Formation Of Interface Dipole Research Articles

Efficient hole injection in organic light-emitting diodes using C60 as a buffer layer for Al reflective anodes

The hole injection of the organic light-emitting diodes with Al as a reflective anode for top-emitting devices was improved by using C60 as a thin buffer layer between Al and a hole transport layer. The driving voltage of the devices with C60 buffer layer was 5.5V compared with 11V for the devices without C60 buffer layer. The decrease of interfacial energy barrier by interface dipole formation between Al and C60 contributed to the low driving voltage of the devices.

Important role of molecular permanent dipoles of the Alq 3/Al interface studied from first-principles

We have studied the atomic geometries and electronic properties of the tris-(8-hydroxyquinoline) aluminum (Alq 3)/Al interface using density functional calculations. The calculated interface dipoles agree reasonably well with the experiments and the origin of the interface dipole formation mainly comes from the alignment of the permanent dipoles of Alq 3 molecules. The HOMO levels of the Alq 3 molecules significantly depend on the orientation of the molecular permanent dipoles and the interfacial gap state observed by experiments is ascribed to the co-existence of the two orientations of the molecular dipoles.

Dependence of Band Offset and Open-Circuit Voltage on the Interfacial Interaction between TiO2 and Carboxylated Polythiophenes

The interface of planar TiO(2)/polymer photovoltaic cells was modified with two carboxylated polythiophenes having different densities of carboxylic acid groups. Both of the interface modifiers increase the photocurrent of the cells but lower the open-circuit voltage. The work function of the TiO(2), measured using a Kelvin probe, increases with increasing density of carboxylic acid groups due to the formation of interfacial dipoles pointing toward the TiO(2) surface. The formation of interfacial dipoles results in a shift in the band offset at the TiO(2)/polymer interface, which explains the decrease in the open-circuit voltage. This work demonstrates that care must be taken when using carboxylic acid side groups to attach polymers to titania surfaces in photovoltaic cells. If the density of attachment groups is just enough to attach the polymer, then the benefits of the interface modifier can be realized without substantially decreasing the open-circuit voltage.

High-efficiency polymer light-emitting diodes based on poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] with plasma-polymerized CHF3-modified indium tin oxide as an anode

We demonstrate that introducing a thin CFx film formed by plasma polymerization of CHF3 on an indium tin oxide (ITO) anode surface for a polymer light-emitting diode with the structure, ITO∕CFx∕poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene](MEH-PPV)∕Ca∕Al, can lead to a high device performance (5.1cd∕A and 24000cd∕m2). The high device performance can be attributed to a better balance between hole and electron fluxes, resulting from a formation of interfacial dipole at the CFx∕MEH-PPV interface to provide a hole blocking effect and an enhancement of electron/hole recombination.

Theoretical Investigation on the Electronic Structure of the Tris-(8-hydroxyquinolinato) Aluminum/Aluminum Interface

We have studied the atomic geometries and the electronic properties of the tris-(8-hydroxyquinolinato) aluminum (Alq3)/Al interface by using density functional theoretical calculations. We examined three surfaces, the close packed Al(111) surface, the Al(332) stepped surface, and the Al adatom adsorbed Al(111) surface to investigate the effect of the surface roughness on the electronic properties of the interfaces. The calculated interface dipoles agree reasonably well with the experimental results and we found that the origin of the interface dipole formation mainly comes from the permanent dipole moment of Alq3 molecules. Although we have examined various possible structures, an interface gap state observed experimentally could not be reproduced by the present calculations.

Effect of thin iridium oxide on the formation of interface dipole in organic light-emitting diodes

The 4,4′-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl was in situ deposited on both iridium-oxide-coated indium-tin-oxide (IrOx–ITO) and O2-plasma-treated ITO (O2–ITO), and their interface dipole energies were quantitatively determined using synchrotron radiation photoemission spectroscopy. The dipole energies of both O2–ITO and IrOx–ITO were same with each other, −0.3eV, meaning the formation of same amount of interface dipole. The secondary electron emission spectra revealed that the work function of IrOx–ITO is higher by 0.5eV than that of O2–ITO, resulting in the decrease of the turn-on voltage via reduction of hole injection barrier.

Photoemission of Alq3 and C60 films on Al and LiF∕Al substrates

Photoemission studies of thin films of Alq3 and C60 deposited on Al and LiF∕Al substrates have been performed in order to deduce the interactions of the organic films with the substrates. For all cases there is evidence of strong interaction resulting in the formation of interfacial dipoles. Attempts to explain the origin of these interfacial dipoles and the type of interface formed in each case have been done through analysis of the valence electronic structure and core levels of the materials. The origin of the interfacial dipoles is mainly covalent interaction when the organic films are deposited on Al substrates, and charge transfer between the organic molecules and the metal through the LiF sandwich layer when the organic films are deposited on LiF∕Al substrates. For thick-enough LiF films, however, there is no interaction between the organic films and the substrates. In no case does the LiF dissociate, unlike what is found for the reverse order of deposition. Two charge-transfer-induced gap states are found for (sub)monolayer films of Alq3 deposited on LiF∕Al. We propose that the formation of two gap states corresponds to negatively charged fac-Alq3.

