Light-emitting Layer Research Articles

Efficient Organic Light-Emitting Diodes Obtained by Introducing Gadolinium (Gd) Complexes Based on Pyrazolone Derivative Ligands as Hole Trappers.

The utilization of lanthanide (Ln) complexes in the realm of organic light-emitting diodes (OLEDs) has garnered extensive interest, particularly in their role as luminescent materials or electron trappers. A series of gadolinium (Gd) complexes with energy levels of high HOMO/LUMO and different triplet state energies were designed and synthesized by introducing substituents with different electronic effects onto the pyrazolone derivative ligands. Subsequently, these complexes were precisely purified by vacuum sublimation and codoped into the light-emitting layer (EML) of the OLEDs. This process was facilitated through the well-matched HOMO/LUMO levels and triplet energies among various functional materials. Consequently, the maximum external quantum efficiencies of blue, red, and green OLEDs were simultaneously enhanced with the ratios of 119%, 28%, and 71%, respectively. This improvement can be credited to the introduction of Gd(III) complex molecules within EMLs, which helps to capture excess holes and improve carriers' balance.

Sterically Tuned Ortho-Phenylene-Linked Donor-Acceptor Benzothiazole-Based Boron Difluoride Complexes as Thermally-Activated Delayed Fluorescence Emitters for Organic Light-Emitting Diodes.

Two donor-acceptor dyes with an ortho-phenylene-linked carbazole electron donor and a benzothiazole-fused boron heterocyclic acceptor were designed, synthesized, and spectroscopically investigated. Due to the steric effects of boron heterocyclic units, the dyes demonstrate different conformations in the crystalline state. The presence of numerous hydrogen-bonding intermolecular interactions and the very weak π-π stacking in the molecular packing results in intense solid-state emission with photoluminescence quantum yields of 40 and 18% for crystals and 50 and 42% for host-based light-emitting layers. The compounds show aggregation-induced emission and thermally activated delayed fluorescence (TADF). The received ionization potential and electron affinity values suggested good charge-injecting ability and bipolar charge-transporting properties of the developed dyes. Transport of holes and electrons was detected in layers of one dye by the time-of-flight measurements. The benzothiazole-based boron difluoride complexes showed high electron mobility of 1.5 × 10-4 and 0.7 × 10-4 cm2 V-1 s-1 at an electric field of 1.35 × 106 V cm-1. Therefore, these dyes were successfully applied as emitters in organic light-emitting diodes with external quantum efficiencies of 15 and 13%, respectively. Our study marks a critical advancement in the area of solid-state emissive boron difluoride dyes, which can be applied as TADF emitters into organic light-emitting diodes. The obtained results reveal that the orientation of the acceptor unit in the ortho-phenylene-linked donor-acceptor dyes makes a significant impact on the TADF activity.

Recent Advances in Fluorescent Polyimides.

Polyimide (PI) refers to a type of high-performance polymer containing imide rings in the main chain, which has been widely used in fields of aerospace, microelectronic and photonic devices, gas separation technology, and so on. However, traditional aromatic PIs are, in general, the inefficient fluorescence or even no fluorescence, due to the strong inter- and intramolecular charge transfer (CT) interactions causing unavoidable fluorescence quenching, which greatly restricts their applications as light-emitting functional layers in the fabrication of organic light-emitting diode (OLED) devices. As such, the development of fluorescent PIs with high fluorescence quantum efficiency for their application fields in the OLED is an important research direction in the near future. In this review, we provide a comprehensive overview of fluorescent PIs as well as the methods to improve the fluorescence quantum efficiency of PIs. It is anticipated that this review will serve as a valuable reference and offer guidance for the design and development of fluorescent PIs with high fluorescence quantum efficiency, ultimately fostering further progress in OLED research.

Polymer Hosts Containing Carbazole-Dibenzothiophene-Based Pendants for Application in High-Performance Solution-Processed TADF-OLEDs.

