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.     

Different orientation of the transition dipole moments of two similar Pt(II) complexes and their potential for high efficiency organic light-emitting diodes

The efficiency of organic light-emitting diodes (OLEDs) is especially limited by their low light outcoupling efficiency. An approach for its enhancement is the use of horizontally oriented emitter molecules with respect to the substrate. In this study we quantitatively determine the orientation of the optical transition dipole moments in doped films of two similar phosphorescent Pt(II) complexes having a linear molecular structure. These emitters are employed in OLED devices and their efficiency is analyzed by optical simulations. For an OLED with slightly more horizontally oriented emitter molecules an external quantum efficiency (η_EQE) of 15.8% at low current-density is realized, indicating a relative improvement of outcoupling efficiency of 5.3% compared to the isotropic case. However, a very similar complex adopting isotropic molecular orientation yields η_EQE of only 11.5% implying an imperfect charge carrier balance in the OLED device and a shift of the recombination zone. Furthermore, we highlight the enormous potential of horizontal molecular orientation of emitting molecules in OLEDs.     

Transparent and color-tunable organic light-emitting diodes with highly balanced emission to both sides

Future lighting applications will strongly benefit from transparent luminescent devices. Here, we demonstrate transparent organic light-emitting diodes (OLEDs), which provide real-time adjustment of the emission color. Making use of the AC/DC concept, two stacked subunits can be addressed independently via an AC signal. Combining blue and yellow emission leads to the possibility to tune the emitted color between deep blue over cold white and warm white to yellow on both emission sides. For such highly complex device architectures, the thickness of each layer needs to be adjusted carefully in order to achieve balanced and efficient emission in both directions. Therefore, optical simulations are carried out to optimize the OLED. Based on these simulations, we present transparent, indium-free OLEDs that achieve a luminous efficacy of 8.7 lm/W in bottom direction and 9.7 lm/W in top direction at a brightness level of 1000 cd/m² for warm white emission and a peak transmission of 56%. Using an emitter combination providing red, green, and blue emission, we were able to achieve a high color-rendering index (CRI) of 84, which further expands the range of possible applications for this promising device concept.     

Highly Efficient Deep Blue Phosphorescent OLEDs Based on Tetradentate Pt(II) Complexes Containing Adamantyl Spacer Groups

Tetradentate Pt(II) complexes are promising emitters for deep blue organic light-emitting diodes (OLEDs) due to their emission energy and high photoluminescence efficiency. However, to obtain a pure blue color, spectral red-shifts, and additional emission peaks at longer wavelengths, originating from strong intermolecular interactions between parallel Pt(II) complexes, must be avoided. Herein, a new class of deep-blue emitting tetradentate Pt(II) complexes consisting of a non-planar ligand and a bulky adamantyl group is reported. The six-membered metallacycle structure renders the Pt(II) complex non-planar. In addition, the bulky adamantyl groups increase intermolecular distances and decrease red-shifts in the emission originating from strong dipole–dipole interactions. Therefore, these Pt(II) complexes exhibit little change in emission color with increasing dopant concentration. OLEDs incorporating these new Pt(II) complexes as emitters exhibit deep blue emission with a Commission International de L'Eclairage (CIE) y under 0.13 and a maximum external quantum efficiency of 22.6%, which is one of the highest observed for deep blue (CIE y < 0.15) phosphorescent OLEDs using Pt(II) complexes. These results provide a new approach for designing Pt(II) complexes for high efficiency deep blue OLEDs.     

Design Strategy of Blue and Yellow Thermally Activated Delayed Fluorescence Emitters and Their All‐Fluorescence White OLEDs with External Quantum Efficiency beyond 20%

Two thioxanthone‐derived isomeric series of thermally activated delayed fluorescence (TADF) emitters 1,6‐2TPA‐TX/3,6‐2TPA‐TX and 1,6‐2TPA‐TXO/3,6‐2TPA‐TXO are developed for organic light‐emitting diodes (OLEDs). Blue emission devices based on symmetrical 3,6‐2TPA‐TX with common vertical transition route realize an extremely high external quantum efficiency (EQE) of 23.7%, and an ever highest EQE of 24.3% is achieved for yellow emission devices based on 3,6‐2TPA‐TXO by solely changing the sulfur atom valence state of the thioxanthone core. In contrast, their corresponding asymmetric isomers are affected by intramolecular energy transfer and more severely by a nonradiative deactivation pathway, to give much low EQE values (<5%). By utilizing 3,6‐2TPA‐TX as a blue emitter and 3,6‐2TPA‐TXO as a yellow emitter, an ever highest EQE of 20.4% is achieved for all‐fluorescence white OLEDs.     

