Oxygen plasma treatment effects of indium-tin oxide in organic light-emitting devices

The effects of oxygen plasma treatment on the surface properties of indium-tin oxide (ITO) substrates and its aging were investigated by X-ray photoelectron spectroscopy (XPS), contact angle, surface energy and polarity measurements. Experimental results demonstrate that the oxygen plasma treatment improves the stoichiometry of the surface and enhances the ITO wetting, so as to improve the surface properties of ITO substrates, due to the introduction of oxygen and the partial removal of hydrocarbon contaminants from the ITO surfaces. With the increment of aging time, however, the improved ITO surface properties are observed to tend to decay. Furthermore, the aging effect of treated ITO substrates on the performance of organic light-emitting devices (LEDs) was studied with respect to the driving voltage, brightness and efficiency. It is found that the device performance is subjected to the ITO surface properties and the ITO substrates aged for various times result in significant differences in electrical and optical characteristics which become worse as the aging time increases. The results indicate that the performance of organic LEDs is closely related to the surface properties of ITO substrates and the interface characteristics of ITO/organic layer.     

Surface modification and characterization of indium–tin oxide for organic light-emitting devices

We used various treatment methods such as ethanol treatment, sodium hydroxide solution treatment, sulfur acid treatment, and oxygen plasma treatment to modify the surface of indium–tin oxide (ITO) substrates for organic light-emitting devices (OLEDs). The surface properties of the treated ITO substrates were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), contact angle, surface energy measurements, four-point probe, X-ray Diffraction (XRD) and ultraviolet-visible spectrophotometer. The results showed that the ITO surface properties were closely related to the treatment methods, and the oxygen plasma is more efficient than other treatments as it leads to smoother surface, better surface stoichiometry, higher work function and surface energy, lower sheet resistance, and higher transmission of the ITO substrates. Moreover, small molecular organic light-emitting devices (SMOLEDs) using different treated ITO substrates as anodes were fabricated and investigated. It was found that surface treatment of ITO substrates has influence upon the injection current, the turn-on voltages of light emission, luminance, efficiency and lifetime. Oxygen plasma treatment on the ITO substrate yields the highest performance of SMOLEDs due to the improvement of interface formation and electrical contact of the ITO substrate with the small molecular material blend in the SMOLEDs.     

Investigation of surface properties of treated ITO substrates for organic light-emitting devices

In this work, a study of the surface properties of treated indium tin oxide (ITO) substrates is reported. We used three different cleaning treatments among others: washing in an ultrasonic bath of acetone and ethyl alcohol or isopropyl alcohol in room temperature as well as dipping into solution (prepared from NH₄OH (25 %), H₂O₂ (30 %) and distilled water) at 60 °C. The relation between ITO morphology and surface properties has been studied by contact angle, and surface energy measurements as well as the surface roughness of ITO samples evaluation by atomic force microscopy (AFM). Experimental results show that the ITO surface properties are closely related to the treatment methods. The ultrasonic degreasing in acetone and ethyl alcohol yields the highest surface energy (38.5 mJ/m²), brings about the maximum reduction in contact angles and causes the smallest increasing the surface roughness of ITO substrates.     

Indium–tin oxide surface treatments: Effects on the performance of liquid crystal devices

In this work, we investigate the effect of indium tin oxide (ITO) substrate cleaning on the surface properties. Wettability technique was used to measure the contact angle and the surface energy of the different treated ITO substrates. It is found that treatment with the methanol without dehydration gives the lowest water contact angle (most hydrophilic surface) and the highest surface energy compared to other solvents. This result was confirmed by impedance measurements performed on nematic liquid crystal cells with ITO electrodes. Indeed, we check the decrease of ionic entities in the interface ITO/liquid crystal. The polarity and dielectric parameters of the used solvents explain the obtained results.     

