Introducing Electron-Withdrawing Linking Units and Thiophene π-Bridges into Polymerized Small Molecule Acceptors for High-Efficiency All-Polymer Solar Cells

A series of narrow bandgap polymer acceptors (PY5-BTZ, PY5-2TZ, PY5-PZ, and PY5-BT) were synthesized by copolymerizing a Y5-like small molecule acceptor (SMA) and different electron-withdrawing (A) units with thiophene π-bridges. Among the polymerized SMAs (PSMAs), the A-unit is bifluoro-benzotriazole for PY5-BTZ, thiazolo[5,4-d]thiazole for PY5-2TZ, pyrazine for PY5-PZ, and benzothiadiazole for PY5-BT. The four PSMA films possess narrower bandgaps of 1.43–1.45 eV. Interestingly, PY5-BTZ shows a higher-lying LUMO energy level of −3.72 eV (−3.76 eV for PY5-2TZ, −3.75 eV for PY5-PZ, and −3.78 eV for PY5-BT) and a stronger absorbance of 1.5 × 105 cm–1 (1.4 × 105 cm–1 for the other three PSMAs). The all-polymer solar cells (all-PSCs) with PY5-BTZ as the polymer acceptor and PBDB-T as the polymer donor exhibit a peak power conversion efficiency of 14.82% with a high open circuit voltage of 0.92 V, which benefits from the higher-lying LUMO level and stronger absorbance of the PY5-BTZ polymer acceptor. The results indicate that introducing weak electron-withdrawing A-units and thiophene π-bridges could be an effective approach in designing and synthesizing high-performance PSMA polymer acceptors for high-efficiency all-PSCs in the future.