Molecular Modification of A–π–D–π–A-Type Small-Molecule Donors for High-Performance Photovoltaics

Molecular engineering serves as a prevalent strategy in solar cells architecture toward robust, reliable, and highly efficient light-electricity conversion devices. Specifically, two well-known strategies, i.e., halogen substitution and π-spacer modification, are extensively introduced. However, the underlying photovoltaics mechanism on benzodithiophene terthiophene rhodamine (BTR) remains lacking. Herein, a combined approach of density functional theory (DFT) and time-dependent DFT calculations is systematically introduced to unravel the implication in terms of structure–property relationships. The results suggest that halogen substitution on BTR molecular backbone can effectively reduce the frontier molecular orbital energy levels of molecule. Moreover, extending the π-spacer can increase the conjugation length of the molecular backbone, which results in improving the photoelectric properties of small molecules. B₃, i.e., the addition of a pair of thiophene rings to the π-spacer of the BTR, with the lowest energy gap and reorganization energy, relatively small exciton binding energy, and the strongest light absorption spectra, is a promising candidate for the donor molecule. In addition, by combining these two modification strategies (i.e., chlorinated B₃), the overall performance of the new B₃-Cl molecule can be further improved compared to B₃. The findings provide a theoretical guidance for the rational design of novel A–π–D–π–A-type small molecules.