Study of MEH–PPV/PCBM active layer morphology and its application for hybrid solar cell performance

Surface morphologies of MEH?CPPV:PCBM active layers were optimized by investigating ITO substrate treated with oxygen and nitrogen plasma. This treatment effectively improved smoothness, transmittance, and contact angle of ITO??s, resulting in good anode contacts for hybrid device structures. The consistently improved performance of hybrid solar cells was also achieved. The surface properties of treated and untreated ITO substrates were compared by contact angle, four point probe, scanning electron microscopy, and atomic force microscopy.     

Surface characterisation and functionalisation of indium tin oxide anodes for improvement of charge injection in organic light emitting diodes

Wettability studies have been performed to probe the surface properties of ITO substrates, aimed to be used as hole injecting electrode in OLEDs. The elimination of organic contaminants upon the cleaning treatment (ultrasonic bath in organic solvents) leads to an increase of the free energy of the ITO surface becoming hydrophilic. The surface energy components calculated from the Van Oss model show the appearance of a basic component upon the cleaning treatment. A thermal treatment at 100 °C for 3 h leads to a decrease of the surface free energy due to surface dehydration. These properties are attributed to the hydroxides formed at the ITO surface inducing improved adhesion at the ITO/polymer interface. The ITO surfaces have been functionalised with a chloroethylphosphonic acid mono-layer to increase their stability. The appearance of an acid-base component leads to a dipolar character of the ITO surface. The formation of a compact layer of a spin coated poly(phenylenevinylene) derivative induces the shielding of the ITO basic character. The weakening of the near infrared absorption associated to ITO free carriers confirms the formation of a dipole layer at the interface with the molecular layer in contact with ITO. Improved injection properties, shown by the current/voltage characteristics, result from the interface modifications.     

以复合材料为基底的ITO薄膜红外隐身特性研究

利用直流磁控溅射法在树脂基复合材料基底上制备了ITO低红外发射率薄膜。选择了低发射率树脂作为基底复合材料的基体树脂。通过对材料表面进行表征、测试材料表面电阻率及红外发射率,研究了材料的红外隐身性能。结果表明,材料的红外发射率随ITO膜表面电阻率的增加先增加,后趋于一定值。     

Indium tin oxide nanorod electrodes for polymer photovoltaics

We have deposited indium tin oxide (ITO) nanorods on glass and glass/ITO substrates by DC sputtering and by e-beam deposition. The properties of the nanorods deposited by different methods and on different substrates have been investigated. The ITO nanorods were also used as an electrode in bulk heterojunction polymer solar cells. We found that the nanorod morphology and sheet resistance had a significant effect on the solar cell performance, with significant improvements in the efficiency compared to commercial ITO film substrates in all cases except for e-beam deposited nanorods on glass that had high sheet resistance. The best power conversion efficiency achieved was 3.2 % (for sputtered ITO nanorods on ITO), compared to 2.1 % for commercial ITO substrates.     

Properties of indium tin oxide thin films prepared on the oxygen plasma-treated polycarbonate substrate

We prepared the indium tin oxide thin (ITO) film on the polymer substrate by using facing target sputtering method. To obtain a smooth surface of the ITO thin film for application of OLEDs, before deposition of the ITO thin film, the polymer substrate was given plasma surface treatment. The electrical and surface properties were measured by a Hall Effect measurement and a contact angle measurement. The structural and optical properties were evaluated by an X-ray diffractometer, an atomic force microscope and a UV/VIS spectrometer, respectively. All ITO thin films deposited on plasma-treated polymer substrate showed an average transmittance over 85% in visible range, and the lowest resistivity was 4.17 × 10^− 4 Ω cm.     

