Localization-dependent charge separation efficiency at an organic/inorganic hybrid interface

Highlights: • Competing relaxation channels after optical excitation of an organic/inorganic hybrid interface by photoluminescence and tr-EST. • Two independent relaxation pathways via the molecular π-systems, decoupled by the spiro linkage of SP6. • Excitons of the 6P unit recombine under light emission and diffuse to the ZnO surface for charge separation. • The other excited state solely decays to a long-lived dark state. By combining complementary optical techniques, photoluminescence and time-resolved excited state absorption, we achieve a comprehensive picture of the relaxation processes in the organic/inorganic hybrid system SP6/ZnO. We identify two long-lived excited states of the organic molecules of which only the lowest energy one, localized on the sexiphenyl backbone of the molecule, is found to efficiently charge separate to the ZnO conduction band or radiatively recombine. The other state, most likely localized on the spiro-linked biphenyl, relaxes only by intersystem crossing to a long-lived, probably triplet state, thus acting as a sink of the excitation and limiting the charge separation efficiency.