Impact of an interface dipole layer on molecular level alignment at an organic-conductor interface studied by ultraviolet photoemission spectroscopy

Abstract

The effect of an interface dipole layer on the energy level alignment at organic-conductor interfaces is studied on a copper phthalocyanine (CuPc) monolayer/electric dipole layer/graphite system via ultraviolet photoemission spectroscopy (UPS) and metastable atom electron spectroscopy. An oriented monolayer of the OTi-phthalocyanine molecule, which has an electric dipole moment, is grown on graphite to yield a welldefined dipole layer with the vacuum side negatively charged. The CuPc monolayer is sequentially deposited on the dipole layer kept at 123 K. This weakly interacting system made of a very thin organic layer on top of a very thin dipole layer is in thermodynamic equilibrium. The UPS data from the system grown with and without the interface dipole layer show that the binding energy of the highest occupied state of the CuPc monolayer decreases when the dipole layer is inserted. The binding energy shift is in excellent agreement with the increase in vacuum level energy of the graphite substrate upon deposition of the dipole layer. The results show that the Fermi level of the CuPc shifts toward the valence states when the interface dipole layer is inserted.

Different approaches to adjusting band offsets at intermolecular interfaces

“…For CuPc deposited on Au [23] (Figure 4a), we found qualitative agreement with prior photoemission [8,[22][23][24][25] and inverse photoemission [23,26] experimental data. The positions of the band edges, as determined by photoemission and inverse photoemission at room temperature, do not directly demonstrate that CuPc is strongly more p-type relative to P(VDF-TrFE).…”

“…Although copper phthalocyanine is considered a planar molecule with no permanent electric dipole [22], the molecular orbital alignment of adsorbed copper phthalocyanine is affected by both interface dipoles [8,13] and molecular orientation [13,22] while interactions or charge transfer influence the highest occupied (HOMO) to lowest unoccupied (LUMO) gap [23]. Indeed, with adsorption on metallic substrates copper phthalocyanine exhibits an electric dipole as the molecular center copper atom is not in the same plane as the surrounding ligand, and adopts a position that is typically closer to the interface [27].…”

“…Both polyaniline [12,[18][19][20][21] and copper phthalocyanine (CuPc) thin films [8,13,[22][23][24][25][26][27] (schematically illustrated in Figure 5) can be produced by vapor deposition compatible with ultrahigh vacuum photoemission. These films can be grown very thin (100 Å or less) and are free of solvent contamination.…”

“…For many large molecular adlayers, including a number of organic and metal-organic species, the energy level alignment (of the adsorbate) is dependent upon the interfacial electronic structure and the interfacial dipole layer, as has been readily demonstrated for many large molecules [5][6][7], including the metal phthalocyanines [8]. Charge injection and the dipoles at the electrode interface are surprisingly complex [9] because of interface chemistry.…”

Low-density band-gap states in pentacene thin films probed with ultrahigh-sensitivity ultraviolet photoelectron spectroscopy

“…Many studies using ultraviolet photoelectron spectroscopy ͑UPS͒ have been performed to clarify the ELA mechanism on weakly interacting organic interfaces, and have shown that the vacuum level ͑VL͒ misalignment and drastic variation of the Fermi level ͑E F ͒ in the organic band gap appear in various organic interface systems. [1][2][3][4][5][6][7][8] Because thermal equilibrium concept strictly requires that the Fermi levels of the organic film and the electrode coincide at the interface after contact, our understanding of the ELA mechanism still lacks consistency with this requirement. 5,6 The mystery on the ELA mechanism must be related to the density of gap states ͑DOGS͒ to control the E F .…”