now we know how to use molybdenum to squeeze each photon to the maximum

The Sun bathes the Earth every second with an unfathomable amount of energy, but human technology suffers from a serious problem of myopia when it comes to capturing it. Until now, traditional solar cells have encountered an insurmountable “physical ceiling” that prevents them from harnessing most of this light. This theoretical limit dictated that no matter what we do, a conventional panel can only harness about a third of the incoming sunlight.

The rules of the game have changed. An international team of researchers has achieved what until recently was considered impossible: developing a system that achieves an energy conversion efficiency close to 130%. In simple terms, the new design is capable of producing more energy carriers than the photons (light particles) it absorbs. The master key behind this science fiction breakthrough is not an exotic new synthetic material, but an old acquaintance of heavy industry: molybdenum.

The quantum relay race. To understand the magnitude of this find, you have to look inside a solar panel. As explained by Kyushu University (Japan)generating electricity from the sun is like a microscopic relay race: photons hit a semiconductor material and pass their energy to electrons, setting them in motion to create a current.

The problem, the university details, is that not all “runners” are the same. Infrared photons have too little energy to activate electrons, while blue light photons have too much, and the excess is wasted uselessly as heat. This frustrating limitation is what physics knows as the limit of Shockley-Queisser.

Jumping over the wall. Scientists have turned to a “dream technology” known as singlet fission (SF). According to the study published in the journal Journal of the American Chemical Society (JACS)singlet fission allows a single high-energy photon to “split” into two smaller energy packets (excitons). It is the equivalent of buying a lottery ticket and getting two prizes.

“We have two main strategies to overcome this limit,” explains Yoichi Sasaki, associate professor at the Faculty of Engineering at Kyushu University. “One is to use singlet fission to generate two excitons from a single photon.”

But there was a catch. Sasaki points out that, under normal conditions, this extra energy is immediately “stolen” by a parasitic mechanism called Förster resonance energy transfer (FRET). The prize disappeared before it could be collected.

This is where the hero of the story comes in. As detailed in the investigation of JACSscientists designed a molybdenum-based metal complex that acts as a “spin-flip” emitter. By absorbing light, an electron in this molybdenum material changes its spin, allowing it to selectively capture that multiplied energy and block the “thief” (FRET). Molybdenum manages, for the first time efficiently, to collect twice as much energy.

The role of molybdenum. Historically, molybdenum has been valued for being an extreme refractory metal. Molybdenum has a brutal fusion of 2620 °Clow thermal expansion and excellent electrical and thermal conductivity. These properties make it indispensable today for manufacturing crucibles that resist molten glass, motherboards for semiconductors, and components for power electronics that must reliably dissipate heat.

This same dimensional stability and thermal conductivity are what have allowed its chemical properties to be refined at the molecular level for the “spin-flip”. However, as Kyushu University warnswe are facing a proof of concept. The impressive 130% yield has been achieved in a laboratory environment, combining the molybdenum complex with tetracene-based materials in a liquid solution. The next great engineering challenge will be to take this solution from the liquid to the solid state.

A quantum leap forged as a team. This milestone was born from collaboration with Johannes Gutenberg University (JGU) in Germany. It was the researcher Adrian Sauer who connected the German studies on molybdenum with the efforts of the Japanese team. The synergy was resounding: the JACS study certifies quantum yields of between 112% and an astonishing 132%, managing to activate an average of 1.3 molybdenum complexes for each photon absorbed.

But the shockwave of this discovery transcends solar panels. Both JACS and Kyushu University highlight that mastering this energy harvesting paves the way to ultra-efficient light-emitting diodes (LEDs) and promises to revolutionize key tools for spintronics and the emerging quantum industry.

The end of the physical ceiling. The limit of 100% efficiency in capturing sunlight has been, for decades, an unbreakable dogma in materials physics. Today we know that it was not a brick wall, but a locked door that only needed the right key.

It is fascinating to see how this key was hidden in molybdenum, an element of the old industrial guard, forged at high temperatures and known for its extreme resistance. By fusing the centuries-old strength of transition metal chemistry with the cutting edge of singlet fission, science has shown that we are still a long way from reaching the ceiling in our race to squeeze every photon that the Sun gives us.

Image | freepik and John Chapman

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