Near-infrared response thienoisoindigo-based small molecule for solution-processed bulk-heterojunction solar cells

5′-Hexyl-2,2′-bithiophene end-capped thienoisoindigo-based small molecule (TIDO2T) is synthesized by Stille coupling reaction. The TIDO2T exhibits good solubility in common organic solvents such as THF, toluene, dichlorobenzene and chloroform. The UV–vis absorption spectrum of TIDO2T shows wide absorption between 427 nm and 871 nm in thin solid film, and the optical band-gap calculated from absorption onset was 1.42 eV. Bulk heterojunctions (BHJ) solar cell was fabricated with TIDO2T as the donor and 6,6-phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor by simple solution spin-coating processing. The device obtained a power conversion efficiency of 1.14% with VOC 0.44 V, JSC 7.76 mA cm−2 and FF 33% under AM 1.5G solar simulator. The external quantum efficiency (EQE) curve extends to near-infrared region (around 900–1000 nm).