Phosphorescent dye-doped hole transporting layer for organic light-emitting diodes

Hole transport materials are critical to the performance of organic light-emitting diodes (OLEDs). While 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) with a high triplet energy is widely used for high efficiency phosphorescent OLEDs, devices using TAPC as a hole transport layer (HTL) have a short operating lifetime due to the build-up of trapped charges at the TAPC/emitting layer (EML) interface during device operation. In this work, to solve the operating stability problem, instead of using conventional HTLs, we use a(fac-tris(2-phenylpyridine)iridium (III))(Ir(ppy)₃) doped layer as an HTL to replace the conventional HTLs. Because of the hole injecting and transporting abilities of the phosphorescent dye, holes can be directly injected into the emitting layer without an injection barrier. OLEDs based on a phosphorescent dye-doped HTL show significant improvement in operational stability without loss of efficiency.     

High performance organic light emitting diodes using substoichiometric tungsten oxide as efficient hole injection layer

In this work we demonstrate the unique hole injection and transport properties of a substoichiometric tungsten oxide with precise stoichiometry, in particular WO_2.5, obtained after the controlled hydrogen reduction during growth of tungsten oxide, using a simple hot-wire vapor deposition technique. We present clear evidence that tungsten suboxide exhibits metallic character and that an almost zero hole injection barrier exists at the anode/polymer interface due to the formation/occupation of electronic gap states near the Fermi level after oxide’s reduction. These states greatly facilitate hole injection and charge generation/electron extraction enabling the demonstration of extremely efficient hole only devices. WO_2.5 films exhibit metallic-like conductivity and, thus, can also enhance charge transport at both anode and cathode interfaces. Electroluminescent devices using WO_2.5 as both, hole and electron injection layer, and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1′,3}-thiadiazole)] (F8BT) as the emissive layer exhibited high efficiencies up to 7 cd/A and 4.5 lm/W, while, stability studies revealed that these devices were extremely stable, since they were operating without encapsulation in air for more than 700 h.     

Stable solution processed hole injection material for organic light-emitting diodes

A new aqueous based polymer, Plexcore® OC AQ1200 (AQ1200) was used as a hole injection layer (HIL) in organic light emitting diodes (OLEDs) and the device results are compared with polyethylene dioxythiophene:polystyrene sulfonate (PEDOT:PSS) in terms of injection efficiency and stability. Dark injection transient measurements show a higher injection efficiency in hole-only devices using AQ1200 HIL compared with PEDOT:PSS. Using AQ1200 as an HIL, high efficiency phosphorescent OLEDs are demonstrated to have a long lifetime, with an estimated operational half lifetime (LT 50) of more than 8000 h from an initial luminance of 1000 cd/m².     

High efficiency green phosphorescent top-emitting organic light-emitting diode with ultrathin non-doped emissive layer

Ultrathin non-doped emissive layer (EML) has been employed in green phosphorescent top-emitting organic light-emitting diodes (TOLEDs) to take full advantages of the cavity standing wave condition in a microcavity structure. Much higher out-coupling efficiency has been observed compared to conventional doped EML with relatively wide emission zone. A further investigation on dual ultrathin non-doped EMLs separated by a special bi-layer structure demonstrates better charge carrier balance and improved efficiency. The resulting device exhibits a high efficiency of 125.0 cd/A at a luminance of 1000 cd/m² and maintains to 110.9 cd/A at 10,000 cd/m².     

Efficient,color-stable flexible white top-emitting organic light-emitting diodes

Flexible white top-emitting organic light-emitting diodes (WTEOLEDs) with red and blue phosphorescent dual-emitting layers were fabricated onto polyethylene terephthalate (PET) substrates. By inserting a 2-nm thin tris(phenypyrazole)iridium between the red and the blue emitters as an electron/exciton blocking layer, significant improvements on luminous efficiency and color stability were observed, reaching 9.9 cd/A (3.74 lm/W) and a small chromaticity change of (0.019, 0.011) in a wide luminance range of 80–5160 cd/m². The origin on color stability was explored by analyzing the electroluminescent spectra, the time-resolved transient photoluminescence decay lifetimes of phosphors, and the tunneling phenomenon. In addition, mechanical bending lifetimes in WTEOLEDs with spin-coated     

Inverted top-emitting organic light-emitting diodes by whole device transfer

We present a method for improving the efficiency of charge carrier injection at both cathode and anode of inverted top-emitting organic light-emitting diode (TOLED). In this method, a bottom-emitting OLED (BOLED) is first fabricated on a flexible substrate, which is then transferred as a whole to a glass substrate for the fabrication of the inverted TOLED (ITOLED). The electrode engineering that is responsible for the improvement, which is not possible in the traditional fabrication of ITOLED, is made realizable by the whole device transfer.     

