Improved Morphology and Efficiency of Polymer Solar Cells by Processing Donor-Acceptor Copolymer Additives

A novel wide-bandgap conjugated polymer PBTA-FPh based on benzodithiophene-alt-benzo[1,2,3]triazole as the main chain and a polar pentafluorothiophenyl (FPh) group in the side chain has been designed and synthesized. In comparison to the pristine polymer PBTA-BO that consists of nonpolar alkyl side chains, the resulting PBTA-FPh exhibits less pronounced aggregation while possessing analogous optical and electrochemical bandgaps. Contact angle measurements demonstrate that the surface energy can be enhanced by incorporating FPh moiety, leading to a better miscibility of PBTA-BO with PC71BM in the presence of a certain amount of PBTA-FPh. The photoactive layer of PBTA-BO:PC71BM:PBTA-FPh with weight ratio of 1:1.2:0.02% exhibits a percolated network with the fibrous features, as revealed by transmission electron microscopy measurements. Of particular interest is the significantly improved photovoltaic performances of polymer solar cell devices for which the power conversion efficiency is enhanced from 6.46% for the control device to 7.91% for device processed with PBTA-FPh as the polymeric additive. These observations indicate that introducing donor-acceptor type of polymeric additive comprising of polar groups in the side chain can be a promising strategy for the fabrication of high-performance polymer solar cells.