Solution-processed Organic Light-emitting Diodes Research Articles

Emission Enhancement by Intramolecular Stacking between Heteroleptic Iridium(III) Complex and Flexibly Bridged Aromatic Pendant Group.

Phosphorescent iridium(III) complexes suffer from a strong aggregation quenching, limiting their use in solution-processed or crystalline organic light-emitting diodes. Here we report how an intramolecular stacking between a flexibly bridged bulky aromatic pendant group and the core of nonionic heteroleptic complex can be exploited to minimize the negative effects of this drawback. The stacked conformation provides a rigid sterical shielding of the polar molecular surface, improving photoluminescence quantum yield of the complex both in solution and crystalline state.

Deep-Blue Thermally Activated Delayed Fluorescence Polymers for Nondoped Solution-Processed Organic Light-Emitting Diodes

The exploitation of deep-blue polymeric emitters is of great importance for the application of solution-processed organic light-emitting diodes (OLEDs) in full color display. The highly efficient deep-blue thermally activated delayed fluorescence (TADF) polymers are rarely reported up to now. Herein, we developed a series of deep-blue TADF polymers to fabricate highly efficient nondoped solution-processed OLEDs. By incorporating appropriate host with high triplet energy and deep-blue emitter with high fluorescent efficiency, the polymers are endowed with distinct TADF features. Using these deep-blue polymeric emitters, the nondoped single polymer based OLEDs achieve a maximum external quantum efficiency of 5.3% with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.15, 0.09), which represents the state-of-the-art device performance for the TADF-based deep-blue polymer light-emitting diodes (PLEDs).

Red to blue emitting cationic iridium complexes with 2-phenyl-4-dimethylaminopyridine as the cyclometalating ligand: Synthesis, characterization and electroluminescent devices

2-Phenyl-4-dimethylaminopyridine (dmappy) with high 3π-π∗ triplet energy is, for the first time, employed as the cyclometalating ligand for constructing cationic iridium complexes with tunable emission properties. Three complexes, namely, [Ir(dmappy)2(bpy)]PF6 (1), [Ir(dmappy)2(pzpy)]PF6 (2) and [Ir(dmappy)2(dma-pzpy)]PF6 (3), have been designed and synthesized (bpy = 2, 2′-bipyridine, pzpy = 2-(1H-pyrazol-1-yl)pyridine and dma-pzpy = 4-dimethylamino-2-(1H-pyrazol-1-yl)pyridine), and their photophysical and electrochemical properties have been comprehensively investigated. By using ancillary ligands with increasing electron-richness (bpy → pzpy → dmapzpy), complexes 1–3 emit red, green and blue light, respectively. Theoretical calculations reveal that for complexes 1 and 2, the light emission stems predominantly from the charge-transfer states (Ir/dmappy → bpy or pzpy), whereas for complex 3, the light emission originates mainly from the dmappy-centered 3π-π∗ state. In the doped film, fluorine-free complex 3 affords comparable emission color, luminescent efficiency and excited-state lifetime to the typical blue-emitting, neutral iridium complex FIrpic that uses fluorinated ligands. Single-layer, solution-processed organic light-emitting diodes based on complex 3 affords blue electroluminescence (EL) peaked at 472 nm, with the EL color competing with that from FIrpic and the peak current efficiency (12.6 cd A−1) being among the highest reported for single-layer, solution-processed blue OLEDs based on ionic iridium complexes.

High-performance solution-processed white organic light-emitting diodes based on silica-coated silver nanocubes.

Solution-processed white organic light-emitting diodes (WOLEDs) with silica-coated silver nanocubes (Ag@SiO2 NCs) incorporated at the interface of a hole transporting layer and emission layer are studied. The concentration of Ag@SiO2 NCs is varied to investigate the effect of Ag@SiO2 NCs on the performances of WOLEDs. Owing to the sharp edges and corners, Ag NCs greatly improve the radiative rate and emission intensity of nearby blue excitons. The blue emission at different Ag@SiO2 NC concentrations determines the performance of the WOLEDs. The emission of the orange excitons is strengthened by the high concentration of Ag@SiO2 NCs, which slightly influences the device performance. On the other hand, the SiO2 shell and some SiO2 nanospheres coexisting with Ag NCs reduce the hole transporting, improving the carrier balance in the WOLEDs. The experimental and simulated results also show that excessive Ag@SiO2 NCs may cause a rough film surface, unbalanced carrier injection, and fluorescence quenching, which decreases the device performance. The optimized WOLED with a proper concentration of Ag@SiO2 NCs has a peak current efficiency of 53.9cd/A, acquiring a significant enhancement factor of 77.3% compared to the control device without Ag@SiO2 NCs.

