A team of researchers combined inorganic semiconductor nanocrystals with organic molecules, allowing them to “upconvert” photons in the visible and near-infrared regions of the solar spectrum, the University of California, Riverside reported. These innovations could boost the energy efficiency of solar cells by 30 percent or more.
“The infrared region of the solar spectrum passes right through the photovoltaic materials that make up today’s solar cells,” said Christopher Bardeen, a professor of chemistry. “This is energy lost, no matter how good your solar cell. The hybrid material we have come up with 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.”
This combination of materials reshapes the solar spectrum so that it can match the photovoltaic materials used to construct the solar cells. The team used cadmium selenide and lead selenide semiconductor nanocrystals; they prepared the hybrids with diphenylanthracene and rubrene. The cadmium selenide nanocrystals were found to have the ability to convert visible wavelengths to ultraviolet photons, and the lead selenide nanocrystals could convert near-infrared photons to visible photons. The researchers directed 980-nanometer infrared light at the hybrid material, which generated upconverted orange/yellow fluorescent 550-nanometer light that nearly doubled the energy of incoming photons. The team was able to raise the upconversion process by up to three orders of magnitude by coating the cadmium selenide nanocrystals with organic ligands.
“This 550-nanometer light can be absorbed by any solar cell material,” Bardeen said. “The key to this research is the hybrid composite material – combining inorganic semiconductor nanoparticles with organic compounds. 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, the inorganics in the composite material take light in; the organics get light out.”