Nanoscale coating could make LED devices more efficient

The project, led by Ryan McCarron, a PhD student at King’s College London, succeeded in introducing a nanostructured film of gold into glass in order to scatter light.

Generally, flat-screen TVs and other devices that incorporate LEDs can only be viewed clearly head-on, with quality dropping off as the viewing angle becomes more acute. As well being a matter of viewing aesthetics though, this limits the efficiency of such devices.

‘If there’s an interface, with glass or some other material on top of the emitting layer, a significant portion of the light coming out will be trapped inside,’ McCarron told The Engineer. ‘Whenever a ray of light approaches glass beyond a certain angle, known as the critical angle, all that light is entirely reflected — none of it is transmitted through.’

In the case of glass that angle is around 42in, whereas for materials with a higher refractive index such as those used in LEDs, it can be more like 15˚, McCarron said.

‘So there’s only a small range of light that will actually be transmitted — the efficiency is drastically lower than it should be theoretically.’

His solution to this problem was to coat the glass with a layer of nanoscale grating incorporating a specific geometry of parallel lines. This facilitates scattering by manipulating so-called ’surface plasmons’ at the interface between materials.

‘The surface plasmons are an intermediate stage between the light on each side of the grating. They propagate along the spaces between the lines in the grating and can become decoupled on the other side,’ McCarron explained..

Currently, the team is using gold, since McCarron said it is easy to work with, but the method is also applicable to cheaper metals such as aluminium.

McCarron added that the method could be used for many other applications, such as bio and chemical sensing, integrated photonics, photovoltaics and organic LEDs.