Asymmetric medium bandgap copolymers and narrow bandgap small-molecule acceptor with over 7% efficiency

Abstract To further investigate non-fullerene polymer solar cells based-thieno[2,3- f ]benzofuran (TBF) polymers, we designed and synthesized two medium bandgap TBF-based polymers, namely TBF-BDD and TBF-BT, containing alkoxyphenyl substituted TBF electron-donor unit, 1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo-[1,2- c :4,5- c ′]dithiophene-4,8-dione (BDD) and 4,7-di(thiophen-2-yl)-5,6-dioctyloxybenzo[ c ][1,2,5]thiadiazole (BT) electron-acceptor segment, respectively. When blended with ITIC (a n-type small molecule acceptor), two polymer:ITIC blends possess better complementary absorption than the absorption with PC 71 BM. The properties, including charge mobilities, and morphologies have been intensively investigated. The polymer solar cells based on TBF-BDD:ITIC and TBF-BT:ITIC (1:1, w/w ) exhibited promising power conversion efficiencies (PCEs) of 7.13% and 7.03%, respectively, under the illumination of AM 1.5 G, 100 mW/cm 2 , which are higher in comparison with fullerene-based devices. Up to now, these PCEs are the highest among TBF-based polymer photovoltaic devices.