The electroluminescent investigation of double layer Eu-complex organic electronic luminescence diodes

Double layer organic electronic luminescence diodes (OLEDs) based on europium(dibenzoylmethanato)₃monophenanthroline [Eu(DBM)₃bath], ITO/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)/Eu(DBM)₃bath/LiF/Al have been fabricated. With increasing the thickness of hole transporting layer, the maximum EL efficiency was increased, and the EL efficiency of 10 cd/A was achieved when the thickness of TPD layer was 80 nm; however, at high current density, the EL efficiency of all devices was decreased drastically. Besides, the evolution of EL emission spectra with increasing operating voltage was found, the mechanisms of the symmetry around the ion improved and the annihilation of excited state of Eu(DBM)₃bath were discussed in explaining this phenomenon.     

利用LiF空穴阻挡层提高有机发光二极管效率

通过引入LiF,明显提高了基于八羟基喹啉铝双层有机发光二极管的发光效率.2 nm 厚的 LiF 空穴阻挡层可将器件的发光效率从 2.6 cd/A 提高到 6.3 cd/A,研究结果表明,LiF 空穴阻挡层可以有效调节空穴的注入与传输,平衡器件中的空穴与电子,提高有机发光二极管的发光效率.     

Transient analysis on stored charges in organic light-emitting diodes and their application in alternating current driven electroluminescence

AC-driven organic light-emitting diodes (OLEDs) can overcome some reliability-related drawbacks to traditional DC-OLEDs. They imply the use of insulating layers in the device. In this work OLEDs containing an insulating layer of poly (methyl methacrylate) (PMMA) in storing charges with the thickness of 2, 6, 8, 10 nm have been prepared and investigated. The emission mechanisms of the device are analyzed considering transient and AC electroluminescence (EL) measurements. We show that charges are stored in the PMMA layer as surface charges and bulk charges. The former contribute to the occurrence of EL spike after the driving pulse with a decay tail for about 80 μs to 10nm PMMA device, and the latter can be released to emit light under reversed voltage more than 2 V because they are immobile unless under the stronger reversed field. Stored charges commonly are harmful for the performance of OLEDs devices due to quenching, nonradiative transition and the thermal energy accumulation even degradation. Whereas when operating under alternating current (AC) stress, we not only obtain the EL peak from injection charges under forward voltage, but also obtain another peak below the built-in voltage 4.3 V, whose peak point lies in −0.2 V. It turns to originate from stored charges so they become beneficial instead of weakening EL for reusing to produce light. All the results pave the way to realize AC driven OLEDs devices using these stored charges, to uncover the AC EL mechanisms and to improve their EL performance.     

Effect of BCP ultrathin layer on the performance of organic light-emitting devices

Based on conventional double layer device, triple layer organic light-emitting diodes （OLEDs） with two heterostructures of indium-tin oxide （ITO）/N,N＇-diphenyl-N,N＇-bis（1-naphthyl）（1,1 ＇-biphenyl）-4,4＇-diamine（NPB）/2,9-dimethyl-4,7-diphenyl- 1,10-phenanthroline （BCP）/8-Hydroxyquinoline aluminum （Alq3）/Mg：Ag USing vacuum deposition method have been fabricated. The influence of different film thickness of BCP layer on the performance of OLEDs has been investigated. The results showed that when the thickness of the BCP layer film gradually varied from 0.1 nm to 4.0 nm, the electroluminescence （EL） spectra of the OLEDs shifted from green to greenish-blue to blue, and the BCP layer acted as the recombination region of charge carriers related to EL spectrum, enhancing the brightness and power efficiency. The power efficiency of OLEDs reached as high as 7.3 lm/W.     

Temperature-Dependent Dynamic Behaviors of Organic Light-Emitting Diode

A systematic investigation of temperature-dependent dynamic behaviors of NPD-Alq₃ organic light-emitting diodes (OLEDs) is carried out. Through an in-depth numerical analysis, it has been found that the luminance decreases and consequently the turn-on voltage increases with decreasing temperature due to a reduction of thermally activated hopping speed, which retards the rise of electroluminescence (EL) upon turn-on as well as the discharge upon turn-off of OLEDs. Most importantly, however, the device efficiency is literally raised as the temperature decreases, a direct consequence of enhanced charge-balance factor. It is also demonstrated that the EL delay upon turn-on is mostly determined by the electron transport through the electron transport layer (ETL), while the fast EL decay (short-lived EL tail) upon turn-off is mainly by the rapid discharge of the steep pileup of carriers at the NPD/Alq₃ interface. The long-lived EL tail is shown to be more pronounced under lower temperatures. In response to a train of voltage pulses, the delay of EL occurring for the first voltage pulse has vanished for the subsequent pulses regardless of temperature due to space charges remaining inside the device after turn-off (in the "off-state"). However, it appears that the pulse-to-pulse interference by the space charge effects is more significant under lower temperatures     

