Hybrid interfaces of conjugate polymers: Band edge alignment studied by ultraviolet photoelectron spectroscopy
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M. Fahlman Department of Science and Technology, ITN, Linko¨ping University, S-581 83 Linko¨ping, Sweden (Received 13 January 2004; accepted 8 April 2004)
The control of hybrid interfaces in polymer-based electronic devices may be enabling in many applications. The engineering of hybrid interface involves (requires) an understanding of the electronic structure of materials—one organic and one inorganic—that form the two halves of hybrid interfaces, as well as the electronic and chemical consequences of the coupling of the two. Although much literature exists describing the interfaces between vapor-deposited organic molecules and model molecules for polymers on the surfaces of clean metals in ultrahigh vacuum, few studies have been reported on spin-coated, semiconducting polymer films on realistic substrates. Spin coating in an inert atmosphere (or even air) is a central part of the process of the fabrication of polymer-based light-emitting devices and other modern polymer-based electronic components. Here, work on the electronic structure of semiconducting (conjugated) polymer films spin-coated onto selected inorganic substrates, carried out using ultraviolet photoelectron spectroscopy, is reviewed and summarized to generate a generalized picture of the hybrid interfaces formed under realistic device fabrication conditions.
I. INTRODUCTION
Electroluminescence (EL) in conjugated polymers has been an evolving research area since the late 1980s, spurred by, among other things, the observation of light emission from simple devices based upon poly(pphenylenevinylene), or PPV.1,2 Although the generation of light by electrical excitation is a phenomenon that has been seen in a wide range of semiconductors, for organic semiconductors it was first reported for anthracene single crystals in the 1960s.3,4 Even in these early studies, the importance of the injection of electrons from one electrode and holes from the other, followed by the capture of oppositely charged carriers (recombination), and the radiative decay of the excited electron-hole state (exciton) produced by this recombination process, was realized. An overview of the recombination of charged carriers by radiative decay of excitons in the context of polymerbased light emitting devices (p-LEDs) may be found elsewhere.5 Here we discuss the current understanding of the electronic structure of hybrid interfaces involved in
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0262 J. Mater. Res., Vol. 19, No. 7, Jul 2004
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functional p-LEDs, as studied by ultraviolet photoelectron spectroscopy (UPS). This review is not intended to be comprehensive, but illustrative, and reflects the work carried out in Linko¨ping, Sweden. However, since essentially all of the work done on this subject, as defined in the title, has been done in Linko¨ping, this is not a severe restriction. It should be pointed out first that much work has been done on what might be called ideal hybrid inter
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