVT</b> 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/<b>PNDI–T-TT</b> device achieves a power conversion efficiency of 6.1%, which is much higher than that of PBDB-T/<b>PNDI–TVT</b> 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.