With LEDs being the preferred long-lasting, low-energy method for replacing less efficient forms of lighting, their uptake has dramatically increased over the past few years. However, despite their luminous outputs having increased steadily over that time, they still fall behind more conventional forms of lighting in terms of brightness. Researchers at Princeton University claim to have come up with a way to change all that by using nanotechnology to increase the output of organic LEDs by 57 percent.
Much of this realized increase has been achieved by using a new method devised by the researchers for the way the light is emitted from the LED. Current commercial LED design actually reduces the amount of light that gets transmitted because light is not channeled from where it is generated at the semiconductor junction, but rather focused by the rudimentary lens of the epoxy resin medium in which the semiconductor is contained. As a result, although LEDs are renowned for their efficiency, only a fraction of the light generated within an LED actually makes it beyond the lens.
“It is exactly the same reason that lighting installed inside a swimming pool seems dim from outside – because the water traps the light,” said professor Stephen Chou, the Joseph C. Elgin Professor of Engineering at Princeton. “The solid structure of a LED traps far more light than the pool’s water.”
An LED is a very efficient way to produce visible light; much more so than incandescent bulbs which actually produce more heat than they do light. An inorganic semiconductor LED is basically just a specialized type of semiconductor diode made from various mixes of metals (usually combinations dominated by Gallium Arsenide and, to a lesser extent, Silicon Carbide).
Simply put, when the diode has power applied in the correct (“forward”) direction, this causes electrons to recombine with holes to release sufficient energy to produce photons which emit light.
Similarly, an organic semiconductor LED (better known as an OLED – the lighting technology behind flexible screen TVs) is made by placing a series of organic thin films between two conductors. Power applied in a comparable manner to an inorganic LED results in a somewhat similar hole/electron recombination, though the injected positive and negative charges recombine in the emissive layer to produce light.
In the Princeton research, the team used a nanoscale structure of its own design dubbed a PlaCSH (plasmonic cavity with subwavelength hole-array) to manipulate light in a way that ordinary material or non-metallic nanostructures cannot. Not only did this vastly improve the efficiency of light output, this method also improved the image clarity of LED displays by around 400 percent compared with standard displays.
The PlaCSH device contains a layer of light-emitting material approximately 100 nanometers thick housed inside a cavity that has one surface made of a thin metal film, while the opposite cavity surface is coated with a metal mesh just 15 nanometers thick. Though tiny, they aren’t the thinnest LEDs so far developed, but by being on a scale smaller than a single wavelength of light, this combination of reflector and mesh guides the light out of the emissive layer with almost no absorption or scattering.
This latest work was the culmination of previous research by Chou and his team where they first developed the PlaCSH structure to be used on solar cells to more efficiently focus incoming light. This increased the absorption to as high as 96 percent of the solar energy being received by the photovoltaic cell, which resulted in an increased efficiency of 175 percent. As such, the team figured that if such a device was so efficient at absorbing light then, logically, it could also be used for light extraction as well.
“From a view point of physics, a good light absorber, which we had for the solar cells, should also be a good light radiator,” Professor Chou said. “We wanted to experimentally demonstrate this is true in visible light range, and then use it to solve the key challenges in LEDs and displays. It is so flexible and ductile that it can be weaved into a cloth.”
The team also claims that the PlaCSH organic LEDs are exceptionally cheap to make as they are made using a system called “nanoimprint,” a technology Professor Chou invented to make nanostructures in a similar way to a printing press produces newspapers.
Professor Chou was recently awarded a grant from the US Department of Energy to further advance the use of PlaCSH, and patent applications for organic and inorganic LEDs using PlaCSH have been filed by Princeton University. The team is now researching the use of PLaCSH in red and blue organic LEDs, as well as the original green LEDs developed.