Fabrication Of Organic Light-emitting Diodes Research Articles

Thermally Induced Phase Separation of the PEDOT:PSS Layer for Highly Efficient Laminated Polymer Light-Emitting Diodes.

Among various conductive polymers, the poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) film has been studied as a promising material for use as a transparent electrode and a hole-injecting layer in organic optoelectronic devices. Due to the increasing demand for the low-cost fabrication of organic light-emitting diodes (OLEDs), PEDOT:PSS has been employed as the top electrode by using the coating or lamination method. Herein, a facile method is reported for the fabrication of highly efficient polymer light-emitting diodes (PLEDs) based on a laminated transparent electrode (LTE) consisting of successive PEDOT:PSS and silver-nanowire (AgNW) layers. In particular, thermally induced phase separation (TIPS) of the PEDOT:PSS film is found to depend on the annealing temperature (Tanneal) during preparation of the LTE. At Tanneal close to the glass transition temperature of the PSS chains, a PSS-rich phase with a large number of PSS- molecules enhances the work function of the PEDOT:PSS on the glass-side surface relative to the air side. By using the optimized LTEs, bidirectional laminated PLEDs are obtained with a total external quantum efficiency of 2.9% and a turn-on voltage of 2.6 V, giving a comparable performance to that of the reference bottom-emitting PLED based on a costly evaporated metal electrode. In addition, an analysis of the angular characteristics, including the variation in the electroluminescence spectra and the change in luminance according to the emission angle, indicates that the laminated PLED with the LTE provides a more uniform angular distribution regardless of the direction of emission. Detailed optical and electrical analyses are also performed to evaluate the suitability of LTEs for the low-cost fabrication of efficient PLEDs.

Inkjet-printed multilayer structure for low-cost and efficient OLEDs

Inkjet printing is considered a key technology in the fabrication of organic light-emitting diodes (OLEDs), but achieving a fully inkjet-printable OLED structure is still a challenge. Here, we propose the fabrication of OLEDs with an inkjet-printed multilayer structure (i.e. anode/hole injection layer (HIL)/electron blocking layer (EBL)/emitting layer (EML)) by properly developing new HIL and EBL inks to achieve uniform and homogeneous printed thin films. In the fabricated multilayer device, the dissolution process of the EBL, which occurs during the printing of the EML, creates a blurred interface, resulting in device performance that achieves maximum current efficiencies of 20 cd/A and 7 cd/A with ITO and printed polymeric anode, respectively. With the aim of simplifying the structure of the device and mimicking the formation of such a blurred interface, another printed multilayer structure (i.e. anode/printed HIL/printed EBL:EML) was proposed, achieving maximum current efficiencies of 13 and 6 cd/A with ITO and polymeric anode, respectively. Such results represent a compromise between simplifying the fabrication process and achieving good electro-optical properties and thus represent a further step towards the fabrication of a fully inkjet-printed ITO-free OLED.

A Fabrication of Organic Light Emitting Diodes (OLEDs) using the Lamination method in a Vacuum-Free Environment

Organic Light Emitting Diodes (OLEDs) have recently become one of the fastest-growing technologies in the world. The challenge in OLED fabrication, especially larger-area OLEDs, is its relatively high costs and complexity. The lamination method at a vacuum-free environment is an approach to simplify and reduce the cost of fabrication. This paper reports our latest progress on OLEDs fabricated using the said method and condition. The processing parameters were explored and optimized. Spin coating the emissive Layer (PFO) at 1300 rpm and the anode (TC-07-S) at 3000 rpm yield the best results in terms of current conduction and success rate. Laminating the OLEDs at 160 °C, with 245 N of force, and for 30 seconds, gave the best results in terms of previously stated parameters. Furthermore, the constituting materials of the OLEDs were explored. It was found that TC-07-S as an anode, PFO as the light-emitting material, a 30-micrometer thick aluminum foil as the cathode, and Kapton as the dielectric and adhesive material yielded the best results. These results may pave the way for other innovative methods to fabricate OLEDs with a simple and affordable processes.

