Anode In Organic Light-emitting Diodes Research Articles

Preserving the work function of Ultra-Violet-ozone treated indium tin oxide by triarylamine-based small molecule modification for solution-processed organic light-emitting diodes with increased external quantum efficiency

UV-ozone treatment is one of the most common ways to increase the work function of indium tin oxide (ITO), which is used as the transparent conducting anode in organic light-emitting diodes (OLEDs). However, the work function increase is time sensitive when the samples are left or processed in air, often returning to a similar value to that measured before UV-ozone treatment. We found that for OLEDs formed by solution processing and containing a poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole injection layer that there was no advantage in terms of device efficiency of using UV-ozone treated ITO. However, if a triarylamine-based small molecule [N1,N3,N5-tris(4-n-butylphenyl)-N1,N3,N5-triphenylbenzene-1,3,5-triamine, 4-n-BTDAB] was introduced onto the UV-ozone treated ITO then the work function was stabilised. When 4-n-BTDAB was introduced between UV-ozone treated ITO and PEDOT:PSS, the resultant solution processed OLEDs were found to have a maximum external quantum efficiency ≈20% higher (corresponding to an absolute increase of ≈2% to around 12%) compared to devices with the same structure but without the triarylamine layer.

The effect of thickness of light emitting layer on physical properties of OLED devices

In this work, we performed to study the effect of thickness (35, 55, 75 and 95 nm) of the light emitted layer (EML) on physical properties of organic light-emitting diode (OLED) devices. We used indium tin oxide (ITO) for the anode in OLED. The ITO substrates were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), and optical absorption (UV–vis). X-ray diffraction studies showed that the ITO films were polycrystalline with a hexagonal or wurtzite structure of indium oxide In2O3. Atomic force microscopy analysis demonstrated that the value of surface roughness ITO is an acceptable value for the proper transfer of the carriers from the boundary of the layers. Optical transmittance spectra of the ITO film showed high transparency of about ∼94% in the visible region.After the characterization of ITO, the OLEDs with the structure of Glass/ITO/ PEDOT:PSS/Alq3/Al were fabricated. Then the current density–voltage, resistance-voltage, power-voltage, and irradiance-voltage of the OLEDs were characterized. The results showed that decreasing EML thickness leads to a reduction of turn-on voltage, resistance, and power. The peak emission intensity of 55 nm thickness EML was stronger and sharper of others.

Nearly 40% outcoupling efficiency in OLEDs with all-metal electrodes

Due to its high transparency and low sheet resistance, indium tin oxide (ITO) has been the material of choice for transparent anodes in organic light-emitting diodes (OLEDs). Indium tin oxide, however, is a source of outcoupling loss due to waveguiding and reduced mechanical stability on flexible/stretchable substrates due to its brittle nature. We demonstrate that highly efficient ITO-free OLEDs can be achieved using high quality silver electrodes and horizontally aligned dipole emitters to avoid plasmonic losses. Using an ultrathin Ag/MPTMS anode and a partially aligned phosphorescent emitter, we demonstrate OLEDs with 30% EQE, luminous efficiency exceeding 130 lm/W, and low leakage current. In addition, we demonstrate OLEDs with an optimized structure showing a 36.1% outcoupling efficiency. Theoretical calculations show that our approach can yield up to 48.4% outcoupling efficiency for perfect horizontal alignment, which exceeds the maximum achievable with ITO. The combination of a silver anode and a horizontal phosphorescent emitter is promising for the future design of ultra-efficient flexible OLEDs.

The application of the nanostructure aluminum in the blue organic light-emitting devices

Abstract A composition of aluminum (Al)/1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) was adopted on the ITO anode of organic light-emitting diodes (OLEDs). With changing the thickness of this anode nanostructure Al, a whole continuous nanofilm or discontinuous nanoparticles would be formed on the ITO surface which can be used to improve the performance of the blue-emitting device. An optical microcavity would be formed by the whole continuous Al nanofilm at the thickness of 15 nm combined with the cathode Al film, which could narrow the emission spectra and enhance the blue emission intensity from the 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl (BCzVBi). A deep blue electroluminescent (EL) emission with an EL efficiency of 5.7 cd/A at a current density of 20 mA/cm2 at saturated blue Commission International de l’Eclairage (CIE) coordinates of (0.137, 0.112) was obtained, of which the full width at half-maxmium (FWHM) of 49 nm was narrower 30% than the reference device with the increased EL intensity. And while the formed discontinuous nanoparticles was optimized at the thickness of 3 nm, the surface plasmon (SP) effect of the Al nanoparticles was utilized to increase the EL efficiency up to 7.5 cd/A which was 36% higher than the reference device at a current density of 20 mA/cm2, and along with the slightly changed CIE coordinates of (0.156, 0.210).

