Danish Transparent Solar Cells Achieve 12.3% Efficiency

Researchers from the EU-funded CitySolar project have developed a new technology that can turn ordinary windows into power-generating units without losing their transparency. It offers an efficiency of 12.3% and a transparency of 30%, the team claims to have set a world record for transparent solar cell modules. These solar cells work like the leaves of a beech tree in spring: They let sunlight pass through while also harvesting energy from the sun’s rays. In that, they are different from BIPV or Building Integrated  Photo Voltaic modules where transparency remains a key issues to solve, even as they offer much higher efficiencies than that promised here.  Transparent solar modules will simply open up hitherto closed options for solar usage.

“Transparent solar cells could be the next big step in building integrated energy solutions,” explains Morten Madsen from the University of Southern Denmark, one of the key researchers behind the project.

Utilizing existing glass surfaces in buildings

Transparent solar cells are desirable because they solve several key challenges. They function as standard windows, allowing light inside, and as electricity-generating units. This dual function enables buildings to produce energy without compromising their architectural aesthetics. “The large glass facades found in modern office buildings can now be used for energy production without requiring additional space or special structural changes,” Morten Madsen points out.

At the same time, the technology enables on-site energy production—unlike distant solar farms, these solar windows generate electricity exactly where needed, reducing transmission losses and minimizing the need for extensive grid upgrades.

Solving the efficiency-transparency trade-off

Until now, the main challenge with transparent solar cells has been balancing efficiency and transparency. Transparent solar windows have struggled to absorb enough energy to generate significant electricity. Conversely, efficient solar windows have often lacked sufficient transparency. The CitySolar project has now successfully overcome this issue. This can potentially transform the building sector, which accounts for approximately 40% of the EU’s energy consumption.

“We are moving towards so-called nearly-zero or zero-energy buildings, but with this technology, we can go even further and create energy-producing buildings. This represents a massive market opportunity,” says Madsen.

A cost-effective solution

The technology combines two types of solar cells—perovskite and organic solar cells—which utilize different parts of the light spectrum. The perovskite layer absorbs near-ultraviolet light, while the organic layer absorbs near-infrared light, leaving the visible spectrum relatively untouched.

“The tandem solar cell mainly harvests energy from the infrared and ultraviolet parts of the sun’s rays, but not from visible light. This allows us to set new efficiency standards for semi-transparent solar windows,” Madsen explains.

The mineral perovskite has proven to be ultra-cheap and highly efficient in converting sunlight into electricity, particularly in the ultraviolet range. Meanwhile, organic solar cells, consisting of carbon-based materials in ultra-thin plastic layers, are also cost-effective. “This means we are dealing with a highly affordable technology,” says Madsen, though he emphasizes that the main commercial challenge lies in balancing cost, aesthetics, and efficiency. While further investment is required to bring the technology to a commercial level, Morten Madsen is optimistic about its future.

“We are in discussions with industry partners about the next steps. We can scale up what we have, but we need business partners. And there are still research improvements to be made—but importantly, we know where the challenges lie and have a clear strategy for overcoming them,” he explains.

About The CitySolar Project

The CitySolar project is coordinated by the Italian National Research Council (CNR, Prof. Aldo Di Carlo). It includes nine partners from seven countries, including leading universities such as the University of Southern Denmark, Friedrich-Alexander-Universität Erlangen-Nürnberg, and the University of Rome Tor Vergata, as well as multiple research centers.

The project is currently at Technology Readiness Level (TRL) 5-6, between the proof-of-concept and prototype stages. By 2050, the EU aims to make all new buildings nearly zero-energy and fully decarbonize the European building sector. The project has received €3.8 million in funding from the EU Horizon 2020 program.