Terminal group modification of alkylthio substituted small molecule donor materials for organic solar cells

Side chains and terminal groups have an important effect on photovoltaic performance of organic solar cells (OSCs) based on oligothiophene donor materials. In order to study the effect of alkylthio substituent and different terminal groups on photovoltaic properties, a series of small molecules were designed and synthesized. Molecule LS0 was modified by alkyl as side chains and 2-ethylrhodanine as terminal groups while LS1a, LS1b, LS2, LS3 were substituted with alkylthio side chains and capped with different terminal groups of 2-ethylrhodanine, 2-hexylrhodanine, 2-ethylhexyl cyanoacetate, and 1,3-indanedione, respectively. Compared with LS0, alkylthio substituted LS1a exhibited higher HOMO and LUMO due to more planar backbone. Capped with different terminal groups, LS1a, LS1b, LS2, and LS3 showed the different optical and photovoltaic properties. Among these molecules, OSCs based on LS1b showed the highest power conversion efficiency (PCE) of 8.38% while others showed PCE of 7.87%, 6.52%, 6.52%, and 4.65% for LSO, LS1a, LS2, and LS3, respectively. The results demonstrated that rational selecting side chains and terminal groups could be a promising strategy to promote the performance of small molecule-based OSCs.