Highly efficient electrophosphorescent polymer light-emitting devices

We report a high performance polymer electroluminescent device based on a bi-layer structure consisting of a hole transporting layer (poly(vinylcarbazole)) and an electron transporting layer poly(9,9-bis(octyl)-fluorene-2,7-diyl) (BOc-PF) doped with platinum(II)-2,8,12,17-tetraethyl-3,7,13,18-tetramethylporphyrin (PtOX). The devices show red electrophosphorescence with a peak emission at 656 nm and a full width at half maximum of 18 nm, consistent with exclusive emission from the PtOX dopants. BOc-PF emission is not observed at any bias. The required doping levels for these phosphorescence-based polymer light-emitting diodes (PLEDs) are significantly lower than for other reported phosphorescence-based PLEDs or organic light-emitting diodes (OLEDs). A doping level of 1% or more give an LED with exclusive PtOX emission, whereas related PLEDs or OLEDs doped with phosphorescent dopants require doping levels of >5% to achieve exclusive dye dopant emission. The device external efficiency was enhanced from 1% to 2.3% when doped with PtOX. The lower doping level in BOc-PF/PtOX based PLEDs decreases triplet–triplet annihilation in these devices, leading to quantum efficiency that is only weakly dependent on current density. The luminescence transient decay time for this device is 500 μs.     

Emission mechanism in the terbium complex doped PVK system

A new kind of rare earth (RE) complex Tb(o-MBA)3phen was synthesized and used as an emitting material in electroluminescence. The material was doped into poly(N-vinylcarbazole) (PVK) as the emitting layer,which was made by spin coating. Three kinds of devices were fabricated with the structures: (A) ITO/PVK:Tb(o-MBA)3phen/LiF/A1; (B) ITO/PVK:Tb(o-MBA)3phen/BCP/AIQ3/LiF/A1; (C) ITO/BCP/PVK:Tb(o-MBA)3phen/A1Q3/LiF/A1. Bright green emission could be obtained from device (A) and (C). The photoluminescence (PL) and electroluminescence (EL) mechanisms of this material had been investigated. Since there was an overlap between the PL spectrum of PVK and the excitation spectrum of the terbium complex, there should be a F6rster energy transfer process between them. The excitation spectrum of PVK doped Tb(o-MBA)3phen system is similar with the excitation spectrum of PVK,yet it is different from that of Tb(o-MBA)3phen. So, the emission of Tb(o-MBA)3phen should partly come from the excitation of PVK while in the organic light-emitting diode (OLED), based on Tb(o-MBA)3phen, the emission mainly comes from the direct recombination of electron and hole. Bright green emission can be obtained from the optimized multi-layer device (C) and the highest EL brightness reached 180 cd/m2 at the voltage of 17 V.     

Optoelectrical characteristics of organic light-emitting devices fabricated with different cathodes

Organic light-emitting devices (OLEDs) with various cathode structures were prepared on indium tin oxide (ITO) substrates by vacuum sublimation technique, and the effects of the device cathodes on the electroluminescence (EL) characteristics of OLEDs were studied in terms of the luminance, efficiency, driving voltage and threshold voltage. The results demonstrate that the optical and electrical performance of OLEDs depend on the properties of the devices' cathodes and the characteristics of the cathode–organic interface and the organic–organic interface. The optoelectrical performance of a device with composite cathodes is better than that of the devices with metal alloy and pure metal cathodes. The improvement in the device performance can be attributed to a more efficient electron injection at the cathode–organic interface, a better balanced hole and electron recombination in the light-emitting layer and fewer accumulated charges near the organic–organic interface.     

Triphenylamine-based trinuclear Pt(II) complexes for solution-processed OLEDs displaying efficient pure yellow and red emissions

Highly efficient phosphors are critical in solution-processed organic light-emitting devices (OLEDs). Multinuclear Ir(III) complexes containing more than one metal center have showed great potential in fabricating high performance OLEDs, yet the electroluminescent (EL) properties of multinuclear Pt(II) complexes are rarely studied. In this work, two neutral trinuclear Pt(II) complexes are synthesized based on the triphenylamine core bearing three bidentate ligand arms. Both the yellow emitter (PyTPt) and deep-red emitter (IqTPt) exhibit improved photoluminescent quantum yields (PLQYs) compared with their corresponding mononuclear Pt(II) complexes. Furthermore, the PLQYs of PyTPt and IqTPt doped films are increased to 0.63 and 0.47, respectively. The solution-processed pure yellow-emitting device based on PyTPt achieves impressively high external quantum efficiency (EQE), current efficiency (CE), and power efficiency (PE) of 16.92%, 56.74 cd/A and 29.09 lm W⁻¹, respectively, which are among the best performance reported for the OLEDs employing multinuclear Pt(II) complexes. The solution-processed device based on IqTPt shows pure red emission with the peak EQE approaching 9.0%. Both PyTPt and IqTPt display much higher EL efficiencies than their corresponding mononuclear Pt(II) complexes. This work demonstrates that it is an attritive strategy to develop multinuclear Pt(II) complexes for high-performance OLEDs.     

