Long-Range Energy Level Shifts Induced by Single Impurity Molecules in C₆₀ Thin Films

Organic photovoltaics (OPV) is a promising technology for low-cost, flexible solar cells with low embodied energy. However, the efficiency remains low due to high exciton binding energies. The main driving force behind exciton dissociation is the energy landscape around the donor-acceptor interface. To optimize device efficiency, we therefore need to better understand the pathways of exciton dissociation at interfaces in OPV materials and how they correlate with the energy landscape. We use scanning tunneling microscopy and spectroscopy to study model systems consisting of thin films of C₆₀ with single molecules of pure and fluorinated zinc phthalocyanine (ZnPc, F₄ZnPc, and F₈ZnPc) added. We measure how a molecule affects the energy levels in the surrounding C₆₀ matrix; they shift by up to 150 meV depending on the degree of fluorination of the impurity molecule. This shift prevails over at least several C₆₀ molecules from the impurity. This large and long-range shift induced by the phthalocyanine, and heavily influenced by the fluorine atoms, opens up new possibilities for controlled design of the energy landscape in the OPV heterojunction to optimize charge transfer efficiency.