Electrode For Organic Light-emitting Devices Research Articles

Research on flexible silver nanowire electrode for organic light-emitting devices

By spin-coating silver nanowires (AgNWs) and polymethyl methacrylate (PMMA), applying pressure imprint and plasma treatment, we obtained flat AgNW thin film with a sheet resistance of 20 Ω/sq and a transmittance of 78% at 550 nm with low surface roughness. No significant change in sheet resistance was observed after cyclic bending (bending radius is 5 mm) test and tape test. After 1 000 bending tests, the change rate of sheet resistance was only 8.3%. The organic light-emitting devices (OLEDs) were prepared by using such AgNW electrodes and a maximum brightness of 5 090 cd/m2 was obtained. Compared with the AgNWs electrode without any treatment, the present AgNW electrodes have lower sheet resistance and better hole injection. Our results show spin-coated with flat layers, embossed and plasma-treated AgNW electrodes are suitable for manufacturing flexible organic optoelectronic devices.

Enhanced efficiency of organic light-emitting devices by using a directly imprinted nanopillared ultrathin metallic electrode.

An ultrathin metal film with high transmittance and conductivity has been demonstrated to be a promising transparent electrode for organic light-emitting devices (OLEDs). However, mediocre surface morphology and continuity of evaporated metal films and the surface plasmon-polaritons (SPPs) energy loss between the metal electrode and organic layer still limit the external quantum efficiency (EQE) of OLEDs. Here, nanoimprint lithography has been directly applied on the ultrathin Au film with underlying uncured photopolymer to fabricate the nanopillared anode. Both the conductivity and transmittance of the nanopillared ultrathin Au film have been improved due to the improvement of continuity and surface smoothness. As we expected, the SPPs mode has been coupled into photons and further extracted from OLEDs by using the nanopillared Au film anode. Finally, 19.2% and 70.1% enhancement of current efficiency were achieved compared to the planar device with ultrathin Au anode and ITO anode, respectively.

Samarium-Doped Indium-Tin-Oxide Electrode for Organic Light-Emitting Devices.

Here in, properties of samarium-doped indium-tin-oxide (ITO:Sm) films deposited by DC-magnetron sputtering, and organic light-emitting devices (OLEDs) using an ITO:Sm film as an anode were investigated. The electrical resistivity of the ITO:Sm films increased with Sm doping. The work function of each ITO:Sm film (5.2-5.7 eV) was higher than that of an ITO film (4.8 eV). The crystal structure of the ITO:Sm films was In₂O₃-type when the doping ratio of Sm in the film was up to 5 at.%, and the structure was amorphous in more strongly samarium-doped films. The properties of the OLEDs were varied by the doping ratio of samarium in the ITO:Sm electrodes. The OLEDs with the ITO:Sm electrode exhibited higher efficiency than that of an ITO-based control device at the same current density.

Microscale‐Patterned Graphene Electrodes for Organic Light‐Emitting Devices by a Simple Patterning Strategy

AbstractGraphene grown by chemical vapor deposition has attracted much attention in optoelectronic application due to its great potential as a large‐area 2D electrode material. However, the clean transfer and effective microscale patterning of graphene still remain great challenges for its use as the electrode in the optoelectronic devices. In this paper, a simple and reliable transfer‐pattern strategy is developed for the microscale‐patterned graphene electrode by using a photoresist as both supporting layer for the graphene transfer and photolithographic mask layer. The microscale‐patterned graphene electrodes transferred onto the desired substrates exhibit low surface roughness of 0.675 nm and mean sheet resistance of 444 Ω ◽−1. 25 µm line width patterned organic light‐emitting devices (OLEDs) arrays with high precision and uniform lighting area have proved the great potential of the transfer‐pattern strategy for high‐resolution OLEDs. Flexible and efficient OLEDs based on patterned graphene anodes can be realized by this strategy. Moreover, a scale of ≈2 in. patterned graphene well demonstrates the feasibility of the transfer‐pattern strategy for large‐area fabrication of the microscale‐patterned graphene.

