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Non‐conjugated luminescent polymers have received much attention due to their excellent flexibility, processability, and biocompatibility. However, the single emission color and low solid‐state luminescence efficiency restrict their further development. Herein, by introducing carboxyl‐terminated alkyl chains on the rigid and helical polymaleimide backbone, a series of non‐conjugated luminescent polymers are designed and synthesized. Due to the "spring lock" formed by the synergism of scalable helical main chains and flexible alkyl side chains, these polymers exhibit effective cluster luminescence with thermally activated delayed fluorescence (TADF) property, thereby significantly improving the solid‐state luminescence efficiency with the quantum yield up to 59.6%. The TADF emission wavelengths of the polymer powders can be tuned from 498 to 639 nm through changing reaction solvents, because the conformation of helical chains forming clusters is sensitive to solvent polarity. Interestingly, metal ions can induce the polymers to construct self‐repairing cluster luminescence gels, which show excellent potential applications in ammonia detection and information encryption. This work not only provides an effective design strategy for the colorful cluster luminescence with high‐efficiency TADF, but also further reveals the cluster luminescence mechanism and enlightens the promising applications of cluster luminescent gels. [ABSTRACT FROM AUTHOR]

Recent advances in organic light‐emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF)‐assisted fluorescence (TAF) attest to the great promise of this technology in practical use. However, the simultaneous realization of high efficiency and device durability in blue OLEDs remains a significant challenge. Clarification of the degradation mechanisms correlated to molecular structure and device configuration is the key to extending the device lifetime. In this study, electron transport materials incorporating two triazine units in close proximity are adopted to use in hole‐blocking and electron‐transporting layers, resulting in superior device performances. In addition, a modified photodegradation experiment reveals that the degradation origins closely relate to charge carriers. The optimization of the device according to the obtained findings leads to 4.5 times extension in the lifetime of the TAF‐OLED using a multiple resonance emitter. These results also provide guidelines for designing robust electron transport materials for blue OLEDs. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) materials are expected to address triplet‐related losses in electrically driven organic lasers, as the electrically generated triplets in the materials can be converted to radiative singlets through reverse intersystem crossing (RISC). This offers a way to bypass triplet absorption and annihilation in organic semiconductor lasers (OSLs). In this work, two versatile TADF emitters 4tCzPz and 4αCbPz for application in organic light‐emitting diodes (OLEDs) and OSLs are presented. Both emitters possess moderately high singlet‐triplet energy gap, ΔEST (≈0.30 eV) and show high photoluminescence quantum yields, ΦPL, in solution and solid‐state and prominent stimulated emission features in solution. Films of 4tCzPz and 4αCbPz doped in mCBP show an amplified spontaneous emission (ASE) threshold of 41.0 and 44.9 µJ/cm2, respectively. The OLEDs with 4tCzPz and 4αCbPz emit with peak wavelengths of 492 and 475 nm, respectively, and show corresponding maximum external quantum efficiencies, EQEmax, of 24.6 and 21.3%. The research shows that D‐A TADF materials hold significant potential not only as emitters for OLEDs but also in OSLs. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) is an emission mechanism whereby both singlet and triplet excitons can be harvested to produce light. Significant attention is devoted to developing TADF materials for organic light‐emitting diodes (OLEDs), while their use in other organic electronics applications such as sensors, has lagged. A family of TADF emitters, TPAPyAP, TPAPyBP, and TPAPyBPN containing a triphenylamine (TPA) donor and differing nitrogen‐containing heterocyclic pyrazine‐based acceptors is developed and systematically studied. Depending on the acceptor strength, these three compounds emit with photoluminescence maxima (λPL), of 516, 550, and 575 nm in toluene. Notably, all three compounds show a strong and selective spectral response to the presence of ZnCl2, making them the first optical TADF sensors for this analyte. It is demonstrated that these three emitters can be used in vacuum‐deposited OLEDs, which show moderate efficiencies. Of note, the device with TPAPyBPN in 2,8‐bis(diphenyl‐phoshporyl)‐dibenzo[b,d]thiophene (PPT) host emits at 657 nm and shows a maximum external quantum efficiency (EQEmax) of 12.5%. This electroluminescence is significantly red‐shifted yet shows comparable efficiency compared to a device fabricated in 4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl (CBP) host (λEL = 596 nm, EQEmax = 13.6%). [ABSTRACT FROM AUTHOR]

