Configuring device architecture with new solution-processable host for high performance low color-temperature OLEDs with ultra-low driving voltage

After the revelation of nonvisual lighting impact on human health, the lighting engineers are more concerned about the human benign light with low cost and high efficiency. The solution-processed fabrication technique with smart device engineering for high efficacy OLED devices is being anticipated to drastically reduce the fabrication cost leading to affordable desired end product. The co-host matrix could be a potential solution to improve device performance multi-folds with suitable band-gap engineering and most effective energy transfer from mixed host to guest. Here, rationally configured device architecture with two novel host materials possessing wide energy gap, high triplet energy, and excellent thermal and morphological stability resulted in highly-efficient solution-processed green, red and low color temperature (CT) OLEDs with sub-bandgap level driving voltage of 2.1 V i.e. record lowest within its own category. Perfect triplet energy match of our newly developed host materials with commercial p-type host m-MTDATA and the common phosphors enabled efficient energy transfer with very low energy loss led to high efficiency of resulting OLED. The resultant solution-processed triplet emitter based green and red OLED devices displayed a maximum efficacy of 75.0 and 30.2 lm W⁻¹ without using light extraction out-coupling techniques, respectively. The designed blue-hazard free 2048 K low CT OLED exhibited a η_PE of 44.5 lmW⁻¹, a η_CE of 47.6 cdA^-1 and an EQE of 20.7% as the maximum value, the highest known so far within its own category. The melatonin suppression sensitivity (MSS) of the engineered low CT OLED is only 1.0% to that of the 480 nm blue light, which is much safer compared to other light sources. The impact of our design engineering was established by fabricating 1 cm × 1 cm device area prototype.     

Versatile,Benzimidazole/Amine‐Based Ambipolar Compounds for Electroluminescent Applications: Single‐Layer,Blue, Fluorescent OLEDs,Hosts for Single‐Layer,Phosphorescent OLEDs

A series of compounds containing arylamine and 1,2‐diphenyl‐1H‐benz[d]imidazole moieties are developed as ambipolar, blue‐emitting materials with tunable blue‐emitting wavelengths, tunable ambipolar carrier‐transport properties and tunable triplet energy gaps. These compounds possess several novel properties: (1) they emit in the blue region with high quantum yields; (2) they have high morphological stability and thermal stability; (3) they are capable of ambipolar carrier transport; (4) they possess tunable triplet energy gaps, suitable as hosts for yellow‐orange to green phosphors. The electron and hole mobilities of these compounds lie in the range of 0.68–144 × 10^?6 and 0.34–147 × 10^?6 cm² V^?1 s^?1, respectively. High‐performance, single‐layer, blue‐emitting, fluorescent organic light‐emitting diodes (OLEDs) are achieved with these ambipolar materials. High‐performance, single‐layer, phosphorescent OLEDs with yellow‐orange to green emission are also been demonstrated using these ambipolar materials, which have different triplet energy gaps as the host for yellow‐orange‐emitting to green‐emitting iridium complexes. When these ambipolar, blue‐emitting materials are lightly doped with a yellow‐orange‐emitting iridium complex, white organic light‐emitting diodes (WOLEDs) can be achieved, as well by the use of the incomplete energy transfer between the host and the dopant.     

Simultaneously enhancement of quantum efficiency and color purity by molecular design in star-shaped solution-processed blue emitters

A series of fluorene-free bipolar star-shaped molecules, Sn-Cz-OXD (n = 1–5), with increasing conjugated length in branches were synthesized as high efficient blue emitters for OLEDs. With the extension of conjugated branches, the solid PL quantum efficiency and external quantum efficiency of Sn-Cz-OXD significantly increased with longer spacer, while the emission spectrum of these materials exhibited a blue-shift with enhanced color purity due to the unique molecular design. All materials maintained exceptionally high thermal stability after prolonged heat treatment at 150 °C in air. The photophysical, electrochemical, thermal properties of these emitters were studied in relation to the molecular structure. Nondoped device based on S4-Cz-OXD with structure ITO/PEDOT:PSS/EML/TPBI/LiF/Al emitted stable pure blue light with CIE coordinates of (0.157, 0.146). It exhibited high current efficiency and external quantum efficiency of 4.96 cd A⁻¹ and 4.20%, respectively. These values are among the best results for solution-processed non-doped blue device based on fluorene-free materials, indicating its potential for commercial applications.     

