Nanotechnology largely eliminates reflection waste from solar cells
The average solar cell that you might see on the roof of a house has an efficiency of about 10 to 15%. That is, only about one in ten of the photons of sunlight striking it are converted into electrons of usable electricity. To a large extent this limitation is set by the inherent properties of the silicon that such cells are made of. In recent years scientists have been looking at ways of improving this situation by using other semiconductors to create cells and even sandwiches of several materials each able to absorb a part of the solar spectrum that the ones above can’t.
One class of semiconductor frequently used in efficient solar cells is the III-Vs. Compounds like gallium arsenide GaAs, that have one atom from group three of the periodic table and one from group five. Using III-V cells, efficiencies of over 40% are possible when the cells are coupled with external optical concentrators.
However even III-V cells are limited by two fundamental physical processes. One is reflection from the surface. Semiconductors have very high refractive indices which means that incident light is reflected far more strongly than it would be from glass or plastic. As much as 30% of the sunlight can be lost in this way.
The second problem relates to the junction. Solar cells are made from junctions between an n-type semiconductor in which electrons are the predominant carrier of electricity and a different version of the same material in which holes carry the charge – so called p-type material. The physics of the p-n junction dictate that there are essentially no charge carriers at all in the junction region which can range from nanometres to microns in size. When a photon of sunlight hits the junction an electron and a hole are created which rapidly migrate to the n and p type material respectively thus creating a current in the external circuit.