Electronic properties of a zinc oxide surface modified by ultra-thin layers of conjugated organic molecules

Thin films of Cu-phthalocyanine (CuPc), perylene and 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA) were thermally deposited in situ in UHV on ZnO(0 0 0 −1) surface. The surface potential, the surface work function Φ and the structure of density of unoccupied electron states (DOUS) located 5–25 eV above the Fermi level ( E F) were monitored during the film deposition, using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. Auger electron spectroscopy (AES) was used to monitor atomic composition of the surfaces under study. The deposition of the perylene and PTCDA films resulted in formation of the DOUS structure typical for these films while the deposition of CuPc films of less than 2 nm thickness resulted in formation of an intermediate DOUS structure which was replaced by a stable DOUS of CuPc along with a further increase of the deposit thickness. Migration of the substrate atoms into the intermediate layer was observed. The electronic work function Φ of the organic films changed during the film deposition until it reached stable values at a film thickness of 8–10 nm. Width of the interface dipole charge transfer layer and formation of intermediate TCS spectra during the film deposition are selected as relevant parameters for further comparison of the features of the interfaces studied.

Pt Schottky diode gas sensors formed on GaN and AlGaN/GaN heterostructure

Exposure of Pt/GaN and Pt/AlGaN/GaN Schottky diodes to H 2 gas at moderately high temperatures around 100 °C resulted in marked increase of forward and reverse currents. Increase was much larger in the Pt/AlGaN/GaN diode than in the Pt/GaN diode. Rapid turn-on responses and somewhat slower turn-off responses were observed with reproducible response magnitudes. A rigorous computer simulation of I– V curves indicated that current changes are due to changes in the Schottky barrier height caused either by H-induced formation of interfacial dipole or by hydrogen passivation of interface states.

Interface dipole at organic/metal interfaces and organic solar cells

In organic-based solar cells, the interface dipole present at the organic/metal interface participates to the collection and injection of charges between the electrode and the active organic material. The origins of the interface dipole is illustrated for a model system of the organic/metal interface composed of the electron–donor molecule p-phenylenediamine (PPDA) interacting with a nickel surface. The interface dipole created at the PPDA/Ni interface is characterized in a joint experimental and theoretical study using photoelectron spectroscopy and density functional theory calculations. The formation of strong interface dipoles upon chemisorption of a PPDA mono-layer is accompanied by a significant decrease (1.5 eV) of the metal work function reaching 3.6 eV.

Electronic properties of interfaces between model organic semiconductors and metals

We present a summary of recent studies of the interface properties of interfaces that are formed between well-established molecular organic semiconductors and various metals. The energy level alignment, the formation of interface dipoles, a possible band bending and interactions at the interfaces have been investigated using photoemission spectroscopy, while polarization-dependent X-ray absorption spectroscopy provided information on the molecular orientation at the interfaces. The observed interface characteristics reach from physisorption to a reaction depending upon both the substrate and the organic molecule under investigation. Different contributions to the often observed interface dipoles are discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Interface dipole at organic/metal interfaces and organic solar cells

In organic-based solar cells, the interface dipole present at the organic/metal interface participates to the collection and injection of charges between the electrode and the active organic material. The origins of the interface dipole is illustrated for a model system of the organic/metal interface composed of the electron-donor molecule p-phenylenediamine (PPDA) interacting with a nickel surface. The interface dipole created at the PPDA/Ni interface is characterized in a joint experimental and theoretical study using photoelectron spectroscopy and density functional theory calculations. The formation of strong interface dipoles upon chemisorption of a PPDA mono-layer is accompanied by a significant decrease (1.5eV) of the metal work function reaching 3.6eV.

Charge transfer and doping at organic/organic interfaces

We studied the electronic properties of technically relevant organic/organic interfaces using photoemission spectroscopy. Representatives of organic semiconductors from the family of the phthalocyanines were evaporated onto PEDOT:PSS [mixture of poly-3,4-ethylenedioxy-thiophene and polystyrenesulfonate] thin films, which are often applied as electrode material in organic semiconductor devices. Besides the formation of interface dipoles we observe energetic shifts of the electronic levels of the semiconductors, which are explained by a doping of the organic semiconductor near the interface due to a mixing of the two organic materials within the interface region.

Thermodynamic equilibrium and metal-organic interface dipole

We determined the interface dipoles at a number of metal-organic interfaces using ultraviolet and x-ray photoelectron spectroscopy. A linear dependence of the dipole on the metal work function is observed. This is consistent with the theory based on the charge transfer and thermodynamic equilibrium across the interface. The agreement suggests that charge transfer is one major factor in the formation of interface dipole. In addition, we find that the pushing back of the electron cloud tail that extends out of the metal surface and the permanent dipole moment of the organic molecule affect the interface dipole.