The film-forming capability of the host plays a crucial role in effectively forming a light-emitting layer through a solution process in organic light-emitting diodes (OLEDs). In this study, we synthesized two side-chain polymer hosts, PCz-DBT and P2Cz-DBT, consisting of carbazole and dibenzothiophene. The synthesis was carried out through radical polymerization using styrene-based host monomers. Their photophysical characteristics and molecular energy levels are similar to those of the reference small molecule hosts, namely, Cz-DBT and 2Cz-DBT. However, compared to the small-molecule hosts Cz-DBT and 2Cz-DBT, the two polymer hosts showed high thermal stability and good film-forming properties in the neat and host-emitter blend films. Specifically, bluish-green multiple-resonance (MR) thermally activated delayed fluorescence (TADF) OLEDs, fabricated via solution processing with an emissive layer based on P2Cz-DBT, exhibited remarkable performance. These devices achieved a maximum external quantum efficiency of 17.4% without utilizing a hole transport layer. This polymer host design strategy is considered to significantly contribute to enhancing the performance of TADF-OLEDs fabricated through solution processing.

All-solution preparation of perovskite light-emitting diodes in ambient air through film-forming regulation of light-emitting layers based on dual-additive doping and oxygen plasma treatment

Organic-inorganic hybrid perovskites are promising for optoelectronics owing to their high color purity, tunable bandgaps, and low nonradiative recombination rates. However, challenges such as rough surfaces and poor film-forming properties constrain the optoelectronic performance of light-emitting devices. Herein, the PEDOT:PSS film treated by oxygen plasma serves as the hole transport layer, while the formation of perovskite films is optimized by dual-additive doping of PEO and TPBi in ambient air. The results show that the PEDOT:PSS films with PSSH-rich after oxygen plasma treatment provide more nucleation sites, favoring the formation of the perovskite film. Meanwhile, the dual-additive prompts to form a dense and uniform film as well. The intrinsic mechanisms involve PEO as a binder to fill grain gaps and improve water-oxygen stability in the perovskite film, while TPBi acts as a passivator to stimulate uniform nucleation and dense crystallization, improving the grain coverage of the light-emitting layer. Under atmospheric conditions, all-solution processed perovskite devices reach a maximum luminance of 810.4 cd m−2 and a maximum current efficiency of 0.437 cd A−1. This work highlights the significance of using oxygen plasma treatment and additives to improve the performance of perovskite films and achieve superior PeLEDs.

Three-Charge (0, -1, -2) Ligand-Based Binuclear and Mononuclear Deep-Red Phosphorescent Iridium Complexes Bearing Benzo[d]oxazole-2-Thiol Ligand.

A new class of three-charge (0, -1, -2) ligand-based binuclear and mononuclear iridium complexes bearing benzo[d]oxazole-2-thiol ligand have been synthesized. Notably, the binuclear complexes (IrIr1 and IrIr2) can be generated at low temperatures by reacting the iridium complex precursors (2a and 2b) with equal amounts of the benzo[d]oxazole-2-thiol ligands, while the corresponding mononuclear complexes (Ir1 and Ir2) are formed at high temperatures. X-ray diffraction analysis shows that the benzo[d]oxazole-2-thiol ligand plays an unusual and interesting bridging role in binuclear complexes and induces rich intermolecular and intramolecular interactions, while in mononuclear complexes, it forms an interesting four-membered ring coordination. More importantly, all complexes experienced efficient deep-red emission in the 628-674 nm range, and the mononuclear complexes have higher luminescent efficiency and longer excited state lifetime than the binuclear complexes. As a result, organic light-emitting diode devices incorporating two mononuclear complexes (Ir1 and Ir2) as guest material of the light-emitting layer can obtain good maximum external quantum efficiency (3.5% and 5.5%) in the deep-red region (629 and 632 nm) with CIE coordinates (0.61, 0.33) and (0.62, 0.34), along with a low turn-on voltage (2.8 V).

