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The offsets between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) at the 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA)/chloroindium phthalocyanine (ClInPc) and PTCDA/zinc phthalocyanine (ZnPc) organic heterojunctions were estimated from a combination of X-ray and ultraviolet photoemission (XPS,UPS) measurements. This combined method allows an improved determination of the electronic structure of such organic/organic‘ interfaces due to the separate determination of the band bending (charge redistribution) following heterojunction formation. Both interfaces have large offsets in the onset for photoemission from their HOMO levels (PTCDA/ZnPc: 0.88 eV; PTCDA/CnlInPc: 0.93 eV). Using thin film absorbance data, the corresponding offsets in LUMO levels were estimated to be 0.66 eV (PTCDA/ZnPc) and 0.34 (PTCDA/ClInPc). The ZnPc/PTCDA interface showed a significant interface dipole (0.25 eV) while the ClInPc/PTCDA contact was essentially dipole free.
Schlaf et al. (Fri,) studied this question.