Charge-transfer (CT) complexes, formed by electron transfer from a donor to an acceptor, play a crucial role in organic semiconductors. Excited-state CT complexes, termed exciplexes, harness both singlet and triplet excitons for light emission, and are thus useful for organic light-emitting diodes (OLEDs). However, present exciplex emitters often suffer from low photoluminescence quantum efficiencies (PLQEs), due to limited control over the relative orientation, electronic coupling and non-radiative recombination channels of the donor and acceptor subunits. Here, we use a rigid linker to control the spacing and relative orientation of the donor and acceptor subunits, as demonstrated with a series of intramolecular exciplex emitters based on 10-phenyl-9,10-dihydroacridine and 2,4,6-triphenyl-1,3,5-triazine. Sky-blue OLEDs employing one of these emitters achieve an external quantum efficiency (EQE) of 27.4% at 67 cd m−2 with only minor efficiency roll-off (EQE = 24.4%) at a higher luminous intensity of 1,000 cd m−2. As a control experiment, devices using chemically and structurally related but less rigid emitters reach substantially lower EQEs. These design rules are transferrable to other donor/acceptor combinations, which will allow further tuning of emission colour and other key optoelectronic properties. The use of rigid linkers to control the relative position and interaction of donor and acceptor units in exciplex emitters leads to the realization of organic light-emitting devices with enhanced external quantum efficiency.

Highly efficient luminescence from space-confined charge-transfer emitters.

The recent progress of efficiency improvement, emission color tuning, and lifetime elongation of blue organic light-emitting diodes (OLEDs) is reviewed. The latter is one of the most important bottlenecks for OLED development. The current status of blue light-emitting material design with emission mechanisms such as fluorescence (F), phosphorescence (Ph), thermally activated delayed fluorescence (TADF), and hybridized local and charge transfer (HLCT) is introduced in the first part of this review. Compared to red and green devices, the long exciton lifetime of the high energy triplet exciton in a blue OLED is the one of the main issues. To avoid the accumulation of high energy triplet excitons in the emitter for blue OLEDs, assisted triplet–triplet fluorescence (TTF) and Hyperfluorescence™ are employed to harvest the triplet excitons. In the second part of this review, we focus on issues from an application viewpoint: what are the requirements of blue OLEDs for display and lighting technologies in terms of efficiency, color, and lifetime? Key performance metrics of blue OLEDs with different technologies over time are summarized. Independent of technology, the trend is similar: the external quantum efficiency improves for the first stage of research, followed by color tuning, and then finally lifetime elongation. The state-of-the-art device performance of blue OLEDs with various emission mechanisms is illustrated. Although Ph- and TADF-emission based devices show satisfactory efficiency and electroluminescence (EL) spectra, despite having a lower efficiency TTF-emission based devices are the mainstream for real applications due to their relatively long operation lifetime. Blue Ph-OLEDs have the potential for lighting applications with suitable material selection and device design. We collected the published results and tried our best to make a fair comparison of the operation lifetime among different technologies. Finally, we discuss the possible future outlook from different viewpoints including new materials, device designs, and applications of blue OLEDs.

Blue organic light-emitting diodes: current status, challenges, and future outlook

Molecular‐Structure and Device‐Configuration Optimizations toward Highly Efficient Green Electroluminescence with Narrowband Emission and High Color Purity

Developing red thermally activated delayed fluorescence (TADF) emitters, attainable for both high-efficient red organic light-emitting diodes (OLEDs) and non-doped deep red/near-infrared (NIR) OLEDs, is challenging. Now, two red emitters, BPPZ-PXZ and mDPBPZ-PXZ, with twisted donor-acceptor structures were designed and synthesized to study molecular design strategies of high-efficiency red TADF emitters. BPPZ-PXZ employs the strictest molecular restrictions to suppress energy loss and realizes red emission with a photoluminescence quantum yield (ΦPL ) of 100±0.8 % and external quantum efficiency (EQE) of 25.2 % in a doped OLED. Its non-doped OLED has an EQE of 2.5 % owing to unavoidable intermolecular π-π interactions. mDPBPZ-PXZ releases two pyridine substituents from its fused acceptor moiety. Although mDPBPZ-PXZ realizes a lower EQE of 21.7 % in the doped OLED, its non-doped device shows a superior EQE of 5.2 % with a deep red/NIR emission at peak of 680 nm.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/ange.201910421

Red/Near-Infrared Thermally Activated Delayed Fluorescence OLEDs with Near 100 % Internal Quantum Efficiency.

In this work, two novel thermally activated delayed fluorescence (TADF) emitters, 2tDMG and 3tDMG, are synthesized for high-efficiency organic light-emitting diodes (OLEDs), The two emitters have a tilted face-to-face alignment of donor (D)/acceptor (A) units presenting intramolecular noncovalent interactions. The two TADF materials are deposited either by an evaporation-process or by a solution-process, both of them leading to high OLED performance. 2tDMG used as the emitter in evaporation-processed OLEDs achieves a high external quantum efficiency (EQE) of 30.8% with a very flat efficiency roll-off of 7% at 1000 cd m-2 . The solution-processed OLEDs also display an interesting EQE of 16.2%. 3tDMG shows improved solubility and solution processability as compared to 2tDMG, and thus a high EQE of 20.2% in solution-processed OLEDs is recorded. The corresponding evaporation-processed OLEDs also reach a reasonably high EQE of 26.3%. Encouragingly, this work provides a novel strategy to address the imperious demands for OLEDs with high EQE and low roll-off.

/pdf/highly-efficient-thermally-activated-delayed-fluorescence-5cf28yop0z.pdf

Highly Efficient Thermally Activated Delayed Fluorescence via an Unconjugated Donor–Acceptor System Realizing EQE of Over 30%

The lack of structural information impeded the access of efficient luminescence for the exciplex type thermally activated delayed fluorescence (TADF). We report here the pump-probe Step-Scan Fourier transform infrared spectra of exciplex composed of a carbazole-based electron donor (CN-Cz2) and 1,3,5-triazine-based electron acceptor (PO-T2T) codeposited as the solid film that gives intermolecular charge transfer (CT), TADF, and record-high exciplex type cyan organic light emitting diodes (external quantum efficiency: 16%). The transient infrared spectral assignment to the CT state is unambiguous due to its distinction from the local excited state of either the donor or the acceptor chromophore. Importantly, a broad absorption band centered at ~2060 cm−1 was observed and assigned to a polaron-pair absorption. Time-resolved kinetics lead us to conclude that CT excited states relax to a ground-state intermediate with a time constant of ~3 µs, followed by a structural relaxation to the original CN-Cz2:PO-T2T configuration within ~14 µs. The development of exciplex-type hosts for thermally activated delayed fluorescence organic light-emitting diodes is hindered by a lack of structural information for these donor:acceptor blends. Here, the authors report the pump-probe Step-Scan Fourier transform IR spectra for a D:A exciplex host.

/pdf/probe-exciplex-structure-of-highly-efficient-thermally-33869nmxz2.pdf

Probe exciplex structure of highly efficient thermally activated delayed fluorescence organic light emitting diodes.

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Probe exciplex structure of highly efficient thermally activated delayed fluorescence organic light emitting diodes.