Solar Cells Info

by Phil Schewe and Ben Stein, April 11, 2007
Source: American Inst. of Physics – Physics News update
http://www.aip.org/pnu/2007/split/819-2.html

Because of its ubiquity in electronics, silicon is the favorite semiconductor used in solar photovoltaic cells. Still, one would like to reduce the amount of Si needed for large-area devices. Furthermore, silicon is a poor light emitter and absorber, and therefore solar cell efficiencies have generally been poor.  The efficiency of thin-film Si cells is even poorer than for wafer-thick Si cells. How to make the cells cheap (using thin films) but also nicely absorptive is an important goal. Scientists at the University of New South Wales in Australia have now enhanced the absorption of sunlight using surface plasmons.

When light strikes a metal sample it can initiate electrical disturbances in the surface, either as localized excitations called surface plasmons or as moving waves called surface plasmon polaritons. The plasmons can be considered as a sort of proxy for the light, except at a shorter wavelength.

If, moreover, the plasmon energy can be efficiently collected and transferred to an underlying waveguide as part of a solar cell, then the cells’ yield can grow. This what the New South Wales researchers do. They use silver nanoparticles to excite surface plasmons, which enhances light trapping. For 1.25-micron-thick thin-film cells, the enhancement was by a factor of 16 for light with a wavelength of 1050 nm.

For wafers, the enhancement was by a factor of 7 for light with a wavelength of 1200 nm. Silicon normally absorbs light only weakly in this part of the spectrum, so the enhancement is significant. Across all wavelengths, the photocurrent enhancement for the 1.25-micron film and the wafer samples was, respectively, 33% and 19%. According to Supriya Pillai (supriyap@student.unsw.edu.au), optimizing the nanoparticle size should bring additional improvements. (Pillai et al., Journal of Applied Physics, upcoming article)