Traditional solar cells respond only to a narrow spectrum of sunlight, making them highly inefficient. In the language of physicists, solar cells convert light with wavelengths corresponding to the energy it takes for electrons to jump from the valence band to the conduction band. Photons with lower energy pass right through the material.Currently, the main difficulty in the process is that addition of oxygen, but the scientist have developed a process that uses short blasts of laser beams to melt the metal and rapidly regrow it to implant oxygen inside.
The new semiconductor material can capture these low-energy photons for electricity, which could make solar cells with efficiencies of around 45 percent, compared with 25 percent for conventional cells that use a single semiconductor and 39 percent for cells with layers of mixed semiconductors.
The new semiconductors have three energy bands instead of the usual two (valence and conduction). The third band lies below the conduction band, effectively splitting the gap between the valence and conduction bands into two smaller parts. "This helps low-energy photons to participate in the process because they can excite [electrons] to the [intermediate] band and then up. It's like a stepping stone," says Wladek Walukiewicz...
The researchers found that introducing a few atoms of oxygen into a zinc-manganese-tellurium (ZnMnTe) alloy splits the compound semiconductor's conduction band into two parts. Similarly, adding nitrogen to a semiconductor such as gallium arsenide phosphide will also give a multi-band semiconductor.
For those of you interested in more details of the science involved, Berkeley Labs has posted a PowerPoint presentation created by the team on its website. If you're simply interested in when such technology might be available commercially, you'll have to wait: Phoenix start-up RoseStreet Labs has licensed the technology, but given its recent development, they're not sure how long it will take to bring it to market. ::Technology Review via Clean Break
Photograph: Gallium arsenide solar cells at the National Renewable Energy Laboratory