Impact of fluorination on the energy level alignment of an F ZnPc/MAPbI₃ interface

We have studied interactions at an interface between a Methylammonium Lead Iodide (MAPbI₃) surface and zinc-phthalocyanine molecules with F substituting peripheral H (F ZnPc; n = 4, 8, 12, and 16) by employing hybrid density functional theory (DFT) based simulations. These calculations show that F ZnPc molecules form a stable interface with MAPbI₃, whose binding strength is comparable to that of the un-substituted (ZnPc) case. As a consequence of fluorination, an increase in the ionization potential/electron affinity (i.e., a systematic lowering of molecular energy levels), as well as interfacial charge transfer, is observed whose magnitude depends upon the degree of fluorination. In contrast to the common belief of unfavorable hole transfer for excessive fluorination, our work unveils that the valence band offset remains favorable for all ranges of substitution (n); thus, hole transfer from MAPbI₃ to F ZnPc is facilitated while the electron transfer process is suppressed. This unusual behavior originates from the intermolecular interaction and substrate-to-molecule electron transfer at the heterojunction, which gradually suppresses the downward shift of F ZnPc energy levels by increasing the value of n. Given the beneficial impacts of fluorination, such as hydrophobicity, our work provides valuable insight for exploiting stable F ZnPc towards high-efficiency perovskite solar cells.