Highly conductive polymer electrodes for polymer light-emitting diodes

Organic light-emitting diodes (OLEDs) offer the advantage of flexibility; however, the use of traditional transparent anode ITO limits further extension of their flexible characteristics. In this study, we propose employing an polymer polybenzodifuranedione (PBFDO) as a flexible transparent anode instead of the rigid ITO. To address the issue encountered during the PBFDO solution spin-coating process, we introduced n-butanol into the PBFDO conductive solution to reduce its viscosity and freezing point by modulating intermolecular hydrogen bonding interactions. Consequently, high-quality PBFDO films with high conductivity, superior transmittance, and low surface roughness were successfully obtained via spin-coating. Moreover, due to its proper work function, regular molecular stacking, and low refractive index properties, PBFDO electrode facilitate efficient carrier injection and transport as well as photon extraction. The resulting device utilizing a PBFDO anode combined with Super Yellow as the light-emitting layer exhibited excellent performance characteristics including a normal threshold voltage of 2.6 V and a maximum luminous efficiency of 12.8 cd A−1 comparable to that device based on the ITO electrode. Furthermore, flexible device also achieved satisfactory performance (7.7 cd A−1) when using the PEN substrate.

Enhancing CdSe/ZnS Quantum-Dot Light-Emitting Diode Performance: The Impact of Thermal Treatment Atmospheres on Fabrication Processes

We explored the thermal treatment impact on the performance of quantum-dot light-emitting diodes (QLEDs). The QLEDs comprised multiple layers: a 2.2-μm thick epoxy buffer layer; a bottom cathode composed of 12-nm MoOx/10-nm Ag/12-nm MoOx; a 20-nm ZnO electron transporting layer (ETL); a 10-nm CdSe/ZnS quantum dot light emission layer (EML); a 40-nm 4,4′,4″-Tris(carbazol-9-yl) triphenylamine hole transporting layer; a 10-nm WOx hole injection layer; and a 100-nm Ag top anode. We applied thermal treatments to the cathode, ETL, and EML separately to assess their effects on the QLEDs. Additionally, we evaluated the impact of the thermal treatment atmosphere. Vacuum thermal treatment on the cathode and EML resulted in minor improvements in QLED performance, whereas treatment of the ETL led to a decline in performance. In contrast, air thermal treatment on the cathode and EML decreased QLED performance but significantly improved it by 15% in current efficiency when applied to the ETL. The performance differences attributable to the thermal treatment atmosphere are likely due to ligand removal and oxidation processes, facilitated by thermal energy and oxygen. Our study highlights that air thermal treatment on the ETL substantially improves QLED performance, offering crucial insights into the significance of thermal treatment in QLED development.

Synthesis of 5-(Aryl)amino-1,2,3-triazole-containing 2,1,3-Benzothiadiazoles via Azide-Nitrile Cycloaddition Followed by Buchwald-Hartwig Reaction.

An efficient access to the novel 5-(aryl)amino-1,2,3-triazole-containing 2,1,3-benzothiadiazole derivatives has been developed. The method is based on 1,3-dipolar azide-nitrile cycloaddition followed by Buchwald-Hartwig cross-coupling to afford the corresponding N-aryl and N,N-diaryl substituted 5-amino-1,2,3-triazolyl 2,1,3-benzothiadiazoles under NHC-Pd catalysis. The one-pot diarylative Pd-catalyzed heterocyclization opens the straightforward route to triazole-linked carbazole-benzothiadiazole D-A systems. The optical and electrochemical properties of the compound obtained were investigated to estimate their potential application as emissive layers in OLED devises. The quantum yield of photoluminescence (PLQY) of the synthesized D-A derivatives depends to a large extent on electron-donating strengths of donor (D) component, reaching in some cases the values closed to 100%. Based on the most photoactive derivative and wide bandgap host material mCP, a light-emitting layer of OLED was made. The device showed a maximum brightness of 8000 cd/m2 at an applied voltage of 18 V. The maximum current efficiency of the device reaches a value of 3.29 cd/A.