Effect of surface treatment and functionalization on the ITO properties for OLEDs

The relation between ITO morphology and surface properties has been studied by wettability measurements and AFM microscopy. Different wet cleaning procedures have been investigated leading to different surface properties: ultrasonic degreasing, RCA and Aqua regia treatment. After full elimination of the organic surface contaminants, the ITO surface shows a main basic surface component. RCA and Aqua regia treatments are the most efficient techniques for cleaning ITO surfaces from organic contamination. Nevertheless, Aqua regia treatment induces ITO surface degradation, which is evidenced by AFM observations and resistance sheet measurements. The ITO surface has been derivatized with a SAM layer of phosphonic acid to improve the ITO/polymer interface. Effect of 2-chloroethylphosphonic acid layer on the different components of the surface energy was determined: acid component is increased whereas basic component is decreased, showing the good coverage of the ITO surface by the SAM layer. A more compact coating of POMX polymer is obtained after SAM functionalization of ITO samples cleaned with RCA treatment.     

Material properties and growth control of undoped and Sn-doped In2O3 thin films prepared by using ion beam technologies

Undoped (IO) and Sn-doped In₂O₃ (ITO) films have been deposited on glass and polymer substrates by an advanced ion beam technologies including ion-assisted deposition (IAD), hybrid ion beam, ion beam sputter deposition (IBSD), and ion-assisted reaction (IAR). Physical and chemical properties of the oxide films and adhesion between films and substrates were improved significantly by these technologies. By using the IAD method, non-stoichiometry and microstructure of the films were controlled by changing assisted oxygen ion energy and arrival ratio of assisted oxygen ion to evaporated atoms. Relationships between structural and electrical properties in ITO films on glass substrates were intensively investigated by using the IBSD method with changing ion energy, reactive gas environment, and substrate temperature. Smooth-surface ITO films (R_rms ≤ 1 nm and R_p-v ≤ 10 nm) for organic light-emitting diodes were developed with a combination of deposition conditions with controlling microstructure of a seed layer on glass. IAR surface treatment enormously enhanced the adhesion of oxide films to polymer substrate. The different dependence of IO and ITO films' properties on the experimental parameters, such as ion energy and oxygen gas environment, will be intensively discussed.     

Surface conditioning of indium-tin oxide anodes for organic light-emitting diodes

Oxygen-plasma treatment of indium-tin oxide (ITO) anodes is now widely used as one of the most effective ways to improve the device performance of organic light-emitting diodes (LEDs). However, the role of oxygen-plasma treatment has not been clearly understood. We have performed detailed studies of the surface and bulk of the ITO thin films exposed to oxygen-plasma. We employed a multitude of experimental techniques, including X-ray and ultraviolet photoelectron spectroscopies, atomic force microscopy, dynamic contact angle measurement, four-point probe and Hall measurements to investigate the changes induced by the plasma. We have also analyzed the device characteristics of polymer LEDs fabricated with these anodes. We found significant modifications of the physico-chemical, morphological, transport and optical properties of the oxygen-plasma treated ITO. Although oxygen-plasma does not show any measurable etching effect, it induces considerable changes leading to an increase in work function, electron carrier concentration and conductivity. It also increases the surface energy and polarity. We relate these modifications to enhancement of the device performance, such as electroluminescence efficiency and lifetime, through their effects on hole injection, and interface structure and stability. Finally, we show that even in the presence of a hole-transport layer such as a poly(styrene sulphonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT:PSS) inserted between the anode and the emissive polymer layer, oxygen-plasma treatment of the ITO anodes is still beneficial for the devices.     

原子力显微镜与X射线光电子能谱对ITO表面改性的研究

采用氧气等离子体(OP)处理对氧化铟锡(ITO)薄膜进行表面改性,通过原子力显微镜(AFM)、X射线光电子能谱(XPS)和四探针等测试手段对薄膜样品进行表征,研究了OP处理对ITO表面性质的影响.实验结果表明OP处理有效去除了ITO表面的污染物,优化了ITO表面的化学组分,降低了ITO表面的粗糙度和方块电阻,改善了ITO的表面形态.与此同时,通过XPS监测研究了OP处理后ITO表面化学组分随老化时间的变化,结果显示经过优化的化学组分随老化时间增加而逐渐退化.另外,以OP处理后经过不同老化时间的ITO样品作为空穴注入电极,制备了有机电致发光器件(OELD),通过测试器件的电压-电流-亮度特性,进一步研究了ITO表面性质对于OELD光电性能的影响.     