Influence of gold nanostructures on excited state intramolecular proton transfer in multidomain HTTH dye

Organic multidomain dyes exhibiting excited state intramolecular proton transfer (ESIPT) are known due to large Stokes shifts and dependence of their luminescence spectral characteristics on the properties of the environment. In this work, influence of gold nanostructures on the spectral characteristics of a “polycarbonate matrix – gold nanostructures – HTTH” system was studied using thiazole dye HTTH as an example. A hypothesis about the possibility of plasmon resonance energy transfer (PRET) between the HTTH molecules in different states, namely the ground state (enol form) and the state after proton transfer (keto form), mediated by gold nanostructures was experimentally tested. Presence of gold nanostructures in the vicinity of HTTH molecules was found to lead to the changes in the ratio of the luminescence peak intensities for the enol and keto form of these molecules. This phenomenon opens up the possibility of additional regulation of the spectral characteristics and may evidence the PRET effect in the systems containing ESIPT-exhibiting dyes and plasmonic nanostructures. The obtained results improve our understanding of the physical processes in the systems similar to the studied one and imply new practical applications of them such as fabrication of organic light-emitting diodes, sensors, super-resolution microscopy tools and ultraviolet-to-visible radiation convertors.

Recent synthetic strategies of small heterocyclic organic molecules with optoelectronic applications: a review.

Over the past few years, there have been tremendous developments in the design and synthesis of organic optoelectronic materials with appealing applications in device fabrication of organic light-emitting diodes, superconductors, organic lasers, organic field-effect transistors, clean energy-producing organic solar cells, etc. There is an increasing demand for the synthesis of green, highly efficient organic optoelectronic materials to cope with the issue of efficiency roll-off in organic semiconductor-based devices. This review systematically summarized the recent progress in the design and synthesis of small organic molecules having promising optoelectronic properties for their potential applications in optoelectronic devices during the last 10-year range (2010-early 2021).

Weak-conjugation linked donor–acceptor emitters for efficient near-ultraviolet organic light-emitting diodes with narrowed full width at half maximum

By weak conjugation, stable near-ultraviolet (NUV) emitters are prepared for organic light-emitting diodes fabrication and highly pure NUV electroluminescence with small efficiency roll-off is achieved with narrowed full-width at half maxima.

Spectroscopic Ellipsometry Studies on Solution-Processed OLED Devices: Optical Properties and Interfacial Layers.

Τhe fabrication of organic light-emitting diodes (OLEDs) from solution involves the major problem of stack integrity, setting the determination of the composition and the characteristics of the resulting interfaces prerequisite for the optimization of the growth processes and the achievement of high devices' performance. In this work, a poly(9,9-dioctylfluorene) (F8) and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) blend is used for the emissive layer (EML), poly-3,4-ethylene dioxythiophene; poly-styrene sulfonate (PEDOT:PSS) is used for a hole transport layer (HTL), and Poly(9,9-bis(3'-(N,N-dimethyl)-N-ethylammoinium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluore-ne))dibromide (PFN-Br) for an electron transport layer (ETL) to produce the OLED device. All the layers are developed using the slot-die process, onto indium tin oxide (ITO)-coated polyethylene terephthalate (PET) flexible substrates, whereas Ag cathode was formed by ink-jet printing under ambient conditions. Spectroscopic ellipsometry measurements were performed upon completion of the successive films' growth, in sequential steps, for the multilayer OLED development. Ellipsometry analysis using different models demonstrate the degree of intermixing within the layers and provide information about the interfaces. These interfacial properties are correlated with the emission characteristics as well as the final performance of the OLED devices.

Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor

Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor

Synthesis and photosensitization study of red luminescent europium (III) complexes with heterocyclic ligands for application in OLEDs

Six complexes of europium (III) with 1-ethyl-6-fluoro-1,4-dihydroxy-4-oxo-7-(1-piperazinyl)-3-quinoline carboxylic acid (L) as main ligand and 1,10-phenanthroline, bathophenanthroline, neocuprine, 2,2′-bipyridyl, 5,6-dimethyl-1,10-phenanthroline as ancillary ligands were synthesized by grinding technique. Through study of thermogravimetric data, it can be concluded that these complexes exhibit excellent thermal stability which favour their use in organic light emitting diode fabrication. Crystalline nature of complexes was studied by powder XRD and SEM. 1H NMR, 13C NMR, FT-IR spectra displayed the coordination site in complexes and confirmed that ligand is present in the zwitterionic form. On monitoring the emission at 370 nm UV excitation, intense red color was observed at 615 nm due to 5D0 → 7F2 electronic transition. Urbach energy, CIE coordinates, Judd-Ofelt parameters, quantum yield, CCT and lasing parameters were calculated. Refractive index and energy band gap values depicted their uses in semiconductor devices. Further investigation of antimicrobial and antioxidant assays certified the impressive antimicrobial and antioxidant activities of these complexes.

