Dithienobenzoselenadiazole-Based Polymer Donors with Tuned Side Chains for Efficient Polymer Solar Cells

Two polymer donors with dithieno[3,2-e:2 ',3 '-g]-2,1,3-benzosele-nadiazole (DTBSe) as electron-deficient (A) units and alkylthiophene derivatives as conjugated pi units between A units and electron-rich units, named as PSe-BO and PSe-HD, respectively, have been designed and synthesized, and the effects of branched alkyl side chains of alkylthiophene derivatives on molecular aggregation, photophysical properties, and photovoltaic performance have been studied. The results indicate that PSe-HD with 2-hexyldecyl side chains possesses a lower HOMO energy level, a smaller bandgap, and a more complementary absorption spectrum than PSe-BO with 2-butyloctyl side chains. Moreover, the PSe-HD:Y6 blend film also shows a better nanofibrous structure and a more orderly molecular orientation than the PSe-BO:Y6 blend film, which leads to higher charge mobility, more balanced charge transport, and less charge recombination of the former. Therefore, the polymer organic solar cell based on PSe-HD:Y6 blends achieves a higher power conversion efficiency of 14.85% than the device based on PSe-BO:Y6 blends owing to thoroughly improved photovoltaic parameters. This study provides an efficient strategy to design polymer donors by introducing DTBSe as A units and synchronously applying side-chain engineering.