Light Transformed For Capture By Solar Cells

When a green laser is shone on cadmium selenide coated with organic materials, it is converted to higher energy light (a) while materials with the wrong coating simply let the light pass through unchanged (b). Zhiyuan Huang, UC Riverside

Scientists have found hybrid materials that allow solar cells to capture light that is currently wasted. The technology marks another step towards producing solar cells that breach what was once considered their maximum possible efficiency, and may bring down prices for solar power in the process.

In Nano Letters, professor Christopher Bardeen of the University of California, Riverside, and colleagues have announced hybrid molecule-nanocrystals that can combine two low-energy photons, producing yellow light in their place. Although other teams have achieved something similar before, Bardeen and his colleagues have managed it with an efficiency that may make the idea commercially viable.

A rainbow's colors are photons of different wavelengths, with different associated energies – longer wavelengths having lower energies. Solar engineers struggle to capture this diversity. Photovoltaic cells work because photons interact with electrons, transferring their energy into electric current.

All solar cells must have a band gap, and can only utilize photons with energy above the gap. On the other hand, any excess energy above the band gap is wasted. This trade-off once led to the belief that no single solar cell could capture more than 34% of the sunlight falling upon it.

If two low-energy photons could be combined into a single photon with energy above the band gap, this excess energy could be captured, allowing the efficiency limit to be broken. It sounds like a fantasy, but materials that achieve this feat, known as upconversion, have been known since the 1960s.

The process occurs when a photon raises the energy level of an electron to an excited state, and a second photon arrives before the electron can drop back to the ground state. The second photon raises the electron's energy to an even higher excited state, and when the electron returns to the ground state, it releases the energy of both in a single, shorter wavelength photon. It's like taking the second caffeine shot before the buzz from your first cup of coffee has worn off.

The problem is that materials in which upconversion has been demonstrated usually don't hold electrons in the excited state long enough for there to be a high chance of another suitable photon to arrive to complete the process. Consequently, only a tiny fraction of the long wavelength light is converted to something we can use.

Bardeen reports that lead selenide nanocrystals coated with organic molecules rubrene and diphenylanthracene upconvert near-infrared photons to orange-yellow light of 550 nanometers, while cadmium selenide shifts visible light into the ultraviolet. “This 550-nanometer light can be absorbed by any solar cell material,” said Bardeen. Most importantly, the coated crystals have an efficiency a thousand times previous versions.

“The inorganic component absorbs two photons and passes their energy on to the organic component for combination,” said Bardeen. “The organic compounds then produce one high-energy photon. Put simply, the inorganics in the composite material take light in; the organics get light out.”

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