Efficient white OLEDs employing red,green, and blue tetradentate platinum phosphorescent emitters

The development of white organic light-emitting diodes (WOLEDs) employing three tetradentate platinum-based emitters, PtN3N-ptb (red), PtON1 (blue), and PtOO8 (green), is described. Spectral and electrical results show that localization of charge buildup, energy transfers between dopants, and concentration-dependence of spectral shape are all major factors in determining the overall color quality of a device. WOLEDs are fabricated by optimizing the thickness of emissive layers, the concentration of dopants, and the stacking order of emissive layers. Ultimately, the device employing 6% PtN3N-ptb(red)/6% PtON1(blue)/6% PtOO8(green) stacking order of emissive layers demonstrates the Commission Internationale de L’Éclairage coordinates (CIE) coordinates of (0.35, 0.35) and Color Rendering Index (CRI) value of 80, peak Power Efficiency (PE) of 41 lm/W, and maximum External Quantum Efficiency (EQE) of 21.0%.     

Chemical degradation processes of highly stable red phosphorescent organic light emitting diodes

We investigate the chemical degradation processes of highly stable red organic light emitting diodes (OLEDs) based on the triplet emitter tris(1-phenylisoquinoline)iridium(III) by laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS). The analysis of LDI-TOF spectra, collected on OLEDs driven at different current densities, shows a direct correlation between the lifetime of the devices and the formation of the three different reaction products: a BPhen dimer, an adduct of BPhen dimer with cesium, and the complex [BAlq₂ + Al(Me-q)₂]⁺ as well. Additionally it was possible to identify another degradation product, whose chemical structure is related to the α-NPD molecule as well to the fluorine of the used p-dopant. This product is only observable in devices aged at very high current densities.     

Efficient phosphorescent white organic light-emitting devices incorporating blue iridium complex and multifunctional orange–red osmium complex

By using the unique properties of the efficient orange–red phosphorescent osmium complex in combination with an efficient blue phosphorescent Iridium complex, efficient white organic light-emitting devices with forward viewing efficiencies up to (17% photon/electron, 36 cd/A, 28 lm/W) and white organic light-emitting devices with color stability vs. brightness can be implemented. Results show that the osmium complex is a multi-functional material that not only has high emission efficiency, but also possesses the effective hole trapping capability, which is useful for balancing hole/electron transport and controlling the emission zones when doped at appropriate locations of the device.     

Influence of the applied charge on the electro-chemical degradation in green phosphorescent organic light emitting diodes

The influence of the current density on the chemical degradation processes of a phosphorescent OLED based on Ir(ppy)₃ emitter is investigated by laser-desorption/ionization time-of-flight mass spectrometry. Comparing the mass spectra collected for unaged and aged OLEDs, the formation of different chemical degradation products could be detected and are identified as dimer and trimer products of BPhen as well as Cs-adducts of these polymers and the well-known emitter-BPhen-adduct ([Ir(ppy)₂BPhen]⁺). In this work, we will show that the formation of [Ir(ppy)₂BPhen]⁺ depends strongly on the amount of the charge flowing through the device, where the other degradation products show a much different behaviour.     

Star-shaped hexaaryltriindoles small molecules: Tuning molecular properties towards solution processed organic light emitting devices

We present a series of differently substituted star-shaped hexaaryltriindoles with tunable light-emitting properties. The deep blue emission is unchanged by donor peripheral substituents while an increasing acceptor character produces a reduction of the optical gap, an increased Stokes shift and eventually leads to the appearance of a new electronic level and to the simultaneous deep blue (413 nm) and green (552 nm) emission in solution. Quenching by concentration increases with the acceptor character but is lower as the tendency of these compounds to aggregate is stronger. Solution processed thin films present optical and morphological qualities adequate for device fabrication and similar electronic structure compared to solutions with an emission range from 423 nm up to 657 nm (red), demonstrating the possibility of tuning the energy levels by chemical functionalization. We have fabricated and characterized single-layer solution processed organic light emitting diodes (OLED) to investigate the influence on transport and emission properties of the substituting species. We analyzed the I–V response using a single-carrier numerical model that includes injection barriers and non-uniform electric-field across the layer. As a result, we obtained the electric field dependence of the mobility for each device. Best results are obtained on the most electron rich derivative functionalized with six donor methoxy groups. This material shows the highest emission efficiency in solid state, due to aggregation-induced enhancement, and better transport properties with the highest mobility and a very low turn-on voltage of 2.8 V. The solution processed OLED devices produce stable deep blue (CIE coordinates (0.16, 0.16)) to white (CIE coordinates (0.33, 0.3)) emission with similar luminous efficiencies.     

