Energy level tuning at inorganic/organic semiconductor heterojunctions

Abstract

In this work we investigate the mechanisms responsible for the energy level alignment at inorganic and organic semiconductors interfaces with photoelectron spectroscopy. We focus on the different contributions that lead to a substantial work function increase (up to 2.5 eV) when depositing thin layers of organic acceptor molecules [1,4,5,8,9,12-hexaazatriphenylenehexacarbonitrile (HATCN) and 2,2'-(perfluoronaphthalene-2,6- diylidene)dimalononitrile (F6-TCNNQ)] on two different inorganic semiconductors (ZnO and GaN). We discuss models to describe this work function increase, which take into account the role of bulk donor concentration of the inorganic semiconductors, their surface state density, and the distance between the inorganic semiconductor and the adsorbed molecules, and we emphasize the importance of surface states on the inorganic semiconductor. The absence of any adsorption-induced core level features for F6-TCNNQ indicates fractional charge transfer, in contrast to HATCN, where additional core level features indicate integer charge transfer. Finally, we demonstrate the utility of this interlayer approach by changing the energy level alignment between the showcase hybrid system ZnO/Sp6