Efficient Blue Organic Light-emitting Diodes Research Articles

Highly efficient pure-blue organic light-emitting diodes based on rationally designed heterocyclic phenophosphazinine-containing emitters

Multi-resonance thermally activated delayed fluorophores have been actively studied for high-resolution photonic applications due to their exceptional color purity. However, these compounds encounter challenges associated with the inefficient spin-flip process, compromising device performance. Herein, we report two pure-blue emitters based on an organoboron multi-resonance core, incorporating a conformationally flexible donor, 10-phenyl-5H-phenophosphazinine 10-oxide (or sulfide). This design concept selectively modifies the orbital type of high-lying excited states to a charge transfer configuration while simultaneously providing the necessary conformational freedom to enhance the density of excited states without sacrificing color purity. We show that the different embedded phosphorus motifs (phosphine oxide/sulfide) of the donor can finely tune the electronic structure and conformational freedom, resulting in an accelerated spin-flip process through intense spin-vibronic coupling, achieving over a 20-fold increase in the reverse intersystem crossing rate compared to the parent multi-resonance emitter. Utilizing these emitters, we achieve high-performance pure-blue organic light-emitting diodes, showcasing a top-tier external quantum efficiency of 37.6% with reduced efficiency roll-offs. This proposed strategy not only challenges the conventional notion that flexible electron-donors are undesirable for constructing narrowband emitters but also offer a pathway for designing efficient narrow-spectrum blue organic light-emitting diodes.

Rational Design of Coumarin-Based Hybridized Local and Charge-Transfer Blue Emitters for Solution-Processed Organic Light-Emitting Diodes.

Hybridized local and charge-transfer (HLCT) with the utilization of both singlet and triplet excitons through the "hot excitons" channel have great application potential in highly efficient blue organic light-emitting diodes (OLEDs). The proportion of charge-transfer (CT) and locally excited (LE) components in the relevant singlet and triplet states makes a big difference for the high-lying reverse intersystem crossing process. Herein, three novel donor (D)-acceptor (A) type HLCT materials, 7-([1,1'-biphenyl]-4-yl(9,9-dimethyl-9H-fluoren-2-yl)amino)-3-phenyl-1H-isochromen-1-one (pPh-7P), 7-([1,1'-biphenyl]-4-yl(9,9-dimethyl-9H-fluoren-2-yl)amino)-3-methyl-1H-isochromen-1-one (pPh-7M), and 6-([1,1'-biphenyl]-4-yl(9,9-dimethyl-9H-fluoren-2-yl)amino)-3-methyl-1H-isochromen-1-one (pPh-6M), were rationally designed and synthesized with diphenylamine derivative as donor and oxygen heterocyclic coumarin moiety as acceptors. The proportions of CT and LE components were fine controlled by changing the connection site of diphenylamine derivative at C6/C7-position and the substituent at C3-position of coumarin moiety. The HLCT characteristics of pPh-7P, pPh-7M, and pPh-6M were systematically demonstrated through photophysical properties and density functional theory calculations. The solution-processed doped OLEDs based on pPh-6M exhibited deep-blue electroluminescence with the maximum emission wavelength of 446 nm, maximum luminance of 8755 cd m-2, maximum current efficiency of 5.83 cd A-1, and maximum external quantum efficiency of 6.54 %. The results reveal that pPh-6M with dominant 1LE and 3CT components has better OLED performance.

Narrow-emitting pyridoimidazole functionalized N-heterocyclic carbene based tetradentate Pt(II) complexes for blue phosphorescent organic light-emitting diodes

Two novel classes of N-heterocyclic carbene (NHC) based tetradentate Pt(II) complexes possessing bulky benzyl groups and pyridoimidazole ligands were designed and synthesized to produce efficient blue phosphorescent organic light-emitting diodes (PHOLEDs). Two blue Pt(II) complexes are reported, namely, platinum (II) [2-(3-(1-(4-(tert-butyl)benzyl)-1H-imidazo [4,5-b]pyridin-3(2H)-yl)phenoxy)-9-(pyridin-2-yl)-9H-carbazole] (Pt(t-BuBnOCzPy)) and platinum (II) [3-(3-(1-benzyl-1H-imidazo [4,5-b]pyridin-3(2H)-yl)phenoxy)-9-(4-(tert-butyl)pyridin-2-yl)-9H-carbazole] (Pt(BnOCz4t-BuPy)), respectively. Notably, pyridoimidazole and bulky benzyl groups efficiently reduced steric hindrance, vibrational peaks, and intermolecular interactions. The photophysical and electrochemical properties of the two Pt(II) complexes were analytically examined experimentally and theoretically. The complexes exhibited blue electroluminescence (EL) emission at 466 nm with a small full-width half maximum (FWHM) of 26 nm and completely suppressed vibrational emission peaks. Blue PHOLEDs, at 5 wt% Pt(t-BuBnOCzPy) and Pt(BnOCz4t-BuPy) doping ratios exhibited maximum external quantum efficiencies (EQEmax) of 19 and 18.6 %, maximum power efficiencies (PEmax) of 25.5 and 24.9 cd m−2, and maximum current efficiencies (CEmax) of 24.3 and 24.3 cd m−2 with Commission International de I'Eclairage (CIEx,y) color coordinates of (0.13, 0.17) and (0.13, 0.17), respectively. These superior results suggest that the designed benzylated NHC pyridoimidazole tetradentate Pt(II) complexes provide a novel means of reducing steric hindrance and intermolecular interactions.