Efficient blue emitters based on 1,3,5-triazine for nondoped organic light emitting diode applications

Two novel efficient blue emitters (TTT-1, TTT-2) containing 1,3,5-triazine, thiophene and triphenylamine have been designed and synthesized. Organic light emitting diodes (OLEDs) using these new triazine derivatives as emissive layers, ITO/TAPC (60 nm)/TTT-1 (Device A) or TTT-2 (Device B) (40 nm)/TPBi (60 nm)/LiF (1 nm)/Al (100 nm), were fabricated and tested. The OLEDs exhibited good performances with low turn-on voltage of 3 V, maximum luminance of ca. 8990 cd/m² for TTT-1 and 15,980 cd/m² for TTT-2, and maximum luminance efficiency of 4.7 cd/A for TTT-1 and 4.0 cd/A for TTT-2, respectively.     

High efficiency and non-color-changing orange organic light emitting diodes with red and green emitting layers

We report a high performance orange organic light-emitting diode (OLED) where red and green phosphorescent dyes are doped in an exciplex forming co-host as separate red and green emitting layers (EMLs). The OLED shows a maximum external quantum efficiency (EQE) of 22.8%, a low roll-off of efficiency with an EQE of 19.6% at 10,000 cd/m², and good orange color with a CIE coordinate of (0.442, 0.529) and no color change from 1000 to 10,000 cd/m². The exciplex forming co-host system distributes the recombination zone all over the EMLs and reduces the triplet exciton quenching processes.     

Efficient and bright polymer light emitting field effect transistors

Light emitting field effect transistors (LEFETs) are emerging as a multi-functional class of optoelectronic devices. LEFETs can simultaneously execute light emission and the standard logic functions of a transistor in a single architecture. However, current LEFET architectures deliver either high brightness or high efficiency but not both concurrently, thus limiting their use in technological applications. Here we show an LEFET device strategy that simultaneously improves brightness and efficiency. The key step change in LEFET performance arises from the bottom gate top-contact device architecture in which the source/drain electrodes are semitransparent and the active channel contains a bi-layer comprising of a high mobility charge-transporting polymer, and a yellow–green emissive polymer. A record external quantum efficiency (EQE) of 2.1% at 1000 cd/m² is demonstrated for polymer based bilayer LEFETs.     

Soluble processed low-voltage and high efficiency blue phosphorescent organic light-emitting devices using small molecule host systems

We report low voltage driving and highly efficient blue phosphorescence organic light emitting diodes (PHOLEDs) fabricated by soluble process. A soluble small molecule mixed host system consisting of hole transporting 4,4’,4’’ tris(N-carbazolyl)triphenylamine (TCTA) and bipolar carrier transporting 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) exhibits high solubility with smooth surface properties. Moreover, this small molecule host shows the smoothest morphological property similar to a vacuum deposited amorphous film. A low driving voltage of 5.4 V at 1000 cd/m² and maximum external quantum efficiency 14.6% obtained in the solution processed blue PHOLEDs are useful for large area low cost manufacturing.     

Polymer-based scattering layers for internal light extraction from organic light emitting diodes

Efficient light extraction for organic light emitting diodes (OLED) using scalable processes and low-cost materials are important prerequisites for the future commercialization of OLED lighting devices. The light-extraction technology exhibited in this paper uses polymer-based high-refractive index scattering layers processed from solution. The scatter matrix formulation incorporates two types of nanoparticles for refractive index tuning and scattering, respectively. Planarization by the same material in order to reduce surface defects was critical for achieving highly increased device yield. Highly efficient and defect-free large-area (1.8 cm²) white OLED devices were fabricated on top of the scattering layer in a bottom emitter configuration. Light extraction enhancement leads to an overall efficiency gain of up to 81% for luminances of 5000 cd m⁻².     

Wide color-range tunable and low roll-off fluorescent organic light emitting devices based on double undoped ultrathin emitters

We demonstrated a novel wide color-range tunable, highly efficient and low efficiency roll-off fluorescent organic light-emitting diode (OLED) using two undoped ultrathin emitters having complementary colors and an interlayer between them. The OLED can be tuned to emit sky blue (0.22, 0.30), cold white (0.29, 0.33), warm white (0.43, 0.42) and yellow (0.40, 0.45) according to the Commission Internationale de L’Eclairage (CIE) 1931 (x, y) chromaticity diagram. The device fabrication was simplified by eliminating doping process in the emission layers. The influence of interlayer thickness on luminous efficiency, efficiency roll-off and color tuning mechanism is thoroughly studied. The recombination zone is greatly broadened in the optimized device, which contributes to stable energy transfer to both emitters and suppressed concentration quenching. With a threshold voltage of 2.82 V, the color tunable organic light emitting diode (CT-OLED) shows a maximum luminance of 39,810 cd/m², a peak external quantum efficiency (EQE) 6% and the efficiency roll-off as low as 11.1% at the luminance from 500 cd/m² to 5000 cd/m². This structure of CT-OLED has great advantages of easy fabrication and low reagent consumption. The fabricated CT-OLEDs are tunable from cold white (0.30, 0.36) to warm white (0.43, 0.42) with correlated color temperature (CCT) 6932 K and 3072 K, respectively, demonstrating that our proposed approach helps to meet the need for lighting with various CCTs.     

