Capturing the High End

Perovskite (say it: pə-ˈräv-ˌskīt, -ˈräf-). It may never become a household word, and the chemical formulas (eg., CH3NH3PbI3−xClx ) are as long as the name. But if you find yourself, in the not-so-distant future, adding new-and-improved solar panels to your roof, they may well contain a perovskite layer. If they do, it will be these “wonder materials” that will make your panels cheap enough to make your investment worthwhile and efficient enough to make you feel smug when you get your electricity bill.

You may ask: Aren’t solar panels dropping in price all the time? What is special about perovskites? The answer is that standard photovoltaic panels only work within a limited range of the sunlight that falls on them. So something like 80% of the energy that directly hits a solar panel is not used for electricity production. This is simply a function of the material – usually silicon – at the heart of the photovoltaic cell. When a photon hits a semiconductor like silicon, it dislodges an electron. Photons at the lower end of the spectrum do not have enough energy to free electrons, but those at the higher end are problematic, as well: Essentially, all the energy over and above that needed to dislodge the electron is wasted. In other words, there is an upper limit to how well silicon can convert sunlight to energy, and thus a limit to the return on the cost.

Perovskites could be the solution to the waste at the high end.

SEM image of perovskite layer shows where the charge separation occurs (diagram) SEM image of perovskite layer shows where the charge separation occurs (diagram)

Because perovskites can work quite well at the high-energy end of the spectrum, they can be combined with silicon cells to give more electricity per square centimeter than silicon alone. And they are cheap and easy to make – no extreme conditions, precious metals or rare earths needed.

The Weizmann Institute’s Profs. David Cahen and Gary Hodes are leaders in the field of photovoltaic materials; between them they have decades of experience in the field. So when they tell you that perovskites are “bigger than apple pie and motherhood – together,” you can be pretty sure they are on to something. They point out that since the first experimental solar cells containing perovskites were demonstrated in 2009, their efficiency has leapt from just a few percent to over 16%. Prof. Henry Snaith, whose Oxford lab has been instrumental in creating that leap, was recently at the Weizmann Institute. He says that there is no reason that that number cannot surpass silicon’s 20%. Theoretically, the sky’s the limit. What is already clear is that perovskites can provide much higher voltage potential than silicon.   

Cahen, Hodes and a host of students and colleagues have recently produced a flurry of papers on perovskite photocells. On the one hand, they are doing the basic chemical analysis that is revealing how these materials function. On the other hand, they are experimenting with perovskites in combination with other materials so as to improve charge separation – the directing of the freed electrons away from the “holes” they leave in the material.

Perovskites (red line) have literally leapt in efficiency, compared to other photovoltaic materials Perovskites (red line) have literally leapt in efficiency, compared to other photovoltaic materials

Their optimism is cautious – as scientists, they know there are still a number of “ifs” and “buts” that must be addressed before perovskites can be installed on every rooftop. But after so many years in which solar technology has hovered just below the verge of economic viability (depending, of course, on the price of oil and on government policy), they are hopeful that perovskites might be the added ingredient that pushes photovoltaics directly into the mainstream.




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