Conjugated Side-Chain-Isolated D–A Copolymers Based on Benzo[1,2-b:4,5-b′]dithiophene-alt-dithienylbenzotriazole: Synthesis and Photovoltaic Properties

Conjugated side-chain-isolated D–A copolymers, based on the donor unit of benzodithiophene (BDT) with a thiophene-conjugated side chain, thiophene π bridge, and the acceptor unit of benzotriazole (BTA) with or without fluorine substitution (PBDT-FBTA and PBDT-HBTA), were designed and synthesized for elucidating their structure–property relationships. The copolymer films demonstrated well-defined absorption peaks with steep absorption edges, consistent with their rigid and ordered structures in the solid films. The substitution of a thiophene-conjugated side chain on the BDT unit in the copolymers aroused 15-nm red-shifted absorption in comparison with its polymer analogues with alkoxy side chains on the BDT unit. Compared to PBDT-HBTA, PBDT-FBTA with two-fluorine-atom substitution on the BTA unit demonstrated a lower highest occupied molecular orbital energy level, higher hole mobility, and significantly better photovoltaic performance. A polymer solar cell (PSC) based on PBDT-FBTA/PC70BM (1:2, w/w) with a 5% 1,8-diiodooctane additive displayed a power conversion efficiency (PCE) of 6.0% with a Jsc of 11.9 mA cm–2, a VOC of 0.75 V, and a fill factor of 67.2%, under the illumination of AM1.5G, 100 mW cm–2. Even at a thicker active layer of 400 nm, the PSC still demonstrated a higher PCE of 4.74%. The results indicate that PBDT-FBTA is a promising polymer donor material for future application of large-area PSCs.