Idemitsu Kosan Co.,Ltd., and Toray Industries, Inc., announced that they have jointly developed an organic electroluminescent (EL) device offering the world’s highest luminous efficiency, with a near practical range of 46 cd/A. The new device employs thermally activated delayed fluorescence (TADF, see note 1) and red fluorescent materials.

This new technology could help reduce the costs and energy consumption of organic EL displays while enabling wide color gamuts. It is the result of combining Idemitsu Kosan’s development of TADF material, offering an exceptional luminous efficiency and lifespan, and Toray’s development of new red fluorescent material with a narrower emission spectrum (see note 2) compared to conventional counterparts, which provides high color purity.

Organic EL displays comprise red, green, and blue light-emitting components. The devices emitting red light mainly employ phosphorescent materials. These materials can convert 100% of electricity to light and enhance luminous efficiency. The downside is that they are expensive because they incorporate rare metals and suffer from low color purity due to wide emission spectra.

Those drawbacks have spurred interest in TADF materials in recent years. Technologies employing TADF materials can convert all power to light, as do phosphorescent ones. Another advantage is that they can attain high color purity when used in combination with fluorescent materials, which have narrow emission spectra. TADF materials also are free of rare metals, so they can help to reduce costs.

Both companies concluded a technical alliance for organic EL materials on September 26, 2017. They have since collaborated in developing new materials by drawing on each other’s organic EL materials, technologies and expertise. The red organic EL device employing TADF materials that the companies developed together attains the same results as mainstream red phosphorescent devices and represents significant progress towards a swiftly materializing new technology. The two companies plan to drive forward to secure adoption of their materials and technologies in mobile and television applications.

Idemitsu Kosan and Toray plan to present the results of their work on the new device during the 26th International Display Workshops at the Sapporo Convention Center on November 27.

Performance Overview Method Deep red organic EL device (top emission setup) LT95(*1) More than 90 hours (current density of 50 mA/cm2 resulting from drive condition) Current efficiency(*2) 46 cd/A (current density of 10 mA/cm2 resulting from drive condition) Chromacity (0.679, 0.320) (CIE1931 color space) *1. Continuous lighting time until initial brightness decreases 5% *2. Current efficiency: Brightness per unit of current (in candelas per Ampere)

Notes:

1. TADF materials

Professor Chihaya Adachi of Kyushu University’s Center for Organic Photonics and Electronics Research led a team that discovered this organic EL material. Organic EL devices produce singlet and triplet excitons at a ratio of 25 to 75, which stems from recombinations of injected holes and electrons. Fluorescent materials normally emit light just from singlet excitons, so only 25% of excitation energy is usable. With TADF materials, however, reverse intersystem crossing from triplet to singlet excitons is efficient. In principle, therefore, 100% of the excitation energy is usable as fluorescence. This material is free from rare metals, unlike phosphorescent materials, so it offers cost and supply advantages. It is the focus of extensive research for advanced light-emitting materials.

2. Emission spectrum

This is a light intensity distribution for light wavelengths. A wide spectrum means a mixing of various colors of light (wavelengths), reducing color purity. A narrow emission spectrum means a high color purity that is nearly monochromatic.

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