Exploring the effects of axial halogen substitutions of subphthalocyanine on the charge transfer nature in subPC/C60 solar cells

Abstract

Boron subphthalocyanine chloride (Cl-subPC) and its derivatives have been extensively studied due to their unique structures and attractive optical performance in organic solar cells (OSCs). However, how axial halogen substitutions of Cl-subPC influence the delicate interfacial charge transfer (CT) is still an open question. In this work, by utilizing long range corrected and constrained density functional theory with polarizable continuum model, we have modelled X-subPC/C60 (X = F, Cl, Br) bi-molecular systems and studied the effect of axial halogen substitutions on CT and charge recombination (CR) rate constants (kCTs and kCRs) with Marcus theory. Fractional transfer charges and the effects of surrounding environment are obtained with high accuracy. The results indicate that cold and hot CTs play different roles to determine exciton dissociations, and the maximum kCTs in U configurations (C60 in the concave of subPC) are much larger than those in B configurations (C60 on the convex of subPC) due to better hybridizations between local excitation and CT states. Furthermore, the order of maximum kCTs in U/B configurations is Cl > F > Br / F > Br > Cl, respectively. Finally, the calculated lifetime (2.32 ps) of excitons in Cl-subPC/C60 B configurations matches with experiment (10 ps), and the much shorter lifetime of exciton in Cl-subPC/C60 U configuration implies that this molecular arrangement may not be dominant at the interface. The theoretical study here gives a deep insight about how halogen substitutions influence the CT nature in X-subPC/C60 OSCs.