Work Function Of Indium Tin Oxide Research Articles

Work Function Modification of Indium–Tin Oxide by Surface Plasma Treatments Using Different Gases

We report on the effects of surface treatment with N2, O2, and N2O plasmas on the work function of indium–tin oxide (ITO). UV photoelectron spectroscopy (UPS) showed that the work function on the ITO surface treated with N2O plasma increased more than that on the samples treated with N2 or O2 plasma. X-ray photoelectron spectroscopy (XPS) showed that the intensity of the O–O bonding peak at 532.3 eV markedly increased owing to the adsorption of O- ions on the ITO surface from breaking bonds in N2O gas by the plasma. The dipole layer formed by O- ions on the ITO surface increases the work function of ITO. Accordingly, N2O plasma treatment leads to a reduction of the potential barrier between the Fermi level of ITO and the highest occupied molecular orbital (HOMO) level of an organic layer when ITO is used as an anode for organic light-emitting devices (OLEDs) and related devices. Therefore, N2O plasma treatment enhances the hole-injection properties from the ITO thin film to the organic layer.

Effects of surface modification of indium tin oxide electrodes on the performance of molecular multilayer organic photovoltaic devices

We investigate the effects of surface modification of indium tin oxide (ITO) on the performance of organic multilayer molecular photovoltaic devices based on pentacene/C60 bi-layer heterojunctions. Values of the open-circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and power conversion efficiency (η) are found invariant of the work function and surface hydrophobicity of ITO. Insensitivity of these parameters to variations of work function in the range of 4.50 to 5.40 eV achieved through the use of surface modifiers are correlated with an invariance of the barrier height (∼0.6 eV) due to Fermi level pinning at the ITO/pentacene interface. Energy barrier heights are extracted independently from the analysis of the electrical characteristics of single-layer diodes based on modified ITO and pentacene using an equivalent circuit model.

Stabilization of the work function of indium tin oxide using organic surface modifiers in organic light-emitting diodes

We herein report on the performance and improved stability of organic light-emitting diodes (OLEDs) in which the transparent indium tin oxide (ITO) electrode is modified using organic surface modifiers based on phosphonic acid anchoring groups. In contrast to air plasma treatment, a commonly used technique to increase the work function of ITO, treatment of the ITO surface with a partially fluorinated phosphonic acid results in a comparable change in work function but with a higher stability over time. The resultant lifetime of OLEDs also increased when this phosphonic acid modified ITO was used.

Work function engineering of electrodes via electropolymerization of ethylenedioxythiophenes and its derivatives

The work functions of two commonly used electrodes, Au and indium-tin-oxide (ITO), can be modified by surface-electropolymerization of ethylenedioxythiophenes (EDOT) or its derivatives. The resulting work function is tunable continually by varying the electropolymerization condition. The value of work function of ITO (or Au) electrodes can be adjusted to decrease by 0.6 eV (or increase by 0.4 eV) when the deposited EDOT film is thin (<40 nm). By contrast, when the EDOT film deposited with thickness greater than 40 nm, the work function is controlled by the intrinsic property of the polymers regardless of the electrode materials. A diode devices with EDOT–COOH modified on ITO surface demonstrates the possibility of improving contact property in organic electronics using the proposed method.

Relationship between work function of indium tin oxide and device performances of C60 modified organic light-emitting diodes

Relationships between the work function of indium tin oxide (ITO) and device performances of C60 modified organic light-emitting diodes were investigated. The work function of ITO was controlled by changing the degree of oxidation of ITO using ultraviolet/O3 treatment. High efficiency could be obtained in the C60 modified device with short ITO treatment time, while driving voltage was lowered in the C60 modified device with long ITO treatment time.

Impedance Spectroscopy Analysis of Organic Light-Emitting Diodes Fabricated on Plasma-Treated Indium-Tin-Oxide surfaces

In this work, impedance spectroscopy analysis was used to study the eects of plasma treatment on the surface of indium-tin oxide (ITO) anodes using CF4, O2 and Ar gases and to model an equivalent circuit for organic light-emitting diodes (OLEDs). The devices with an ITO/TPD/Alq3/LiF/Al structure could be modeled as a simple combination of resistors and capacitors. For modifications of the ITO surface, the samples were treated in an inductively-coupled tubular reactor system. The OLEDs fabricated on plasma-treated ITO anodes showed a lower impedance and a higher capacitance. The changes in the impedance and the capacitance were attributed to removal of contaminants and to changes in the work function of ITO. The impedance spectroscopy analysis showed that the devices with plasma-treated ITO anodes had dierent values of the contact resistance (RC), the parallel resistance (RP) and the parallel capacitance (CP).

