Boosting Efficiency of Near‐Infrared Organic Light‐Emitting Diodes with Os(II)‐Based Pyrazinyl Azolate Emitters

Tremendous effort has been devoted to developing novel near‐infrared (NIR) emitters and to improving the performance of NIR organic light‐emitting diodes (OLEDs). Os(II) complexes are known to be an important class of NIR electroluminescent materials. However, the highest external quantum efficiency achieved so far for Os(II)‐based NIR OLEDs with an emission peak wavelength exceeding 700 nm is still lower than 3%. A new series of Os(II) complexes ( 1 – 4 ) based on functional pyrazinyl azolate chelates and dimethyl(phenyl)phosphane ancillaries is presented. The reduced metal‐to‐ligand charge transfer (MLCT) transition energy gap of pyrazinyl units in the excited states results in efficient NIR emission for this class of metal complexes. Consequently, NIR OLEDs based on 1 – 4 show excellent device performance, among which complex 4 with a triazolate fragment gives superior performance with maximum external quantum efficiency of 11.5% at peak wavelength of 710 nm, which represent the best Os(II)‐based NIR‐emitting OLEDs with peak maxima exceeding 700 nm.     

Molecularly Engineered Near‐Infrared Light‐Emitting Electrochemical Cells

The development of near‐infrared (NIR) luminescent materials has emerged as a promising research field with important applications in solid‐state lighting (SSL), night‐vision‐readable displays, and the telecommunication industry. Over the past two decades, remarkable advances in the development of light‐emitting electrochemical cells (LECs) have stunned the SSL community, which has in turn driven the quest for new classes of stable, more efficient NIR emissive molecules. In this review, an overview of the state of the art in the field of near‐infrared light‐emitting electrochemical cells (NIR‐LEC) is provided based on three families of emissive compounds developed over the past 25 years: i) transition metal complexes, ii) ionic polymers, and iii) host–guest materials. In this context, ionic and conductive emitters are particularly attractive since their emission can be tuned via molecular design, which involves varying the chemical nature and substitution pattern of their ancillary ligands. Herein, the challenges and current limitations of the latter approach are highlighted, particularly with respect to developing NIR‐LECs with high external quantum efficiencies. Finally, useful guidelines for the discovery of new, efficient emitters for tailored NIR‐LEC applications are presented, together with an outlook towards the design of new NIR‐SSL materials.     

Near‐Infrared Light‐Emitting Diodes (LEDs) Based on Poly(phenylene)/Yb‐tris(β‐Diketonate) Complexes

Near‐infrared‐emitting electroluminescent (EL) devices using blue‐light‐emitting polymers blended with the Yb complexes Yb(DBM)₃phen (DBM = dibenzoylmethane), Yb(DNM)₃phen (DNM = dinaphthoylmethane), and Yb(TPP)L(OEt) (L(OEt) = [(C₅H₅)Co{P(O)Et₂}₃]^–) have been studied. EL devices composed of Yb(DNM)₃phen blended with PPP‐OR11 showed enhanced near‐IR output at 977 nm when compared to those fabricated with Yb(DBM)₃phen/PPP‐OR11 blends. The maximum near‐IR external efficiencies of the devices with Yb(DBM)₃phen and Yb(DNM)₃phen are, respectively, 7 × 10^–5 (at 6 V and at 0.81 mA mm^–2) and 4 × 10^–4 (at 7 V, and 0.74 mA mm^–2). The optimal blend composition for EL device performance consisted of PPP‐OR11 blended with 10–20 mol‐% Yb(DNM)₃phen. A device fabricated using Yb‐(TPP)L(OEt)/PPP‐OR11 showed significantly enhanced near‐IR output efficiency, and future efforts will focus on devices fabricated using porphyrin‐based materials.     

