The Renaissance of Oligothiophene‐Based Donor–Acceptor Polymers in Organic Solar Cells

Abstract The power conversion efficiencies (PCEs) of organic solar cells (OSCs) have increased rapidly owing to the development of non‐fullerene acceptors (NFAs). However, the development of polymer donors lags behind significantly. Currently, the polymer donors are dominated by a handful of thiophene‐substituted benzo[1,2‐b:4,5‐b']dithiophene (BDTT) polymers, which suffer from lengthy synthesis and high production cost. Compared with BDTT‐based polymers, oligothiophene‐based donor‐acceptor polymers feature much easier synthesis, which were the prevailing polymer donors in fullerene‐based OSCs, but almost disappeared in non‐fullerene OSCs. Herein, two oligothiophene‐based donor‐acceptor polymers (PTTz‐3HD and PTTz‐4HD) are reported to re‐evaluate this kind of polymer in non‐fullerene OSCs. Benefiting from the exquisite alkyl chain design, the polymer PTTz‐3HD exhibits more planar conformation, stronger aggregation, and higher crystallinity, which in turn contributes to the formation of an optimal active layer morphology when blended with NFA. As a result, a PCE of 16.1% and 16.7% is achieved by PTTz‐3HD in binary and ternary OSCs, respectively. Of particular note, the product of short‐circuit current density and fill factor of PTTz‐3HD is fully comparable to those of BDTT‐based polymers. These results suggest the renaissance of oligothiophene‐based donor‐acceptor polymers in OSCs and demonstrate a promising avenue to access high‐efficiency OSCs from low‐cost materials.