Non-doped Organic Light-emitting Diodes Research Articles

Synthesis, photoluminescence and electroluminescence properties of a new blue emitter with aggregation-induced emission and thermally activated delayed fluorescence characteristics

The emitters with aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) characteristics are in high demand in organic light-emitting diodes (OLEDs) owing to their strong fluorescence and high exciton utilization under electrical excitation. Herein, a blue emitter, 10-(3-((3,5-di(9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (m-CZ-DPS-DMAC), was synthesized by incorporating carbazole as skeleton, acridine as electron donor, and diphenyl sulfone as electron acceptor. m-CZ-DPS-DMAC emits weak fluorescence in good solvent, while it is obviously enhanced in the aggregate state, which is typical of AIE molecules. Meanwhile, the energy levels of the singlet and triplet states (ΔEST) of the molecule is relatively small, and it also exhibits obvious temperature dependence and oxygen sensitivity, which directly proves its TADF properties. In view of the above properties, a series of non-doped and doped OLEDs were prepared using m-CZ-DPS-DMAC as light-emitting layers. Among them, non-doped OLED (device A) displays blue emission (488 nm) with the turn-on voltage (Von), the maximum luminance (Lmax), the maximum current efficiency (CEmax), the maximum power efficiency (PEmax) and the maximum external quantum efficiency (EQEmax) of 2.6 V, 3460 cd m−2, 26.09 cd A–1, 29.26 lm W−1 and 10.05%, respectively. Doped OLED (device C) constructed based on m-CZ-DPS-DMAC doped 30% in DPEPO shows the satisfactory performance with the maximum emission peak of 486 nm, the Von of 2.8 V, the Lmax of 4571 cd m−2, the CEmax of 21.37 cd A–1, the PEmax of 22.37 lm W−1, and the EQEmax of 9.44%, respectively. The outstanding performance of m-CZ-DPS-DMAC proves that it is a potential material for designing blue OLEDs with AIE-TADF properties.

Simple excited-state modification toward a deep-blue hybridized local and charge-transfer (HLCT) fluorophore and non-doped organic light-emitting diodes

A deep-blue HLCT emitter DCZ2F showing the state-of-the-art EQE of 5.62% and CIE coordinates of (0.163, 0.035) was achieved in non-doped OLEDs.

Structurally Regulated Carbazole-Pyridine Derivatives Based on Space-Crowded Theory for Efficient Narrowband Ultraviolet Nondoped Organic Light-Emitting Diodes from the High-Lying Reverse Intersystem Crossing Process.

Achieving ultraviolet and narrowband emission simultaneously in nondoped organic light-emitting diodes (OLEDs) remains a tremendous challenge. Here, a "space-crowded donor-acceptor-donor" molecular design strategy is proposed for developing ultraviolet pure organic fluorophores by the nearby substituted positions at the phenyl linker between carbazole and pyridine units. Benefitting from the large steric hindrance effect, multiple intramolecular interactions, and low-frequency vibronic coupling dominated excited state property, all the emitters exhibit excellent fluorescence efficiencies at the solid state as well as the narrow full width at half maximums (FWHMs). Moreover, the effect of different substitution positions of pyridine on the structure-property relationship is also revealed. Consequently, the nondoped OLEDs exhibit an electroluminescence emission peak of 397 nm with FWHMs of 17 and 22 nm. Due to the high-lying reverse intersystem crossing process, external quantum and exciton utilization efficiencies of 3.6 and 54.55%, respectively, have been achieved based on the emitter with para-linkage. These findings may pave an avenue for the development of high-performance narrowband ultraviolet materials and OLEDs.

Novel Ir(III) Complexes with NHC-Based Ancillary Ligands for Efficient Nondoped OLEDs

Nondoped organic light-emitting diodes (OLEDs) are of paramount importance for display and lighting applications owing to their advantages of facile fabrication and outstanding stability. However, nondoped OLEDs achieving extraordinary electroluminescence (EL) performance and low turn-on voltage (Von) remain sparse. Here, three Ir(III) complexes featuring N-heterocyclic carbene (NHC) auxiliary ligands functionalized with electron-deficient aromatic sulfonyl or phosphine oxide groups are reported as promising emitters for nondoped OLEDs. All Ir(III) complexes exhibit green emission with relatively high neat film efficiency. Although the photoluminescence spectra of three complexes reveal similarities, there are distinct differences in the nondoped EL performance. The nondoped device N3 based on tBu-Ir-ISO displays the most eminent EL performances and presents a low Von of 2.1 V, a power efficiency of 30.7 lm W-1, and a maximum current efficiency of 27.0 cd A-1, which can be attributed to steric hindrance and balanced carrier-transporting ability induced by electron-deficient substituents. Moreover, doped devices D1-D3 also realize excellent EL performance. It is believed that the strategy reported herein is a simple and efficient way of constructing excellent Ir(III) complexes for nondoped phosphorescent OLEDs.

