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Crystals that convert light into more useful wavelengths

Organic crystals in solid solution have been led to search for higher photon upconversion materials, which convert long wavelength light currently lost into shorter wavelength light that is more useful. Scientists at the Tokyo Institute of Technology have revisited a previously deemed dull approach to materials – using a molecule originally developed for organic LEDs – achieving exceptional performance and efficiency. Their discoveries pave the way for many new photonic technologies, such as better solar cells and photocatalysts for the production of hydrogen and hydrocarbons.

Light is a powerful source of energy that can, if harnessed properly, be used to cause stubborn chemical reactions, generate electricity, and operate optoelectronic devices. However, in most applications not all wavelengths of light can be used. This is because the energy carried by each photon is inversely proportional to its wavelength, and chemical and physical processes are only triggered by light when the energy supplied by individual photons exceeds a certain threshold.

This means that devices like solar cells cannot take advantage of all of the color contained in sunlight because it includes a mixture of high and low energy photons. Scientists around the world are actively exploring materials to achieve photon upconversion (PUC), whereby photons with lower energies (longer wavelengths) are captured and re-emitted as photons with higher energies (shorter wavelengths). One promising way to accomplish this is triplet-triplet annihilation (TTA). This process requires the combination of a sensitizer material and an annihilator material. The sensitizer absorbs low energy photons (long wavelength light) and transfers its excited energy to the annihilator, which emits higher energy photons (shorter wavelength light) due to TTA .

Finding good solid materials for the PUC has been difficult for a long time. Although liquid samples can achieve relatively high PUC efficiency, working with liquids, especially those including organic solvents, is inherently risky and time consuming in many applications. However, previous attempts to create PUC solids generally suffered from poor crystal quality and small crystal domains, leading to short travel distances of triplet excited states and therefore low PUC efficiency. In addition, in most of the previous solid samples of PUC, stability under continuous photoirradiation has not been tested and experimental data has often been acquired in inert gas atmospheres. Therefore, low efficiency and insufficient stability of materials has been a concern for a long time.

Today, in a recent study led by Associate Professor Yoichi Murakami of Tokyo Tech, Japan, a team of researchers found the answer to this challenge. Posted in Material horizon, their (open access) article describes how they focused on van der Waals crystals, a classic class of materials that was not considered for the quest for high efficiency PUC solids. After discovering that 9- (2-naphthyl) -10-[4-(1-naphthyl)phenyl]anthracene (ANNP), a hydrocarbon molecule originally developed for blue organic LEDs, was a great annihilator to embody their concept, they tried mixing it with platinum octaethylporphyrin (PtOEP), a Basic sensitizer that absorbs green light.

The team found that the aggregation of sensitizing molecules could be completely avoided by using the crystalline phase of a solid van der Waals solution with a sufficiently low proportion of PtOEP to ANNP (approximately 1: 50,000). They carried out a thorough characterization of the resulting crystals and found out why the use of the annihilator ANNP prevented the aggregation of the sensitizer when other existing annihilators had failed to do so in previous studies. In addition, the solid crystals produced by the team were very stable and exhibited exceptional performance, as Murakami points out: “The results of our experiments using simulated sunlight indicate that solar concentrating optics such as lenses are not. more necessary to efficiently convert terrestrial sunlight. “

Overall, this study brings van der Waals crystals back into the PUC game as an effective way to create exceptional solid materials using multipurpose hydrocarbon annihilators. “The proof of concept we presented in our article is a major technical leap forward in the quest for high performance solid PUCs, which will open up various photonic technologies in the future,” concludes Murakami. Hopefully, further research on this topic will allow us to effectively transform light into its most useful forms.

– This press release was provided by the Tokyo Institute of Technology


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