Observed for the first time in lead hybrid perovskites, these volatile disturbances may explain why these materials are exceptionally good at converting sunlight into electricity in solar cells.
Polarons are transient distortions in the atomic lattice of a material that develop within a few trillionths of a second around a moving electron and then vanish quickly.
Volatile as they are, they influence a material’s actions and may even be the reason why solar cells made of lead hybrid perovskites achieve exceptionally high laboratory efficiencies.
Today, for the first time, scientists at the SLAC National Accelerator Laboratory of the Department of Energy and Stanford University have used X-ray lasers from the laboratory to observe and directly test polar formation.
On Jan. 4, 2021, they announced their findings in Nature Materials.
“These materials have taken the field of solar energy research by storm because of their high efficiencies and low costs, but people are still arguing about why they work,”These materials have taken the field of solar energy research by storm because of their high efficiencies and low costs, but people are still arguing about why they work.
For a few years,”The idea that polarons might be involved has been around for a few years,”the idea that polarons might be involved has been around. “But our experiments are the first to directly observe the formation of these local distortions, including their size, shape and how they evolve.”
Exciting, nuanced and hard to comprehend,
Crystalline materials named after the mineral perovskite, which has a similar atomic structure, are Perovskites. Around a decade ago, scientists started integrating them into solar cells, and the performance of these cells has steadily improved in converting sunlight into energy, even though their perovskite components have several defects that should impede electricity flow.
Lindenberg said these materials are notoriously complex and difficult to comprehend.
Although scientists find them fascinating because they are both powerful and easy to produce, they are also highly unstable, break when exposed to air and contain lead that must be kept out of the atmosphere, increasing the prospect that they may make solar cells cheaper than today’s silicon cells.
Using a ‘electron camera’ or X-rays, previous studies at SLAC investigated the existence of perovskites.
Among other things, they have shown that in perovskites, light swirls atoms around, and have measured the lifespan of sound waves that bring heat through the materials.
Lindenberg’s team used the Linac Coherent Light Source (LCLS) laboratory for this research, a strong free-electron X-ray laser that can image materials in near-atomic detail and track atomic motion in a millionth of a billionth of a second.
They examined single material crystals, synthesized at Stanford by Associate Professor Hemamala Karunadasa’s group.
A tiny sample of the material was bombarded with light from an optical laser and the X-ray laser was then used to observe how the material behaved over tens of trillionths of a second.
Expanding distortion bubbles.
Burak Guzelturk, a scientist at DOE’s Argonne National Laboratory who was a postdoctoral fellow at Stanford at the time of the experiments, says, “When you introduce a charge into a material by bombarding it with light, as happens in a solar cell, electrons are released, and those free electrons begin to move around the material,”
They are soon surrounded and engulfed by a kind of local distortion bubble – the polaron – which travels with them,”Soon they are surrounded and engulfed by a kind of bubble of local distortion – the polaron – that travels with them,” “Some people have argued that this ‘bubble’ protects the electrons from scattering off defects in the material and helps explain why they travel so efficiently to the solar cell contact to flow out as current.”
As Lindenberg puts it, the hybrid perovskite lattice structure is flexible and soft – like “a strange combination of a solid and a liquid at the same time,” and that’s what enables the polarons to shape and expand.
Their observations showed that, on the scale of a few angstroms, about the distance between atoms in a solid, polaronic distortions begin very small and extend rapidly outward in all directions, to a diameter of approximately 5 billionths of a meter, an increase of about 50 times.
This causes about 10 layers of atoms, over tens of picoseconds, or trillionths of a second, to be pushed slightly outward inside a roughly spherical area.
In reality, this distortion is zii zi