A solid material that ‘converts’ visible light photons into UV light photons could change the way we use sunlight

The importance of solar energy as a renewable energy source is increasing. Sunlight contains high-energy UV light with a wavelength below 400 nm, which can be widely used, for example, for photopolymerization to form resin and activation of photocatalysts to drive reactions that generate green hydrogen or useful hydrocarbons (fuels, sugars, olefins, etc.). The latter is often called “artificial photosynthesis”. Photocatalytic reaction by UV light to effectively kill viruses and bacteria is another important application. Unfortunately, only about 4% of Earth’s sunlight is in the UV range of the electromagnetic spectrum. This leaves much of the sunlight spectrum untapped for these purposes.

Photon upconversion (UC) could be the key to solving this problem. It is the process of converting long wavelength, low energy photons (such as those present in visible light) to short-wavelength, high-energy photons (such as those found in UV light) via a process called “triplet-triplet annihilation” (TTA). Previous work in this area has reported UV-visible UCs using organic solvent solutions which required the solution to be first deoxygenated and then sealed in an airtight container to avoid exposure to oxygen which deactivated and degraded the UC samples of TTA-based photons. Not only did these materials lack photostability in the presence of oxygen, they also did not perform effectively with incident solar-intensity light. These problems have presented obstacles in practical applications of photon CU.

Now, two Tokyo Tech scientists—Prof. Yoichi Murakami and his graduate student Riku Enomoto – have found a solution to these problems – a revolutionary solid film that can perform UV visible photonic UC for low incident light while remaining photostable for an unprecedented amount of time in air. They describe this revolutionary invention in their article published in the Journal of Materials Chemistry C.

Professor Murakami explains the novelty of their research: “Our invention will enable the practical use of the visible portion of low-intensity light, such as sunlight and LED mood lighting, for applications that are efficiently made with UV light, and its photostability—shown to be at least over 100 hours, even in the presence of air—is the highest ever reported in any TTA-based photon UC material. , regardless of the shape of the material, as long as we could study.

In addition to this record photostability, these films had an ultra-low excitation threshold (only 0.3x the intensity of the sun) and a high UC quantum efficiency of 4.3% (normalized UC emission efficiency of 8. 6%) – both in the presence of air – making it a unique material, as most materials in this class lose their UC photonic capability when exposed to air.

To prepare this material, the researchers fused a sensitizer (i.e. a molecular chromophore capable of absorbing longer wavelength photons) with a much larger amount of an annihilator (i.e. i.e., an organic molecule that received the energy excited by the sensitizer triplet and then caused the TTA process); the combination of sensitizer and annihilator was selected by the researchers. This two-component melt was then cooled on a controlled temperature gradient surface to form a solid-state UV visible photonic UC thin film.

That new technique—temperature gradient solidification—is highly controllable and repeatable, meaning it is compatible with realistic industrial processes. Professor Murakami tells us: “We believe that temperature-controlled solidification can provide a solid basis for developing advanced photon UC films, that too on a solid substrate without using organic solventsas demonstrated for the first time in this work.”

Finally, to demonstrate the UV visible photon UC of the thin film, the researchers used it with simulated sunlight of solar intensity consisting only of visible light to successfully cure and solidify a resin that would otherwise require UV light for the same process.

This study presented, for the very first time, a new class of UC solids with unprecedented photostability that can realistically be used for the upconversion of low intensity visible light photons to UV light photons in the presence of air.

“Our research will not only broaden the exploration of a new class of UV light generators materials but will also help greatly expand the utility of abundant low visible light to applications that are driven by UV light,” concludes Professor Murakami.