Molecular design of organic small molecules based on diindole–diimide with fused aromatic heterocycles as donors for organic solar cells

A series of diindole–diimide based small molecules with different aromatic end groups have been designed and investigated theoretically as donor materials for organic solar cells (OSCs). Their optical, electronic, and charge transport properties have been investigated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The calculation results showed that the designed molecules can lower the band gap and extend the absorption spectrum towards longer wavelengths. The optical, electronic, and charge transport properties of the designed molecules are affected by the introduction of different aromatic end groups. We have also predicted the mobility of molecules with benzo[ d ]thiazole and benzo[ c ]thiophene end groups. Their hole mobility values are 0.157 and 6.57 × 10 −2 cm 2 V −1 s −1 , respectively. Our results reveal that the designed molecules are promising hole transport materials and donors with intense broad absorption spectra for OSCs.