Highly-flexible, ultra-thin, and transparent single-layer graphene/silver composite electrodes for organic light emitting diodes

Transparent conductive electrode (TCE) platforms are required in many optoelectronic devices, including organic light emitting diodes (OLEDs). To date, indium tin oxide based electrodes are widely used in TCEs but they still have few limitations in term of achieving flexible OLEDs and display techniques. In this paper, highly-flexible and ultra-thin TCEs were fabricated for use in OLEDs by combining single-layer graphene (SLG) with thin silver layers of only several nanometers in thickness. The as-prepared SLG + Ag (8 nm) composite electrodes showed low sheet resistances of 8.5 Ω/□, high stability over 500 bending cycles, and 74% transmittance at 550 nm wavelength. Furthermore, SLG + Ag composite electrodes employed as anodes in OLEDs delivered turn-on voltages of 2.4 V, with luminance exceeding 1300 cd m−2 at only 5 V, and maximum luminance reaching up 40 000 cd m−2 at 9 V. Also, the devices could work normally under less than the 1 cm bending radius.

Green and low-cost approach to modify the indium tin oxide anodes in organic light-emitting diodes by electrochemical treatment in NaCl aqueous solution

We demonstrate an environment-friendly, simple, and low energy cost approach as an alternative to conventional O2 plasma treatment to modify the surface of indium tin oxide (ITO) anodes for use in organic light-emitting diodes (OLEDs). ITO is electrochemically treated in NaCl aqueous solution. A chlorinated ITO (Cl-ITO) electrode with a work function of 5.41eV was obtained, which is 0.66eV higher than that of pre-cleaned ITO. The increase of work function is due to the anodic oxidation reactions occurred on the surface of ITO. The power dissipation is only ∼3mW in our approach, which is five orders of magnitude lower than that of O2 plasma treatment (∼100W). We fabricated the OLEDs with the configuration of Cl-ITO/NPB(35nm)/CBP:Ir(ppy)3 (15nm, 8wt%)/TPBi:Ir(ppy)3 (10nm, 8wt%)/TPBi (10nm)/Bphen (50nm)/Cs2CO3 (2nm)/Al (100nm), where NPB is N, Ń-di-1-naphthyl-N, Ń-diphenylbenzidine, CBP is 4′-bis(carbazol-9-yl)biphenyl, TPBi is 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole), Ir(ppy)3 is bis(3-phenylpyridine) iridium(III) and Bphen is 4,7-diphenyl-1,10-phenanthroline. A maximum power efficiency of 95.0lmW−1 and external quantum efficiency (EQE) of 24.2% were achieved, respectively, which was slightly higher than that of the OLED fabricated on O2-plasma-treated ITO (91.2lmW−1, EQE=23.1%).

Preparation of an indium zinc oxide–silver–indium zinc oxide multilayer film and its application in organic light-emitting diodes

Indium zinc oxide (IZO) is one of the most promising anode materials for organic light-emitting diodes (OLEDs). In the present paper, IZO–Ag–IZO multilayer films were prepared by sputtering and their electrical and optical properties were investigated. The multilayer that consisted of a 30-nm-thick top IZO layer sputtered under Ar, a 14-nm-thick Ag layer, and a 30-nm-thick IZO sputtered under O2 was observed to exhibit the highest figure of merit among the investigated multilayer films because of its low sheet resistance (3.6 Ω/sq) and high transmittance (86.9%). OLEDs fabricated using the developed IZO/Ag/IZO anode exhibited a higher luminance and a greater current density than those fabricated using an anode composed of a single layer of IZO or indium tin oxide. As a result, we observed that the IZO/Ag/IZO multilayer exhibits good electrical and optical properties as a transparent conductive film and is suitable for use as an anode in OLEDs.

