Use In Organic Light-emitting Diodes Research Articles

A stable and true-blue emissive hexacoordinate Si(IV) N-heterocyclic carbene complex and its use in organic light-emitting diodes.

A neutral hexacoordinate Si(IV) complex containing two tridentate N-heterocyclic carbene ligands is synthesised and characterized by X-ray crystallography, optical spectroscopy, electrochemistry and computational methods. The stable compound exhibits remarkable deep-blue photoluminescence particularly in the solid state, which enables its use as an electroluminescent material in organic light-emitting diodes.

Lithographic convex pattern as a wavelength-independent light extraction structure for efficient organic light-emitting diodes

Micrometer-scale ITO convex matrixes are used to improve the light extraction efficiency of OLEDs. Through optical manipulation, this scheme enhances the EQEs by up to 34.5% for blue, 22.4% for green, 28.6% for red, and 31.3% for NIR OLEDs.

Tailored Near‐Infrared Luminescent Materials with Fast Reverse Intersystem Crossing for Efficient Deep‐Red/Near‐Infrared Organic Light‐Emitting Diodes with High Efficiencies and Small Roll‐Offs

AbstractRobust organic luminescent materials radiating deep‐red/near‐infrared (DR/NIR) light are highly desired in organic light‐emitting diodes (OLEDs). However, most reported DR/NIR materials show only moderate electroluminescence (EL) efficiencies accompanied by rock‐ribbed efficiency roll‐offs, which immensely limit their practical use in OLEDs. In this work, two tailor‐made NIR luminescent materials with thermally activated delayed fluorescence (TADF) property are developed based on rigid and strong electron‐accepting core of 2,7‐bis(4‐cyanophenyl)dibenzo[a,c]phenazine‐11,12‐dicarbonitrile and electron donors of triphenylamine or N,N‐di([1,1′‐biphenyl]‐4‐yl)phenylamine. They exhibit high photoluminescence quantum yields, fast reverse intersystem crossing processes and large horizontal dipole ratios. Their nondoped OLEDs show NIR lights with maximum external quantum efficiencies (ηext,maxS) of 0.59% at 856 nm and 1.14% at 850 nm. And their doped OLEDs radiate DR to NIR lights (646–737 nm) and attain superb ηext,maxs of up to 30.3% at 646 nm with small efficiency roll‐offs. The state‐of‐the‐art EL performances of both NIR TADF materials demonstrate their high potential as emitters in OLEDs and the insights gained in this work will enlighten the further development of efficient NIR luminescent materials.

Sustainable use of organic light-emitting diode (OLED) and liquid crystal display (LCD) glass wastes as cement replacement in concrete

With the persistently increasing use of electronic and display devices, there has been a significant surge in OLED and LCD glass wastes. One way to efficiently utilize such wastes is incorporating them in mortars or concrete as supplementary cementing material (SCM) by replacing a part of cement. The use of SCMs is customary in contemporary concretes. Different industrial by-products and waste materials have been investigated earlier for such applications. In this paper, the use of OLED and LCD glass wastes as a partial replacement for cement binders has been reviewed. Mechanical properties, durability attributes, microstructural characteristics, and structural performance of the cementitious composites incorporating LCD and OLED powders have been assessed. It has been concluded that adding LCD and OLED leads to improved mechanical strength after 28-day age. The modified composites have better durability, enhanced microstructure, lower porosity, and superior overall performance. The enhanced mechanical and durability properties with LCD and OLED addition is attributed to the pozzolanic activity due to high amounts of alumina and amorphous silica. Incorporating LCD and OLED wastes in concrete as a partial cement replacement can also reduce environmental burden and concrete cost, thus promoting sustainable development.

Tuning the emission and exciton utilization mechanisms of pyrazine-based multi-carbazole emitters and their use in organic light-emitting diodes

We report there pyridine-based emitters where the efficiency of the TADF differs significantly, leading to divergent OLED efficiencies.

Investigating the role of heavy metal complexes as efficient triplet emitters for OLED applications

