Highly Efficient Hole Injection Using Polymeric Anode Materials for Small‐Molecule Organic Light‐Emitting Diodes

A novel, highly efficient hole injection material based on a conducting polymer polythienothiophene (PTT) doped with poly(perfluoroethylene‐perfluoroethersulfonic acid) (PFFSA) in organic light‐emitting diodes (OLEDs) is demonstrated. Both current–voltage and dark‐injection‐current transient data of hole‐only devices demonstrate high hole‐injection efficiency employing PTT:PFFSA polymers with different organic charge‐transporting materials used in fluorescent and phosphorescent organic light‐emitting diodes. It is further demonstrated that PTT:PFFSA polymer formulations applied as the hole injection layer (HIL) in OLEDs reduce operating voltages and increase brightness significantly. Hole injection from PTT:PFFSA is found to be much more efficient than from typical small molecule HILs such as copper phthalocyanine (CuPc) or polymer HILs such as polyethylene dioxythiophene: polystyrene sulfonate (PEDOT‐PSS). OLED devices employing PTT:PFFSA polymer also demonstrate significantly longer lifetime and more stable operating voltages compared to devices using CuPc.     

新型双空穴注入型高效有机电致发光二极管

采用一种新型有机电致发光二极管（OLED）的阳极结构,在玻璃衬底上以半透明的A1膜为出光面,通过在空穴注入层（HIL）和空穴传输层（HTL）中间插入MoOa层,制备了底发射OLED。制备的器件结构为Glass／Al（15nm）／HAT—CN（IOnm）／M003（30nm）／NPB（30nm）／Alq3（60nm）／B...     

Triplet-induced degradation: An important consideration in the design of solution-processed hole injection materials for organic light-emitting devices

Solution-processed organic light-emitting devices (OLEDs) still require improvements in their operational lifetime in order for them to become commercially viable. One factor that limits the lifetime of these devices is the instability of the hole injection layer (HIL). Therefore, understanding its degradation mechanism is crucial for the development of more stable solution-processed OLEDs. In this work, we use an archetypal fluorescent OLED in conjunction with an experimental solution-processed HIL in order to elucidate the degradation mechanism in these HILs. Our studies show that degradation is caused by triplet excitons. This new triplet-induced hole injection degradation is expected to be a common phenomenon in OLEDs, and therefore should have important implications for the design of stable HILs.     

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 aqueous composite hole injection layer of PEDOT:PSS+MoOx for efficient ultraviolet organic light-emitting diode

Hole injection layer (HIL) plays a crucial role in governing external quantum efficiency (EQE) of ultraviolet organic light-emitting diodes (UV OLEDs). We develop a solution-processed aqueous composite HIL of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) incorporated MoO_x (PEDOT:PSS+MoO_x) and cast successful application to UV OLEDs. PEDOT:PSS+MoO_x is characterized in detail with scanning electron microscopy, atomic force microscopy, UV–visible absorption spectra, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and impedance spectroscopy measurements. The results show that PEDOT:PSS+MoO_x features superior film morphology and exceptional electronic properties such as enhanced surface work function and promoted hole injection capacity. With PEDOT:PSS+MoO_x as HIL, the UV OLED gives maximum EQE of 4.4% and radiance of 12.2 mW/cm² as well as improved durability. The electroluminescence peaks at 376 nm with full width at half maximum of 34 nm and stable voltage-dependent spectra. Our results pave a way for exploring efficient UV OLEDs with solution-processable techniques.     

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.     

Opto-electronic properties of poly (fluorene) co-polymer red light-emitting devices on flexible plastic substrate

In this paper, we report on the multilayer poly (fluorene) co-polymer red light-emitting devices (PLEDs) fabricated on flexible plastic substrates. An organic hole transport layer (HTL) is inserted between PEDOT:PSS hole injection (HIL) and light-emissive layers (LEL). Since the highest occupied molecular orbital (HOMO) of the HTL is located between those of HIL and LEL, the insertion of HTL reduces the effective HOMO level offset between HIL and LEL, reducing the device operation voltage and producing comparable or better device efficiencies in comparison with the conventional PEDOT:PSS-only devices. Maximum emission efficiency, /spl sim/0.8 cd/A, power efficiency, /spl sim/0.7 lm/W, and external quantum efficiency, /spl sim/1.5%, have been obtained for multilayer red PLEDs.     

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。     

利用新型的空穴注入层材料提高有机发光器件性能

研究使用新材料2-TNATA作空穴注入层制备OLED,发现空穴注入层厚度的最佳参数为35 nm,器件的发光光谱随空穴注入层厚度并不发生显著变化,微腔作用对发光光谱的影响基本可以忽略.并将2-TNATA作为空穴注入层的器件同CuPc制作的器件进行了对比,发现使用2-TNATA能获得更佳的器件性能.     