A novel asymmetric fused chromophore for non-doped organic light-emitting diode with small efficiency roll-off

We have designed and synthesized a novel asymmetric amine‒carbonyl fused emitter, BACQ, having a hybridized local and charge-transfer emissive state. From the single-crystal analysis, the BACQ exhibited non-planar configuration with multiple C–H‧‧‧O and C–H‧‧‧π interactions, which could rigidify the molecule and provide good thermal stability. Non-doped device based on BACQ present sky-blue emission with maximum luminance of 11180 cd m−2, and external quantum efficiency of 2.48% with small roll-off.

Propeller-shape isomers with turn-on through-space charge transfer for solution-processed non-doped organic light-emitting diodes

Purely organic thermally activated delayed fluorescence (TADF) emitters have developed rapidly in the last decade. Recently, the TADF materials with through-space charge transfer (TSCT) attract great attention due to the ease of tuning the optoelectronic properties for organic light-emitting diodes (OLEDs). Herein, we report the design and synthesis of three propeller-shape emitters via isomer engineering, which are named as TRZ-o-SDMAC, TRZ-m-SDMAC and TRZ-p-SDMAC respectively. Surprisingly, TRZ-o-SDMAC and TRZ-m-SDMAC simultaneously exhibit TADF and aggregation-induced emission. In contrast, the para-linked emitter TRZ-p-SDMAC exhibited no delayed fluorescence with only the nanosecond-scale (12 ns) transient decay. The solution-processed non-doped TADF-OLEDs based on the meta-linked compound TRZ-m-SDMAC render a maximum external quantum efficiency of 20.3 %, which is substantially high than those of the ortho- and para-linked analogues. This work manifests the unique feature of turn-on TSCT and thus TADF via isomeric strategy.

Asymmetric anthracene host decorated with benzothienocarbazole for high efficiency blue organic light-emitting diodes

In this work, we describe an asymmetric anthracene-based 5-(4-(10-phenylanthracen-9-yl)phenyl)-5H-benzo [4,5]thieno[3,2-c]carbazole (ATDBT) for high efficiency doped and non-doped organic light-emitting diodes (OLEDs). Anthracene triggered triplet-triplet annihilation and structurally induced bipolar characteristics of the ATDBT significantly accelerated the triplet to singlet up-conversion process. The fabricated devices eventually achieved high efficiency with a small efficiency roll-off at high luminance. The doped device with ATDBT as the host has a maximum external quantum efficiency (EQE) of more than 8%, pure-blue emission, and maximum luminance of more than 40,000 cd/m2. The non-doped ATDBT device also showed a high EQE of 4.9%. As a result, ATDBT-type asymmetric molecular structures are vital for the development of high-efficiency doped and non-doped blue OLEDs.

Highly Efficient Near-Infrared Thermally Activated Delayed Fluorescent Emitters in Non-Doped Electroluminescent Devices.

Constructing organic near-infrared (NIR) luminescent materials to confront the formidable barrier of "energy gap law" remains challenging. Herein, two NIR thermally activated delayed fluorescence (TADF) molecules named T-β-IQD and TIQD were developed by connecting N,N-diphenylnaphthalen-2-amine and triphenylamine with a novel electron withdrawing unit 6-(4-(tert-butyl)phenyl)-6H-indolo[2,3-b]quinoxaline-2,3-dicarbonitrile. It is confirmed NIR-TADF emitters concurrent with aggregation-induced emission effect, J-aggregate with intra- and intermolecular CN⋅⋅⋅H-C and C-H⋅⋅⋅π interactions, and large center-to-center distance in solid states can boost the emissive efficiencies both in thin films and non-doped organic light-emitting diodes (OLEDs). Consequently, the T-β-IQD-based non-doped NIR-OLED achieved the maximum external quantum efficiency (EQEmax ) of 9.44 % with emission peak at 711 nm, which is one of the highest efficiencies reported to date for non-doped NIR-OLEDs.