Enhanced Light Extraction from Mechanically Flexible, Nanostructured Organic Light‐Emitting Diodes with Plasmonic Nanomesh Electrodes

Characteristics of light extraction from nanostructured organic light‐emitting diodes (OLEDs) incorporating plasmonic metal nanomesh anodes are investigated for achieving high‐performance, flexible OLEDs. Given that flexibility of devices is limited by rigid and fragile constituents such as indium tin oxide electrodes, alternative transparent, and flexible electrodes including nanomesh‐type metal films have been widely studied, where high transparency, low resistance, and good flexiblity were regarded as essential requirements of high‐performance flexible applications due to the expectation that they are closely related to the device performance. However, here it is reported that a metal nanomesh optimized for high transmittance cannot promise high device efficiency when it is used as an anode of OLEDs. Systematic experiments together with analytic simulations elucidate that the surface plasmon loss is singnificantly reduced by metallic cathode nanostructures that were inherently formed from a periodic nanomesh anode, which results in huge enhancement of the light extraction, though the optimal period of the nanomesh anode for high light extraction is inconsistent with that for high transmittance. Further performance characterization of nanostructured OLEDs with metal nanomesh anodes provides insight into strategies to realize highly efficient flexible OLEDs.

ITO-free organic light-emitting diodes with MoO3/Al/MoO3 as semitransparent anode fabricated using thermal deposition method

In this paper, semitransparent electrodes with the structure substrate/MoO3/Al/MoO3 (OMO) were fabricated via the thermal deposition method for use as the anode in organic light-emitting diodes (OLEDs). The optical transmittance of the metal layer was enhanced by depositing metal oxidation (MoO3) and metal (Al) layers. The optimal thickness of the Al thin films was determined to be 15nm for high optical transmittance and good electrical conductivity. The optimized films show the typical sheet resistance of 7Ω/sq and a high transmittance of 70% at 550nm. The indium-tin-oxide (ITO)-free OLEDs with the fabricated composite anodes on a glass substrate exhibited the high luminance and current efficiency of 21,750cd/m2 and 3.18cd/A, respectively. In addition, bending effects on the polyethersulfone (PES) substrate/MoO3/Al/MoO3 and PES substrate/MoO3/Al structures were investigated. Cracks formed on the surface of the samples with a bending radius smaller than or equal to 1cm. MoO3 covering the Al layer modifies the surface of the electrode and enhances durability. The surface roughness of the bi-layer films was higher than that of the tri-layer films. Therefore, OLEDs with OMO anode outperform those with bi-layer films anode.

MoO3/Ag/MoO3 anode for organic light-emitting diodes and its carrier injection property

We report on the application of the dielectric/metal/dielectric (DMD) structure consisting of a molybdenum trioxide (MoO3)/silver (Ag)/MoO3 stack as the transparent electrode in organic light-emitting diodes (OLEDs). Bright emission similar to that of the indium–tin-oxide anode (ITO) device was obtained from the OLEDs with the DMD anode. Also, the barrier height at the interface of DMD/bis[N-(1-naphthyl)-N-phenyl] benzidine (α-NPD) is similar to that at the ITO/α-NPD interface. The DMD electrode is a promising anode for OLEDs.

Bilayer graphite-oxide anode for organic light-emitting diode

A solution-processing method for obtaining graphite oxide (GO), which is used for the organic light-emitting diodes (OLEDs), is presented herein. The use of poly(arylene ether)s as the solid carbon source is demonstrated for the first time. Raman spectroscopy, Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), luminance, and electroluminescence (EL) reveal the essential properties. The respective percentages of the peak areas of sp2 C and sp3 C in the GO are calculated to be 54.47 and 37.32%. The GO had a sheet resistance of 97 Ω/square determined by the four-probe method and a conductivity of 5.538 × 101 Ω−1 cm−1 determined by the Hall-effect method.