Novel P-I-N-P top-emitting organic light-emitting diodes with enhanced efficiency and stability

An additional p-doping layer is added to the P-I-N stack of top-emitting organic light-emitting diodes (TEOLEDs) to control the electron tunneling current and improve interfacial stability. In addition, double p-doped layers, which are adjacent to the bottom-anode, are introduced to simultaneously optimize robustness and doping efficiency of p-type doping. In the emissive layer (EML), a second assistant emitter molecule is used which transfer its triplet energy to the actual emitter which is lower in energy, thus increasing the luminous efficacy. Such a co-doped dual-emitter layer is able to separate polarons and excitons and thus reduces chemical degradation. Compared to conventional P-I-N TEOLEDs, our novel P-I-N-P device shows negligible increase of driving voltage at low bias but offers significantly increased efficiencies. In addition, the P-I-N-P stack renders the electrical properties less sensitive to thickness variations and prolonged operation, which is attributed to the existence of a one-sided abrupt N-P tunneling junction beneath the top cathode contact.     

High efficiency p-i-n organic light-emitting diodes with a novel n-doping layer

This study demonstrated p-i-n organic light-emitting diodes (OLEDs) incorporating a novel n-doping transport layer which is comprised of cesium iodide (CsI) doped into tris-(8-hydroxyquinoline) aluminum (Alq₃) as n-doping electron transport layer (n-ETL) and a p-doping hole transport layer (p-HTL) which includes molybdenum oxide (MoO₃) doped into 4,4′,4″-tris[2-naphthyl(phenyl)amino] triphenylamine (2-TNATA). The device with a 15 wt.% CsI-doped Alq₃ layer shows a turn on voltage of 2.4 V and achieves a maximum power efficiency of to 4.67 lm/W as well, which is significantly improved compared to these (3.6 V and 3.21 lm/W, respectively) obtained from the device with un-doped Alq₃. This improvement is attributed to an increase in the number of electron carriers in the transportation layer leading to an efficient charge balance in the emission zone. A possible mechanism behind the improvement is discussed based on X-ray photoelectron spectroscopy (XPS).     

High-efficiency and reduced efficiency roll-off of top-emitting organic light-emitting diodes

Top-emitting organic light-emitting diodes (TEOLEDs) have attracted extensive attention for their high brightness and flat-panel display. However, the efficiency roll-off at high brightness is the issue that needs to be resolved for further practical applications using TEOLED devices. Herein, a serial of high-efficiency tandem TEOLED introducing a fullerene/zinc-phthalocyanine organic semiconductor heterojunction as a charge generation layer is demonstrated. With unique photovoltaic properties, the charge generation layer can absorb part of the photons emitted by the emission layer (Ir(ppy)3) and generate electrons and holes. By optimizing the thickness of the charge generation layer, the pure green electroluminescent TEOLED device manufactured has a high brightness of 156 000 cd/cm2 and a maximum current efficiency of 86 cd/A. Importantly, relying on the energy between the photovoltaic and the microcavity effects, only 1.5% of the efficiency roll-off is obtained at 1 000—10 000 cd/cm2. Introducing fullerene/zinc-phthalocyanine as the charge-generating layer provides a promising alternative for developing high-efficiency tandem TEOLED devices.     