New Phenanthro[9,10-d]oxazole-Based Fluorophores with Hybridized Local and Charge-Transfer Characteristics for Highly Efficient Blue Non-Doped OLEDs with Negligible Efficiency Roll-Off.

It is vital to develop highly efficient non-doped blue organic light-emitting diodes (OLEDs) with high color purity and low-efficiency roll-off for applications in display and lighting. Herein, two blue D-A fluorophores TPA-PO and TPA-DPO are designed and synthesized, in which phenanthro[9,10-d]oxazole (PO) acts as the acceptor and triphenylamine as the donor. TPA-PO and TPA-DPO display good thermal stability and efficient luminescence efficiency in neat film. Results based on photophysical property and theoretical calculation demonstrate that TPA-PO and TPA-DPO possess the hybridized local and charge-transfer (HLCT) feature, which can utilize the triplet exciton to achieve highly efficient electroluminance (EL). The non-doped OLEDs with TPA-PO/TPA-DPO as pure emissive layer show the uniform EL emission peak at 468 nm, corresponding to CIE coordinates of (0.168, 0.187) and (0.167, 0.167), respectively. The TPA-DPO-based non-doped OLEDs provide the maximum external quantum efficiency (EQE) of 7.99 % and high exciton utility efficiency of 48.4 %~72.6 %. Moreover, the TPA-DPO-based device exhibits low-efficiency roll-off, still maintaining the EQE of 6.03 % at the high luminance of 5000 cd m-2. Those findings state clearly that PO is a promising building block of blue fluorophore with a potential HLCT feature to be applied in non-doped OLEDs.

Utilization of hot excitons to fabricate high efficiency blue fluorescence organic light-emitting diodes

Organic light emitting diodes (OLEDs) are a new generation of display and lighting technology. The practical application requires OLEDs to have high efficiency, long life and low cost. By comparison, fluorescent OLEDs have the advantages of long lifetime and low cost, but the problem of low efficiency. Hot excitons and their utilization are effective methods to improve the efficiency of fluorescent OLEDs. This paper introduces our recent research results on blue fluorescent OLEDs by the design of device structures to efficiently utilize hot excitons. We have carried out detailed studies on mechanism and find that the internal conversion between triplet energy levels is the main loss channel of hot excitons. Through the strategic design of device structures, we have achieved a breakthrough in the efficiency of blue fluorescent OLEDs. The maximum external quantum efficiency was as high as 14.8% and kept 14.4% at the luminance of 1000 cd/m2.

Carbazole-based thermally activated delayed fluorescent emitters for efficient pure blue organic light-emitting diodes

Carbazole-based thermally activated delayed fluorescent emitters for efficient pure blue organic light-emitting diodes

Efficient deep-blue luminescence based on dual-channel intra/intermolecular exciplexes

Highly efficient and stable blue organic light-emitting diodes (OLEDs) cannot be easily obtained simultaneously. In particular, the efficiency roll-off as a reference index to evaluate the lifetime of deep-blue OLED at high luminescence is still severe. A novel molecule (CzSiTrz) connected with carbazole and triazine fragments by a nonconjugated silicon atom is designed. An intramolecular charge transfer emission and intermolecular exciplex luminescence in the aggregated state are obtained, resulting in a dual-channel intra/intermolecular exciplex (DCIE) emission with fast and efficient reverse intersystem crossing (RISC). Deep-blue OLED with Commission Internationale de l'Eclairage (CIE) coordinates of (0.157, 0.076) and a record-high external quantum efficiency (EQE) of 20.35% at high luminance (5000 cd m-2) is accomplished. Simple molecular synthesis and device fabrication of this strategy give a unique approach to realizing high-performance deep-blue electroluminescence.