Roll-to-Roll Fabrication of PEDOT:PSS Stripes Using Slot-Die Head With <inline-formula> <tex-math notation="LaTeX">$\mu$ </tex-math> </inline-formula>-Tips for AMOLEDs

Stripe coating offers an approach to remove the pixel bank structure used to confine ink droplets. For potential application in solution-processable active-matrix organic light-emitting diode displays, we fabricate fine and dense stripes of poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate)(PEDOT:PSS) using a slot-die head with the dual plate (shim plate with slit channels and meniscus-guiding plate with μ-tips as narrow as pixels). We analyze the flow distribution of the aqueous PEDOT:PSS solution near the μ-tip by varying the dual-plate configuration (μ-tip width, μ-tip length, and shim plate thickness) together with the process variable (i.e., coating speed) and offer design guidelines for the roll-to-roll fabrication of a fine stripe pattern. It can be achieved by reducing the μ-tip length as well as the shim plate thickness and increasing the coating speed until the flow breaks up. This scheme also enables us to raise the stripe density without defects. It is found that the μ-tip length is crucial for coatings of dense stripes. If μ-tip is long, it is not feasible to fabricate dense stripes regardless of the shim plate thickness due to an increase in resistance to flow. It is also found that the wettability of the solution serves as one of the critical parameters that determine the stripe profiles. We have fabricated 100 stripes with the average width of 227 μm and interstripe width nonuniformity as low as 9.2%. Finally, we have fabricated organic light-emitting diodes (OLEDs) atop the conductive PEDOT:PSS stripes and successfully obtained light emission from OLED stripes.

Enhanced near-infrared electroluminescence from a neodymium complex in organic light-emitting diodes with a solution-processed exciplex host

We report enhanced near-infrared (NIR) electroluminescence from a Nd3+-complex with thenoyltrifluoroacetone and 1,10-phenanthroline ligands. The NIR-emitting complex was blended into an exciplex-forming co-host system comprising 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene as the electron transport material and 4,4′,4″-tris(carbazol-9-yl)triphenylamine as the hole transport material in solution-processed small molecule organic light-emitting diodes (OLEDs). This binary ambipolar host system favors direct charge trapping and exciton formation on the Nd3+-complex molecules. Efficient energy transfer from the singlet and triplet exciplexes formed between the host molecules to the Nd3+ ions contributes to the enhanced luminescence efficiency. The photoluminescence quantum yield of this blend is 1.2%, and the optimized OLED shows a maximum electroluminescence external quantum efficiency of 0.034%. The device also exhibits a low efficiency roll-off of only 12% over a current density range of 100 mA/cm2, due to the reduced triplet-polaron annihilation.

Front Cover: Simply Structured Near‐Infrared Emitters with a Multicyano Linear Acceptor for Solution‐Processed Organic Light‐Emitting Diodes (Chem. Eur. J. 4/2019)

Front Cover: Simply Structured Near‐Infrared Emitters with a Multicyano Linear Acceptor for Solution‐Processed Organic Light‐Emitting Diodes (Chem. Eur. J. 4/2019)

Highly-efficient solution-processed green phosphorescent organic light-emitting diodes with reduced efficiency roll-off using ternary blend hosts

We have investigated the effect of various materials mCP DpAn-5BzAc poly(9-vinylcarbazole) and TCTA as the hosts on the performance of solution-processed green phosphorescent organic light-emitting diodes (PhOLEDs).

Highly efficient electroluminescence from evaporation- and solution-processable orange–red thermally activated delayed fluorescence emitters

Orange and red thermally activated delayed fluorescence emitters have been developed for high performance vacuum-deposited and solution-processable organic light-emitting diodes.

