Tuning the absorption range of naphthothiophene diimide-based acceptors for organic solar cells

Abstract

In this research, we focused on the tuning of the absorption properties of acceptor materials based on naphthothiophene diimide (NTI) with a triad structures (NTI-π-NTI) by modifying the central π-unit. Novel NTI-based acceptors with a spirobifluorene (SF) unit (SF-NTIs) were designed and synthesized. Their absorption range (400–600 nm) were clearly different from that of previously reported acceptor with a NTI-indacenodithiophene-NTI structure (IDT-NTI, 650–800 nm). Although, the SF-NTI with the same solubilizing groups as IDT-NTI (2-ethylhexyl groups) is not soluble enough for solution processes and favorable molecular orientation properties for OSC devices, the 1-hexylheptyl substituted SF-NTI showed a good solubility and favorable “face-on” orientation in the active layer films. The fabricated OSCs showed power conversion efficiency as high as 5.2%, and excellent thermal stability. These results indicate that the absorption of the NTI-triads can be tuned by modifying the central π-unit and that their orientation nature and solubility can be controlled by altering the alkyl groups on the imide moieties. These properties allow us to develop efficient NTI-based acceptors with various absorption ranges.