A New Approach to Increasing Solar Cell Efficiency

Researchers have developed a method that allows the conversion efficiency of sunlight into energy to exceed 100%. This was achieved using a novel molecular system capable of increasing the amount of energy harvested from solar radiation. This breakthrough could lay the foundation for next-generation solar technologies.

Limitations of Traditional Solar Cells

Solar energy is considered a key tool in reducing fossil fuel use and combating climate change. Despite the Sun constantly delivering vast amounts of energy to Earth, modern solar cells can capture only a small fraction of this flow. This is due to a physical limit that has long been considered insurmountable.

The Study and a New Mechanism

In a study published on March 25 in the Journal of the American Chemical Society, scientists from Kyushu University (Japan), together with colleagues from Johannes Gutenberg University in Mainz (Germany), proposed a way to overcome this barrier. They used a molybdenum metal complex known as a "spin-flip" emitter to capture additional energy generated during singlet fission (SF). This process enables a higher light conversion efficiency, previously thought unattainable.

With this technology, they achieved an energy conversion efficiency of about 130%, surpassing the traditional 100% limit and paving the way for more advanced solar cells.

How Solar Cells Work and Energy Losses

Solar cells generate electricity when photons from sunlight hit a semiconductor and transfer energy to electrons, causing them to move and create an electric current. However, not all photons are equally useful: low-energy photons (infrared) cannot activate electrons, while high-energy photons (such as blue light) lose excess energy as heat. As a result, solar cells utilize only about a third of the incoming sunlight. This limitation is known as the Shockley–Queisser limit.

Singlet Fission as a Way to Boost Energy

There are two main strategies to overcome this limit: converting low-energy infrared photons into higher-energy visible photons, and using singlet fission to generate two excitons from a single photon. Normally, each photon creates only one spin-singlet exciton, but with SF, this exciton can split into two lower-energy spin-triplet excitons, potentially doubling the available energy. Some materials, like tetracene, support this process, but efficiently capturing such excitons has been challenging.

Overcoming Energy Losses Due to FRET

Some energy can be lost through the Förster Resonance Energy Transfer (FRET) mechanism before exciton multiplication occurs. To address this, the researchers used a molybdenum "spin-flip" emitter that selectively captures multiplied triplet excitons after fission. In this system, the electron changes its spin when absorbing or emitting near-infrared light, allowing efficient extraction of triplet energy generated during SF. Precise tuning of energy levels minimized FRET losses and ensured effective harvesting of multiplied excitons.

Experimental Results and Future Prospects

Combined with tetracene-based materials in solution, the system successfully collected energy with a quantum yield of about 130%. This means that approximately 1.3 molybdenum metal complexes were activated for every photon absorbed, exceeding the usual limit and demonstrating the creation of more energy carriers than incoming photons.

The research is currently at the proof-of-concept stage, but the results could inspire further work on integrating singlet fission and metal complexes. This opens up prospects not only for solar energy, but also for LEDs and new quantum technologies. In the future, the integration of these materials into solid-state systems is planned to further improve energy transfer efficiency and move closer to practical applications in solar cells.