Energy can never be created or destroyed. That’s basic Physics 101. You simply cannot create energy out of thin air. Yet researchers at Kyushu University in Japan say they have developed a technology that pushes the energy conversion efficiency of solar cells to 130%!
At first glance, the results of the research, conducted with collaborators at Johannes Gutenberg University in Germany, sound fanciful at best. However, the reality is far more nuanced. Using a molybdenum-based “spin-flip” metal complex paired with a singlet fission material, the scientists managed to generate more usable energy carriers than incoming photons.
Let’s break things down.
At any given moment during the day, the Earth receives roughly 89,000 terawatts of solar energy – almost 5,000 times the global human energy consumption annually. However, modern solar technologies capture only a fraction of it.
Photovoltaic solar cells, the kind that most likely come to mind when you think of solar panels, convert only about 20% of the sunlight that hits them into usable electricity. The conversion limitations primarily stem from the Sun itself.
Solar cells convert light into electricity through a relatively simple process. Photons, which are packets of light energy, stream in from the Sun and strike a semiconductor material, typically silicon. When a photon hits, it transfers its energy to an electron in the semiconductor, knocking it loose and setting it in motion. The energized moving electrons constitute an electric current.
The problem is that photons are not all equal. They arrive with wildly different energy levels depending on their wavelength. Infrared photons, at the low-energy end of the spectrum, do not carry enough energy to knock electrons loose at all. Instead, they pass through or are absorbed as heat, wasted. Blue light photons, on the other hand, carry far more energy than is needed to free an electron. The excess is shed as heat, also wasted.
This fundamental mismatch between the energy supply and the semiconductor’s electron threshold imposes a hard ceiling on efficiency known as the Shockley-Queisser limit. For a standard single-junction solar cell, that ceiling is around 33%.
Even with perfect engineering, you cannot extract more than a third of incoming solar energy this way. This is why even the very best commercially available solar panels do not surpass 25% conversion efficiency.
Now, under normal conditions, one photon excites one electron, creating a single unit of usable energy, known as an exciton. Even when a photon with more energy than needed hits the solar cell, only one exciton is generated. The rest of the energy is wasted as heat. So it’s always one photon, one exciton. This has always been considered a given. But what if it were not? This question forms the basis of the Kyushu research. The team’s approach centers on a phenomenon called singlet fission.
Singlet fission is a process in which a single high-energy exciton splits into two lower-energy excitons. Instead of producing one exciton per photon, the process allows a single high-energy photon to result in two lower-energy excitons.
“We have two main strategies to break through this limit,” explains Associate Professor Yoichi Sasaki of Kyushu University’s Faculty of Engineering. “One is to convert lower-energy infrared photons into higher-energy visible photons. The other, what we explore here, is to use singlet fission to generate two excitons from a single exciton photon.”
In theory, this could double the number of usable charge carriers. In practice, however, the process has a major flaw: those extra excitons are notoriously difficult to capture. The singlet fission concept is not new. The problem has always been capture. Before the two new excitons can be extracted and put to work, they tend to be hijacked by competing mechanisms, such as Förster resonance energy transfer (FRET), in which energy is effectively “stolen” before it can be used.
This is where the researchers’ innovation comes in, bringing with it the elegance of physics. Their solution: a molybdenum-based “spin-flip” emitter, a system that selectively captures these otherwise lost triplet excitons.
During absorption and emission, an electron within the complex flips its spin. This property makes it uniquely suited to accepting the triplet excitons produced by singlet fission while ignoring the competing FRET pathway. The result is a measurable quantum yield of around 130%. This means that, on average, 1.3 excitons are successfully harvested for every photon absorbed.
So … is that 130% solar conversion efficiency for solar cells? Absolutely not. An energy efficiency of 130% would violate the law of conservation of energy, the bedrock of physics. What the researchers achieved was 130% quantum yield, a measure not of energy, but of charge carriers per photon.
“Quantum efficiency usually should not be higher than 100%, but [quantum yield] can be, if a proper definition is provided, that is, depending on how it is defined,” explains Dr. Jin Zhang, Professor of Chemistry and Biochemistry at the University of California – San Diego, who was not a part of the research.
“What, then, is the ‘breakthrough’?” you may ask. Simply put, the solar cells do not absorb more sunlight than usual. Instead, they extract more usable charge carriers from the same absorbed light, recovering energy that would normally be lost as heat from high-energy photons.
Now that the “130%” definition is clear, it becomes easier to appreciate what the researchers have actually accomplished.
They have demonstrated a viable pathway to capture and use excitons that were previously inaccessible. By suppressing energy losses and improving how high-energy photons are handled, the system addresses one of the core inefficiencies in solar conversion. Blue light photons, which currently overshoot the threshold and shed the excess as heat, could instead be split into two usable excitons each, reducing heat loss and increasing current.
Realistic projections suggest that a well-engineered singlet fission-enabled solar cell could meaningfully push efficiencies beyond those of current commercial panels, with some models approaching 35-45% under ideal conditions. That’s up to double the efficiency in some models.
For now, the paper, published in the Journal of the American Chemical Society, is still in the proof-of-concept stage. The experiments were conducted in solution at the molecular level, meaning the technology is still several important steps away from a solid-state solar cell.