7‐3: OLED Camera‐Under Panels with Improved Imaging Quality

In this paper, we report the novel design and fabrication of OLED (organic light‐emitting diode) Camera‐under panels (CUP) with improved transmittance and imaging quality. We utilized TFT (thin film transistor)‐outside structure and patterned cathode in the CUP region. The diffraction simulation, transmittance test, Point spreading function (PSF) test, and scene photos proved the efficiency of our solution. Remarkable enhancement of imaging performance, which was identical to that of a normal AA‐hole OLED panel, was achieved. The results confirm the effectiveness of our CUP design strategy and also the potential of our design to be commercially used in the industry.

Efficient Blue Electrophosphorescence and Hyperphosphorescence Generated by Bis-tridentate Iridium(III) Complexes.

Four blue-emissive iridium(III) complexes bearing a 3,3'-(1,3-phenylene)bis[1-isopropyl-6-(trifluoromethyl)-3H-imidazo[4,5-b]pyridin-2-ylidene]-based pincer chelate, which are derived from PXn·H3(PF6)2, where n = 1-4, and a cyclometalating chelate given from 9-[6-[5-(trifluoromethyl)-2λ2-pyrazol-3-yl]pyridin-2-yl]-9H-carbazole [(PzpyCz)H2], were successfully synthesized and employed as both an emissive dopant and a sensitizer in the fabrication of organic light-emitting diode (OLED) devices. These functional chelates around a IrIII atom occupied two mutually orthogonal coordination arrangements and adopted the so-called bis-tridentate architectures. Theoretical studies confirmed the dominance of the electronic transition by the pincer chelates, while the dianionic PzpyCz chelate was only acting as a spectator group. Phosphorescent OLED devices with [Ir(PX3)(PzpyCz)] (B3) as the dopant gave a maximum external quantum efficiency (EQE) of 21.93% and CIExy of (0.144, 0.157) and was subjected to only ∼10% of roll-off in efficiency at a high current density of 1000 cd m-2. Blue-emissive narrow-band hyperphosphorescence was also obtained using B3 as an assistant sensitizer and ν-DABNA as a terminal emitter, giving both an improved EQE of 26.17% and CIExy of (0.116, 0.144), confirming efficient Förster resonance energy transfer in this hyperdevice.

Coinage Metal Complexes of Germylene and Stannylene.

Group 11 metal (M = Cu, Ag, Au) complexes of heavier tetrylenes (namely, germylene and stannylene) have been studied for several years now. However, the field is mainly unexplored for their potential application either in homogeneous catalysis or in photophysical properties regardless of how the current reports allude to the rich and fascinating chemistry of group 14 elements. In this mini-review, we attempted to summarize the recent advances of heavier tetrylene-stabilized coinage metal complexes, which are majorly dominated by structure elucidation studies. This comprehensive summary intends to help researchers design and fine-tune the tetrylene ligand framework that can lead to coinage metal complexes for future applications as photoemitters in organic light-emitting diode fabrication and efficient catalysts in homogeneous catalysis.

Monomeric carbazolylcyanobenzenes as thermally activated delayed fluorescence emitters: effect of substitution position on photoluminescent and electroluminescent properties

In this study, two new donor-acceptor type thermally activated delayed fluorescence materials using carbazole as a donor and cyano substituted benzene as an acceptor were synthesized. Structural parameters, molecular orbital densities, photophysical characteristics were determined. By changing the substitution position of the cyano groups on the phenyl rings, the relationship between structure and properties was also be disclosed. These materials were tested as emissive species for the fabrication of organic light-emitting diodes. The blue devices showed EL emission peaks at 464 and 488 nm with the CIE coordinates of (0.17, 0.18) and (0.21, 0.31), respectively.

Hot exciplexes in U-shaped TADF molecules with emission from locally excited states

Fast emission and high color purity are essential characteristics of modern opto-electronic devices, such as organic light emitting diodes (OLEDs). These properties are currently not met by the latest generation of thermally activated delayed fluorescence (TADF) emitters. Here, we present an approach, called “hot exciplexes” that enables access to both attributes at the same time. Hot exciplexes are produced by coupling facing donor and acceptor moieties to an anthracene bridge, yielding an exciplex with large T1 to T2 spacing. The hot exciplex model is investigated using optical spectroscopy and quantum chemical simulations. Reverse intersystem crossing is found to occur preferentially from the T3 to the S1 state within only a few nanoseconds. Application and practicality of the model are shown by fabrication of organic light-emitting diodes with up to 32 % hot exciplex contribution and low efficiency roll-off.