Preparation and Characterization of Dense Films of Poly(amidoamine) Dendrimers on Indium Tin Oxide

Indium tin oxide (ITO) substrates were modified with a layer of poly(amidoamine) (PAMAM) dendrimers to change their surface properties and, in particular, the substrates' work function. The functionalization procedure involved the electrostatic adsorption of positively charged PAMAM dendrimers of generation five onto negatively polarized ITO surfaces. Three different PAMAM dendrimers were used: PAMAM-NH2 and PAMAM-OH with terminal amine and hydroxyl groups, respectively, as well as Q-PAMAM-NH2, which had been prepared from PAMAM-NH2 by quaternization of the dendrimer's terminal and internal amine groups with methyl iodide. The resulting organic films were analyzed by contact angle goniometry, X-ray photoelectron spectroscopy, ellipsometry, and Kelvin probe force microscopy to confirm the presence of a dense layer. A Langmuir isotherm was derived from surface densities of fluorescence-labeled PAMAM-NH2 dendrimers from which we deduced an equilibrium binding constant, K(eq), of (1.3 +/- 0.3) x 10(5) M(-1). Kelvin probe measurements of the contact potential difference revealed a high reduction of the work function from 4.9 eV for bare ITO to 4.3 eV for ITO with a dense film of PAMAM-NH2 of generation five. PAMAM-OH and Q-PAMAM-NH2 resulted in slightly smaller work function changes. This study illustrates that the work function of ITO can be tuned by adlayers composed of PAMAM dendrimers.

Contact resistance between pentacene and indium–tin oxide (ITO) electrode with surface treatment

Contact resistance between indium–tin oxide (ITO) electrode and pentacene was studied by transmission line method (TLM). Organic solvent cleaned, inorganic alkali cleaned, and self-assembled monolayer (with OTS: octadecyltrichlorosilane) modified ITO electrode structures were compared. Pentacene layer of 300 Å thickness was vacuum deposited on patterned ITO layer at 70 °C with a deposition rate of 0.3 Å/s. Alkali cleaned and SAM modified ITO gave a lower contact resistance of about 6.34 × 10 4 Ω cm 2 and 1.88 × 10 3 Ω cm 2, respectively than organic solvent cleaned ITO of about 6.58 × 10 5 Ω cm 2. Especially with the SAM treatment, the work function of ITO increased closer to the highest occupied molecular orbital (HOMO) level of pentacene, which lowers the injection barrier between ITO and pentacene. It was also believed that pentacene morphology was improved on SAM modified ITO surface due to the lowering of the surface energy. We could obtain the low contact resistance with SAM treatment which is comparable to the measured value of gold–pentacene contact, 1.86 × 10 3 Ω cm 2. This specific contact resistance is still much higher than that of amorphous silicon thin film transistor (0.1–30 Ω cm 2).

Comment on “Mechanism for the increase of indium-tin-oxide work function by O2 inductively coupled plasma treatment” [J. Appl. Phys. 95, 586 (2004)

A high work function on indium tin oxide (ITO) surfaces with O2 inductively coupled plasma (ICP) treatment was obtained in the paper: “Mechanism for the increase of indium tin oxide work function by O2 inductively coupled plasma treatment” [Lee et al., J. Appl. Phys. 95, 586 (2004)]. They attributed this to the higher work function of the larger upward band bending and the larger electron affinity of the ITO near the surface by the ICP treatment. In this Comment, we indicate the mistake in their analysis of the observed result by synchrotron radiation photoemission spectroscopy.

Effects of Sulfide Treatment of Indium Tin Oxide on Efficiency of Polymer Light-Emitting Diodes

The effects of the sulfide treatment of indium tin oxide (ITO) on the efficiency of polymer light-emitting diodes (PLEDs) were investigated in this study. We found that an increase in the work function of ITO and a decrease in the number of oxygen vacancies (VO) near the ITO surface region (due to the occupation of VO by sulfur) result in an increase in device efficiency. It is worth noting that a suitable sulfide treatment time is an important issue for producing high-external-quantum-efficiency PLEDs.