Effects of hole carrier injection and transport in organiclight-emitting diodes

In this paper, we examine the effects of hole carrier injection and mobility on both the electroluminescence (EL) quantum efficiency and the operating voltage of bilayer organic light-emitting diodes (OLED's). We find that hole-injection is limited by the nature of the hole injecting interface and significantly affects the operating voltage, but not the quantum efficiency of the OLED. Hole mobility is found not to affect the device quantum efficiency. We demonstrate the characteristics of an ideal ohmic contact by measuring space-charge-limited currents in a trap-free hole transporting polymer layer     

量子点发光二极管中载流子注入机理的研究

针对量子点(QDs)发光二极管(QLED)中载流子注 入不平衡的问题,对载流子的注入机理进行了研 究。在隧穿注入和空间电荷限制电流(SCLC)模型的基础上,仿真分析了空穴和电子在QDs 层的注入情况,制备 了QLED的样品。CdSe/CdS作为QDs层,PEDOT:PSS作为空穴注入层(HIL),TPD作为 空穴传输层(HTL),Alq₃作为电子传输层(ETL)。优选的QDs层厚为25nm时,确定了TPD和Alq₃的理论最优厚分别为48nm。研究发现, 当驱动电压低于6.5V时,隧穿注入电流在载流子的传输过 程中起主导作用;高于6.5V时,SCLC在载流子的传输过程中起主导 作用。实验结果表明,当 Alq₃厚为20nm时,器件发出QDs的红光,随着Alq₃厚度的增加, 器件开始出现绿光,实验结果与仿 真结果基本吻合。研究结果对QLED的制备具有理论借鉴意义。     

载流子平衡方法提高量子点发光二极管效率的研究

尝试采用三种方式来平衡载流子的浓度,以提高量子点发光二极管(QLED)的外量子效率等性能:在正装结构(ITO/HIL/HTL/QD/ETL/EIL/金属阴极)的QLED的发光层和电子传输层中间插入超薄聚甲基丙烯酸甲脂(PMMA)电子阻挡层;在空穴注入和传输层方面,通过使用更加优化的HIL等来提高空穴注入和传输几率;在QD发光层方面,用短链配体来置换量子点的长链配体以增加载流子向量子点发光层中的传输效率等。在进行量子点配体交换的同时带来了量子点在正交溶剂中的可溶性优势,有利于QLED器件的全溶液法制备。     

Bandgap engineering in Alq3- and NPB-based organic light-emitting diodes for efficient green, blue and white emission

Bandgap engineering by insertion of a hole-blocking layer (HBL) between the hole-injection layer (HIL) and hole-transporting layer (HTL) for green and blue-emitting organic light-emitting device is shown to improve the current efficiency by 30% and 70%, respectively. The improvement was attributed to a better electron–hole balance in the device. Two different organic materials, 2,9-dimethyl-4,7-diphenylphenanthroline and tris-(8-hydroxyqunoline) aluminum were used as HBL. Variation of HBL and HTL thickness was shown to adjust the current efficiency and operating voltage, respectively. Insertion of a thin HBL between HIL and HTL for blue-emitting devices also resulted in appearance of multicolor emission which was used to produce a pure white light.     

Colloidal quantum-dot LEDs with a solution-processed copper oxide (CuO) hole injection layer

Solution-processed copper oxide (CuO) thin films are introduced as a hole injection layer (HIL) for quantum dot-based light-emitting diodes (QD-LEDs). AFM, XPS and UPS measurements are utilized for the characterization of the thermally-annealed CuO films. The optimized CuO-based QD-LEDs exhibited an external quantum efficiency (EQE) of 5.37% with a maximum brightness over 70,000 cd/m². The key parameters including the current efficiency and power efficiency of CuO-based QD-LEDs are comparable to the commonly-used PEDOT:PSS-based QD-LEDs using the same structure, further demonstrating that CuO is an effective hole injection layer for QD-LED applications.     

Organometal halide perovskite as hole injection enhancer in organic light-emitting diode

We introduce an organometal halide perovskite (CH₃NH₃PbI₃), as a hole injection layer (HIL) to accelerate hole injection and transport in tris-(8-hydroxyquinoline) aluminum-based organic light-emitting diodes (OLEDs). The excellent charge mobility of CH₃NH₃PbI₃ along with the better interface contacts induced by the CH₃NH₃PbI₃ HIL improved the charge balance and thus enhanced device performance compared with that of OLEDs without a HIL and with a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) HIL. Maximum luminance of 19110 cd m⁻² and power efficiency of 3.210 lm W⁻¹ were obtained. Also, besides more balanced charge recombination, the non-aqueous fabrication of the perovskite HIL and the chemical stability of indium tin oxide in contact with CH₃NH₃PbI₃ led to increased device stability and durability, giving a half-life time as long as 31.7 h.     

