The stuff of dreams

(From "Horizons" no. 107, December 2015) ​​​​For some years now, growing numbers of physicists, chemists and engineers have been lulled into a trance by the sirens of a new, exotically named family of materials: the perovskites. This group of oxides is to be found at the heart of a wide range of research projects in fields as diverse as they are promising, including solar energy, microelectronics and lasers.

The first mention of this class of compounds dates back to 1839. At that time ‘perovskite' referred to the rock calcium titanate (CaTiO3), taking the name of the Russian mineralogist Lev Perovski. The term now covers a wide range of materials containing two groups of oxidised atoms and featuring the same cubic crystalline structure.

Bespoke material

Its crystalline structure is actually very common. “It’s probably the most widely found crystal structure on earth”, says Jean-Marc Triscone, a physicist at the University of Geneva. But it’s here that things start to become interesting. “By minutely adjusting the base elements, a radical change can be brought to every property of the material”, he continues. It’s as simple as taking a magnetic perovskite and substituting one of its elements for another. The result is a completely different material, one that in the process may, for example, have lost in magnetism but gained in conductivity. Perovskites can even be combined, giving rise to new materials with unforeseen properties. “It’s like Lego: you can stack them up and create new structures And not only are the new structures perfect, because the crystal components are all identical, but they also take on different properties from the parents”.

Physicists harbour the crazy desire of being able to create a 100% bespoke material that can be built on a needs basis. In Geneva, for example, Triscone is trying to assemble different perovskites into an ambient-temperature superconductor (a material though which electric current passes without any wasteful resistance). Others are hoping to fit the particle accelerators at CERN with new magnets based on the superconductive oxides that won Georg Bednorz and Alex Müller of IBM Zurich the Nobel Prize for Physics in 1987. These oxides look much like a stack of perovskites.

And these are just some of the multitudinous examples of potential applications ranging from the design of lasers and LEDs to new types of computer memory. Another promising field of application is centred on ferroelectric perovskites, the crystals of which are composed of ions, giving them natural electric polarisation. By applying an electric field, the orientation of ferroelectric domains can be changed, subtly altering the crystal structure of the perovskite. This in turn modifies the thermal properties of the material. The result is an ‘intelligent’ insulator that can actively compensate for the significant temperature changes suffered by microprocessors and components of satellites and motor vehicles. Nevertheless, “for now the effect has only been observed at very low temperatures of around 80 kelvins [Ed: -193° C]”, says Christian Monachon, a Swiss physicist working at the University of California, Berkeley, USA. This is not an insurmountable obstacle, however, “my research leads me to believe that we can obtain materials with variable thermal conductivity, by using barium titanate, for example, which I’m currently studying”.

The sun is shining on perovskites

The greatest interest of perovskites lies in photovoltaic applications. In the space of five years, the performance of perovskite solar cells has quadrupled, almost reaching that of silicon cells and therefore heralding change.

But scientists haven’t always felt persuaded by this application. “Since the 1980s, researchers have focused on designing lasers”, says the expert Jacky Even of the National Applied Sciences Institute in Rennes, France. It was only in 2009 that the marriage of perovskites and sunshine occurred to a team at Toin University in Yokohama, Japan, which then tried to integrate perovskites into a photovoltaic cell. “It was not an idea well-suited to the exceptional properties of these materials”, Even says. “They wanted to promote the light-absorbing properties of the coloured solar cells, but the results were weak and the article went unnoticed for years”.

This all changed in 2012, when the idea was taken up independently by two specialists in photovoltaics, Henry Snaith at the University of Oxford and his former mentor Michael Grätzel at EPFL. They both started from the concept of a coloured
cell developed by Grätzel in the 1990s and entered into competition to design a new
type of solar cell based on a perovskite whose oxygen atoms are replaced by iodine
or bromine.

Just as with silicon in classic solar cells, the perovskite absorbs light and transports
electric charges between electrodes. Once more the key here is in the way perovskites
form modules. A hybrid perovskite combining organic and inorganic groups becomes
a photovoltaic material able to absorb 10 times more light than silicon and that can transport electric charges more effectively than classic colours. “This was a real conceptual leap that’s led to a new branch of photovoltaics”, says Even.

Competing with silicon

Since then, a battle has been raging between the teams, and their ranks have swelled as others join them. At the end of September 2015, Grätzel announced at a congress
in Lausanne that his team had achieved a performance of 20.8%. This compares with
the 25.6% of silicon cells which have been under development for more than 50 years.
“Competition is stiff and there is a lot at stake”, says Joël Teuscher, a researcher in the Photochemical Dynamics Group at EPFL. “But it is also healthy”.

Today, the performance race seems to be on the home straight. Specialists are
now asking more fundamental questions. “We are still looking to understand how this works exactly”, says Teuscher. “This is a very passionate period in which work is becoming interdisciplinary”. These questions will also help researchers resolve inherent problems of these materials such as their instability (they are fragile and soluble) and also the presence of lead in the crystals, something which may hamper
future commercial applications. Although as Even says, “a car battery contains 8 kg
of lead, whereas a square metre of solar panels only has half a gram”!

Perovskites are indeed the stuff that a scientist’s dreams are made of, but not all
applications will be successful. For every advantage there are at least as many
problems. “Perovskites are opening up a fascinating path”, says Triscone. “And it
doesn’t matter if many research projects lead nowhere; it will perhaps only take one
to revolutionise physics”.

Fabien Goubet is a science journalist for Le Temps.