Boron-based non-fullerene small molecule acceptors via nitrogen substitution: a theoretical study

Acceptors play vital roles in absorbing sunlight and consequently producing charge in organic solar cells (OSCs). Boron-based non-fullerene acceptors have received particular attention due to their tunable electronic structure and high optoelectronic performance. Herein, a set of boron-based acceptors (M-BNP4P-1 and its nine derivatives) with double B ← N bridged bipyridine unit were theoretically studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The calculation results showed that nitrogen substitutions could precisely tune the energy levels of the frontier molecular orbitals in a wide range. The designed acceptors A1a, A1b, A2a, and A3a exhibit enhanced electron mobility of up to two orders of magnitude (about a few tenths of cm² V⁻¹ S⁻¹) compared to the parent molecule M-BNBP4P-1 (A0). When nitrogen is substituted at the para position of the nitrogen atom of the pyridine ring of A0, the resulting acceptor A2d exhibits the maximum absorption wavelength of 727 nm with a considerably large oscillator strength. Moreover, the interfacial properties of PTB7-Th/A0 and PTB7-Th/A2d composed of A0 and A2d paired with the donor PTB7-Th have been systematically studied. Importantly, PTB7-Th/A2d exhibits charge transfer states as high as 67%, facilitating charge transfer via a direct excitation mechanism. Furthermore, the hot exciton mechanism and intermolecular electric field mechanism are more favourable in PTB7-Th/A2d. Our results provide a new design strategy to tune the optoelectronic properties of organic semiconductors and a valuable skeleton for the design of new and high-performance organoboron small molecular acceptors.