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Chiral Polymers Take Fast Track to High-Efficiency Circularly Polarized OLEDs

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BEIJING, Dec. 30, 2021 — In experimental work, researchers in the group of Chen Chuanfeng from the Institute of Chemistry of the Chinese Academy of Sciences have demonstrated devices using chiral thermally activated delayed fluorescent (TADF) polymers. The devices, chiral-polymers-based circularly polarized (CP) OLEDs, demonstrated highly efficient circularly polarized electroluminescence (CPEL) properties.

According to the researchers, the work is the first report of CPEL to be detected from CP-OLEDs fabricated with chiral TADF-active polymers.

CPEL based on OLEDs has attracted interest due to its efficient ability to generate circularly polarized light directly. This gives it wide potential for applications such as 3D displays, optical data storage, and optical spintronics.

Graphic showing the donor-acceptor copolymerization strategy that researchers from the Institute of Chemistry of the Chinese Academy of Sciences used to show chiral thermally activated delayed fluorescent (TADF)-active polymers for high efficiency, circularly polarized OLEDs. The researchers report the work is the first detection of circularly polarized electroluminescence from circularly polarized OLEDs fabricated with chiral TADF-active polymers. Courtesy of Angewandte Chemie.
Graphic showing the donor-acceptor copolymerization strategy that researchers from the Institute of Chemistry of the Chinese Academy of Sciences used to show chiral thermally activated delayed fluorescent (TADF)-active polymers for high-efficiency, circularly polarized OLEDs. The researchers report that the work is the first detection of circularly polarized electroluminescence from circularly polarized OLEDs fabricated with chiral TADF-active polymers. Courtesy of Angewandte Chemie.
TADF materials have been used in CP-OLEDs to achieve highly efficient CPEL since Chuanfeng’s group reported TADF-based CP-OLEDs in 2018. The materials have been regarded as “third-generation” luminescent materials since they can use both singlet and triplet excitons via the reverse intersystem crossing process — which is defined as a photophysical process of energy transfer from the excited triplet state back to the singlet.

As a result, TADF-based OLEDs can theoretically achieve 100% internal quantum efficiency (QE), the researchers said. In a photodetector, QE describes the ratio of the number of charge carriers at either terminal and the number of photons that hit the device’s photoreactive surface.

The research team designed and synthesized two pairs of chiral TADF polymers to make its devices. It used the strategy of chiral donor-acceptor (D*-A)  copolymerization. The chiral donor moiety, which is part of the donor molecule that is also part of the other molecule, featured a rigid triptycene scaffold. This scaffold can provide chirality, and it can also break the conjugation of the polymer backbone.

The researchers used a benzophenone diphenyl sulfone unit and a benzophenone unit as the acceptor moieties to prepare the pairs of polymers. Next, they fabricated solution-processed CP-OLEDs using the pairs as the light-emitting materials.

The corresponding devices achieved outstanding performance. They exhibited high maximum external quantum efficiency of up to 22.1% and high maximum brightness of up to 34,350 cd/m2.

Additionally, an intense mirror image CPEL signal has been detected in such high-efficiency CP-OLED devices, the researchers said. They believe the experimental results proved the effectiveness of a new strategy of D*-A copolymerization for achieving CPEL.

The research was published in Angewandte Chemie (www.doi.org/10.1002/anie.202110794).

Photonics.com
Dec 2021
GLOSSARY
photochemistry
The study of chemical reactions stimulated by the properties of light.
polarization
With respect to light radiation, the restriction of the vibrations of the magnetic or electric field vector to a single plane. In a beam of electromagnetic radiation, the polarization direction is the direction of the electric field vector (with no distinction between positive and negative as the field oscillates back and forth). The polarization vector is always in the plane at right angles to the beam direction. Near some given stationary point in space the polarization direction in the beam...
circularly polarized light
A light beam whose electric vectors can be broken into two perpendicular elements that have equal amplitudes and that differ in phase by l/4 wavelength.
electroluminescence
The nonthermal conversion of electrical energy into light in a liquid or solid substance. The photon emission resulting from electron-hole recombination in a PN junction is one example. This is the mechanism employed by the injection laser.
Research & TechnologyeducationDisplayschiralityphotochemicalphotochemistrypolymersmaterialsOLEDsLEDspolarizationcircular polarizationcircularly polarized lightelectroluminescenceelectroluminescence and photoreflectanceTADFchemicalsAsia Pacific

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