Kyoto University developing ultra-narrowband emitting organic LEDs with a full width at half maximum of 5.5 nm

A research group including Professor Takuji Hatakeyama, Associate Professor Masashi Mamada, Assistant Professor Junki Ochi, Master's Student Kota Kataoka (at the time of the research), and Doctoral Student Taehwan Lee of the Graduate School of Science at Kyoto University has advanced a molecular design concept known as multiple resonance. Through this approach, they successfully developed organic materials that exhibit emission with an extremely narrow full width at half maximum (FWHM; the width of the emission spectrum at half of its peak height). The findings were published in Science.

Organic light-emitting diodes (OLEDs) are widely commercialized in displays for devices such as smartphones. However, they face a persistent challenge. Because their FWHM is wide, the emitted colors suffer from color bleeding. In 2016, Hatakeyama and his colleagues discovered a new molecular design guideline, which drastically accelerated the development of materials showing narrowband emission over the past decade.

The multiple-resonance (MR) effect developed by Hatakeyama's team suppresses the interaction between molecular vibrations and the excited states of electrons (high-energy states) involved in light emission, thereby preventing the FWHM from broadening. Specifically, by localizing electrons, which previously existed between the carbon atoms of organic molecules, directly onto the carbon atoms, molecular vibrations are dampened, effectively narrowing the FWHM.

While this approach previously led to the development of narrowband emitting materials with an FWHM of approximately 20 nm, their spectral width remained significantly broader compared to stimulated emission light, such as lasers, which amplify a single wavelength of light. Consequently, there has been a call for further narrowing of the bandwidth.

In this study, the team proposed a new molecular design guideline. By linking multiple basic MR units together, they expanded the spatial separation region between the HOMO (Highest Occupied Molecular Orbital) and the LUMO (Lowest Unoccupied Molecular Orbital). This allowed them to delocalize excitons while fully maintaining the MR effect. The effectiveness of this guideline was demonstrated through a detailed evaluation of the emission characteristics of newly synthesized molecules.

The developed molecule, designated as m-CzB10-Mes, is a ladder-type medium-sized molecule whose molecular skeleton is linked by two bonds in a ladder-like fashion. Such molecules generally have restricted synthetic pathways and suffer from low yields. However, by utilizing a "one-shot borylation" method independently developed by the research team, the team successfully introduced boron atoms into the target positions with a high yield of over 99%. This enabled the efficient synthesis of a hetero-nanocarbon molecule containing 10 boron atoms.

The emission FWHM reached 6.9 nm in toluene and dropped to 5.5 nm in a low-polarity solvent. This yielded an extremely sharp emission spectrum, even when compared to DABNA1 (which has an FWHM of 22 nm in toluene), a representative conventional MR molecule. It showed an extremely fast TADF process with a delayed fluorescence lifetime of 1 microsecond or less. Furthermore, it realized performance comparable to existing D-A-type materials in TADF characteristics as well, succeeding in achieving both narrowband light emission and a fast TADF process. Additionally, the emission spectrum exhibited a sharpness comparable to that of naturally amplified light (such as superluminescence) resulting from stimulated emission when the laser dye was strongly excited.

This shows the possibility that LEDs can realize monochromaticity close to a laser, which has been considered difficult with organic materials, and will contribute greatly to future light-emitting molecule design and the development of next-generation display technologies.

Hatakeyama stated, "Research using functional groups is also being done in various countries, but our method delocalizes across a wide space without breaking the multiple resonance effect, completely eliminating the influence of the light emission excitation process and molecular structure changes. At present, we believe this is the optimal solution."

Journal Information
Publication: Science
Title: Organic spontaneous emission approaching the monochromatic limit
DOI: 10.1126/science.aee0001