New blue phosphorescent Pt(II) complex with pyridyltriazole-based tetradentate ligand for organic light-emitting diodes

We designed and synthesized Pt-Pytz, a blue phosphorescent Pt (II) complex having 6,6-(1-methoxyethane-1,1-diyl)di (2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine) as a tetradentate ligand, and investigated its photophysical, electrochemical, and electroluminescent properties. The complex showed an emission maximum at 449 nm in dichloromethane solution, with a high photoluminescence quantum efficiency of 0.81. Furthermore, the photoluminescence spectrum of Pt-Pytz in the film state contains no additional excimer emission in the long-wavelength region, indicating suppressed molecular aggregation in the solid state. An organic light-emitting diode based on Pt-Pytz as a blue dopant (10% doping) achieved the best external quantum efficiency of 7.0% with a power efficiency of 6.4%, a current efficiency of 11.2 cd/A, and the CIE coordinates of (0.146, 0.218).     

Correlations of impedance-voltage characteristics and carrier mobility in organic light emitting diodes

The correlation of accumulation charges at the interfaces of organic layers and carrier mobility in organic light emitting devices (OLEDs) were investigated through the impedance versus voltage (Z-V) characteristics of devices. The properties of devices with various combinations of cathode structures, HTLs and ETLs were investigated to understand the impedance transition in Z-V characteristics of OLEDs. It was observed that there is an extra impedance transition before devices turn on when the hole mobility in the HTL is much greater than the electron mobility in the ETL in the devices, which makes the Z-V characteristics a potential tool to compare the electron mobility in ETL and hole mobility in HTL.     

以iridium磷光为红绿蓝染料的白光器件研究

以磷光染料iridium (III) bis [(4,6-di-fluoropheny)-pyridinato-N,C2′] picolinate(Flrpic)掺在4.4′-bis (9-carbazolyl)-2,2′-dimethyl-biphenyl(CDBP)中作为蓝光发光层,tris (2-Phenylpyridine) iridium(Irlppy)3和bis (1-phenyl-isoquinoline) acetylacetonate iridium (III)(Ir(piq)2)(acac)共掺在4,4′-N,N′-dicarbazole-biphenyl(CBP)中作为绿光和红光发光层,制备了高效白光器件.通过控制染料的浓度和发光层的厚度调节颜色,实现白光发射.器件的最大亮度为17 V时37 100 cd/m2,最大效率为5 V时7.37 lm/W.当亮度从1 000 cd/m2 到30 000 cd/m2色坐标由(0.41,0.42) 变到(0.37,0.39).     

Novel benzothiadiazine 1,1-dioxide based bipolar host materials for efficient red phosphorescent organic light emitting diodes

In this work, three novel bipolar host materials TPA-SA, 3CBZ-SA and 4CBZ-SA have been designed and synthesized by incorporating triphenyl amine and carbazole as donor and benzothiadiazine 1,1-dioxide as an acceptor. These molecules exhibit moderately high triplet energies and bipolar carrier transport characteristics (ambipolarity) which is useful for the exothermic energy transfer to the dopants and also for the balanced carrier injection/transport in the emissive layers. These materials exhibited good performances in PhOLEDs and furnished external quantum efficiency in the range of 10.0–15.0%. Notably, a red phosphorescent device using TPA-SA as the host doped with Ir(pq)₂(acac) exhibited a maximum EQE, power efficiency and current efficiency of 15.0%, 16.0 lm/W, and 25.3 cd A⁻¹, respectively.     

Efficiency enhancement of organic light emitting diode via surface energy transfer between exciton and surface plasmon

Organic light emitting diodes (OLEDs) with surface plasmon (SP) enhanced emission have been fabricated. Gold nanoclusters (GNCs) deposited using thermal evaporation technique has been used for localization of surface plasmons. Size of GNCs and distance of GNCs from the emissive layer have been optimized using steady state and time resolved photoluminescence (PL) results. 3.2 Times enhancement in PL intensity and 2.8 times enhancement in electroluminescence intensity of OLED have been obtained when GNCs of size 9.3 nm has been introduced at a distance of 5 nm from emissive layer. Distance dependence of energy transfer efficiency between exciton and SPs was found to be of 1/R⁴ type, which is typically the dependence for dipole-surface energy transfer.     

