Photon Upconversion Could Expand Solar Hydrogen Potential

When it comes to converting sunlight into useful energy, a lot of effort has been focused on photovoltaics, as this is a method that can convert a lot of the Sun’s energy output into electricity.

However, this does not mean this is the most efficient option for all applications. For example, if the goal is to produce green hydrogen, this creates a multi-step process where efficiency is lost at each step: sunlight -> power -> transmission -> electrolysis -> hydrogen.

This is why different approaches have been investigated, notably using sunlight directly to convert water into hydrogen, a process known as photocatalysis.

The problem is that even with the right catalysts, most of the sunlight is in the visible and infrared ranges, which are simply not energetic enough to split water molecules into hydrogen. So even with silicon carbide boosting photocatalysis efficiency, it is still not ideal. In large part, only the ultraviolet (UV) part of the light spectrum is strong enough.

This is why the discovery by Japanese researchers at Kyushu University and the Institute for Molecular Science, SOKENDAI, that a new solid-state method could be used to increase photon energy levels could be a game-changer for future green hydrogen production. They published their results in the prestigious journal Nature Communications¹, under the title “Sterically protected π-electron systems for efficient solid-state photon upconversion”.

From Visible To UV Light

Photocatalysis of water into hydrogen could radically boost green energy production. This is because green hydrogen is a key missing element for storing energy over weeks and months of low sunlight or no wind, and also the perfect fuel for decarbonizing sectors like shipping and air flight, either directly or through the production of ammonia and artificial fuel. But unfortunately, only UV is strong enough to perform photocatalysis.

“Although inorganic photocatalysts using ultraviolet (UV) light have achieved high-efficiency photocatalytic water splitting, they suffer from the low UV fraction in sunlight (about 3% for the 300–400 nm range).”

But the alternative could be not a better catalyst, but converting the much more abundant visible light into UV, or “photon upconversion”.

The researchers focused on a process called triplet–triplet annihilation-based photon upconversion (TTA-UC). In its simplest explanation, this merges two low-energy photons into a single, higher-energy photon by having them absorbed by an acceptor molecule before being reemitted.

Source: Nature

Optimizing Photon Upconversion Stability

From Liquid To Crystals

So far, methods for upconversion using molecules like  1,4-bis((triisopropylsilyl)ethynyl)naphthalene (TIPS-Nph) and 2,5-diphenyloxazole (PPO) have good quantum yields (ΦUC), but solvent volatility poses a critical limitation for device applications and long-term use.

A practical solution is instead needing stable materials that can be deployed at scale, with minimal to no maintenance, so that entire fields of photocatalytic converters can be deployed to mass produce green hydrogen.

In crystals and solid acceptors, a phenomenon called singlet quenching can reduce quantum yield.

The researchers have used alkyl-chain substitution (adding longer carbon chains) in the organic molecules used as acceptors to increase both stability and reduce occurrences of singlet quenching.

Source: Nature

Measuring Crystal Performances

The researchers used a molecule called DHI (5,10-dihydroindeno[2,1-a]indene) with an almost perfect 96% quantum yield when in liquid form (solution). But it normally declines very badly in yield when in crystal form.

When adding the extra carbon chains to the molecule, the crystal form of DHI can reach quantum yields as high as 64%-69%. These high results indicate that the donor molecules are uniformly dispersed within the acceptor crystal, enabling efficient triplet sensitization.

Source: Nature

The material could also be produced with simple film-forming techniques, such as room-temperature casting and spin coating, without requiring any special heating treatment, making it more likely to be relevant for any future industrial large-scale application.

The process is also oxygen-tolerant and even requires it, which means it does not need to occur in a sealed, oxygen-free environment, another important element to achieve for commercial applications.

“TTA-UC is turned on when oxygen in the system is consumed by conversion to singlet oxygen. The iBu-DHI/Ir(ppy)3 film showed upconversion in air even under intense irradiation (λdt = 370 nm, Iex = 2.0 W cm–2) for more than 1 h. ”

Source: Nature

Crystals’ performance generally depends on the microscopic structure at the atomic level. So the researchers first conducted theoretical calculations to determine the likely structure of these crystals.

