Predicting Operational Stability for Organic Light-Emitting Diodes with Exciplex Cohosts

Positional Isomeric Cyano-Substituted Bis(2-phenylpyridine)(acetylacetonate)iridium Complexes for Efficient Organic Light-Emitting Diodes with Extended Color Range

Bis(2-phenylpyridine)(acetylacetonate)iridium, Ir(ppy)2(acac), is a benchmark green emitter for phosphorescent organic light-emitting diodes (PhOLEDs). In this work, we reported three positional isomeric cyano-substituted Ir(ppy)2(acac) complexes, i.e., Ir(3-CN), Ir(4-CN), and Ir(10-CN), with the emission in the yellow to red region (544-625 nm). Through theoretical investigation and single-crystal analysis, it was found that the introduction of cyano substitution at various positions of the ppy ligand allows for tuning the electron distribution and coordination bond length of Ir complexes. Therefore, the charge transfer property of Ir complexes is enhanced such that the energy gap of the cyano-substituted Ir(ppy)2(acac) complexes was reduced. In addition, Ir(3-CN), Ir(4-CN), and Ir(10-CN) exhibited high PLQYs of 83, 54, and 75%, respectively, with the phosphorescence lifetime in the range of 0.79-2.08 μs. Notably, the device utilizing Ir(3-CN) as the emitter exhibited a maximum external quantum efficiency (EQE) of 25.4%, current efficiency of 56.9 cd A-1, power efficiency of 68.7 lm W-1, and brightness of 61,340 cd m-2 at 8 V. The EQE of this device remained 24.3 and 19.9% at luminances of 1,000 and 10,000 cd m-2, corresponding to the efficiency roll-off of 4.3 and 21.7%, respectively. Comparing to the Ir complexes using the ligand with an extended conjugated structure, our results demonstrated a simple molecular design strategy for phosphorescence emitters with reduced molecular weight for efficient PhOLEDs in the yellow to red color region.

Discrete Mononuclear Platinum(II) Complexes Realize High-Performance Red Phosphorescent OLEDs with EQEs of up to 31.8% and Superb Device Stability

Designing mononuclear platinum(II) complexes that do not rely on intermolecular aggregation for high-performance red organic light-emitting diodes remains a formidable challenge. In this work, three robust red-emitting Pt(II) complexes are created by utilizing a rigid 4-coordination configuration, where the ligands are formed by linking electron-donor of triphenylamine (TPA) moieties with electron-acceptor of pyridine, isoquinoline, and/or δ-carboline units. The thermal stability, electrochemical, and photophysical properties of the complexes are thoroughly examined. The complexes display efficient red phosphorescence, with high photoluminescence quantum yields and short excited lifetimes. The OLEDs dope with these complexes exhibit high maximum external quantum efficiencies (EQEs) of up to 31.8% with minimal efficiency roll-off even at high brightness. Significantly, the devices demonstrate exceptional long operational lifetime, with a T90 lifetime of over 14000 h at initial luminance of 1000 cd m-2 , indicating the potential for these complexes to be practically utilizes.

Highly efficient blue quantum-dot light-emitting diodes based on a mixed composite of a carbazole donor and a triazine acceptor as the hole transport layer

Solution-processed thermally activated delayed fluorescence (TADF) exciplexes were employed as the hole transport layer (HTL) of blue quantum dot (QD) light-emitting diodes (QLEDs) by blending polymer donors of poly(N-vinylcarbazole) (PVK) with small molecular acceptors of 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T). As a result, the PVK:T2T HTL can harvest holes and electrons leaking from the QD active layer to form exciplex excitons and then this harvested exciton energy can be effectively transferred to the adjacent QD emitters through the Förster resonance energy-transfer process. Furthermore, the TADF exciplexes can enhance the hole mobility of the HTL due to the charge transfer process from the PVK donor to the T2T acceptor under an external electric field. The maximum current efficiency (CE) and external quantum efficiency (EQE) of the fabricated blue ZnCdS/ZnS core/shell QLEDs increase from 4.14 cd A^-1 and 7.33% for the PVK HTL to 7.73 cd A^-1 and 13.66% for the PVK:(5 wt%)T2T HTL, respectively. Our results demonstrate that the TADF exciplex HTL would be a facile strategy to design high-performance blue QLEDs.

