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New Quantum Dot Material Boosts Solar Cell Efficiency

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Engineers at University of Toronto in Canada and King Abdullah University of Science and Technology (KAUST) in Saudi Arabia developed a film made of nanoscale semiconductors called quantum dots for inexpensive and more efficient solar cells. The team led by Toronto engineering professor Ted Sargent published its findings in a letter to the journal Nature Nanotechnology.

Quantum dots are semiconductors a few nanometers in size — one nanometer equals one billionth of a meter — that use the entire solar spectrum, including visible and invisible wavelengths, to generate electric power. The Toronto and KAUST researchers generated quantities of quantum dots using chemical methods called colloidal synthesis, but with new techniques that make the technology more feasible for commercial production of inexpensive solar cells.

Lead Toronto co-author Susanna Thon says, “Previously, quantum dot solar cells have been limited by the large internal surface areas of the nanoparticles in the film, which made extracting electricity difficult.” Among the obstacles the researchers had to overcome were were poor surface quality of the photovoltaic material that trapped electrons generated by the quantum dots, as well as the low density of the dots in the films, which limited the amount of light that could be captured.

The team introduced chlorine atoms soon after creating the dots, with which lead Toronto co-author Alex Ip says, “we’re able to patch the previously unreachable nooks and crannies that lead to electron traps.” Ip adds that the use of short organic linkers made it possible to bind quantum dots in the film closer together. Co-author Aram Amassian of KAUST showed that the organic ligand exchange for linking the quantum dots was necessary to achieve the densest film.

The solar cell made with the quantum dots achieved a 7 percent efficiency, which represents a 37 percent increase in efficiency over the previous certified rating for a colloidal quantum dot device. And unlike current slow and expensive semiconductor growth techniques, colloidal quantum dot films can be made quickly and at low cost much like paint or ink. This discovery makes it possible, says the team, for solar cells that can be produced on flexible substrates in the same way newspapers are rapidly printed in mass quantities.

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