AIE-active deep red/near-infrared electroluminescent emitters with fine regulation of excited state

Deep red/Near-infrared (DR/NIR) emitters with high photoluminescent quantum yield (PLQY) in the aggregate state are still challenge for achieving highly efficient non-doped organic light emitting diodes (OLEDs). Herein, weutilizethemethodologyofregulatingexcited-state property toadjusttheproportionof radiative and nonradiativetransitionataggregation, three aggregation-inducedemission(AIE)activeemitters (TNZNp, TNZTPAN and TNZTPEDA) withdiversesumof benzene ring rotors areprepared. TNZTPEDA with more rotors suffers from fast nonradiative transition which leads to the low fluorescent efficiency in the unimolecular and aggregate levels. Oppositely, decreasing rotors to enhance obviously solid-state PLQY of TNZNp, but the radiative transition rate is still lower than the nonradiative rate. After introducing rotational electronic donor to improving locally excited-state proportion and suppress intermolecular interaction, simultaneously, TNZTPAN has the best PLQY of 56 % due to the faster radiative transition than nonradiative transition at aggregation. Three emitters are used to manufacture non-doped OLEDs, TNZTPAN shows the maximal electroluminescence peaking at 692 nm with Commission International de L’Eclairage coordinates of (0.68, 0.30), the external quantum efficiency of 3.54 % represents a higher level among the reported non-doped DR/NIR-type OLEDs. The results of this work demonstrate the feasibility of regulating excited-state property to prepare highly bright emitters, which plays important role in designing organic DR/NIR emitters with high fluorescent efficiency at aggregation in the future.

High-performance non-doped near ultraviolet OLEDs with the EQE ∼ 6 % and CIEy ∼ 0.03 from high-lying reverse intersystem crossing

Developing high-performance organic fluorophors with short-wavelength emission is both significant and challenging subject in the application of organic light-emitting diodes (OLEDs). In this work, we proposes a “coupling hardness with distortion” design concept to construct an efficient near ultraviolet (NUV) fluorophor mP2MPC with 1,4-dimethyl-2,5-diphenylbenzene as a rotatable bridge via the meta-positons linking rigid and bulky phenanthroimidazole and carbazole groups. As a result, the non-doped OLED with it as emitter not only exhibits an emission peak at 395 nm with the Commission Internationale de l’éclairage coordinates of (0.163, 0.028), but also realizes a high forward-viewing external quantum efficiency of 6.09 % due to the high-lying reverse intersystem crossing. The value still keeps as high as 5.56 % at 1000 cd m−2, showing a small efficiency roll-off. This is one of the highest efficiencies among the non-doped devices with the electroluminescence peak below 400 nm. Moreover, the green and red phosphorescent OLEDs with mP2MPC as host can also show excellent electroluminescence performance with the forward-viewing external quantum efficiencies over 21.5 %, illustrating that mP2MPC should be a prospective material for OLEDs.

Construction of biomass-based AIEgens with dehydroabietic acid triarylamine and tetraphenylethene Moieties for non-doped OLEDs

Dehydroabietic acid triarylamine (DTPA) synthesized from renewable natural rosin is an excellent hole-transporting material but is affected by aggregation-caused quenching in the solid state. Herein, two new biomass-based AIE luminogens (AIEgens), namely DTPA-TPE and 2DTPA-TPE, were synthesized through the attachment of DTPA to tetraphenylethene (TPE). Their aggregates in poor solvents and thin films have a strong emission with a fluorescence quantum yield of up to 67.4%. DTPA AIEgens exhibit superior thermal and morphological stability, showing high thermal degradation (T5d up to 412.0 °C) and glass transition (Tg up to 150.8 °C) temperatures. Electroluminescence (EL) devices with compounds used either as emitting layers (EMLs) or as EMLs and hole-transporting layers (HTLs) were constructed. The device with 2DTPA-TPE as the EML exhibits the best performance (turn-on voltage of 2.4 V, maximum luminance of 12,414 cd m−2, current efficiency of 6.2 cd A−1, and power efficiency of 6.5 lm W−1). The devices without the HTL (N,N′-di-1-naphthyl-N,N′-diphenylbenzidine (NPB)) exhibit performances comparable with or better than those of the devices with NPB. These results indicate that these compounds are promising luminescent materials for non-doped organic light-emitting diodes.