Influence of heat treatment on indium–tin-oxide anodes and copper phthalocyanine hole injection layers in organic light-emitting diodes

Modifications of indium–tin-oxide (ITO) and copper phthalocyanine (CuPc) layers by heat treatment aimed at lowering driving voltage in organic light-emitting diodes (OLEDs) are examined. Significant changes were observed in the surface morphology and carrier injection properties of ITO and CuPc layers after annealing at T = 250 °C for 0–60 min in a glove box. In the case of ITO annealing, although the ITO work function gradually decreased and the surface of the ITO layer became smoother than that of an unannealed ITO layer, we observed an appreciable decrease in the driving voltage with an increase in annealing time. In the case of CuPc annealing, on the other hand, we observed deterioration of the OLED's characteristics. All devices demonstrated an increase in driving voltage due to the pronounced crystallization of the CuPc layer.

Current-voltage behavior in hole-only single-carrier devices with self-assembling dipole molecules on indium tin oxide anodes

The authors report the use of chemically modified indium tin oxide (ITO) with different binding groups (–COCl and –PO2Cl2) of p-chlorobenzene derivatives forming effective monolayers to control the work function of ITO and hence to enhance the hole injection. The enhanced hole injection is studied by measuring current density–voltage (J-V) characteristics. The behavior of J-V characteristics caused by varying the ITO work function in hole-only single-carrier devices with a hole transport layer of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine is examined. Upon grafting with p-chlorophenylphosphoryl dichloride, the J-V characteristics show a space-charge-limited conduction behavior. Such modified ITO anodes lead to improvements in the device properties.

유기 디스플레이 소자를 위한 Self Assembled Monolayer의 표면개질을 이용한 ITO의 일함수 증가

Indium tin oxide (ITO) used as an electrode in organic light emitting diodes (OLEDs) and organic thin film transistors (OTFTs) was modified by a self-assembled monolayer (SAM). For device fabrication, surface of the ITO was modified by immersion in a solution including various phosphonic acid at room temperature in order to increase work function of an electrode. The work function of ITO with SAM was measured by Kelvin probe. Work function increase of 0.88 eV was observed in ITO with various SAM. Therefore, ohmic contact is achieved in an interface between ITO and organic semiconductors (pentacene). We analyzed the origin of work function increase of ITO with SAM by X-ray photoelectron spectroscopy. We confirmed that increase of oxygen bonding energy attributed to increase the work function of ITO. These results suggested that ITO with the SAM gives a high possibility for high performance of OLEDS and OTFTs.

Mechanisms of Performance Improvement for Polymer Light-Emitting Diodes Fabricated on (NH[sub 4])[sub 2]S[sub x]-Treated Indium-Tin Oxide Substrates

Photoelectron spectroscopy measurements show that (NH 4 ) 2 S x surface treatment may lead to an increase in the work function of indium-tin oxide (ITO), due to the occupation of oxygen vacancies (V O ) by sulfur and a reduction in the amount of V O near the ITO surface region. Comparison between the current-luminance-voltage characteristics of polymer light-emitting diodes fabricated on ITO substrates with and without (NH 4 ) 2 S x treatment, shows that (NH 4 ) 2 S x treatment led to the enhancement of external quantum efficiency, due to the reduction of forward-biased leakage current (resulting from the reduction of V O -related defects near the ITO surface region).

