Process optimization of RTA on the characteristics of ITO-coated GaN-based LEDs

We present a detailed study on the optimization of rapid thermal annealing (RTA) on GaN-based light emitting diodes (LEDs). 14 mil × 28 mil GaN-based LED chips are fabricated with indium tin oxide (ITO) layer treated by RTA under various temperatures and times. Through the optical and electrical property analyses of ITO film, it is found that the transmittance and sheet resistance are improved after RTA process due to the better ITO crystallization and bigger grain size, compared with ITO treated by conventional furnace annealing. By employing electroluminescence measurement for the LED chips with RTA treatment, the forward voltage is found to be low as a result of low sheet resistance and contact resistance, and light output power (LOP) is high due to high ITO transmittance and good current density uniformity. Under RTA temperature of 550 °C and time of 3 min, the optimized LOP and forward voltage at 60 mA injection current are 71.2 mW and 2.97 V, respectively. Moreover, the reliability of the chips with RTA is better than those with furnace annealing.     

Influence of CuS membrane annealing time on the sensitivity of EGFET pH sensor

In this study, CuS membranes were deposited on glass substrates using spray pyrolysis, and then annealed at 150 °C for durations of 5, 15 and 30 min. The precursor materials for the synthesis of the CuS membranes include copper chloride and sodium thiosulfate dissolved in deionized water. 0.4 M concentration of the mixed solution was subsequently sprayed on the glass substrates at 200 °C. Afterwards, the as-deposit membranes were implemented as extended gate field effect transistors (EGFET) for pH sensors. The structure, morphology, resistance, sensitivity and linearity of the membranes were determined. The structural characteristics showed that the maximum average grain size of the CuS membrane was 24.2 nm after 30 min of annealing. The sensitivity of the membranes was measured in buffer solutions of pH 2, 4, 6, 8, 10 and 12. The sensitivity increased with annealing time until maximum values of 31.5 µA/pH and 27.8 mV/pH were attained at 30 min, with minimum resistance of 400 Ω and minimum hysteresis of 11.4 mV. Thus, the ideal annealing time for optimum sensitivity, resistance and hysteresis is 30 min.     

Transparent conductors with an ultrathin nickel layer for high-performance photoelectric device applications

A thin nickel (Ni) layer of thickness 5 nm was inserted in between the indium tin oxide (ITO) layers of thickness 50 nm each so as to increase the conductivity of ITO without affecting much of its transmittance nature. ITO layers with and without Ni film were prepared by reactive DC sputtering on both Si and glass substrates. The influence of Ni layer on the optical and electrical properties of prepared devices was investigated. Due to the insertion of thin Ni layer, the resistivity of ITO/Ni/ITO sample (3.2×10⁻⁴ Ω cm) was reduced 10 times lesser than that of ordinary ITO layer (38.6×10⁻⁴ Ω cm); consequently it increased the mobility of ITO/Ni/ITO device. The external and internal quantum efficiencies (EQE and IQE) of ITO/Ni/ITO device exhibited better performance when compared to ITO layer that has no Ni film. At wavelengths of 350 and 600 nm, the photoresponses of ITO/Ni/ITO device were predominant than that of reference ITO device. This highly conductive and photoresponsive Ni inserting ITO layers would be a promising device for various photoelectric applications.     

Reaction of NiO film on flexible substrates with buffer solutions and application to flexible arrayed lactate biosensor

In this study, we proposed a flexible arrayed lactate biosensor based on pH-sensitive nickel oxide (NiO) films, and also investigated the adhesion between NiO film and different flexible substrates under reactions with different solutions. L-lactate dehydrogenase (LDH) and nicotinamide adenine dinucleotide (NAD⁺) were co-immobilized on pH-sensitive NiO film by using a crosslinking reagent, 3-glycidoxypropyltrimethoxysilane (GPTS). It was found that GPTS concentration would affect the sensing characteristics for the lactate biosensor. The lactate biosensor with 1:2 volume ratio of GPTS-toluene mixture had the best average sensitivity and linearity. In the range of lactate concentration from 0.2 mM to 3 mM, arrayed flexible lactate biosensor based on the NiO/PET structure had 38.218 mV/mM of average sensitivity. In addition, the average sensitivity of the sensor based on the NiO/Pt/PET structure was 32.483 mV/mM.     