Interface exciplex represents a promising host material for organic light‐emitting diodes (OLEDs) with barrier‐free charge injection and highly confined recombination region. However, the efficiency of radiative recombination in pristine exciplex is usually low and needs to be improved by doping various emitters. In this study, the interface exciplex OLEDs doped with fluorescence, phosphorescence, and thermally activated delayed fluorescence (TADF) emitters is fabricated to investigate the relationship between their excited‐state properties and electroluminescence efficiencies. A maximum external quantum efficiency of 20% is achieved in interface exciplex OLEDs doped with TADF emitter, which corresponds to nearly 100% exciton utilization and is superior to those of fluorescence and phosphorescence emitters. Furthermore, optical spectroscopy and magneto‐electroluminescence method are used to study the advantages of TADF emitter in interface exciplex host. The large dipole of TADF emitter is beneficial for harvesting energy from the charge‐transfer state at the interface, and its reverse intersystem crossing avoids the accumulation of triplet excitons that leads to triplet‐triplet annihilation in interface exciplex OLEDs. These results demonstrate that the photophysical process needs to be carefully considered in designing high‐performance emitters for exciplex host materials, and it may bring in‐depth understanding on improving exciton utilization and electroluminescence efficiency in interface exciplex OLEDs. [ABSTRACT FROM AUTHOR]

Observation of thermally activated delayed fluorescence (TADF) in conjugated systems redefined the molecular design approach to realize highly efficient organic light emitting diodes (OLEDs) in the early 2010s. Enabling effective reverse intersystem crossing (RISC) by minimizing the difference between singlet and triplet excited state energies (ΔEST) is proven to be a widely applicable and fruitful approach, which results in remarkable external quantum efficiencies (EQE). The efficacy of RISC in these systems is mainly dictated by the first‐order mixing coefficient (λ), which is proportional to spin‐orbit coupling (HSO) and inversely proportional to ΔEST. While minimizing ΔEST has been the focus of the OLED community over the last decade, the effect of HSO in these systems is largely overlooked. Here, molecular systems with increased HSO are designed and synthesized by substituting selected heteroatoms of high‐performance TADF materials with heavy‐atom selenium. A new series of multicolor TADF materials with remarkable EQEs are achieved. One of these materials, SeDF‐B, results in pure blue emission with EQEs approaching 20%. Additionally, flexible graphene‐based electrodes are developed for OLEDs and revealed to have similar performance as standard indium tin oxide (ITO) in most cases. These devices are the first report of TADF based OLEDs that utilize graphene‐based anodes. [ABSTRACT FROM AUTHOR]

There is increasing interest in thermally activated delayed fluorescence (TADF) in materials, and to understand its mechanism in the excited state dynamics. Recent challenges include color purity, efficient deep‐blue emission, fast exciton decay lifetimes, high reverse intersystem crossing rates (kRISC), low‐efficiency roll‐off in organic light‐emitting diodes (OLEDs), and long device lifetimes. Here, a series of compounds having benzonitrile and carbazole rings are examined, that provide a detailed understanding of the excited states, and a guideline for high‐performance TADF. A dense alignment of the excited states with several different characters within a small energy range results in high kRISC of >2 × 106 s−1, while maintaining radiative rate constants (kr) >107 s−1. OLEDs based on the optimum compound exhibit a low‐efficiency roll‐off and a CIEy (y color coordinate of Commission Internationale de l'Éclairage) <0.4. TADF‐assisted fluorescence (TAF) OLED exhibits a maximum external quantum efficiency of 22.4% with CIE coordinates (0.13,0.15). This work also provides insights for device engineering and molecular designs. [ABSTRACT FROM AUTHOR]

Due to the energy gap law, the direct fabrication of efficient organic afterglow materials with long emission wavelengths at ambient conditions remains challenging. Here, luminescent dopants with moderate kRISC values of 100–101 s−1 are designed to harvest triplet energies, simultaneously improving afterglow efficiency and maintaining emission lifetimes >0.1 s. Organic matrices with large dipole moments are selected to populate the triplet excited states of the luminescent dopants and suppress their nonradiative decay and quenching. The dopant‐matrix systems exhibit TADF‐type organic afterglow with quantum efficiency of 20% to 60% and emission wavelengths exceeding 600 nm. Because of their singlet excited state nature, the TADF‐type afterglow emitters can efficiently transfer excited energy to rhodamine B or cyanine 5.5 fluorescence dyes for the construction of red and near‐infrared afterglow materials which display promising bioimaging applications. [ABSTRACT FROM AUTHOR]