Thermally cross-linkable spirobifluorene-core-based hole transport layer with high solvent-resistivity for solution processible OLEDs

The fabrication of organic light-emitting diodes (OLEDs) composed of multi-layered structure through a solution-based process suffers from the dissolution of the preformed lower parts during the coating of upper layers. To prevent this problem during the solution process, a promising approach of introducing a cross-linkable layer with a high solvent-resistivity has been proposed. Herein, thermally cross-linkable spirobifluorene-core-incorporated hole transport layers (HTLs) with a cross-linking temperature of 180 °C are designed for solution-processible OLEDs composed of multi-layered structures. The enhanced morphology stability and solvent-resistive property of the synthesized HTLs are evaluated through a rinsing-test with an organic solvent used for the emitting layer. Considering the charge transport property of HTLs in OLEDs, the appropriate energy level and triplet energy values of the synthesized HTLs promote the efficient cascade hole migration. Consequently, the OLED composed of thermally cross-linkable HTL shows higher quantum efficiency (QE) of 16.5% and lower operation voltage of 5.1 V at 1000 cd/m2, compared to that composed of a commercialized polymer, poly (9-vinylcarbazole) (PVK). Thus, in this study, the feasibility of the potential application of thermally cross-linkable HTL with the spirobifluorene-core-units as charge transport layers for solution-based optoelectronic devices composed of multi-layered structures has been verified.

Investigations on optical properties of Eu0.5 Sm0.5 (TTA)3 tppo hybrid organic complexes molecularly doped in PMMA and PS matrices.

An attempt has been made to synthesize orange-red light-emitting rare earth-doped polymer matrices Eu0.5 Sm0.5 (TTA)3 tppo (Eu=europium, Sm=samarium, TTA=thenoyltrifluoroacetone, tppo=triphenylphosphine oxide) hybrid organic complex by solution technique. Blended thin films were made by molecularly doping the complex in polymethylmethacrylate (PMMA) and polystyrene (PS) polymers at different weight percentages (5%, 10%). These films were solvated in basic and acidic media to explore the effect of solvent on its luminescence properties. UV-visible absorption spectra of these solvated films portray two peaks corresponding to π→π* and n→π* optical transitions in the range 240-275nm and 370-390nm in basic medium. Energy band gap of these thin films in basic medium was found between 3.16eV to 3.20eV and 3.12eV to 3.22eV in acidic medium. Photoluminescence spectra of all films in dichloromethane showed an intense peak at 614nm, whereas in formic acid, the same were found at 475nm, which fell in the orange-red and blue region of the visible spectrum, respectively. CIE coordinates of these solvated films in various solvents and at different weight percentages revealed tunable orange-red to blue light emission with change in polarity of the solvent. Therefore the synthesized complexes blended in the polymers can be shaped into flexible films for fabrication of organic light-emitting diodes (OLEDs) with consistent results, proving their prospect as colour tunable light emissive materials for OLED devices, lasers, displays, and solid-state lighting.

Synthesis and photoluminescence analysis of europium(III) complexes with pyrazole acid and nitrogen containing auxiliary ligands

Four red photoluminescent europium(III) complexes have been synthesized by using fluorinated carboxylate ligand, 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic acid as main ligand and bathophenanthroline, 1,10-phenanthroline and 2,2-bipyridyl as auxiliary ligands, through solution precipitation approach. The complexes possess an optimum thermal stability, which is sufficient for organic light emitting diodes fabrication. Under ultraviolet excitation, the complexes display an intense emission peak of europium ion, which makes them a promising red color emitter in display devices. Internal quantum efficiency, color coordinates, Judd–Ofelt parameters, and energy transfer mechanism have been explored. The investigated antimicrobial and antioxidant activity suggest that the synthesized complexes are potent antimicrobial and antioxidant agents. Research Highlights A new ligand L has been synthesized by an ecofriendly method. Four Eu(III) complexes have been synthesized through a facile solution precipitation method by using ligand L and auxiliary ligands. CHN, EDAX, IR, NMR, TG-DTG, DR, UV–vis, and PL spectroscopic techniques and antioxidant analysis are employed. Potential candidate for display devices, laser devices, and antimicrobial agents.

Versatile Pt(II) Pyrazolate Complexes: Emission Tuning via Interplay of Chelate Designs and Stacking Assemblies.

