The use of charge transfer interlayers to control hole injection in molecular organic light emitting diodes

In order to improve the performance of the indium–tin oxide (ITO) electrode frequently used as the anode in electroluminescent devices, we report its modification using ultrathin films of C₆₀ and 11,11,12,12-tetracyanonaphtho-2,6-quinodimethane (TNAP). In both cases the interaction between the film and the ITO substrate is found to shift the work function of the electrode, thereby modifying the barrier to hole injection in the model system ITOTPDAlq₃Al (where TPD is N,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine and Alq₃ is tris(quinolin-8-olato) aluminium). Scanning Kelvin probe measurements show that the ITO work function is increased by as much as 0.25 eV with a TNAP overlayer, whilst C₆₀ overlayers are found to reduce the work function by a comparable amount. The former has been attributed to a charge transfer effect, however, for C₆₀ overlayers the variation in the electrostatic potential across the interface cannot be attributed to charge transfer alone. The performance of devices incorporating these modified ITO electrodes are rationalised in terms of the work function modification, film thicknesses and the hole transport properties of the two films.     

Millivolt Modulation of Plasmonic Metasurface Optical Response via Ionic Conductance

A plasmonic metasurface with an electrically tunable optical response that operates at strikingly low modulation voltages is experimentally demonstrated. The fabricated metasurface shows up to 30% relative change in reflectance in the visible spectral range upon application of 5 mV and 78% absolute change in reflectance upon application of 100 mV of bias. The designed metasurface consists of nanostructured silver and indium tin oxide (ITO) electrodes which are separated by 5 nm thick alumina. The millivolt‐scale optical modulation is attributed to a new modulation mechanism, in which transport of silver ions through alumina dielectric leads to bias‐induced nucleation and growth of silver nanoparticles in the ITO counter‐electrode, altering the optical extinction response. This transport mechanism, which occurs at applied electric fields of 1 mV nm^?1, provides a new approach to use of ionic transport for electrical control over light–matter interactions.     

Electronic transport and optical properties of thin oxide films

In this work, we present optical characterization of films of two transparent conductive oxides (ITO: indium tin oxide and ZnO: zinc oxide) including absorption coefficient and optical gap energy. We have also investigated the transport properties of ITO and ZnO films through measurements of electrical conductivity and thermoelectric power versus temperature. These measurements enabled us to investigate conduction mechanisms for metal-nonmetal transitions. Undoped ZnO thin films show a metal-semiconductor transition at temperatures beyond 350 K. We have conducted a similar study on ITO films where we demonstrated, for the first time, the existence of a conductivity transition below 400 K, which indicates a high absolute thermoelectric power at temperatures above the transition temperature.     

Charge transfer through amino groups-small molecules interface improving the performance of electroluminescent devices

A carboxylic group functioned charge transporting was synthesized and self-assembled on an indium tin oxide (ITO) anode. A typical electroluminescent device [modified ITO/TPD (50 nm)/Alq₃ (60 nm)/LiF (2 nm)/(120 nm)] was fabricated to investigate the effect of the amino groups-small molecules interface on the characteristics of the device. The increase in the surface work function of ITO is expected to facilitate the hole injection from the ITO anode to the Hole Transport Layer (HTL) in electroluminescence. The modified electroluminescent device could endure a higher current and showed a much higher luminance than the nonmodified one. For the produced electroluminescent devices, the I-V characteristics, optical characterization and quantum yields were performed. The external quantum efficiency of the modified electroluminescent device is improved as the result of the presence of the amino groups-small molecules interface.     

Optimization of indium tin oxide by pulsed DC power on single junction amorphous silicon solar cells

We investigated the optimal deposition conditions of a thin indium tin oxide (ITO) film on an amorphous silicon (a-Si) single-junction solar cell using pulsed DC magnetron sputtering. Thin ITO films were deposited while power, deposition time, pressure, gas flow and temperature were varied to find such conditions. The efficiency of a-Si solar cells with ITO films was 6.65% at the optimal conditions — a pulsed DC power of 40 W, a deposition time of 460 s, a pressure of 0.53 Pa, gas flow of 16 sccm and 151 °C. On the other hand, an a-SiGe tandem solar cell with the ITO films made at the optimal conditions yields an efficiency of 7.20%. We have also examined the surface morphology of ITO coated a-Si solar cells, using atomic force microscopy. Interestingly, a change in power does not alter the surface morphology at small length scales, whereas at large scales, the lower power sample had a lower surface roughness than the samples made with higher powers. We also find that for the range of deposition conditions examined, the value of the roughness exponent does not change with α ? 2/3 and a thin layer of ITO does not modify the surface morphology significantly.     