Solution-processed MoOx hole injection layer towards efficient organic light-emitting diode

Low cost, high throughout and large scale production techniques, such as roll-to-roll, printing and doctor blading, boost the favorite of electronic devices with all solution process. While MoO_x are conventionally formed via high temperature and vacuum deposition, we develop a novel, lower-temperature, solution-processable MoO_x hole injection layer (HIL) and cast successful application in organic light-emitting diodes (OLEDs). The characterization of MoO_x is presented in detail using X-ray diffraction, atomic force microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and impedance spectroscopy measurements. The results show MoO_x features amorphous phase structure, superior film morphology and exceptional electronic properties. With solution-processed MoO_x as HIL, highly efficient OLED is demonstrated. The luminous efficiency has been enhanced by 56% in comparison with that of the counterpart using evaporated MoO_x. The main reasons for the substantially improved performance are the tailored surface work function and appropriate hole injection capacity correspondingly result in optimizing carrier balance in OLED. Our results pave a way for advancing MoO_x-based organic electronic devices with solution-processable techniques.     

Solution-processed quantum dot light-emitting diodes based on NiO nanocrystals hole injection layer

Nickel oxide (NiO), as a kind of p-type transition metal oxide (TMO) has shown promising applications in photoelectric devices. In our work, the NiO nanocrystals (NCs) are fabricated by a simple solvothermal method using tert-butyl alcohol and nickel acetylacetonate as precursors at 200 °C for different reaction times. The diameters and valence band edge of the prepared NiO NCs are increased with the increase reaction time from 12 h, 24 h–36 h. The band gaps of the NiO NCs were decreased with the increase time. Selected area electron diffraction (SAED) shows that the NiO NCs is polycrystalline structure. X-ray diffraction (XRD) indicates that the NiO NCs is cubic crystal form. X-ray photoelectron spectroscopy (XPS) shows that the as-prepared NiO NCs have a core of NiO and some form of Ni₂O₃ and NiOOH states on its surface. Further, the obtained NiO NCs is applied on quantum dot light-emitting diode (QLED) as hole injection layer (HILs), showing excellent hole injection properties. Particularly, the NiO NCs for 24 h obtains the best results due to its high band gap and pure cubic crystal phase. Highly bright orange-red QLED with peak luminance up to ∼25580 cd m⁻², and current efficiency (CE) of 5.38 cd A⁻¹ are achieved successfully based on the high performance NiO HIL, further, the device obtained relative long operational lifetime of 11491 h, which has been improved by more than 6- fold as compared to 1839 h for the device based on PEDOT.     

双空穴注入层对有机电致蓝光器件性能的改善

CuI/CuPc被采用作为有机电致蓝光CBP:BCzVBi器件 的双空穴注入层。采用双空穴注入层后使得CBP:BCzVBi蓝光器件的启亮电压降低至 3.4 V,较采用CuPc单空穴注入层的CBP:BCzVBi蓝光器件低0.4 V。在驱动电流20 mA/cm²的情况下,与单空穴注入层器件 相比,采用该双空穴注入层结构使得器件电流效率提升约19%,亮度 增加约17%,驱动电压降低0.9 V。采用Fowler -Nordheim (F-N)隧穿注入理论对器件空穴注入电流的影响因素进行了分析,发现双空穴 注入层形成的能级台阶可以有效地改善发光器件的空穴注入效率,进而起到改善器件发光电 流效率和降低驱动电压的目的。     

有机发光器件中空穴注入对负电容的影响

对不同结构的有机发光器件(OLED)进行了电容-电压(C-V)特性测量,研究了不同空穴注入结构对OLED负电容的影响。结果表明,负电容的产生与OLED内部电场的分布有着密切的关系,负电容开始出现的频率与电压的平方根呈指数关系。与超薄的单层空穴注入层相比,掺杂的空穴注入层不仅能降低器件的驱动电压,而且其载流子传输特性和出现负电容时的初始电压对频率有着更强的依赖性。     

High-contrast top-emitting organic light-emitting diodes with AlO1.086 dark-and-conductive electrodes

To improve the poor contrast of conventional organic light-emitting diodes (OLEDs) resulting from highly reflective metal electrode, a dark-and-conductive electrode with an average reflectance of 28.1% and a resistivity of 4.6 × 10⁻⁴ Ω/cm was fabricated by fine-tuning O₂/Ar flow ratio on aluminum electrode sputtering. X-ray photoelectron spectroscopy analysis indicates pure aluminum and aluminum oxide coexist in the fabricated dark-and-conductive electrodes. With the proposed dark-and-conductive AlO_1.086 electrodes, top-emitting OLEDs exhibit significantly improved contrast, whereas maintain moderate luminous efficiency. The demonstrated AlO_1.086 dark-and-conductive electrodes can potentially replace the circular polarizers for high-contrast OLED display applications.     