Effect of pH on the properties of electrochemically prepared ZnO thin films

Zinc Oxide (ZnO) thin films have been electrochemically deposited on fluorine doped tin oxide (FTO) coated glass substrates from an aqueous electrolyte. Deposition potential −0.96 V was optimized by cyclic voltammetry experiment for slow scan rate 5 mV/s with moderate agitation of electrolyte. The effect of pH on the electrodeposition of ZnO is studied by cyclic voltammetry, X-ray diffraction (XRD), scanning electron microscopy (SEM), optical spectroscopy and photoelectrochemical I-t transient characteristics. It is revealed that the pH of the electrolyte has significant influence on the surface morphology and structural properties. Highly crystalline ZnO layers with hexagonal crystal structure deposited for all pH of the solutions. A systematic shift observed in the reflections (002) and (101) is correlated with an effective tensile strain developed in the crystal lattice. A remarkable improvement in the crystallinity was noticed in the as-deposited ZnO samples with increasing pH and upon heat treatment. Optical direct band gap ~ 3.26–3.33 eV and transmittance ~70 −80% was measured by optical spectroscopy. PL measurement showed the band edge emission at 375–382 nm and a visible light emission at 410–550 nm. The intensities of emission peaks are found to be affected by the pH of bath. The compact, densely packed and well adherent thin films of ZnO electrodeposited in zinc nitrate bath for pH 2.0, 3.5 and 6.0. The surface morphology has been changed from granular to disc shaped and finally a large hexagonal sheets were obtained with an increase in the pH of bath. Nearly stoichiometric ZnO thin films are electrodeposited at −0.96 V versus Ag/AgCl reference electrode for pH 6.0. The photoelectrochemical (PEC) measurement (I-t transient curve) shows the enhancement in photocurrent with increasing the pH of zinc nitrate solution. After heat treatment the photocurrent is increased by 54%, 98% and 130% in the samples deposited from 2.0, 3.5 and 6.0 pH of the bath. I-V measurements were further confirmed the current enhancement in all samples after heat treatment.     

Influence of plasma treatment of ITO surface on the growth and properties of hole transport layer and the device performance of OLEDs

Surface energy of indium tin oxide (ITO) surfaces treated by different plasmas, including argon (Ar–P), hydrogen (H₂–P), carbon tetrafluoride (CF₄–P), and oxygen (O₂–P), was measured and analyzed. The initial growth mode of hole transport layers (HTLs) was investigated by atomic force microscope observation of thermally deposited 2 nm thick N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) on the plasma treated ITO surfaces. The results show that different plasma treatments of ITO influence the growth of HTLs in significantly different ways through the modification of surface energy, especially the polar component. The O₂–P and CF₄–P were found to be most effective in enhancing surface polarity through decontamination and increased dipoles, leading to more uniform and denser nucleation of NPB on the treated ITO surfaces. It was further found that increased density of nucleation sites resulted in a decreased driving voltage of OLEDs. Under the same fabricating conditions, a lowest driving voltage of 4.1 V was measured at a luminance of 200 cd/m² for the samples treated in CF₄–P, followed by the samples treated in O₂–P (5.6 V), Ar–P (6.4 V), as-clean (7.0 V) and H₂–P (7.2 V) plasma, respectively. The mechanisms behind the improved performance were proposed and discussed.     

Post-fabrication electric field and thermal treatment of polymer light emitting diodes and their photovoltaic properties

This work presents post-fabrication electric field and heat treatment methods developed for polymer light emitting diodes (PLEDs), which have degraded due to exposure to oxygen and water vapors during low-cost fabrication performed in standard room conditions. Investigated PLEDs have structures composed of indium tin oxide (ITO), poly(3,4-ethylenedioxythiophene), poly(styrenesulfonate), (PEDOT:PSS), poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV), and aluminum (Al). Heat treatment restores the light emitting function of dysfunctional PLEDs but also causes a high turn-on voltage of 10 V. Electric field treatment utilizing ?1 V reduces this high turn-on voltage to 3 V. This procedure also improves open circuit voltages from 5 mV to 55 mV, and short circuit currents from 0.5 nA to 5 nA when PLEDs are operated as photovoltaic cells under a light intensity of 500 mW/m². Repeated I–V sweep measurements additionally show improved stability and uniformity. The reasons for these improvements, the usage of an optimal treatment temperature of 130 °C, and the usage of treatment voltages of 0 and ?1 V are discussed.     

