Researchers Discover “Spin Flip” in Metal Complexes That Push Solar Energy Conversion to 130%

Solar power has become the most affordable and popular option for renewable energy generation in the fight against climate change. However, the core design of solar cells has created a physical ceiling on the efficiency of this energy source, despite the abundance of sunlight in some regions on earth. This ceiling has regularly been touted as impossible to break, but a research team led by Kyushu University in Japan in partnership with Johannes Gutenberg University (JGU) Mainz from Germany have just done the impossible. To understand this “miracle” in science, let’s first look at an overview of how solar cells work to generate electricity.

How Solar Cells Generate Electricity

Solar cells convert sunlight into electricity directly through the photovoltaic effect using semiconductor materials, such as silicon. Sunlight contains many energized particles known as photons that radiate from the sun outward. When they reach a solar panel, these particles hit the semiconductor and knock electrons loose from the material’s surface to create a flow of electricity that forms direct current, which flows in one direction.

However, sun rays contain multiple particles that vary in their energy and ability. There are the low-energy particles, such as infrared photons, which are not powerful enough to excite and dislodge electrons from the semiconductor’s atoms to create an electric flow. On the other hand, high-energy particles, such as blue light photons, are too powerful, so most of their energy is lost as heat when it hits the semiconductor.

So only the middle-spectrum light photons efficiently excite the solar cell semiconductors to dislodge them and create an electric current flow. This means the most efficient solar panels theoretically only harvest about a third of the sun’s energy, and this is known as the Shockley–Queisser limit. The definition of this term is the maximum theoretical efficiency of a single junction solar cell, which is around 33.7%. In real life, solar panel efficiency stands at around 15-23%, depending on the material (monocrystalline, polycrystalline, etc.)

How To Break Through the Shockley–Queisser Limit

The researchers had two ways to break the Shockley–Queisser limit.

1. First is to increase the energy of the low-energy infrared photons into visible high-energy photons that are powerful enough to dislodge semiconductor electrons in the solar cell.

2. To use Singlet Fission (SF) to generate two lower energy excitons from a single exciton photon of the high energy particles, such as blue light.

The research team used option 2. A single photon generates at most one spin-singlet exciton after electronic excitation, and SF can split high-energy singlet exciton into two usable lower-energy spin-triplet excitons, effectively doubling the energy output from each cell. The challenge here is to capture these two SF-born excitons because their energy can be easily stolen and wasted by a mechanism known as FRET (Forster Resonance Energy Transfer) just before fission occurs. Organic semiconductor materials like tetracene can capture these excitons, but the process is still challenging.

To overcome this, the team settled on metal complexes because they have molecule structures that can be designed flexibly to harvest the SF-born excitons. They discovered a molybdenum-based “spin-flip” emitter as the best harvesting material for these electrons. In this metal complex's molecules, an electron flips its spin during the emission or absorption of near-infrared light, meaning it is capable of accepting the spin-triplet excitons produced by the SF process. This solved the first challenge.

The next one was to prevent energy stealing by the FRET process, and the team only had to tune the energy levels of the spin-triplet excitons produced by the SF process to overcome energy capturing by FRET to eliminate wastage. This allowed the multiplied excitons to be selectively attracted by the molybdenum-based harvester.

What This Means For Solar Efficiency Figures

This discovery achieved a quantum yield of 130%, which means about 1.3 molybdenum-based metal complexes were excited by each absorbed photon. 130% exceeds the 100% limit, meaning if used in solar cells, these complexes would generate more energy carriers than the photons they absorb from the sun. However, this work is still mostly at the proof-of-concept stage.

The next step for the team is to build and assemble actual solar cells to test this concept, and they plan to do this by bringing the two types of materials together in the solid state to push for efficient energy transfer and eventually integrate them into fully operational solar cells.

Remember, the Shockley–Queisser limit is around 33.7%. Spin Flip emitters capture the higher energy band of light photons, meaning only the low-energy non-visible light will not be absorbed. So this limit might increase to around 67%, resulting in a double power output from the same surface area.

For solar farms, this means they can generate double energy from the same real estate space, resulting in higher returns and a grid that relies less on carbon-based fuels. For homeowners, industries, and other consumers of solar energy, this breakthrough can mean less reliance on the grid even during rainy, snowy, and cold winter months. These solar energy consumers will also be able to generate and sell more energy to the grid using the same surface area during summer, resulting in a higher net income or reduced bills.

The only challenge might come about in the storage department, but battery technology is also advancing, and these two might intersect at just the right moment in time to push solar energy to new limits and become the mainstream source of energy globally.

What Next For Spin-Flip Metal Complexes

The researchers involved in this study hope their discovery will inspire other educational institutions and companies to further explore this intersection of metal complexes and singlet fission to advance semiconductor technologies that deal with photon emission or absorption, such as solar panels and light emitting diodes. It could also be useful in discovering the next generation of quantum technologies.

This research was published in the Journal of the American Chemical Society on March 25, 2026 (DOI: 10.1021/jacs.5c20500) with the title being “Exploring Spin-State Selective Harvesting Pathways from Singlet Fission Dimers to a Near-Infrared-Emissive Spin-Flip Emitter.”