Light-emitting diodes , analog

Thin polymer films have many possible technical applications. Transistors and light-emitting diodes are the obvious ones. In ultra-thin films, one may even approach an electronics of molecular dimension. Molecular electronics will be a future challenge for basic and applied science. Nature applies it on a large scale in the reaction centers of the photosynthetic process, where photoinduced mobile charges are separated in some analogy to the separation of the photo-(p-n)-pair in the junction zone of a semiconductor (see Section 13.3.1). 

More recently, emission of light by a metal-CP-metal sandwich has been observed [235]. This is again a thin-film device, analogous to conventional MIM devices [230]. To some extent, such a light-emitting diode (LED) can be considered as the reciprocal of a photovoltaic cell. In the latter, absorption of a photon creates an electron-hole pair that is collected in the external circuit, whereas in the former, recombination of an electron and a hole that have been injected from the electrodes generates an emitted photon. LEDs using CPs are discussed in Section V.C. 

Different two layer light emitting diodes (LEDs) have been tested using the aromatic oxadiazole polymers as electron transporting layer, with PPV as emissive layer. It was found that the use of the aromatic oxadiazole polymers can realise efficient light emission using stable aluminum metal as cathode. No definite enhancement of electroluminescence efficiency has yet been observed. This can probably be explained by the fact that PPV and the oxadiazole polymers 18-20 have similar reduction potentials, and therefore by analogy similar LLMO levels. 

Bnilt-in displays are available as liqnid-crystal displays (LCDs), light-emitting diodes (LEDs), or planar gas-discharge units. Accuracies are generally from 0.1 to 1.0 percent. Analog-to-digital circuitry is kept to a minimum to reduce cost. 

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