Zero-Zero Energy-Dominated Degradation in Blue Organic Light-Emitting Diodes Employing Thermally Activated Delayed Fluorescence

Blue-emitting organic light-emitting diodes (OLEDs) fall significantly behind other OLEDs in operational stability. To better understand the key factors governing the stability of blue OLEDs employing thermally activated delayed fluorescence (TADF), nine efficient sky-blue to green TADF emitters with different frontier orbital energy levels and different TADF lifetimes have been designed and synthesized on the basis of charge-transfer (CT) acridine/phenyltriazine derivatives. Among them, ToDMAC-TRZ, a molecule composed of a 9,9-dimethyl-2,7-di-o-tolyl-9,10-dihydroacridine donor and a 2,4,6-triphenyl-1,3,5-triazine acceptor, shows a quantum yield of nearly 1 and a TADF lifetime as short as 0.59 μs in thin film. However, the stability of OLEDs is independent of the frontier orbital energy levels and TADF lifetime of the emitter. In contrast, the device half-life is found to decrease by five-sixths as the 0-0 energy of the singlet excitons increases by about 0.06 eV, which can be well-explained by the Arrhenius equation employing a photoreaction model. Whether in photoluminescence or electroluminescence, the contribution of long-lifetime triplet excitons to degradation is much lower than expected, which can be accounted for by how the solid-state solvation effect reduces the energy of the ³CT state and how most molecules have a low-lying locally excited triplet state.

Acridine Based Small Molecular Hole Transport Type Materials for Phosphorescent OLED Application

Two small molecular hole-transporting type materials, namely 4-(9,9-dimethylacridin-10(9H)-yl)-N-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-N-phenylaniline (TPA-2ACR) and 10,10′-(9-phenyl-9H-carbazole-3,6-diyl)bis(9,9-dimethyl-9,10-dihydroacridine) (PhCAR-2ACR), were designed and synthesized using a single-step Buchwald–Hartwig amination between the dimethyl acridine and triphenylamine or carbazole moieties. Both materials showed high thermal decomposition temperatures of 402 and 422 °C at 5% weight reduction for PhCAR-2ACR and TPA-2ACR, respectively. TPA-2ACR as hole-transporting material exhibited excellent current, power, and external quantum efficiencies of 55.74 cd/A, 29.28 lm/W and 21.59%, respectively. The achieved device efficiencies are much better than that of the referenced similar, 1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC)-based device (32.53 cd/A, 18.58 lm/W and 10.6%). Moreover, phenyl carbazole-based PhCAR-2ACR showed good device characteristics when applied for host material in phosphorescent OLEDs.

Improved Efficiency and Lifetime of Deep-Blue Hyperfluorescent Organic Light-Emitting Diode using Pt(II) Complex as Phosphorescent Sensitizer

Although the organic light-emitting diode (OLED) has been successfully commercialized, the development of deep-blue OLEDs with high efficiency and long lifetime remains a challenge. Here, a novel hyperfluorescent OLED that incorporates the Pt(II) complex (PtON7-dtb) as a phosphorescent sensitizer and a hydrocarbon-based and multiple resonance-based fluorophore as an emitter (TBPDP and ν-DABNA) in the device emissive layer (EML), is proposed. Such an EML system can promote efficient energy transfer from the triplet excited states of the sensitizer to the singlet excited states of the fluorophore, thus significantly improving the efficiency and lifetime of the device. As a result, a deep-blue hyperfluorescent OLED using a multiple resonance-based fluorophore (ν-DABNA) with Commission Internationale de L'Eclairage chromaticity coordinate y below 0.1 is demonstrated, which attains a narrow full width at half maximum of ≈17 nm, fourfold increased maximum current efficiency of 48.9 cd A-1 , and 19-fold improved half-lifetime of 253.8 h at 1000 cd m-2 compared to a conventional phosphorescent OLED. The findings can lead to better understanding of the hyperfluorescent OLEDs with high performance.