Organic Light‐Emitting Diode Sensing Platform: Challenges and Solutions

The organic light‐emitting diode (OLED)‐based sensing platform is gaining momentum due to unique attributes of the compact OLEDs that are used as excitation sources. This paper, however, points to issues related to this sensing platform that will affect many (bio)chemical sensing applications, in particular in photoluminescence (PL)‐based sensors operated in the advantageous time domain, where pulsed OLEDs are utilized. The issues are related to the post‐pulse electroluminescence (EL) profile, i.e., transient EL, which depends on the OLED materials and structure, and to the long‐wavelength tail of the typically broad‐band EL spectrum. Depending on materials and device structure, the transient EL may exhibit spikes peaking at ～100–200 ns and μs‐long tails. As shown, these interfere with the determination of PL decay times (that are related to analyte concentrations) of sensing elements. The results also indicate that the long‐wavelength tail of the EL spectrum contributes to the interfering post‐pulse μs‐long EL tail. Hence, it is shown that the choice of OLED materials, the use of microcavity (μC) OLEDs with tunable, narrower EL bands, and the use of UV OLEDs alleviate these issues, resulting in more reliable data analysis. Furthermore, a 2‐D uniform 2 μm‐pitch microlens array that was previously used for improving light extraction from the OLEDs (J.‐M. Park et al., Optics Express 2011 , 19, A786) is used for directional PL scattering toward the photodetector, which leads to a ～2.1–3.8 fold enhancement of the PL signal. This behavior is shown for oxygen sensing, which is the basis for sensing of bioanalytes such as glucose, lactate, ethanol, cholesterol, and uric acid.     

High‐Efficiency Near‐Infrared Fluorescent Organic Light‐Emitting Diodes with Small Efficiency Roll‐Off: A Combined Design from Emitters to Devices

The simultaneous realization of high quantum yield and exciton utilizing efficiency (η_r) is still a formidable challenge in near‐infrared (NIR) fluorescent organic light‐emitting diodes (FOLEDs). Here, to achieve a high quantum yield, a novel NIR dye, 4,9‐bis(4‐(diphenylamino)phenyl)‐naphtho[2,3‐c ][1,2,5]selenadiazole, is designed and synthesized with a large highest occupied molecular orbital/lowest unoccupied molecular orbital overlap and an aggregation‐induced emission property, which demonstrates a high photoluminescence quantum yield of 27% at 743 nm in toluene and 29% at 723 nm in a blend film. For a high η_r, an orange‐emitting thermally activated delayed fluorescent material, 1,2‐bis(9,9‐dimethyl‐9,10‐dihydroacridine)‐4,5‐dicyanobenzene, is chosen as the sensitizing host to harvest triplet excitons in devices. The optimized devices achieve a good η_r of 45.7% and a high external quantum efficiency up to 2.65% at 730 nm, with a very small efficiency roll‐off of 2.41% at 200 mA cm^?2, which are among the most efficient values for NIR‐FOLEDs over 700 nm. The effective utilization of triplet excitons via the thermally activated delayed fluorescence‐sensitizing host will pave a way to realize high‐efficiency NIR‐FOLEDs with small efficiency roll‐off.     

Efficient Near‐Infrared Emission by Adjusting the Guest–Host Interactions in Thermally Activated Delayed Fluorescence Organic Light‐Emitting Diodes

Thermally activated delayed fluorescence materials can effectively achieve high efficiency by harvesting singlet and triplet excitons in organic light‐emitting diodes (OLEDs). However, the choice of host material has a huge impact on the efficiency of the device, especially for the near‐infrared (NIR) luminescent material. In this contribution, a series of host materials are used to match the thermally activated delayed fluorescence emitter, 3,4‐bis(4‐(diphenylamino)phenyl)acenaphtho[1,2‐b]pyrazine‐8,9‐dicarbonitrile (APDC‐DTPA), for fabricating NIR OLEDs. All the host materials have the higher triplet energy than that of APDC‐DTPA. As the organometallic compound of Zn(BTZ)₂ has relatively stronger dipole moment, the electroluminescence spectral peak of doped device shows strong bathochromic shift exceeding 700 nm and changes with doping concentration. Finally, the extremely high external quantum efficiency of 7.8% (with 10 wt% of doping concentration) and 5.1% (with 20 wt% of doping concentration) are achieved with the emission peaks of 710 and 728 nm, respectively, which are superior to that of the device based on the other host materials. The approach is feasible to achieve bathochromic shift and highly efficient fluorescent OLEDs.     

