Hole Injection Layer Material Research Articles

Theoretical insight on electronic and optical properties of some phosphorescent iridium(III) complexes for organic light-emitting diode devices

Organic light-emitting diode (OLED) structures have been extensively investigated because of their potential applications in various fields. It is known that organic or metal-mediated organic compounds are used in the layers of OLEDs. Within OLED materials, iridium(III) compounds have attract much attention owing to their many advantages. Therefore, we predicted the OLED behaviors of twenty-five phosphorescent iridium(III) complexes using theoretical chemical methods. All theoretical calculations were performed with the B3LYP hybrid functional using Gaussian 16 and Amsterdam Modeling Suite 2023 programs. While the 6–31G(d) basis set for non-metal atoms and LANL2DZ basis set for iridium metal was preferred in the computations carried out by using Gaussian program, whereas in the calculations performed with the Amsterdam Modelling Suite program, the TZP basis set was used for all atoms. From the theoretically obtained results, it is seen that Ir6, Ir7, Ir22-Ir25 complexes can be proposed as good candidate for hole injection layer materials in OLED based on indium tin oxide as an anode. Within complexes investigated, it has been determined that Ir16 and Ir17 complexes are the most suitable candidates for electron transport layer materials, whereas Ir16 complex can be used as both a hole transfer layer and ambipolar materials. Furthermore, it has been predicted that Ir4, Ir7, Ir8, and Ir23 complexes could be preferred as hole blocking layer compounds. From the detailed analysis of the singlet-triplet transitions of the complexes studied, it could be said that all iridium(III) complexes investigated could be used as highly efficient phosphorescent OLED molecules.

High-Yield Exfoliation of Stanene Nanodots for High-Performance Organic Light-Emitting Diodes.

Stanene nanodots (SnNDs) derived from layered tin have attracted considerable interest due to their conveniently tunable bandgap and topological superconductivity. However, high-yield exfoliation of ultrathin SnNDs is still a challenge due to the short layer spacing and strong binding energy. In this work, atomically thin SnNDs with a uniform size of 2.3 nm are successfully prepared by utilizing imidazolium ionic liquid-assisted exfoliation. The obtained SnNDs possess a wide bandgap of 2.69 eV, along with notable solvent compatibility (well dispersed in both polar and nonpolar solvents) and excellent stability. Furthermore, we construct Ir(ppy)3-based green OLED with hybridizing SnNDs and graphene oxide (GO) as the hole injection layer (HIL). It proves that the application of SnNDs helps to modulate the work function and passivate surface defects of GO, increasing hole mobility and thereby improving the device performance. Compared to the PEDOT:PSS-based control device, the optimized SnNDs-GO-based OLED demonstrates an improvement of 6.56, 41.06, and 8.16% in current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE), respectively. This work not only introduces a new approach to preparing 2D SnNDs but also creates a novel HIL material for OLED devices.

Enhancing the performance of CdSe/ZnS quantum-dot light-emitting diodes by utilizing hole injection layers fabricated with a solution-processed mixture of vanadium oxide and molybdenum oxide

We developed a high-performance CdSe/ZnS quantum-dot light-emitting diode (QLED) incorporating a hole injection layer (HIL) composed of a solution-processed vanadium oxide (VO) and molybdenum oxide (MoO) mixture. The QLED with a VO:MoO ratio of 1:1 in the HIL demonstrated a current efficiency of 62.0 cd/A, representing a 60% improvement over the QLED utilizing poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the HIL. Besides improved performance, the substitution of the highly reactive PEDOT: PSS with chemically stable metal oxides in HIL fabrication is anticipated to enhance device reliability. The highest occupied molecular orbital level of the VO:MoO ratio of 1:1 HIL was higher than that of HILs with other mixing ratios. Additionally, the VO:MoO ratio of 1:1 HIL exhibited higher oxidation number states compared to HILs with other mixing ratios. These characteristics contributed to the superior performance of the QLED with the VO:MoO ratio of 1:1 HIL. Our approach, which employs chemically stable metal oxides as HIL materials for high-performance QLEDs, can advance future QLED technology.

