Researchers in Denmark are experimenting with a selenium–silicon tandem solar cell.

While an increasingly common clean energy resource for individual homes and grid-scale production alike, solar cells are shockingly inefficient – at best capturing less than 30% of the energy from the sunlight that strikes them.

Rasmus Nielsen and his team of physicists and engineers at the Technical University of Denmark have found a possible method to boost that efficiency to 40% by creating a selenium-silicon tandem solar cell – building up multi-material layers with different photon-absorption properties that increase the wavelengths of sunlight that can be captured and transformed into electricity.

For this new study, published in the journal PRX Energy, the team went back to selenium, a semiconducting material that was used before silicon to make early solar cells. Selenium's different photon-absorbing properties allow for the creation of a dual-material solar cell with a wider bandgap.

Selenium is found in a few rare minerals and is most commonly obtained during the electrolytic refining of copper. Selenium also has a much lower melting point, making it less energy-intensive to process than silicon.

"Tandem solar cells, which integrate two absorbers with different bandgaps into a single device, hold promise for achieving significantly higher device efficiencies; nevertheless, only a few such devices have reached commercialization," the report said. "The main challenge for realizing the next-generation, low-cost tandem solar cell is the identification of a wide-bandgap top cell that is process compatible with a low-bandgap bottom cell while maintaining high performance, low cost and long-term stability."

To fabricate their prototype, Nielsen and his team sandwiched a thin film of crystallized selenium on top of a standard silicon solar cell base coated with various conductive oxide layers, a combination which was able to generate 1.68 volts of electricity with a conversion efficiency of 2.7%.

While a far cry from silicon's maximum power-conversion efficiency of 26.8%, the voltage aligns with energy goals for multi-material cells. The team believes that a tenfold increase in efficiency can be achieved by changing the conductive materials that connect the silicon with the selenium, thereby reducing voltage losses from electrical resistance in the circuit and increasing electron transport.

Experimenting with physical material structures and device simulation, the team is still investigating architecture and interfaces, which is providing valuable insight into the key challenges that must still be addressed to realize higher efficiencies.

The team's calculations suggest with confidence that with updates and refinement of the cell structure, the performance of their selenium-silicon cell could soon rival today's best silicon cells with an increased efficiency rating of 40%.