磁控溅射法制备TiO2空穴缓冲层的有机发光器件

采用磁控溅射方法在ITO表面制备了不同厚度的TiO2超薄膜用做有机发光二极管（OLEDs）的空穴缓冲层,使OLEDs（ITO／TiO2／TPD／Alq3／Al）的发光性能得到很大改善。研究TiO2缓冲层厚度对器件性能影响的结果表明,当TiO2缓冲层厚度为1nm,电流密度为100mA／cm^2时,器件的发光效率为2cd／A,比未加缓冲层器件的发光效率增加了近一倍。这是由于加入适当厚度的TiO2缓冲层限制了空穴的注入并且提高了空穴与电子注入之间的平衡。     

Influence of microstructure on the electrochemical performance of tin-doped indium oxide film electrodes

The effect of the microstructure of tin-doped indium oxide (ITO) films on their electrochemical performance was studied using three redox probes, tris(2,2'-bipyridyl ruthenium(II) chloride (Ru(bpy)3(2+/3+)), ferrocyanide (Fe(CN)6(4-/3-)), and ferrocenemethanol (FcCH2H(0/+)). ITO films were deposited using dc magnetron sputtering under a variety of conditions that resulted in films having different degrees of crystallinity, crystallographic texture, sheet resistance, surface roughness, and percent tin. It was found that the electron transfer for all three redox probes used in this study was more efficient at polycrystalline films than at amorphous ITO films. This effect is more pronounced at faster scan rates. The crystallographic texture of the ITO films, surface roughness, and a change in sheet resistance from 7.9 to 13.7 ohms/square did not have an effect on electron-transfer kinetics. ITO films deposited using a 1 wt % SnO2 target and having sheet resistance comparable to films deposited using a 10 wt % SnO2 target had dramatically different microstructure from the films with higher weight percent Sn and were shown to perform poorly when used as electrode materials. We believe that the dramatic differences in electron-transfer kinetics observed at the various ITO films can be attributed to either the different density of defect sites along the grain boundaries or defect sites caused by substitutional Sn in the film.     

Enhanced stability of organic thin films for electroluminescence by photoirradiation

Enhanced stability against heat and moisture in organic vapor-deposited TPD thin film having hole transport property has been achieved by UV irradiation. The crystallization of TPD film without any treatment was started within a week even at RT and accelerated with increasing temperature under the air. By photoirradiation using UV light, however, the TPD thin film was found to be difficult to form crystallized state and showed good contact with substrate even at 60 °C and in water for more than one month. Furthermore, the molecular interdiffusion by heat between organic layers in Alq3/TPD bilayer was remarkably suppressed by the UV irradiation due to the enhanced stability of the TPD film.     

Strain and temperature effects in indium-tin-oxide sensors

Indium-tin-oxide (ITO) thin films strain gages were prepared by reactive sputtering onto both high purity alumina and lanthanum stabilized zirconia substrates. We report the piezoresistive response of these ITO gages in the temperature range 350-1500 °C up to 1000 microstrain (με). Strain response and gage factors are reported for ITO in both tension and compression. The data suggest that not only is there an annealing component but also a strain component of the piezoresistive change with time at temperature. The effects in both tension and compression are reported.     

High-performance transparent conducting oxide nanowires

We report the growth and characterization of single-crystalline Sn-doped In2O3 (ITO) and Mo-doped In2O3 (IMO) nanowires. Epitaxial growth of vertically aligned ITO nanowire arrays was achieved on ITO/yttria-stabilized zirconia (YSZ) substrates. Optical transmittance and electrical transport measurements show that these nanowires are high-performance transparent metallic conductors with transmittance of approximately 85% in the visible range, resistivities as low as 6.29 x 10(-5) Omega x cm and failure-current densities as high as 3.1 x 10(7) A/cm2. Such nanowires will be suitable in a wide range of applications including organic light-emitting devices, solar cells, and field emitters. In addition, we demonstrate the growth of branched nanowire structures in which semiconducting In2O3 nanowire arrays with variable densities were grown epitaxially on metallic ITO nanowire backbones.     

Properties of 2,6-di-tert.-butyl-4-(2,5-diphenyl-3,4-dihydro-2H-pyrazol-3-yl)-phenol as hole-transport material for life extension of organic light emitting diodes

We studied structural and optical properties of 5′ replaced pyrazoline by hindered phenol 2,6-di-tert.-butyl-4-(2,5-diphenyl-3,4-dihydro-2H-pyrazol-3-yl)-phenol (HPhP) films for application in organic light-emitting diode (OLED) as a hole transport layer (HTL). Analysis of impedance and current-voltage characteristics of ITO/HPhP/Al structure has shown that the current is limited by a space charged region with exponential distribution of traps near Fermi level. Characteristics of electroluminescence structure ITO/HPhP/Alq₃/poly(ethylene glycol) dimethyl ether/Al was studied and analyzed. We performed the comparative analysis of luminescence time decay in two types of electroluminescent devices with HTL from pyrazolines derivative with hindered phenol and without it. We showed that hindered phenol in HTL slows down the degradation processes in OLED.     

