New method to boost solar-cell efficiency

Scientists have created a one-step process for producing highly efficient materials that let the maximum amount of sunlight reach a solar cell.
Researchers from Rice University found a simple way to etch nanoscale spikes into silicon that allows more than 99 per cent of sunlight to reach the cells' active elements, where it can be turned into electricity.
The research by chemist Andrew Barron and Rice graduate student and lead author Yen-Tien Lu appears in the Royal Society of Chemistry's Journal of Materials Chemistry A.
The more light absorbed by a solar panel's active elements, the more power it will produce. Coatings in current use that protect the active elements let most light pass but reflect some as well.

Various strategies have cut reflectance down to about 6 per cent, Barron said, but the anti-reflection is limited to a specific range of light, incident angle and wavelength.
Black silicon reflects almost no light. Black silicon is simply silicon with a highly textured surface of nanoscale spikes or pores that are smaller than the wavelength of light.

The texture allows the efficient collection of light from any angle - from sunrise to sunset.
The new method of creating black silicon in one step makes it far more practical than previous methods, researchers said.
Researchers have replaced the two-step process that involved metal deposition and electroless chemical etching with a single step that works at room temperature.

The chemical stew that makes it possible is a mix of copper nitrate, phosphorus acid, hydrogen fluoride and water. When applied to a silicon wafer, the phosphorus acid reduces the copper ions to copper nanoparticles.
The nanoparticles attract electrons from the silicon wafer's surface, oxidising it and allowing hydrogen fluoride to burn inverted pyramid-shaped nanopores into the silicon.
Fine-tuning the process resulted in a black silicon layer with pores as small as 590 nanometres (billionths of a metre) that let through more than 99 per cent of light.
By comparison, a clean, un-etched silicon wafer reflects nearly 100 per cent of light.

Barron said the spikes would still require a coating to protect them from the elements, and his lab is working on ways to shorten the eight-hour process needed to perform the etching in the lab.