The first solar battery was made by Bell Telephone Laboratories in New Jersey, the foremost physics laboratory in the world in 1954. Then, a number of electrical engineers, physicists, and chemists created a simple but practical silicon solar cell.
Silicon is both abundant-it’s found in sand-and almost perfectly suited to absorb the energy of sunlight. Bell Lab researchers had been using it to develop the transistor when they stumbled across some very useful properties.
Sunlight is composed of a spectrum of wavelengths, some visible to the eye as colours, and some invisible, like infrared and UV radiation. When the atoms in a material absorbs solar energy-in the form of a discrete package of energy called a photon-electrons orbiting the atoms get excited, becoming energetic enough to break free of the atom and create an electric current.
The excited electron can be captured in an electric circuit, and its energy can be used to do work like powering a radio or motor. But if it’s allowed to revert to its unexcited state, it loses the energy it absorbed from the photon. To ensure the excited electrons do not lose their energy, the silicon is treated with impurities, like boron or phosphorous, that creates a junction through which the electron cannot fall to its unexcited state.
Once a conductive material is attached to the silicon solar cell and hooked up to an appliance, the stream of excited electrons travels from the solar cell to the appliance in an electric current. The circuit is completed by connecting the appliance back to the solar cell, replenishing the silicon’s lost electrons, and ready for another photon to arrive and begin the process all over again.
The concept is simple-U of T chemistry and physics professor R.J. Dwayne Miller creates solar cells for his class in minutes-but like Bell’s historic solar battery, most of these simple cells are less than 10 per cent efficient at capturing sunlight and turning it into an electric current.
Raising solar cell efficiency is Dr. Ted Sargent’s goal. Last year, the U of T’s electrical and computer engineering professor discovered that a kind of nano-particle called quantum dots can act like silicon atoms when exposed to light: their electrons become excited and flow in a current. Unlike silicon, these quantum dots are sensitive to infrared light, greatly expanding the range of harvestable sunlight.
According to Sargent, today’s solar cells, which do not capture infrared light, lose more than half the energy we could harvest from sunshine. A plastic solar cell with Sargent’s quantum dots could harvest both visible light and infrared light, potentially reaching 30 per cent efficiency. Plastic solar cells operate alone at five to six per cent efficiency and, unlike silicon-based cells, are being investigated by a number of researchers as they are cheaper to manufacture.
But Sargent’s quantum dots could take 10 years to go from lab bench to commercial product. Still, the research world of solar technology is buzzing with the possibilities of spray-on solar cells and fabrics that collect solar energy, even if these innovations may be decades away.
-S.H.