Investigation of the properties of indium tin oxide-organic contacts for optoelectronic applications

This paper presents some investigations on the electrical transport properties of ITO/single (double) layer organic semiconductor (m-DNB, benzil, PTCDA, Alq3) contacts in SIS-like (ITO/organic/Si) and MIS-like (ITO/organic/metal) heterostructures. The I-V characteristics have emphasised the injection properties of different contacts and the effect of space charge limited currents in correlation with the type and preparation conditions of the contacts. We have studied the influence of the type of contact (In/ITO; In/Al) on the electrical conduction in Alq3/PTCDA/Si/In heterostructure. In a planar grid contact configuration for In/Al/PTCDA/Al/In structure we have observed the effect of the low electric field on the shape of the I-V characteristic.     

UV light emitting transparent conducting tin-doped indium oxide (ITO) nanowires

Multifunctional single crystalline tin-doped indium oxide (ITO) nanowires with tuned Sn doping levels are synthesized via a vapor transport method. The Sn concentration in the nanowires can reach 6.4 at.% at a synthesis temperature of 840?°C, significantly exceeding the Sn solubility in ITO bulks grown at comparable temperatures, which we attribute to the unique feature of the vapor-liquid-solid growth. As a promising transparent conducting oxide nanomaterial, layers of these ITO nanowires exhibit a sheet resistance as low as 6.4 Ω/[Symbol: see text] and measurements on individual nanowires give a resistivity of 2.4 × 10(-4) Ω cm with an electron density up to 2.6 × 10(20) cm(-3), while the optical transmittance in the visible regime can reach ～ 80%. Under the ultraviolet excitation the ITO nanowire samples emit blue light, which can be ascribed to transitions related to defect levels. Furthermore, a room temperature ultraviolet light emission is observed in these ITO nanowires for the first time, and the exciton-related radiative process is identified by using temperature-dependent photoluminescence measurements.     

有机发光器件(ITO/TPD/Alq3/Mg/Al)的光电性能研究

为了研究有机电致发光器件光电性能随工作参数的变化，对ITO／TPD(50nm)／AIq3(50nm)／Mg／Al的实验数据进行分析，发现该器件在低压时属于注入电流限制，高压时为陷阱电荷限制(TCLC)。另外，采用实验数据验证复合理论，发现通过电场数据和电流密度数据(F^2／J)能够直接地反映器件量子效率随电流密度的变化趋势。     

Properties of ITO on plastics and glass by Nd:YAG laser deposition at 355 and 532 nm

Indium tin oxide (ITO) was deposited on polycarbonate (PC), polyethylene terephthalate (PET) and glass substrates at room temperature and in a low-pressure oxygen environment by a pulsed Nd:YAG laser at 355 and 532 nm. The ITO film resistivity varied with the oxygen pressure, which achieved the lowest value of 1.5 × 10⁻³ Ω cm from the four-point probe measurements. The highest optical transmittance which depended on the target-to-substrate distance, was determined from UV-vis-NIR spectrophotometer. The highest optical transmission was 94% at 5 cm. The carrier concentration, of the order of 10¹⁹ cm⁻³ was determined from the Hall-effect measurements. Those films deposited at 355 nm of laser wavelength did show some better properties as compared to 532 nm. Attempts were made to use these ITO-coated plastic substrates for the fabrication of a organic light-emitting device that was based on single-layer, molecularly doped (poly(N-vinyl carbazole)) (PVK) with a mixture of tris(8-hydroxyquinoline) aluminum (III) (Alq₃) and N,N'-bis (3-methylphenyl)-N,N'-bis-(phenyl)-benzidine (TPD) of 1:1 ratio.     

Surface modification of indium tin oxide anodes by self-assembly monolayers: Effects on interfacial morphology and charge injection in organic light-emitting diodes

