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 JSC 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 (Eg < 1.3 eV)–based organic solar cells.