Electrical pumped energy up-conversion: A non-linear electroluminescence process mediated by triplet-triplet annihilation

Excitons dominate the optoelectronic properties of organic devices. Normal organic light-emitting devices (OLEDs) generally linearly use exciton to generate electroluminescence (EL), in which one photon is produced from per exciton. Eliminating bi-excitons quenching is of great concern for efficient devices. Here, we have theoretically and experimentally investigated a non-linear EL process mediated by triplet-triplet annihilation (TTA) in rubrene/C₆₀-based OLEDs. This non-linear EL process realizes the electrically pumped up-conversion by promoting two low-energy triplet excitons into one high-energy exciton, thus extremely lowering the working voltage of resulting OLEDs. It is clearly seen that this up-conversion EL intensity essentially exhibits two distinct regimes at different current densities, i.e. a quadratic dependence at low current density where mono-triplet decay is dominant, and a linear dependence at high current density where bi-triplet decay becomes dominant. These results further our understanding of the non-linear optoelectronic process. In particular, our results demonstrate that energetically utilizing the TTA provides possibility of fabricating low-driving voltage, high efficiency OLEDs via non-linear EL process.     

Investigation of the energy transfer mechanism in OLEDs based on a new terbium β-diketonate complex

This study is focused on the contribution of radiative and non-radiative processes to the electroluminescence emission of OLEDs based on a new terbium(III) complex: {Tris(acetylacetonate)[1,2,5]thiadiazole[3,4-f][1,10]phenanthroline}terbium(III) or [Tb(ACAC)₃TDZP]. The effects of the energy transfer mechanism are discussed based on photoluminescence and electroluminescence measurements. The terbium complex showed an intense photoluminescence with high color purity in the green region, characteristics of the Tb(III) ion narrow line transitions. However, when used in a double-layer OLED its electroluminescence showed an orange broad band emission which can be attributed to the electrophosphorescence of the ligands and to an inefficient energy transfer from the organic ligand to the Tb(III) ion. Alternatively, devices with a Tb(III) complex acting as a dopant (7.6%) in a matrix of CBP used as the active layer showed an improvement in the energy transfer process, resulting in the appearance of the characteristic emission lines of the Tb(III) ion.     

Mechanistic Study on High Efficiency Deep Blue AIE‐Based Organic Light‐Emitting Diodes by Magneto‐Electroluminescence

Aggregation‐induced emission (AIE) materials are highly attractive because of their excellent properties of high efficiency emission in nondoped organic light‐emitting diodes (OLEDs). Therefore, a deep understanding of the working mechanisms, further improving the electroluminescence (EL) efficiency of the resulting AIE‐based OLEDs, is necessary. Herein, the conversion process from higher energy triplet state (T₂) to the lowest singlet state (SS₁) is found in OLEDs based on a blue AIE material, 4′‐(4‐(diphenylamino)phenyl)‐5′‐phenyl‐[1,1′:2′,1′′‐terphenyl]‐4‐carbonitrile (TPB‐AC), obviously relating to the device efficiency, by magneto‐EL (MEL) measurements. A special line shape with rise at low field and reduction at high field is observed. The phenomenon is further clarified by theoretical calculations, temperature‐dependent MELs, and transient photoluminescence emission properties. On the basis of the T₂‐S₁ conversion process, the EL performances of the blue OLEDs based on TPB‐AC are further enhanced by introducing a phosphorescence doping emitter in the emitting layer, which effectively regulates the excitons on TPB‐AC molecules. The maximum external quantum efficiency (EQE) reaches 7.93% and the EQE keeps 7.57% at the luminance of 1000 cd m^?2. This work establishes a physical insight for designing high‐performance AIE materials and devices in the future.     

Organic light-emitting devices based on new rare earth complex Tb（p-CIBA）3phen

A new rare earth complex Tb（p-CIBA）3phen was synthesized and introduced into organic tight emitting devices （OLEDs） as emitting material. The Tb（p-CIBA）3phen was doped into PVK to improve the filmforming and hole-transporting property. Two kinds of devices were fabricated. The device structure is as the following. Single-layer device： ITO/PVK： Tb （p-CIBA） 3 phen /LiF/Al; double-layer device： ITO/PVK： Tb（p-CIBA）3phen/AIQ/LiF/AI. The performances of both devices were investigated carefully. We found that the emission of PVK was completely restrained,and only the green emission was observed from the electroluminescence. The full width at half maximum （FWHM） was less than 10 nm. The highest EL brighthess of the single-layer device is 25.4 cd/cm^2 at a fixed bias of 18 V,and the highest EL brightness of the double-layer device reaches 234.8 cd/cm^2 at a voltage of 20 V.     