Organic light‐emitting diode behaviors of some synthesized platinum(II)‐based complexes

AbstractOrganic light‐emitting light diodes (OLEDs) have been increasingly used in displays, replacing liquid‐crystal displays (LCD) and light‐emitting diodes (LEDs) panels. The increase in commercial use of OLED has led to the search for OLED with high performance. For that reason, the OLED properties of monomers and dimers of some synthesized platinum(II)‐based complexes were estimated by using different computational chemistry tools with different codes. The electron/hole reorganization energies, the adiabatic/vertical ionization potentials, the adiabatic/vertical electron affinities, the chemical hardness values, the dipole moments, the frontier orbital shapes/energy levels, the energy gaps, the emission wavelengths, spin‐orbit matrix elements, the rates of reverse intersystem crossing and intersystem crossing of the investigated complexes were determined. From the theoretically obtained data, it was found that Pt(hppz)2 and Pt(fppz)2 complexes can be used as electron transfer material. Furthermore, it was stated that Pt(f2bipz)(bpy) is both electron‐blocking layer and hole blocking layer materials. Moreover, it was noted that that PtOEP complex can be utilized as a good electron injection layer and hole injection layer material. Addition to these, it was emphasized that that Pt(f2bipz)(bpy) can be considered as a good candidate for near infrared organic light emitting diodes and thermal activated delayed fluorescent organic light emitting diodes. In light of computational chemistry, it should be expected that the study will provide a great contribution to studies related to organic light emitting diodes.

Highly Efficient and Stable Green Phosphorescent Light‐Emitting Diodes Based on Solution‐Processable Ir(III) Complexes with Electron‐Transporting Ancillary Ligands

AbstractTwo green heteroleptic Ir(III) complexes are reported, namely, bis[5‐(2‐ethylhexyl)‐8‐trifluoromethyl‐5H‐benzo[c][1,5]‐naphthyridin‐6‐one](2‐(5‐phenyl‐1,3,4‐oxadiazol‐2‐yl)phenolate)iridium (III) (Ir(CF3BNO)2‐pop) and bis[5‐(2‐ethylhexyl)‐5H‐benzo[c][1,5]naphthyridin‐6‐one](2‐(5‐phenyl‐1,3,4‐oxadiazol‐2‐yl)phenolate)iridium (III) (Ir(BNO)2‐pop) for solution‐processed phosphorescent organic light‐emitting diodes (PHOLEDs). Both the Ir(III) complexes exhibit high external quantum efficiency (EQE) and intense green emission upon using poly(3,4‐ethylenedioxythiophene):polystyrene sulfonic acid (PEDOT:PSS) as a hole injection layer (HIL); Ir(CF3BNO)2‐pop based PHOLEDs perform better with an EQE of 23% and a current efficiency of 82 cd A−1 than Ir(BNO)2‐pop based PHOLEDs. In addition, to improve device operational lifetime, a crosslinkable HIL material, 9,9'‐(cyclohexane‐1,1‐diylbis(4,1‐phenylene))bis(N‐phenyl‐N‐(4‐(((4‐vinylbenzyl)oxy)methyl)phenyl)‐9H‐carbazol‐3‐amine) (CPCzDPAX‐1) is designed and synthesized. Device operational lifetimes of Ir(CF3BNO)2‐pop and Ir(BNO)2‐pop PHOLEDs incorporating the CPCzDPAX‐1 HIL show 14‐fold and 6‐fold improvement, compared to the corresponding devices incorporating the PEDOT:PSS HIL. To the best of the authors’ knowledge, this is the best device lifetime performance by using the combination of new Ir(III) complexes and HIL materials for solution‐processed green PHOLEDs.