Luminescent color control and electroluminescent characteristics of iridium(III) complexes containing a four-membered chelating ring from ancillary ligand

Six dpppy (P,P-diphenyl-N-(pyridin-2-yl)phosphinic amide)- and dppq (P,P-diphenyl-N-(quinolin-2-yl)phosphinic amide)-based iridium(III) complexes 1a, 1b, 2a, 2b, 3a and 3b containing four-membered chelate architecture were synthetized. Their crystal structure and photophysical properties were investigated. The crystal structure shows that two coordinating N atoms from the ancillary ligand dpppy and dppq form a four-membered chelate architecture with the centered iridium(III). By changing the substituent groups and conjugation of the cyclometalated ligand, the control of the luminescent color of the complexes was achieved. In CH2Cl2 solution, complexes 1a, 1b, 2a, 2b, 3a and 3b emitted phosphorescence peaked at 509, 506, 479, 478, 620 and 618 nm with CIE color coordinates of (0.29, 0.60), (0.27, 0.61), (0.13, 0.37), (0.14, 0.35), (0.67, 0.33) and (0.67, 0.33), respectively. The quantum chemical calculation shows that their emitting light is primarily from 3LC π−π* states of the cyclometalated ligand. Electroluminescent devices with dual light-emitting layers were fabricated and exhibited mild efficiency roll-off.

High-Performance Ionogel-Embedded electroluminescent device reliably operating in harsh environments

Recently, there have been notable strides in stretchable, adaptable displays capable of conforming to curved surfaces, including human interfaces, architecture, and vehicles. However, previous research mostly focused on coplanar structures, where the light-emitting layer is sandwiched between facing electrodes, leading to limited performance. This study introduces a breakthrough approach by embedding electrodes within the light-emitting layer and utilizing polymer electrolytes to increase lighting volume, resulting in exceptionally high brightness (3,260 cd/m2), stretchability (500 %), durability (5,000 cycles) and doubled power efficiency. Furthermore, the device with buried electrodes proves reliable even in harsh environments, such as ice or boiling water, commonly encountered in real-life scenarios. This concept paves the way for a new era of ACEL applications, including shape-adaptable outdoor lighting and displays, light-emitting soft robotics, and actuators capable of operating in challenging environmental conditions previously unattainable with other types of light-emitting devices.

Near-infrared phosphorescent OLEDs exhibiting over 10% external quantum efficiency and extremely long lifetime using resonant energy transfer with a phosphorescent assist dopant

We have introduced a three-component light-emitting layer consisting of a host, a phosphorescent assist dopant, and a phosphorescent emitter to simultaneously realize high efficiency and long lifetime in NIR organic LEDs. An efficient energy transfer between the two phosphorescent materials was observed with 1 wt% doping ratio of the NIR emitter, significantly enhancing the external quantum efficiency reaching 10.5% with an emission peak at 769 nm. The device showed a low drive voltage and a long operational lifetime, with a luminance decay of less than 11% from the initial value after 4800 h under accelerated conditions of 100 mA cm−2.

230 nm wavelength range far-UVC LED with low Al-composition differentiation between well and barrier layers of MQWs

Abstract Reducing the average Al composition of Al x Ga1−x N/Al y Ga1−y N multiple quantum wells (MQWs) is an effective approach to increase the current injection efficiencies of far-UV-C LEDs (far-UVC LEDs). A reduction can be realized by decreasing the Al-composition differentiation between the well and barrier layers. Compared to conventional MQWs, a 230 nm wavelength far-UVC LED equipped with a single-Al-composition and a 39 nm thick light-emitting layer exhibits a higher external quantum efficiency (EQE). The EQE of far-UVC LEDs with low Al-composition differentiation (∼1%) is enhanced to approximately 0.6% and 1.4% under continuous wave operations at 230 nm and 236 nm wavelengths, respectively.

Effects of Electron-Withdrawing Strengths of the Substituents on the Properties of 4-(Carbazolyl-R-benzoyl)-5-CF3-1H-1,2,3-triazole Derivatives as Blue Emitters for Doping-Free Electroluminescence Devices.

The synthesis of four 4-(carbazolyl-R-benzoyl)-5-CF3-1H-1,2,3-triazoles with extra groups ((3-methyl)-phenyl-, 4-fluorophenyl-, quinolinyl-, or (3-trifluoromethyl)-phenyl-) in the acceptor fragment has been reported. The effects of substituents with different electron-withdrawing strengths on the thermal, electrochemical, photophysical, and electroluminescence properties of the synthesized compounds are discussed. The results of X-ray analyses and density functional theory (DFT) calculations support unusual molecular packing and electronic properties. The compounds are capable of glass formation with glass transition temperatures ranging from 54-84 °C. Ionization potentials of the compounds are in the range of 5.98-6.22 eV and electron affinities range from 3.09 to 3.35 eV. Under ultraviolet excitation, the neat films of the compounds exhibit blue emission with photoluminescence quantum yields ranging from 18 to 27%. The films of selected compounds are used for the preparation of host-free light-emitting layers of organic light-emitting diodes with very simple device structures and an external quantum efficiency of 4.6%.