Voltage-Dependent Emission Varying from Blue to Orange-Red from a Nondoped Organic Light-Emitting Diode with a Single Emitter.

Organic light-emitting diodes (OLEDs) with tunable emission colors, especially white OLEDs, have rarely been observed with a single emitter in a single emissive layer. In this paper, we report a new compound featuring a D–A–D structure, 9,9′-(pyrimidine-2,5-diylbis(2,1-phenylene))bis(3,6-di-tert-butyl-9H-carbazole) (PDPC). A nondoped OLED using this compound as a single emitter exhibits unique voltage-dependent dual emission. The emission colors range from blue to orange–red with an increase in voltage, during which white electroluminescence with a Commission Internationale De L’Eclairage (CIE) coordinate of (0.35, 0.29) and a color render index (CRI) value of 93 was observed. A comparative study revealed that the dual emission simultaneously originates from the monomers and excimers of the emitter. This study provides insight into understanding the multimer-excited mechanism and developing novel color-tunable OLEDs.

Substituent effects on the luminescence and charge transport properties of novel bis-lactam-based molecules

A series of bis-lactam-based molecules were synthesized and applied in organic field-effect transistors (OFETs) and organic light-emitting diodes (OLEDs). Among the derivatives, 1,5-dioctyl-3,7-bis(9-phenyl-9H-carbazol-2-yl)-1,5-naphthyridine-2,6-dione (NTD-pCz) exhibited the highest maximum hole mobility of 0.11 cm2 V−1 s−1, with on–off current ratios (Ion/Ioff) of >105 in OFETs, which was attributed to NTD-pCz exhibiting the lowest reorganization energy, extended π-conjugation with a relatively small dihedral angle between the NTD core and the side group, and the strongest intermolecular interaction in the thin-film state. In addition, NTD-pCz exhibited the highest maximum external quantum efficiency of 3.56%, with a current efficiency of 9.95 cd A−1, when incorporated into nondoped OLEDs, which is ascribed to its excellent solid-state photoluminescence quantum yield of 83%. These results reveal the potential of NTD-based molecules for use in efficient next-generation multifunctional optoelectronic devices.

Chrysene-Cored Fluorescent Dendrimers as Nondoped Deep-Blue Emitters for Solution-Processable Electroluminescent Devices

AbstractNew solution-processable chrysene-basked deep-blue fluorescent dendrimers consisting of chrysene as a core end-capped by carbazole dendrons were designed, synthesized, and characterized. These dendrimers exhibit strong deep-blue emissions in solution or thin-film states with decent hole mobility and high thermal and electrochemical stability. They can be effectively used as nondoped emitters in organic light-emitting diodes (OLEDs). The nondoped OLEDs, which have a simple structure, showed good electroluminescence (EL) performance (luminance: 2334–2400 cd m–2; external quantum efficiency: 1.88-2.51%; turn-on voltage: 3.8–4.0 V) and deep-blue EL spectra (CIE y: 0.065–0.075) with a narrow full width at half maximum of 61–65 nm.

Multifunctional luminophores with dual emitting cores: TADF emitters with AIE properties for efficient solution- and evaporation-processed doped and non-doped OLEDs