Enhanced performance of flexible quantum dot light-emitting diodes using a low-temperature processed PTAA hole transport layer

Low-temperature processing is important for improving the stability and performance of flexible quantum dot light-emitting diodes (QLEDs). In this study, QLEDs were fabricated using poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA) as a suitable hole transport layer (HTL) material owing to its low-temperature processability and vanadium oxide as the low-temperature solution-processable hole injection layer material. The maximum luminance and highest current efficiency of the QLEDs on a glass substrate with an optimal PTAA HTL was 8.9 × 104 Cd/m2 and 15.9 Cd/A, respectively, which was comparable to that of conventional devices. The QLEDs on a flexible substrate showed a maximum luminance of 5.4 × 104 Cd/m2 and highest current efficiency of 5.1 Cd/A. X-ray and ultraviolet photoelectron spectroscopies were used to investigate the chemical state and interfacial electronic structure according to the materials and the state changes of the HTL, respectively. The interfacial electronic structure showed that PTAA exhibited a better hole transport ability owing to its low hole injection barrier ({Phi }_{{text{h}}}). Moreover, QLEDs with a PTAA HTL could operate as photosensors under reverse bias conditions. These results indicate that the low-temperature-processed PTAA HTL is suitable for improving the performance of flexible QLEDs.

The Selection of Anode and Cathode Materials for Top Emission Organic Light-Emitting Diodes

In this study top emission organic light-emitting diodes (TE-OLED) were successfully fabricated on flexible PET/multilayer disordered silver nanonetwork (MDSN) substrate with conventional LiF/Al as a semitransparent cathode and Ag as a reflective anode. The effects of the hole injection layer, anode buffer layer, and an electron injection layer on the luminescence characteristics of TE-OLEDs were investigated. At first, the thickness of cathode Al is adjusted based upon the overall transmittance of the top-emission cathode and its conductivity. There is a high energy barrier for the hole between the work function of the anode Ag (4.2 eV) and the NPB highest occupied molecular orbital (HOMO) energy level (5.5 eV), which is not favorable for hole injection. This study tested four types of hole injection layer (HIL) materials. Finally, MoO3 was selected as an optimal HIL material, and the optimum thickness was adjusted, enabling the hole to be injected smoothly from the anode to the NPB and then to an emitting layer. The TE-OLED luminance reached 268 cd/m2. There is a high energy barrier between the work function of Ag and MoO3 HOMO (5.3 eV)—about 1.1 eV—which is still not conducive to the hole injection, so a thin layer of high work function metal Au (work function 5.1 eV) was added to the top of the anode silver, which more matches with the MoO3 energy level. It can make the hole easier to inject from the anode to MoO3 (HIL) and protect the silver from oxidation. At 8 V, the luminance is increased to 413.7 cd/m2, and the current efficiency is 0.81 cd/A. The luminance is significantly improved. An electron transport/hole blocking layer TPBi (10 nm) was added to enhance the electron transport capability and effectively block the holes with higher electron mobility and a higher HOMO energy level of TPBi. So that more holes can remain in the emitting layer and increase the chance of the electron-hole recombination to improve the luminance and current efficiency of TE-OLED. At 8 V, the luminance and current efficiency can reach 611.4 cd/m2 and 0.95 cd/A, respectively.

Enhanced Hole Injection Characteristics of a Top Emission Organic Light-Emitting Diode with Pure Aluminum Anode.

A top emitting organic light-emitting diode (OLED) device with pure aluminum (Al) anode for high-resolution microdisplays was proposed and fabricated. The low work function of the Al anode, even with a native oxide formed on the Al anode surface, increases the energy barrier of the interface between the anode and hole injection layer, and has poor hole-injection properties, which causes the low efficiency of the device. To enhance the hole-injection characteristics of the Al anode, we applied hexaazatriphenylene hexacarbonitrile (HATCN) as the hole-injection layer material. The proposed OLED device with a pure Al anode and native oxide on the anode surface improved efficiency by up to 35 cd/A at 1000 nit, which is 78% of the level of normal OLEDs with indium tin oxide (ITO) anode.

