A team of chemists at the University of California, Riverside, have devised a way to make solar energy conversion more efficient.

Currently, labor cost and the cost of the land make up a large bulk of the expense when it comes to installing solar cells. The researchers believe solar energy could be made cheaper if less land had to be purchased to host the solar panels, and if each solar cell could produce more power.

Images of upconversion in a cuvette contaning lead selenide/rubrene selenide mixture. The yellow spot is the emission from the rubrene originating from (a) an unforcused continuous wave 800-nm laser with an intensity of 300 W/cm2, (b) a focused continuous wave 980-nm laser with an intensity of 2000 W/cm2. Image credit: Zhiyuan Huang, UC RiversideImages of upconversion in a cuvette contaning lead selenide/rubrene selenide mixture. The yellow spot is the emission from the rubrene originating from (a) an unforcused continuous wave 800-nm laser with an intensity of 300 W/cm2, (b) a focused continuous wave 980-nm laser with an intensity of 2000 W/cm2. Image credit: Zhiyuan Huang, UC RiversideThe researchers report in Nano Letters that by combining inorganic semiconductor nanocrystals with organic molecules, they have succeeded in “upconverting” photons in the visible and near-infrared regions of the solar spectrum.

“The infrared region of the solar spectrum passes right through the photovoltaic materials that make up today’s solar cells,” says Christopher Bardeen, a chemistry professor. “This is energy lost, no matter how good your solar cell is.”

The hybrid material the researchers devised first captures two infrared photons that would normally pass right through a solar cell without being converted to electricity, then adds their energies together to make one higher energy photon. This upconverted photon is readily absorbed by photovoltaic cells, generating electricity from light that normally would be wasted.

During experimentation, the researchers worked with cadmium selenide and lead selenide semiconductor nanocrystals. The organic compounds they used to prepare the hybrids were diphenylanthracene and rubrene. The cadmium selenide nanocrystals could convert visible wavelengths to ultraviolet photons, while the lead selenide nanocrystals could convert near-infrared photons to visible photons.

The researchers directed 980-nm infrared light at the hybrid material, which then generated upconverted orange/yellow fluorescent 550-nm light, almost doubling the energy of the incoming photons. According to the research team, they were able to boost the upconversion process by as much as three orders of magnitude by coating the cadmium selenide nanocrystals with organic ligands, providing a route to higher efficiencies.

“This 550-nm light can be absorbed by any solar cell material,” says Bardeen. “The key to this research is the hybrid composite material—combining inorganic semiconductor nanoparticles with organic compounds.” He says that organic compounds cannot absorb in the infrared but are good at combining two lower energy photons to a higher energy photon. By using a hybrid material, the inorganic component absorbs two photons and passes their energy on to the organic component for combination. The organic compounds then produce one high-energy photon. Put simply, he says, the inorganics in the composite material take light in; the organics get light out.

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