CINNO Research Industry Information, a collaboration between R & D team of Professor Takiji Kaneda of Kansai Gakuin University and a subsidiary of JNC Corporation-JNC Petrochemical Co., Ltd. has successfully developed an organic display for color purity exceeding quantum dots and LED Blue luminescent material.
Need to develop OLED display light-emitting materials with excellent color purity and luminous efficiency;
Utilizing the characteristics of nitrogen and boron, we have successfully developed an organic series of blue light-emitting materials (ν-DABNA) with a color purity that exceeds the Gallium series LED and Cadmium series quantum dots.
It is expected to achieve high color gamut, high brightness, low power consumption, and blue light reduction in organic EL displays.
Organic EL (OLED) displays, as a new type of display technology that replaces liquid crystal displays, are being pushed into practical use. However, organic light-emitting materials have the disadvantage that the color purity of light emission is low (the light emission spectrum is wide). If the color purity is low, it is necessary to remove unnecessary colors from the light emission spectrum (Spectre) by using an optical filter (Filter) to improve the color purity when applied to the display screen. As a result, the brightness and luminous efficiency of the display screen are large. Decreased. In addition, the improvement of color purity through a filter is still limited, so there is a problem that it is difficult to improve the color gamut of a display screen, and a light-emitting material with high color purity needs to be developed.
Professor Putian and his research and development team introduced 2 boron and 4 nitrogen at appropriate positions of the light-emitting molecules, coupled with the effect of the resonance effect, successfully controlled the cause of the broad luminescence spectrum-namely the telescopic vibration, and successfully developed Organic series of blue light-emitting materials (v-DABNA) with color purity exceeding Gallium series LEDs and Cadmium series quantum dots.
The R & D team successfully developed DABNA, which is the prototype of ν-DABNA in 2016, and successfully applied it to the organic EL display of high-end smart phones. The color purity and luminous power of ν-DABNA developed this time far exceed DABNA. It is expected to achieve high color gamut, high brightness, low power consumption, and blue light reduction in organic EL displays.
The results of this research and development were released on July 15, 2019 (UK time) in the online breaking news edition of the British scientific magazine Nature Photonics.
Research background and process:
Compared with the liquid crystal display, the organic EL (OLED) display has the advantages of excellent contrast (Contrast), no viewing angle limitation, fast response and other advantages, and has a wide range of applications in smart phones, TVs, industrial displays. As light-emitting materials for organic EL displays, three types of materials, fluorescent materials, phosphorescent materials, and thermally activated delayed fluorescent (TADF) materials, can be used as organic series light-emitting materials, but all have full width at half maximum (Full Width at Half Maxima) is large and the color purity is low.
In general, the display light is displayed by mixing the three primary colors of red, green, and blue to display a variety of colors. If the color purity is low, there may be a problem that the color cannot be reproduced. The image quality (color reproducibility) will also be degraded. Organic EL displays sold on the market generally use optical filters to remove unwanted light from the light emission spectrum, improve color purity (that is, reduce the width of the spectrum), and then use it. At this time, if the width of the original spectrum is wide, the proportion of light to be removed will also increase, and the problems of display screen brightness and luminous efficiency will greatly decrease. In addition, the color purity improved by the filter is limited, so there is also a problem that it is difficult to improve the color gamut of the display screen, so it is urgent to develop a light emitting material with high color purity. In addition, in this context, as technologies to replace organic EL, micro-LEDs using Gallium series light emitting diodes (LEDs) and QD-OLEDs using Cadmium series quantum dots R & D is in full swing.
