Benzotrithiophene polymers with tuneable bandgap for photovoltaic applications

Four benzotrithiophene (BTT) based donor–acceptor polymers have been prepared by alternating with diketopyrrolopyrrole (DPP), thiazolo[5,4-d]thiazole (TTz), thieno[3,4-c]pyrrole-4,6-dione (TPD) and thiadiazolo[3,4-g]quinoxaline (DTBTQx). The BTT-based polymers possess good solubility in common organic solvents and excellent thermal stability. PBTTDPP and PBTTDTBTQx possessed an optical bandgap of 1.27 eV and 1.20 eV, with absorption spectra extending to the near-infrared region. The bulk-heterojunction solar cells delivered a highest PCE of 1.39%, with a short circuit current of 5.46 mA cm⁻², an open-circuit voltage of 0.51 V and a fill factor of 0.50.