Energy‐level alignment at metal–organic and organic–organic interfaces

Abstract

This article reports on the electronic structure at interfaces found in organic semiconductor devices. The studied organic materials are C60 and poly (para‐phenylenevinylene) (PPV)‐like oligomers, and the metals are polycrystalline Au and Ag. To measure the energy levels at these interfaces, ultraviolet photoelectron spectroscopy has been used. It is shown how the energy levels at interfaces deviate from the bulk. Furthermore, it is demonstrated that the vacuum levels do not align at the studied interfaces. The misalignment is caused by an electric field at the interface. Several effects are presented that influence the energy alignment at interfaces, such as screening effects, dipole layer formation, charge transfer, and chemical interaction. The combination of interfaces investigated here is similar to interfaces found in polymer light‐emitting diodes and organic bulk heterojunction photovoltaic devices. The result, the misalignment of the vacuum levels, is expected to influence charge‐transfer processes across these interfaces, possibly affecting the electrical characteristics of organic semiconductor devices that contain similar interfaces. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2549–2560, 2003

Formation of the Interfacial Dipole at Organic‐Organic Interfaces: C60/Polymer Interfaces

“…Note that the same alignment of the C 60 energy levels has been observed at its interfaces with oligomer homologues of poly(para-phenylenevinylene), indicating that CT -states of C 60 (at 4.5 eV) are involved in formation of a dipole at these interface as well. [9] The energy level alignment at interfaces of C 60 and F8 further supports this proposed model for organic-organic interfaces. As shown in Figure 3 the energy of the positive polaronic level of F8 falls at 5.2 eV, which is 0.7 eV higher than the energy of the CT -states of C 60 .…”

“…The examples that have been reported show typically marginal vacuum level shifts, unless the interface is composed of a donor-acceptor pair of materials. [7][8][9] Concerning donor-acceptor interfaces, a generalized description of the energetics at such organic-organic interfaces with the IDIS model has been proposed. [10] The interactions at such interfaces ware assumed sufficiently strong to induce a density of interfacial states that is large enough to accommodate for significant charge exchange across the interface and, in result, the formation of an interfacial dipole layer.…”

Polymer Solar Cells

“…In another study, however, deviations from this pinning behavior have been found [105]. Thus, the individual energy level alignments between organic/metal interfaces are critical [72,[106][107][108][109][110][111][112]. Interfacial dipoles formed at the organic semiconductor/electrode interface change the effective metal work function and thus affect the V OC as well [72,108,109,113,114].…”

Ambipolar Charge Transport in Films of Methanofullerene and Poly(phenylenevinylene)/Methanofullerene Blends

“…First, it has been shown by photoelectron spectroscopy that a dipole of some 0.25 eV, pointing towards the PPV, is created at the interface between a film of PPV oligomers and C 60 . [57] Second, the higher polarizability of the PCBM (through the higher value of e) means that the cost of charging the molecule is less. Finally, the large density of states in the fullerene compared to the polymer [58,59] may create a significant entropic driving force.…”

“…[63,64] The formation of a CT state is consistent with the evidence for interface dipoles at PPV/fullerene interfaces. [57] Optical excitation of this band should result in weakly bound polaron pairs that may be easily separated to deliver electrons and holes to the respective materials. One tentative explanation for the observed hole-transport behavior in MDMO-PPV:PCBM blends is, therefore, that hole polarons generated in the MDMO-PPV phase may be transferred into the interfacial CT state, for which transition there is no energy barrier, and are then transported by hopping between the CT complexes.…”