Near infra-red transparent Mo-doped In2O3 by hetero targets sputtering for phosphorescent organic light emitting diodes

We report a highly near infrared (NIR) transparent MoO₃-doped In₂O₃ (IMO) film prepared by hetero target sputtering for use as a transparent anode in phosphorescent organic light emitting diodes (OLEDs). Effective activation of Mo dopant in the In₂O₃ matrix and good crystallinity with the (2 2 2) preferred orientation from by rapid thermal annealing (RTA) led to the lowest resistivity of 4.25 × 10^?4 Ohm cm and sheet resistance of 16.9 Ohm/square, comparable to a conventional ITO anode without lose of transparency in the NIR region. Due to high carrier mobility in the IMO matrix, IMO film exhibited higher transmittance in the visible and NIR regions compared to ITO film even though it has a similar resistivity. Both synchrotron X-ray scattering and high resolution transmission electron microscope examinations showed that the optimized IMO film annealed at 600 °C had a rectangular shaped columnar structure with a strongly preferred (2 2 2) orientation. Identical current density–voltage–luminance and quantum efficiency of the phosphorescent OLED fabricated on an IMO anode were comparable to those of the OLED on a reference ITO anode due to the high transparency and low resistivity of the IMO anode.     

Improved electrodeposition of CdS layers in presence of activating H2SeO3 microadditive

CdS thin films were deposited electrochemically onto indium tin oxide (ITO)/glass substrates from aqueous solutions containing 0.01 M CdCl₂, 0.05 M Na₂S₂O₃ and 0.02 M Edta-Na₂ at −1.2 mV versus saturated sulfate reference electrode. Depositions were carried out at various temperatures (20, 50 and 80 °С) and different pH (2.5, 3.5 and 4.5) in a three electrode electrochemical cell. All above mentioned electrochemical syntheses were reproduced in presence of H₂SeO₃ microadditive to compare resulted CdS layers. Electrodeposited CdS thin films were characterized by different instrumental techniques to know the influence of deposition conditions on the quality of the obtained layers. It was found that the presence of 0.05–0.5 mM of H₂SeO₃ in the electrolyte changes the mechanism of the CdS film formation that facilitates nucleation and a growth of a more dense and uniform polycrystalline CdS film. Addition of 0.5 mM of H₂SeO₃ into the initial solution allowed us to obtain nearly stoichiometric (sulfur content ~52 at%) CdS films at reduced temperature value of 50 °C vs. higher temperature values used in a conventional electrodeposition process of CdS layers. No Se-containing phases were detected by EDX, Raman and XRD analyses in the CdS films. The presence of H₂SeO₃ tends to rearrange polytype crystalline structure of CdS to more stable hexagonal structure. The band gap value of CdS was increased from 2.3 eV to 2.5 eV as a result of H₂SeO₃ addition.     

Sub 30 nm silver nanowire synthesized using KBr as co-nucleant through one-pot polyol method for optoelectronic applications

Ultrathin, high aspect ratio silver nanowires synthesized in solution has attracted significant attention for fabricating optoelectronic devices with good properties and relatively low cost, and has been the most promising material to replace ITO. In this paper, a novel method is proposed to synthesis ultrathin nanowires through facile one-pot polyol method using KBr as a co-nucleant to NaCl. The effect of three common nucleants NaCl, FeCl₃ and KBr on nucleation rate and product size were well studied. KBr is found to be beneficial for decreasing product size but unable to generate nanowires effectively alone. By adjusting the exact concentration of KBr, silver nanowires with an average length of 21 μm and average diameter of 26 nm could be synthesized conveniently. Conductive film prepared using these nanowires had transmittance over 90% and sheet resistance about 10 Ω/□, which is better than the average level of common ITO-glass and highly potential to be applied in optoelectronic applications.     

Enhanced thermal stability of organic solar cells on nano structured electrode by simple acid etching

The long-term thermal stability of organic photovoltaic device has been emerged as a crucial characteristic to satisfy the demand of industry requiring lifespan of 20 year. In this paper, etched indium–tin-oxide (ITO) nanoelectrodes are employed to enhance the thermal stability and power conversion efficiency in poly(3hexylthiophene):methanofullerene bulk-heterojunction solar cells. A simple etching process for the ITO electrode was carried out using a hydrochloric acid solvent in order to significantly increase the roughness of the ITO surface. This nanostructured ITO not only induced efficient light harvesting, but also acted as a barrier that suppressed the PCBM phase separation, resulting in the lifespan increased from 33 h to 77 h during thermal annealing.     