Efficient Near‐Infrared Light‐Emitting Diodes based on In(Zn)As–In(Zn)P–GaP–ZnS Quantum Dots

Near‐infrared (NIR) lighting plays an increasingly important role in new facial recognition technologies and eye‐tracking devices, where covert and nonvisible illumination is needed. In particular, mobile or wearable gadgets that employ these technologies require electronic lighting components with ultrathin and flexible form factors that are currently unfulfilled by conventional GaAs‐based diodes. Colloidal quantum dots (QDs) and emerging perovskite light‐emitting diodes (LEDs) may fill this gap, but generally employ restricted heavy metals such as cadmium or lead. Here, a new NIR‐emitting diode based on heavy‐metal‐free In(Zn)As–In(Zn)P–GaP–ZnS quantum dots is reported. The quantum dots are prepared with a giant shell structure, enabled by a continuous injection synthesis approach, and display intense photoluminescence at 850 nm with a high quantum efficiency of 75%. A postsynthetic ligand exchange to a shorter‐chain 1‐mercapto‐6‐hexanol (MCH) affords the QDs with processability in polar solvents as well as an enhanced charge‐transport performance in electronic devices. Using solution‐processing methods, an ITO/ZnO/PEIE/QD/Poly‐TPD/MoO₃/Al electroluminescent device is fabricated and a high external quantum efficiency of 4.6% and a maximum radiance of 8.2 W sr^?1 m^?2 are achieved. This represents a significant leap in performance for NIR devices employing a colloidal III–V semiconductor QD system, and may find significant applications in emerging consumer electronic products.     

Highly Efficient Solid‐State Near‐Infrared Emitting Material Based on Triphenylamine and Diphenylfumaronitrile with an EQE of 2.58% in Nondoped Organic Light‐Emitting Diode

The development of efficient near‐infrared (NIR) emitting material is of current focus. Donor–acceptor (D–A) architecture has been proved to be an effective strategy to obtain narrow energy gap. Herein, a D–A‐type NIR fluorescent compound 2,3‐bis(4′‐(diphenylamino)‐[1,1′‐biphenyl]‐4‐yl)fumaronitrile (TPATCN) is synthesized and fully characterized. As revealed by theoretical calculations and photophysical experiments, TPATCN exerts the advantages of the relatively large dipole moment of the charge transfer state and a certain degree of orbital overlap of the local excited state. A highly mixed or hybrid local and charge transfer excited state might occur to simultaneously achieve both a large fraction of singlet formation and a high quantum efficiency in D–A system. TPATCN exhibits strong NIR fluorescence with the corresponding thin film quantum efficiency of 33% and the crystal efficiency of 72%. Remarkably, the external quantum efficiency of nondoped NIR organic light‐emitting diode (OLED) reaches 2.58% and remains fairly constant over a range of 100–300 mA cm^?2, which is among the best results for NIR OLEDs reported so far.     

Simplified Bilayer White Phosphorescent Organic Light‐Emitting Diodes

We report on highly efficient blue, orange, and white phosphorescent organic light‐emitting diodes consisting only two organic layers. Hole transporting 4, 4,’ 4”‐tris (N‐carbazolyl)triphenylamine (TcTa) and electron transporting 2‐(diphenylphosphoryl) spirofluorene (SPPO1) are used as an emitting host for orange light‐emitting bis(3‐benzothiazol‐2‐yl‐9‐ethyl‐9H‐carbazolato) (acetoacetonate) iridium ((btc)2(acac)Ir) and blue light‐emitting iridium(III)bis(4,6‐difluorophenyl‐pyridinato‐N,C2’) picolinate (FIrpic) dopant, respectively. Combining these two orange and blue light‐emitting layers, we successfully demonstrate highly efficient white PHOLEDs while maintaining Commission internationale de l'éclairage coordinates of (, ). Accordingly, we achieve a maximum external quantum, current, and power efficiencies of 12.9%, 30.3 cd/A, and 30.0 lm/W without out‐coupling enhancement.     

