Absence of final-state screening shifts in photoemission spectroscopy frontier orbital alignment measurements at organic/semiconductor interfaces

Abstract

Chloroindium phthalocyanine (ClInPc) thin films were grown by vapor deposition on highly oriented pyrolytic graphite (HOPG) substrates. The alignment of the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO) of the phthalocyanine layer relative to the Fermi level of the HOPG substrate was determined by combined X-ray and ultraviolet photoemission spectroscopy (XPS, UPS) measurements. The measurements revealed that the electronic structure of this interface is almost free of band bending, which results in only very weak shifts of the ClInPc-related XPS core-level peaks. Therefore, the influence of final-state screening shifts on orbital alignment measurements at organic/semiconductor interfaces with photoemission spectroscopy (PES) can be investigated in this system. The comparison of our results with PES measurements of monolayers and multilayers of xenon on palladium substrates revealed that final-state screening plays no or only an insignificant role within the accuracy that can be achieved in such measurements.