54.3: Realization of Uniform OLED Pixels based on Multi‐nozzle by Inkjet printing

Inkjet printing has been used as an additive manufacturing method to fabricate organic light emitting diode (OLED). By changing the type and proportion of the solvent of the inks and adjusting the printing parameters of the ink droplets, uniform ejection of multi‐nozzle is achieved and the volume deviation of the jet is controlled at about 5% when poly(3,4‐ethy lenedioxythiohene) : poly (styrenesulfonate) (PEDOT:PSS), as the hole injection layer materials, was dissolved in isopropanol and ethylene glycol (volume ratio 2: 1). Moreover, the OLED devices of monochrome pixel was fabricated based on the multi‐nozzle, and the turn‐on voltage is reduced to 3.1v along with the improvement of the brightness and efficiency of the device.

Efficient blue organic light-emitting diodes with low operation voltage by improving the injection and transport of holes

In this work, we reported the highly efficient organic electroluminescent (EL) devices based on blue phosphorescent material FIr6. By utilizing p-type dopant HAT-CN as hole injection layer material, the injection of holes from the anode was significantly improved. In addition, doping HAT-CN into hole transport layer (HTL) and inserting thin TcTa film between HTL and light-emitting layer (EML) improved the transport of holes. The stepwise energy levels between functional layers reduced the injection barriers of carriers, thus improving the balance of carriers within EML. Finally, highly efficient blue EL device realized the maximum current efficiency, power efficiency, external quantum efficiency (EQE) and brightness up to 39.14 cd/A, 38.68 lm/W, 24.6% and 17201 cd/m2, respectively. Even at the high brightness of 1000 cd/m2 (3.7 V), current efficiency and EQE as high as 36.73 cd/A and 23.1%, respectively, can still be retained by the same device. Our investigation provides an idea for the design of high-performance devices with low operation voltage and slow efficiency roll-off.

Tetra-methyl substituted copper (II) phthalocyanine as a hole injection enhancer in organic light-emitting diodes

We have enhanced hole injection and lifetime in organic light-emitting diodes (OLEDs) by incorporating the isomeric metal phthalocyanine, CuMePc, as a hole injection enhancer. The OLED devices containing CuMePc as a hole injection layer (HIL) exhibited higher luminous efficiency and operational lifetime than those using a CuPc layer and without a HIL. The effect of CuMePc thickness on device performance was investigated. Atomic force microscope (AFM) studies revealed that the thin films were smooth and uniform because the mixture of CuMePc isomers depressed crystallization within the layer. This may have caused the observed enhanced hole injection, indicating that CuMePc is a promising HIL material for highly efficient OLEDs.

Efficient hole injection material for low operating voltage blue fluorescent organic light emitting diodes

In this paper, we demonstrate a design and synthesis of efficient hole injecting material, 5,10,15-trimethyl-5H-diindolo[3,2-a:3’,2’-c] carbazole (TMDI), for blue fluorescent organic light emitting diodes (OLEDs). Our synthesized TMDI material has good hole mobility of about 8.9 x10−4cm2/Vs and compatible highest occupied molecular orbital level of −5.1eV, which are important parameters for excellent hole injection layer (HIL) in the OLED applications. The fabricated blue fluorescent OLEDs with TMDI yield about 8% reduction in operating voltage and 39% of improvement in power efficiency as compare to the widely used HIL material 4,4’,4”-tris-(N-(naphthalen-2-yl)-N-phenyl-amino) triphenylamine. Such performance enhancements are mainly attributed to good hole mobility of TMDI.

Pressure-assisted fabrication of organic light emitting diodes with MoO3 hole-injection layer materials

In this study, pressures of ∼5 to ∼8 MPa were applied to organic light emitting diodes containing either evaporated molybdenum trioxide (MoO3) or spin-coated poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulphonate) (PEDOT:PSS) hole-injection layers (HILs). The threshold voltages for both devices were reduced by about half, after the application of pressure. Furthermore, in an effort to understand the effects of pressure treatment, finite element simulations were used to study the evolution of surface contact between the HIL and emissive layer (EML) under pressure. The blister area due to interfacial impurities was also calculated. This was shown to reduce by about half, when the applied pressures were between ∼5 and 8 MPa. The finite element simulations used Young's modulus measurements of MoO3 that were measured using the nanoindentation technique. They also incorporated measurements of the adhesion energy between the HIL and EML (measured by force microscopy during atomic force microscopy). Within a fracture mechanics framework, the implications of the results are then discussed for the pressure-assisted fabrication of robust organic electronic devices.

