New state of matter unlocks a secret of perovskite solar cells

Perovskite solar cells are advancing at a rapid rate, and is drawing interest from scientists working to not just boost their performance but better understand how they offer such incredible, ever-increasing efficiencies. By turning their tools to perovskite crystals scientists have discovered unexpected behavior that represents an entirely new state of matter, which they say can help drive the development of advanced solar cells and other optical and electronic devices.

One of the reasons there is such interest around perovskite solar cells is the counter-intuitive way they are able to offer such excellent performance in spite of defects in their crystal structure. While much research focuses on fixing these defects to boost their efficiency, through chemical treatments, molecular glue or even sprinklings of chili compounds, the fact remains that the material is a far more effective semiconductor than it should be.

“Historically, people have been using bulk semiconductors that are perfect crystals,” says senior author Patanjali Kambhampati, an associate professor in the Department of Chemistry at McGill University. “And now, all of a sudden, this imperfect, soft crystal starts to work for semiconductor applications, from photovoltaics to LEDs. That’s the starting point for our research: how can something that’s defective work in a perfect way?”

This work is actually a continuation of previous research demonstrating that while perovskites might look like a solid substance, they actually have some characteristics of a liquid. This duality is largely attributed to an atomic lattice structure that deforms as it encounters free electrons, a phenomenon known as polaron formation. This is likened to the way a trampoline stretches and changes shape if you were to throw a large rock on its center.

Diagram depicting the distortion of the atomic lattice structure in perovskite crystals, and the formation of a quantum-dot-like particle

Colin Sonninchsen

Where a trampoline would gradually dissipate this energy as the rock stops bouncing and nestles in its center, the reverse was found to be true of the deforming atomic lattice structure of perovskite crystals. The team observed this process in action using a form of pump/probe spectroscopy to study the electronic dynamics of the perovskite crystals, and surprisingly found an overall increase in energy following the deformation.

The scientists say this is the result of the perovskite crystals behaving like quantum dots, which themselves have shown promise as a way of improving solar cell technology. These tiny, flat semiconducting crystals are so small that they restrict the movement of electrons in a unique way, which gives them distinct characteristics.

Known as quantum confinement, this phenomenon had previously only been observed in particles measuring a few nanometers in size. The fact that it has now been seen at play within perovskite crystals that are much larger than this amounts to the discovery of a new state of matter, according to the scientists.

“What the polaron does is confine everything into a spatially well-defined area,” says Kambhampati. “One of the things our group was able to show is that the polaron mixes with an exciton to form what looks like a quantum dot. In a sense, it’s like a liquid quantum dot, which is something we call a quantum drop. We hope that exploring the behavior of these quantum drops will give rise to a better understanding of how to engineer defect-tolerant optoelectronic materials.”

The research was published in the journal Physical Review Research.

Source: McGill University

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