Perovskite Quantum Dots and Their Potential for Optical Cooling

Researchers from Chiba University investigated a promising avenue for solid-state optical cooling with perovskite quantum dots. This study, published in the journal Nano Letters, focuses on anti-Stokes photoluminescence, a phenomenon where materials release photons with higher energy than they absorb. It has the potential to completely transform current cooling technologies.

Their research shows the advantages and disadvantages of this innovative cooling strategy, with promising future developments in energy-saving technologies.

Since heat tends to deteriorate materials and reduce performance in various ways, cooling systems are an essential component of many contemporary technologies. However, cooling can frequently be a laborious and expensive procedure. As a result, researchers have been looking for creative and effective ways to cool down materials.

Solid-state optical cooling is a well-known application that uses the extremely unusual phenomena known as anti-Stokes (AS) emission. The electrons in materials typically go into an “excited” state when they absorb photons from incoming light.

In a perfect world, some extra energy is released as light as the electrons return to their initial state, and the remainder is transformed into heat. Electrons in materials that experience AS emission interact with “phonons,” or crystal lattice vibrations, to produce more energetic photons than those in the incident light. Theoretically, these materials could cool down rather than heat up when exposed to light if AS emission efficiency is near 100%.

A group of researchers led by Professor Yasuhiro Yamada from the Graduate School of Science at Chiba University examined this phenomenon in depth in a potential perovskite-based material structure.

This team, which included Takeru Oki from the Graduate School of Science and Engineering, Chiba University, Dr. Kazunobu Kojima from the Graduate School of Engineering, Osaka University, and Dr. Yoshihiko Kanemitsu from the Institute for Chemical Research, Kyoto University aimed to clarify the optical cooling phenomena in a unique configuration of perovskite quantum dots (very tiny CsPbBr₃ crystals) embedded within a Cs₄PbBr₆ host crystal matrix (designated as CsPbBr₃/Cs₄PbBr₆ crystal).

Efforts to achieve optical cooling in semiconductors have encountered several difficulties, primarily due to challenges in reaching nearly 100% emission efficiency, and true cooling has been elusive. Though quantum dots are promising for their high emission efficiency, they are notoriously unstable, and exposure to air and continued illumination degrade their emission efficiency. Thus, we focused on a stable structure known as ‘dots-in-crystals,’ which may overcome these limitations.

Yasuhiro Yamada, Professor, Graduate School of Science, Chiba University

The application of semiconducting quantum dots poses an unresolved challenge. Excitons, which are pairs of electrons and positively charged “holes,” are produced when light strikes a semiconductor.

Usually, excitons release light as they recombine. Nevertheless, a process known as Auger recombination, which releases energy as heat rather than light, becomes more noticeable at high exciton concentrations. Due to this process, high-intensity light irradiation frequently causes heating rather than cooling in semiconductor quantum dots.

The researchers employed time-resolved spectroscopy to ascertain the circumstances under which Auger recombination was more common. Even at moderate light levels, these studies demonstrated that heating was inevitable, suggesting that low-intensity light trials were necessary to detect true optical cooling.

Unfortunately, optical cooling loses some of its effectiveness at low intensities. Under ideal circumstances, their sample showed a theoretical chilling limit of roughly 10 K from room temperature.

Another goal of the investigation was to make more accurate temperature readings than in past documented attempts. To achieve this, they created a technique that uses the shape of the emission spectrum to determine the temperature of materials with high emission efficiency. Several samples showed true optical cooling, and the researchers discovered that when the intensity of the excitation light grew, a change from cooling to heating took place.

Previous reports of optical cooling in semiconductors lacked reliability, primarily due to flaws in temperature estimation. Our study, however, not only established a reliable method but also defined the potential and limitations of optical cooling through time-resolved spectroscopy, marking a significant achievement in the field.

Yasuhiro Yamada, Professor, Graduate School of Science, Chiba University

This work makes future investigations aimed at reducing Auger recombination to enhance the cooling capabilities of dots-in-crystal configurations possible. If optical cooling advances sufficiently to be widely used, it may serve as the basis for several energy-saving devices that support international sustainability objectives.

Canon Foundation, the International Collaborative Research Program of the Institute for Chemical Research, Kyoto University, JST-CREST, and KAKENHI, funded the study.

Journal Reference:

Yamada, Y., et al. (2024) Optical Cooling of Dot-in-Crystal Halide Perovskites: Challenges of Nonlinear Exciton Recombination. Nano Letters. doi.org/10.1021/acs.nanolett.4c02885.