Folding solar cells

The most expensive part of conventional solar cells is rapidly becoming the raw silicon wafers which they are made from. One obvious way of reducing the amount of silicon a cell needs is to make it thinner. The problem is that silicon needs to be about 0.12mm thick to absorb 90% of the light hitting it, so making the cells any thinner whilst still being flat is difficult.

Xiaoying Guo and collegues in the University of Illinois have a solution to this problem - if you can build structures which trap light in lots of reflections, then it will pass through the solar cell more than once so a thin cell will be much more likely to absorb the light. The problem is that you can't make flat stuctures which do this.

They got around this problem by folding silicon foils only 1.25 microns thick into structures such as cylinders and spheres, doing the actual folding by using drops of water placed on the silicon. Water has surface tension which pulls on the silcon, causing it to bend. As the droplet dries out, it gets smaller, moving the point that is being bent.

To show that this method works, the researchers have used a cylindrical solar cell to generate twice as much power as the equivalent area of flat cell.