Recent advances in Schottky barrier concepts

Theoretical models of Schottky-barrier height formation are reviewed. A particular emphasis is placed on the examination of how these models agree with general physical principles. New concepts on the relationship between interface dipole and chemical bond formation are analyzed, and shown to offer a coherent explanation of a wide range of experimental data.

Distinguishing between interface dipoles and band bending at metal/tris-(8-hydroxyquinoline) aluminum interfaces

Using a combination of photoelectron spectroscopies and ellipsometry to study the thin-film growth of Alq3 (tris-(8-hydroxyquinoline) aluminum) on Ag, we have been able to distinguish between molecular orbital energy shifts due to “band bending” and interface dipole formation. We have observed large binding energy shifts (≈0.5 eV) of the organic molecular levels at low coverages. We conclude that these shifts are consistent with the formation of a polarized molecular layer, or interface dipole, and are inconsistent with an electrostatic band-bending model.

The effect of structural distortions on the electronic structure of carbon nanotubes

We calculated the effects of structural distortions on the electronic structure of carbon nanotubes. The main effect of nanotube bending is an increased mixing of σ and π states. This mixing leads to an enhanced density-of-states in the valence band near the Fermi energy. While in a straight tube the states accessible for electrical conduction are essentially pure C(2p π) states, they acquire significant C(2sp σ) character upon bending. Bending also leads to a charge polarization of the C–C bonds in the deformed region reminiscent of interface dipole formation. Scattering of conduction electrons at the distorted regions may lead to electron localization.

Energy-level alignment at model interfaces of organic electroluminescent devices studied by UV photoemission: trend in the deviation from the traditional way of estimating the interfacial electronic structures

The electronic structures of model interfaces of organic electroluminescent (EL) devices were investigated with UV photoemission spectroscopy (UPS). Interfaces of TTN (tetrathianaphthacene) and TCNQ (tetracyanoquinodimethane) were also studied as extreme cases for hole transport and electron transport material, respectively. For all organic/metal interfaces studied, the work function of metal electrode was changed by deposition of organic layer, i.e., the vacuum level was shifted at the interface, indicating the invalidity of the traditional energy level alignment model where a common vacuum level was assumed at organic/metal interface. At TCNQ/Au, DP-NTCI/Al, which are acceptor/metal interfaces, upward shift of the vacuum level of organic layer relative to that of metal was observed, suggesting the formation of interfacial dipole due to electron-transfer from metal to acceptor. At other organic/metal interfaces, TPD(N, N'-diphenyl-N, N'-(3-methylphenyl)-1, 1'-biphenyl-4, 4'-diamine)/Au or ITO (indium tin oxide), ALq/sub 3/ (tris(8-hydroxyquinolino) aluminum)/Al, DP-NTCl(N, N'-diphenyl-1,4,5,8- naphthyltetracarboxylimide)/Al or Au, downward shift of the vacuum level was observed. Such downward shift has been also observed in our previous study for porphyrin/metal interfaces, and seems to be a trend for organic/metal interfaces at which no electron-transfer from metal to organic layer occurs. This trend suggests that the traditional model tends to underestimate (overestimate) the barrier height for hole (electron) injection. On the other hand, the vacuum level shift at ALq/sub 3//TPD interface was less than 0.1 eV, leading to an apparent applicability of the traditional model. However, it is not always the case for organic/organic interfaces: finite shift of 0.2 eV was observed at TTN/TCNQ interface due to electron-transfer from TTN to TCNQ. Possible origins of vacuum level shift at organic/metal interfaces were also discussed.

Electronic structure of 8-hydroxyquinoline aluminum (alq 3)/metal interfaces studied by UV photoemission

The electronic structures of tris(8-hydroxyquinoline) aluminum (Alq 3) / metal (Au, Al) interfaces as a model interface of organic electroluminescent (EL) devices were investigated by ultraviolet photoemission spectroscopy (UPS). We found abrupt shifts of the vacuum level of ca. 1 eV at the interfaces in contrast to the traditional assumption with a common vacuum level at the interface. The shift indicates the formation of interfacial dipole with the metal side negatively charged. At Alq 3 / Al interface, the estimated energy position of the lowest unoccupied molecular orbital (LUMO) was very close to the Fermi level of the substrate metal, corresponding to the electron-injecting nature of the interface. This is in contrast to the poor electron-injecting character expected by assuming a common vacuum level where Fermi level of the substrate should be around the center of the gap. The electronic structure of Alq 3 as a solid is also discussed in comparison with the results by semi-empirical molecular orbital calculation.

FORMATION OF SURFACE DIPOLES FOR SMALL PALLADIUM CLUSTERS DEPOSITED ON α-(0001)Al2O3

The Kelvin-probe method is utilized to measure the work function of a single-crystal aluminum covered with palladium clusters. It is found that formation of interface dipoles occurs by charge transfer from Al 2 O 3 to Pd clusters, particularly for those less than 2 nm in diameter. These results provide valuable clue to the understanding of metal-support electronic interactions, which is important in catalysis.