Imagine light poles on the street generating their own electricity. Or, the façade of buildings producing power for our daily use. And the batteries in cars and buses getting recharged without being plugged into a source of power. These are not science fiction scenarios but on the verge of becoming a reality. These surfaces will indeed generate solar power if they are coated with a solution containing a wonder material that goes by the name of perovskite.
Perovskite may be the Holy Grail that the solar power industry has been seeking for the last two decades. The silicon-based solar cell that constitutes the bulk of the present photovoltaic market is close to reaching the theoretical limit of efficiency. To boost efficiency, scientists have adopted the tandem cell strategy, wherein a layer of another suitable material is coated on the silicon cells. Different types of materials have been tried and the most promising among them is perovskite. Recently, a start-up spawned by Oxford University, announced a record-breaking efficiency of 29.5 per cent for a solar cell, compared to 15-25 per cent for today’s commercial cells. This feat was accomplished by coating the silicon cells with a thin film of perovskite.
Perovskite was originally a mineral discovered in the Ural Mountains of Russia in 1839 and named after the Russian mineralogist, Lev Perovski. Today it is synthesised in the laboratories and refers to a class of materials that have the same crystal structure as calcium titanate, the original mineral. The most widely studied perovskite today is methylammonium lead iodide, a compound that has lead and iodine. The biggest advantage of perovskites over silicon cells is higher efficiency. Coating existing silicon cells with perovskites will straightaway boost the power production by at least 20 per cent. The higher efficiency of perovskites is due to the fact the coated cells are able to absorb more of solar radiation. Sunlight consists of a wide spectrum of light ranging from violet to red. The thin layer of perovskite absorbs the violet end of the spectrum with shorter wavelengths and the silicon substrate absorbs the higher wavelength red end.
Efficiency is not the only bright thing about perovskites. They are cheaper to produce than silicon and have a lower environmental footprint. Producing solar cell-grade silicon from quartz or sand is a very energy-intensive process. Perovskites can also be used on a stand-alone basis as a thin-film cell and this has exciting possibilities. A solution of perovskite can be spray coated on virtually any surface turning it into a potential power generator. This flexibility of perovskite can lead to solar panels that can be rolled up. People can even wear apparels coated with perovskite. These applications are truly mind-boggling but still a decade away. The first commercial application of perovskite would be a thin film grafted on to a silicon cell.
Not everything about perovskites is hunky-dory. There are serious questions about its stability and toxicity. Whereas silicon cells are extremely robust and have a proven life of 25 years and more, perovskites are known to degrade in humid and saline environments. Also, some experts are worried that lead, a known toxin, might get leached out into the environment. Tin is being studied as an alternative for lead, but the results are not very encouraging so far. Much research is happening at a frenetic pace to address these two issues. At least a dozen start-ups are working on perovskite cells with the Oxford University spin-off currently ahead of the pack. The first commercial perovskite cells are expected to be rolled out early next year.
The photovoltaic industry owes its origin to the photoelectric effect discovered by Albert Einstein in 1905, for which he received the 1921 Nobel Prize. The first practical solar cell was produced by Bell Labs in 1954. Since then photovoltaic cells have matured considerably. With silicon technology approaching saturation, scientists have been looking at an alternate class of materials to enhance the performance. Perovskites first made its appearance in 2009 with a measly efficiency of 3.8 per cent. The last decade has seen dramatic improvements and we are now poised to usher in the third generation photovoltaic cells spearheaded by perovskite.
The price of photovoltaic electricity dropped by more than 80 per cent in the last decade. Solar power is already cheap and according to the International Energy Association (IEA) has become the cheapest ever in history. But the share of solar energy in global electricity production was just a little more than 2 per cent in 2020. Solar power capacity is set to double in the next 5 years and together with wind power will knock coal off its pedestal as the most dominant source of electricity production.
(The author is a Mumbai-based freelance engineering consultant and science writer. Views are personal and do not necessarily reflect those of Outlook Magazine.)
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