Light behaves differently in macro- and nano- scale environments, and this could be a starting point for those researchers who want to improve the solar cells’ efficiency. It was a starting point for some Stanford engineers, who found out that light ricocheting inside an ultra-thin polymer film solar cell behaves differently than if the film wasn’t so thin. The difference is enormous.
Research based on classic assumptions about how the light behaves inside a solar cell says it isn’t possible to increase the cell’s efficiency by means of maximizing the internal reflection of the cell, or “light trapping”. “The longer a photon of light is in the solar cell, the better chance the photon can get absorbed,” said Shanhui Fan, associate professor of electrical engineering. Previous experiments with light trapping at macroscale levels in silicon solar cells proved that there is a limit to what it can do to efficiency at this size.
The film that the Stanford scientists manufactured was roughed up a bit to absorb more than 10 times the energy predicted by classical calculations.
Due to the light’s dual nature (sometimes acting as a stream of photons and other times as a wave of energy), it doesn’t always go in a straight line inside certain environments, the scientists concluded. When Zongfu Yu, one of the researchers, investigated the light’s behavior inside a material whose thickness was far lower than its wavelength, he found out that it could be retained for longer times between the material’s boundaries. He and his colleague Shanhui Fan, associate professor of electrical engineering, measured a 12-fold increase in the absorption of light within the thin film, compared to the macroscale limit.
“We all used to think of light as going in a straight line,” Fan said. “For example, a ray of light hits a mirror, it bounces and you see another light ray. That is the typical way we think about light in the macroscopic world.
“But if you go down to the nanoscales that we are interested in, hundreds of millionths of a millimeter in scale, it turns out the wave characteristic really becomes important.”
Yu placed the organic thin film between two layers of material acting as confining layers once the light entered the rough-surfaced upper layer into the film. That layer sent the incoming light in different directions.
Manufacturing a solar cell so thin could have beneficial consequences on the industry, if the cells rendered efficient enough for mass fabrication. The materials used are much less than if the cell was built around bulky silicon and the prices could surely drop, not to mention the overall increase flexibility and usability of the cells.