The end-capped group effect on dithienosilole trimer based small molecules for efficient organic photovoltaics

Abstract

Three new small molecules, FBT–tDTS, DFBT–tDTS and RHO–tDTS, were designed and synthesized by using a rigid dithienosilole trimer (tDTS) as the novel donor unit and 5-fluorobenzothiadiazole (FBT), 5,6-difluorobenzothiadiazole (DFBT) and 3-ethylrhodanine (RHO) as acceptor units, respectively. According to the density functional theory calculations, RHO–tDTS has a more rigid structure due to the linkage between the donor and acceptor units. As a result, RHO–tDTS exhibits a red-shifted absorption spectrum and a higher hole mobility in comparison with FBT–tDTS and DFBT–tDTS. Small molecule solar cells based on RHO–tDTS show a power conversion efficiency of 7.56%, significantly performing better than the other two analogues. The mechanism of the good photovoltaic performance of RHO–tDTS was discussed. We also demonstrated that the end-capped groups on small molecules play an important role in tuning the performance of the organic photovoltaic devices.