What’s the smallest thing you can see? A grain of sand? A speck of dust? The latter is around 0.005 mm or five millionths of a metre. For materials scientists, these length scales are gargantuan: they need to zoom in much further, to see things that are 10,000 times smaller. Down at this scale, the arrangement of the atoms is revealed, which allows one to see the fundamental structure of the material. And that’s a key resource to have if you find a useful property of a material, but can’t fully explain the origin.
Take perovskite materials, for example. Their application as the active medium in solar cells is the fastest advancing technology in the sector, reaching an efficiency performance of 29.1 per cent. Their relatively cheap production cost makes them highly attractive but it’s unclear why this particular crystal structure works so well at allowing electrons to flow, therefore generating electricity, under exposure to light. If you can crack that, then you can design and (hopefully) grow the perfect perovskite crystal for high efficiency.
Enter a team led by SLAC National Accelerator Laboratory and Stanford University, who chose their local x-ray free-electron laser (X-FEL) as the super-zoom tool of choice for exploring lead hybrid perovskites. The advantage of an X-FEL is that, in addition to atomic spacing spatial resolution, the flash of x-ray laser light is so short that it can freeze-frame the motion of electrons. With repeat measurement and a slight delay on the x-ray flash you can build up a slo-mo movie of what the material is doing as electrons move through.
In their work, published in January in Nature Materials, the team applied this technique to measure the formation, evolution and disappearance of a tiny bubble-like deformation in the lattice atomic structure of perovskite as electrons passed through, known as a polaron. The surprise result was that the polaron rapidly grew – distorting up to ten layers of atoms – to a much larger size than ever thought before. So, is this the origin of the solar cell efficiency? Further work is needed. What a cliff-hanger.
This piece is a preview from the Witness section of New Humanist spring 2021. Subscribe today.