Improved Performance of White Phosphorescent Organic Light‐Emitting Diodes through a Mixed‐Host Structure

Highly efficient white phosphorescent organic light‐emitting diodes with a mixed‐host structure are developed and the device characteristics are studied. The introduction of a hole‐transport‐type host (N, N’‐dicarbazolyl‐3‐3‐benzen (mCP)) into an electron‐transport‐type host (m‐bis‐(triphenylsilyl)benzene (UGH3)) as a mixed‐host emissive layer effectively achieves higher current density and lower driving voltage. The peak external quantum and power efficiency with the mixed‐host structure improve up to 18.9% and 40.9 lm/W, respectively. Moreover, this mixed‐host structure device shows over 30% enhanced performance compared with a single‐host structure device at a luminance of 10,000 cd/m² without any change in the electroluminescence spectra.     

Novel Heterolayer Organic Light‐Emitting Diodes Based on a Conjugated Dendrimer

We demonstrate a novel organic light‐emitting diode (LED) heterolayer structure that contains a conjugated dendrimer as the light‐emitting molecule. The LED was prepared by spin‐coating two dendrimer layers from the same solvent. The device consists of a graded bilayer structure formed from a neat dendrimer film covered with a film consisting of the same dendrimer but doped with the electron‐transporting material 2‐(4‐biphenylyl)‐5‐phenyl‐1,3,4‐oxadiazole (PBD). In this device, the heterojunction interface present in conventional bilayer organic light‐emitting diodes is eliminated, and is replaced by a graded interlayer. By optimizing the concentration of PBD in the dendrimer, a peak electroluminescence (EL) external quantum efficiency of 0.16 % at 600 cd m^–2 was obtained. The EL quantum efficiency is significantly enhanced in comparison with devices based on a single layer, a conventional bilayer, and a single‐layer doped with PBD. The EL quantum efficiency is a factor of eight larger than that of a conventional bilayer LED made with the conjugated dendrimer as the emissive layer and poly(methylmethacrylate) (PMMA) doped with PBD as the electron‐transporting layer. The best blended device exhibited only one third of the efficiency of the graded device. The improvement in the operating characteristics of the graded device is attributed to the efficient device structure, in which exciton formation is improved by a graded doping profile of electron‐ and hole‐transporting components.     

Tunable Near‐Infrared Organic Nanowire Nanolasers

Organic semiconductor nanowires have inherent advantages, such as amenability to low‐cost, low‐temperature processing, and inherent four‐level energy systems, which will significantly contribute to the organic solid‐state lasers (OSSLs) and miniaturized laser devices. However, the realization of near‐infrared (NIR) organic nanowire lasers is always a big challenge due to the difficultly in fabrication of organic nanowires with diameters of ≈100 nm and material issues such as low photoluminescence quantum efficiency in the red‐NIR region. What is more, the achievement of wavelength‐tunable OSSLs has also encountered enormous challenge. This study first demonstrates the 720 nm NIR lasing with a low lasing threshold of ≈1.4 µJ cm^?2 from the organic single‐crystalline nanowires, which are self‐assembled from small organic molecules of (E )‐3‐(4‐(dimethylamino)‐2‐methoxyphenyl)‐1‐(1‐hydroxynaphthalen‐2‐yl)prop‐2‐en‐1‐one through a facile solution‐phase growth method. Notably, these individual nanowires' Fabry–Pérot cavity can alternatively provide the red‐NIR lasing action at 660 or 720 nm from the 0–1 or 0–2 radiative transition channels, and the single (660 or 720 nm)/dual‐wavelength (660 and 720 nm) laser action can be achieved by modulating the length of these organic nanowires due to the intrinsic self‐absorption. These easily‐fabricated organic nanowires are natural laser sources, which offer considerable promise for coherent light devices integrated on the optics microchip.     

