ABSTRACT
INTRODUCTION Since the first report of successful organic light-emitting diodes (OLEDs) with low operating voltage [1], rapid progress has been made in this field leading to the commercialization of organic displays [2]. The main advantages of OLEDs are their ease of fabrication via simple techniques (vacuum evaporation or spin coating), the possibility of large light-emitting surfaces and flexible polymer substrates, and a great variety of luminescent materials, with emission covering the entire visible spectrum. Particularly, white light emission has been obtained using low molecular weight materials in a multilayer structure [3] or polymers in a doped single layer [4] showing a possible application of OLEDs to lighting, for example in LCD backlighting. Finally, due to the development of efficient encapsulation techniques, lifetimes greater than 10000 hours are commonly obtained in industrial production. Despite improvement of the devices performance, the physical processes leading to organic electroluminescence are not completely understood. Particularly, the lack of electrical model describing injection and transport of charges inside the organic layers prevents the optimization of components. Studies of the current-voltage characteristics have shown that either injection at the electrodes (via tunnelling or thermoionic emission) [5] or internal space charge effects (space-charge limited current) in these lowmobility materials (typically l0 to le cm2V-Is-') may be predominant [6]. Measurement of the electric field distribution inside the organic layers could give important information on the transport processes. However, due to the thickness of OLEDs (typically 100 nm) conventional techniques usually applied to insulators are not valid. In this paper we present an alternative non-invasive method based on electroabsorption spectroscopy, which was first suggested by Campbell et al.[7]. We will describe the experimental rig and present our first results on a typical ITO/Organic/Metal structure.