A symmetrical carbazole/benzonitrile-based high triplet energy bipolar host designed by a para-linkage strategy for efficient blue and green phosphorescent organic light-emitting diodes

Based on a para-linkage concept, a symmetrical carbazole-benzonitrile derivative of 4-(9H-carbazol-9-yl)-3,5-diphenyl-2,6-difluoronitrile (CzPhFBN) was designed and prepared as a host for phosphorescent organic light-emitting diodes (PhOLEDs). In the CzPhFBN molecular, a cyano (CN) unit and a carbazole (Cz) unit were attached to the central phenyl ring at the para positions. Simultaneously, the Cz group was surrounded by two phenyl units to increase structure distortion for high triplet energy and CN unit was surrounded by two F units to improve electron transporting behavior. The photophysical properties and carrier transport behavior of CzPhFBN were investigated. The results demonstrated that CzPhFBN offered a high triplet energy of exceeding 3.0 eV and bipolar carrier transporting property. By incorporating CzPhFBN as the host, the blue FIrpic- and green Ir(ppy)3-based PhOLEDs with a common device configuration delivered the maximum efficiencies of 11.1%/19.7 cd A−1/12.1 lm W−1 and/13.1%/43.6 cd A−1/28.6 lm W−1, respectively. These results indicate that this study provides a potential design strategy of high triplet energy bipolar host materials with a simple and elegant structure.

True- to Sky-Blue Emitters Bearing the Thiazolo[5,4-d]thiazole Electron Acceptor for Single and Tandem Organic Light-Emitting Diodes

A series of emitters displaying efficient photoluminescence (PL) and electroluminescence (EL) into the true- to sky-blue regions is herein described and employed as electroluminescent materials for the fabrication of efficient blue organic light-emitting diodes (OLEDs). The compounds possess linear, yet a twisted, donor−π–acceptor−π–donor (D−π–A−π–D) architecture, where D and A are a (substituted) carbazole (Cz) and thiazolo[5,4-d]thiazole (TzTz) moiety as the electron donor and accepting unit, respectively. D = 9H-carbazol-9-yl and 3,6-di-tert-butyl-9H-carbazol-9-yl for compounds TzTz-PCz2 and TzTz-PtbCz2, respectively. In dilute CH2Cl2 solutions, both compounds display intense (PL quantum yield = 60–74%) and short-lived (τ = ca. 0.8–2.0 ns) luminescence arising from an intramolecular charge-transfer (1ICT) excited state and falling into the sky-blue to the greenish-blue region (λem = 448–502 nm). Compared to the previously reported TzTz-TPA parental emitters that features two triphenylamine (TPA) donor groups, the highly twisted nature and less extended π-conjugation of the donor–acceptor TzTz-PCz2 and TzTz-PtbCz2 counterparts herein presented enable to widen the HOMO–LUMO energy gaps and warrant blue shifting of the emission. Finally, OLED devices fabricated by employing the herein proposed emitters are presented as well with both single- and tandem-device architectures. The so-prepared OLEDs display true-blue to sky-blue EL peaking at λEL,max = 460 nm (TzTz-PCz2) and 467–470 nm (TzTz-PtbCz2) and achieve maximum external quantum efficiency and luminance of 10.2% and 44,851 cd m–2 (for TzTz-PtbCz2) in the case of a tandem device, respectively, with a mitigated roll-off efficiency. These results demonstrate that the tandem device employing these TzTz donor–acceptor blue emitters raises the feasibility of fluorescent compounds to compete with other potential emitters in practical applications.

Asymmetric Decoration of a meta-Linked Bitriazine-Based Host for Highly Efficient and Stable Blue Phosphorescent Organic Light-Emitting Diodes

High triplet energy hosts for blue phosphorescent organic light-emitting diodes were developed by decorating a meta-linked bitriazine core with carbazole and tetraphenylsilyl functional groups. A symmetric host with two carbazole units as the two triazine units of the core and an asymmetric host with one carbazole unit and one tetraphenylsilyl unit as the two triazine units were prepared. The triplet energy of these two hosts was 2.97 eV, suitable for triplet exciton harvesting of blue phosphors. Comparing the two host designs, the asymmetric decoration of the two triazine units with carbazole and tetraphenylsilyl units was superior to the symmetric decoration of the two triazine units with two carbazoles in terms of high external quantum efficiency (EQE) and long-term device stability. A high EQE of 19.7% and a long device lifetime of 2093.6 h at 100 cd m-2 were achieved using the asymmetrical host.