Heat sink performances of GaN/InGaN flip-chip light-emitting diodes fabricated on silicon and AlN submounts

Fabricating flip-chip light emitting diodes (FCLEDs) with two good thermal conductivity materials of silicon and aluminum nitride (AlN) as submount are investigated on its output power and heat sink capacity. It is known that many advantages exist in FCLED structures. In addition to the upward emitting light, the downward propagating light is reflected up by a high reflectance contact, increasing the light extraction. The heat generated in the LED flows directly through the interconnect metal of the submount, improving thermal conduction. Except blue shift at the low current injection region (0–0.3 A), the heat induced bang gap narrowing (red shift) at high current injection region (0.3–0.7 A) is observed with a red shift of 8.92 nm for conventional LED, 4.62 nm for silicon submount FCLED, and only 2.87 nm for AlN submount FCLED. The light intensity of FCLEDs with silicon and AlN submounts exhibits 1.6 and 7 times at an injection current of 0.35 and 0.7 A, respectively, larger than that of conventional LED.     

Universally applicable small-molecule co-host ink formulation for inkjet printing red,green, and blue phosphorescent organic light-emitting diodes

For organic light-emitting diodes (OLEDs), inkjet printing technology is being developed as an alternative to the traditional vacuum evaporation, because of its precise patterning, high-efficient material utilization, large-area compatibility and low-cost. In this work, we report a universal ink formulation of small-molecule co-host and binary solvents for red, green and blue phosphorescent OLEDs. Moreover, the effect of hole-transporting layers on the ink spreading, film uniformity and exciton confinement ability is investigated. Furthermore, a large-area (170 mm × 170 mm) and homogeneous light-emitting film is inkjet-printed. Finally, red, green and blue OLEDs are successfully constructed using these optimized ink formulations on the solvent resistance hole-transporting layer. This work can reduce the complexity to adjust the host materials and solvents for different color inks, and could be applied in large-area and low-cost OLED displays with high resolution.     

Stable efficiency roll-off in blue phosphorescent organic light-emitting diodes by host layer engineering

The device characteristics of blue phosphorescent organic light-emitting diodes (PHOLEDs) with mixed host structure were investigated by changing the combination and the composition of host materials in emissive layer. The distributed recombination zone and balanced charge carrier injection within emissive layer were achieved through mixed host optimization with a hole transport-type and an electron transport-type host materials, therefore the device performances were greatly enhanced, with external quantum and power efficiencies of 21.8% and 53 lm/W. Moreover, mixed host blue PHOLEDs exhibited a extremely low stable efficiency roll-off with quantum efficiencies of 20.3% and 18.6% at a luminance of 1000 and 10,000 cd/m².     

Efficient blue organic light-emitting diodes using newly-developed pyrene-based electron transport materials

We synthesized new kinds of pyrene-based electron transport materials: 1,6-di(pyridin-3-yl)-3,8-di(naphthalen-1-yl)pyrene (N1PP) and 1,6-di(pyridin-3-yl)-3,8-di(naphthalen-2-yl)pyrene (N2PP). The external quantum efficiencies of the device with these electron transport materials increase by more than 50% at 1 mA cm^?2 compared with those of the device with representative Alq₃ as an electron transport material. The enhanced quantum efficiency is due to a balanced charge recombination in an emissive layer. Electron mobilities in N1PP and N2PP films are three times higher than that in Alq₃. Highly enhanced power efficiency is achieved due to a low electron injection barrier and a high electron mobility. We find that the luminance degradation in the blue OLEDs is correlated with the HOMO energy levels of electron transport materials.     