Tailoring the Electronic Landscape of Quantum Dot Light-Emitting Diodes for High Brightness and Stable Operation

The charge injection imbalance into the quantum dot (QD) emissive layer of QD-based light-emitting diodes (QD-LEDs) is an unresolved issue that is detrimental to the efficiency and operation stability of devices. Herein, an integrated approach to harmonize the charge injection rates for bright and stable QD-LEDs is proposed. Specifically, the electronic characteristics of the hole transport layer (HTL) is delicately designed in order to facilitate the hole injection from the HTL into QDs and confine the electron overflow toward the HTL. The well-defined exciton recombination zone by the engineered QDs and HTL results in high performance with a peak luminance exceeding 410 000 cd/m², suppressed efficiency roll-off characteristics (ΔEQE < 5% between 200 and 200 000 cd/m²), and prolonged operational stability. The electric and optoelectronic analyses reveal the charge carrier injection mechanism at the interface between the HTL and QDs and provides the design principle of QD heterostructures and charge transport layers for high-performance QD-LEDs.

Pyridine-Based Bipolar Hosts for Solution-Processed Bluish-Green Thermally Activated Delayed Fluorescence Devices: A Subtle Regulation of Chemical Stability and Carrier Transportation

Versatile host materials with good chemical stability and carrier-transporting ability are quite responsible for achieving stable solution-processed thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). Herein, we reported three bipolar dendritic hosts with or without the electron-withdrawing pyridine moiety via 6-site-linkages, namely, 3,3'-bis(3,3″,6,6″-tetra-tert-butyl-9'H-[9,3':6',9″-tercarbazol]-9'-yl)-1,1'-biphenyl (mCDtCBP), 3,3″,6,6″-tetra-tert-butyl-9'-(6-(3-(3,3″,6,6″-tetra-tert-butyl-9'H-[9,3':6',9″-tercarbazol]-9'-yl)phenyl)pyridine-2-yl)-9'H-9,3':6',9″-tercarbazole (mCDtCBPy), and 6,6'-bis(3,3″,6,6″-tetra-tert-butyl-9'H-[9,3':6',9″-tercarbazol]-9'-yl)-2,2'-bipyridine (mCDtCBDPy), exhibiting outstanding solubility, thermal stability as well as electrochemical stability. According to the calculation of bond dissociation energy (BDE), photodegradation results, and carrier dynamics evaluation, a significant relationship between device stability and the pyridine-based dendritic hosts was uncovered. Using mCDtCDPy with the highest electron mobility as the host, the solution-processed bluish-green TADF-OLED showed the shortest operational lifetime due to the unbalanced charge fluxes despite its highest anionic BDE for good chemical stability. However, the device based on mCDtCBPy exhibited twice longer lifetime than that based on mCDtCBP in spite of their similar balanced charge transportation, highlighting the importance of higher anionic BDE of the C-N bond in the device degradation process. Our findings unveiled a potential approach to achieve a subtle regulation of chemical stability and carrier transportation for realizing stable solution-processed TADF-OLEDs.

A π-D and π-A Exciplex-Forming Host for High-Efficiency and Long-Lifetime Single-Emissive-Layer Fluorescent White Organic Light-Emitting Diodes

Exciplex-forming hosts with thermally activated delayed fluorescence (TADF) provide a viable opportunity to unlock the full potential of the yet-to-be improved power efficiencies (PEs) and stabilities of all-fluorescent white organic light-emitting diodes (WOLEDs), but this, however, is hindered by the lack of stable blue exciplexes. Here, an advanced exciplex system is proposed by incorporating bipolar charge-transport π-spacers into both the electron-donor (D) and the electron-accepter (A) to increase their distance for hypsochromic-shifted emission while maintaining the superior transporting ability. By using spirofluorene as the π-spacer, 3,3'-bicarbazole as the D-unit, and 2,4,6-triphenyl-1,3,5-triazine as the A-unit, a π-D and π-A exciplex with sky-blue emission and fast reverse intersystem crossing process is thereof constructed. Combining this exciplex-forming host, a blue TADF-sensitizer, and a yellow conventional fluorescent dopant in a single-emissive-layer, the fabricated warm-white-emissive device simultaneously exhibits a low driving voltage of 3.08 V, an external quantum efficiency of 21.4%, and a remarkable T80 (time to 80% of the initial luminance) of >8200 h at 1000 cd m^-2 , accompanied by a new benchmark PE of 69.6 lm W^-1 among all-fluorescent WOLEDs.

