Transparent Conductive Aluminum-doped Zinc Oxide Research Articles

Preparation of highly transparent conductive aluminum-doped zinc oxide thin films using a low-temperature aqueous solution process for thin-film solar cells applications

In this work, we developed a facile route to fabricate highly transparent conductive AZO thin films by using an aqueous solution process followed by an ultraviolet (UV) exposure technique at an annealing temperature as low as 200 °C, where the aluminum citrate complex was used as the Al doping source. Various of deposition parameters in the AZO thin films fabrication process were studied and optimized. The mechanisms on UV exposure and heat treatment to the conductivity improvement and conductivity stability of the AZO films were analysed. Results showed that the AZO thin films under the best parameters conditions demonstrated an optical transmittance higher than 85% in the visible spectra region and a lowest electrical resistivity of 4.8 × 10−3 Ω cm, while exhibited densely oriented columnar grains and uniform surface morphology as well as uniform composition distribution. The UV exposure could remove carbon species from the surface of the AZO thin films to reduce oxygen-related defects and release free carriers at the boundaries and interfaces, thereby improving the conductivity of the AZO thin films. The simultaneous treatment of the AZO thin film by ultraviolet exposure and heat treatment could remove carbon species at a deeper thickness, thereby improving the conductive stability of the AZO thin films. Kesterite Cu2ZnSnS4 thin film solar cells incorporating the optimal AZO thin films as top electrodes demonstrated a best power conversion efficiency (PCE) of 7.15%, which was comparable to the PCE value obtained by using the sputtering deposited AZO thin films as top electrodes.

Aluminum-doped ZnO thin films deposited on flat and nanostructured glass substrates: Quality and performance for applications in organic solar cells

Transparent conductive aluminum-doped zinc oxide (AZO) thin films were deposited on a flat and nanostructured borosilicate glass substrates by using DC-magnetron sputtering. The aluminum doping concentration was kept at 3.3 wt% and the film thickness (100–400 nm) was varied. The thin films were then annealed at 250 °C for 1 h in nitrogen and/or argon atmosphere, respectively. AZO thin films exhibited hexagonal wurtzite structure of ZnO with an intense (0 0 2) diffraction peak, indicating that they have c-axis preferred orientation. Optical transmittance was observed to be greater than 80% in the visible range for films deposited on both flat and nanostructured glass substrates. The lowest resistivity of 9.7 × 10−4 Ω cm was observed for AZO film of 400 nm thickness on flat glass substrate, annealed in Nitrogen atmosphere. The power conversion efficiency (PCE) of 0.27% and 2.24% were recorded for organic solar cell devices based on AZO deposited on nanostructured and flat borosilicate glass, respectively. In comparison with the latter, the PCE of ITO based device was recorded to be 3.17%. Due to their good optical and electrical properties, AZO thin films are promising candidates as transparent electrodes in organic solar cells.

Aluminum-Doped Zinc Oxide Transparent Electrode Prepared by Atomic Layer Deposition for Organic Light Emitting Devices

Transparent conductive aluminum-doped zinc oxide (AZO) films are being introduced as alternatives to indium tin oxide (ITO) films, because they do not contain indium, which is expensive and toxic. In this study, the structural, electrical, and optical properties of AZO electrodes fabricated by atomic layer deposition (ALD) at a low temperature of 150 °C were examined by X-ray photoemission spectroscopy and scanning electron microscopy. The H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O purge time was changed in the ZnO cycle to alter the orientation of crystal phases and the film's electrical conductivity. An optimized AZO electrode, which had an Al:Zn mole ratio of 1:49, was prepared with a 20 s H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O purge time. The resulting transparent electrode had a low resistivity (1.25 mΩ·cm ± 0.2 mΩ·cm) and a high transmittance (83.2% at 550 nm). The AZO film exhibited a high work function of 4.7 eV. Consequently, an classic organic light-emitting device (OLED) with an N,N'-bis-(1naphthl)-diphenyl-1,1' -diphenyl-4,4' -diamine and tris(8-quinolinolato) aluminum structure was fabricated on a glass substrate using the optimized AZO anode, and a maximum current efficiency of 3.9 cd/A was achieved. These results suggest that this method for preparing transparent conductive films via ALD can be used to create anodes for OLEDs.