Dependence of Light‐Emitting Characteristics of Blue Phosphorescent Organic Light‐Emitting Diodes on Electron Injection and Transport Materials

We investigate the light‐emitting performances of blue phosphorescent organic light‐emitting diodes (PHOLEDs) with three different electron injection and transport materials, that is, bathocuproine(2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline) (Bphen), 1,3,5‐tri(m‐pyrid‐3‐yl‐phenyl)benzene (Tm3PyPB), and 2,6‐bis(3‐(carbazol‐9‐yl)phenyl)pyridine (26DCzPPy), which are partially doped with cesium metal. We find that the device characteristics are very dependent on the nature of the introduced electron injection layer (EIL) and electron transporting layer (ETL). When the appropriate EIL and ETL are combined, the peak external quantum efficiency and peak power efficiency improve up to 20.7% and 45.6 lm/W, respectively. Moreover, this blue PHOLED even maintains high external quantum efficiency of 19.6% and 16.9% at a luminance of 1,000 cd/m² and 10,000 cd/m², respectively.     

Emission Area Patterning of Organic Light‐Emitting Diodes (OLEDs) via Printed Dielectrics

Solution‐processibility is one of the distinguished traits of organic light‐emitting diodes (OLEDs) compared to existing solid‐state LED technologies. It allows new opportunities which can simplify the fabrication and potentially reduce the cost of manufacturing process. Emission area patterning is one of the crucial fabrication steps and it usually involves subtractive methods, such as photolithography or etching. Here, printing techniques are used to pattern the emission area of blade‐coated OLED layers. The print qualities of a number of printing schemes are characterized and compared. Spray coating and screen printing are used to deposit dielectrics with desired patterns on the OLED layers. At luminance of 1000 cd m⁻² the OLEDs patterned using spray‐coated and screen‐printed dielectric show current density of 8.2 and 10.1 mA cm⁻², external quantum efficiency (EQE) of 2.1% and 2.1%, and luminous efficacy of 5.5 and 6.3 lm W⁻¹, respectively. The OLED characteristics and features of each printing scheme in depositing the dielectric layer are discussed. The printing methods are further applied to demonstrate displays with complex shapes and a seven‐segment display.     

Squaraine‐Doped Functional Nanoprobes: Lipophilically Protected Near‐Infrared Fluorescence for Bioimaging

Hydrophobically stabilized near‐IR fluorescence from self‐assembled nanoprobes composed of amphiphilic poly(maleic anhydride‐alt‐octadec‐1‐ene) (PMAO) and lipophilized squaraine dopants is reported. From comparative studies with varying lipophilicity of squaraine dyes as well as of nanoparticulate polymer matrices, it is found that dual protection by simultaneous lipophilization of the dye‐polymer pair greatly improves the chemical stability of labile squaraine dyes, to produce efficient NIR fluorescence in physiological aqueous milieux. The surface properties of negatively charged PMAO nanoparticles are readily modified by coating with an amine‐rich cationic glycol chitosan with biofunctionality. Efficient cellular imaging and in vivo sentinel lymph node mapping with size and surface‐controlled nanoprobes demonstrate that lipophilic dual protection of NIR fluorescence and the underlying functional nanoprobe approach hold great potential for bioimaging applications.     

Self‐Assembling Azaindole Organogel for Organic Light‐Emitting Devices (OLEDs)

This study reports on the use of a self‐assembling organogel, 5‐(4‐nonylphenyl)‐7‐azaindole ( 1 ), as a new emitter in small‐molecule organic light emitting devices (OLEDs). The theoretical calculations along with the photophysical characterization studies suggest the coexistence of the monomer and dimer species at high concentration of compound 1 . The presence of this type of dimer (formed via H‐bonding) is responsible for the increased emission. However, the most notable feature is the 3D network of vastly interconnected fibers formed in the organogel that modifies the photophysical properties. Based on this, several OLED architectures are made in order to understand the mechanism involved in the electroluminescence (EL) behavior of 1 . Although the position of the EL spectra differs from that of the photoluminescence (PL) spectra, the trends observed in the device properties perfectly match with dimer formation. In this framework a better device performance is associated to a higher efficiency of dimer formation, which optimizes in the OLED prepared from the organogel. Therefore, these results show that the rational combination of a moiety showing a strong PL intensity increased upon aggregation with organogel properties is an efficient strategy to create alternative emitters for OLED devices.     

