Effect of geometrical orientation on the charge-transfer energetics of supramolecular (tetraphenyl)-porphyrin/C₆₀ dyads

The charge transfer (CT) excited state energies of donor-acceptor (D/A) pairs determine the achievable open-circuit voltage of D/A-based organic solar cell devices. Changes in the relative orientation of donor-acceptor pairs at the interface influence the frontier orbital energy levels, which impacts the dissociation of bound excitons at the D/A-interface. We examine the effect of relative orientation on CT excited state energies of porphyrin-fullerene dyads. The donors studied are base- and Zn-tetraphenyl porphyrin coupled to C₆₀ as the acceptor molecule in an end-on configuration. We compare the energetics of a few low-lying CT states for the end-on geometry to our previously calculated CT energetics of a co-facial orientation. The calculated CT excitation energies are larger for the end-on orientation in comparison to the co-facial structure by about 0.7 eV, which primarily occurs due to a decrease in exciton binding energy in going from the co-facial to the end-on orientation. Furthermore, changes in relative donor-acceptor orientation have a larger impact on the CT energies than changes in donor-acceptor distance.