Thickness of ITO thin film influences on fabricating ZnO nanorods applying for dye-sensitized solar cell

Background and purposeIn order to compare the substrates influence on the properties of ZnO films and nanostructures, in this paper, the ITO substrates with different thicknesses were investigated.MethodITO thin films of different thickness (200 nme500 nm) were deposited on glass substrates by DC sputtering, on which ZnO nanorods were fabricated from as-deposited ZnO films by reducing annealing method.ResultsIt was found that the structural and electrical properties of ITO films were significantly influenced by the ITO film thickness. The roughness of ITO films was increased with increase in thickness. The Hall mobility of ITO films was also increased with the increase of film thickness; in contrast, the resistivity was decreased. The highest Hall mobility of 29.2 cm²/V s and the lowest resistivity of 1.303 × 10⁻⁴ Ω cm were obtained from 500 nm-thick ITO film. The structural properties of ZnO nanorods were significantly influenced by the ITO film thickness. The density of ZnO nanorods gradually decreased with the increase in thickness of ITO film.ConclusionThe overall conversion efficiency of demonstrated dye-sensitized solar cell was 2.11% with a fill factor 0.526, indicating high potential to be used as photoanodes in dye-sensitized solar cell applications.     

Surface treatment of polycarbonate and polyethersulphone for SiNx thin film deposition

Silicon nitride thin films were deposited with good adhesion on plasma treated polyethersulphone (PES) and polycarbonate (PC) substrates by in-situ rf magnetron sputtering. The surfaces of the PES and PC substrates were performed by plasma treatment at various rf powers and processing time in Ar, O₂ atmosphere. From the X-ray Photoelectron Spectroscopy (XPS) examination of the surface of the treated substrates, it was found that the ratio of oxide containing bonds increased with increasing rf power. The surface roughness of the PES and PC substrates increased with increasing rf power. The plasma treated surface of the substrates became hydrophilic as measured by the water contact angle. The water contact angle for the PES and PC substrates decreased with increasing rf power and processing time, significantly. The lowest value of the contact angle of 14.09° was observed at rf power of 200 W. It was observed that the adhesion properties between the SiN_x films and substrates were enhanced by the plasma treatment.     

Work function and valence band structure of tin-doped indium oxide thin films for OLEDs

Transparent conducting ITO thin film has been widely used as anode material in OLEDs due to its good optical transparency, low electrical resistivity, ease of patterning, high work function and efficient hole injection properties. The interface between ITO and organic layer in OLED device is thus important and can influence the electrical and luminescent properties. In this report, ITO substrates were treated with 20% H₃PO₄ solution. The corresponding changes in crystalline morphology were studied by X-ray diffraction. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were performed at ∼10⁻⁹ Torr to study the work function and the valence band structure of ITO substrates. It was found that work function became slightly lower after the treatment, probably caused by the formation of metal complex compounds and metal hydroxides. The binding energy of In 3d_5/2 shifted from 444.6 to 445.3 eV. This shifting was referred to the formation of In-OH bonding. It would be easier to provide electron by In-OH bonding than by In-O-In or Sn-O-Sn when photons reached ITO surface. The interface between ITO and CuPc was improved through polar surface and less aggregation. In addition, the OLED devices exhibited improved performance in both external quantum efficiency and luminescence efficiency.     

