Waste Light Has Been Converted To Energies Solar Cells Can Use For The First Time

The Molecular Photonics Laboratories at UNSW has been used to combine low-energy 'waste' light into a shorter wavelengths solar cells can use. UNSW Sydney/Exciton Science

Much of the radiation the Sun showers on the Earth can't be captured by existing solar panels. For the first time, scientists have found a way to transform this low-energy radiation into something silicon cells turn into electricity. The efficiency demonstrated so far is very low, but the team responsible think this will change. If so, it will represent another step towards solar electricity so cheap fossil fuels can't compete.

Most objects absorb high-energy (short-wavelength) electromagnetic radiation and release it as longer wavelength heat. Going the other way, turning low-energy photons to higher energy ones, is much harder.

Nevertheless, certain materials absorb long-wavelength (low-energy) photons and combine them for release with shorter wavelengths, a process known as upconversion. Scientists have previously made crystals that combine the energies of two near-infrared photons to make one yellow one. The key to upconversion is for the crystal to hold the first low-energy photon long enough for a second one to arrive so they can be combined.

While an exciting demonstration of what is possible, that research, and others like it, all used radiation not far beyond the range we can see. Professor Tim Schmidt of the University of New South Wales is seeking to do something much more useful: upconvert the far-infrared photons that are too low energy to be captured by silicon solar cells, technically known as being below silicon's bandgap. Unfortunately, this is also much more difficult.

“There is a general law that the lifetime of an excited state decreases exponentially with its energy,” Schmidt told IFLScience. So the less energy you're trying to hold onto, the harder it is to keep it until a new photon arrives.

Nevertheless, Schmidt and co-authors have announced in Nature Photonics they have upconverted radiation below silicon's band gap for the first time. To their surprise, part of the secret was adding oxygen, which at higher energies interferes with storing the energy from the first-arriving photon.

The molecules involved in upconverting far-infrared light look like they belong in a space invaders game. The five stage process is 1. Photon absorption. 2. Triplet energy trapping. 3. Triplet energy transfer to the emitter (V79). 4. Triplet–triplet annihilation 5. Upconverted photon emission. An alternative energy transfer pathway involving singlet oxygen is shown. Gholizadeh et al/Nature Photonics

Currently, far-infrared sunlight passes through solar cells. The idea is to coat their backs with an upconverting material and then make use of rebounding higher energy light. If done perfectly, Schmidt told IFLScience, this would increase the maximum possible portion of sunlight silicon solar cells capture from 29 percent to 38 percent. The benefits could be almost as big for emerging solar technologies such as perovskites.

The same technology could have other uses, for example making the silicon in charge-coupled device cameras sensitive to the far infrared.

Where space is limited, such as the roofs of electric cars, higher efficiency solar cells could open whole new markets. For utility solar farms and most rooftop solar, cost is the major factor. Nevertheless, higher efficiency solar cells need less land, installation, and mounting, and these savings may more than offset the price of adding an upconverting layer, helping bring solar energy's cost down further.

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