Three silane derivatives including dodecyltrichlorosilane (DDTS), phenyltriethoxysilane (PTES) and 3-aminopropyl-methyl-diethoxysilane (APMDS) were used to modify the indium tin oxide (ITO) surfaces. The effects of various terminal groups of the self-assembled monolayers (SAMs) on the growth behavior and interfacial morphologies of N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (NPB) film deposited on the SAM-modified ITO were studied, as well as their effects on the performance of organic light-emitting diodes (OLED) devices. The results show that the growth behavior of NPB film over-deposited on the SAM-modified ITO is mainly determined by the wettability of the surface. The covering ability and thermal stability of NPB film on the SAM-modified ITO decrease in the order: bare ITO > ITO/PTES > ITO/APMDS > ITO/DDTS. However, the covering characteristic of NPB films on these substrates did not show direct relation to the transport of carriers across the anode/NPB interface as evaluated from the cyclic voltammogram and OLED performance. The turn-on voltages for these SMA-modified OLED devices increase in the order: ITO/PTES < ITO/DDTS ≤ bare ITO < ITO/APMDS. The enhancing effect of PTES on the hole injection is ascribed to the similar structure of PTES to NPB. On the contrary, the inhibition effect of APMDS is caused from the interaction of the lone-pair electrons of amine group to the transport carriers. Since these devices are known to be hole dominant, the luminance efficiency increase in a similar order as that for the turn-on voltage: ITO/PTES < ITO/DDTS ≤ bare ITO < ITO/APMDS.     

6-N,N-diarylsubstituted 1H-pyrazolo[3,4-b]quinoxalines-novel materials for single-layered photovoltaic devices

At present work organic single layered photovoltaic device, which can use polaron-assisted transport has been developed using a ITO/PEDOT:PSS (poly(3,4-ethylene dioxythiophene)-poly-(styrene sulphonate)/Active Layer/Al) architecture. We have achieved a large photovoltaic response for single layered device (yielding up to 0.82%) by varying the values of state dipole moments of the chromophore. Interaction of chromophore with polymer chains and resulting polaronic transport mechanism seems to be very important as well.     

Characterization of nanocrystalline indium tin oxide thin films prepared by ion beam sputter deposition method

Indium tin oxide (ITO) thin films were deposited on glass substrates by ion beam sputter deposition method in three different deposition conditions [(i) oxygen (O₂) flow rate varied from 0.05 to 0.20 sccm at a fixed argon (1.65 sccm) flow rate, (ii) Ar flow rate changed from 1.00 to 1.65 sccm at a fixed O₂ (0.05 sccm) flow rate, and (iii) the variable parameter was the deposition time at fixed Ar (1.65 sccm) and O₂ (0.05 sccm) flow rates]. (i) The X-ray diffraction (XRD) patterns show that the ITO films have a preferred orientation along (400) plane; the orientation of ITO film changes from (400) to (222) direction as the O₂ flow rate is increased from 0.05 to 0.20 sccm. The optical transmittance in the visible region increases with increasing O₂ flow rate. The sheet resistance (R_s) of ITO films also increases with increasing O₂ flow rate; it is attributed to the decrease of oxygen vacancies in the ITO film. (ii) The XRD patterns show that the ITO film has a strong preferred orientation along (222) direction. The optical transmittance in the visible spectral region increases with an increase in Ar flow rate. The R_s of ITO films increases with increasing Ar flow rate; it is attributed to the decrease of grain size in the films. (iii) A change in the preferred orientations of ITO films from (400) to (222) was observed with increasing film thickness from 314 to 661 nm. The optical transmittance in the visible spectral region increases after annealing at 200 °C. The R_s of ITO film decreases with the increase of film thickness.     

Monitoring interface traps in operating organic light-emitting diodes using impedance spectroscopy

Electronic trap densities at the indium tin oxide (ITO)/hole transport layer (HTL) interface in operating organic light-emitting diodes (OLEDs) are characterized in situ using impedance spectroscopy. For OLEDs with a high density of active trap states, negative values of the frequency derivative of resistance are clearly observable for frequencies on the order of 10 kHz, whereas positive values are observed when the trap density is low With this technique, it is revealed that the trap density is minimized via the introduction of a TPD-Si₂ (4,4′-bis[(p-trichlorosilylpropylphenyl) phenylamino]-biphenyl) passivation layer at the ITO/HTL interface or by the application of large electric fields during device operation. Furthermore, impedance spectroscopy illustrates that the ITO/HTL interface is not a simple series resistance when traps are present since they are shown not to contribute to high frequency conduction. Overall, this paper demonstrates that the parasitic effects of interface traps can mask the underlying negative capacitive transport in OLEDs and presents a technique capable of monitoring the trap density of buried interfaces in organic electronic devices.     

Enhancing the tuning range of a single resonant band long period grating while maintaining the resonant peak depth by using an optimized high index indium tin oxide overlay

We report laboratory test results of a long period grating (LPG) that can maintain a constant resonant peak depth over an enhanced tuning range when it is coated with an indium tin oxide (ITO) electrode that has optimized thickness and refractive index. The authors have experimentally demonstrated a LPG coated with ITO that can be tuned in excess of 200 nm with an ambient refractive index change of less than 0.01. To the best of the authors' knowledge, this is the highest sensitivity reported for a LPG to date. In addition to the tuning performance, the resonant peak remains within 1 dB of its maximum depth for at least 100 nm of the tuning range.