Approaches for Long Lifetime Organic Light Emitting Diodes

Organic light emitting diodes (OLEDs) have been well known for their potential usage in the lighting and display industry. The device efficiency and lifetime have improved considerably in the last three decades. However, for commercial applications, operational lifetime still lies as one of the looming challenges. In this review paper, an in-depth description of the various factors which affect OLED lifetime, and the related solutions is attempted to be consolidated. Notably, all the known intrinsic and extrinsic degradation phenomena and failure mechanisms, which include the presence of dark spot, high heat during device operation, substrate fracture, downgrading luminance, moisture attack, oxidation, corrosion, electron induced migrations, photochemical degradation, electrochemical degradation, electric breakdown, thermomechanical failures, thermal breakdown/degradation, and presence of impurities within the materials and evaporator chamber are reviewed. Light is also shed on the materials and device structures which are developed in order to obtain along with developed materials and device structures to obtain stable devices. It is believed that the theme of this report, summarizing the knowledge of mechanisms allied with OLED degradation, would be contributory in developing better-quality OLED materials and, accordingly, longer lifespan devices.

Achieving Enhanced Thermally Activated Delayed Fluorescence Rates and Shortened Exciton Lifetimes by Constructing Intramolecular Hydrogen Bonding Channels

A fast radiative rate, highly suppressed nonradiation, and a short exciton lifetime are key elements for achieving efficient thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) with reduced efficiency roll-off at a high current density. Herein, four representative TADF emitters are designed and synthesized based on the combination of benzophenone (BP) or 3-benzoylpyridine (BPy3) acceptors, with dendritic 3,3″,6,6″-tetra-tert-butyl-9'H-9,3':6',9″-tercarbazole (CDTC) or 10H-spiro(acridine-9,9'-thioxanthene) (TXDMAc) donors, respectively. Density functional theory simulation and X-ray diffraction analysis validated the formation of CH···N intramolecular hydrogen bonds regarding the BPy3-CDTC and BPy3-TXDMAc compounds. Notably, the construction of intramolecular hydrogen bonding within TADF emitters significantly enhances the intramolecular charge transfer (ICT) strength while reducing the donor-acceptor (D-A) dihedral angle, resulting in accelerated radiative and suppressed nonradiative processes. With short TADF exciton lifetimes (τ_TADF) and high photoluminescence quantum yields (ϕ_PL), OLEDs employing BPy3-CDTC and BPy3-TXDMAc dopants realized maximum external quantum efficiencies (EQEs) up to 18.9 and 25.6%, respectively. Moreover, the nondoped device based on BPy3-TXDMAc exhibited a maximum EQE of 18.7%, accompanied by an extremely small efficiency loss of only 4.1% at the luminance of 1000 cd m^-2. In particular, the operational lifetime of the sky-blue BPy3-CDTC-based device was greatly extended by 10 times in contrast to the BP-CDTC-based counterpart, verifying the idea that the in-built intramolecular hydrogen bonding strategy was promising for the realization of efficient and stable TADF-OLEDs.

Bis-tridentate IrIII Phosphors Bearing Two Fused Five-Six-Membered Metallacycles: A Strategy to Improved Photostability of Blue Emitters

Iridium complexes bearing chelating cyclometalates are popular choices as dopant emitters in the fabrication of organic light-emitting diodes (OLEDs). In this contribution, we report a series of blue-emitting, bis-tridentate Ir^III complexes bearing chelates with two fused five-six-membered metallacycles, which are in sharp contrast to the traditional designs of tridentate chelates that form the alternative, fused five-five metallacycles. Five Ir^III complexes, Px-21-23, Cz-4, and Cz-5, have been synthesized that contain a coordinated dicarbene pincer chelate incorporating a methylene spacer and a dianionic chromophoric chelate possessing either a phenoxy or carbazolyl appendage to tune the coordination arrangement. All these tridentate chelates afford peripheral ligand-metal-ligand bite angles of 166-170°, which are larger than the typical bite angle of 153-155° observed for their five-five-coordinated tridentate counterparts, thereby leading to reduced geometrical distortion in the octahedral frameworks. Photophysical measurements and TD-DFT studies verified the inherent transition characteristics that give rise to high emission efficiency, and photodegradation experiments confirmed the improved stability in comparison with the benchmark fac-[Ir(ppy)₃ ] in degassed toluene at room temperature. Phosphorescent OLED devices were also fabricated, among which the carbazolyl-functionalized emitter Cz-5 exhibited the best performance among all the studied bis-tridentate phosphors, showing a maximum external quantum efficiency (EQE_max ) of 18.7 % and CIE_x,y coordinates of (0.145, 0.218), with a slightly reduced EQE of 13.7 % at 100 cd m^-2 due to efficiency roll-off.