Hole Injection Layer In Organic Light-emitting Diodes Research Articles

The Optimization of Hole Injection Layer in Organic Light-Emitting Diodes.

Organic light-emitting diodes (OLEDs) are widely recognized as the forefront technology for displays and lighting technology. Now, the global OLED market is nearly mature, driven by the rising demand for superior displays in smartphones. In recent years, numerous strategies have been introduced and demonstrated to optimize the hole injection layer to further enhance the efficiency of OLEDs. In this paper, different methods of optimizing the hole injection layer were elucidated, including using a suitable hole injection material to minimize the hole injection barrier and match the energy level with the emission layer, exploring new preparation methods to optimize the structure of hole injection layer, and so on. Meanwhile, this article can help people to understand the current research progress and the challenges still faced in relation to the hole injection layer in OLEDs, providing future research directions to enhance the properties of OLEDs.

Hole Injection Characteristics and Annealing Temperature Dependence for Organic Light‐Emitting Diodes Using Spontaneous Polarization

An efficient carrier injection at the electrode/organic semiconductor interface of organic light‐emitting diodes (OLEDs) is challenging. It is well‐known that an electric dipole at the electrode/semiconductor interface improves the carrier injection from the electrode because of a vacuum‐level shift. In this study, poly (vinylidene fluoride‐trifluoroethylene) [P(VDF/TrFE)] is used as a polar polymer with spontaneous polarization. P(VDF/TrFE) is spin‐coated on an indium tin oxide (ITO) electrode surface as a hole injection layer for OLEDs. The P(VDF/TrFE) layer enhances the hole injection from the ITO electrode by causing a vacuum‐level shift. As a result, the device driving voltage can be lowered. In addition, annealing temperature dependence is observed. To investigate the origin of the hole injection effect, the P(VDF/TrFE)‐layered ITO surface and the thin film structure of the P(VDF/TrFE) layer are measured. The P(VDF/TrFE) layers at different annealing temperatures show no difference in surface properties but show significant differences in the thin film structure. The hole injection of OLEDs is significantly enhanced because of the structural change in ultrathin P(VDF/TrFE) layer.

UV-ozone treated Ti3C2Tx-MXene nanosheets as hole injection layer for organic light-emitting diodes

In this work, we successfully synthesized Ti3C2Tx-MXene nanosheets with a lateral dimension of 1 ∼ 2 μm by using wet-chemical etchant and sonication-assisted liquid-phase exfoliation method. After UV-ozone treatment, the work function for Ti3C2Tx-MXene nanosheets was adjusted to 5.14 eV well matching to indium tin oxide (ITO) anode (4.80 eV). Using Ti3C2Tx-MXene nanosheets as hole injection layer (HIL), Ir(ppy)3-based green organic light-emitting diodes (OLEDs) were fabricated, and the resulting devices exhibit a low turn-on voltage of 3.0 V and the high current efficiency, power efficiency and external quantum efficiency of 72.79 cd/A, 54.44 lm/W and 20.34 %, which are obviously better than these of 67.38 cd/A, 50.38 lm/W and 18.65 % for PEDOT:PSS-based reference device. Further, FIrpic- and Ir(pq)2acac-based blue and red OLEDs with Ti3C2Tx-MXene nanosheets HIL were demonstrated, which also show the significantly improved device efficiency of 44.56 cd/A, 32.44 lm/W and 22.07 % for blue device and 39.17 cd/A, 38.54 lm/W and 21.90 % for red device than these for corresponding PEDOT:PSS-based devices, proving the universal applicability of Ti3C2Tx-MXene nanosheets as HIL in OLEDs. The higher performance for Ti3C2Tx-MXene nanosheets-based devices can be attributed to the appropriate work function, high transmittance, high carrier mobility and good film forming property for Ti3C2Tx-MXene nanosheets. This is the first report about the MXene using as HIL in OLEDs, which opens up a new way to develop high-performance holes injection materials for solution-processed OLEDs.

