ABSTRACT
Jerome Cornil, Donizetti dos Santos, and Roberto Lazzaroni University of Mons-Hainaut, Mons, Belgium
Massimo Malagoli and Jean-Luc Bredas University of Arizona, Tucson, Arizona
I. INTRODUCTION
The reports of efficient electroluminescence from organic conjugated molecules [ 1 ] and polymers [2] have triggered over the recent years spectacular developments in the field of organic-based electro-optic devices, such as light-emitting diodes (LEDs), photodiodes, and solid-state lasers. The typical architecture of a single-layer LED consists of an organic film sandwiched between two metallic contacts [3,4]. A first process in the operation of an LED is the injection of electrons and holes in the organic layer under the application of a voltage in forward bias between the two electrodes. In order to reduce the energy barriers at the metal-organic interfaces and hence the driving voltage required for producing light out of the plastic layer, the metal used at the cathode (typically Al, Ca, Mg, or alloys) thereof has a low work function with the Fermi energy matching at best the energy of the lowest unoccupied level of the organic compound; in contrast, the metal at the anode (indium-tin-oxide, or ITO, in most devices due to its metallic and transparent character) has a larger work function with a Fermi energy lying close to the energy of the highest occupied level of the molecule. The nature of the chemical interactions taking place at the metal-organic interface plays a critical role in determining the efficiency of emitting devices and has been extensively described in previous theoretical and experimental works [3,5].