高效暖白光器件的廉价制备及其相关材料研究

低色温光源由于其对抗黑变激素具有较低的抑制作用而成为生理友好照明的首选。同时,高的能量效率对于节能也至关重要。本课题采用温和的溶液旋涂方法分别制备了含互补色、三基色和四基色磷光染料的单层有机白光二极管(WOLED)。经过优化WOLED的结构,实现了宽亮度范围内100~10 000cd/m2的低色温(low-CCT)白光发射。CCT低至2 500K以下、显示指数(CRI)高达到83、电流效率在亮度为1 000cd/m2时达到了17.8cd/A,与传统的白炽灯功效相当。高发光性能、廉价制备成本及生理友好的特性表明,本工作制备的器件是益于人类健康的照明光源尤其是夜间照明光源的理想选择。     

High efficiency warm white phosphorescent organic light emitting devices based on blue light emission from a bipolar mixed-host

In this study, we demonstrate a high-efficiency and low turn-on voltage warm white phosphorescent organic light emitting devices (PH-WOLEDs) based on a blue mixed-host emission layer (EML) and an orange ultrathin layer. The device has a simple structure and would simplify the fabrication process and reduce fabrication costs. The concept is based on the design a high-efficiency blue mixed-host EML, using an electron-transport material, 4,6-Bis(3,5-di(pyridin-4-yl) phenyl)-2-(3-(pyridin-3-yl) phenyl) pyrimidine (B4PYMPM) to enhance the carrier balance ability of the hole-transport material 1,3-Bis(carbazol-9-yl) benzene (MCP) which operates as the mixed-host and when the MCP: B4PYMPM ratio in the mixed-film was 4:1 got better effects. Based on the blue EML, we realized WOLEDs, characterized by a peak power efficiency of 71.3 lm/W at 3.1 V and a low turn-on voltage of 2.65 V. The mixed-host blue EML exhibited a much higher performance compared to the MCP host. Stable warm white light emission with Commission International de L'Eclairage (CIE) coordinates from (0.37, 0.45) to (0.38, 0.47) for a luminance value ranging from 1000 to 10,000 cd/m² was obtained.     

Spectroscopic and electrical characteristics of highly efficient tetraphenylsilane-carbazole organic compound as host material for blue organic light emitting diodes

Thermal, electrical and spectroscopic properties have been studied for bis(3,5-di(9H-carbazol-9-yl) phenyl)diphenylsilane (SimCP2) which has exhibited high external quantum efficiency of 17.7% and power efficiency of 24.2 lm/W when it is used as host material for iridium bis(4,6-difluorophenypyridinato)picolate (FIrpic) blue emitter. They are compared with 1,3-bis (9-carbazolyl) benzene (mCP) and 3,5-bis (9-carbazolyl) tetraphenylsilane (SimCP) which have been also used as host for blue emitters. SimCP2 exhibits a highest glass transition temperature (148 °C) and is morphologically more stable. The electron and hole mobilities are higher (4.8 × 10⁻⁴ and 2.7 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively, at electric field of 9 × 10⁴ V cm⁻¹) than those of mCP and SimCP. The zero-phonon S₁ emission band is observed at 344 nm, while the T₁ emission band at 412 nm, i.e., this material preserves the characteristics of wide band-gap of 3.56 eV and high T₁ triplet energy of 3.01 eV. From the intensity ratio of the T₁ emission to the S₁ emission, it is suggested that the intersystem crossing rate is smaller for SimCP2 than for mCP and SimCP. From these results, we clarify the reasons why SimCP2 is superior to mCP and SimCP as the host material for blue phosphorescence emitter in organic light emitting diodes.     

Efficient hole-transporter for phosphorescent organic light emitting diodes with a simple molecular structure

N,N-diphenyl-4-(quinolin-8-yl)aniline (SQTPA), which composes a triphenylamine group and a quinoline group, has been synthesized and employed as a hole-transporter in phosphorescent OLEDs. It has been proved that SQTPA has efficient hole-transport property with a hole-mobility of 3.60 × 10⁻⁵ cm²/V s at the electric field of 800 (V/cm)^1/2, which is higher than that of NPB (1.93 × 10⁻⁵ cm²/V s). Blue, orange and green phosphorescent OLEDs have been fabricated based on FIrpic, Ir(2-phq)₃, Ir(ppy)₃ with typical structures by using SQTPA as the hole-transporter. The SQTPA-based devices show maximum external quantum efficiencies and power efficiencies of 17.5%, 32.5 lm/W for blue, 12.3%, 20.5 lm/W for orange and 20.3%, 64.5 lm/W for green. The performances of SQTPA-based devices are much better than that of NPB-based phosphorescent OLEDs with similar structures. Thought of its very simple molecular structure and easy synthetic route, SQTPA should be an efficient hole-transporter for phosphorescent OLEDs.