A flexible plasma-treated silver-nanowire electrode for organic light-emitting devices

Silver nanowires (AgNWs) are a promising candidate to replace indium tin oxide (ITO) as transparent electrode material. However, the loose contact at the junction of the AgNWs and residual surfactant polyvinylpyrrolidone (PVP) increase the sheet resistance of the AgNWs. In this paper, an argon (Ar) plasma treatment method is applied to pristine AgNWs to remove the PVP layer and enhance the contact between AgNWs. By adjusting the processing time, we obtained AgNWs with a sheet resistance of 7.2Ω/□ and a transmittance of 78% at 550 nm. To reduce the surface roughness of the AgNWs, a peel-off process was used to transfer the AgNWs to a flexible NOA63 substrate. Then, an OLED was fabricated with the plasma-treated AgNWs electrode as anode. The highest brightness (27000 cd/m2) and current efficiency (11.8 cd/A) was achieved with a 30 nm thick light emitting layer of tris-(8-hydroxyquinoline) aluminum doped with 1% 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5 H,11H-(1)-benzopyropyrano(6,7-8-I,j)quinolizin-11-one. Compared to thermal annealing, the plasma-treated AgNW film has a lower sheet resistance, a shorter processing time, and a better hole-injection. Our results indicate that plasma treatment is an effective and efficient method to enhance the conductivity of AgNW films, and the plasma-treated AgNW electrode is suitable to manufacture flexible organic optoelectronic devices.

Efficiency Enhancement of Organic Light-Emitting Devices Fabricated Using a 1,4,5,8,9,11-Hexaazatriphenylene Hexacarbonitrile Silver Electrode.

A 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN)/Ag/HAT-CN transparent electrode for organic light-emitting devices (OLEDs) was investigated. The current density of OLEDs was enhanced with an increase in the Ag layer thickness of a HAT-CN/Ag/HAT-CN anode. While the luminance of the OLEDs with a HAT-CN/Ag/HAT-CN anode slightly decreased with increasing thickness of the upper HAT-CN layer, the current efficiency of the OLEDs with thicknesses for the Ag layer of 20 nm and for the upper HAT-CN layer of 100 nm was almost two times higher than that of OLEDs with an ITO anode. Electroluminescence spectra of the OLEDs with a HAT-CN/Ag/HAT-CN anode were shifted due to the micro-cavity effect in the stacked anode, which was confirmed by the transmittance spectra.

Enhancing light out-coupling of organic light-emitting devices using indium tin oxide-free low-index transparent electrodes

With its increasing and sufficient conductivity, the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been capable of replacing the widely used but less cost-effective indium tin oxides (ITOs) as alternative transparent electrodes for organic light-emitting devices (OLEDs). Intriguingly, PEDOT:PSS also possesses an optical refractive index significantly lower than those of ITO and typical organic layers in OLEDs and well matching those of typical OLED substrates. Optical simulation reveals that by replacing ITO with such a low-index transparent electrode, the guided modes trapped within the organic/ITO layers in conventional OLEDs can be substantially suppressed, leading to more light coupled into the substrate than the conventional ITO device. By applying light out-coupling structures onto outer surfaces of substrates to effectively extract radiation into substrates, OLEDs using such low-index transparent electrodes achieve enhanced optical out-coupling and external quantum efficiencies in comparison with conventional OLEDs using ITO.

Effects of defects and non-coordinating molecular overlayers on the work function of graphene and energy-level alignment with organic molecules

To elucidate the features of graphene as an electrode material, we studied the effect of defects and molecular overlayers on the work function of graphene using density-functional theory. We found that in-plane geometrical deformations (such as Stone–Thrower–Wales defects, carbon vacancies, and hydrogenated edges) have only a marginal effect. In contrast, intercalated alkaline atoms (K or Li) and overlayers of superhalogen species (BF 4 and PF 6) radically change the work function. We show that the geometry of the sp 2 carbon surface remains robust after electron transfer to superhalogens, and the Fermi level could be well aligned with the energy levels of organic molecules. These methods for work function control can be used for the application of graphene materials as transparent electrodes for organic light-emitting devices.