High efficiency organic light-emitting diodes with in situ synthesized Cu(I) complex emitter: How to do chemical reaction in a vacuum chamber?

Cu(I) complexes are considered as idea emitters in organic light-emitting diodes (OLEDs) due to their low cost and theoretical high internal quantum efficiency, which are two important issues have to be concerned for OLEDs commercialization. However, most Cu(I) complexes are unstable toward sublimation and hence not amenable to the vacuum deposition method typically used to fabricate OLEDs. To solve this problem, a codeposition route that involves codeposition of CuI and pyridine derivative has been proposed to synthesis Cu(I) complex emitter in situ. Since chemical reactions were conducted in a vacuum chamber, we systematically studied the effect of reactant chemical structure, reaction ratio, and deposition rate on the in situ synthesized Cu(I) complex and its application as an emitter in OLEDs. With an optimal chemical reaction condition, the device showed a high external quantum efficiency (EQE) up to 14.2% at a brightness of 100 cd/m², corresponding to a current and power efficiency of 45.2 cd/A and 33.3 lm/W, respectively. The performance is comparable to those efficient OLEDs with iridium complex emitter, while using a CuI dopant that having only one ten-thousandth of price to bis(2-phenylpyridine) (acetylacetonate)iridium.     

Facile Synthesis of Highly Efficient Lepidine‐Based Phosphorescent Iridium(III) Complexes for Yellow and White Organic Light‐Emitting Diodes

Highly efficient lepidine‐based phosphorescent iridium(III) complexes with pentane‐2,4‐dione or triazolpyridine as ancillary ligands have been designed and prepared by a newly developed facile synthetic route. Fluorine atoms and trifluoromethyl groups have been introduced into the different positions of ligand, and their influence on the photophysical properties of complexes has been investigated in detail. All the triazolpyridine‐based complexes display the blueshifted dual‐peak emission compared to the pentane‐2,4‐dione‐based ones with a broad single‐peak emission. The complexes show emission with broad full width at half maximum (FWHM) over 100 nm, and the emissions are ranges from greenish–yellow to orange region with the absolute quantum efficiency (Φ_PL) of 0.21–0.92 in solution, i.e., Φ_PL = 0.92 ( 18 ), which is the highest value among the reported neutral yellow iridium(III) complexes. Furthermore, high‐performance yellow and complementary‐color‐based white organic light‐emitting diodes (OLEDs) have been fabricated. The FWHMs of the yellow, greenish–yellow OLEDs are in the range of 94–102 nm, which are among the highest values of the reported yellow or greenish–yellow‐emitting devices without excimer emission. The maximum external quantum efficiency of monochrome OLEDs can reach 24.1%, which is also the highest value among the reported yellow or greenish–yellow devices. The color rendering indexes of blue and complementary yellow‐based white OLED is as high as 78.     

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.     

Dependence of Pt(II) based phosphorescent emitter orientation on host molecule orientation in doped organic thin films

We report that the molecular orientation of a disk-shaped Pt(II) complex dopant in organic thin films is linearly proportional to the orientation of the host molecules. We ascribe this relationship to the parallel alignment of the Pt complex with the host molecules induced by a π-π interaction. This would be caused by their planar and conjugated structure, indicating that the intermolecular interaction and steric effect play an important role. This finding can be applied to obtain a horizontal emitter orientation, resulting in highly efficient OLEDs based on Pt(II) complexes.     

Synthesis,Photophysical, and Electroluminescent Device Properties of Zn(II)‐Chelated Complexes Based on Functionalized Benzothiazole Derivatives

New Zn(II)‐chelated complexes based on benzothiazole derivatives, including substituted functional groups such as methyl ( MeZn ), methoxy ( MeOZn ), or fluorenyl unit ( FuZn ), are investigated to produce white‐light emission. 2‐(2‐Hydroxyphenyl)benzothiazole derivatives in toluene and DMSO exhibit excited‐state intramolecular proton transfer (ESIPT), leading to a large Stokes shift of the fluorescence emission. However, in methanol they exhibit no ESIPT due to the intermolecular hydrogen bonding between the 2‐(2‐hydroxyphenyl)benzothiazole derivative and methanol. Their Zn(II)‐chelated complexes exhibit the absorption band red‐shifted at 500 nm in nonpolar solvent and the absorption band blue‐shifted at about 420 nm in protic solvent. In multilayer electroluminescent devices, methyl‐substituted Zn(II)‐chelated complex ( MeZn ) exhibits excellent power efficiency and fluorene‐substituted Zn(II)‐chelated complex ( FuZn ) has a high luminance efficiency (1 cd m^?2 at 3.5 V, 10 400 cd m^?2 at 14 V). The EL spectra of Zn(II)‐chelated complexes based on benzothiazole derivatives exhibit broad emission bands. In addition, their electron‐transport property for red–green–blue (RGB) organic light‐emitting diodes (OLEDs) is systematically studied, in comparison with that of Alq₃. The results demonstrate the promising potential of MeZn as an electron‐transporting layer (ETL) material in preference to Alq₃, which is widely used as an ETL material.     

