Enhancing the photovoltaic performance of quinoxalino[2,3-b′]porphyrinatozinc-based donor–acceptor copolymers by using 4,4′-bipyridine as a linear bidentate ligand additive

In this study, two donor–acceptor polymers, P(QP-TT) and P(QP-TT-Zn), were synthesized based on the 2,2′:5′,2′′-terthiophene (TT) donor unit and quinoxalino[2,3-b′]porphyrin (QP) or quinoxalino[2,3-b′]porphyrinatozinc (QP-Zn) acceptor unit. Bulk heterojunction polymer solar cells (PSCs) were fabricated. An alternative strategy for optimizing the photovoltaic devices by using a linear bidentate ligand additive, 4,4′-bipyridine (Bipy), is reported. The Bipy additive is totally different from the most widely used solvent additives in that it can form a coordination effect with metalloporphyrin-based polymers and leads to more ordered arrangements of polymers. Photovoltaic devices based on P(QP-TT-Zn) showed a notable improvement of power conversion efficiency (PCE) when a small amount of Bipy was added to the blend solution. Further optimization combined with thermal annealing and methanol treatment showed a significant improvement of PCE from 0.85 to 3.51%, which is the highest value among PSC devices based on porphyrin-based conjugated polymers. However, application of the same optimization methods to devices based on P(QP-TT) showed no improvements of photovoltaic performance. Morphology analysis revealed that blend films of P(QP-TT-Zn):PC71BM showed desired interpenetrating networks with some ordered packing in the thin film after adding the Bipy additive. The results suggest that the improvements of the photovoltaic performance are potentially due to the coordination effect between the nitrogen of Bipy and central zinc of P(QP-TT-Zn).