How do we get across? Impact of bridging aromatic units on the optical, electronic, and photovoltaic properties of donor-acceptor polymers

A series of donor - pi - acceptor (D- pi -A) type polymers (P1, P2, and P3) containing benzodithiophene (BDT) as donor unit and benzothiadiazole (BT) as acceptor unit were designed and synthesized by incorporating diverse electron-rich pi bridges as thiophene (T), selenophene (Se) and thieno[3,2-b] thiophene (TT) via Pd-catalyzed Stille cross-coupling reaction. The effect of the various ff-bridges on the optical and electrochemical properties of P1, P2, and P3 was examined, and for P1 and P2, the effect of pi-bridges on photovoltaic properties was examined. Density functional theory (DFT) calculations were performed on tetramer model structures to understand the effect of pi-bridges on the structural, electronic, and optical properties of the resulting polymers. Insertion of different pi-bridges results in polymers with up-shifted highest occupied molecular orbital (HOMO). Thiophene pi-bridged polymer P1:PC71BM showed the best photovoltaic performance with a power conversion efficiency (PCE) of 3.48 %, a short-circuit density (JSC) of 9.19 mA/cm2, an open-circuit voltage (VOC) of 0.75 V, and a fill factor (FF) of 50.30 %. The highest PCE obtained from a P2:PC71BM-based photovoltaic device was 2.42 %.