Work function tunning of lithium-doped vanadium oxide for functioning as hole injection layer in organic light-emitting diodes

The hole-injection layer (HIL) plays an important role in the electroluminescent property of organic light-emitting diodes (OLEDs). In this paper, Li-doped VOx was prepared by solution processing under low-temperature annealing conditions and used as a new kind of HIL in OLEDs. It demonstrated the best phosphorescent OLED performance when the molar ratio of Li:VOx was 3%. The turn-on voltage was 3.7 V, the maximum current efficiency (CE) was 69.5 cd A−1 and the maximum power efficiency (PE) was 48.4 lm∙W−1. In contrast to the OLEDs with pristine VOx as HIL, the CE and PE of the OLEDs with Li:VOx were increased by 14.7% and 40.7% respectively. Based on the Fowler-Nordheim tunneling mechanism, doping Li+ in VOx improved the efficiency of hole injection and balanced carriers by adjusting the work function, thereby improving the efficiency of the device. The experimental results indicated that Li-doped VOx film prepared by solution processing is a very potential material as the HIL of high-performance OLED.

Synergistic effects of Li-doped NiO film prepared by low-temperature combustion as hole-injection layer for high performance OLED devices

Abstract The hole-injection layer (HIL) has vital impacts on the electroluminescence performance of organic light-emitting diodes (OLEDs). Herein, low-temperature solution-processed Li-doped nickel oxide (Li:NiOx) was fabricated as a novel HIL for OLEDs. As a result, the phosphorescent green OLED with 5% molar ratio Li:NiOx exhibits the best performance with low turn-on voltage of 3.1 V, maximum current efficiency of 55.8 cd A−1, maximum power efficiency of 57.2 lm·W−1 and maximum external quantum efficiency of 17.1%. While it is only 52.1 cd A−1, 50.9 lm·W−1 and for the reference device with undoped NiOx film, corresponding a 7%, 12% and 8% enhancement in efficiency respectively . It reveals that the improved efficiency of the Li:NiOx film HIL based device with is mainly ascribed to the synergistic effect of extrinsic doping in the aspects of achieving enhanced hole injection. Our work demonstrates that the solution-processed Li-doped NiOx film is a promising HIL for high-performance OLEDs.

Revealing the influence of hole injection material's molecular orientation on OLED's performance

Abstract In this study, we introduced N,N′-bis(4-trifluoromethoxybenzyl) naphthalene-1,4,5,8-tetracarboxylic acid di-imide (NDI-BOCF3) and N,N′-Bis(4- trifluoromethoxyphenyl) naphthalene-1,4,5,8-tetracarboxylic acid diimide (NDI- POCF3) as hole injection layers (HILs) in organic light emitting diodes (OLEDs) and investigated the relationship between HIL's molecular orientation and the hole injection characteristics. NDI-BOCF3 and NDI-POCF3 have similar optical and electrochemical properties; however, it is quite surprising that both of these materials demonstrated totally different function when used as HILs in OLEDs. NDI-BOCF3 is observed to enhance the hole injection, in contrast, NDI-POCF3 hinders hole injection. X-ray Diffraction (XRD), Electrostatic Surface Potential (ESP) calculations and Ultraviolet Photoelectron Spectrometry (UPS) were applied to explore the mechanism of NDI-BOCF3 and NDI-POCF3 as HILs. By comparing molecular orientation and electrostatic potential distribution of these two materials, we identified that the change in anode's work function caused by different molecular orientation of HILs results in completely different hole injection properties.

Two-In-One Method for Graphene Transfer: Simplified Fabrication Process for Organic Light-Emitting Diodes.

Graphene as one of the most promising transparent electrode materials has been successfully applied in organic light-emitting diodes (OLEDs). However, traditional poly(methyl methacrylate) (PMMA) transfer method usually results in hardly removed polymeric residues on the graphene surface, which induces unwanted leakage current, poor diode behavior, and even device failure. In this work, we proposed a facile and efficient two-in-one method to obtain clean graphene and fabricate OLEDs, in which the poly(9,9-di-n-octylfluorene-alt-(1,4-phenylene-(4-sec-butylphenyl)imino)-1,4-phenylene) (TFB) layer was inserted between the graphene and PMMA film both as a protector during the graphene transfer and a hole-injection layer in OLEDs. Finally, green OLED devices were successfully fabricated on the PMMA-free graphene/TFB film, and the device luminous efficiency was increased from 64.8 to 74.5 cd/A by using the two-in-one method. Therefore, the proposed two-in-one graphene transfer method realizes a high-efficient graphene transfer and device fabrication process, which is also compatible with the roll-to-roll manufacturing. It is expected that this work can enlighten the design and fabrication of the graphene-based optoelectronic devices.

