One of the world’s most promising materials in the quest to produce more efficient, robust and cheaper solar cells is defective. And that’s a good thing.
The material is a semiconductor known as perovskite, a yellow, brown or grayish-black calcium, titanium, oxide mineral that is also an imperfect, soft crystal.
Perovskites have generated a lot of excitement over the past decade because they can act as semiconductors even when there are defects in their crystal structure. This is a big deal because getting most other semiconductors to work well requires painstaking and expensive manufacturing techniques to produce crystals that are as defect-free as possible.
Now researchers at McGill University are closer to understanding how perovskites pull off this trick.
“It’s the starting point for the perovskite field to understand how a defective material can work like an excellent semiconductor in application,” says McGill chemist Patanjali Kambhampati. “It’s our aim to advance our understanding of the physics and chemistry of the material that gives rise to these remarkable properties.”
In traditional bulk solids, electrons are free to move around as unbound waves. In a quantum confined material, these electron waves are confined in space to yield new physical properties which can be exploited for light absorption and emission.
Until now, this phenomenon had only been observed in particles a few nanometres in size – for example in quantum dots on a flatscreen TV.
When particles are nanosized, the movement of electrons are constrained in a way that gives them different properties than larger pieces of the same material – properties that can be fine-tuned to produce effects such as the emission of light in precise colours.
The researchers used a technique known as state-resolved pump/probe spectroscopy to show a similar type of confinement happens in bulk caesium lead bromide perovskite crystals.
State-resolved pump/probe spectroscopy uses optical pulses 50 femtoseconds long to examine the dynamics of material. The instrument revealed that the crystals showed quantum dot-like behaviour in pieces of perovskite significantly larger than quantum dots.
This behaviour could have important implications for green energy. In addition to their role on the “supply side” of the energy problem, these materials also offer value on the “demand side,” says Kambhampati.
“Quantum dots are excellent light emitters and can be developed into energy efficient sources of room lighting – one of our largest energy drains,” he says.
This discovery follows the McGill team’s earlier research demonstrating that perovskites, while appearing to be a solid, can also act like a liquid. That’s because perovskites have an atomic lattice that can distort in response to the presence of free electrons.
Kambhampati likens it to a trampoline absorbing the impact of a rock thrown into its centre. Just as the trampoline will eventually bring the rock to a standstill, the distortion of the perovskite crystal lattice has a stabilizing effect on the electron.
While the trampoline analogy would suggest a gradual dissipation of energy, moving from an excited state back to a more stable one, the pump/probe spectroscopy data in fact revealed the opposite. To the researchers’ surprise, their measurements showed an overall increase in energy.
University of Toronto physical chemist Dwayne Miller calls the work a “careful study” and says its finding “changes significantly the view of dynamic polaron formation and photocarrier trapping in this material.
“Trapping processes lead to losses in solar energy conversion or other applications such that this new observation could well help advance control of energy loss channels in this class of semiconductors.”