• Three dual emitting core emitters are developed by coupling single core emitters. • The emitters possess high absorption coefficients and PLQY values in neat films. • Photophysical investigations demonstrate both AIE and TADF properties. • Highly efficient OLEDs with ultra-low efficiency roll-offs are achieved. Three novel emitters with dual emitting cores, namely 3,3′,5,5′-tetra(triphenylamine-4-yl)-[1,1′-biphenyl]-2,2′,6,6′-tetracarbonitrile (DDTPAIPN), 3,3′-di(triphenylamine-4-yl)-5,5′-di(3,5-(9,9′-dicarbazolyl)phenyl)-[1,1′-biphenyl]-2,2′,6,6′-tetracarbonitrile (DTPAmCPIPN), and 3,3′,5,5′-tetra(3,5-(9,9′-dicarbazolyl)phenyl)-[1,1′-biphenyl]-2,2′,6,6′-tetracarbonitrile (DDmCPIPN) are designed and synthesized by coupling their respective single core emitters. Compared with their single core counterparts, the three novel compounds exhibit significantly improved thermal stability, absorption coefficients, and photoluminescence efficiencies in neat films. Photophysical measurements show that these three emitters exhibit both aggregation-induced emission and thermally activated delayed fluorescence properties, which is beneficial for non-doped organic light-emitting diodes (OLEDs). The new compounds show relatively balanced charge carrier transport abilities as revealed by unipolar devices. The solution- and evaporation-processed doped and non-doped OLEDs made from these three luminophores achieve excellent electroluminescence (EL) performances. The peak current, power, and external quantum efficiencies of a DTPAmCPIPN-based solution-processed doped device reach 59.2 cd A −1 , 61.3 lm W −1 , and 17.2%, respectively, with an emission peak at 548 nm. The evaporation-processed doped device based on DTPAmCPIPN exhibits maximum EL efficiencies of up to 63.7 cd A −1 , 61.1 lm W −1 , and 19.2%. Moreover, the non-doped OLEDs also possess superior EL efficiencies with extremely small efficiency roll-offs. The molecular design strategy in this work is demonstrated to be effective in developing new emitters for efficient and stable OLEDs.

High Efficiency and Low Roll-Off Non-Doped Near Ultraviolet Organic Light-Emitting Diodes with the Eqe Exceeding 6% and Ciey Smaller than 0.03

High Efficiency and Low Roll-Off Non-Doped Near Ultraviolet Organic Light-Emitting Diodes with the Eqe Exceeding 6% and Ciey Smaller than 0.03

Novel deep-blue hot exciton material for high-efficiency nondoped organic light-emitting diodes

A deep-blue OLED based on a novel hot exciton material exhibited an EQEmaxof 7.51% with CIE coordinates of (0.154, 0.075).

Highly Efficient Non-Doped Organic Light-Emitting Diodes Based on Long-Range Coupling and Efficient Energy Transfer

Highly Efficient Non-Doped Organic Light-Emitting Diodes Based on Long-Range Coupling and Efficient Energy Transfer

Recent progress in thermally activated delayed fluorescence emitters for nondoped organic light-emitting diodes.

Nondoped organic light-emitting diodes (OLEDs) have drawn immense attention due to their merits of process simplicity, reduced fabrication cost, etc. To realize high-performance nondoped OLEDs, all electrogenerated excitons should be fully utilized. The thermally activated delayed fluorescence (TADF) mechanism can theoretically realize 100% internal quantum efficiency (IQE) through an effective upconversion process from nonradiative triplet excitons to radiative singlet ones. Nevertheless, exciton quenching, especially related to triplet excitons, is generally very serious in TADF-based nondoped OLEDs, significantly hindering the pace of development. Enormous efforts have been devoted to alleviating the annoying exciton quenching process, and a number of TADF materials for highly efficient nondoped devices have been reported. In this review, we mainly discuss the mechanism, exciton leaking channels, and reported molecular design strategies of TADF emitters for nondoped devices. We further classify their molecular structures depending on the functional A groups and offer an outlook on their future prospects. It is anticipated that this review can entice researchers to recognize the importance of TADF-based nondoped OLEDs and provide a possible guide for their future development.

Thermally activated delayed fluorescence polymers for high-efficiency solution-processed non-doped OLEDs: Convenient synthesis by binding TADF units and host units to the pre-synthesized polycarbazole-based backbone via click reaction

At present, TADF polymers with multiple functional components are still limited due to their complicated synthetic process and low synthetic repeatability. To overcome these problems, a post-functionalization strategy was exploited to synthesize the self-host TADF conjugated polymers. A series of yellow-green TADF polymers, PCz-mCP-PxzTrz-X, X = 10, 20, 30 or 40, were successfully synthesized by grafting stoichiometric TADF units (phenoxazinetriphenyltriazine, PxzTrz) and host units (1,3-bis(N-carbazolyl)benzene, mCP) onto the side chain of a pre-synthesized polycarbazole-based backbone via a click reaction. The carbazole-containing main chain and mCP-containing side chains served as an “encapsulant”, which can transport carriers and prevent the TADF units from concentration quenching. Using PCz-mCP-PxzTrz-30 as the light-emitting layer, the solution-processed non-doped organic light-emitting diode displayed an external quantum efficiency of 15.2% at 100 cd m−2. Our synthetic method could offer a useful strategy to construct self-host TADF polymers containing multiple functional components.