Monothiatruxene‐Based,Solution‐Processed Green,Sky‐Blue,and Deep‐Blue Organic Light‐Emitting Diodes with Efficiencies Beyond 5% Limit

The development of blue materials with good efficiency, even at high brightness, with excellent color purity, simple processing, and high thermal stability assuring adequate device lifetime is an important remaining challenge for organic light‐emitting didoes (OLEDs) in displays and lightning applications. Furthermore, these various features are typically mutually exclusive in practice. Herein, four novel green and blue light‐emitting materials based on a monothiatruxene core are reported together with their photophysical and thermal properties, and performance in solution‐processed OLEDs. The materials show excellent thermal properties with high glass transition temperatures ranging from 171 to 336 °C and decomposition temperatures from 352 to 442 °C. High external quantum efficiencies of 3.7% for a deep‐blue emitter with CIE color co‐ordinates (0.16, 0.09) and 7% for green emitter with color co‐ordinates (0.22, 0.40) are achieved at 100 cd m^?2. The efficiencies observed are exceptionally high for fluorescent materials with photoluminescence quantum yields of 24% and 62%, respectively. The performance at higher brightness is very good with only 38% and 17% efficiency roll‐offs at 1000 cd m^?2. The results indicate that utilization of this unique molecular design is promising for efficient deep‐blue highly stable and soluble light‐emitting materials.     

Mixed Ruddlesden–Popper and Dion–Jacobson Phase Perovskites for Stable and Efficient Blue Perovskite LEDs

Producing efficient blue and deep blue perovskite LEDs (PeLEDs) still represents a significant challenge in optoelectronics. Blue PeLEDs still have problems relating to color, luminance, and structural and electrical stability so new materials are needed to achieve better performance. Recent reports suggest using low n states (n = 1, 2, 3) to achieve blue electroluminescence in Ruddlesden–Popper (RP) perovskite films. However, there are fewer reports on the other quasi-2D structure, Dion–Jacobson (DJ) perovksites, despite their highly desirable optical properties, due to the difficulty in achieving charge injection. To resolve this issue, herein, w e have mixed DJ phase precursors, propane-1,3-diammonium (PDA) bromide into RP phase perovskites and fabricated low-dimensional PeLEDs. It is found that these specific precursors aid in suppressing both the low n (n = 1) and high n (n ≥ 4) quasi-2D RP phases and is an effective strategy in blue-shifting sky-blue RP perovskites into the sub-470 nm region. With optimization of the PDA concentration and device layers, it is achieved an external quantum efficiency of 1.5% at 469 nm and stable electroluminescence for the first deep blue PeLED to be reported using DJ perovskites.     

p-Doped p-phenylenediamine-substituted fluorenes for organic electroluminescent devices

Two novel p-phenylenediamine-substituted fluorenes have been designed and synthesized. Their applications as hole injection materials in organic electroluminescent devices were investigated. These materials show a high glass transition temperature and a good hole-transporting ability. It has been demonstrated that the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) doped p-phenylene-diamine-substituted fluorenes, in which F4-TCNQ acts as p-type dopant, are highly conducting with a good hole-transporting property. The organic light emitting devices (OLEDs) utilizing these F4-TCNQ-doped materials as a hole injection layer were fabricated and investigated. The pure Alq₃-based OLED device shows a current efficiency of 5.2 cd/A at the current density of 20 mA/cm² and the operation lifetime is 1500 h with driving voltage increasing only about 0.7 mV/h. The device performance and stability of this hole injection material meet the benchmarks for the commercial requirements for OLED materials.     