Flexible top-emitting warm-white organic light-emitting diodes with highly luminous performances and extremely stable chromaticity

The flexible top-emitting white organic light-emitting diode (FTEWOLED) with a very high efficiency but a significant color alteration is achieved with a blue/red/blue sandwiched tri-emission-layer. The voltage-dependent recombination region alternation and the emission mechanism are systematically investigated through a delta-doping method and the time-resolved transient photoluminescence lifetime measurement. By locating the main exciton recombination region at the 4,4′,4″-Tris(carbazol-9-yl)-triphenylamine (TCTA) and 9,9-spirobifluoren-2-yl-diphenyl-phosphine oxide (SPPO1) interface, replacing the carrier-trapping red dopant guest with an orange guest that utilizes energy transfer mechanism, and using a P–I–N structure together with the FIrpic blue guest dopant to balance the electron and hole carriers, an extremely color stable and a very high efficient FTEWOLED is fabricated, with the resulting high current and power efficiencies of 22.7 cd/A and 14.27 lm/W, and a warm white illumination with a small chromaticity variation of (−0.0087, +0.0015) over a broad luminance range of more than four orders of magnitude. In addition, the performances can be further improved to 23,340 cd/m², 24.49 cd/A and 15.39 lm/W with a slight concentration alteration of the orange emitter.     

Effective hole-injection layer for non-doped inverted top-emitting organic light-emitting devices

Non-doped inverted top-emitting organic light-emitting diode with high efficiency is demonstrated through employing an effective hole-injection layer composed of MoO_x. One reason for high efficiency lies on the energy-level matching between MoO_x and hole-transport, and another is due to the Ohmic contact formed between MoO_x and Ag. Both of them lead to an improvement of the hole-injection capability from Ag top anode. Moreover, the symmetrical current of “hole-only” device with MoO_x shows better hole-injection capability, which is independent of the deposition sequence. The optimized device with MoO_x hole-injection layer exhibits maximum current efficiency of 3.7 cd/A at a raised luminance level of 14,900 cd/m² and a maximum luminance of 47,000 cd/m² under 18 V.     

Lithium phenolate complexes for an electron injection layer in organic light-emitting diodes

We synthesized π-conjugated lithium phenolate complexes, lithium 2-(2-pyridyl)phenolate (LiPP), lithium 2-(2′, 2′′-bipyridine-6′-yl)phenolate (LiBPP), and lithium 2-(isoquinoline-1′-yl)phenolate (LiIQP). These complexes showed lower sublimation temperatures of 305–332 °C compared to 717 °C of LiF. The organic light-emitting devices (OLEDs) using these complexes as an electron injection layer exhibited high efficiencies which are comparable to that of the device using LiF. Especially, a 40-nm thick film of LiBPP or LiPP was effective as an electron injection material, providing low driving voltages, while such a thick film of LiF serves as a complete insulator, resulting in high driving voltages.     

Ionic liquid modified zinc oxide injection layer for inverted organic light-emitting diodes

We have demonstrated a novel approach for fabricating efficient hybrid organic–inorganic light emitting diodes (HyLEDs) by introducing dopants into solutions processable metal oxides as an interfacial layer. The doped ZnO is prepared by adding ionic liquid (IL) to a precursor solution for the ZnO. In this way a heavily doped ZnO:ILs cathode was obtained that enhances the electron injection properties and assures a good wetting of the organic active materials.     

High-performance inverted top-emitting green electrophosphorescent organic light-emitting diodes with a modified top Ag anode

Green electrophosphorescent inverted top-emitting organic light-emitting diodes with a Ag/1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) anode are demonstrated. A high current efficacy of 124.7 cd/A is achieved at a luminance of 100 cd/m² when an optical outcoupling layer of N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl-4,4′-diamine (α-NPD) is deposited on the anode. The devices have a low turn-on voltage of 3.0 V and exhibit low current efficacy roll-off through luminance values up to 10,000 cd/m². The angle dependent spectra show deviation from Lambertian emission and color change with viewing angle. Hole-dominated devices with Ag/HAT-CN electrodes show current densities up to three orders of magnitude higher than devices without HAT-CN.