The team built an inverted pyramid solar cell to capture more light.

Image: Lintao Hou

Light hitting a traditional flat solar

cell (top) and the researchers' inverted

pyramid design (bottom).

Images: Lintao Hou

When sunlight hits a solar panel, a lot of it bounces right back off and its potential to be turned into electricity is lost. However a team of researchers in China and Sweden designed a simple but effective new solar cell design that captures much of that reflected light, potentially tripling the amount of energy that could be captured per square foot.The team's design essentially turned a small pyramid inside-out, and coated its inside with a kind of flexible solar cell. They built and tested four different pyramids with progressively sharper "peaks" to measure how they compared to traditional flat solar cells."For flat solar cell[s], the light shines into the device only once and a large part of the light is reflected out, whereas for pyramid solar cell, the light is reflected many times within the structure," said Lintao Hou, a researcher at China's Jinan University, who helped develop the solar cell design. He added also that square based pyramids are ideal because they can be placed next to each other without leaving any gaps.They found that the pointiest pyramids were the most efficient at harvesting light because it reflected back onto the solar cells more times. At their sharpest angle, 30 degrees from vertical, Hou and his team converted 5.5 percent of the light energy that hit the solar cell into electricity, a major improvement over the peak of about 1.6 percent for a flat cell.However there were some economic tradeoffs the sharper the solar cells got. As they made their pyramids pointier, they had to shrink the size of their bases, otherwise they would be adding area to the solar cells throwing off their results. On the plus side, this meant that they could squeeze more cells into a smaller area. However, if a builder wanted to cover an entire roof with these cells, they would need many more than if they were just laying flat."In principle, the narrower the angle, the higher the efficiency," Hou said. "But when the angle is small, it surely consumes more polymer materials and leads to the increase of the production cost."The other downside to the pointiest pyramids is that the shadows cast by their sides keep sunlight from falling on the whole surface of the cell. The deepest parts of the cells wold only be illuminated for a short period of time each day, when the sun is almost directly overhead. A shallower cell would be illuminated for more time, however it would reflect less light back onto itself.Taking all of this into account, the researchers figure that a pyramid whose peak is at a 90 degree angle would give the highest energy output while maximizing the economy materials. In fact, it's efficiency at converting light into electricity is only slightly higher than that of a flat solar cell, but it would take up only about 70 percent of the space. However deeper cells would likely be more efficient on a solar panel that tracks with the sun, minimizing the shadows cast by the sides of its cells.The kind of solar cells they tested are flexible polymer cells . They're durable, easy to work with and can shaped into pyramids without much difficultly, but are pretty inefficient at turning light into electricity no matter what shape they're in. The best flat polymer cells developed in labs convert about 9 percent of light energy that hits them into electricity, while the industry standard for silicon photovoltaics, the kind seen on the tops of houses, is already about 18 percent. Hou said also that silicon would be hard to shape into inverted pyramids because it is so brittle. Hou and his team's results can be found in the most recent Journal of Physics D.