MoO3修饰氧化石墨烯作为空穴注入层影响研究

研究了MoO₃修饰氧化石墨烯(GO)作为空穴注入层的影响。采用旋涂的方法制备了GO, 再真空蒸镀修饰层MoO₃,得到了空穴注入能力强和透过率高的复合薄膜。MoO₃的厚分 别采用0、3、5和8nm。通过优化MoO₃的厚度发现,当MoO₃的厚为5nm时,复合薄膜 的透过率达到最大值,在 550nm的光波长下透光率为88%,且此时采用 复合薄膜作为空穴注入层制备的结构为 ITO/GO/MoO₃(5nm)/NPB(40nm)/Alq₃(40nm)/LiF(1nm)/Al(100nm)的有机电致发光器件(OLED)性能 最佳。通过对OLED进一步的优化,改变Alq₃的厚度,分别取50、60和70nm,测量其电压 、电流、亮度、色坐标和电致发光(EL)光谱等参数发现,当Alq₃的厚为50nm时器件性能最 佳。最终制备了结构为ITO/GO/MoO₃(5nm)/NPB(50nm)/Alq₃(50nm)/LiF(1nm)/Al(100 nm)的OLED,在电压为10V时,最大电流效率达到5.87cd/A,与GO单独作为空穴注入层制备的器件相比,提高了50%。     

空穴注入层2T-NATA对OLED器件性能的影响

为了提高有机电致发光器件OLED的发光效率,引入2T-NATA作为空穴注入层,制备了结构为ITO/2T-NATA(Xnm)/NPB(25nm)/Alq3:C545T(20nm:质量分数4.5%)/Alq3(30nm)/LiF(1nm)/Al(100nm)的绿光器件,其中X为空穴注入层2T-NATA厚度。分析了2T-NATA的蒸镀厚度分别0,5,10,15,20,25,30,35nm时器件的发光性能。结果表明,2T-NATA的HOMO能级较好的与ITO功函数匹配,降低了空穴注入势垒,引入空穴注入层2T-NATA提高了器件的发光亮度和效率。当2T-NATA厚度为15nm时,器件的效果最好,起亮电压只需2.87V,亮度最高达到18000cd/m2,是不引入空穴注入层亮度的5倍多,在12V时发光效率可达11.4cd/A。     

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.     

Solution-based nanostructure to reduce waveguide and surface plasmon losses in organic light-emitting diodes

Organic light-emitting diodes (OLEDs) typically have low out-coupling efficiency. In this paper, a solution-based nanoparticle layer is presented as a nanostructure to enhance the out-coupling efficiency of OLEDs. Silica nanoparticles (NPs) are randomly distributed on indium tin oxide by spin-coating a silica NP solution. By further spin-coating poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as a hole injection layer, a randomly corrugated PEDOT:PSS layer is fabricated. A nanostructured OLED having the corrugated PEDOT:PSS layer above the NP layer shows enhanced external quantum efficiency and power efficiency because the trapped light of the waveguide and surface plasmon modes is extracted by Bragg diffraction. The nanostructured OLED shows no angular dependence due to the broad periodicities of the corrugation. The simply fabricated and cost-effective silica NP layer nanostructure, which does not require a lithography step, has potential to enhance the efficiency of both white OLED displays and lighting.     

High‐Efficiency and Stable Quantum Dot Light‐Emitting Diodes Enabled by a Solution‐Processed Metal‐Doped Nickel Oxide Hole Injection Interfacial Layer

Stabilization is one critical issue that needs to be improved for future application of colloidal quantum dot (QD)‐based light‐emitting diodes (QLEDs). This study reports highly efficient and stable QLEDs based on solution‐processsed, metal‐doped nickel oxide films as hole injection layer (HIL). Several kinds of metal dopants (Li, Mg, and Cu) are introduced to improve the hole injection capability of NiO films. The resulting device with Cu:NiO HIL exhibits superior performance compared to the state‐of‐the‐art poly(3,4‐ethylenedioxythiophene):poly(styrene‐sulfonate) (PEDOT:PSS)‐based QLEDs, with a maximum current efficiency and external quantum efficiency of 45.7 cd A^?1 and 10.5%, respectively. These are the highest values reported so far for QLEDs with PEDOT:PSS‐free normal structure. Meanwhile, the resulting QLED shows a half‐life time of 87 h at an initial luminance of 5000 cd m^?2, almost fourfold longer than that of the PEDOT:PSS‐based device.