Japan scientists achieve sunlight-to-UV conversion with 1.9% efficiency, and it could reshape solar-powered technology

Scientists at Kyushu University in Japan have pulled off something that sounds almost like a physics magic trick taking ordinary visible sunlight and converting it into ultraviolet light using a newly designed solid-state material.

The material works by combining the energy of two low-energy photons to produce a single, higher-energy UV photon, a process called photon upconversion. Published on June 23 in Nature Communications, the study marks the first time this kind of conversion has been achieved efficiently in a solid material using nothing more than natural outdoor sunlight, at a conversion efficiency of 1.9%.

Why UV light matters and why converting sunlight to UV is so hard

Ultraviolet light might have a bad reputation for causing sunburn, but it is genuinely indispensable in a range of technologies.

It is widely used in air purification systems, in curing dental fillings, hardening gels in nail products, and in resin-based 3D printing. The problem is that UV light makes up only about 6% of the sunlight that actually reaches Earth's surface, and only a portion of that is practically useful.

So harnessing the sun to generate UV on demand has long been an attractive but difficult goal.The underlying physics of what the Kyushu team has done involves a phenomenon called triplet-triplet annihilation (TTA).

In simple terms, a donor molecule absorbs visible light and shifts its electrons into a high-energy state. That energy is then passed to a nearby acceptor molecule. When two of these energised acceptor molecules interact, they pool their energy and release it as a single UV photon higher in energy than either of the original light particles that went in.

It is, as the researchers themselves describe it, a little like combining two cups of warm water and expecting to get one cup of boiling water.

It should not work in everyday life, but in the quantum world, it does.

The molecular design that made solid-state upconversion work

The real challenge was not making this happen in a liquid that had already been done. The difficulty was making it work in a solid. In liquid systems, molecules can move around freely, which makes it easier for the energised triplet states to find each other. But those same liquid systems tend to rely on toxic solvents and are prone to evaporation, making them impractical for real-world devices.In solid materials, molecules are packed tightly together. That density creates a different problem: the electron clouds of neighbouring molecules overlap too strongly, causing the excited triplet states to collapse before they can combine and produce UV light. Finding the sweet spot close enough for energy transfer, but not so close that everything fizzles out had stumped researchers for years.The Kyushu team solved this by working with an organic semiconductor called dihydroindenoindenedene (DHI).

They attached alkyl chains to specific carbon atoms on the DHI molecule the sp3 carbons, which have four bonds pointing outward in fixed three-dimensional directions. This structural feature created precisely controlled gaps between neighbouring molecules in the solid. The molecules ended up close enough to hand off energy to each other, but spaced just far enough apart to prevent the electronic overcrowding that kills efficiency.The resulting material, as detailed in the Nature Communications paper by Harada et al., produced bright light emission, maintained long-lived excited states, and achieved a solid-state fluorescence quantum yield exceeding 60%. When combined with a donor molecule, the full system reached that 1.9% visible-to-UV upconversion efficiency under real outdoor sunlight conditions.

What 1.9% actually means and why it matters

That number might not sound impressive at first glance. But context matters here.

Yoichi Sasaki, the study's corresponding author and Associate Professor at Kyushu University's Faculty of Engineering, points out that roughly two UV photons are produced for every hundred visible-light photons absorbed and crucially, the whole process runs on natural sunlight alone, without any need for concentrated or artificial light sources.

Most solid-state materials that attempt photon upconversion cannot match even this performance, even when exposed to light intensities far beyond what the sun provides outdoors.The team has already filed a patent application for the material. Beyond its scientific novelty, it has practical advantages too the synthesis process is relatively straightforward and relies on inexpensive starting materials, making eventual scale-up a realistic prospect.

Potential applications: from clean air to solar-driven chemistry

The researchers see several near-term application areas where sunlight-driven UV generation could make a real difference. Solar-driven photocatalysis is one using sunlight to trigger chemical reactions that currently require dedicated UV lamps.

Indoor air purification is another, since UV light is effective at breaking down airborne pathogens and pollutants. Low-intensity UV-based 3D printing is a third area, where the ability to use ambient or solar UV rather than powered UV lamps could simplify and cheapen the process considerably.All of these applications currently depend on either direct UV from the sun which is limited and variable, or on UV lamps that consume electricity.

A material that harvests visible sunlight and converts it into usable UV on its own could eventually reduce or replace both.

A research journey more than 14 years in the making

The breakthrough also carries a personal dimension. The foundational work for this project began in 2012, when Nobuo Kimizuka, now Professor Emeritus at Kyushu University's Research Centre for Negative Emissions Technologies, started exploring photon upconversion through triplet energy migration in molecular self-assemblies.

Over the following years, his team made steady progress in solutions and gels, but the solid-state problem remained unsolved.That changed in the final stretch before Kimizuka's retirement in 2024. Graduate students Naoyuki Harada, Hayato Shoyama, and Nutnicha Boonmong, along with then-Assistant Professor Kiichi Mizukami, joined Sasaki in a concentrated push to bring the research across the finish line. They handed the completed draft to Kimizuka just eleven days before he left the lab."This discovery is the culmination of over 14 years of our research and marks a major milestone in photon-upconversion and molecular self-assembly research," Kimizuka said. For a field that has spent over a decade trying to make solid-state photon upconversion work under real-world conditions, the timing felt like more than just a scientific result it was, as the team put it, a retirement gift.