Three homoleptic Pt(II) metal complexes [Pt(imPz)2] (1), [Pt(imiz)2] (2), and [Pt(imMz)2] (3) were synthesized from the treatment of Pt(DMSO)2Cl2 and functional imidazolyl pyrazole in refluxing tetrahydrofuran (THF). Alternatively, the heteroleptic Pt(II) complexes [Pt(imPz)(fppz)] (4), [Pt(imiz)(fppz)] (5), and [Pt(imMz)(fppz)] (6) were obtained from the treatment of a common intermediate [Pt(fppzH)Cl2] with a corresponding imidazolyl chelate. Pt(II) complexes 1, 2, and 5 were studied by single-crystal X-ray diffraction to reveal the corresponding packing arrangement in their crystal lattices, among which both homoleptic complexes 1 and 2 formed monomeric species, while heteroleptic 5 aligned as a dimer with a nonbonding Pt···Pt contact of 3.574 Å. Subsequent photophysical examinations showed that the homoleptic 1-3 and heteroleptic 4-6 exhibited the structured sky-blue ππ* emission and structureless light-green-emitting metal-metal-to-ligand charge transfer (MMLCT) emission in the solid state, respectively. A shortened Pt···Pt interaction of approximately 0.34-0.35 nm was confirmed in thin films of all heteroleptic Pt(II) complexes 4-6 by grazing-incidence X-ray diffraction (GIXD) analyses. Finally, Pt(II) complex 6 was employed as a dopant in the fabrication of organic light-emitting diode (OLED) devices with varied doping ratios, among which OLEDs with only 1 wt % 6 in the SimCP host exhibited a maximum external quantum efficiency (EQE) of 5.8% and CIEx,y values of 0.20, 0.31. In contrast, OLEDs using a nondoped architecture (i.e., 100% of 6 in the emitting layer (EML)) achieved a maximum EQE of 26.8%, current efficiency (CE) of 91.7 cd A-1, and power efficiency (PE) of 80.1 lm·W-1 and CIEx,y values of 0.41, 0.55, manifesting their versatility in various degrees of stacking assemblies and hence facile color-tuning capability on OLEDs.

Highly Transparent, Highly Thermally Stable Nanocellulose/Polymer Hybrid Substrates for Flexible OLED Devices

Flexible organic light-emitting diode (OLED) devices based on polymer substrates have attracted worldwide attention. However, the current OLED polymer substrates are limited due to weak thermal stability, which is not compatible with the high temperature in OLED fabrication. Here, we developed a novel nanocellulose/polyarylate (PAR) hybrid polymer substrate with both high transparency and excellent thermal properties. Benefiting from the nanometer scale of the cellulose nanofibrils (CNFs) and the efficient interfacial interaction with PAR, the substrate exhibited greatly improved thermal stability, with a glass transition temperature of 192 °C, the thermal decomposition temperature of 501 °C, and upper operating temperature up to over 220 °C. Meanwhile, the hybrid substrate exhibits outstanding mechanical properties. Notably, no apparent transparency loss was observed after the CNF addition, and the hybrid substrate maintains a high transmittance of 85% and a low haze of 1.75%@600 nm. Moreover, OLED devices fabricated on the hybrid substrates exhibit a much improved optoelectrical performance than that of the devices fabricated on the conventional poly(ethylene terephthalate) (PET) substrates. We anticipate this research will open up a new route for fabricating flexible high-performance OLEDs.

Rational Design of Axially Chiral Platinabinaphthalenes with Aggregation-Induced Emission for Red Circularly Polarized Phosphorescent Organic Light-Emitting Diodes.

Circularly polarized luminescent (CPL) materials have received a lot of interest due to their potential applications in next-generation displays. However, the development of easily accessible red circularly polarized phosphorescent emitters for practical organic light-emitting diodes fabrication remains a grand challenge. In this paper, we report a new family of CPL-active platinum complexes based on the binaphthalene chiral platform. These axially chiral platinabinaphthalenes were facile synthesized by directly incorporating platinum(II) into the π-conjugated backbone of a commercially available enantiopure binaphthalene derivate. These complexes exhibit aggregation-induced circularly polarized phosphorescence enhancement with high quantum yields of up to 66% and luminescence dissymmetry factors of around 2.6 × 10-3. Moreover, solution-processable circularly polarized organic light-emitting diodes (CPOLEDs) using these complexes as emitters show good performance with the maximum luminance of up to 3500 cd m-2 and dissymmetry factor values of around 1.0 × 10-3. These findings by the rational design of axially chiral platinabinaphthalenes are important for the development of high-performance CPL complexes for CPOLEDs.