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Department of Chemical Engineering and Biotechnology

 
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The research, by scientists in our optoelectronic materials group, exposes a link between the so-called ‘photon recycling’ process and the energy efficiency of LEDs and solar cells.

The phenomenon of photon recycling – where a material emits and then reabsorbs light before it can escape to the surroundings – has been observed in several materials that show promise for next generation solar panels and light emitting diodes (LEDs).

In their paper published today in Physical Review Letters, the researchers from the Cambridge Department of Chemical Engineering and Biotechnology, and Cavendish Laboratory, present a new analytical method that enables them to count the number of photon recycling events occurring in a material.

They found that photon recycling is significantly more important in LEDs than solar panels and can make devices more efficient, by allowing the electrons responsible for light generation to remain energised for longer within a material.

The researchers also demonstrated that, surprisingly, in some circumstances, photon recycling can have a detrimental effect on efficiency, by making the material less able to absorb or emit new light.

“Management of light is paramount in devices like solar cells and LEDs,” says Sam Stranks, who leads the Optoelectronic Materials and Device Spectroscopy group at the Department of Chemical Engineering and Biotechnology and Cavendish Laboratory. “This work allows us to better understand what light does in these devices — and in particular guides how we can best use and re-use photons to maximise device performance.”

Their work significantly expands our understanding of the role of recycled light within devices that are key to a low carbon economy.

“Academic interest in photon recycling has been growing in the last few years, but until now we have lacked a full picture of its role in solar panels, LEDs and other devices,” says Alan Bowman, PhD student and the paper's first author. “Our new analysis provides a simple method to understand photon recycling, allowing for a direct comparison of its importance in different situations. As a result of this straightforward approach, we’ve developed a far better conceptual understanding of photon recycling.”

Light enters the solar panel from above and is absorbed in the central layer, creating high energy charges (shown as + and –). When charges meet they can emit light (red lines). This light escapes the panel (left of diagram) or is reabsorbed and recycled into new charges (right of diagram).

Read the full paper

Quantifying Photon Recycling in Solar Cells and Light-Emitting Diodes: Absorption and Emission Are Always Key
Alan R. Bowman, Miguel Anaya, Neil C. Greenham, and Samuel D. Stranks
Phys. Rev. Lett. 125, 067401

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