Energy level alignment at polymer/electrode interfaces in light-emitting devices studied by photoelectron spectroscopy

ABSTRACTThe application of ele ctroactive organic materials in devices (light emitting diodes, photovoltaic cells) often requires electrodes with a low workfunction. Commonly, aluminum or alkaline earth metals are used, which usually exhibit strong interaction with the organic material, such as the formation of new covalent bonds or doping. This results in a strong modification of the electronic structure of the organic/metal interface, and in most cases does not yield the energy level alignment expected for the unreacted interface. As a n alternative to the above-mentioned metals we propose the use of samarium, with a workfunction of 2.7 eV, for the following reason: we have studied the interface formation between Sm and p -sexiphenyl (6P, which exhibits intense blue electroluminescence), with ultraviolet photoelectron spectroscopy. Sm was deposited stepwise onto thin films of 6P in ultrahigh vacuum, and the photoelectron spectra were recorded after each step. We did not find any indication for a strong interaction between the two materials. Metallic Sm is formed instantaneously and the valence electronic structure of 6P remains unchanged upon the metal deposition. The weak interaction at this interface allows one to determine the energy level alignment between a metal and an organic material in a direct manner from the photoelectron spectra, without the need for making any assumptions on the workfunction or ionization potential.

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Conjugated Polymer Surfaces and Interfaces for Light-Emitting Devices

MRS Bulletin ◽

10.1557/s0883769400033625 ◽

1997 ◽

Vol 22 (6) ◽

pp. 46-51 ◽

Cited By ~ 15

Author(s):

W.R. Salaneck ◽

J.L. Brédas

Keyword(s):

Conjugated Polymers ◽

Photoelectron Spectroscopy ◽

Electronic Materials ◽

Structural Information ◽

Molecular Solids ◽

Chemical Modeling ◽

Polymer Leds ◽

Light Emitting ◽

Light Emitting Devices ◽

Surfaces And Interfaces

Since the discovery of high electrical conductivity in doped polyacetylene in 1977, π-conjugated polymers have emerged as viable semiconducting electronic materials for numerous applications. In the context of polymer electronic devices, one must understand the nature of the polymer surface's electronic structure and the interface with metals. For conjugated polymers, photoelectron spectroscopy—especially in connection with quantum-chemical modeling—provides a maximum amount of both chemical and electronic structural information in one (type of) measurement. Some details of the early stages of interface formation with metals on the surfaces of conjugated polymers and model molecular solids in connection with polymer-based light-emitting devices (LEDs) are outlined. Then a chosen set of issues is summarized in a band structure diagram for a polymer LED, based upon a “clean calcium electrode” on the clean surface of a thin film of poly(p-phenylene vinylene) (PPV). This diagram helps to point out the complexity of the systems involved in polymer LEDs. No such thing as “an ideal metal-on-polymer contact” exists. There is always some chemistry occurring at the interface.

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Energy level alignment between C60 and Al using ultraviolet photoelectron spectroscopy

Applied Surface Science ◽

10.1016/j.apsusc.2005.12.095 ◽

2006 ◽

Vol 252 (22) ◽

pp. 8015-8017 ◽

Cited By ~ 15

Author(s):

J.H. Seo ◽

S.J. Kang ◽

C.Y. Kim ◽

S.W. Cho ◽

K.-H. Yoo ◽

...

Keyword(s):

Energy Level ◽

Photoelectron Spectroscopy ◽

Ultraviolet Photoelectron Spectroscopy ◽

Energy Level Alignment

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Energy level alignment at C 60 /Hf using x-ray and ultraviolet photoelectron spectroscopy

10.1117/12.663489 ◽

2006 ◽

Author(s):

C. Y. Kim ◽

J. H. Seo ◽

S. W. Cho ◽

K. -H. Yoo ◽

C. N. Whang ◽

...

Keyword(s):

Energy Level ◽

Photoelectron Spectroscopy ◽

Ultraviolet Photoelectron Spectroscopy ◽

X Ray ◽

Energy Level Alignment

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Investigations on the Aluminum/Para-Hexaphenyl Interface in Light Emitting Devices

MRS Proceedings ◽

10.1557/proc-488-509 ◽

1997 ◽

Vol 488 ◽

Cited By ~ 4

Author(s):

N. Koch ◽

L.-M. Yu ◽

J.-L. Guyaux ◽

Y. Morciaux ◽

G. Leising ◽

...

Keyword(s):

Photoelectron Spectroscopy ◽

Active Material ◽

Direct Interaction ◽

Metal Electrode ◽

Insulating Layer ◽

Aluminum Films ◽

Light Emitting ◽

Light Emitting Devices ◽

Thin Aluminum

AbstractBlue light emitting devices (LED) with para-hexaphenyl (PHP) as the active material and aluminum as cathode exhibit very high quantum efficiencies. To further optimize device performance it is crucial to understand the physical properties of the involved interfaces. We have performed Rutherford-Backscattering experiments on actual devices to show the importance of oxygen in the interface formation at the cathode as this leads to the formation of a layer of AlxOy between PHP and aluminum. In devices, where the organic film is exposed to air before the metal electrode is evaporated, an insulating layer on the metal-side therefore is inherent. It has been shown that the introduction of an intermediate layer between active material and electrodes results in a higher quantum efficiency of the LED, the most common concepts being charge-transport-layers, or insulators on the other hand. Our results underline the need for a better control of the LED processing. Ultraviolet- and X-ray photoelectron spectroscopy in situ growth studies of thin aluminum films on PHP have been made to reveal the change in the electronic structure of the active medium in a LED in the absence of oxygen. Also the direct interaction of oxygen with this organic material is investigated by photoelectron spectroscopy.

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