Showing papers on "Indium tin oxide published in 1999"

Electrical, optical, and structural properties of indium–tin–oxide thin films for organic light-emitting devices

Abstract: High-quality indium–tin–oxide (ITO) thin films (200–850 nm) have been grown by pulsed laser deposition (PLD) on glass substrates without a postdeposition annealing treatment. The structural, electrical, and optical properties of these films have been investigated as a function of target composition, substrate deposition temperature, background gas pressure, and film thickness. Films were deposited from various target compositions ranging from 0 to 15 wt % of SnO2 content. The optimum target composition for high conductivity was 5 wt % SnO2+95 wt % In2O3. Films were deposited at substrate temperatures ranging from room temperature to 300 °C in O2 partial pressures ranging from 1 to 100 mTorr. Films were deposited using a KrF excimer laser (248 nm, 30 ns full width at half maximum) at a fluence of 2 J/cm2. For a 150-nm-thick ITO film grown at room temperature in an oxygen pressure of 10 mTorr, the resistivity was 4×10−4 Ω cm and the average transmission in the visible range (400–700 nm) was 85%. For a 170-n...

Built-in field electroabsorption spectroscopy of polymer light-emitting diodes incorporating a doped poly(3,4-ethylene dioxythiophene) hole injection layer

Abstract: We report electroabsorption measurements of polymer light-emitting diodes, (LEDs), fabricated with poly(4-4′-diphenylene diphenylvinylene), PDPV, as the emissive layer, Ca–Al cathodes, and indium tin oxide (ITO) anodes, with and without a doped conducting polymer hole injection/transport layer, namely poly(3,4-ethylene dioxythiophene), PEDOT, doped with poly(styrene sulfonate), PSS−. In these structures, the bias at which the electroabsorption signal is null corresponds to the difference between the electrodes’ work functions. We find that such a built-in voltage increases by 0.5 V when a PEDOT:PSS film is incorporated between the ITO electrode and the emissive layer. This leads to a marked reduction of the anode barrier height at the hole-injecting interface, and accounts for a variety of improvements brought about by the PEDOT insertion, namely: (a) the increase of luminescence efficiency, (b) the reduction of the turn-on voltage, and (c) the increase of the device lifetime.

Indium tin oxide thin films for organic light-emitting devices

Abstract: High-quality indium tin oxide (ITO) thin films (150–200 nm) were grown on glass substrates by pulsed laser deposition (PLD) without postdeposition annealing. The electrical, optical, and structural properties of these films were investigated as a function of substrate temperature, oxygen pressure, and film thickness. PLD provides very uniform ITO films with high transparency (⩾85% in 400–700 nm spectrum) and low electrical resistivity (2–4×10−4 Ω cm). The Hall mobility and carrier density for a 170-nm-thick film deposited at 300 °C are 29 cm2/V s and 1.45×1021 cm−3, respectively. Atomic force microscopy measurements of the ITO films indicated that their root-mean-square surface roughness (∼5 A) is superior to that (∼40 A) of commercially available ITO films deposited by sputtering. ITO films grown at room temperature by PLD were used to study the electroluminescence (EL) performance of organic light-emitting devices. The EL performance was comparable to that measured with commercial ITO anodes.

Characterization of treated indium-tin-oxide surfaces used in electroluminescent devices

Abstract: The influence of oxidative and reductive treatments of indium–tin–oxide (ITO) on the performance of electroluminescent devices is presented. The improvement in device performance is correlated with the surface chemical composition and work function. The work function is shown to be largely determined by the surface oxygen concentration. Oxygen-glow discharge or ultraviolet–ozone treatments increase the surface oxygen concentration and work function in a strongly correlated manner. High temperature, vacuum annealing reduces both the surface oxygen and work function. With oxidation the occupied, density of states (DOS) at the Fermi level is also greatly reduced. This process is reversible by vacuum annealing and it appears that the oxygen concentration, work function, and DOS can be cycled by repeated oxygen treatments and annealing. These observations are interpreted in terms of the well-known, bulk properties of ITO.

Red organic light-emitting diodes using an emitting assist dopant

Abstract: We propose an emitting assist (EA) dopant system for obtaining organic light-emitting diodes (OLEDs) with pure red emission. The EA dopant (rubrene) did not itself emit but assisted the energy transfer from the host (Alq3) to the red emitting dopant (DCM2). The cell structure used was {indium tin oxide/hole injection layer [(20 nm), CuPc/hole transport layer (50 nm), NPB/emitting layer (40 nm), Alq3+DCM2 (2%)+rubrene (5 wt %)]/MgIn}. (CuPc: Copper (II) phthalocyanine, NPB: N, N′-Di(naphthalen-1-yl)-N, N′-diphenyl-benzidine, DCM2: 4-Dicyanomethylene- 2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-8-yl)vinyl]-4H-pyran). A stable red emission (chromaticity coordinates: x=0.64, y=0.36) was obtained in this cell within the luminance range of 100–4000 cd/m2. When the cell was not doped with rubrene, the emission color changed from red to orange as the luminance increased. The EA dopant system is a promising method for obtaining red OLEDs.

