Toronto-based company Luxell Technologies has developed a technique to improve the efficiency of thin-film electroluminescent (EL) displays by controlling the band-gap within the active layer.
According to Alexey Krasnov, Luxell's principal scientist, by gradually increasing the concentration of selenium towards the centre of the semiconducting active layer, efficiency and therefore the brightness of a display can be improved by a factor of at least two.
In a thin-film EL device, light is emitted as a result of electrons striking manganese within the active layer. The active layer is encapsulated by two Y2O3 or Al2O3 insulator layers.
"The problem is that efficiency is best at the edge of the layer," said Krasnov. "Deeper in the layer, electrons create a positive space charge which reduces kinetic energy. To maintain this you need to apply a higher voltage."
The gradual increase in selenium is controlled through deposition. This reduces the band gap and therefore increases efficiency. Krasnov has published a paper in Applied Physics Letters describing the technique.
The concentration of selenium is monitored using a scanning electron beam microscope. Luxell used an alloy containing zinc, sulphur and selenium, with manganese as the dopant. As the selenium concentration increases, the quantity of sulphur is reduced. Brightness is twice that of a ZnS layer.
"The method gives a crisp image and a wide viewing angle, with a fast response and high contrast," said Krasnov. "There is a great potential for full-colour application if you introduce colour dopants into this graded active layer."