Simultaneously obtaining high efficiency and deep blue emission in organic light emitting diodes (OLEDs) remains a challenge. To overcome the demands associated with deep blue thermally activated delayed fluorescence (TADF) emitters, two deep blue TADF materials namely, DBA–BFICz and DBA–BTICz, are designed and synthesized by incorporating oxygen‐bridged boron (DBA) acceptor with heteroatoms, oxygen and sulphur‐based donors, BFICz and BTICz, respectively. Both TADF materials show deep blue photoluminescence emissions below 450 nm by enhancing the optical band gap over 2.8 eV through deeper highest occupied molecular orbital (HOMO) level of heteroatom based donor moieties. At the same time, the photoluminescence quantum yields (PLQYs) of both TADF materials remain over 94%. The TADF device with DBA–BFICz as an emitter exhibits a good external quantum efficiency (EQE) of 33.2%. Since both new TADF materials show deep blue emissions and high efficiencies, hyperfluorescence (HF) OLED devices are fabricated using ν‐DABNA as a fluorescence dopant. DBA–BFICz as a TADF sensitized host in HF–OLED reveals an outstanding EQE of 38.8% along with narrow full width at half maximum of 19 nm in the bottom emission pure blue OLEDs. This study provides an approach to develop deep blue TADF emitters for highly efficient OLEDs. [ABSTRACT FROM AUTHOR]

To reduce cost and improve environmental sustainability, there continues to be an important need for the development of efficient organic light‐emitting diodes (OLEDs) that do not rely on heavy metal‐containing compounds. In particular, the efficiency of fluorescent near‐infrared (NIR) OLEDs continues to lag well‐behind that of their platinum‐containing counterparts. Low efficiencies in this spectral range mainly arise from the low quantum yields of fluorescent NIR emitters due to the energy gap law and inefficient harvesting of triplet excitons. In this paper, a thermally activated delayed fluorescent (TADF) material is used as the assistant dopant to demonstrate pure NIR‐emitting fluorescent OLEDs with an external quantum efficiency of up to 3.8%, with an electroluminescence maximum at 840 nm and a spectral full‐width at half‐maximum of < 40 nm. The efficiency is more than three times higher than that of the best previously reported fluorescent OLEDs in this spectral range and approaches that achievable with the best platinum‐containing phosphorescent emitters. [ABSTRACT FROM AUTHOR]

Hyperfluorescence is emerging as a powerful strategy to develop optoelectronic devices with high‐color purity and enhanced stability. It requires appropriate integration of a sensitizer displaying efficient thermally activated delayed fluorescence (TADF) and an emitter displaying strong, narrow‐band fluorescence. Here, through a joint computational and experimental approach, an unprecedented, end‐to‐end systems level description of the electronic and optical processes that take place in a hyperfluorescent emissive layer composed of a TADF sensitizer, 2,5‐bis(2,6‐di(9H‐carbazol‐9‐yl)phenyl)‐1,3,4‐oxadiazole (4CzDPO), and a fluorescent emitter, 2,5,8,11‐tetra‐tert‐butylperylene (TBPe) is provided. The photophysical properties measurement of the emissive layer is combined with the computational determination of the electronic properties, film morphology, and excitation transfer phenomena. The Förster resonance energy transfer rates from 4CzDPO to TBPe are on the order of 1011 s−1, considerably higher than the radiative and nonradiative recombination rates for 4CzDPO. These features ensure nearly complete energy transfer to TBPe, leading to a five‐fold increase in the photoluminescence quantum yields in the 4CzDPO:TBPe system in comparison to neat films of 4CzDPO. This approach highlights the factors that can provide efficient energy transfer from TADF molecules to fluorescent emitters, suppress energy transfer among TADF molecules, and avoid the need for a host material within the emissive layer. [ABSTRACT FROM AUTHOR]

