Improving self-assembly behavior and photovoltaic performance of the indacenodithiophene-based small molecules via increasing dipole moment of the terminal group

Abstract Two small molecules (SMs) of IDT(BT-T3Cz) 2 and IDT(DFBT-T3Cz) 2 with a D(A-Ar) 2 framework were designed and synthesized as a novel class of photovoltaic donor materials, which contain a donor (D) core of indacenodithiophene (IDT), two acceptor (A) arms of benzothiadiazole (BT) or fluorinated benzothiadiazole (DFBT), and two terminals of 3-carbazole (Cz). The impacts of 3-carbazole and fluorine atom on the optical absorption, electrochemical property, hole mobility, film morphology, and solar cell performance were primarily studied. A broad absorption profile in a range of 350–750 nm and a low HOMO energy level around −5.30 eV were observed for both SMs. In comparison to the reported SM of IDT(BT-Th 2 ) 2 with 5-alkyl-thiophene terminal, both SMs with 3-carbazole terminal exhibited stronger molecular packing and better self-assembly behavior owing to the increasing dipole moment of carbazole. Furthermore, the IDT(DFBT-T3Cz) 2 showed a higher power conversion efficiency (PCE) of 5.99% with an increasing fill factor ( FF ) of 59.6% than IDT(BT-T3Cz) 2 in their solar cells under the illumination of AM 1.5G at 100 mWcm −2 . To the best of our knowledge, this is one of the highest PCE value reported for the IDT based SMs in solution-processed bulk heterojunction solar cells. And this work provides an efficient strategy to improve self-assembly behavior and photovoltaic performance via increasing dipole moment of the terminal groups.