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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).

Published: December 2021
Glossary
chirality
Chirality is a property of certain molecules and objects in which they are non-superimposable on their mirror images. In other words, a chiral object or molecule cannot be exactly superimposed onto its mirror image, much like a left and right hand. The term "chirality" comes from the Greek word cheir, meaning hand, emphasizing the handedness or asymmetry of the object or molecule. A molecule or an object with this property is said to be chiral, while its non-superimposable mirror image is...
photochemical
The term photochemical pertains to chemical processes or reactions that are initiated or influenced by the absorption of light. Photochemical reactions involve the interaction of light, often in the form of ultraviolet or visible radiation, with molecules, leading to changes in their chemical structure or properties. These reactions are distinct from thermal or non-light-induced chemical reactions. Key characteristics of photochemical reactions include: Light absorption: Photochemical...
photochemistry
The study of chemical reactions stimulated by the properties of light.
polarization
Polarization refers to the orientation of oscillations in a transverse wave, such as light waves, radio waves, or other electromagnetic waves. In simpler terms, it describes the direction in which the electric field vector of a wave vibrates. Understanding polarization is important in various fields, including optics, telecommunications, and physics. Key points about polarization: Transverse waves: Polarization is a concept associated with transverse waves, where the oscillations occur...
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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