Polymerization could be a feasible method to overcome the rigid structure induced self‐quenching effect in conventional thermally activated delayed fluorescence (TADF) emitters. Despite steady progress in TADF polymer research, developing an efficient red TADF polymer still remains a great challenge because of the large non‐radiative internal conversion rate governed by the energy gap law. Herein, a novel strategy for constructing a red TADF conjugated polymer is presented by means of embedding quinoxaline‐6,7‐dicarbonitrile (QC) as an acceptor into a polycarbazole (PCz) backbone and attaching donor 9,10‐dihydroacridine (A) as a pendant. The obtained polymers PCzAQCx with the appropriate molar content of the AQC unit (x ≥ 0.5) exhibit efficient TADF features with a dominant emissive peak at 627−661 nm and a photoluminescence quantum yield of up to 76% in neat film. The non‐doped electroluminescent devices with the polymers produce red emissions with a maximum external quantum efficiency (EQE) of up to 12.5% and the emission peak at 620 nm, which represents state‐of‐the‐art performance for solution‐processed devices based on red TADF polymers. Furthermore, combined with a blue TADF emitter, the bright white devices with tunable spectra cover the whole visible–near infrared range from 400 to 900 nm and a record‐high EQE of up to 22.4% is achievable. [ABSTRACT FROM AUTHOR]

Recent studies have demonstrated that in thermally activated delayed fluorescence (TADF) materials, efficient reverse intersystem crossing occurs from nonradiative triplet exited states to radiative singlet excited states due to a small singlet–triplet energy gap. This reverse intersystem crossing significantly influences exciton annihilation processes and external quantum efficiency roll‐off in TADF based organic light‐emitting diodes (OLEDs). In this work, a comprehensive exciton quenching model is developed for a TADF system to determine singlet–singlet, singlet–triplet, and triplet–triplet annihilation rate constants. A well‐known TADF molecule, 3‐(9,9‐dimethylacridin‐10(9H)‐yl)‐9H‐xanthen‐9‐one (ACRXTN), is studied under intensity‐dependent optical and electrical pulse excitation. The model shows singlet–singlet annihilation dominates under optically excited decays, whereas singlet–triplet annihilation and triplet–triplet annihilation have strong contribution in electroluminescence decays under electrical pulse excitation. Furthermore, the efficiency roll‐off characteristics of ACRXTN OLEDs at steady state is investigated through simulation. Finally, singlet and triplet diffusion length are calculated from annihilation rate constants. [ABSTRACT FROM AUTHOR]

New emitters that can harvest both singlet and triplet excited states to give 100% internal conversion of charge into light, are required to replace Ir based phosphors in organic light emitting diodes (OLEDs). Molecules that have a charge transfer (CT) excited state can potentially achieve this through the mechanism of thermally activated delayed fluorescence (TADF). Here, it is shown that a D-A charge transfer molecule in the solid state, can emit not only via an intramolecular charge transfer (ICT) excited state, but also from exciplex states, formed between the molecule and the host material. OLEDs based on a previously studied D-A-D molecule in a host TAPC achieves >14% external electroluminescence yield and shows nearly 100% efficient triplet harvesting. In these devices, it is unambiguously established that the triplet states are harvested via TADF, but more interestingly, these results are found to be independent of whether the emitter is the ICT state or the D-A-D/host exciplex. [ABSTRACT FROM AUTHOR]

Understanding the delayed fluorescence mechanism behind the creation of emissive singlets from the non-emissive triplets in exciplexes is vital for the fabrication of highly efficient blue fluorescent emitters, and subsequent white light applications. In this article we report the spectroscopic investigation of the exciplex formed between 4,4′,4′′-tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA) and 2-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) in a 50:50 blended film. The mechanism behind extra singlet production in the blend is of E-type nature, that is, "thermally activated" delayed fluorescence. The exciplex singlet-triplet energy splitting is estimated to be around 5 meV, smaller than previously estimated at ≈ 50 meV. The absence of a well defined separation between prompt emission and emission components with very long lifetimes, >100 ns, is indicative of such a small exchange energy, and arises through multiple cycling between the resonant singlet and triplet manifolds before eventually being emitted from a singlet state. An observed redshift of the exciplex emission with time and increasing temperature is attributed to different exciplex species being formed between the m-MTDATA and PBD molecules. [ABSTRACT FROM AUTHOR]