Anthracene-based fluorescent emitters for efficient blue organic light-emitting diodes

Anthracene-based fluorescent emitters for efficient blue organic light-emitting diodes

Peripheral engineering of platinum(ii) dicarbene pincer complexes for efficient blue hyperphosphorescent organic light-emitting diodes

Pt(ii) dicarbene pincer complexes served as a promising dopant sensitizer to an MR-TADF terminal emitter.

Rational molecular design of phenanthroimidazole–azine derivatives for efficient non-doped blue organic light-emitting diodes with low-efficiency roll-off

A family of phenanthroimidazole–azine derivatives (TrBPI, 4PyBPI, 2PyBPI, and 2PyTPI) bearing various kinds of azine acceptors (2,4-diphenyltriazine, 4,6-diphenylpyrimidine, and 2,6-diphenylpyrimidine) are successfully developed.

A sensitization strategy for highly efficient blue fluorescent organic light-emitting diodes

Highly efficient blue fluorescent materials have recently attracted great interest for organic light-emitting diode (OLED) application. Here, two new pyrene based organic molecules consisting of a highly rigid skeleton, namely SPy and DPy, are developed. These two blue light emitters exhibit excellent thermal stability. The experiment reveals that the full-width at half-maximum (FWHM) of the emission spectrum can be tuned by introducing different amounts of 9,9-diphenyl-N-phenyl-9H-fluoren-2-amine on pyrene units. The FWHM of the emission spectrum is only 37 nm in diluted toluene solution for DPy. Furthermore, highly efficient blue OLEDs are obtained by thermally activated delayed fluorescence (TADF) sensitization strategy. The blue fluorescent OLEDs utilizing DPy as emitters achieve a maximum external quantum efficiency (EQE) of 10.4% with the electroluminescence (EL) peak/FWHM of 480 nm/49 nm. Particularly, the EQE of DPy-based device is boosted from 2.6% in non-doped device to 10.4% in DMAc-DPS TADF sensitized fluorescence (TSF) device, which is a 400% enhancement. Therefore, this work demonstrates that the TSF strategy is promising for highly efficient fluorescent OLEDs application in wide-color-gamut display field.Graphical

Highly efficient stable blue organic light-emitting diodes based on a novel anthracene[2,3-b]benzofuran framework

Organic blue fluorescent emitters’ emissions are highly desirable for high-resolution organic light-emitting diode (OLED) display technology. In this work, a design strategy to realize narrowband emission of organic blue fluorescent emitters by connecting carbazole and amine, named ABFCz and ABFAn, respectively, with anthracene[2,3-b]benzofuran as rigid groups is reported for the first time. The developed materials can achieve high efficiency and long life. Studies of the doped devices show that ABFCz-based blue-emitting OLEDs achieve a maximum current efficiency of 6.96 cd A−1 with a narrow full width at half maximum (FWHM) of 44 nm, as well as a Commission Internationale de l’Eclairage (CIE) coordinate of (0.15, 0.14). A maximum external quantum efficiency (EQEmax) of 7.7% is also achieved. Meanwhile, the ABFAn emitter-based devices exhibit blue emission with a maximum current efficiency of up to 9.20 cd A−1 (EQEmax of 11.4%) with a FWHM of 44 nm, as well as a CIE coordinate of (0.14, 0.13). The devices are based on an ABFAn operating lifetime test LT90 (luminance decays to 90% of the initial brightness) of 202 h at an initial luminance of 1000 cd m−2. The results show that anthracene[2,3-b]benzofuran can be modified with a higher glass transition temperature (T g) to achieve high-efficiency and stable-operating-lifetime blue light devices. Our work may provide a new approach for designing high-efficiency blue materials.

Highly Efficient Solution-Processed Deep Blue Organic Light-Emitting Diodes with an External Quantum Efficiency of 17.2% Alleviate the Hole Accumulation with a Modifying Layer Prepared by the Spontaneous Spreading Method

Highly Efficient Solution-Processed Deep Blue Organic Light-Emitting Diodes with an External Quantum Efficiency of 17.2% Alleviate the Hole Accumulation with a Modifying Layer Prepared by the Spontaneous Spreading Method

Highly Efficient Solution-Processed Blue Phosphorescent Organic Light-Emitting Diodes Based on Co-Dopant and Co-Host System.