P‐160: Application of Graphene Oxide to Organic Light‐Emitting Diodes

AbstractWe verified the possibility of the use of graphene oxide (GO) as a hole‐injecting material for organic light‐emitting diodes (OLEDs). The performance of OLEDs with a GO hole‐injection layer (HIL) is comparable to that of conventional devices with a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HIL. We expect that GO can resolve the intrinsic problems of the common solution‐processable HIL material, PEDOT:PSS.

High Efficiency New Hole Injection Materials for Organic Light Emitting Diodes Based on Dimeric Phenothiazine and Phenoxazine Moiety Derivatives

New hole injection materials for organic light emitting diodes (OLEDs) based on phenothiazine and phenoxazine were synthesized, and the electro-optical properties of synthesized materials were examined by through UV-visible (UV-vis), photoluminescence (PL) spectrum and cyclic voltammetry (CV). 1-BPNA-t-BPBP and 1-BPNA-t-BPBPOX showed T(g) of 127 and 200 degrees C, which are higher than that (110 degrees C) of 2-TNATA, a commercial hole injection layer (HIL) material. The highest occupied molecular orbital (HOMO) level of the synthesized materials of 1-BPNA-t-BPBP and 1-BPNA-t-BPBPOX were 4.97 and 4.91 eV, indicating values well-matched between HOMO (4.8 eV) of ITO and HOMO (5.4 eV) of NPB, hole transporting layer (HTL) material. As a result of using the synthesized materials in OLED device as HIL, 1-BPNA-t-BPBPOX of 2.43 Im/W was higher than 2-TNATA of 1.98 Im/W and 1-BPNA-t-BPBP of 1.39 Im/W in power efficiency. These results indicated that 1-BPNA-t-BPBPOX shows higher excellent power efficiency which is about 18% improved over 2-TNATA a commercial HIL material.

An Enhanced Operational Stability of Organic Light Emitting Devices with Polymeric Buffer Layer

A uniform and robust polymer-based hole injection layer (HIL) was formed by spin-casting process in an ambient air condition. The rough surface of indium tin oxide (ITO) was covered by this polymer-based HIL which plays an important role as a buffer layer. The utilization of this buffered HIL polymer resulted in an enhanced long-term stability of organic light emitting devices (OLEDs). In addition, we found that the precise control of hole current flow is possible by doping of 9,10-dicyanoanthracene (DCA) with moderate electron affinity (∼3.5 eV) plausibly due to a strong dipole interaction with the polymeric HIL material. As a result, a new buffer layer doped with DCA showed a substantially controlled hole injection into an emitting layer (EML) which causes a much more enhanced lifetime of the blue OLED by a factor of 4.3.

Organic Light Emitting Diodes: Effect of Annealing the Hole Injection Layer on the Electrical and Optical Properties

Organic Light Emitting Diodes (OLED) are receiving increased attention due to tremendous application potential these devices hold in the areas of large area displays and lighting applications. However, the problems of efficiency, stability and shelf life are major challenges for making OLEDs an attractive alternative. The simple device structure involving anode, emissive layer and cathode is no longer the norm. Recently, various buffer layers like Hole Injection Layer (HIL), Hole transport Layer (HTL), Electron Injection Layer (EIL), Electron Transport Layer (ETL) etc. are being widely used as integral parts of the OLED architecture to enhance the performance parameters. The nomenclature of these layers is often confusing and sometimes used by different authors to mean different layers and a common and universal nomenclature for layers is still wanting. Applying a buffer layer, often called as the hole injecting layer (HIL) between anode and emissive layer is a general technique for increasing the efficiency and stability of organic light emitting diodes. Poly- (3,4-ethyhylene dioxythiophene): poly- (styrenesulphonate) (PEDOT:PSS) is a very common and popular such HIL used in OLEDs. In this chapter, a basic structure of OLEDs has been discussed in perspective with this HIL material and the effect of annealing this PEDOT: PSS layer on the characteristics of the device at different temperatures ranging from 100°C to 300°C in vacuum. Devices fabricated in clean room conditions are characterized for their electrical and optical properties. Equivalent circuits of the devices are deduced using impedance spectroscopy and discussed. Surface morphology of the HIL layers using atomic force microscopy (AFM) provides reasons for the variation of the device properties with the annealing of HIL.