We present here two iridium metal complexes, namely complex A and B, based on two different cyclometalated ligands 2-(4-fluorophenyl) H-imidazo [1,2-a] pyridine (pip), and 2-phenylpyrdine (ppy), and an ancillary ligand acetylacetone (acac) ligands for their use in organic light-emitting diodes. The structural and purity analysis of the complexes have been estimated via NMR spectroscopy and elemental analysis. UV–visible and photoluminescence spectroscopy were used to investigate the optical characteristics of the molecules. Both complexes exhibit nearly identical absorption spectra comprising of sharp bands in the ultraviolet region i.e., 245 nm–300 nm for the complex A and 232 nm and 304 nm for the complex B which are assigned to spin allowed π – π* transitions of cyclometalated (C^N) ligand and relatively weaker bands in the lower energy range i.e., 300 nm to 450 nm for the complex A and 403 nm and 457 nm for the complex B, which are ascribed to metal-ligand charge transfer (MLCT) transitions. Upon excitation, both of the complexes reveal green photoluminescence with their emission peaks at 535 nm and 520 nm for the complexes A and B, respectively. The thermal analysis of both complexes performed under a nitrogen atmosphere exhibit excellent thermal stability up to 300 °C. The electrochemical behavior of the complexes has been analyzed through cyclic voltammetry measurements and the corresponding values of HOMO/LUMO levels were found to be 5.09/2.52 eV for the complex A and 5.21/2.92 eV for the complex B. Furthermore, the diffusion of ions at the surface of electrodes and the corresponding values of diffusion coefficients have been estimated using the Randles-Sevcik equation.

Highly Efficient and Stable Organic Light-Emitting Diodes with Inner Passivating Hole-Transfer Interlayers of Poly(amic acid)-Polyimide Copolymer.

Ensuring the long‐term stability of high‐performance organic light‐emitting diodes (OLEDs) has remained a great challenge due to their limited lifetime and durability. Herein, a novel functional interlayer consisting of a poly(amic acid)‐polyimide copolymer is introduced for use in OLEDs. It is shown that an OLED sample with a polyimide‐copolymer interlayer exhibits high peak brightness of nearly 96 000 cd m−2 and efficiency of ≈92 cd A−1, much higher than those (≈73 000 cd m−2 and ≈83 cd A−1) of a well‐organized reference OLED. Moreover, the growth of dark spots is strongly suppressed in the sample OLED and the device lifetime is extended considerably. Further, highly stable and uniform large‐area OLEDs are successfully produced when using the interlayer. These improvements are ascribed not only to the excellent film‐forming and hole‐transferring properties but also to the inner passivating capability of the polyimide‐copolymer interlayer. The results here suggest that the introduction of an inner passivating/encapsulating hole‐transferable polyimide‐copolymer interlayer together with conventional external encapsulation technology represents a promising breakthrough that enhances the longevity of high‐performance next‐generation OLEDs.

Highly twisted α-diketone-based thermally activated delayed fluorescence emitters and their use in organic light-emitting diodes

We have designed a highly twisted small thermally activated delayed fluorescence (TADF) emitter PXZ-α-DK based on an α-diketone (α-DK) as a strong acceptor and phenoxazine (PXZ) as a strong donor to obtain red-shifted emission in comparison with the equivalent α-diketone linked to 9,9-dimethyl-9,10-dihydroacridine (DMAC). The PXZ-α-DK shows emission at 586 nm and DMAC-α-DK shows emission at 548 nm in 1,3-bis(N-carbazolyl)benzene (mCP) host at 1.5 wt% doping of the emitter, with short-delayed lifetimes of 6.9 μs for PXZ-α-DK and 7.6 μs for DMAC-α-DK. Organic light-emitting diodes (OLEDs) fabricated using these emitters show green electroluminescence at 555 nm for DMAC-α-DK, with a maximum external quantum efficiency, EQEmax, of 6.3%, and orange electroluminescence at 585 nm for PXZ-α-DK, with an EQEmax of 0.8%. We corroborate the optoelectronic properties of these emitters with DFT calculations.

Emerging Biomedical Applications of Organic Light‐Emitting Diodes

AbstractAs solid‐state light sources based on amorphous organic semiconductors, organic light‐emitting diodes (OLEDs) are widely used in modern smartphone displays and TVs. Due to the dramatic improvements in stability, efficiency, and brightness achieved over the last three decades, OLEDs have also become attractive light sources for compact and “imperceptible” biomedical devices that use light to probe, image, manipulate, or treat biological matter. The inherent mechanical flexibility of OLEDs and their compatibility with a wide range of substrates and geometries are of particular benefit in this context. Here, recent progress in the development and use of OLEDs for biomedical applications is reviewed. The specific requirements that this poses are described and compared to the current state of the art, in particular in terms of the brightness, patterning, stability, and encapsulation of OLEDs. Examples from several main areas are then discussed in some detail: on‐chip sensing and integration with microfluidics, wearable devices for optical monitoring, therapeutic devices, and the emerging use in neuroscience for targeted photostimulation via optogenetics. The review closes with a brief outlook on future avenues to scale the manufacturing of OLED‐based devices for biomedical use.