多层白色有机发光器件的结构和性能优化

以红、蓝、绿为基,制备了不同发光层组合次序的有机发光器件,研究了各发光层的顺序及厚度对器件性能的影响,并在此基础上构成了白色有机发光器件.通过改变关键发光层的厚度,来调节不同颜色之间的平衡,从而达到色度很好的向色;由于关键发光层的厚度很薄,因此得到的器件在高电压的色度漂移也很小.优化的白光器件在200 mA/cm2时,电流效率为3.78 cd/A,色坐标为x=0.345,y=0.323.根据激子产生和扩散理论,讨论了器件性能对于各发光层的厚度及激子扩散长度的依赖关系,拟合结果与实验结果吻合.     

Co‐deposited Cu(I) Complex for Tri‐layered Yellow and White Organic Light‐Emitting Diodes

Four compounds 4‐[3,6‐di(carbazol‐9‐yl)carbazol‐9‐yl]isoquinoline (TCIQ), 3‐[3,6‐di(carbazol‐9‐yl)carbazol‐9‐yl]pyridine (TCPy), 4‐(carbazol‐9‐yl)isoquinoline (4CIQ), and 3‐(carbazol‐9‐yl)pyridine (CPy) containing pyridyl or isoquinolyl were designed and synthesized to co‐deposition with copper iodide (CuI) to form luminescent Cu(I) complex doped film in situ, which could be utilized as the emissive layer in organic light‐emitting diodes (OLEDs). It is found that simple tri‐layered yellow and white OLEDs can be achieved by co‐depositing CuI and TCIQ with tuning ratios. The compound TCIQ serves a dual role as both a ligand for forming the emissive Cu(I) complex and as a host matrix for the formed emitter in yellow OLEDs, and a third role as a blue emitter in white OLEDs.     

Highly efficient,orange–red organic light-emitting diodes using a series of green-emission iridium complexes as hosts

We investigated highly efficient phosphorescent organic light-emitting diodes (OLEDs) based on an orange–red emission iridium complex as the guest and five green emission iridium complexes as the host material, respectively. For comparison, a device using a common fluorescent host CBP (4,4′-bis(N-carbazolyl)-1,1′-biphenyl) has also been fabricated. Results show that the steric hindrance and exciton transporting property of the iridium complex host are found to be critical to this kind of doping system, a proper steric hindrance and improved exciton transporting ability result in reducing of triplet–triplet annihilation, thus improving of the device performance. In addition, all devices using iridium complexes as host have better performance than that of CBP, which arised from the fact that those green emission iridium complexes have a lower triplet excited energy befitting for energy confinement and a higher highest occupied molecular orbital (HOMO) level for hole injection.     

Novel Deep-Blue Hybridized Local and Charge-Transfer Host Emitter for High-Quality Fluorescence/Phosphor Hybrid Quasi-White Organic Light-Emitting Diode

A new hybrid local and charge transfer (HLCT) molecule 2TPA-PPI is obtained for constructing the high-performance organic light-emitting diodes (OLEDs) in this work. 2TPA-PPI possesses the sufficient emission/charge-transporting properties, thus it is used as a neat emitter achieving an efficient deep-blue OLED with very high external quantum efficiency (EQE) up to 10.7%, as well as a multi-functional emitting host matrix constructing the high-performance phosphorescent OLEDs. More importantly, a high-efficiency candle light-style OLED adopting the HLCT/phosphor hybrid strategy is realized, where 2TPA-PPI acts as not only a blue emitter, but also a universal host sensitizing both yellow and red phosphors. This quasi-white OLED represents almost the highest EQE/PE level of 25.2%/49.7 lm W⁻¹ at the practical luminance level of 1000 cd m⁻² for the white OLEDs with the excellent color rendering index values of more than 80 reported.     

Peripheral Decoration of Multi-Resonance Molecules as a Versatile Approach for Simultaneous Long-Wavelength and Narrowband Emission

High device efficiency and color-purity are the two essentials for high-quality organic light-emitting diodes (OLEDs). Multi-resonance (MR) molecules show great potentials for high color-purity OLEDs due to their sharp emission bands. However, most MR molecules exhibit emission limited from deep-blue to green spectral region. Herein, through peripherally decorating MR emitter with electron donors, a new approach enabling the emission spectra of MR emitters red-shift while retaining narrowband emission is demonstrated. By manipulating the numbers and electron-donating abilities of the peripheries, the first narrowband yellow emitter with emission maxima of 562 nm and a full-width at half-maximum (FWHM) of 30 nm is realized. Highly efficient OLEDs with an external quantum efficiency of over 24% and excellent color purity are fabricated by employing these newly developed MR molecules as emitters.     