Study of Configuration Differentia and Highly Efficient,Deep‐Blue,Organic Light‐Emitting Diodes Based on Novel Naphtho[1,2‐d]imidazole Derivatives

Two novel naphtho[1,2‐d]imidazole derivatives are developed as deep‐blue, light‐emitting materials for organic light‐emitting diodes (OLEDs). The 1H‐naphtho[1,2‐d]imidazole based compounds exhibit a significantly superior performance than the 3H‐naphtho[1,2‐d]imidazole analogues in the single‐layer devices. This is because they have a much higher capacity for direct electron‐injection from the cathode compared to their isomeric counterparts resulting in a ground‐breaking EQE (external quantum efficiency) of 4.37% and a low turn‐on voltage of 2.7 V, and this is hitherto the best performance for a non‐doped single‐layer fluorescent OLED. Multi‐layer devices consisting of both hole‐ and electron‐transporting layers, result in identically excellent performances with EQE values of 4.12–6.08% and deep‐blue light emission (Commission Internationale de l'Eclairage (CIE) y values of 0.077–0.115) is obtained for both isomers due to the improved carrier injection and confinement within the emissive layer. In addition, they showed a significantly better blue‐color purity than analogous molecules based on benzimidazole or phenanthro[9,10‐d]imidazole segments.     

Water‐Soluble Lacunary Polyoxometalates with Excellent Electron Mobilities and Hole Blocking Capabilities for High Efficiency Fluorescent and Phosphorescent Organic Light Emitting Diodes

High performance solution‐processed fluorescent and phosphorescent organic light emitting diodes (OLEDs) are achieved by water solution processing of lacunary polyoxometalates used as novel electron injection/transport materials with excellent electron mobilities and hole blocking capabilities. Green fluorescent OLEDs using poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐(1,4‐benzo‐{2,1′,3}‐thiadiazole)] (F8BT) as the emissive layer and our polyoxometalates as electron transport/hole blocking layers give a luminous efficiency up to 6.7 lm W^?1 and a current efficiency up to 14.0 cd A^?1 which remained nearly stable for about 500 h of operation. In addition, blue phosphorescent OLEDs (PHOLEDs) using poly(9‐vinylcarbazole) (PVK):1,3‐bis[2‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazo‐5‐yl]benzene (OXD‐7) as a host and 10.0 wt% FIrpic as the blue dopant in the emissive layer and a polyoxometalate as electron transport material give 12.5 lm W^?1 and 30.0 cd A^?1 power and luminous efficiency, respectively, which are among the best performance values observed to date for all‐solution processed blue PHOLEDs. The lacunary polyoxometalates exhibit unique properties such as low electron affinity and high ionization energy (of about 3.0 and 7.5 eV, respectively) which render them as efficient electron injection/hole blocking layers and, most importantly, exceptionally high electron mobility of up to 10^?2 cm² V^?1 s^?1.     

High-efficiency hybrid organic–inorganic light-emitting devices

We report efficient red, orange, green and blue organic–inorganic light emitting devices using light emitting polymers and polyethylenimine ethoxylated (PEIE) interlayer with the respective luminance efficiency of 1.3, 2.7, 10 and 4.1 cd A⁻¹, which is comparable to that of the analogous conventional devices using a low work-function metal cathode. This is enabled by the enhanced electron injection due to the effective reduction of the ZnO work-function by PEIE, as well as hole/exciton-blocking function of PEIE layer. Due to the benign compatibility between PEIE and the neighboring organic layer, the novel phosphorescent organic–inorganic devices using solution-processed small molecule emissive layer show the maximum luminance efficiency of 87.6 cd A⁻¹ and external quantum efficiency of 20.9% at 1000 cd m⁻².     

