Influence of altering chlorine substitution positions on the photovoltaic properties of small molecule donors in all-small-molecule organic solar cells

In-depth understanding of the structure-property relationship in organic solar cells (OSCs) is of great importance in the development of the OSC field. In this study, we designed and synthesized two new chlorinated small molecule donor materials, named SM1-alpha-Cl (with alpha-chlorinated thiophene as a conjugated side chain) and SM1-beta-Cl (with beta-chlorinated thiophene as a conjugated side chain), by a side chain isomerization strategy. The morphology results indicate that the two small molecules present different aggregation characteristics, and the SM1-beta-Cl:Y6 blend film displays enhanced molecular stacking properties, benefiting the charge transport. Besides, the ultrafast spectroscopic study indicates a higher charge transfer (CT) state yield in the SM1-beta-Cl based blend. Therefore, the devices based on SM1-beta-Cl:Y6 demonstrate efficient exciton dissociation, balanced electron/hole mobility and a higher power conversion efficiency (PCE) of 12.14%, while SM1-alpha-Cl:Y6 based devices give an inferior PCE of 8.93%. These results imply that side-chain isomerization of small molecule donors could be an effective method to fine-tune the morphological features and improve the photovoltaic performance.