Observation of strong band bending in perylene tetracarboxylic dianhydride thin films grown on $SnS2$

Perylene tetracarboxylic dianhydride (PTCDA) thin films were grown in several steps on tin disulfide $(SnS2)$ single crystals and characterized by combined x-ray and ultraviolet photoemission spectroscopy (XPS), (UPS) in order to characterize the frontier orbital line-up and the interface dipole at their interface. Due to the large difference between the work functions of PTCDA (4.26 eV) and $SnS2$ (5.09 eV) this experiment represents a model system for the investigation of band bending related phenomena in organic semiconductor heterojunctions. Our results show that the equilibration between the Fermi levels of both materials in contact is achieved almost solely by band bending (bulk charge redistribution) in the PTCDA layer. No significant interface dipole was detected which means that the PTCDA molecular orbitals and the $SnS2$ bands align at the vacuum level corresponding to the electron affinity rule. Our experiments clearly demonstrate the importance of an additional XPS measurement which (in most cases) allow the measurement of band bending with much higher accuracy than could be achieved in experiments carried out by UPS alone. These experiments also show that, due to the different depth sensitivity of high binding energy cutoff (secondary edge) and XPS core levels (or UPS valence bands), it is very important to grow relatively thick overlayers in order to measure orbital alignment and interface dipole correctly.