Bright idea: a 330nm-thick film of organometal halide perovskite fabricated on a glass sheet. This film is the active element of new 15% efficient solar cells. Photograph: Boshu Zhang, Wong Choon Lim Glenn & Mingzhen Liu
The worst part of my job as a materials scientist is going to conferences. They are often turgid affairs conducted in the ballrooms of hotels so identical to one another that you can't tell whether you are in Singapore or Manchester. The same speakers are there, for the most part droning on about the same thing they droned on about at the last conference. I should know, I am one of them.
But occasionally, just occasionally, someone says something so radically new that it causes you sit up and actually listen. Your neighbors are no longer fiddling with their smartphones; there is the proverbial buzz in the air.
This was the scene at the Materials Research Society conference in Boston last December, where a breakthrough in perovskite solar cells was announced. If perovskites mean nothing to you read on, as they may have a very big impact on your future fuel bill.
If we could capture approximately 1% of the sunlight falling on to the British Isles and turn it into electricity we would meet our current energy demands. The reason no one suggests doing this rather than building wind, nuclear or conventional power stations is the cost. We currently use silicon solar cells to turn sunlight into electricity but they are expensive and require subsidies.
Silicon is a poor conductor of electricity because all of its four outer electrons are bound up in the chemical bonds holding the crystal together. However, by adding a tiny amount of phosphorous, which has five outer electrons, you effectively add a free electron to the crystal and make it conduct moderately well. Similarly, you can add boron, which has only three outer electrons, and effectively do the same thing, only now the conducting charge is called an electron hole.
The magic comes when you put a phosphorous silicon layer next to a boron silicon layer: the holes and the electrons cancel each other out at the junction but create an electric field that means that electrons only like to flow in one direction across the junction. This is called a diode.
There are many flavours of diodes, each having a different junction architecture. Light-emitting diodes (LEDs) emit light when electrons flow across the junction but the opposite effect also works: light hitting the diode creates an electric current, and this is how a solar cell works.
Silicon solar cells are as intricately micro-engineered as the silicon chips in your smartphone or tablet, but instead of being the size of a postage stamp they are the size of a table. They are made in enormous billion-dollar clean-room and high-temperature fabrication facilities, which are expensive precisely because controlling the purity of the silicon and the doping levels of phosphorous and boron is not trivial. It is also energy intensive. Although prices have been coming down, fossil fuels are still so cheap it is very hard for silicon solar cells to compete.
Materials scientists have been exploring other semiconductor technologies for a long while, trying to find something cheaper and better than silicon. Until recently the best bet was dye-sensitised solar cells. These have been around since the 1980s, but they have not managed to make a big impact on the energy market because, although they are cheap because they don't require billion-dollar clean-room facilities, their efficiency is generally low. This is where the new perovskite solar cells come in. There are exotic-sounding compounds, such as methylammonium trihalogen plumbates, that have a quite simple crystal structure called a perovskite. Like the dye-sensitised solar cells, these solar cells are easy and cheap to make, but they have another trick up their sleeve – they don't need a complicated diode architecture to achieve high efficiency.
Materials researchers in Oxford, led by Dr Henry Snaith, have recently shown that they can make simple perovskite solar cells with efficiency pushing 20%. This is big news, because 20% makes them competitive with existing commercial silicon solar cells while being much cheaper to make in high volumes. They are also more suitable for incorporating into roofing materials and glass panels than silicon and so have the clear potential of being as fundamental to our city architecture as steel, concrete and asphalt. In other words, they could well be the materials that will make it possible to collect the 1% of solar energy we need as a nation, at a cost that can compete with fossil fuels.
Hearing research results such as this makes you grudgingly admit conferences are worth going to, and indeed gets you wondering whether we might look back in 10 years and pinpoint this as the time the solar energy revolution really ignited. One of my industry colleagues believes so; after the talk he immediately Skyped his research group, told them to stop what they were doing and get working on perovskite solar cells. The race to commercialize them is on.
http://www.theguardian.com/technology/2014/mar/02/perovskite-lightbulb-moment-abundant-solar-power-britain?CMP=fb_gu
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