Generating electricity from low energy light has been a major challenge for researcher working in the field of solar power. Last year in 2020, scientists had a major breakthrough in using low-energy light to generate electricity for solar power.
The energy from the sun is not just visible light. the spectrum is broad, including infrared light which gives us heat and ultraviolet which can burn our skin.
The human eye can only see visible light, but light comes in many other "colors"—radio, infrared, ultraviolet, X-ray, and gamma-ray—that are invisible to the naked eye.
On one end of the spectrum there is infrared light, which, while too red for humans to see, is all around us and even emitted from our bodies. Warm-blooded animals, including humans, radiate infrared light. That's why infrared cameras are helpful for thermal imaging and night vision when searching for people or animals.
Einstein's won the Nobel Prize for discovering that the energy of the electrons ejected from a photoelectric plate depended on frequency which is just inverse of wavelength, but not on light intensity (amplitude), as wave theory predicted. So shorter the wavelength of incident light means higher the frequency of the light thus more energy possessed by ejected electrons.
The Effect of Solar Energy Wavelength on Electron Energy
Einstein's explanation of the photoelectric effect helped establish the quantum model of light
When photons are incident on a conducting material, they collide with the electrons in the individual atoms. If the photons have enough energy, they knock out the electrons in the outermost shells. These electrons are then free to circulate through the material. Depending on the energy of the incident photons, they may be ejected from the material altogether.
According to Planck's law, the energy of the incident photons is inversely proportional to their wavelength. Short-wavelength radiation occupies the violet end of the spectrum and includes ultraviolet radiation and gamma rays. On the other hand, long-wavelength radiation occupies the red end and includes infrared radiation, microwaves and radio waves.
Sunlight contains an entire spectrum of radiation, but only light with a short enough wavelength will produce the photoelectric or photovoltaic effects. This means that a part of the solar spectrum is useful for generating electricity. It doesn't matter how bright or dim the light is. It just has to have – at a minimum – the solar cell wavelength. High-energy ultraviolet radiation can penetrate clouds, which means that solar cells should function on cloudy days – and they do.
Solar Energy Wavelength and Cell Efficiency
In short, PV cells are sensitive to light from the entire spectrum as long as the wavelength is above the band gap of the material used for the cell, but extremely short wavelength light is wasted. This is one of the factors that affects solar cell efficiency. Another is the thickness of the semiconducting material. If photons have to travel a long way through the material, they lose energy through collisions with other particles and may not have enough energy to dislodge an electron.
The reflectivity of the solar cell is third factor affecting efficiency of the solar cell. A specific percentage of incident light bounces off the cell's surface without hitting an electron. The solar cell manufacturers generally apply coating of a nonreflective, light-absorbing material to reduce losses from reflectivity and increase efficiency. This is the main reason solar cells are appear black.
The sun is a very large, naturally occurring, fusion reactor in the sky.
It constantly releases a vast amount of energy. Even by the standards of sky fusion reactors it’s pretty powerful and outshines at least 90% of the stars out there.
How powerful is our Sun?
Take for an example
We may combined together all the energy humanity produces in from burning coal, oil, and natural gas and all the energy from the fission of uranium and other elements in nuclear reactors and all the energy spun out of wind and then sum up the total for an entire year, it would come to less than the amount of sunlight energy hitting the earth every seven seconds.
Sunlight comes in form of photons, which is ‘the smallest amount of light possible’.
These photons have different wavelengths. Photon with longer wavelengths have less energy and with shorter wavelengths have more energy.
If a photon doesn’t have enough energy our eyes can’t see it. The same goes if it has too much energy. Only Photon with a specific energy range are visible to our eyes and we call them visible light.
Photons that don’t have enough energy to be seen are called infrared and those with too much are called ultraviolet.
In an ordinary semiconductor photovoltaic cell, incoming photons excite electrons from the cell’s valence band to its conduction band. The electrons are then collected at an electrode to generate a current. Unfortunately, photons with less energy than the semiconductor’s “bandgap” cannot excite electrons, while photons with more energy than the bandgap lose their surplus energy as heat. The result is that most of the incident solar energy is lost.
Sunlight energy that reaches the ground is around 4% ultraviolet, 43% visible light, and 53% infrared. Solar panels mostly convert visible light into electrical energy, and they also can make use of almost half the infrared energy. But solar panels only use a small portion of ultraviolet.
