Observation of strong band bending in perylene tetracarboxylic dianhydridethin films grown on SnS2

Perylene tetracarboxylic dianhydride (PTCDA) thin films were grown in sever al steps on tin disulfide (SnS2) single crystals and characterized by combi ned x-ray and ultraviolet photoemission spectroscopy (XPS), (UPS) in order to characterize the frontier orbital line-up and the interface dipole at th eir interface. Due to the large difference between the work functions of PT CDA (4.26 eV) and SnS2 (5.09 eV) this experiment represents a model system for the investigation of band bending related phenomena in organic semicond uctor 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 in terface dipole was detected which means that the PTCDA molecular orbitals a nd the SnS2 bands align at the vacuum level corresponding to the electron a ffinity rule. Our experiments clearly demonstrate the importance of an addi tional XPS measurement which (in most cases) allow the measurement of band bending with much higher accuracy than could be achieved in experiments car ried out by UPS alone. These experiments also show that, due to the differe nt depth sensitivity of high binding energy cutoff (secondary edge) and XPS core levels (or UPS valence bands), it is very important to grow relativel y thick overlayers in order to measure orbital alignment and interface dipo le correctly. (C) 1999 American Institute of Physics. [S0021-8979(99)07015- 2].