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December 6, 2021
by Liu Jia, Chinese Academy of Sciences
A beam of light can be divided into left-handed and right-handed polarized light by physical methods. Chiral fluorescent materials can emit different left-handed and right-handed polarized light when they are excited, which is called circularly polarized luminescence (CPL). To take full advantage of this property, circularly polarized organic light-emitting diodes (CP-OLEDs) can be fabricated with chiral fluorescent emitters to obtain circularly polarized electroluminescence (CPEL). CPEL has attracted extensive attention.
In order to achieve highly efficient CPEL, thermally activated delayed fluorescent (TADF) materials have been widely used in CP-OLEDs since Prof. Chen Chuanfeng’s group from the Institute of Chemistry of the Chinese Academy of Sciences (ICCAS) firstly reported TADF-based CP-OLEDs in 2018. TADF emitters have been regarded as third-generation luminescent materials since they can utilize both singlet and triplet excitons via reverse intersystem crossing (RISC) process. Therefore, TADF-based OLEDs can achieve theoretically 100 percent internal quantum efficiency (IQE).
In a new study published in Angewandte Chemie, Prof. Chen’s group reported new progress on chiral-polymers-based CP-OLEDs which achieved intense CPEL properties. This is the first report of CPEL detected from CP-OLEDs fabricated with chiral TADF-active polymers.
The researchers designed and synthesized two pairs of chiral TADF polymers with a strategy of chiral donor-acceptor (D*-A) copolymerization, which was first proposed by the group. The chiral donor moiety has a rigid triptycene scaffold which can provide chirality as well as break the conjugation of the polymer backbone. The benzophenone diphenyl sulfone unit and benzophenone unit were used as the acceptor moieties to prepare the two pairs of polymers.
The researchers then fabricated solution-processed CP-OLEDs using the two pairs of polymers as the emitting materials. The corresponding devices achieved outstanding performances with high maximum external quantum efficiency (EQEmax) of up to 22.1 percent, high maximum brightness (Lmax) of up to 34,350 cd/m2. Intense mirror-image CPEL signal was detected in such highly-efficient CP-OLED devices.
It is believed that the experimental results proved the effectiveness of the new strategy of D*-A copolymerization for realizing CPEL. The study provides a new perspective for the research field of CPEL.
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