Innovative quantum dots in solar panels

High on any environmentalist's wish list is a solar cell perfectly tuned to absorb the full spectrum of wavelengths of the Sun's rays. A new technique reported in Nature Materials improves the efficiency of solar cells by making use of 'quantum dots' — semiconducting crystals billionths of a metre across — could move this desire one step closer to reality.

Engineers have been trying to fabricate solar cells using quantum dots for around a decade. By varying the size of the dots, it is possible to control what wavelengths of light a solar cell absorbs. This ability has the potential to increase efficiency, explains Edward Sargent, an electrical engineer at the University of Toronto, Ontario, who led the study.

The main technical challenge is in suspending the quantum dots in a film and in coating them with molecules, usually organic ligands, which ease the flow of electrons through the charged crystals, but add bulk to the film, limiting the quantum dots performance.

Sargent and his team swapped the bulky inorganic ligands with small inorganic ions — chloride, bromide and iodide — which each have a single free electron to bond to the quantum dot crystals. To verify that the inorganic ions were in place, Kang Wei Chou and Aram Ammassian, two material scientists at the King Abdullah University of Science and Technology, in Thuwal, Saudi Arabia, used a scanning transmission electron microscope to image the surface of the crystals down to individual atoms.

Solar cells made using this innovative have a solar-power conversion efficiency of up to 6%, which is the highest achieved so far. To be commercially competitive with conventional solar cells, they need to reach 10%, adds Sargent. "But here we have paved the way for future improvements."

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