Anomalous hole injection deterioration of organic light-emitting diodes with a manganese phthalocyanine layer

Metal phthalocyanines (MPcs) are well known as an efficient hole injection layer (HIL) in organic devices. They possess a low ionization energy, and so the low-lying highest occupied molecular orbital (HOMO) gives a small hole injection barrier from an anode in organic light-emitting diodes. However, in this study, we show that the hole injection characteristics of MPc are not only determined by the HOMO position but also significantly affected by the wave function distribution of the HOMO. We show that even with the HOMO level of a manganese phthalocyanine (MnPc) HIL located between the Fermi level of an indium tin oxide anode and the HOMO level of a N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine hole transport layer the device performance with the MnPc HIL is rather deteriorated. This anomalous hole injection deterioration is due to the contracted HOMO wave function, which leads to small intermolecular electronic coupling. The origin of this contraction is the significant contribution of the Mn d-orbital to the MnPc HOMO.

Carrier injection efficiencies and energy level alignments of multilayer graphene anodes for organic light-emitting diodes with different hole injection layers

The carrier injection efficiencies of organic light-emitting diodes with a multilayer graphene (MLG) anode were compared for different hole injection layers (HIL). The energy level alignments at the hole injecting interface were also studied by using the ultraviolet photoelectron spectroscopy (UPS). We employed 1,1-bis[4-[N,N′-di(p-tolyl)amino]phenyl]cyclohexane (TAPC) as the hole transporting materials, while N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) or 1,4,5,8,9,12-hexaaza-triphenylene-2,3,6,7,10,11-hexacarbonitrile (HAT-CN) was used as the HIL. The current–voltage characteristics of hole-only devices showed that the MLG anode was only slightly inferior to the indium-tin oxide (ITO) anode in hole injection performance for all the HIL layers. The best efficiency was observed for both MLG and ITO anodes with HAT-CN HIL. We compared UPS-measured energy level alignment of TAPC/HIL/MLG interface for different HILs and concluded that the unique charge generation interface at TAPC/HAT-CN played a crucial role for the improved performance of HAT-CN HIL.

The application of single-layer graphene modified with solution-processed TiOx and PEDOT:PSS as a transparent conductive anode in organic light-emitting diodes

There are many challenges for a direct application of graphene as the electrodes in organic electronics due to its hydrophobic surfaces, low work function (WF) and poor conductance. The authors demonstrate a modified single-layer graphene (SLG) as the anode in organic light-emitting diodes (OLEDs). The SLG, doped with the solution-processed titanium suboxide (TiOx) and poly(3,4-ethylenedio-xythiophene)/poly(styrene sulfonic acid) (PEDOT:PSS), exhibits excellent optoelectronic characteristics with reduced sheet resistance (Rsq), increased work function, as well as over 92% transmittance in the visible region. It is notable that the Rsq of graphene decreased by ∼86% from 628Ω/sq to 86Ω/sq and the WF of graphene increased about 0.82eV from 4.30eV to 5.12eV after a modification by using the TiOx–PEDOT:PSS double interlayers. In addition, the existence of additional TiOx and PEDOT:PSS layers offers a good coverage to the PMMA residuals on SLG, which are often introduced during graphene transfer processes. As a result, the electrical shorting due to the PMMA residues in the device can be effectively suppressed. By using the modified SLG as a bottom anode in OLEDs, the device exhibited comparable current efficiency and power efficiency to those of the ITO based reference OLEDs. The approach demonstrated in this work could potentially provide a viable way to fabricate highly efficient and flexible OLEDs based on graphene anode.

Electron-irradiated n+-Si as hole injection tunable anode of organic light-emitting diode

Traditionally, n-type silicon is not regarded as a good anode of organic light emitting diode (OLED) due to the extremely low hole concentration in it; however, when doped with Au element which acts as carrier generation centers, it can be, as shown in our previous work. In this study, we demonstrate a new kind of carrier generation centers in n+-type silicon, which are the defects produced by 5 MeV electron irradiation. The density of carrier generation centers in the irradiated n+-Si anode can be controlled by tuning the electron irradiation time, and thus hole injection current in the OLEDs with the irradiated n+-Si anode can be optimized, leading to their much higher maximum efficiencies than those of the OLEDs with non-irradiated n+-Si anode. For a green phosphorescent OLED with the irradiated n+-Si anode, the current efficiency and power efficiency reach up to 12.1 cd/A and 4.2 lm/W, respectively.

