An Electron Acceptor with Broad Visible–NIR Absorption and Unique Solid State Packing for As-Cast High Performance Binary Organic Solar Cells

A novel acceptor–donor–acceptor (A–D–A) type electron acceptor 6TIC-4F with terthieno[3,2-b]thiophene (6T) as the core unit is rationally designed and synthesized, which exhibits an extraordinarily narrow bandgap (≈1.24 eV) and strong absorption between 650 and 1000 nm. X-ray crystallographic analysis reveals that it has unique intermolecular π–π stacking. The solar cells based on the as-cast 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):6TIC-4F binary blends exhibit an excellent power conversion efficiency (PCE) of 11.14% with a high J_SC of 23.00 mA cm⁻², and a high fill factor of 0.67, which represents one of the best PCE values for low bandgap (E_g < 1.3 eV)–based organic solar cells.