Low Voltage Electroluminescence

The advent of the blue light emitting diode (LED) and of organic electroluminescent structures has strongly revitalized interest in this luminescence mechanism. For the first time, efficient light-emitting structures can be realized which do not require either high or low pressure. In addition: hitherto luminescent devices always used a cascade: in fluorescent lamps, first a discharge is generated and the resulting invisible radiation is converted into visible light, resulting in a (considerable) energy

loss. In cathode-ray tubes, first an electron beam consisting of electrons with rather high kinetic energy is generated; this subsequently impinges on the luminescent material. In the phosphors, electron bombardment finally leads to excitations where electrons in the conduction band are coupled to holes in the valence band (excitons). These excitons are transferred to activator ions. Although operating at physical limits, the phosphors’ energy efficiency is limited to about 20% [5.225]. As result, there are no white light emitting devices with energy efficiency greater than 50%. Low voltage electroluminescent devices may be a way around this problem. In such devices, the step leading to emission is the recombination of electrons in conduction band states with holes in valence band states and, in principle, only the band gap energy is required to excite the luminescence. Within limits, the color of the emission can be selected by choosing the appropriate semiconductor. The generation of luminescence can be very energy efficient; the main issue is getting the light out of the emitting device. Blue emitting LEDs are available with wall plug efficiency approaching 30%; red emitting LEDs are even approaching 60% efficiency.

Apart from efficiency, the power of LEDs is also an important driving force. LEDs with input power of 30 W, with external energy efficiency of about 10%, have been demonstrated in 2003 by the Japanese company Nichia.

In this section, only luminescent materials that can be used in inorganic low voltage electroluminescent devices will be discussed in some detail. Phosphors are used for two reasons:

— Inorganic LEDs generally generate narrow line emission. Combination of LEDs emitting in different spectral regions to generate white results in white light of low quality: it cannot reproduce all colors in a natural way.

— Efficient LEDs are not yet available in all colors required. Green is a special concern.

Phosphors for LEDs have to fulfill rather harsh conditions. The Stokes Shift must be small, the absorption must be high and, in addition, as the excitation densities are of the order of 20 W cm-2 (the area which emits light is much smaller than e. g. in fluorescent lamps), the luminescent materials must remain efficient up to high temperatures, should not show saturation (meaning a less than linear increase in output power with input power at high excitation densities) and must be radiation stable.

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