Colloidal quantum dots irradiated with a UV light. Different sized quantum dots emit different color light due to quantum confinement. (Credit: Antipoff http://en.wikipedia.org/wiki/File:Quantum_Dots_with_emission_maxima_in_a_10-nm_step_are_being_produced_at_PlasmaChem_in_a_kg_scale.jpg)Researchers in the University of Toronto’s Edward S. Rogers Sr. Department of Electrical & Computer Engineering have designed and tested a new class of solar-sensitive nanoparticle, an air-stable n-type colloidal quantum dot (CQD) solid.
This new form of solid, stable light-sensitive nanoparticles could lead to cheaper and more flexible CQD solar cells, as well as better gas sensors, infrared lasers, infrared light emitting diodes and more. The work, led by post-doctoral researcher Zhijun Ning and Professor Ted Sargent, was published this week in Nature Materials (see footnote).
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Quantum dot solar cells are an emerging field in solar cell research that uses quantum dots as the absorbing photovoltaic material, as opposed to better-known bulk materials such as silicon, copper indium gallium selenide (CIGS) or CdTe. Quantum dots have bandgaps that are tunable across a wide range of energy levels by changing the quantum dot size. This is in contrast to bulk materials, where the bandgap is fixed by the choice of material composition. This property makes quantum dots attractive for multi-junction solar cells, where a variety of different bandgap materials are used to improve efficiency by harvesting select portions of the solar spectrum.Collecting sunlight using these tiny colloidal quantum dots depends on two types of semiconductors: n-type, which are rich in electrons; and p-type, which are poor in electrons. The problem? When exposed to the air, n-type materials bind to oxygen atoms, give up their electrons, and turn into p-type. Ning and colleagues modeled and demonstrated a new colloidal quantum dot n-type material that does not bind oxygen when exposed to air.
Maintaining stable n- and p-type layers simultaneously not only boosts the efficiency of light absorption, it opens up a world of new optoelectronic devices that capitalize on the best properties of both light and electricity. For the average person, this means more sophisticated weather satellites, remote controllers, satellite communication, or pollution detectors.
“This is a material innovation, that’s the first part, and with this new material we can build new device structures,” said Ning. “Iodide is almost a perfect ligand for these quantum solar cells with both high efficiency and air stability—no one has shown that before.”
Ning’s new hybrid n- and p-type material was used to build an air-processed inverted quantum junction device, which shows the highest current density from any CQD solar cell and a solar power conversion efficiency as high as 8%.
But improved performance is just a start for this new CQD solar cell architecture. The powerful little dots could be mixed into inks and painted or printed onto thin, flexible surfaces, such as roofing shingles, dramatically lowering the cost and accessibility of solar power for millions of people.
“The field of colloidal quantum dot photovoltaics requires continued improvement in absolute performance, or power conversion efficiency,” said Sargent. “The field has moved fast, and keeps moving fast, but we need to work toward bringing performance to commercially compelling levels.”
This research was conducted in collaboration with Dalhousie University, King Abdullah University of Science and Technology and Huazhong University of Science and Technology.
Ning, Z., Voznyy, O., Pan, J., Hoogland, S., Adinolfi, V., Xu, J., Li, M., Kirmani, A., Sun, J., Minor, J., Kemp, K., Dong, H., Rollny, L., Labelle, A., Carey, G., Sutherland, B., Hill, I., Amassian, A., Liu, H., Tang, J., Bakr, O., & Sargent, E. (2014). Air-stable n-type colloidal quantum dot solids Nature Materials DOI: 10.1038/nmat4007