Tris(5‐phenyl‐1H‐1,2,4‐triazolyl)iridium(III) Complex and Its Use in Blue Phosphorescent Organic Light‐Emitting Diodes to Provide an External Quantum Efficiency of up to 27.8%

AbstractAlthough studies have investigated various N‐heterocyclic‐based Ir(III)‐cyclometalates that exhibit high efficiency and electrochemical stability, blue‐phosphorescent Ir(III) complexes with further improved efficiency and blue‐emission purity are needed for use as emitters in organic light‐emitting diodes (OLEDs). Therefore, the preparation and comprehensive characterization of a phenyl‐1,2,4‐triazole‐based facial‐homoleptic iridium(III) complex, TzIr (fac‐tris(1‐(2,6‐diisopropylphenyl)‐3‐methyl‐5‐phenyl‐1H‐1,2,4 triazolyl)iridium(III)) is herein described. The complex shows a definite blue‐emission band (λem = 456 nm) in the solution and film states at ambient temperature. Moreover, this complex exhibits considerably high phosphorescent quantum efficiency (80%) and thermal stability (Td5 = 362 °C). Multilayer phosphorescent OLEDs using TzIr as the emitter and 3,3‐di(9H‐carbazol‐9‐yl)biphenyl (mCBP)/ 9‐(3′‐(9H‐carbazol‐9‐yl)‐5‐cyano‐[1,1′‐biphenyl]‐3‐yl)‐9H‐carbazole‐3‐carbonitrile (CNmCBPCN) as the mixed host are fabricated. The devices exhibit outstanding performance, with a high external quantum efficiency (27.8%), current efficiency (38.7 cd A−1), and CIEy (y coordinate value of the Commission Internationale de l'Eclairage color space) less than 0.18. The results of this study indicate that this novel 1,2,4‐triazole‐based dopant, TzIr, is a promising candidate for developing highly efficient blue‐phosphorescent emitters for use in OLEDs.

Thermally Assisted Fluorescent Polymers: Polycyclic Aromatic Materials for High Color Purity and White-Light Emission.

Thermally activated delayed fluorescence (TADF) sensitization of fluorescence is a promising strategy to improve the color purity and operational lifetime of conventional TADF organic light-emitting diodes (OLEDs). Here, we propose a new design strategy for TADF-sensitized fluorescence based on acrylic polymers with a pendant energy-harvesting host, a TADF sensitizer, and fluorescent emitter monomers. Fluorescent emitters were rationally designed from a series of homologous polycyclic aromatic amines, resulting in efficient and color-pure polymeric fluorophores capable of harvesting both singlet and triplet excitons. Macromolecular analogues of blue, green, and yellow fourth-generation OLED emissive layers were prepared in a facile manner by Cu(0) reversible deactivation radical polymerization, with emission quantum yields up to 0.83 in air and narrow emission bands with full width at half-maximum as low as 57 nm. White-light emission can easily be achieved by enforcing incomplete energy transfer between a deep blue TADF sensitizer and yellow fluorophore to yield a single white-emissive polymer with CIE coordinates (0.33, 0.39) and quantum yield 0.77. Energy transfer to the fluorescent emitters occurs at rates of 1-4 × 108 s-1, significantly faster than deactivation caused by internal conversion or intersystem crossing. Rapid energy transfer facilitates high triplet exciton utilization and efficient sensitized emission, even when TADF emitters with a low quantum yield are used as photosensitizers. Our results indicate that a broad library of untapped polymers exhibiting efficient TADF-sensitized fluorescence should be readily accessible from known TADF materials, including many monomers previously thought unsuitable for use in OLEDs.

The Leap from Organic Light-Emitting Diodes to Organic Semiconductor Laser Diodes

In recent years, organic light-emitting device technology has expanded from organic light-emitting diodes (OLEDs) to organic semiconductor laser diodes (OSLDs) with the progress of sophisticated mo...

Comparison of ophthalmic toxicity of light-emitting diode and organic light-emitting diode light sources

The use of organic light-emitting diodes (OLEDs) has rapidly increased in recent years. However, the effect of OLEDs on human health has not been studied yet. We investigated morphologic and functional changes after OLEDs exposure of human ocular cells, including corneal, conjunctival, lens, and retinal pigment epithelial cells, and mouse eyes. In corneal and conjunctival epithelial cells, the levels of reactive oxygen species production and interleukin-8 expression after white light-emitting diodes (LED) exposure were significantly greater than those after OLED exposure. Although no gross morphologic changes of the eyelid or cornea were found in LED- or OLED-exposed mice, oxidative stress on ocular surface was significantly increased, and the outer nuclear layer (ONL) was significantly shorter in both light-treated groups than the control group. Moreover, ONL thickness was significantly lower in the LED group than the OLED group. The electroretinography response was significantly lower in light exposure group, and there was significant difference between LED- and OLED-treated mice. Although OLED exhibits certain ocular toxicity, it can be less toxic to eyes than LED. The higher blue-wavelength energy of LED light might be the reason for its higher toxicity relative to OLED.