A study of low temperature crystallization of amorphous thin film indium–tin–oxide

Abstract: Deposition of tin-doped–indium-oxide (ITO) on unheated substrates via low energy processes such as electron-beam deposition can result in the formation of amorphous films The amorphous-to-crystalline transformation was studied in this system using in situ resistivity, time resolved reflectivity, glancing incidence angle x-ray diffraction, and transmission electron microscopy The resistivity of 180 nm thick In2O3(99 wt %SnO2) was monitored during isothermal anneals at 125, 135, 145, and 165 °C The dependence of the resistance on the volume fraction of crystalline phase was established using glancing incidence angle x-ray diffraction and a general two phase resistivity model for this system was developed These studies show that, upon annealing, as-deposited amorphous ITO undergoes both a structural relaxation and crystallization Structural relaxation of the amorphous material includes local ordering that increases the ionized vacancy concentration which, in turn, increases the carrier density in the

Indium tin oxide contacts to gallium nitride optoelectronic devices

Abstract: We have fabricated GaN-based light-emitting diodes using transparent indium tin oxide (ITO) p contacts. ITO-contacted devices required an additional 2 V to drive 10 mA, as compared to similar devices with metal contacts. However, ITO has lower optical absorption at 420 nm (α=664 cm−1) than commonly used thin metal films (α=3×105 cm−1). Uniform luminescence was observed in ITO-contacted devices, indicating effective hole injection and current spreading.

A photoelectron spectroscopy study on the indium tin oxide treatment by acids and bases

Abstract: We report on the chemical adsorption of acids and bases on indium tin oxide (ITO). Ultraviolet photoelectron spectroscopy was used to measure the work function of the treated ITO and atomic surface concentrations were determined by x-ray photoelectron spectroscopy. The acid treatments yield work-function shifts as high as 0.7 eV compared to the nontreated ITO. Huge shifts in the work function are also obtained for the treatments with bases and are opposite to those obtained with the acids. These dramatic shifts are indicative of a double ionic surface layer. The importance of an appropriate plasma treatment prior to the chemical adsorption of acids or bases is discussed in terms of surface acido-basicity.

Improved operational stability of polyfluorene-based organic light-emitting diodes with plasma-treated indium–tin–oxide anodes

Abstract: We report the influence of various surface treatments of indium–tin–oxide (ITO) anodes on the operational stability of high-efficiency (up to 8.2 lm/W) green-emitting polymer light-emitting diodes (PLEDs), employing a doped poly(3,4-ethylene dioxythiophene), hole transport layer, a polyfluorene based emissive layer, and Ca–Al cathodes. The anodes were modified by physical (oxygen-plasma), chemical (aquaregia), and combined treatments. Oxygen plasma improves the stability under constant current with respect to all other anodes, with half-brightness (100 cd/m2) lifetimes two to five times longer than for untreated samples, and 1000 times longer than for aquaregia samples. We derive two major indications for optimization of PLEDs. First, thermal management of the diode is of the uppermost importance and there is significant scope for improvement. Second, the ITO anode and in general the electrical properties of the hole-injecting contact are crucial to device operation, even in the presence of a hole transpo...

Surface preparation and characterization of indium tin oxide substrates for organic electroluminescent devices

