Silicon solar cells built on a nanostructured substrate (top left) have a surface patterned with nanoscale domes (top right). The scale bar in both electron-microscope images is 500 nanometers. The diagram shows the layers of the device, from bottom to top: a quartz substrate, a reflective layer of silver, a transparent conducting oxide, the active layer of amorphous silicon, and another oxide layer. Credit: ACS/Nano Letters

The accumulation of dust on the surface of a solar cell can block light and cut into cell efficiency. Researchers at Stanford have demonstrated that solar cells patterned at the nanoscale with domed structures absorb more light and, as a bonus, are self-cleaning.

The nanoscale patterning is not just on the surface of the cell but is applied to every layer. The cells are built on a substrate patterned with nanoscale cones. The bottom layer is a film of silver 100 nanometers thick that acts as an electrical contact and a light reflector; atop this is a film of amorphous silicon sandwiched between transparent conducting layers. Though the substrate is jagged, the accumulation of layers results in domed structures that happen to resemble the mushroom-like structures other researchers have been developing for self-cleaning surfaces. An added layer of hydrophobic molecules makes the cells nearly superhydrophobic: water droplets roll along the surface, pulling dust away with them.

These nanodome structures not only repel water, but help trap light. Because they're so small--about 500 nanometers in diameter--the nanodomes interact with light in a cool way, absorbing 94 percent of all light from the infrared to the ultraviolet. A flat solar cell made from the same materials absorbs only 65 percent of light in the same broad spectrum. So far the overall power conversion efficiency of the cells is 5.9 percent. The lead researcher, Stanford materials science professor Yi Cui, says these patterning techniques could be applied to other solar materials. This work is described online in the journal Nano Letters.