Solar power is cheap (actually, it’s free) and it’s available over most of the Earth’s surface. The trick is being able to easily harness the energy that the Sun pours down onto our planet. If humans were able to collect and store this abundant source of energy, we could completely let go of petroleum fuels and nuclear power. The World Meteorological Organization estimates that over 120 Watts per square meter are soaked up by the earth during sunlit hours. How can we harness this clean, inexpensive energy source? Solar cells are electrical devices designed to transform the energy from incoming sunlight into electrical power. Unfortunately, right now the efficiency of solar panels – efficiency being the percentage of the sunlight which is converted to electricity, as a function of how much total sunlight is soaked up by the panel – is still very low, despite decades of research.
Solar energy still has promise. A recent report published in the science journal Chemistry of Materials outlines a new approach to solar cell circuitry. It’s a commonly known fact that as the circuitry of computers and other electronic devices gets smaller, the overall power and capability of the device increases. This is because more circuitry can be deposited in a given area. Computers that once took up the size of a large room now fit in the palm of your hand. A new type of circuitry has now been developed for solar panels, which involves shrinking the size of the electronic circuit to the true nanoscale: wires that are only 10^-9 meters in size. This allows the fabrication of extremely dense pockets of circuitry that are very efficient at soaking up incoming sunlight and facilitating the transformation into electricity.
Key to this success was the development of a new method of “growing” the circuits. If you or I were to build a circuit, we would probably start with a lot of loose wire and individual components, and then lay them down in the correct pattern and connect everything / solder all the wires together. That solution is best for the distance scale in which humans interact – where most things are larger than a millimeter. However, if the wires are on the nanoscale – 10,000 times smaller than a millimeter – it becomes extremely difficult, if not impossible, to individually select the wires and “lay them down” in the correct placement, for obvious reasons. Researchers are therefore now using the properties of a natural gemstone, sapphire, to enable the circuitry to be grown in the correct fashion.
Sapphires might seem like an expensive component for a solar panel. However, the sapphire only serves as the surface for the circuit. It acts as a template and can therefore be cut extremely thin. This wafer of sapphire is extremely well-ordered on the microscopic scale, which is a property common to gemstones and most crystals. By using standard deposition techniques, scientists can cover the sapphire surface with a material called zinc oxide. The zinc oxide begins to crystallize on the sapphire and grows in the shape of a thin wire along the natural fault lines present in the sapphire crystal. Each sapphire has a well-defined lattice structure which is common to all sapphires. The nanocrystals of zinc oxide follow this pathway and so the resulting nanowires of zinc grow into that pattern, with no need to pick up them and place them into a circuit: the circuit grows itself.
The important breakthrough in this discovery is that up until now, nanowires had only been able to be grown on the surface of semiconductors such as silicon. This hampered their use as the growing surface would leak current during some applications. Now that sapphire (an insulator) can now be used, the only conductive pathway remaining is the spiderweb of zinc nanowires. This state-of-the-art fabrication technique should allow access to all manner of new devices, including high efficiency solar panels. By keeping the individual wires extremely small, a large amount of circuitry can be packed into a given volume, giving the incoming sunlight the best opportunity to be transformed into useful electricity before it escapes from the solar panel.
The source of this article can be found at:
“Towards Industrial-Scale Fabrication of Nanowire-Based Devices”.
Chemistry of Materials, published by the American Chemical Society.