Highly Efficient New Hole Injection Materials for OLEDs Based on Dimeric Phenothiazine and Phenoxazine Derivatives

New hole injection layer (HIL) materials for organic light-emitting diodes (OLEDs) based on phenothiazine and phenoxazine were synthesized, and the electro-optical properties of the synthesized materials were examined by UV−vis and photoluminescence spectroscopy, and by cyclic voltammetry. 10,10′-bis(4-tert-butylphenyl)-N7,N7′-di(naphthalen-1-yl)-N7,N7′-diphenyl-10H,10′H-3,3′-biphenoxazine-7,7′-diamine (1-PNA-BPBPOX) showed glass transition temperatures (Tg) of 161 °C, which was higher than that (110 °C) of Tris(N-(naphthalen-2-yl)-N-phenyl-amino) triphenylamine (2-TNATA), a commercial HIL material. The HOMO levels of the synthesized materials were 4.9−4.8 eV, indicating a good match between the HOMO of indium tin oxide (ITO) (4.8 eV) and the HOMO of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) (5.4 eV), a common hole transfer layer (HTL) material. Because the synthesized materials showed minimal absorption at wavelengths shorter than 450 nm, they have good potential for use as effective HIL ...

Synthesis and Electro-Optical Properties of Carbazole Containing Copolymers with Different Conjugated Structures for Polymer Light-Emitting Devices

New copolymers such as Poly(9-(4-(2-ethyl-hexyloxy)-3,5-dimethylphenyl)carbazole-alt-aminonaphtalene) [ P1 ] and Poly(4-(bis-methylbiphenyl-alt-9H-carbazol-9-yl)benzenamine) [ P2 ] with chromophores separated by non-conjugated methylene spacers, and Poly(4-(dioctylfluorene-alt-9H-carbazole-9yl)benzenamine) [ P3 ] based on alternating fluorene and amine units in the polymer backbone were synthesized by palladium catalyzed polycondensation reaction. Optical properties of these polymers were characterized in terms of UV/visible absorption and photoluminescence (PL) emission in solution and film state. Maximum PL emission (λmax,PL) of synthesized copolymers were observed in the blue region of 426 nm–460 nm in solution state, but the three polymers all showed large red shift in film state with the green emission. The electro-optical properties of these polymers in polymer light-emitting diode (PLED) devices were investigated from the viewpoint of utilizing the new polymers as emitting layer and hole injection/transport layer. The polymer P3 was found to function both as hole injection and hole transport layer with performance better than that of well known PEDOT:PSS hole injection layer material.

Effect of the plasma treatment of anode electrode of the organic light-emitting diodes on the growth of hole-injection layer

In this paper, we focused on the effects of the plasma treatment of the anode electrode of organic light emitting diodes (OLED) on the growth of hole-injection layer (HIL). The CF 4 plasma (CF 4-P) treatment, which is known for efficient method to enhance the performance of OLED, was not effective on the OLED with the HIL material copper phthalocyanine (CuPc). The CF 4-P treated OLED showed remarkably reduced electroluminescence (EL) characteristics while the O 2 plasma treated OLED showed improved EL efficiency. The dependence of the CuPc growth on the polarity of substrate induced the morphological difference of the HIL, and finally resulted in the different device characteristics.