Effect of halogen/chalcogen substitution on the dielectric constant of asymmetric acceptor alloy to improve the efficiency and stability of inverted organic photovoltaics.

The dielectric constant (ε r) 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 ε r 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 ε r 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 (τ rise = 0.39 ps) and a longer charge carrier lifetime (τ decay = 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.