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Light can act as both a wave and a particle, and this fact is put to good use in solar power technologies: capturing the sun’s energy with a few shiny panels seems somewhat magical, but it’s simply a manipulation of nature’s atomic behavior. Quantum scale photosynthesis could be key to developing new designs for solar energy and nanoscale devices.
A number of biological systems residing in low light conditions have unique protein structures for photosynthesis that utilize quantum dynamics to convert 100% of absorbed light into electrical energy. They exhibit incredible efficiencies and could lead to a new understanding of renewable solar energy.
Those studying quantum biology have demonstrated that quantum wave-like entities transport energy in the early stages of photosynthesis. During the process, photons are absorbed by pigments like chlorophyll and create excited molecular states known as excitons. These excitons carry energy as quantum waves through pigment protein-complexes (PPC), where their energy is used to release electrons for photosynthetic chemistry.
A key problem in both nature and man-made solar cells is the trapping and dissipation of excitons during the process. Research from the University of Cambridge has found a mechanism in PPCs that prevents this by reversing the flow of escaped energy.
Researchers there studied light-harvesting proteins in green sulfur bacteria and found its exceptional ability to harvest light was down to an intricate process of energy transport that falls outside classical physics – instead it depends strongly on quantum physics, specifically quantum coherence.
In photosynthesis, quantum coherence sees particle-like excitons traversing the molecular structure via multiple channels concurrently. Normally, quantum coherence is fragile and easily destroyed, but in this instance, it actually increases the speed of energy flow across molecules and prevents it from getting stuck.
The researchers believe their work could solve some of the key issues in solar cell technology, and the aim now is to stabilize quantum coherence, especially at ambient temperatures. This is an important goal for quantum-based advanced solar cells and for quantum computing and nanotechnology too.
In solar cells, incoming light, exhibiting wave-like properties, is concentrated by mirrors and lenses, passing through the silicon wafer. Once inside the cell, it collides with electrons – displaying particle-like properties – knocking the electrons loose and freeing them to create an electric current between the panel and batteries or the grid.
This set-up works well, but there is room for improvement; most solar panels are incredibly inefficient, capturing between five and 19% of the potential energy.
Next-generation solar cells employ quantum dots – nanosized semiconductors, which are so small that only a handful of electrons can live there. This residence is so cramped that the dot behaves like an artificial atom in that its electrons reside only at specific, quantized energy levels. These energy levels define exactly which wavelengths of light the dot will absorb.
So, instead of using sheets of silicon sandwiched between glass panels, quantum dots utilize a matrix of finely-tuned crystals. It is possible to tune the wavelengths at which the dot absorbs light by altering its size, and the hope is that by mixing dots of different sizes it will be possible to absorb sunlight across a wide range of wavelengths.
Although such quantum solar cells are still in development, they are likely to be cheaper and more efficient; since more electrons than normal can be excited, efficiency could increase to up to 65%.
Quantum physics could power the future; the strange conduct of quantum physics may seem too unpredictable to rely on for our energy needs, but new technology based on quantum dots could capitalize on this behavior.
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