Si photodetectors imprinted with ITO nanodomes for enhanced photodetection at NIR wavelengths

The p–n junction Si photodetector coated with ITO nanodomes was fabricated using large scale applicable nano imprint method. The ITO/Ni/ITO layers were deposited on both the ITO nanodomes and on the planar Si substrate. All the ITO nanodomes were electrically connected via the bottom ITO layer. The FESEM images showed the uniform array and identical surroundings of ITO nanodomes structures along with Ni and ITO layers. The highest rectification ratio of 1511.10 was obtained as the ITO nanolens focused the incident light effectively into the rectifying junction. The planar device showed the lowest reverse saturation current of 2.4 µA. The quantum efficiencies of the ITO nanodome coated Si detector were greater than that of planar ITO film coated Si devices. The presented photodetector showed the maximum internal quantum efficiency of 72.2% at 600 nm. The photoresponse of the device were excellent at a NIR wavelength of 900 nm. The Si photodetector with ITO nanodome coating would be the best device for long wavelength photodetection applications.     

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 × 10⁴ Ω cm² and 1.88 × 10³ Ω cm², respectively than organic solvent cleaned ITO of about 6.58 × 10⁵ Ω cm². 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 × 10³ Ω cm². This specific contact resistance is still much higher than that of amorphous silicon thin film transistor (0.1–30 Ω cm²).     

Preparation,structural and optical investigations of ITO nanopowder and ITO/epoxy nanocomposites

Indium Tin Oxide (ITO) nanopowder was synthesized by a sol–gel method. It was characterized by X-ray diffraction (XRD) and transmission electron microscopy. ITO/epoxy nanocomposites (ITO–EP-NCs) were prepared by mechanically dispersing the as-prepared ITO nanopowder into epoxy matrix. The XRD patterns show structural changes depending on ITO content. The interdependence of structural, morphological, optical properties and the dispersed concentration of ITO nanoparticles were investigated. The UV–visible absorption spectra revealed that the ITO–EP-NCs exhibit enhanced UV light absorption properties and wide absorption bandwidth ranging up to 400 nm from 2 wt% ITO loading. Thus, it indicated that UV and IR-shielding properties have been improved with the incorporation of ITO nanoparticles into the epoxy matrix.The gap energy of epoxy matrix was reduced by adding the ITO-NPs, leading to the improvement of its electrical conductivity. Indeed, the AC electrical conductivity of ITO–EP-NCs showed a critical percolation threshold p_c=0.21 wt% ITO. For low loading (<2 wt% ITO), the ITO–EP-NCs have combined good transparency in the visible range and enhanced electrical conductivity, which are required for optoelectronics devices.     

Effect of enhanced-mobility current path on the mobility of AOS TFT

In this study, the mobility enhancement in an Amorphous Oxide Semiconductor Thin Film Transistor (AOS TFT), particularly the effect of enhanced-mobility current path was investigated. In the TFT structure, the a-IGZO single active channel layer was replaced by double layers. Indium Tin Oxide (ITO) was employed as an enhanced-mobility current path material and was embedded in an amorphous Indium Gallium Zinc Oxide (a-IGZO) channel layer of a conventional bottom gate structure TFT. To analyze the effect of the length of an additional current path, the a-IGZO channel length was fixed at 80 μm, and the length of the ITO enhanced-mobility current path was increased to 20, 40, and 60 μm. As a result, the mobility increased monotonically with the length of the enhanced-mobility current path and was predictable from the rule of mixture. The maximum saturation mobility of 28.3 cm²/V s resulted when the length of the enhanced-mobility current path was 60 μm. This value is more than double that of a single path TFT. Such enhancement in mobility is attributed to the high conductivity of ITO and a good conduction band match between a-IGZO and ITO.     

Organic light-emitting diodes fabricated on recessed anodes for light extraction enhancement

Circular recesses have been fabricated on indium tin oxide (ITO) anodes to enhance light extraction of organic light-emitting diodes (OLEDs). The effects of recess depth and recess coverage ratio on the performance of a green OLED were systematically investigated. Results showed that the current efficiency could be enhanced from 40.7 cd/A of a planar device to 47.2 cd/A of the device with a recess depth of 100 nm and a recess coverage ratio of 14.1%. The enhanced light extraction by the recess wall effect was realized to be the major factor leading to the improved efficiency. The efficiency is however limited by the accompanying increase in electrical resistivity of the ITO films at deep recesses and high recess coverage ratios. Despite of the insignificant efficiency enhancement (up to 16%) in this study, this recessed ITO approach provides a simple architecture to enhance waveguide mode light extraction without adding an internal medium.     