Construction of White‐Light‐Emitting Silk Protein Hybrid Films by Molecular Recognized Assembly among Hierarchical Structures

The fabrication of bio‐hybrid functional films is demonstrated by applying a materials assembly technique. Based on the hierarchical structures of silk fibroin materials, functional molecular/materials, i.e., quantum dots (QDs), can be fixed to amino acid groups in silk fibroin films. It follows that white‐light‐emitting QD silk hybrid films are obtained by hydrogen bond molecular recognition to the –COO^– groups functionalized to blue luminescent ZnSe (5.2 nm) and yellow luminescent CdTe (4.1 nm) QDs in a molar ratio of 30:1 of ZnSe to CdTe QDs. Simultaneously, a systematic blue shift in the emission peak is observed from the QD solution to QDs silk fibroin films. The significant blue shift hints the appearance of the strong interaction between QDs and silk fibroins, which causes strong white‐light‐emitting uniform silk films. The molecular recognized interactions are confirmed by high resolution transmission electron microscopy, field scanning electron microscope, and attenuated total internal reflectance Fourier transform infrared spectroscopy. The QD silk films show unique advantages, including simple preparation, tunable white‐light emission, easy manipulation, and low fabrication costs, which make it a promising candidate for multicomponent optodevices.     

A new spiro[fluorene-9,9′-xanthene]-based host material possessing no conventional hole- and electron-transporting units for efficient and low voltage blue PHOLED via simple two-step synthesis

A new spiro[fluorene-9,9′-xanthene]-based host material SFX-PF without possessing conventional hole- and electron-transporting units has been developed, via very simple two-step synthesis, for efficient and low voltage blue phosphorescent organic light-emitting device (PHOLED). The blue device exhibited a low turn-on voltage of 2.8 V, and high maximum current efficiency, power efficiency, and external quantum efficiency up to 29.3 cd/A, 28.9 lm/W, and 14.7%, respectively. At the luminance level of 1000 cd/m², the driving voltages are still lower than 4.0 V with 12.9% roll-off of the external quantum efficiency. Based on our previous studies, these investigations provide the clues that SFX could be a new building block for designing blue phosphorescent host materials.     

Effect of water-soluble vitamins on the structure and properties of poly(3,4-ethylenedioxythiopehene):poly(styrenesulfonate)

Intrinsically conducting polymers can have important application in biology because they can be conductive and have good biological compatibility. Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) has been the most popular conductive polymer in biological application due to its solution processability in water. PEDOT:PSS can be used as electrode materials or active materials of biological devices or circuits. It is important to study the effect of biomaterials on the structure and properties of PEDOT:PSS films. In this work, water-soluble vitamins that are biomaterials needed for organisms are used to treat PEDOT:PSS. They can significantly enhance the conductivity of PEDOT:PSS from 0.3 S cm⁻¹ up to higher than 1000 S cm⁻¹. The conductivity enhancement depends on the structure of vitamins. The highest conductivity enhancement was observed for PEDOT:PSS treated with vitamin B3. The vitamin-induced changes in the structure and properties of PEDOT:PSS were studied by UV–Vis absorption spectroscopy, temperature-dependence of resistance measurements, atomic force microscopy and cyclic voltammetry. The characterizations indicate that vitamins can induce phase segregation between PEDOT and PSS and the conformational change of the PEDOT chains. These discoveries are important to understand the application of PEDOT:PSS in biology and the development of new biological application of PEDOT:PSS.     

Hybridization of CsPbBr1.5I1.5 perovskite quantum dots with 9,9-dihexylfluorene co-oligomer for white electroluminescence

Quaternary lead halide CsPbBr_1.5I_1.5 perovskite quantum dots (PQDs) with cubic shapes and emission wavelength at 566 nm were synthesized by hot injection and carefully characterized to better compensate with blue co-oligomer for white light emission. With a very simple device structure, the electroluminescent hybrid device exhibits a turn-on voltage of 4.7 V, a maximum luminance ∼1200 cd/m², and steady CIE coordinates of (0.28, 0.33). Compared to the photoluminescence spectrum, the increased electroluminescence from PQDs in the HFSO/PQD composite film of the device strongly suggests that the emission from CsPbBr_1.5I_1.5 PQDs are partially due to the direct charge injection.     