High Efficiency Blue Organic LEDs Achieved By an Integrated Fluorescence–Interlayer–Phosphorescence Emission Architecture

This paper presents a new strategy to develop efficient organic light‐emitting devices (OLEDs) by doping fluorescent‐ and phosphorescent‐type emitters individually into two different hosts separated by an interlayer to form a fluorescence–interlayer–phosphorescence (FIP) emission architecture. One blue OLED with FIP emission structure comprising p‐bis(p‐N,N‐diphenylaminostyryl)benzene (DSA‐Ph) and bis[(4,6‐di‐fluorophenyl)‐pyridinate‐N,C²']picolinate (FIrpic) exhibiting a peak luminance efficiency of 15.8 cd A^?1 at 1.54 mA cm^?2 and a power efficiency of 10.2 lm W^?1 at 0.1 mA cm^?2 is successfully demonstrated. The results are higher than those of typical phosphorescent OLEDs with a single emission layer by 34% and 28%, respectively. From experimental and theoretical investigations on device performance, and the functions of the used emitters and interlayer, such enhancement should ascribe to the appropriate utilization of the two types of emitters. The fluorescent emitter of DSA‐Ph is used to facilitate the carrier transport, and thus accelerate the generation of excitons, while the phosphorescent emitter of FIrpic could convert the generated excitons into light efficiently. The method proposed here can be applied for developing other types of red, green, and white OLEDs.     

Highly efficient thienylquinoline-based phosphorescent iridium(III) complexes for red and white organic light-emitting diodes

Highly efficient 2-(thiophen-2-yl)quinoline-based phosphorescent iridium(III) complexes bearing 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine or picolinic acid as ancillary ligands are designed and synthetised. The variation of ancillary ligands is attempted to finely tune the photophysical properties of these complexes, especially the solution phosphorescent quantum yields (ΦPL), full width at half maximum (FWHM), etc. The picolinic acid-based complex displays the slightly red-shifted dual-peak emission compared to triazolpyridine-based one. The complexes show bright emission with broad FWHM up to 83 nm, and the emissions are in red region with the very high absolute ΦPL up to 0.76 in solution. Moreover, high-performance red and three-color-based white organic light-emitting diodes (OLEDs) with excellent color stability have been fabricated. The maximum external quantum efficiencies of red and white OLEDs can reach 16.2% and 15.1%, respectively. The maximum current efficiency and power efficiency of white OLED are as high as 35.5 cd A⁻¹ and 34.0 lm W⁻¹, respectively. Especially, the designed white OLED exhibits excellent spectral stability under wide operating voltage range, and the 1931 Commission Internationale de L'Eclairage of white OLED only changes from (0.43, 0.42) to (0.44, 0.44), the color rendering index is in a narrow range of 75–77.     

利用PtOEP掺杂改善Spiro发光器件的发光效率

为了利用有机三线态发光提高有机发光器件的发光效率,用磷光材料掺杂到聚合物主体中作为发光层,制备有机电致发光器件.在测量器件的电流-电压特性、发光亮度-电压特性和电致发光谱的基础上,计算了器件的外量子效率,研究了磷光材料的掺杂浓度对器件发光效率的影响.结果表明,对特定的材料体系,适当控制掺杂浓度,可以同时观察到荧光和磷光光谱,使掺杂器件的外量子效率在纯聚合物发光器件的基础上得到明显提高.     

Efficiency analysis of organic light-emitting diodes based on optical simulation

In spite of huge progress in improving the internal quantum efficiency of organic light-emitting diodes (OLEDs), these devices still suffer from poor light out-coupling. Loss mechanisms are for example waveguiding in the organic layers and the substrate as well as the excitation of surface plasmons at metallic electrodes. Their relative strength and the mutual dependence on the OLED structure have been studied both experimentally and by numerical simulation. Here, we consider the impact of the radiative quantum efficiency of the emitter material on predictions of light extraction from OLEDs. Competing processes resulting in non-radiative recombination of charge carriers usually reduce the emitter quantum efficiency in a real device. We show that optical simulation leads to erroneous conclusions when neglecting these competing processes. Furthermore, we demonstrate a method, which allows determining both the radiative quantum efficiency and the charge recombination factor via simulation based analysis of experimental data. This analysis of device efficiency is applied on a set of red-emitting electrophosphorescent devices.