ZnO-based transparent anodes for organic light-emitting devices

ABSTRACTZnO/Au/ZnO (ZAZ) electrodes grown on flexible PEN substrates were evaluated as transparent electrodes for organic light-emitting devices (OLEDs). OLEDs fabricated with the ZAZ electrodes showed reduced leakage in contrast to control OLEDs on ITO and reduced ohmic losses at high current densities. At a luminance of 25000 cd/m2, the lum/W efficiency of the ZAZ electrode based device was 5% greater than for the device on ITO. The ZAZ electrodes also allow for a broader spectral output in the green wavelength region of peak photopic sensitivity compared to ITO. The results have implications for electrode choice in display technology.

Molybdenum-Doped Zinc Oxide Electrodes for Organic Light-Emitting Devices

The properties of molybdenum-doped ZnO (MZO) films deposited by radio-frequency (rf) magnetron sputtering and organic light-emitting devices (OLEDs) using an MZO film as an anode were investigated. A transparent conducting MZO film with high transparency and relatively low resistivity was prepared by rf magnetron sputtering at room temperature. The structural, electrical, and optical properties of the films deposited under different Ar pressures were investigated. X-ray diffraction patterns showed that the nature of the films is polycrystalline with a hexagonal structure and a preferred orientation along the c axis. Thus, MZO films may be a potential material for OLEDs.

Transparent manganite films as hole injectors for organic light emitting diodes

Organic light-emitting diodes (OLEDs) are nowadays one of the most attractive devices based on organic semiconductors due to their successful application in the display technology. Electroluminescence in OLEDs is mainly governed by the fluorescence from excited singlet states, which have large transition probabilities providing the major radiative pathway. The “forbidden” triplet state emission can be activated by increasing spin–orbit coupling via dye doping. The singlet–triplet exciton formation statistics is usually given by 1:3 partition due to the quantum constrains. Injection of carriers with finite spin polarisation should influence and modify the recombination statistics and can be used for tuning of the device efficiency. In this context, the development of a new class of electrodes able to guarantee both efficient charge and spin injection becomes of paramount importance. We show that strongly spin polarised colossal magnetoresistance manganite La 0.7Sr 0.3MnO 3 (LSMO) can successfully substitute conventional ITO electrodes in OLEDs. Highly transparent, metallic and ferromagnetic LSMO layers were used in combination with standard Al and spin polarised Co top electrodes. Electrical and optical characterisations of the OLEDs with spin polarised electrodes indicate the applicability of the new manganite electrodes for organic light-emitting devices.

Aluminum-doped zinc oxide films as transparent conductive electrode for organic light-emitting devices

Highly transparent conductive, aluminum-doped zinc oxide (ZnO:Al) films were deposited on glass substrates by midfrequency magnetron sputtering of metallic aluminum-doped zinc target. ZnO:Al films with surface work functions between 3.7 and 4.4 eV were obtained by varying the sputtering conditions. Organic light-emitting diodes (OLEDs) were fabricated on these ZnO:Al films. A current efficiency of higher than 3.7 cd/A, was achieved. For comparison, 3.9 cd/A was achieved by the reference OLEDs fabricated on commercial indium–tin–oxide substrates.

Surface and interface analysis for copper phthalocyanine (CuPc) and indium-tin-oxide (ITO) using X-ray photoelectron spectroscopy (XPS)

Indium-tin-oxide (ITO) has been widely used as a hole injection electrode for organic light-emitting devices (OLEDs), but the work function of ITO film usually mismatches the highest occupied molecular orbital (HOMO) of the hole transport materials. Copper phthalocyanine (CuPc) has been used as a hole injection buffer to enhance the hole injection from ITO to hole transport layer (HTL). A thin CuPc layer was thermally evaporated onto ITO-coated glass substrate and the surface and interface electron states of the CuPc/ITO close contact were measured and analyzed by X-ray photoelectron spectroscopy (XPS) technology. Results show that, in the interface of CuPc and ITO film, the role of oxygen is very crucial, the oxygen in ITO film may diffuse into the organic semiconductor and cause the alteration of electron states in organic materials.