Improved Electro-optical Performance of OLEDs Using PdCo Alloy Nanoparticles Supported on Polypropylenimine Dendrimer–Grafted Graphene

We fabricated organic light emitting diodes (OLEDs) with PdCo alloy nanoparticles supported on polypropylenimine dendrimer–grafted graphene (PdCo/PPI–g-G) that was doped at different concentrations (0, 0.02, 0.03, 0.06 and 2 wt%) in a poly(3,4-ethylene dioxythiophene): polystyrene sulfonic acid (PEDOT:PSS) as a host layer. PdCo PPI–g-G and PEDOT:PSS were studied as hole injection layers in OLEDs. The efficiency of hole injection from the anode is strongly dependent on the concentration of the PdCo/PPI–g-G. While a graphene oxide and pure PEDOT:PSS layer leads to higher driving voltage and decreased luminescence efficiency. PdCo/PPI–g-G of 0.02 wt% noticeably enhanced the hole injection and resulted in lower turn-on voltage, increased luminance, and current efficiencies. The 0.02 wt% PdCo/PPI–g-G based device has also a luminance of 3705 cd/m2 and maximum efficiency of 1.9 cd/A at 13 V which are the highest values among other PdCo/ PPI–g-G based devices.

MoS2-nanosheet/graphene-oxide composite hole injection layer in organic light-emitting diodes

In this work, composite layers comprising two-dimensional MoS2 and graphene oxide (GO) were employed as hole injection layers (HILs) in organic light-emitting diodes (OLEDs). MoS2 was fabricated by the butyllithium (BuLi) intercalation method, while GO was synthesized by a modified Hummers method. The X-ray diffraction patterns showed that the intensity of the MoS2 (002) peak at 14.15° decreased with increase in GO content; the GO (001) peak was observed at 10.07°. In the C 1s synchrotron radiation photoemission spectra, the contributions of the C-O, C=O, and O-C=O components increased with increase in GO content. These results indicated that GO was well mixed with MoS2. The lateral size of MoS2 spanned from a few hundreds of nanometers to 1 μm, while the size of GO was between 400 nm and a few micrometers. Thus, the coverage of the MoS2-GO composite on the ITO surface improved as the GO content increased, owing to the large particle size of GO. Notably, GO with large size could fully cover the indium tin oxide film surface, thus, lowering the roughness. The highest maximum power efficiency (PEmax) was exhibited by the OLED with MoS2-GO 6:4 composite HIL, indicating that similar contents of MoS2 and GO in MoS2-GO composites provide the best results. The OLED with GO HIL showed very high PEmax (4.94 lm W−1) because of very high surface coverage and high work function of GO. These results indicate that the MoS2-GO composites can be used to fabricate HILs in OLEDs.

Cu-Doped nickel oxide prepared using a low-temperature combustion method as a hole-injection layer for high-performance OLEDs

Low-temperature combustion methods to deposit s-NiOx and s-Cu-NiOx on ITO glass to fabricate efficient green phosphorescent OLEDs.

Performances of Liquid‐Exfoliated Transition Metal Dichalcogenides as Hole Injection Layers in Organic Light‐Emitting Diodes

2D transition metal dichalcogenide (TMD) nanosheets, including MoS2, WS2, and TaS2, are used as hole injection layers (HILs) in organic light‐emitting diodes (OLEDs). MoS2, WS2, and TaS2 nanosheets are prepared using an exfoliation by ultrasonication method. The thicknesses and sizes of the TMD nanosheets are measured to be 3.1–4.3 nm and more than 100 nm, respectively. The work functions of the TMD nanosheets increase from 4.4–4.9 to 4.9–5.1 eV following ultraviolet/ozone (UVO) treatment. The turn‐on voltages at 10 cd m−2 for UVO‐treated TMD‐based devices decrease from 7.3–12.8 to 4.3–4.4 V and maximum luminance efficiencies increase from 5.74–9.04 to 12.01–12.66 cd A−1. In addition, this study confirms that the stabilities of the devices in air can be prolonged by using UVO‐treated TMDs as HILs in OLEDs. These results demonstrate the great potential of liquid‐exfoliated TMD nanosheets for use as HILs in OLEDs.

