Reports indicate that a new solar-cell design based on a carpet of silicon rods could produce electricity at a fraction of the cost of conventional solar devices. According to a report in Nature News, the carpets have yet to be made into a working solar cell, but preliminary measurements of their ability to absorb light and generate current suggest they could become a cheap replacement for existing technology. The idea behind photovoltaic solar cells is that when sunlight strikes a material, it dislodges electrons, which start to flow in one direction. The electrons leave behind empty, positively-charged ‘holes’ that move the other way, effectively creating a current. It’s a simple idea, but it isn’t cheap. The material of choice for cells is often thin silicon wafers, which are efficient at absorbing light but expensive to produce.
Now, Harry Atwater and his colleagues at the California Institute of Technology in Pasadena have found an alternative that uses one hundredth of the material of current silicon technology.
Using a well-established technique for assembling nanowires on a surface, the researchers grew a ‘carpet’ of micrometre-scale silicon rods lined up like hairs standing on end. They then embedded the rods in a transparent polymer. This configuration alone is not enough to absorb light efficiently. Although the array can absorb light coming in at shallow angles, light beating down from directly overhead tends to pass between the rods and is lost. “Not being able to absorb light at noon isn’t a great property for a solar cell,” said Atwater. To solve this problem, the team sprinkled aluminium oxide particles into the transparent polymer. These particles scatter incoming light so that it bounces around inside the array, increasing the light’s chances of striking a silicon wire. “As a result, up to 85 percent of usable incident sunlight can be absorbed effectively,” said Atwater. Atwater is pleasantly surprised to find that the silicon carpet outperforms conventional wafers at absorbing infrared frequencies. According to him, the result flies in the face of the “established dogma” that the most highly absorbing surface is a roughly textured wafer. “We’ve broken through what people thought was the absorption limit,” he said.
“The fundamental issue with all solar cells is that they are too expensive, and reducing the material and the cost, but keeping the efficiency, has been the main thrust of research in this field,” said physicist Ken Durose, an expert on solar cells at Durham University, UK.