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Most solar cells, charge-coupled device (CCD) cameras and photodiodes (a semiconductor that converts light into electrical current) are made from silicon, which cannot respond to light less energetic than the near infrared. This means that some parts of the light spectrum are going unused by many of our current devices and technologies.
Teams across the United States and Australia have used the strategy, called photochemical upconversion, to change invisible infrared light into “more energetic, visible light” so that it can be used to generate electricity.
This is the first time light of this type has been able to be captured, and while the efficiency of the technology needs more work before commercialisation is possible, it bodes well for the future of solar power.
The simple secret ingredient
Previous attempts to harness infrared light for solar power were unsuccessful, only managing to upconvert near infrared light instead.
However, researchers at RMIT University and UNSW University in Australia, and at the University of Kentucky in the US, found that oxygen could be used to help the process. Normally oxygen is a hindrance to these reactions, but at low energies it can be harnessed positively.
‘The first study, led by researchers at RMIT University, Australia's UNSW, and the University of Kentucky, found that low-energy light that's invisible to the human eye can be "upconverted" using oxygen to generate electricity, which could allow solar panels to generate more energy using the same amount of sunlight.”
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All photovoltaic solar cells transmit photons with energies below the absorption threshold (bandgap) of the absorber material, which are therefore usually lost for the purpose of solar energy conversion. Upconversion (UC) devices can harvest this unused sub-threshold light behind the solar cell, and create one higher energy photon out of (at least) two transmitted photons. This higher energy photon is radiated back towards the solar cell, thus expanding the utilization of the solar spectrum. Key requirements for UC units are a broad absorption and high UC quantum yield under low-intensity incoherent illumination, as relevant to solar energy conversion devices, as well as long term photostability. Upconversion by triplet–triplet annihilation (TTA) in organic chromophores has proven to fulfil the first two basic requirements, and first proof-of-concept applications in photovoltaic conversion as well as photo(electro)chemical energy storage have been demonstrated.
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"I'm very hopeful and think that we can improve the efficiency quickly. I think it's quite exciting for everyone"- UNSW professor and lead researcher Elham Gholizadeh
Photochemical upconversion is a strategy for converting infrared light into more energetic, visible light, with potential applications ranging from biological imaging and drug delivery to photovoltaics and photocatalysis. Although systems have been developed for upconverting light from photon energies in the near-infrared, upconversion from below the silicon bandgap has been out of reach. Here, we demonstrate an upconversion composition using PbS semiconductor nanocrystal sensitizers that absorb photons below the bandgap of silicon and populate violanthrone triplet states below the singlet oxygen energy. The triplet-state violanthrone chromophores luminesce in the visible spectrum following energy delivery from two singlet oxygen molecules.
Journal :Nature Photonics
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“What’s interesting is that often without oxygen, lots of things work well,” says contributing author Professor Jared Cole from RMIT University, “and as soon as you allow oxygen in, they stop working.
“It was the Achilles heel that ruined all our plans but now, not only have we found a way around it, suddenly it helps us.”
Although it may be a while before this technology can be put to widespread use, the breakthrough continues what has been a good year so far for solar power innovation. Double-sided solar panels have begun to be phased in, which are 35 per cent more efficient than standard panels. Elsewhere the largest solar project neared completion in Abu Dhabi, providing record-breaking levels of energy.
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Another recent study led by the Okinawa Institute of Science and Technology in Japan found that solar panels may be more inexpensive and efficient if built with a material called perovskites rather than silicon, which most current solar panels are made of. The solar cells currently on the market utilize silicon, which makes them expensive to fabricate when compared to more traditional power sources.
That's where another, relatively new-to-science, material comes in -- metal halide perovskite. When nestled at the center of a solar cell, this crystalline structure also converts light to electricity, but at a much lower cost than silicon. Furthermore, perovskite-based solar cells can be fabricated using both rigid and limber substrates so, alongside being cheaper, they could be more light-weight and flexible. But, to have real-world potential, these prototypes need to increase in size, efficiency, and lifespan.
"Scaling up is very demanding; any defects in the material become more pronounced so you need high-quality materials and better fabrication techniques," Luis Ono, one of the Okinawa Institute study's authors, said in a statement.
Two major breakthroughs in solar cell technology could vastly improve the way energy is harvested from the sun.
The two studies, published in Nature Energy and Nature Photonics, will transform the efficiency and significantly reduce the cost of producing solar cells, scientists say.