Getting the Right Twist: Influence of Donor–Acceptor Dihedral Angle on Exciton Kinetics and Singlet–Triplet Gap in Deep Blue Thermally Activated Delayed Fluorescence Emitter

Here, a novel deep blue emitter SBABz4 for use in organic light-emitting diodes (OLED) is investigated. The molecular design of the emitter enables thermally activated delayed fluorescence (TADF), which we examine by temperature-dependent time-resolved electroluminescence (trEL) and photoluminescence (trPL). We show that the dihedral angle between donor and acceptor strongly affects the oscillator strength of the charge transfer state alongside the singlet-triplet gap. The angular dependence of the singlet-triplet gap is calculated by time-dependent density functional theory (TD-DFT). A gap of 15 meV is calculated for the relaxed ground state configuration of SBABz4 with a dihedral angle between the donor and acceptor moieties of 86{\deg}. Surprisingly, an experimentally obtained energy gap of 72+/-5 meV can only be explained by torsion angles in the range of 70{\deg}-75{\deg}. Molecular dynamics (MD) simulations showed that SBABz4 evaporated at high temperature acquires a distribution of torsion angles, which immediately leads to the experimentally obtained energy gap. Moreover, the emitter orientation anisotropy in a host matrix shows an 80% ratio of horizontally oriented dipoles, which is highly desirable for efficient light outcoupling. Understanding intramolecular donor-acceptor geometry in evaporated films is crucial for OLED applications, because it affects oscillator strength and TADF efficiency.

Recent Advances of Exciplex-Based White Organic Light-Emitting Diodes

Recently, exciplexes have been actively investigated in white organic light-emitting diodes (WOLEDs), since they can be effectively functioned as (i) fluorescent or thermally activated delayed fluorescent (TADF) emitters; (ii) the hosts of fluorescent, phosphorescent and TADF dopants. By virtue of the unique advantages of exciplexes, high-performance exciplex-based WOLEDs can be achieved. In this invited review, we have firstly described fundamental concepts of exciplexes and their use in organic light-emitting diodes (OLEDs). Then, we have concluded the primary strategies to develop exciplex-based WOLEDs. Specifically, we have emphasized the representative WOLEDs using exciplex emitters or hosts. In the end, we have given an outlook for the future development of exciplex-based WOLEDs.

Mono substituted pyrenes as multifunctional materials for OLED: Analysis of the substituent effects on the charge transport properties using DFT methods

Abstract In this work, with a general interest to understand the charge transport properties of mono substituted pyrene molecules which can be used as multifunctional materials in Organic Light Emitting Diodes (OLED), we have analyzed the charge transport properties calculated from DFT methods for a number of the mono substituted pyrene molecules whose crystal structures have been obtained from Cambridge structural data base (CSD). The data for the analysis was obtained by optimizing the geometries of the molecules in gas phase and subsequently generating the frontier orbitals and calculation of reorganization energies; transfer integrals in the solid state for various path ways were also calculated. The drift mobilities were then estimated from this data. The reasons for the variation of the drift mobilities in this group are then analyzed and the pattern reported. This work would be useful for generating new efficient pyrene derivatives and for improving existing ones for use in OLED.

Highly efficient and spectrally stable white organic light-emitting diodes using new red heteroleptic Iridium(III) complexes

Highly efficient red phosphorescent iridium(III) complexes, (MPQ)2Ir(Pppy) and (MPQ)2Ir(TMSppy) based on 4-methyl-2-phenylquinoline (MPQ) as a cyclometalated main ligand and 2-[(1,1′-biphenyl)4-yl]pyridine (Pppy) or 2-[4-(trimethylsilyl)phenyl]pyridine (TMSppy) as a second cyclometalated ligand were synthesized for use in organic light-emitting diodes (OLEDs). The iridium(III) complex with MPQ and acetylacetone (acac) as an ancillary ligand, (MPQ)2Ir(acac), was also synthesized for comparison. The geometrical configuration of the heteroleptic complex (MPQ)2Ir(acac)is N,N-trans, whereas (MPQ)2Ir(Pppy) and (MPQ)2Ir(TMSppy)show all-facial geometries. Among the iridium(III) complexes, red OLEDs using the (MPQ)2Ir(Pppy) exhibited significantly more improved luminance and external quantum efficiency with markedly prevented concentration self-quenching at high doping concentration by introduction of Pppy. Moreover, a high maximum external quantum efficiency (EQEmax) of 28.0% with a high color-rendering index (CRI) of 81 was achieved in the white OLEDs (WOLED) fabricated using the bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) [FIrpic], tris(2-phenylpyridine)iridium(III) [Ir(ppy)3], and (MPQ)2Ir(Pppy), and it is one of the highest values among the reported three primary color component WOLED. Also, the WOLED fabricated using the (MPQ)2Ir(Pppy) showed excellent spectral stability at different voltages.

