Scientists continue to tinker with recipes for turning sunlight into electricity. By testing new materials and components, in varying sizes and combinations, their goal is to produce solar cells that are more efficient and less expensive to manufacture, allowing for wider adoption of renewable energy.
The latest development in that effort comes from researchers in St. Petersburg, Russia. The group recently created a tiny prototype of a high-efficiency solar cell using gallium phosphide and nitrogen. If successful, the cells could nearly double today’s efficiency rates—that is, the degree to which incoming solar energy is converted into electrical power.
The new approach could theoretically achieve efficiencies of up to 45 percent, the scientists said. By contrast, conventional silicon cells are typically less than 20 percent efficient.
“Silicon is a very cheap material and it’s well developed, but it’s not highly efficient,” said Ivan Mukhin, a researcher at ITMO University and a lab director at St. Petersburg Academic University. “If we can improve efficiency, you can lower the price of producing solar cells… and help reduce the price of producing energy.”
Mukhin and his colleagues published their results this month in the journal Solar Energy Materials and Solar Cells. Their research builds on work by Zhores Alferov, the late Russian physicist and Nobel Prize winner. Alferov predicted the possibility of combining silicon with A3B5 materials, a family of semiconductors, to improve efficiencies. The new prototype is the first to demonstrate the concept using diluted gallium phosphide—a polycrystalline compound semiconductor—with nitrogen atoms.
The prototype cell is just 1 square centimeter in size. (For context, a typical solar cell is about 16 square centimeters, and dozens are used in a single solar panel.) Mukhin and his team began with a silicon substrate, or wafer, which is a thin slice of crystalline silicon. On top of that, they grew a layer of pale orange gallium phosphide. The compound integrates well into silicon, but gallium phosphide has limited light-trapping properties.
However, scientists found a way around that. When combined with nitrogen, the compound demonstrated a direct bandgap and was “great” at absorbing light, they said. The cell’s single photoactive layer showed a solar efficiency of 2 percent. Now, the researchers are working to grow additional photoactive layers on the substrate. So-called “multi-junction” solar cells absorb different wavelengths of incoming sunlight, which makes them more efficient than the more common single-junction silicon cell.
Mukhin acknowledged that the materials his team studies are still significantly more expensive than silicon. He said one way to lower costs could be to combine the new solar cells with concentrated solar power technologies. Using mirrors or lenses, these systems concentrate a large area of sunlight onto a receiver, which could reduce the number of costly cells needed to generate electricity. Likewise, solar-panel equipment that positions cells to face the sun could also maximize sunlight and minimize the number of cells required.
“Right now, this is basic fundamental research to show that you really can grow gallium phosphide on silicon,” Mukhin said of his team’s work. And so far, he said, it seems like “a very promising way to improve the final efficiency of your solar cell.”