Efficient and inexpensive solar cells will play a key role in energy generation in this century if harmful fossil fuel emissions are to be reduced. Nanoparticles are promising candidates for use in solar cell active layers due to their low manufacturing cost, size-tunable optical and electronic properties, and potentially high carrier mobilities. However, nanoparticle solar cell research to date has been focused on nanoparticles of cadmium compounds, which pose an environmental hazard. I am investigating solar cell devices that incorporate silicon and germanium nanoparticles, both of which are naturally abundant and non-toxic.
Spherical silicon and germanium nanoparticles are synthesized from silane and germanium tetrachloride using a simple, low-pressure, radio-frequency plasma process. The nanoparticles are nearly monodisperse, and their diameters may be tuned from 3-10 nm by adjusting the plasma conditions. Several solar cell structures incorporating these particles have been conceived, including all-inorganic and hybrid nanoparticle-polymer devices. Prior to constructing these devices, however, it is important to study the absorption and conductivity of the nanoparticle layers that will be used.
Recent co-planar conductivity measurements of films of germanium nanoparticle agglomerates embedded in non-conducting polystyrene matrices show surprisingly high conductivities. Measurements in the dark yield conductivity values of roughly 10 -5 S/cm. Measurements of conductivities in solar-simulating AM1.5 light are consistently twice as large as the dark conductivities, indicating that the germanium particles are photosensitive (a necessary property for any solar cell material). These conductivities approach those of amorphous silicon—the standard in solar cell research—suggesting that germanium nanoparticles are particularly promising for solar cells.