Side Chain Engineered Naphthalene Diimide-Based Terpolymer for Efficient and Mechanically Robust All-Polymer Solar Cells

The recent evolution of new polymer acceptors (P_As) using small-molecule building blocks with high light absorption has significantly increased the power conversion efficiency (PCE) of all-polymer solar cells (all-PSCs), but their mechanical properties are typically poor. Thus, poly­[[N,N′-bis­(2-octyldodecyl)-naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)] (N2200) is still considered one of the most successful P_As in all-PSCs. Herein, we report the development of new naphthalene dimide-based P_As (NDI-P_As) that enable the achievement of both superior PCEs and mechanical robustness of all-PSCs compared to those of N2200-based devices. Our approach is very simple and effective for constructing a series of P_As [PNDIHD/DT-x, where x = 0–1], consisting of two NDI units with different side chains (2-hexyldecyl (HD) and 2-decyltetradecyl (DT)). The PNDIHD/DT-0.41-based all-PSCs with the poly­[(2,6-(4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)-benzo­[1,2-b:4,5-b′]­dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis­(2-ethylhexyl)­benzo­[1′,2′-c:4′,5′-c′]­dithiophene-4,8-dione))] (PBDB-T) polymer donor achieve a PCE of 9.3%, which outperforms the N2200-based device (PCE = 7.7%). This is mainly attributed to the enhanced charge generation and transport abilities of the PNDIHD/DT-0.41-based all-PSCs, as a result of optimal domain size and purity as well as high electron mobility. Importantly, the PBDB-T:PNDIHD/DT-0.41 blend shows excellent mechanical robustness, with a crack onset strain (COS) of 30% and a toughness of 7.5 MJ m^–3. In addition, a flexible polymer solar cell (FPSC) device with the PBDB-T:PNDIHD/DT-0.41 blend shows a high initial PCE of 6.73%, which is maintained over 6% even after bending 1500 times.