Abstract: The cleanliness of indium tin oxide (ITO) sub- strates used in organic light-emitting diodes (OLEDs) is in- vestigated by contact angle measurement and by X-ray pho- toemission spectroscopy (XPS). It was found that ultraviolet (UV) ozone treatment is quite effective in removing organic contamination on the ITO surface. The degree of surface con- tamination was checked by changes in contact angles and by XPS. Strong correlation can be established between these two techniques. OLEDs fabricated from UV-irradiated ITO sub- strates exhibit low turn-on voltage and superior brightness. There are now widespread interests in using organic or poly- meric fluorescent thin films for fabricating electroluminescent (EL) devices (1, 2). The basic structure of an organic EL de- vice consists of one or more layers of organic fluorescent materials sandwiched between an anode and a metal cathode. In most cases, a thin film of indium tin oxide (ITO), with a thickness in the order of 0:1 mm, is used as the anode ma- terial. The ITO layer can be prepared by standard sputtering techniques onto a glass plate or a plastic substrate. Since ITO is a transparent conductor, it also functions as the viewing side for the EL device. Although there are now numerous re- ports on EL devices fabricated by many different materials, few reports have been devoted to the preparation of ITO for EL devices. Since organic EL devices are thin film devices, a small amount of contamination on the surface of the an- ode can severely alter the work function, or the interfacial barrier height between the organic layer and the anode. Un- less the anode is thoroughly cleaned, the electrical and optical characteristics can be highly unstable and unpredictable. It is, therefore, of utmost importance that the ITO substrate is carefully cleaned before deposition of the organic layers. A variety of methods (3-8) have been developed in the preparation of ITO surfaces for organic light-emitting diodes (OLEDs). A summary of these methods is shown in Table 1.

Surface energy and polarity of treated indium–tin–oxide anodes for polymer light-emitting diodes studied by contact-angle measurements

Abstract: We present contact-angle hysteresis and surface energy of differently treated indium–tin–oxide (ITO) thin films obtained from contact angles for liquids with different polar character We find that the hysteresis and the polar and dispersion component of the surface energy depend strongly on the surface treatments Oxygen-plasma treatments induce the highest polarity and the highest total surface energy, and we suggest that this improves the interface formation with polymers, and therefore, the performance of light-emitting diodes We discuss the results in relation to the ITO surface roughness and chemical heterogeneity modified by the different treatments

Combined photoemission/in vacuo transport study of the indium tin oxide/copper phthalocyanine/N,N′-diphenyl-N,N′-bis(l-naphthyl)-1,1′biphenyl-4,4″diamine molecular organic semiconductor system

Abstract: Ultraviolet photoemission spectroscopy (UPS) was used to study the indium tin oxide/copper phthalocyanine (CuPc) and CuPc/N,N′-diphenyl-N,N′-bis(l-naphthyl)-1,1′biphenyl-1-4,4″diamine interfaces, which are commonly used as an anode/hole injection layer/hole transport layer combination in organic light emitting devices. In order to assess the validity of the transport barriers measured using UPS, in vacuo I–V measurements have been performed on simple devices grown and measured in the same system as the samples studied using UPS. I–V characteristics were modeled using numerical simulations. The parameters used in the simulated curves which best fit the measured I–V characteristics agree quantitatively with the UPS measured barriers.

Influence of the hole transport layer on the performance of organic light-emitting diodes

Abstract: We investigate the influence of the hole-transporting layer (HTL) on the performance of bilayer vapor-deposited organic light-emitting diodes. Three different HTL materials were used: m-MTDATA, triphenyl-diamine, and naphthyl-phenyl-diamine. In all cases, Alq3 was the electron-transporting layer (ETL). We measure and compare the current density-voltage (J–V) and luminance–voltage (L–V) characteristics of these devices and we conclude that the operating voltage is controlled by the type of HTL used and the nature of the hole-injecting indium tin oxide/HTL interface. We found that the device quantum efficiency depends not only on the electron transport characteristics of the ETL but also on the energetics of the HTL/ETL interface. Analysis of the J–V characteristics suggests that current flow in bilayer devices cannot be described sufficiently by a single carrier theory; both hole and electron currents should be considered.

Organic Two-Layer Light-Emitting Diodes Based on High-Tg Hole-Transporting Polymers with Different Redox Potentials

Abstract: A series of soluble arylamine-based hole-transporting polymers with glass transition temperatures in the range of 130−150 °C have been synthesized. The synthetic methodology allows facile substitution of the aryl groups on the amine with electron-withdrawing and electron-donating moieties, which permits tuning of the redox potential of the polymer. These polymers have been used as hole-transport layers (HTLs) in two-layer light-emitting diodes ITO/HTL/Alq/Mg [ITO = indium tin oxide, Alq = tris(8-quinolinato)aluminum]. The maximum external quantum efficiency of the device increases if the redox potential of the HTL is increased to facilitate reduction of the positive charge carriers at the HTL/Alq interface. A fluorinated hole-transport polymer with a relatively large redox potential (390 mV vs ferrocenium/ferrocene) yielded the device with the highest external quantum efficiency of 1.25% photons/e-. The device stability, however, follows the opposite trend. The device with the most electron-rich HTL exhibited the best performance after prolonged usage.