Highly efficient hot exciton deep-blue OLEDs based on triphenyl-phosphine oxide modified anthracene derivatives

The utilization of triplet state excitons is important for the efficiency of organic light-emitting diodes (OLEDs). In this work, two isomers (p-PIAnPO and m-PIAnPO) of anthracene-based derivatives with triphenylphosphine oxide (PO) and phenanthroimidazole (PI) in different positions were designed and synthesized. In particular, PO provides large spatial hindrance and promotes hole transport, and PI has excellent bipolar transport ability, which helps to balance carrier transport. The para-linkage (p-PIAnPO) favors conjugation expansion and achieves high luminescence efficiencies, whereas the meta-linkage (m-PIAnPO) provides a highly twisted molecular conformation, thus effectively interrupting conjugation and leading to deep-blue emission. These molecules exhibit excellent thermal stability and efficient electroluminescence properties. Non-doped OLEDs using p-PIAnPO and m-PIAnPO as the light-emitting layers achieve maximum external quantum efficiencies of 6.0% and 4.2% with the λEL at 466 nm and 448 nm, corresponding to high exciton utilization efficiency (EUE) of 59%–88% and 54%–81%. In addition, p-PIAnPO-based OLEDs achieve an ultra-low efficiency roll-off of 1.7% at 1000 cd m−2. Theoretical calculations and experimental results show that the high EUE is mainly attributed to the high-lying reverse intersystem crossing from T3 to S1 namely hot exciton channel.

Highly efficient co-doped blue phosphorescent organic electroluminescent devices due to improved carriers balance and broadening recombination zone

High performance blue phosphorescent organic electroluminescent devices were realized by utilizing trivalent terbium complex and iridium complex as co-dopant and emitter, respectively. In this case, the co-doped trivalent terbium complex molecules within light-emitting layer function as deeper electron trappers due to its low-lying energy levels. Compared with reference device, co-doped devices displayed relatively higher electroluminescent performances due to improved carriers’ balance as well as broadening recombination zone. Finally, the optimal device with main emission peak at 462 nm obtained the maximum brightness, external quantum efficiency, current efficiency and power efficiency up to 24396 cd/m2, 26.3%, 46.93 cd/A and 50.82 lm/W, respectively. Meanwhile, even at the practical brightness of 1000 cd/m2 (4.3 V), prominent external quantum efficiency and current efficiency as high as 24.0% and 40.73 cd/A, respectively, can still be retained because the broadening recombination zone helps to decrease exciton density thus suppress exciton quenching.

Regulating the continuous and concentrated distribution of Quasi-2D perovskite phases to achieve Sky-Blue Light-Emitting diodes with efficiency approaching 17%

Blue perovskite light-emitting diodes (PeLEDs) are increasingly crucial for manufacturing high-definition full-color displays due to their outstanding electroluminescent properties. Quasi-two-dimensional (quasi-2D) perovskite stands out as the most promising material for blue light-emitting layer, given its robust quantum/dielectric confinement. In this study, we present the preparation of PeLEDs with a tunable blue emission wavelength using pure bromide perovskite components by adjusting the content of halogenated organic ammonium bromide [2-bromoethylamine hydrobromide (BEABr)]. The bifunctional ligand BEABr not only achieves an excellent phase distribution of quasi-2D perovskite but also reduces the weak van der Waals gap between individual perovskite layers, facilitating effective energy transfer in perovskite films. Additionally, the bifunctional groups passivate perovskite defects and inhibit phase separation, leading to efficient and stable sky-blue luminescence. The BEABr-doped sky-blue PeLED demonstrated superior device performance, achieving a maximum external quantum efficiency of 16.98 % at 493 nm. This work serves as inspiration for further exploration of efficient and stable commercial blue PeLED devices.

