Ultrathin-Film Nanowire Designs

Nanocrystals and nanowires are utilized in a new generation of solar collectors (a nanometer is one billionth of a meter). In conventional solar cells, at the P-N junction one photon splits one electron from its hole companion as it travels to the electron-capturing electrode. If solar collectors are made of semiconducting nanocrystals that disperse the light, according to TU Delft s professor Laurens Siebbeles, an avalanche effect results and one photon can release two or three electrons, because this effect maximizes photon absorption while minimizing electron-hole recombination. This effect of the photon-scattering nanoparticles substantially increases cell efficiency. 

Nanowires increase efficiency because they directly deliver the electrons from the interface of the nanowire with the polymer to their electrode, while the electrode holes travel in the opposite direction to the tip of the wire and pass through the very thin polymer layer before reaching their electrode. Earlier carbon nanotube and nanowire designs were not directly connected to their electrodes and therefore did not provide the electrons with a direct path. 

Another advantage of the nanowire design is that the nanowires can be grown on cheap metal substrates that can conform to irregular and curved electrode surfaces such as the roofs of homes or cars. 

Nanosolar s ultrathin-film design is made of copper indium gallium sele-nide (CIGS) and is claimed to achieve up to 19.5% efficiency. Although such 

A side profile of gallium arsenide nanowires growing on a silicon substrate. The nanowires grow upward from the substrate, creating a surface that s able to absorb more sunlight than a flat surface is. (Credit McMaster University.) 

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