Organic Light‐Emitting Diodes: Organic Light‐Emitting Diodes with a White‐Emitting Molecule: Emission Mechanism and Device Characteristics (Adv. Funct. Mater. 4/2011)

The unique and unprecedented electroluminescence behavior of the white‐emitting molecule 3‐(1‐(4‐(4‐(2‐(2‐hydroxyphenyl)‐4,5‐diphenyl‐1H‐imidazol‐1‐yl)phenoxy)phenyl)‐4,5‐diphenyl‐1H‐imidazol‐2‐yl)naphthalen‐2‐ol (W1), fluorescence emission from which is controlled by the excited‐state intramolecular proton transfer (ESIPT) is investigated. W1 is composed of covalently linked blue‐ and yellow‐color emitting ESIPT moieties between which energy transfer is entirely frustrated. It is demonstrated that different emission colors (blue, yellow, and white) can be generated from the identical emitter W1 in organic light‐emitting diode (OLED) devices. Charge trapping mechanism is proposed to explain such a unique color‐tuned emission from W1. Finally, the device structure to create a color‐stable, color reproducible, and simple‐structured white organic light‐emitting diode (WOLED) using W1 is investigated. The maximum luminance efficiency, power efficiency, and luminance of the WOLED were 3.10 cd A^?1, 2.20 lm W^?1, 1 092 cd m^?2, respectively. The WOLED shows white‐light emission with the Commission Internationale de l′Eclairage (CIE) chromaticity coordinates (0.343, 0.291) at a current level of 10 mA cm^?2. The emission color is high stability, with a change of the CIE chromaticity coordinates as small as (0.028, 0.028) when the current level is varied from 10 to 100 mA cm^?2.     

Interlayer Engineering with Different Host Material Properties in Blue Phosphorescent Organic Light‐Emitting Diodes

We investigated the light‐emitting performances of blue phosphorescent organic light‐emitting diodes, known as PHOLEDs, by incorporating an N,N’‐dicarbazolyl‐3,5‐benzen interlayer between the hole transporting layer and emitting layer (EML). We found that the effects of the introduced interlayer for triplet exciton confinement and hole/electron balance in the EML were exceptionally dependent on the host materials: 9‐(4‐tert‐butylphenyl)‐3.6‐bis(triphenylsilyl)‐9H‐carbazole, 9‐(4‐tert‐butylphenyl)‐3.6‐ditrityl‐9H‐carbazole, and 4,4’‐bis‐triphenylsilanyl‐biphenyl. When an appropriate interlayer and host material were combined, the peak external quantum efficiency was greatly enhanced by over 21 times from 0.79% to 17.1%. Studies on the recombination zone using a series of host materials were also conducted.     

Built‐In Haze Glass‐Fabric Reinforced Siloxane Hybrid Film for Efficient Organic Light‐Emitting Diodes (OLEDs)

Substrates with high transmittance and high haze are desired for increasing the light outcoupling efficiency of organic light‐emitting diodes (OLEDs). However, most of the polymer films used as substrate have high transmittance and low haze. Herein, a facile route to fabricate a built‐in haze glass‐fabric reinforced siloxane hybrid (GFRH) film having high total transmittance (≈89%) and high haze (≈89%) is reported using the scattering effect induced by refractive index contrast between the glass fabric and the siloxane hybrid (hybrimer). The hybrimer exhibiting large refractive index contrast with the glass fabric is synthesized by removing the phenyl substituents. Besides its optical properties, the hazy GFRH films exhibit smooth surface (R_sq = 0.2 nm), low thermal expansion (13 ppm °C⁻¹), high chemical stability, and dimensional stability. Owing to the outstanding properties of the GFRH film, OLED is successfully fabricated onto the film exhibiting 74% external quantum efficiency enhancement. The hazy GFRH's unique optical properties, excellent thermal stability, outstanding dimensional stability, and the ability to perform as a transparent electrode enable them as a wide ranging substrate for the flexible optoelectronic devices.