www.stuff.co.nz – The solar power market is dominated by solar panels using silicon to capture the sun’s light and convert some of the energy into electricity. An alternative to silicon has emerged in recent years.
Perovskite is a calcium titanium oxide mineral found in abundance in several parts of the world. The mineral is named after Russian mineralogist, Lev Perovski (1792-1856).
The efficiency of a solar panel is defined as the percentage of energy in the sunlight it receives which is converted to electrical energy. The best silicon solar panels have an efficiency of about 25 per cent.
A flow of electrons is a flow of charged particles, otherwise known as an electric current.
In the dark, the electrons in a solar cell stay bound to their respective atoms, however sunlight liberates some of these electrons which then haphazardly move about the crystal lattice until they reach an electrode which swiftly removes them to deliver the current.
It requires a certain minimum energy to liberate electrons from their host atoms so not all of the incoming solar energy can be converted to electrical energy. This is referred to as the band gap.
The electrons’ drunken movement through the lattice, many colliding with obstacles en route, results in further energy loss. The better the crystal quality the lower this loss will be. For these reasons no solar cell can be 100 per cent efficient.
The electric power of a solar cell is the number of electrons reaching the electrode per second (the current) multiplied by the energy of the electrons (the voltage).
In 2009 a researcher from Toin University in Japan announced that he had discovered how to make perovskite minerals suitable for use in solar cells. Perhaps commercially unwisely, he reported this widely and because the chemical processing of perovskite is cheap compared with silicon, his discovery triggered an avalanche of interest in laboratories around the world.
At the forefront of this stampede is Oxford University’s Clarendon Laboratory. Three researchers from this laboratory recently published an article in Scientific American, reporting on progress to date.
The processing of silicon for use in solar cells requires it to be subjected to temperatures of about 900 degrees Centigrade whereas perovskite only requires 100 degrees, so there are savings in manufacture.
Perovskite has a better crystalline structure than silicon, mainly free of defects and perovskite cells are flexible so can be rolled in sheets.
Perovskite solar cells have disadvantages. The production method to date has not succeeded in making cells of the crystalline film large enough to be commercial.
In addition, the film is very sensitive to moisture and has to be encased in a watertight seal – this is also true of silicon. A small amount of lead is required to be added to the perovskite to assist its performance, but lead is toxic so sealing of the cells is very important.
More work has to be done on stabilizing the crystalline structure which shows signs of breaking up after only 2000 hours of operation. Compare this with the 54,000 hour, 25 year warranty which is normal for silicon panels.
There is an effect caused by charged molecules and not electrons, migrating from one side of the cell to the other, giving the appearance of a greater flow of electrons (current) but which do not contribute to the actual current. This can lead to inflated estimates of the power delivered. A testing standard has to be agreed to allow the data to be corrected for this. These drawbacks can be overcome, although the best efficiency for perovskite solar cells is still only 20 per cent, they think that 30 per cent is achievable.
The Oxford team make the point that the cost of solar panels includes installation, materials, labour, permits and inspection. They estimate that the cost alone, of the solar cells, is only about 20 per cent of the total installed cost per watt of electrical power, so reductions in manufacturing costs may not lead to huge reductions in total costs.
It is thought the future of solar cells is in a hybrid panel made of both perovskite and silicon. This is because perovskite is effective at converting shorter wavelengths of sunlight, blue and violet, into electricity, whereas silicon is better at converting longer wavelengths.
A tandem cell, with a perovskite top layer and a silicon lower layer, could be the answer. The Oxford researchers think this tandem solution will be the next generation of higher efficiency solar cells, but make the point that a lot of work still needs to be done.