December 1, 2015 report
Quantum dots used to convert infrared light to visible light
(Phys.org)—A team of researchers at MIT has succeeded in creating a double film coating that is able to convert infrared light at modest intensities into visible light. In their paper published in the journal Nature Photonics, the team describes their film, how well it works and the possible uses for it.
One way to make a solar cell more efficient would be to increase the amount of light energy that is able to be captured and converted by it to electrical energy, and one of the ways to make that happen would be to find a way to capture and use photons that are below their normal bandgap. To accomplish that goal the team at MIT sought to upconvert photons in infrared light to visible light.
Their approach was to use two films placed on top of a clear plate of glass. The film layer on the bottom was made using a type of quantum dot—an inorganic semiconductor lead sulphid which had been coated with a single layer of fatty acids to make the surface passive. The top film layer was crystalline and made of rubrene, an organic molecule.
In the two film approach, the top film absorbs infrared light, and the energy from it is then transferred to the bottom film. That energy, which exists in the form of excitons is then diffused as it passes through the rubrene—a process called triplet–triplet annihilation. They tested the films by shining an infrared laser at the finished product and found it glowed with visible light—it works, the team notes, because the collision of two low-energy excitons create high-energy excitons, i.e. singlets, which can emit visible light. The team reports that the films are quite efficient at converting the infrared light to visible light.
Besides the possible use in solar cells the films might also be used in cameras, allowing, for example, a visual representation of what is going on in the fog covering a roadway, or in creating better night-vision goggles. The team has plans to improve their film approach, hopefully optimizing it to make it more efficient and perhaps allowing for converting longer infrared wavelengths.
Optical upconversion via sensitized triplet–triplet exciton annihilation converts incoherent low-energy photons to shorter wavelengths under modest excitation intensities1, 2, 3. Here, we report a solid-state thin film for infrared-to-visible upconversion that employs lead sulphide colloidal nanocrystals as a sensitizer. Upconversion is achieved from pump wavelengths beyond λ = 1 μm to emission at λ = 612 nm. When excited at λ = 808 nm, two excitons in the sensitizer are converted to one higher-energy state in the emitter at a yield of 1.2 ± 0.2%. Peak efficiency is attained at an absorbed intensity equivalent to less than one sun. We demonstrate that colloidal nanocrystals are an attractive alternative to existing molecular sensitizers, given their small exchange splitting, wide wavelength tunability, broadband infrared absorption, and our transient observations of efficient energy transfer. This solid-state architecture for upconversion may prove useful for enhancing the capabilities of solar cells and photodetectors.
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