The low-lying HOMO level of the blue emitter and the interfacial miscibility of organic materials result in inferior hole injection, and long exciton lifetime leads to triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA), so the efficiencies of blue phosphorescent organic light-emitting diodes (PhOLEDs) are still unsatisfactory. Herein, we design co-host and co-dopant structures to improve the efficiency of blue PhOLEDs by means of solution processing. TcTa acts as hole transport ladder due to its high-lying HOMO level, and bipolar mCPPO1 helps to balance carriers’ distribution and weaken TPA. Besides the efficient FIr6, which acts as the dominant blue dopant, FCNIrPic was introduced as the second dopant, whose higher HOMO level accelerates hole injection and high triplet energy facilitates energy transfer. An interesting phenomenon caused by microcavity effect between anode and cathode was observed. With increasing thickness of ETL, peak position of electroluminescence (EL) spectrum red shifts gradually. Once the thickness of ETL exceeded 140 nm, emission peak blue-shifts went back to its original position. Finally, the maximum current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) of blue phosphorescent organic light-emitting diode (PhOLED) went up to 20.47 cd/A, 11.96 lm/W, and 11.62%, respectively.

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.

Quantum Dot-Based White Organic Light-Emitting Diodes Excited by a Blue OLED

In this study, white organic light-emitting diodes (OLEDs) consisting of red quantum dots (RQD) and green quantum dots (GQD) were investigated. These are the most exciting new lighting technologies that have grown rapidly in recent years. The white OLED development processes used consisted of the following methods: (a) fabrication of a blue single-emitting layer OLED, (b) nanoimprinting into QD photoresists, and (c) green and red QD photoresists as color conversion layers (CCL) excited by blue OLEDs. To fabricate the blue OLED, the HATCN/TAPC pair was selected for the hole injection/transport layer on ITO and TPBi for the electron transport layer. For blue-emitting material, we used a novel polycyclic framework of thermally activated delayed fluorescence (TADF) material, ν-DABNA, which does not utilize any heavy metals and has a sharp and narrow (FWHM 28 nm) electroluminescence spectrum. The device structure was ITO/HATCN (20 nm)/TAPC (30 nm)/MADN: ν-DABNA (40 nm)/TPBi (30 nm)/LiF (0.8 nm)/Al (150 nm) with an emitting area of 1 cm × 1 cm. The current density, luminance, and efficiency of blue OLEDs at 8 V are 87.68 mA/cm2, 963.9 cd/m2, and 1.10 cd/A, respectively. Next, the bottom emission side of the blue OLED was attached to nanoimprinted RQD and GQD photoresists, which were excited by the blue OLED in order to generate an orange and a green color, respectively, and combined with blue light to achieve a nearly white light. In this study, two different excitation architectures were tested: BOLED→GQD→RQD and BOLED→RQD→GQD. The EL spectra showed that the BOLED→GQD→RQD architecture had stronger green emissions than BOLED→RQD→GQD because the blue OLED excited the GQD PR first then RQD PR. Due to the energy gap architectures in BOLED-GQD-RQD, the green QD absorbed part of the blue light emitted from the BOLED, and the remaining blue light penetrated the GQD to reach the RQD. These excited spectra were very close to the white light, which resulted in three peaks emitting at 460, 530, and 620 nm. The original blue CIE coordinates were (0.15, 0.07). After the excitation combination, the CIE coordinates were (0.42, 0.33), which was close to the white light position.

Structurally modified [1,2,4]triazolo[1,5‑a]pyridine derivatives as promising materials for highly efficient blue fluorescent organic light-emitting diodes

Here, we designed and synthesized three donor–acceptor (D-A) emitters, 1-phenyl-2-(4-(2-phenyl-[1,2,4]triazolo[1,5-a]pyridin-6-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (m-PPI-TP), 1-phenyl-2-(4-(2-phenyl-[1,2,4]triazolo[1,5-a]pyridin-7-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (p-PPI-TP) and 1-phenyl-2-(4-(2-phenyl-[1,2,4]triazolo[1,5-a]pyridin-8-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (o'-PPI-TP), based on [1,2,4]triazolo[1,5-a]pyridine (TP) as electron-transporting unit and 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole (PPI) as hole-transporting moiety serving as emitting block concurrently. All three compounds were demonstrated to be thermally stable with decomposition temperatures (Td) in the range of 454–476 °C and high photoluminescence quantum yields over 86%. Besides, high exciton utilization efficiency (EUE) via “hot exciton” process was proved according to photophysical experiments and theoretical calculations analysis. What’s more, the structure–property relationships between different substituted sites of the TP building block and PPI unit were also unveiled. Finally, the non-doped blue and doped deep-blue OLEDs were fabricated with high external quantum efficiency (EQE) approaching 8%. The yellow and red phosphorescent OLEDs (PHOLEDs) based on m-PPI-TP as the host material achieved the best EL performance with EQE of 26.55% and 20.35%, respectively.