Bi3+, Eu 3+, Tb3+ 掺杂Lu3TaO7的发光性能研究

采用固相反应法制备了Bi ³⁺ 、Eu³⁺ 、Tb³⁺ 掺杂的Lu₃TaO₇。测量了样品的X射线衍射谱、激发和发射光谱及荧光衰减曲线。三种离子掺杂的Lu₃TaO₇均呈现出强的荧光发射,其中Bi³⁺具有峰位在431 nm处的一强发射宽带,衰减寿命为16.8 μs,Eu ³⁺ 、Tb ³⁺ 则表现出稀土离子的特征锐发射峰,衰减寿命分别为1.26 ms和1.20 ms。因此,它们均是具有潜在应用前景的重闪烁体材料。     

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.     

Employing ∼100% Excitons in OLEDs by Utilizing a Fluorescent Molecule with Hybridized Local and Charge‐Transfer Excited State

In principle, the ratio (Φ) of the maximum quantum efficiencies for electroluminescence (EL) to photoluminescence (PL) can be expected to approach unity, if the exciton (bound electron–hole pair) generated from the recombination of injected electrons and holes in OLEDs has a sufficiently weak binding energy. However, seldom are examples of Φ > 25% reported in OLEDs because of the strongly bound excitons for most organic semiconductors in nature. Here, a twisting donor–acceptor triphenylamine‐thiadiazol molecule (TPA‐NZP) exhibits fluorescent emission through a hybridized local and charge‐transfer excited state (HLCT), which is demonstrated from both fluorescent solvatochromic experiment and quantum chemical calculations. The HLCT state possesses two combined and compatible characteristics: a large transition moment from a local excited (LE) state and a weakly bound exciton from a charge transfer (CT) state. The former contributes to a high‐efficiency radiation of fluorescence, while the latter is responsible for the generation of a high fraction of singlet excitons. Using TPA‐NZP as the light‐emitting layer in an OLED, high Φ values of 93% (at low brightness) and 50% (at high brightness) are achieved, reflecting sufficient employment of the excitons in the OLED. Characterization of the EL device shows a saturated deep‐red emission with CIE coordinates of (0.67, 0.32), accompanied by a rather excellent performance with a maximum luminance of 4574 cd m^?2 and a maximum external quantum efficiency (η_ext) of ～2.8%. The HLCT state is a new way to realize high‐efficiency of EL devices.     

含氮杂菲类中性配体的铕配合物电致发光研究

文章用一系列含氮杂菲类中性配体的铕配合物Eu（DBM）3pyzphen、Eu（DBM）3dpmq、Eu（DBM）3dppz和Eu（DBM）3dptp制作了有机EL器件,研究了它们的电学特性。采用的器件结构为ITO/TPD/CBP：Eu（DBM）3L/BCP/Alq3/LiF/Al（L=pyzphen、dpmq、dppz和dptp）,其中基于Eu（DBM）3pyzphen的器件获得了992cd/m2的最大亮度和2.9cd/A的最大效率。在此基础上分析了第二配体对器件性能的影响。     

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.     

Structure optimization of organic light-emitting devices

A triple layer organic light-emitting diode (OLED) with two heterostructure of indium-tin oxide (ITO)/N,N’-diphenyl-N, N’-bis(1-naphthyl) (1,1’-biphenyl)-4,4’-diamine (NPB)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/ 8-Hydrox- yquinoline aluminum (Alq3)/Mg:Ag has been fabricated by using the vacuum deposition method. The influence of different film thickness of BCP layer on the performance of the OLEDs has been investigated. The results show that when the thickness of the BCP layer film gradually r...     

掺Eu3+, Sm3+, Tb3+的TTFA配合物敏化发光效应的实验研究

为了研究稀土离子的α-噻吩甲酰三氟丙酮(α-thienyltrifluoroacetone,TTFA)配合物中敏化发光效应,对自行制备出的稀土离子Eu3+,Sm3+和Tb3+单掺TTFA配合物Eu(TTFA)3,Tb(TTFA)3,Sm(TTFA)3及3种稀土离子两两共掺的配合物的荧光光谱进行了分析,得出了稀土离子Eu3+,Sm3+,Tb3+与配体TTFA的敏化特性以及稀土离子Eu3+,Sm3+和Tb3+之间的敏化特性。Eu(TTFA)3,Tb(TTFA)3,Sm(TTFA)3及3种稀土离子共掺的配合物中,稀土离子Eu3+,Sm3+,Tb3+与配体TTFA及Eu3+,Sm3+,Tb3+之间有明显的敏化效应。结果表明,将其用于聚合物光纤放大器具有良好的发展前景。