Abstract
The dielectric constant (εr) of non-fullerene acceptors is a key parameter in organic solar cells, significantly influencing exciton dissociation efficiency and charge recombination dynamics. Substituting bromine, characterized by moderate electronegativity and high polarizability, at the o-benzodipyrrole core of acceptors effectively modulates their optical properties, molecular packing, and dielectric constants. The asymmetric monobrominated CBrB–Cl acceptor exhibits a more red-shifted absorption spectrum, enhanced crystallinity, and a higher εr of 3.92, contributing to a high efficiency of 17.69% in the inverted PM6:CBrB–Cl device. Further, a selenium-incorporated CB–Se acceptor was selected to form a well-matched dual asymmetric acceptor alloy. The combination of heavy bromine and selenium atoms in the CBrB–Cl:CB–Se alloy increase the εr of the PM6:CBrB–Cl:CB–Se blend film to 4.23, which in turn reduces exciton binding energy, promotes efficient charge separation, and suppresses charge recombination, as evidenced by a faster charge separation time (τrise = 0.39 ps) and a longer charge carrier lifetime (τdecay = 309 ps). The inverted PM6:CBrB–Cl:CB–Se device achieved a high PCE of 18.33%, which is considered to be one of the highest among inverted OPV devices. This work reveals that bromine/selenium substitution enhances the dielectric constant and crystalline packing, thereby advancing molecular design and improving device performance. The asymmetric CBrB–Cl and CB–Se form a highly crystalline acceptor alloy with favorable morphology and enhanced dielectric constant. The inverted PM6:CBrB–Cl:CB–Se device achieves an impressive PCE of 18.33% with high thermal stability.