Positional Effects of Alkyl Chain on Photovoltaic Performances of Quinoxaline-Based Polymers

Controlling the alkyl chains is an efficient and simple method for modifying various aspects of conjugated polymers. In particular, we focused on the positioning effects of the alkyl chains on the photovoltaic features of p-type polymer donors. To achieve this, two D–A-type quinoxaline-based polymers with the same chemical composition, except for the location of the alkyl chains, were designed and synthesized. A basic D–A-type polymeric structure was generated by assembling electron-donating benzodithiophene (BDT) and monocyanated quinoxaline-based monomers through a thiophene bridge. Subsequently, the selective incorporation of branched 2-butyloctyl chains into either the thiophene bridges or thiophene side groups at the 2,3-positions of the quinoxaline unit afforded two target polymers, PBT-dQxCN and PBT-uQxCN, respectively. Most of all, changing the position of the alkyl chains leads to a significant alteration in the molecular planarity, thereby making a difference in the diverse properties of the polymers. Owing to the favorable optical, electrochemical, and morphological properties of PBT-uQxCN caused by the more planar conformation, the conventional nonfullerene acceptor-based device made from PBT-uQxCN exhibited higher power conversion efficiency (PCE) of 11.51%, compared to that based on PBT-dQxCN (9.11%). These results clearly reveal the importance of controlling the alkyl chain positions to enhance the photovoltaic characteristics of quinoxaline-based polymers.