By Peter Weiss
Researchers in Germany have modified standard solar cell designs so that they can be made from cheaper materials. That could be important, because price has long been a major obstacle to wider use of photovoltaics.
Typically in commercial solar cells, photons from the sun jolt electrons loose from chemical bonds in a layer of silicon several hundred micrometers thick. Dislodged electrons escape the silicon layer into an adjacent substance that lets them flow freely.
To ensure that liberated electrons don’t get trapped by defects within the silicon’s crystalline structure, manufacturers deposit the material with as few defects as possible. That precision in manufacturing adds to the cell’s cost.
Instead, says Rolf Könenkamp of Portland (Ore.) State University, it’s possible to “use extremely inexpensive and essentially bad material to make a solar cell.”
Such a material must be thin so its defects don’t impede the light-generated electrons. However, a thin light-absorbing layer can be too transparent.
At the Hahn-Meitner Institute in Berlin, Könenkamp and his colleagues created an electron-accepting layer of titanium dioxide with a rough surface. Onto the bumpy surface, the team deposited an imperfect, 150-nanometer-thick layer of the light-absorbing semiconductor cadmium telluride.
In the June Semiconductor Science and Technology, the researchers report that the electron-accepting titanium dioxide’s bumpiness bounces photons around within the light-absorbing layer, upping the chance for electrons to be freed.
Meanwhile, the cadmium telluride layer’s thinness permits those electrons to escape into the titanium dioxide.
Other teams had demonstrated this roughened-surface approach using more exotic solar-cell technologies, says Könenkamp. However, this new work shows that the approach also holds promise for improving upon well-established methods for making cells.
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