Functionalization of Organometallic Complexes Aimed at Solution-Processed Organic Light-Emitting Diode: Strategic Molecular Designs of Phosphorescent Dendritic Emitters

Functionalization of Organometallic Complexes Aimed at Solution-Processed Organic Light-Emitting Diode: Strategic Molecular Designs of Phosphorescent Dendritic Emitters

Through-space charge transfer hexaarylbenzene dendrimers with thermally activated delayed fluorescence and aggregation-induced emission for efficient solution-processed OLEDs.

Through-space electron interaction plays a critical role in determining the optical and charge transport properties of functional materials featuring π-stacked architectures. However, developing efficient organic luminescent materials with such interactions has been a challenge because of the lack of well-established prototypical molecules. Here we report the design of through-space charge transfer hexaarylbenzenes (TSCT-HABs) containing circularly-arrayed electron donors (acridan/dendritic triacridan) and acceptors (triazine), which exhibit both thermally activated delayed fluorescence (TADF) and aggregation-induced emission (AIE) effects for high-efficiency solution-processed organic light-emitting diodes (OLEDs). Spatial separation of donors and acceptors in the TSCT-HABs induces a small singlet-triplet energy splitting of 0.04-0.08 eV, leading to delayed fluorescence with microsecond-scale lifetimes. Meanwhile, the TSCT-HABs display the AIE effect with emission intensity enhanced by 6-17 fold from solution to the aggregation state owing to their propeller-shaped configuration. Solution-processed OLEDs based on the TSCT-HABs show maximum external quantum efficiency up to 14.2%, making them among the most efficient emitters for solution-processed TADF OLEDs.

Water-soluble pH neutral triazatruxene-based small molecules as hole injection materials for solution-processable organic light-emitting diodes

Water-soluble and pH neutral triazatruxene-based small molecules have been used as hole injection materials in solution-processable organic light-emitting diodes, achieving even better performance compared with PEDOT:PSS.

High performance solution-processed green phosphorescent organic light-emitting diodes with high current efficiency and long-term stability

Highly efficient (D–π–A)-type host and green Ir(iii) complexes are introduced for solution-processed PHOLEDs that achieve high CE with considerably high EQE. The devices with symmetrical complex show more stable than those with asymmetrical complex.

Novel molecular triad exhibiting aggregation-induced emission and thermally activated fluorescence for efficient non-doped organic light-emitting diodes.

A light-emitting molecular triad (BPCP-2CPC) with dual functionality was successfully synthesized and applied to solution-processed non-doped organic light-emitting diodes. The BPCP-2CPC triad contains 9-phenyl-9H-carbazole units as a host moiety tethered to a green-emitting core (BPCP) through a cyclohexane unit and exhibits thermally activated delayed fluorescence (TADF) and aggregation-induced emission (AIE) characteristics simultaneously. The BPCP-2CPC-based non-doped TADF-OLED devices showed a high external quantum efficiency (EQE) of 13.4%.

Phosphorescent and TADF polymers and dendrimers in solution-processed self-host organic light-emitting diodes: structure analysis and design perspectives

The relation between chemical structure and physical and electroluminescence properties for dendrimeric and polymeric emitters is examined; balanced charge transport is necessary for achieving the most efficient self-host devices with low efficiency roll-off.

Lighting with organophosphorus materials: solution-processed blue/cyan light-emitting devices based on phosphaphenalenes.

A family of electroluminescent organophosphorus materials for solution-processed organic light-emitting diodes is reported. The investigated systems present a six-membered phosphorus heterocycle fused with a pyrrole or benzopyrrole moiety. The materials exhibit high thermal stability and are soluble in a variety of organic solvents. Already in very simple OLED architectures, the novel electroluminescent materials display blue or cyan luminescence; i.e. color coordinates x = 0.18; y = 0.21 and x = 0.22; y = 0.30, respectively, with performances that are in the range of state-of-the-art phosphazene materials. In electrofluorochromic devices, the color emission coordinates of the organophosphorus materials are tuned upon applying an electric potential. These findings set the bedrock for the development of new generations of organophosphorus light-emitting devices.

Leakage-free solution-processed organic light-emitting diode using a ternary host with single-diode emission area up to 6 × 11.5 cm2.