Influence of indium tin oxide treatment using UV–ozone and argon plasma on the photovoltaic parameters of devices based on organic discotic materials

AbstractRecently, we reported that the open circuit voltage, V0C, in two‐layer organic solar cells depends on the indium tin oxide (ITO) treatment. However, the V0C variation details were not clearly understood. The study of the electronic structure at the ITO/organic interface was carried out by using the Kelvin probe method. The results obtained show a strong dependence of the organic vacuum level on the ITO work function, which, in turn, depends on the anode treatment. An abrupt variation of the vacuum level was observed after deposition of a few organic monolayers, which could be attributed to the presence of dipoles at the interface. We demonstrate that the shift magnitude depends on the nature and duration of the ITO treatment. To this end, two different ITO treatments were used. The first one, based on UV–ozone exposure, is known to increase the ITO work function, WITO. The second of these uses a microwave argon plasma generated by a repartee cyclotron excitation. This was adapted for decreasing the WITO value. An investigation on the origin of these variations (‘up and down’) is presented in this study. Copyright © 2005 Society of Chemical Industry

Indium-tin oxide anodes modified by self-assembly for light-emitting diodes based on blue-emitting polyfluorenes

We present the electro-optical properties of light-emitting diodes (LEDs) based on two blue-emitting polyfluorenes, with aluminum cathodes and having indium-tin oxide (ITO) anodes modified by self-assembled monolayers (SAM) of benzenesulphenyl chloride derivatives. The modification of ITO surface was confirmed by contact angle measurements. Electroabsorption measurements were used to assess the modification of ITO work function upon SAM formation. We find that the use of a benzenesulphenyl chloride derivative containing a nitro group is as effective at improving LEDs characteristics as the use of a hole-injection layer of well known PEDOT:PSS.

Fabrication of nickel oxide and Ni-doped indium tin oxide thin films using pyrosol process

Organic light emitting diodes (OLEDs) need indium tin oxide (ITO) anodes with highly smooth surface. The work function of ITO, about 4.8 eV, is generally rather lower than the optimum level for application to OLEDs. In this work, NiO was deposited by pyrosol process on pyrosol ITO film to increase the work function of the ITO for improving the performance of OLEDs. It was confirmed that NiO was successfully deposited on pyrosol ITO film and the NiO deposition increased the work function of pyrosol ITO, using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and atmospheric photoelectron spectroscopy. Furthermore, doping ITO with Ni succeeded in producing the Ni-doped ITO film with high work function and lower sheet resistance.

Importance of ITO surface conditions for the interaction with thin CuPc layers

Abstract The electronic interaction between the transparent anode material indium tin oxide (ITO) and the common organic hole injection material copper phtalocyanine has been investigated by means of angle-resolved UV photoemission. The study was made on two differently treated ITO substrates, both cleaned in common organic solvents and one of them heated in air after the cleaning. The heat treatment efficiently removes surface carbon-containing contamination, and has also an oxidizing effect. As a result, the ITO work function is increased by 0.4–0.6 eV. This surface preparation has significant impact on the electronic properties of adsorbed CuPc. We find that the Pc HOMO level, important for transport properties, is shifting by 0.5 eV depending on surface conditions. This is interpreted as an effect of Fermi level pinning within the gap of the spin-split Cu derived b1g molecular orbitals.

Work-function changes of treated indium-tin-oxide films for organic light-emitting diodes investigated using scanning surface-potential microscopy

Local surface work-function (WF) measurement on indium-tin-oxide (ITO) films prepared by different cleaning methods for use as anode materials in organic light-emitting diodes were studied using scanning surface-potential microscopy. The ITO WF changes with standard wet-cleaning treatment correspond directly to asperities on the film surface. The maximum difference value (MDV) in the local WF reached 0.41eV. However, after wet-cleaning ITO with the ultraviolet ozone or O-plasma treatment, the WF distribution is more uniform than the original distribution. Owing to the sufficient ITO surface oxidization, the mean local WF value increases effectively to more than 5.00eV and the MDV is less than 0.05eV. Furthermore, the changes in roughness and conductivity on the ITO surface at different treatment times are also discussed. The proper exposure time for the O-plasma treatment can thereby be determined.

Direct Measurement of Surface Complex Loading and Surface Dipole and Their Effect on Simple Device Behavior

Tin complexes of phenoxide ligands having a range of dipole moments were prepared on the surface of indium-tin oxide (ITO). Surface complex loadings and stoichiometries were measured by quartz crystal microgravimetry. Work functions of ITO substrates treated with these various surface complexes were measured using a Kelvin probe. Surface complex dipole moments were then calculated based on measured surface loadings. Changes in the ITO work function effected by surface phenoxide complex introduction correlate with these surface complex dipole moments and with total surface dipole per unit area, and current densities in simple hole-only diode devices also correlate with these total surface dipoles.