Research content:
Professor Putian and his research team have developed an organic series of blue light-emitting materials (v-DABNA) with extremely high color purity (refer to the right of the figure below). Hitherto, as the blue light-emitting material of organic EL, polycyclic aromatic hydrocarbons (Hydrocarbon) -based pyrene and perylene inducers have been used. However, it brings about the problem of the emission spectrum with the Full Width at Half Maxima of about 40nm (refer to the left of the figure below). The reason is that HOMO and LUMO mainly exist between different carbon atoms, accompanied by When emitting light, when the singlet state (S1) is excited to the ground state (S0) (S1 → S0 transition, equivalent to the electron migration from LUMO to HOMO), the electron density between carbon atoms changes great. Due to the migration of S1 → S0, if the density between the carbon atoms becomes larger, the active force between the carbon atoms will also change, although it will also be accompanied by the carbon-carbon bond stretching vibration, according to the energy of the vibration (Energy) ( 1300-1700 cm-1), the width of the emission spectrum increases. On the other hand, regarding v-DABNA, due to the multiple resonance effects of boron and nitrogen, HOMO and LUMO are locally distributed on different carbon atoms, and there are almost no electrons between the carbon atoms generated by the S1 → S0 migration. The density changes, so there is no telescopic vibration (refer to the figure on the right). Although the migration of S1 → S0 will generate twisted (twisted) vibrations of the entire molecule, the energy of the vibration (Energy) is extremely small (~ 20cm-1), so it shows an extremely narrow full width at half maximum of 14-18nm. Luminescence spectrum. In addition, v-DABNA has excellent TADF characteristics, and in terms of practical brightness (300cdm-2), it has far more than 30% of the external quantum luminous efficiency of conventional blue particles.
Future directions:
The v-DABNA developed this time has both the color purity and highest level of power exceeding GaN LEDs and Cadmium series quantum dots. To this end, it is expected to achieve high color gamut and high brightness of organic EL displays. , Low power consumption, and blue light reduction. In addition, regarding the display screens on the market, how to improve the performance of the blue light emitting element is its "Bottle Neck". For this reason, by rationally optimizing the element structure and production process, it is expected that the cost of the display screen can be reduced in the future. The molecular design established through this research will also develop more light-emitting materials with excellent properties in the future.
Explanation of terms:
DABNA
“Ultrapure Blue Thermally Activated Delayed Fluorescence Molecules: Efficient HOMO–LUMO Separation by the Multiple Resonance Effect” (Adv. Mater. 2016, 28, 2777. doi: 10.1002 / adma.201505491
Thermally activated delayed fluorescence (TADF material)
TADF material is a fluorescent material, which can effectively convert triplet excitation to singlet excitation. In theory, all electricity can be converted into light. Moreover, it also has the advantage that it does not use iridium like phosphorescent materials. (Iridium), platinum (Platina) and other rare elements. The TADF material was discovered by Professor Anda Chiba (Kyushu University's Advanced Organic Optoelectronics Research Center) and his team. Now researchers at home and abroad are actively researching with Professor Anda as the center.
Full Width at Half Maxima
Also called "half-width, half-peak width", it is a standard for calculating the width of a mountain-shaped function. Here, it refers to the width (full width) of the spectrum at a value 1/2 of the maximum value of the light emission intensity in the light emission spectrum. The blue light source of the organic EL display on the market uses a fluorescent material with a relatively narrow full width at half maximum.
HUMO
Among the molecular orbitals occupied by electrons, the highest energy orbital-acronym for Highest Occupied Molecular Orbital. Slightly lower energy levels than LUMO mentioned below. Organic EL elements are in an unstable state (excitation state), that is, a state in which electrons migrate from HOMO to LUMO, and when electrons migrate from LUMO to HOMO (that is, a stable state), light is generated. In the excited state, since the electrons occupying molecular orbitals become integrated, they are called semi-occupied orbitals (SOMO: Single Occupied Molecular Orbital).
LUMO
Among the molecular orbitals that are not occupied by electrons, the lowest energy orbit is the abbreviation of Lowest Unoccupied Molecular Orbital. In the excited state, the electrons occupying molecular orbitals become one body, so they are semi-occupied orbits (SOMO).
External quantum power
One of the indicators of efficiency used in photoelectric conversion elements (organic EL, light-emitting diode-LED, etc.). It is expressed by the ratio of the number of electrons injected from the outside and the number of light quantums released to the outside in a unit time. In organic EL, the values will vary due to the composition of the protons; in general, fluoresceins are ~ 10%, phosphorescent and TADF protons are ~ 30%. The ratio of the number of electrons injected from the outside to the photon digits occurring inside the element is called "internal quantum power", and the phosphorescent and TDAF element can reach 100%. Since it is impossible to extract all the generated light from the front of the panel, the external quantum power of the display is much lower than the internal quantum power. External quantum power = internal quantum power * light extraction rate.