Recycling indium tin oxide (ITO) anodes for use in organic light-emitting diodes (OLEDs)

Electrodes in many thin-film optoelectronic devices are made of layers of indium tin oxide (ITO), which are suitably conductive, can be deposited by established methods, and are nearly transparent and colorless. Regrettably, indium is an uncommon element, its price continues to rise steeply, and producing ITO electrodes consumes a large fraction of the total energy needed to make thin-layer devices. Nevertheless, ITO electrodes are widely used in devices, both in research and industrial production. The growing commercial importance of these devices creates an urgent need for ways to ensure sustainability, such as by recovering ITO from devices no longer needed or even by recycling intact ITO electrodes. Recent work has shown that patterned ITO electrodes can be recovered from organic thin-film photovoltaic devices (OPVs) after simple exposure to water or dilute aqueous base, and then reused in new devices with essentially no decrease in performance. We now report that similar approaches can be used to recover and reuse ITO electrodes in organic light-emitting diodes (OLEDs), which are currently manufactured in much greater numbers than OPVs. Use of atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) to study the surfaces of ITO electrodes recovered from OLEDs revealed minor changes in morphology, but the performance of the recycled electrodes in new OLEDs proved to be virtually unchanged, even after multiple cycles of recovery and reuse.

Highly-flexible, ultra-thin, and transparent single-layer graphene/silver composite electrodes for organic light emitting diodes

Transparent conductive electrode (TCE) platforms are required in many optoelectronic devices, including organic light emitting diodes (OLEDs). To date, indium tin oxide based electrodes are widely used in TCEs but they still have few limitations in term of achieving flexible OLEDs and display techniques. In this paper, highly-flexible and ultra-thin TCEs were fabricated for use in OLEDs by combining single-layer graphene (SLG) with thin silver layers of only several nanometers in thickness. The as-prepared SLG + Ag (8 nm) composite electrodes showed low sheet resistances of 8.5 Ω/□, high stability over 500 bending cycles, and 74% transmittance at 550 nm wavelength. Furthermore, SLG + Ag composite electrodes employed as anodes in OLEDs delivered turn-on voltages of 2.4 V, with luminance exceeding 1300 cd m−2 at only 5 V, and maximum luminance reaching up 40 000 cd m−2 at 9 V. Also, the devices could work normally under less than the 1 cm bending radius.

Green and low-cost approach to modify the indium tin oxide anodes in organic light-emitting diodes by electrochemical treatment in NaCl aqueous solution

We demonstrate an environment-friendly, simple, and low energy cost approach as an alternative to conventional O2 plasma treatment to modify the surface of indium tin oxide (ITO) anodes for use in organic light-emitting diodes (OLEDs). ITO is electrochemically treated in NaCl aqueous solution. A chlorinated ITO (Cl-ITO) electrode with a work function of 5.41eV was obtained, which is 0.66eV higher than that of pre-cleaned ITO. The increase of work function is due to the anodic oxidation reactions occurred on the surface of ITO. The power dissipation is only ∼3mW in our approach, which is five orders of magnitude lower than that of O2 plasma treatment (∼100W). We fabricated the OLEDs with the configuration of Cl-ITO/NPB(35nm)/CBP:Ir(ppy)3 (15nm, 8wt%)/TPBi:Ir(ppy)3 (10nm, 8wt%)/TPBi (10nm)/Bphen (50nm)/Cs2CO3 (2nm)/Al (100nm), where NPB is N, Ń-di-1-naphthyl-N, Ń-diphenylbenzidine, CBP is 4′-bis(carbazol-9-yl)biphenyl, TPBi is 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole), Ir(ppy)3 is bis(3-phenylpyridine) iridium(III) and Bphen is 4,7-diphenyl-1,10-phenanthroline. A maximum power efficiency of 95.0lmW−1 and external quantum efficiency (EQE) of 24.2% were achieved, respectively, which was slightly higher than that of the OLED fabricated on O2-plasma-treated ITO (91.2lmW−1, EQE=23.1%).