In an LED, light is generated by means of an inorganic semiconductor. To put it simply, a semiconductor consists of two differently composed (doped) areas that are immediately adjacent to each other. Electrons can move in both areas, however, they move at different quantum-mechanical energy levels. In the so-called n area there is a surplus of electrons, while in the p area there is a surplus of defect electrons (so-called holes). In the contact area between the two layers, diffusion processes lead to a balance between the two types of charge. On account of the electrostatic conditions created, this process is self-inhibiting and separates the two areas electrically from each other; it is also said that the diode blocks the current.
If a voltage source is then applied to the diode in such a way that its negative pole is connected to the n area (cathode), the external voltage works with the internal voltage, so to say, and the blocking layer can be overcome or suspended, and the diode becomes conductive. If the polarity of the voltage source is reversed, the external voltage reinforces the blocking effect, and the diode remains blocked for the flow of current. A diode can thus be compared with a non-return valve in hydraulics.At the transition point between the n and p areas, the electrons switch from a high energy level to a low quantum-mechanical energy level. Since no energy can be lost in nature, the “surplus” energy must be dissipated. This can happen in the form of dissipated heat, for example.
With light emitting diodes, the materials of the diodes are such (doped) that the surplus energy is emitted as light. As the differential energy at the transition point of the electrons from n to p is approximately constant for all pairs of materials, and the energy is always emitted in the form of a light quantum, the spectrum of the emitted light is fairly narrow. In other words, a normal LED always emits a fixed colour of light. White light can be generated with single-colour LED by means of additive colour synthesis. Here, for example, the emissions of red, green and blue LED are mixed with each other. If the intensities of the individual colours are matched to each other, the human eye perceives such mixed light as white.
However, the light from such a light source is not of optimum quality. Its spectrum is very discontinuous on account of the individual peaks of the LED. In particular the colour rendering of such a system is thus relatively poor. For this reason, such systems are usually only used today for creating dynamic coloured light.
Significantly better white LED light can be achieved with the help of conversion phosphors. Here, the relatively high-energy (short-wave) light of a blue LED is used to induce the phosphor to emit light. Usually phosphors with yttrium, aluminium and gallium (YAG phosphors) are used. They emit light in the yellow-green range, and the mixture of their emissions together with unconverted blue components of the LED result in white light. Depending on the quality of the phosphor and the LED, good to very good white spectra can be created; on account of the wide spectral emission of the phosphor, its spectra are relatively homogeneous. As a rule, however, a distinct blue peak can still be detected.
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Spectrum of an LED - wavelength [ nm ]
Current developments in the field of phosphors therefore aim to improve the spectral composition and thus try to raise the quality of the generated light.
OLED (Organic Light Emitting Diode) are millimetre-thin glass wafers with organic materials enclosed in them. These form layers which are around 400 nanometres thin and which current can flow through. The organic layers are enclosed by an anode and a cathode layer, which function as the electrical contacts from both sides. The organic layer contains molecules which begin to glow when an electric current passes through them. The particular molecular structure determines the colour of the light. The organic layers are coated to protect them from external effects.
An organic light emitting diode consists of several organic semiconducting layers between two electrodes, at least one of which is transparent. In the manufacture of an OLED, successive organic layers are applied to a conductive substrate, followed by another conductive electrode. In general, two different classes of material are used in the manufacture of organic, light-emitting components: polymer substances and what are known as small molecule materials, which have no orientation properties and thus form amorphous layers.
