Light ‘trick’ could lead to better solar cells, LEDs

Light ‘trick’ could lead to better solar cells, LEDs

Technology News |
By Rich Pell

By transforming the diffusion curve the result is more light energy being held inside an opaque layer which could lead to better yields for solar cells or LEDs. Even in a medium characterized by randomness, like a collection of non-organized particles that all scatter light, the net spreading of light is uniform. This is typical for diffusion.

The randomness in the UT Complex Photonics Group’s experiments exists in a layer of white paint. Light that is falling on the collection of zinc oxide particles the paint is made of, will be scattered by the particles and will start interfering with light, scattered from neighbouring particles. Nevertheless, it will spread out in a uniform way. Theoretically, the energy density will show a linear fall-off with penetration depth.

The scientists of the Complex Photonics Group (MESA+ Institute for Nanotechnology) did not take this for granted and worked on a way to turn the falling curve into a rising one so as to enhance the energy level inside the layer. Following the fundamental diffusion curve, the energy density rises until half of the layer and then falls off.

Experimental setup for measuring light falling on and moving through an opaque layer, using fluorescent microscopy to monitor the results.

The scientists have not altered the layer only the light. The ‘wave front shaping’ technique used was developed earlier on, leaving the way open to program the light waves in such a way that they choose the best pathways and show a bright light spot at the backside of the layer. The technique is also suitable for active control of the diffusion process.

To prove that light moves according to the desired curve the scientists mix the paint particles with fluorescent nano-size spheres that act as reporters inside the layer. The local energy levels inside the layer are shown by the fluorescent spheres emitting light, with a highly sensitive camera at the backside of the layer measuring total fluorescent intensity.

The blue curve shows the expected fall-off of energy density versus penetration depth, the red one is the enhanced curve introducing far more energy inside the layer.

The measured energy levels highly agree with the enhanced diffusion curve so more light energy can be entered into a scattering medium. In solar cells, more light would be available for the conversion into electrical energy. White LEDs can be made more cost-effective, and better lasers with a high yield can be developed. In medical applications, better control of the illumination of tissue is possible. First of all, the scientists prove that it is possible to ‘trick’ light inside complex media.  

For more, see the paper “Coupling of energy into the fundamental diffusion mode of a complex nanophotonic medium” published in the New journal of Physics.

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