Ulrich Hörmann, Stefan Zeiske, Soohyung Park, Thorsten Schultz, Sebastian Kickhoefel, Ullrich Scherf, Sylke Blumstengel, Norbert Koch, Dieter Neher

• Electroluminescence (EL) spectra of hybrid charge transfer states at metal oxide/organic type-II heterojunctions exhibit bias-induced spectral shifts. The reasons for this phenomenon have been discussed controversially and arguments for either electric field-induced effects or the filling of trap states at the oxide surface have been put forward. Here, we combine the results of EL and photovoltaic measurements to eliminate the unavoidable effect of the series resistance of inorganic and organic components on the total voltage drop across the hybrid device. For SnOx combined with the conjugated polymer [ladder type poly-(para-phenylene)], we find a one-to-one correspondence between the blue-shift of the EL peak and the increase of the quasi-Fermi level splitting at the hybrid heterojunction, which we unambiguously assign to state filling. Our data are resembled best by a model considering the combination of an exponential density of states with a doped semiconductor. Published under license by AIP Publishing.