A 3D simulation model to study all-inorganic CsPbX3 (X = Br and I) perovskites-based light-emitting diodes with different hole-transporting layers

The development of numerical models is essential for optimizing perovskite light-emitting diodes (PeLEDs) and explaining their physical mechanism for further efficiency improvement. This study reports, for the first time, on a detailed device modelling of an all-inorganic perovskite LED consisting of CsPbX3 (X = Br and I) as light emitting layer (LEL) with different hole transporting layers (HTLs), employing COMSOL Multiphysics simulation package. Therefore, a 3D simulation model is served to investigate the appropriate HTLs that meet the design requirements of a PeLED in terms of band off-set engineering. For this purpose, a series of all-inorganic halide perovskites with different HTLs such as PEDOT: PSS, CuSCN and MoO3 are simulated under the same theoretical settings, and the performances of LEDs are compared with each other. This is done through studying their electronic properties using current density–voltage (J-V) curves and internal quantum efficiency (IQE) measurements. The results obtained from the J-V curves reveal that all the CsPbBr3-based samples with different HTLs exhibit the same turn-on voltage (V on) of approximately 4.2 V, while this value increases to 5.8 V for the CsPbI3-based samples. Compared with the PeLEDs based on CsPbI3, the PeLEDs based on CsPbBr3 indicate lower V on due to the formation of shorter charge carrier injection barriers at their interfaces. Furthermore, among the various simulated structures, the highest IQE is obtained for perovskite CsPbI3-based LED with MoO3 HTL (5.21%). The effect of different parameters on the performance of the proposed configurations are also investigated, and it turns out that the thickness of LELs and lifetime of charge carriers have a decisive role to play in the efficiency of PeLEDs. This theoretical study not only successfully explains the working principle of PeLEDs but also clearly shows researchers how to produce high-performance LEDs in the laboratory by knowing the physical properties of materials and accurately adjusting energy band alignments.

Multilayered inverted polymer-based light emitting diodes fabricated by meniscus coating and transfer-printing technique

We have developed polymer-based inverted organic LEDs (iOLED) with a multilayered structure using orthogonal solvent, transfer-printing, improved meniscus-coating method that moves back and forth. Reciprocating the glass rod back and forth repeatedly, the material usage of electron-injection layers, polymeric light-emitting layer and polymeric hole-transporting layers were decreased to 1/10 for the film deposited onto the glass slide and to ca. 1/20 for the film deposited onto the elastomer stamp compared with the conventional spin-coating. We obtained higher external quantum efficiency (EQE) and lower operation voltage with solution processable metal oxide hole-injection layer and silver anode. The improvements in the threshold voltage and the maximum EQE were observed for the device with electron-transporting EIL, such as alcohol soluble phenanthroline derivative, which is an effective way to reduce the number of multilayers and the tact time for fabricating iOLEDs.

Donor engineering to regulate fluorescence of a symmetrical structure based on a fluorene bridge for white light emission.

A white organic light-emitting device (WOLED) obtained using blue and yellow complementary colors possesses extremely high optical efficiency. We designed and prepared a completely symmetric D-π-D type efficient blue light small molecule FFA based on octylfluorene as a π bridge, where the undoped device showed efficient blue organic light-emitting device (OLED) performance with a maximum emission wavelength of 428 nm, Commission Internationale de l'Eclairage (CIE) coordinates of (0.17, 0.11) and one of the narrowest full width at half maximum (FWHM) of 35 nm. To improve the matching measure of complementary color materials for achieving white light emission, a yellow light small molecule FCzA was prepared by adjusting the conjugation degree of peripheral electron-donating groups based on the same fluorene-based π bridge with FFA. Undoped devices based on FCzA demonstrated an electroluminescence (EL) emission peak at 576 nm with CIE coordinates of (0.43, 0.49) and a relatively wide FWHM of 130 nm. Ultimately, the white OLED device was modulated with CIE coordinates located at (0.33, 0.38) via proportional regulation with a mixture of FFA and FCzA in a ratio of 10 : 3 as the light-emitting layer.