Electron drift mobility and electroluminescent efficiency of tris(8-hydroxyquinolinolato) aluminum

Abstract: The electron drift mobility in films of tris(8-hydroxyquinolinolato) aluminum (Alq) deposited at different rates (0.2, 0.4, and 0.7 nm/s) on silicon has been determined by the time-of-flight technique. It has been found that the drift mobility of electrons in Alq increased by about two orders of magnitude as the deposition rate decreased from 0.7 to 0.2 nm/s. Further, the electron drift mobility in all Alq samples increased linearly with the square root of the applied electric field. Electroluminescent devices with a structure of indium tin oxide/α-naphthylphenylbiphenyl amine (NPB, 90 nm)/Alq (90 nm)/Mg:Ag were fabricated at different Alq deposition rates. The device efficiency was found to increase with increasing electron mobility in Alq. As the electron is the minority carrier in the present device, an increase in electron mobility in Alq would thus lead to an increase in device efficiency.

Material and method for printing high conductivity electrical conductors and other components on thin film transistor arrays

Abstract: A process sequence is disclosed for fabricating arrays of Thin Film Transistors by printing metallic conductors for the gate and data lines and possibly the Indium Tin Oxide Pixel electrode as well. The process eliminates conventional step-and-repeat photolithographic patterning, and provides high conductivity metallization for large arrays. These arrays may be used in displays, detectors and scanners.

Electrochemical Charge Injection into Immobilized Nanosized Gold Particle Ensembles: Potential Modulated Transmission and Reflectance Spectroscopy

Abstract: Self-assembled multilayer thin films of nanometer-sized gold particles linked with organic dithiols have been prepared on glass, indium tin oxide, and gold substrates. The gold particles within these structures retain their integrity and no sintering occurs as demonstrated by their optical absorbance, ellipsometry, and potential modulated transmission and reflectance spectroscopy. The optical response to electrochemical charge injection into the outermost layer of gold particles is completely different from that of bulk gold electrodes, and it is concluded that the particles have to be regarded as discrete, immobilized quantum-dots.

A Simple Colloidal Route to Nanocrystalline ZnO/CuInS2 Bilayers

Abstract: Thin semiconductor CuInSe2 and CuInS2 films (CIS) with bandgap values (Eg) of around 1.04 eV (for selenide) and 1.5 eV (for sulfide) represent an important class of the currently developed light absorbers for solar energy harvesting. Conversion efficiencies of 12±13 % were achieved on large area modules, whereas close to 18 % was achieved with laboratory cells, indicating a large potential for CIS-derived photovoltaic materials. For their preparation, a broad range of physical and electrochemical deposition routes are available. Typically, CIS films are created via a rapid thermal sintering of elemental Cu, In and Se layers evaporated on Mo-coated glass substrates. The photovoltaic cell is then completed by overcoating the CIS-macrograins with a thin CdS buffer layer and a metal± organic chemical vapor deposition derived, transparent Al/ ZnO window electrode. In this contribution, we address a low cost colloidal route to nanocrystalline ZnO/CIS bilayers on indium tin oxide (ITO) glass. For the film deposition, concentrated coating colloids, with size-quantized CuInS2 particles were developed. It is well-established that size quantization in semiconductors (i.e. increasing bandgap energy with decreasing semiconductor dimension) takes place at particle dimensions smaller than the Wannier±Mott (WM) exciton of the corresponding macroscopic bulk phase. By knowledge of the high frequency dielectric constant, e¥, and the reduced effective exciton mass, m = 1/(m e + m ±1 h ), one can calculate the WM-exciton Bohr radius according to RB = (e¥/m) aB, with aB being the Bohr radius of the hydrogen atom. Taking the CIS bulk values of e¥ = 11, me = 0.16 and mh = 1.3, we calculated the WM-exciton size to be 8.1 nm, which predicts a blue shift in the optical absorption threshold (below 826 nm = 1240/1.5 eV) for CIS-particle sizes below 8 nm. Figure 1 shows changes in the optical absorption spectrum during the CIS condensation. Condensation was induced on addition of bis(trimethylsilyl)sulfide to a mixture of Cu(I)±P(OPh)3 and In(III)±P(OPh)3 complexes (Cu/ In = 1) in Ar saturated acetonitrile (for details see Experimental). At sulfide concentrations ~25 % (with respect to the present metals), the absorption spectrum exhibits a shoulder located at 370 nm that is strongly blue-shifted with respect to the bulk crystals (a gap energy difference of more than 2 eV). On further addition of the sulfide source (50 %), the absorption shoulder shifts from 370 nm to 400 nm, and the optical density rises due to increasing particle concentration. Under stoichiometric conditions (100 % S corresponds to the Cu:In:S stoichiometry of 1:1:2), a steep tail is observed with the absorption onset located near 580 nm. A remarkable dynamic color change accompanies this condensation process which can be seen with the naked eye. On each dropwise addition of the sulfide source, the color of the reacting solution rapidly changes from colorless to yellow to orange to red and becomes colorless or yellow again later on (build-up and decay of size-quantized cluster±cluster aggregates). There is a crossover concentration value around 50 % S, above which the sol does not self-bleach anymore, retaining a constant orange color. On the further allat-once addition of 50 % S, it takes at least 10 hours for the stoichiometric orange colloid to become deep red. Solvent removal from the above 0.05 M solutions on a rotary evaporator yields 0.5 M stable lacquers which can be used to produce single step coated, 1 mm thick compact CIS-layers with good adhesion to glass or nanocrystalline ZnO films. The preparation of nanocrystalline ZnO films from particulate colloids follows a recently published procedure (see also Experimental). Figure 2 shows the UV± Vis optical spectrum of a 2 mm thick (single step dip-coated, and at 400 C pre-sintered) ZnO film displaying the characteristic absorption edge at around 380 nm. After dip-withdrawing the ZnO film in CIS-colloid, and subsequent sintering in vacuum at 400 C for 20 minutes, a dark brown colored ZnO/CIS bilayer is formed. Its optical absorption exhibits a shoulder at 750 nm indicating that coalescence of small CIS particles to larger nanocrystals took

