Carbon makes electricity from the sun
The sun is our ultimate source of energy, but capturing its rays using solar cells is unsatisfactory. First solar cells are expensive, and even when the sun shines brightly, the amount of electricity produced is small. Research now suggests that carbon, the main constituent of all organic compounds on the earth, at last makes the sun an economical electrical powerhouse.
Solar cells produce electricity when the sun’s energy is absorbed to move electrons into higher energy states. In these excited states, the electrons move through the cell material towards an electrode, generating an electric current.
Today’s commercial solar cells are made from inorganic semiconductor materials, where there is a good match between the energy in sunlight and the energy which can be absorbed by electrons. Recently, though, scientists have discovered that some plastic-like organic materials can also be used to absorb solar light and convert into electricity.
In these organic polymers the absorbed light is converted to an excited, negatively charged electron bound to a positive ‘hole’. This bound electron-hole pair is termed a ‘exciton’.
“In normal solar cells the electron and the hole are not bound as an exciton, and can drift apart to opposite electrodes to develop a voltage. This is the origin of the classical photovoltaic effect exploited in solar cells,” explains Professor Gehan Amaratunga, who leads a research team in Cambridge University. “Therefore, to have a polymer solar cell it is essential that the exciton is split or disassociated so that electron and hole can drift apart to form the negative and positive terminals of the cell. Exciton dissociation takes place best at the junction between the polymer and a metal which accepts the electrons.”
Professor Amaratunga’s group in the Department of Engineering thought that carbon could make a good electron acceptor in the polymer. “We introduced carbon into the body of the polymer in the form of tiny channels which could accept the electrons and allow them to travel to the electrode. It is as if there were electron acceptor sites throughout the entire polymer, and not just at its surface. We used a form of carbon called single wall nanotubes, where carbon atoms arranged in a flat sheet are rolled up to form a tube.”
The researchers sandwiched the polymer-carbon nanotube blend between a layer of aluminium on one side and indium-tin oxide on the other to act as the electrodes.
They found that when illuminated, the cells produced an electric current around two orders of magnitude larger than for a more typical cell made with only the polymer. “These results are very exciting,” says Professor Amaratunga. “It appears that the nanotubes improve the transport of free electrons to the electrodes, opening the way for a new class of photocell with improved performance. You get more electricity for the size of your cell, and being much cheaper, they make solar power more of an economical option.”
These organic solar cells have an energy payback time of as little as three months. In other words, in three months they convert the same amount of energy that is needed for their manufacture. Even though their energy efficiency is only 2-3%, the best solar cells have an energy payback time of five to six years and are far more expensive.
“These new organic cells could be especially important in less developed countries where the initial investment available for developing an electric power infrastructure is limited,” notes Professor Amaratunga.
Perhaps carbon, all too condemned for its polluting effects in soot and greenhouse gases, could contribute to renewable energy production too.