Optimizing carrier balance of a red quantum-dot light-emitting electrochemical cell with a carrier injection layer of cationic Ir(III) complex

Solid-state light-emitting electrochemical cells (LECs) with promising features of solution processability, low-voltage operation and compatibility with inert cathode metals have shown great potential in display and lighting applications in recent years. Among the reported emissive materials for LECs, ionic transition metal complexes (iTMCs) have relatively higher electroluminescence (EL) efficiencies due to their phosphorescent property. However, the red iTMCs generally exhibit moderate color saturation and low emission efficiency, limiting their display applications. To improve color saturation and device efficiency of red LECs, efficient quantum dots (QDs) with narrow emission bandwidth are good alternative emissive materials. In this work, efficient and saturated red QD LECs employing iTMC carrier injection layers to provide in situ electrochemical doping are demonstrated. The thicknesses of iTMC and red-QD layers are systematically adjusted to achieve the best carrier balance. In the optimized device, the iTMC carrier injection layer facilitates hole injection into the red-QD layer while electrons are injected from the cathode into the red-QD layer directly since the electron injection barrier is low. The Commission Internationale de I'Eclairage (CIE) coordinates of the EL spectra approach the red standard point of National Television System Committee (NTSC). High external quantum efficiency and current efficiency reaching 9.7% and 16.1 cd A⁻¹, respectively. These results confirm superior carrier balance in such a simple iTMC/QD bilayer device structure. Furthermore, compared with iTMC LECs, less degree of device efficiency roll-off upon increasing device current is observed in QD LECs since a shorter excited-state lifetime of fluorescent QDs reduces the probability of collision exciton quenching. Saturated and efficient red EL with mitigated efficiency roll-off from red-QD LECs employing iTMC carrier injection layers confirms that they are good candidates of saturated light sources for displays.     

Multimode Emission from Lanthanide-Based Metal–Organic Frameworks for Advanced Information Encryption

Although remarkable progress on luminescent materials is made in advanced optical information storage and anti-counterfeiting applications, many challenges still remain in these fields. Currently, most luminescent materials are based on a single photoluminescent model that can be easily imitated by substitutes. In this work, a series of multimodal emission lanthanide-based metal–organic frameworks (MOFs) are developed, where they emit red and green light originating from Eu³⁺ and Tb³⁺ under ultraviolet light irradiation. Meanwhile, under 980 nm near-infrared laser irradiation, these MOFs show cyan upconversion cooperative luminescence derived from Yb³⁺ and characteristic upconversion luminescence from lanthanide activators (Eu³⁺, Tb³⁺, or Ho³⁺), respectively. Based on the integrated optical functionality, the functional information storage applications are successfully designed, which indicates that multimodal emission features can be easily detected under ultraviolet lamps (254 or 393 nm) or 980 nm near-infrared laser. And, the unique optical features show a high level of security in the advanced information storage application, which would be sufficiently complex to be forged.     

Efficient non-doped pure blue organic light-emitting diodes based on an asymmetric phenylenevinylene with cis double bond

We reported an asymmetric phenylenevinylene with a cis double bond 2-(4-(p-tolyl)styryl)-1,4-dip-tolylbenzene (cis-TSDTB) and its use as efficient deep-blue emitter for organic light-emitting diodes (OLEDs) applications. The crystal structure of cis-TSDTB showed torsion configuration and asymmetric geometry, which make it packing in a reduced intermolecular interaction arrangement. And its single crystals showed excellent fluorescence owing to this unique molecular configuration. Typical OLEDs using cis-TSDTB as non-doped emitters exhibited saturated blue light with the CIE 1931 coordinates of (0.15, 0.10), which is quite close to the National Television Standards Committee (NTSC) blue standard. High luminescence efficiency (3.4 cd A⁻¹) and high brightness (9855 cd m⁻²) have been realized in the device. All of these outstanding results indicated that cis-phenylenevinylene will be a promising candidate as blue light-emitting materials.