Electronic, optical, and charge transport properties of dimerized small-molecule acceptors: the role of end-group engineering

Dimerized small-molecule acceptors (DSMAs) have attracted increasing attention in organic solar cells (OSCs) due to the advantages of long-term morphology stability and exceptional repeatability. However, the power conversion efficiencies of the DSMA-based OSCs are highly dependent on the dimerization modes and the underlying structure-performance relationship remains unclear. Here, we have revealed the role of end-group (EG) engineering of the A-D-A small-molecule acceptors (SMAs) in tuning the electronic, optical, and electron transport properties of vinyl-bridged DSMAs by multiscale theoretical calculations. The results point out that the EG engineering can effectively modulate the lowest unoccupied molecular orbital (LUMO) electron density at the linkage atoms of the SMAs, leading to a broad range of super-exchange (SE) couplings for intramolecular electron transfer between two SMA units among the studied DSMAs. Consequently, the LUMO energy and distribution are greatly changed, which further change the excited state energy and oscillator strength. In addition, the different EGs have important influences on the intermolecular electronic couplings and connectivity. Notably, compared to the previously reported DSMA of BB-V, the new-designed NB-V demonstrates simultaneous improvements in light absorption and electron mobility due to well-balanced intramolecular and intermolecular electronic couplings. This work provides helpful insights into the development of DSMAs for high-efficiency OSCs.