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

Chloroindium phthalocyanine (ClInPc) thin films were grown by vapor deposit ion on highly oriented pyrolytic graphite (HOPG) substrates. The alignment of the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUM O) of the phthalocyanine layer relative to the Fermi level of the HOPG subs trate was determined by combined X-ray and ultraviolet photoemission spectr oscopy(XPS, UPS) measurements. The measurements revealed that the electroni c 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. There fore, the influence of final-state screening shifts on orbital alignment me asurements at organic/semiconductor interfaces with photoemission spectrosc opy (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 insignif icant role within the accuracy that can be achieved in such measurements. ( C) 1999 Elsevier Science B.V. All rights reserved.