Study of the substrate treatment effect on initial growth of indium tin-oxide films on polymer substrate using in situ conductance measurement

Changes in the initial growth mode of ion beam sputtered indium tin oxide (ITO) films on polycarbonate (PC) substrates were investigated by an in situ measurement of electrical conductance. The PC substrates were irradiated with l keV Ar ions in an oxygen environment (ion assisted reaction: IAR), prior to the film deposition for changing the surface energy. The electrical conduction modes in ITO films were discussed in terms of the film thickness and the surface energy of PC substrates. It was found that, in the initial part of the film growth, ITO nucleation density increased with the increase of the surface energy of PC. The change of the growth mode was discussed in both viewpoints of thermodynamics and atomic kinetics theories and verified by AFM (atomic force microscope) observations. Thermal stability of ITO films was investigated to observe the effect of the growth mode change by IAR pre-treatment of polymer substrate.     

Electric properties and surface characterization of transparent Al-doped ZnO thin films prepared by pulsed laser deposition

We grew 2 wt.% Al-doped ZnO (AZO) films on 5.08 cm-diameter polymer substrates at room temperature by the pulsed laser deposition (PLD) technique added the beam-rastering function. The structural properties, surface morphology, resistivity, mobility and chemical bonding states of AZO/polymer films were measured. The structuring of polymer surface by atmospheric plasma can occur at nm scales and can influence adhesion, optical and wettability properties of the materials. With increasing plasma treatment power, surface hydrophilicity and roughness for PET and PES polymer increased, respectively.     

Layer‐by‐Layer Assembled Oxide Nanoparticle Electrodes with High Transparency,Electrical Conductivity,and Electrochemical Activity by Reducing Organic Linker‐Induced Oxygen Vacancies

Solution‐processable transparent conducting oxide (TCO) nanoparticle (NP)–based electrodes are limited by their low electrical conductivity, which originates from the low level of oxygen vacancies within NPs and the contact resistance between neighboring NPs. Additionally, these electrodes suffer from the troublesome trade‐off between electrical conductivity and optical transmittance and the restricted shape of substrates (i.e., only flat substrates). An oxygen‐vacancy‐controlled indium tin oxide (ITO) NP‐based electrode is introduced using carbon‐free molecular linkers with strong chemically reducing properties. Specifically, ITO NPs are layer‐by‐layer assembled with extremely small hydrazine monohydrate linkers composed of two amine groups, followed by thermal annealing. This approach markedly improves the electrical conductivity of ITO NP‐based electrodes by significantly increasing the level of oxygen vacancies and decreasing the interparticle distance (i.e., contact resistance) without sacrificing optical transmittance. The prepared electrodes surpass the optical/electrical performance of TCO NP‐based electrodes reported to date. Additionally, the nanostructured ITO NP films can be applied to more complex geometric substrates beyond flat substrates, and furthermore exhibit a prominent electrochemical activity. This approach can provide an important basis for developing a wide range of highly functional transparent conducting electrodes.     

ZnO nanowires hydrothermally grown on PET polymer substrates and their characteristics

Zinc oxide nanowires (ZnO NWs) were successfully synthesized on the ITO/PET polymer substrates by a hydrothermal method. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy investigations were carried out to characterize the crystallinity, surface morphologies, and orientations of these NWs, respectively. The influence of NW surface morphologies on the optical and electrical properties of ZnO NWs was studied. The hydrothermally grown ZnO NWs with direct band gap of 3.21 eV emitted ultraviolet photoluminescence of 406 nm at room temperature. Field emission measurements revealed that the threshold electric fields (Eth, current density of 1 mA/cm2) of ZnO NWs/ITO/PET and ZnO NWs/ZnO/ITO/PET are 1.6 and 2.2 V/microm with the enhancement factors, beta values, of 3275 and 4502, respectively. Furthermore, the field emission performance of ZnO NWs deposited on the ITO/PET substrate can be enhanced by illumination with Eth of 1.3 V/microm and displays a maximum emission current density of 18 mA/cm2. The ZnO NWs successfully grown on polymer substrate with high transmittance, low threshold electric field, and high emission current density may be applied to a flexible field emission display in the future.