Highly efficient polymer solar cells with improved molecular stacking and appropriate active layer morphology by side chain engineering of small molecular acceptors

Abstract For indacenodithiophene (IDT) core-based small molecule acceptors (SMAs), side chains attached onto the sp3 carbon of the IDT unit can not only improve the film-forming characteristics of SMAs, but also adjust the molecular stacking, optical properties, and morphology of the active layer of polymer solar cells (PSCs). In this paper, we obtained an efficient IDT core-based SMA C4Ph-IDT-DFIC with a phenyl at the tail of alkyl side chain. Compared with IDIC-4 F with the phenyl group inside the alkyl chain, the C4Ph-IDT-DFIC exhibited different absorption spectras, energy levels, molecular stacking and morphological properties when blended with donor material PM6. And the C4Ph-IDT-DFIC showed a wider absorption range in the 500–806 nm with lower optical band gap of 1.54 eV (~40 nm red-shifted than IDIC-4 F), due to the improved molecular stacking. Moreover, PM6:C4Ph-IDT-DFIC devices showed higher and more balanced charge mobilities. PSCs based on PM6:C4Ph-IDT-DFIC exhibited a power conversion efficiency (PCE) of 10.47% with a short-circuit current (Jsc) of 17.78 mA cm−2, while the PM6:IDIC-4 F device yielded a PCE of 9.26% with a Jsc of 15.18 mA cm−2. Our work provides a feasible molecular design strategy to adjust the molecular stacking and improve photovoltaic performance of PSCs.