HOMO/LUMO alignment at PTCDA/ZnPc and PTCDA/ClInPc heterointerfaces determined by combined UPS and XPS measurements

The offsets between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) at the 3,4,9,10-perylenetetraca rboxylic dianhydride (PTCDA)/chloroindium phthalocyanine (ClInPc) and PTCDA /zinc phthalocyanine (ZnPc) organic heterojunctions were estimated from a c ombination of X-ray and ultraviolet photoemission (XPS,UPS) measurements. T his combined method allows an improved determination of the electronic stru cture of such organic/organic' interfaces due to the separate determination of the band bending (charge redistribution) following heterojunction forma tion. Both interfaces have large offsets in the onset for photoemission fro m their HOMO levels (PTCDA/ZnPc: 0.88 eV; PTCDA/CnlInPc: 0.93 eV). Using th in film absorbance data, the corresponding offsets in LUMO levels were esti mated to be 0.66 eV (PTCDA/ZnPc) and 0.34 (PTCDA/ClInPc). The ZnPc/PTCDA in terface showed a significant interface dipole (0.25 eV) while the ClInPc/PT CDA contact was essentially dipole free.