Effect of alkyl side chains of twisted conjugated polymer donors on photovoltaic performance

This is an exciting topic on maximizing the photovoltaic performance for a given conjugated skeleton by optimizing the side chains. In this work, we report two conjugated polymers (P3TCO-D and P3TCO-DMO) with the same conjugated backbone, the only difference in alkyl chains of carbonyl units (n-decyl and 3,7-dimethyloctyl). Both polymers have similar molecular weights, polydispersity indexes, and molecular energy levels. The P3TCOD has a stronger aggregation behavior than P3TCO-DMO. Interestingly, both pure polymer films have similar surface morphology. After blending with an electron acceptor of IT-4F, P3TCO-D: IT-4F film shows remarkable increased surface roughness of 23.2 nm and phase-separation. In contrast, 3TCO-DMO: IT-4F film displays slightly increased surface roughness of 3.82 nm and good phase-separation. In polymer solar cells, the P3TCO-D-based device gives a decent power conversion efficiency (PCE) of 10.1%, while the P3TCO-DMO-based device produces a better PCE of 12.8%. This work demonstrates that the optimization of side chains in the twisted conjugated polymers is very important to improve their photovoltaic performances.