Energy level alignment at interfaces of organic semiconductor heterostructures

We have used ultraviolet photoelectron spectroscopy (UPS) to investigate the interfaces of two organic semiconductor heterostructures. UPS was used to determine the relative energies of the highest occupied molecular orbitals of the organic semiconductors, and to measure the interface dipoles at each interface. The two systems studied were the 4-4′-N,N′-dicarbazolyl-biphenyl(CBP)/tris(8-hydroxy-quinoline)aluminum (Alq3) interface, and the copper phthalocyanine (CuPc)/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′biphenyl-4,4″diamine (α-NPD) interface. The assumption of a common vacuum level was found to be valid, within experimental uncertainty, for both interfaces.