Electroluminescence and the Photovoltaic Effect

In Chaps. 5-10, we treated primarily individual, intrinsic properties of organic solids phonons, excitons, spin excitations, semiconducting properties, metallic conductivity, and superconductivity. Our main interest was directed in particular to excitations in the bulk of the organic solids. 

In this chapter, we will deal with the combination of electrical and optical properties. Here, along with the bulk properties, also the interfaces between the electrical contacts and the organic solids, as well as the interfaces between different organic solids, will play a central role. We restrict ourselves in this chapter to two electroop-tical effects electroluminescence and the photovoltaic effect The two phenomena are complementary  

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The thickness of the organic layer or, in multilayer devices, of the organic layers, is as a rule in the range between 10 and a few 100 nanometers. The electrooptics of organic devices is thus also a nanotechnology. The optimisation of the contacts and the layer thicknesses is - along with the intrinsic materials parameters - of central importance for the efficiency of the devices, i.e. for the luminous yield of electroluminescent devices or for the electric power of photovoltaic cells. The devices must therefore be optimised by both controlled variation of the layers and layer thicknesses and by comparison with simulations. 

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Deep-level states play an important role in solid-state devices through their behavior as recombination centers. For example, deep-level states are tmdesirable when they facilitate electronic transitions that reduce the efficiency of photovoltaic cells. In other cases, the added reaction pathways for electrons result in desired effects. Electroluminescent panels, for example, rely on electronic transitions that result in emission of photons. The energy level of the states caused by introduction of dopants determines the color of the emitted light. Interfacial states are believed to play a key role in electroluminescence, and commercieil development of this technology will hinge on understanding the relationship between fabrication techniques and tile formation of deep-level states. Deep-level states also influence the performance of solid-state varistors. 

Besides the value of LB-films as model systems in fundamental research there are a number of potential applications for these layers. The incorporation of LB monolayers and multilayers into both metal/LB-film/metal and metal/LB-film/semiconductor (MIS) devices has recently been attracting considerable attention [237]. Structures in the first category may find application as the basis for simple photovoltaic cells or switches. When deposited onto semiconducting substrates, the fine control of the LB layer thickness permits the optimization of the efficiency of both photovoltaic and electroluminescent structures. Thicker films can be used to control the surface conductivity of a variety of semiconductors and as the basis for a field effect transistor. The three particular examples presented in this section should serve to indicate the usefulness of monomolecular insulating films in the field of microelectronics. 

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