Halogen-Free Polymerized Small Molecular Acceptor with Thiophene[3,2-<i>b</i>]thiophene as a π-Bridge for Efficient All-Polymer Solar Cells

Polymerized small molecular acceptors (PSMAs) have undergone a rapid development in the past few years, and developing halogen-free PSMAs with simple π-bridges is important for the environment and industrial production. Herein, two PSMAs, namely, PY2T and PYTT, were designed by employing 2,2′-dithiophene and thiophene[3,2-b]thiophene (TT) as π-bridges, respectively. The rigid planar structure of TT causes a red-shift in PYTT absorption, expanding the absorption range. Cyclic voltammetry and theoretical calculation results both demonstrated that PYTT had increased frontier energy levels relative to PY2T, due to the stronger electron-donating property of TT. The PM6:PYTT blend showed a higher charge separation efficiency and mobilities, much finer morphology, tighter π–π stacking, and larger crystalline coherence length. The power conversion efficiency of PYTT (15.38%) was higher than that of PY2T (13.99%), principally due to the increase of the short-circuit current density. This work indicates that TT is a promising π-bridge for designing high-performance halogen-free PSMAs.