Transparent Al-doped ZnO anodes in organic light-emitting diodes investigated using a hole-only device

Al-doped ZnO (AZO) films with a thickness of ∼400nm were prepared by sputtering on glass substrates for use as transparent anodes of organic light-emitting diodes (OLED) devices. The operation voltages (at 100cd/m2) of OLED devices with AZO and ITO anode materials were 10.5 and 5.5V, respectively. The maximum luminance output of the AZO device was 6450cd/m2 (achieved at 12.5V) and that of the ITO device was 9830cd/m2 (achieved at 10.5V). We demonstrate that a hole-only device method can be used to estimate the suitability of AZO and ITO anodes in the OLED devices and to verify experimental results. The AZO thin films with low price and non-toxicity may be suitable as alternative anodes in OLED devices under high voltage.

The effect of Ag layer thickness on the properties of WO 3/Ag/MoO 3 multilayer films as anode in organic light emitting diodes

Transparent conductive WO 3/Ag/MoO 3 (WAM) multilayer electrodes were fabricated by thermal evaporation and the effects of Ag layer thickness on the optoelectronic and structural properties of multilayer electrode as anode in organic light emitting diodes (OLEDs) were investigated using different analytical methods. For Ag layers with thickness varying between 5 and 20 nm, the best WAM performances, high optical transmittance (81.7%, at around 550 nm), and low electrical sheet resistance (9.75 Ω/cm 2) were obtained for 15 nm thickness. Also, the WAM structure with 15 nm of Ag layer thickness has a very smooth surface with an RMS roughness of 0.37 nm, which is suitable for use as transparent conductive anode in OLEDs. The current density−voltage−luminance ( J− V− L) characteristics measurement shows that the current density of WAM/PEDOT:PSS/TPD/Alq 3/LiF/Al organic diode increases with the increase in thickness of Ag and WO 3/Ag (15 nm)/MoO 3 device exhibits a higher luminance intensity at lower voltage than ITO/PEDOT:PSS/TPD/Alq 3/LiF/Al control device. Furthermore, this device shows the highest power efficiency (0.31 lm/W) and current efficiency (1.2 cd/A) at the current density of 20 mA/cm 2, which is improved 58% and 41% compared with those of the ITO-based device, respectively. The lifetime of the WO 3/Ag (15 nm)/MoO 3 device was measured to be 50 h at an initial luminance of 50 cd/m 2, which is five times longer than 10 h for ITO-based device.

Modulation of indium–tin oxide work function by a versatile self-assembled monolayer measured with the scanning Kelvin nanoprobe

In molecular optoelectronics, high-quality contacts at electrode|organics interfaces are crucial for charge carriers to efficiently flow through and therefore play a critical role on device performance. Electrode surface morphology, adhesibility, wettability, and work function are thus many parameters that must be accurately controlled, which is achievable using self-assembling monolayer (SAM) surface chemistry. Herein, we employ this technique to alter the electronic and surface energy-related properties of indium–tin oxide (ITO). In comparison to unmodified ITO, the newly introduced SAM-derivatized surface exhibits limited wettability and considerably higher work function (ΔΦ = ~1.2 eV). Several applications are proposed for this organic coating, notably at the anode of organic light-emitting diode (OLED) devices for decreasing the hole injection barrier or as an atmospherically stable protective layer in the coatings industry.

Effect of Thicknesses on the Structure, Conductivity, and Transparency of Al-doped ZnO Anodes in Organic Light-Emitting Diodes

Al-doped ZnO (AZO) films of various thicknesses were prepared by sputtering on glass substrates for use as transparent anodes of OLED devices. The effect of AZO film thickness on the structure, conductivity, and transparency of AZO films was investigated. The performance of OLEDs based on AZO anodes with various thicknesses is also discussed. The surface roughness of AZO is a crucial factor for the performance of OLEDs. The AZO thin film may be a potential alternative to an ITO anode due to its low price, and non-toxicity.

Manganese-doped indium oxide and its application in organic light-emitting diodes

Thin films of manganese-doped indium oxide (IMO) deposited by electron beam evaporation have been investigated as anodes in organic light-emitting diodes (OLEDs). The IMO films have a high work function of 5.35 eV, a desirable surface morphology with an average roughness of 1.1 nm, a high average optical transmittance of 87.2% in the visible region, and a maximum optical transmittance of 92% at 460 nm. It is demonstrated that an IMO anode can effectively improve hole injection at the anode/organic interface, resulting in OLEDs with an increased electroluminescent efficiency.