They then tested the crystal with X-ray crystallography and found that the X-ray diffraction patterns of the single crystals and those of the spin-coated films were similar, demonstrating why this method worked.

Source: Nature

This is not to say that the crystals could not be optimized further, with even higher yield theoretically possible with a more precise method to control the creation of the individual crystals and their organization in a thin layer.

“The performance of the present solid-state Vis-to-UV TTA-UC system could be further improved by optimizing the donor molecular structure and employing a controlled crystallization process.”

Future Applications

Currently, hydrogen production is dominated by “gray hydrogen” produced from fossil fuels, and a small but growing portion is made from renewable energy, or “green hydrogen”, which still struggles to be economically competitive with other fuels.

Ultimately, directly taking the sunlight and using it to produce hydrogen, without massive power transmission, batteries, cables, and electro-catalysts, could reduce the total price of such an installation greatly. No intermediary steps will also improve the overall energy efficiency of green hydrogen production, a serious issue with methods using electrocatalysis.

“The design principle of the π-protected DHI chromophores developed in this study will be widely extended to various chromophores. It enables excellent TTA-UC properties in thin films prepared by simple spin-coating and drop-casting methods, paving the way for broad applications and promising to revolutionize photofunctional chemistry involving excited triplets.”

Such novel solid-state materials with good stability could make commercially viable next-generation photonic materials by converting low-intensity, abundant photons into hydrogen-generating, high-intensity UV photons.

Investing In Advanced Solar Energy

First Solar

Currently, most of the world’s photovoltaic panels are produced in China, thanks to the country’s extensive ecosystem in the production of polysilicon and the manufacturing of solar cells.

However, other technology than silicon-based solar cells exists, and one of the survivors of the solar industry in the West, First Solar, is leading in this field, using cadmium telluride solar cells. They are both easier to produce (thin-film technology) and have a higher efficiency than silicon-based cells, albeit with higher costs for their raw material.

This type of cell is also more durable, which can change the equation for both homeowner and utility companies when calculating the lifetime cost of a solar cell and its depreciation. This is especially true as the quick progress in solar cell yield and declining costs has been slowing down in the past few years.

Source: First Solar

As cadmium telluride cell production is a mostly automated manufacturing process, it is relatively less sensitive to differences in labor costs. This can make its production in Western countries a lot more competitive, especially when they are sold locally, and it removes shipping costs from the equation.

Instead of multiple factories, with each actor specialized in one segment like polysilicon purification, and with many days to produce a solar cell, First Solar can go from raw materials to finished product in less than 4 hours.

Source: Department Of Energy

In the long run, First Solar expects to be able to fully recycle the cadmium telluride from old cells, and 90% of the total solar cells. The remaining 5-10% of the recycled module scrap consists primarily of glass fine particles, which are captured by dust control systems and High-Efficiency Particulate Air (HEPA) filtration systems.

This could reduce material costs, remove the ecological cost of resource extraction, and remove any risks of pollution.

“With every module sold, we also sold the service that we pick up the modules at the end of life and recycle them. That was basically 8 years before regulation came in in Europe. We now have the electronic waste directive where PV is part of that.”

Andreas Wade – Global Sustainability Director at First Solar Future Techs

Besides cadmium telluride, First Solar is also exploring even more advanced solar cell technology, like perovskite and cadmium telluride-perovskite hybrid cells, which could have higher efficiency and even more durability.

In the long run, the experience of First Solar in producing thin-film photovoltaic panels could also be applied to photocatalyst cells for hydrogen production.

Overall, First Solar is a great stock for investors looking to invest in the solar energy boom with a focus on Western producers, instead of the more geopolitically sensitive Chinese producers.

(You can read more about First Solar in our investment report dedicated to the company and about solar energy in our report “The Solar Age – A Bright Future To Mankind”)

Latest First Solar (FSLR) Stock News and Developments

Study Referenced

^{1. Harada, N., Shoyama, H., Boonmong, N. et al. Sterically protected π-electron systems for efficient solid-state photon upconversion. Nature Communications. 17, 5134 (2026). https://doi.org/10.1038/s41467-026-73898-0}