A light-emitting device based on a CdTe nanocrystal/polyaniline composite

Abstract: CdTe nanocrystal/polyaniline composite films as well as films of closely packed CdTe nanocrystals were used to fabricate light-emitting devices with low turn-on voltages [∽2.5 V for a device with a Mg cathode and an indium tin oxide (ITO) anode]. In principle, the emitted colour is tunable from green to red, depending on the size of the nanocrystals. The use of the particle loaded polyaniline film as the light-emitting layer results in a considerable enhancement of the quantum efficiency in comparison with the polymer-free device with closely packed CdTe nanocrystals. The light-emitting devices are found to work more efficiently in a pulsed mode than under continuous operation.

Increase of charge carriers density and reduction of Hall mobilities in oxygen-plasma treated indium–tin–oxide anodes

Abstract: We report investigations of the electronic transport properties carried out by means of the Hall technique for indium–tin–oxide thin films on glass after a variety of surface treatments. We find that oxygen-plasma treatments induce a significant increase in the carrier concentration, and a less significant decrease of mobilities with respect to “as-received” or aquaregia treated substrates. We consider that this is indicative of an increased concentration of defects, as a result of the plasma exposure.

Effect of hydrogen partial pressure on optoelectronic properties of indium tin oxide thin films deposited by radio frequency magnetron sputtering method

Abstract: In this study, indium tin oxide (ITO) films were made from an oxidized target with In2O3 and SnO2 in a weight proportion of 9:1 using the radio frequency magnetron sputtering method. Hydrogen was added to the gas mixture during the preparation of the ITO films. In order to study the effect of hydrogen partial pressure on the structural and optoelectronic properties of the ITO films, we have varied the hydrogen partial pressure in the gas mixture over the range 0–1.6×10−5 Torr and kept the substrate temperature constant at 300 °C during film growth. The x-ray diffraction patterns of ITO films prepared at different hydrogen partial pressures show that the films have (111) and (100) preferred orientations. Hall effect measurements reveal that the addition of hydrogen in the sputtering gas mixture shows an increase in the number of charged carriers in the ITO films. However the carrier mobility did not increase considerably. At optimal conditions, ITO films with resistivity of 2.7×10−4 Ω cm and transparency of over 89% in the visible wavelength region were achieved.

Effect of substrate absorption on the efficiency of laser patterning of indium tin oxide thin films

Abstract: Maskless laser patterning of indium tin oxide thin films for flat panel display applications was studied as a function of wavelength using different harmonics of a diode-pumped Q-switched Nd:YLF laser. Electrically isolating lines could be written at all wavelengths used. However, while lines written at the infrared and the visible wavelengths exhibited a ripplelike morphology due to incomplete material removal, ultraviolet laser irradiation produced residue-free etch lines with superior smoothness even at higher scan speeds. The threshold fluences for material removal at different wavelengths were found to correlate with the optical properties of the indium tin oxide film. In addition, numerical simulations of laser-induced temperature rise yielded peak surface temperatures well above the vaporization temperature of the indium tin oxide film, indicating that, at all wavelengths studied, material removal occurs via thermal vaporization. The calculations also revealed that the absorption of the ultraviolet...