P‐139: Improved Power Efficiency of Organic Light‐Emitting Diodes using Solution‐Processed CuSCN Hole Injection Layer

We have introduced Copper(.) Thiocyanate (CuSCN) which is a wide bandgap, p‐type semiconductor material, effectively operating as a hole injection layer in organic light‐emitting diodes (OLEDs) with solution process. The electrical and optical characteristics of CuSCN as a hole injection layer in OLED has been investigated.

Field effect transistor behavior of organic light-emitting diodes with a modified configuration of ITO anode

We have developed the organic light-emitting diodes (OLEDs) with a modified configuration of ITO anode in which a thin channel was etched to form a bottom-contact field effect transistor (FET) using ITO and MgAg as a source/drain electrode and a gate electrode, respectively. The hole injection layer in OLEDs functioned as an active layer of FET and the other organic layers as insulator-like layer. The devices were found to exhibit a behavior of FET due to horizontal charge migration between source and drain, and an electro-optical transfer characteristic due to vertical charge transport and recombination. We have investigated the dependence of drain current on the channel length from 5 to 30 μm and found that the modified channel length could change drain current directionally and quantitatively.

Tungsten oxide doped N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine as hole injection layer for high performance organic light-emitting diodes

By introducing tungsten oxide (WO3) doped N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine (NPB) hole injection layer, the great improvement in device efficiency and the organic film morphology stability at high temperature were realized for organic light-emitting diodes (OLEDs). The detailed investigations on the improvement mechanism by optical, electric, and film morphology properties were presented. The experimental results clearly demonstrated that using WO3 doped NPB as the hole injection layer in OLEDs not only reduced the hole injection barrier and enhanced the transport property, leading to low operational voltage and high efficiency, but also improved organic film morphology stability, which should be related to the device stability. It could be seen that due to the utilization of WO3 doped NPB hole injection layer in NPB/tris (8-quinolinolato) aluminum (Alq3)-based device, the maximum efficiency reached 6.1 cd A−1 and 4.8 lm W−1, which were much higher than 4.5 cd A−1 and 1.1 lm W−1 of NPB/Alq3 device without hole injection layer. The device with WO3 doped NPB hole injection layer yet gave high efficiency of 6.1 cd A−1 (2.9 lm W−1) even though the device was fabricated at substrate temperature of 80 °C. These results adequately indicated that WO3 doped NPB was a promising hole injection layer for high efficiency and high stability OLEDs.

High-T g N-Triarylamine Derivatives as a Hole Injecting Layer in Organic Light-Emitting Diodes

N-triarylamines, m-MTDAPB and 1-TNAPB, with high glass transition temperature and hole affinity were evaluated as a hole injection layer (HIL) in organic light-emitting diodes (OLEDs) with the configuration of ITO/N-triarylamine/α-NPD (HTL)/Alq3 (EML)/LiF/Al. The performance of N-triarylamines as HIL in OLEDs was investigated in terms of turn-on voltage, luminescence, color purity, etc. Better EL performance was observed in OLED with m-MTDAPB as HIL, showing turn-on voltage of 6 V and maximum luminescence of 2,976 cd/m2 at 13.5 V. This might be attributed to well-matched HOMO energy offsets at the interfaces of anode/HIL/HTL layers in OLED with the configuration ofITO/m-MTDAPB/α-NPD (HTL)/Alq3 (EML)/LiF/Al, compared to OLED with1-TNAPB as HIL.

A thermally stable hole injection material for use in organic light-emitting diodes

A thermally stable hole injection material, 4,4′-bis-(3,5-bis-(diphenylamino)-phenyl)-1,1′-binaphthyl(PPBN), was synthesized and the device performances of the organic light-emitting diodes(OLEDs) with PPBN as a hole injection layer were investigated. Glass transition temperature of PPBN could be enhanced to 148 °C by the introduction of a rigid diphenylbinaphthyl core in the molecular structure. The use of PPBN as a hole injection layer for OLEDs lowered the driving voltage and enhanced the light-emitting efficiency. The power efficiency of PPBN based devices with tris(8-hydroxyquinoline)aluminium as an emitting material could be improved by 80% compared with that of 4,4′,4″-tris(2-naphthylphenylamino)triphenylamine based devices.