Low-temperature nickel-induced nano-crystallization of silicon on PET by MIC,hydrogenation and mechanical stress

The effects of RF-Plasma hydrogenation and applied mechanical strain on the crystallization of silicon layers deposited on plastic substrates have been investigated where the maximum temperature remained below 170 °C for the entire process. The structural properties of the samples have been studied by optical, scanning-electron and transmission-electron microscopy where the nano-crystallinity of the silicon layers has been confirmed. The maximum average diameter of the silicon grains was 4.5 nm and occurred for an applied tensile strain of 4%. In addition, a thin-film transistor on a plastic substrate has been fabricated and found to possess an electron mobility of 2.4 cm²/V s.     

A fiber-optic pH sensor based on relative Fresnel reflection technique and biocompatible coating

A biocompatible fiber-optic pH sensor based on Fresnel reflection technique and a sensing coating is presented. Sodium alginate and polyethylenimine are alternatively deposited on the sensing fiber end to form the sensing coating via a layer-by-layer electrostatic self-assembly technique. An optical switch is added to the measurement system for the convenience of fast calibration. A linear, monotonic and fast response in a large pH range (from pH 5.87 to pH 10.55) is obtained with the resolution of 0.01 pH unit. The sensitivity of the pH sensor is 0.018 R.I.U/pH. It is not influenced by fluctuations of light source.     

Optoelectronic characteristics of MEH-PPV polymer LEDs with thin,doped composition-graded a-SiC:H carrier injection layers

The optoelectronic characteristics of poly(2-methoxy-5-(2′ethyl-hexoxy)-1,4-phenylene-vinylene) (MEH-PPV) polymer LEDs (PLEDs) have been improved by employing thin doped composition-graded (CG) hydrogenated amorphous silicon–carbide (a-SiC:H) films as carrier injection layers and O₂-plasma treatment on indium–tin-oxide (ITO) transparent electrode, as compared with previously reported ones having doped constant-optical-gap a-SiC:H carrier injection layers. For PLEDs with an n-type a-SiC:H electron injection layer (EIL) only, the electroluminescence (EL) threshold voltage and brightness were improved from 7.3 V, 3162 cd/m² to 6.3 V, 5829 cd/m² (at a current density J = 0.6 A/cm²), respectively, by using the CG technique. The enhancement of EL performance of the CG technique was due to the increased electron injection efficiency resulting from a smoother barrier and reduced recombination of charge carriers at the EIL and MEH-PPV interface. Also, surface modification of the ITO transparent electrode by O₂-plasma treatment was used to further improve the EL threshold voltage and brightness of this PLED to 5.1 V, 6250 cd/m² (at J = 0.6 A/cm²). Furthermore, by employing the CG n[p]-a-SiC:H film as EIL [hole injection layer (HIL)] and O₂-plasma treatment on the ITO electrode, the brightness of PLEDs could be enhanced to 9350 cd/m² (at a J = 0.3 A/cm²), as compared with the 6450 cd/m² obtained from a previously reported PLED with a constant-optical-gap n-a-SiCGe:H EIL and p-a-Si:H HIL.     

Electrical and optical properties of Cu2ZnSnS4 grown by a thermal co-evaporation method and its diode application

Cu₂ZnSnS₄ (CZTS) is low cost and constitutes non-toxic materials abundant in the earth crust. Environment friendly solar cell absorber layers were fabricated by a thermal co-evaporation technique. Elemental composition of the film was stated by energy dispersive spectroscopy (EDS). Some optical and electrical properties such as absorption of light, absorption coefficient, optical band gap charge carrier density, sheet resistance and mobility were extracted. Optical band gap was found to be as 1.44 eV, besides, charge carrier density, resistivity and mobility were found as 2.14×10¹⁹ cm⁻³, 8.41×10⁻⁴ Ω cm and 3.45×10² cm² V⁻¹ s⁻¹, respectively. In this study Ag/CZTS/n-Si Schottky diode was fabricated and basic diode parameters including barrier height, ideality factor, and series resistance were concluded using current–voltage and capacitance–voltage measurements. Barrier height and ideality factor values were found from the measurements as 0.81 eV and 4.76, respectively, for Ag/CZTS/n-Si contact.