The electrical current leakage and stability are studied for solution-processed OLEDs with areas of 4.45 mm2, 3 × 3.2 cm2, and 6 × 11.5 cm2. The emission layer of the OLED has a ternary or binary mixed host with hole-transporting molecules tris(4-carbazoyl-9-ylphenyl)amine (TCTA) and 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi), together with the electron-transporting molecule 2,7-bis(diphenylphosphoryl)-9,9′-spirobi[fluorene] (SPPO13). The phosphorescent emitters are Ir(mppy)3 for green and bis[4-(4-tert-butylphenyl)thieno[3,2-c]pyridine][N,N′-diisopropylbenamidinato]iridium(iii) (PR-02) for orange. Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl))diphenylamine)] (TFB) is used as the hole transport layer and PEDOT:PSS is used as the hole injection layer. On top of the emission layer, CsF/Al is deposited by thermal evaporation as the cathode. All organic layers are deposited by blade coating and the initial current leaking defects can be avoided by careful control of the coating conditions. The detrimental burning point caused by a local current short developed after long-time operation can be avoided by reducing the operation voltage using a ternary mixed host. The operation voltage is only 4 V at 100 cd m−2 and 5 V at 250 cd m−2 for the green emitting device. Furthermore, the crystallization defect is reduced by the ternary host. For the orange emitting device, the binary host is good enough with an operating voltage of 5 V at 100 cd m−2. For an area as large as 6 × 11.5 cm2, the OLED shows good stability and there is no burning point after an operation of over 1600 hours.

Recent Advances in Conjugated TADF Polymer Featuring in Backbone-Donor/Pendant-Acceptor Structure: Material and Device Perspectives.

Along with the persistent research interest in organic light-emitting diode (OLED) display and lighting technology, a new studying topic is now focused on developing thermally activated delayed fluorescence (TADF) polymer emitters, with the purpose to achieve high-performance cost-effective, solution-processed OLEDs (s-OLEDs) purely from fluorescent-type materials. However, research in this topic is in its infancy about the designing rules of polymer structures, photophysical mechanisms and the correlated devices. In this Personal Account, mainly from our personal experience we will shortly introduce the historical developments, status and perspectives about one representative kinds of TADF polymers, i. e. the conjugated TADF polymers featuring in backbone-donor/pendant-acceptor (BDPA) structure scaffold, which shows very promising electroluminescent (EL) performance even using simple s-OLED structure. Special attention is focused on illustrate the molecular designing & synthesis motivation, chemistry & device tactics towards solving the limiting factors about the quantum yields and aggregation-quenching tendency in solid states. Further challenges and strategies towards optimizing their overall EL performance, e. g. simultaneous achieving extremely high external quantum efficiency, power efficiency and low roll-off rate, are also discussed.

Efficient solution-processed red organic light-emitting diode based on an electron-donating building block of pyrrolo[3,2-b]pyrrole

Abstract An electron-rich moiety, pyrrolo[3,2-b]pyrrole, has been used as a new donor block in organic lighting-emitting diodes (OLEDs). Combined with a strong electron-withdrawing group of 2,1,3-benzothiadiazole, a novel red fluorophore BTZPP was synthesized. The solution-processed red-emitting device based on BTZPP showed encouraging performances with external quantum efficiency of up to 3.4%. This evidently indicates that the pyrrolo[3,2-b]pyrrole group can be a promising electron-donating moiety for enriching the material types and promoting the development of OLEDs.

Hyperbranched polymers with aggregation-induced emission property for solution-processed white organic light-emitting diodes

In this work, a new series of hyperbranched polymers of PFTPE-Ir(piq)3-X(X = 1, 5, 10) were designed and synthesized, in which tris(1-phenylisoquinoline)iridium(Ш) (Ir(piq)3) acts as red emission core and PFTPE acts as branches. The photophysical study reveals that these hyperbranched polymers exhibit aggregation-induced emission (AIE) characteristic, inducing in much higher photoluminescent quantum yield (ΦY) in neat film than that in dilute tetrahydrofuran (THF) solution. The white-light OLEDs using PFTPE-Ir(piq)3-X as emission layer show rather weaker efficiency roll-off. Especially, the white-light OLED based on PFTPE-Ir(piq)3-5 as emission layer shows a maximum luminance of 4686 cd/m2, a maximum luminous efficiency of 2.43 cd/A, a maximum external quantum efficiency of 1.08% and the Commission Internationale de l’Eclairage coordinate of (0.26, 0.36).