Organic crystals show promise for more efficient photon upconversion

In many applications, lower energy, long-wavelength light is wasted. This makes devices such as solar cells less efficient, as they can’t fully benefit from both the high and low energy photons that sunlight produces. Now, via a solid material, researchers in Tokyo have discovered a way to save those longer wavelengths and make them useful.

According to a team at the Tokyo Institute of Technology (Tokyo Tech), specifically, as individual photons’ energy is inversely proportional to its wavelength “and chemical and physical processes are triggered by light only when the energy provided by individual photons exceeds a certain threshold,” not all wavelengths of light can be utilized. For years, scientists pursuing more-efficient solar cells have sought solid materials that can achieve photon upconversion (PUC), which involves photons with lower energies and thus longer wavelengths being captured and re-emitted as those with higher energies and shorter wavelengths. As opposed to existing methods that involve liquid samples, which can be “inherently risky and cumbersome in many applications” despite touting high PUC efficiency, solid crystal materials have always been deemed more promising. However, challenges including poor crystal quality have led to “short travelling distances of triplet excited states and thus low PUC efficiency.”

In their recent study—published in Materials Horizons—the Tokyo Tech researchers demonstrated that van der Waals solid-solution organic crystals can essentially transform long-wavelength light that is typically wasted into shorter wavelength light, “yielding triplet-sensitized [PUC] organic crystals with extraordinary performance.” At a solar excitation intensity of only 0.175 sun, the crystals achieved a high upconversion quantum yield of 16% out of a theoretical 50%.

“[This is] a major technical leap forward in the quest for high-performance PUC solids, which will open up diverse photonics technologies in the future,” says Yoichi Murakami, an associate professor at Tokyo Tech and co-author of the study. He notes that the findings could boost and enhance existing technologies, such as solar cells, as well as photocatalysts for hydrogen and hydrocarbon productions.