Experimental demonstration of the universal energy level alignment rule at oxide/organic semiconductor interfaces

Abstract

In this work, our experimental demonstration of the universal energy level alignment rule at oxide/organic semiconductor interfaces is reported. Photoemission spectroscopy is used to show the three different regimes of the energy level alignment: the lowest unoccupied molecular orbital (LUMO) is pinned to the substrate Fermi level at the extreme low end of work functions; the energy offset of the highest occupied molecular orbital (HOMO) follows the Schottky-Mott rule; the HOMO is pinned to the substrate Fermi level at the extreme high end of work functions. To demonstrate this, fullerene C${}_{60}$ was deposited on eight different types of transition-metal oxides, ZrO${}_{2}$, TiO${}_{2}$, NiO, Co${}_{3}$O${}_{4}$, CuO, V${}_{2}$O${}_{5}$, MoO${}_{3}$, and WO${}_{3}$, followed by in situ ultraviolet photoemission spectroscopy.