The effects of the argon plasma treatments on transparent conductive aluminum-dope zinc oxide thin films prepared by the pulsed DC magnetron sputtering

Transparent conductive aluminum-doped zinc oxide (ZnO:Al or AZO) thin films were fabricated by the pulsed DC magnetron sputtering deposition. The prepared AZO thin films were carefully treated with the argon plasma treatments in the vacuum state with a variety of the plasma conditions, i.e., the control power and the operated pressure. The plasma-treated AZO samples were then investigated for their crystal structures by X-ray diffraction (XRD). Their physical morphologies and surface topologies were examined by field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). Electrical properties of the samples were measured with the four-point probe and the Hall effect measurements. Finally, their optical transmission was observed with the UV-Vis-NIR spectrophotometry. The results from the as-deposited and the plasma-treated AZO thin films will be compared and discussed.

Aerosol assisted chemical vapour deposition of transparent conductive aluminum-doped zinc oxide thin films from a zinc triflate precursor

The use of zinc triflate (trifluoromethanesulfonate), [Zn(OTf)2] as a precursor in the aerosol assisted chemical vapour deposition of zinc oxide thin films is described. Aluminum doped zinc oxide (AZO) thin films are also shown to be deposited when aluminum acetylacetonate [Al(acac)3] was introduced into the precursor solution, illustrating the versatility of this system. Film characterization techniques include glancing angle X-ray powder diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and optical and electrical measurements. AZO films with an Al content of 7at.% were found to have favourable transparent conducting oxide properties with simultaneous high transparency (>80%) in the visible light region and a low electrical resistivity (1.96×10−3Ωcm).

High Performance AZO Thin Films Deposited by RF Magnetron Sputtering at Low Temperature

Transparent conductive aluminum-doped zinc oxide (AZO) films had been prepared on glass substrates by radio frequency magnetron sputtering with different sputtering powers using an ultra- high density ceramic target at 100°C Crystallinity levels, microstructures, optical and electrical properties of these thin films were systematically investigated by the scanning electron microscope, X-ray diffraction diffractometer, UV-visible spectrophotometer and four-point probe method. All the AZO thin films exhibited preferential orientation along the (0 0 2) plane with hexagonal wurtzite structure. The average transmittance of the thin films is over 86%, the highest transmittance 88.97% and the lowest resistivity 3x10-4Ω·cm are simultaneously obtained at 200W. With the sputtering power increase, the grain growth was deteriorated and the resistivity increased from 3x10-4Ω·cm to 2.5x10- 3Ω·cm. The band gaps of AZO films range from 3.81eV to 3.89eV. This article discussed some important patents related to the methods of depositing AZO thin films. Keywords: AZO thin films, electrical properties, low temperature, magnetron sputtering, microstructure, optical properties.

Single-crystal indium phosphide nanowires grown on polycrystalline copper foils with an aluminum-doped zinc oxide template

The growth of indium phosphide (InP) nanowires on transparent conductive aluminum-doped zinc oxide (AZO) thin films on polycrystalline copper (Cu) foils was proposed and demonstrated. AZO thin films and zinc oxide (ZnO) thin films, as comparison, were deposited on Cu foils by radio frequency magnetron sputtering. Subsequently, InP was grown by metal organic chemical vapor deposition with gold catalysts. InP nanowire networks formed on the AZO thin films, while no InP nanowires grew on the ZnO thin films. Morphological, crystalline, and optical properties of the InP nanowires on AZO thin films were compared with those of InP nanowires grown on silicon (Si) substrates. Zinc diffusion from AZO thin films into InP nanowire networks was suggested as the cause of substantial modifications on the optical properties of the InP nanowires on AZO thin films; redshift in photoluminescence spectra and a larger relative TO/LO intensity ratio in Raman spectra were observed, in comparison to those of the InP nanowires grown on Si substrates. In this paper, we proposed and demonstrated a new route to grow semiconductor nanowires on metals that potentially provide low-cost and mechanically flexible substrates and establish a reliable electrical contact by utilizing conductive oxide thin films as a template, which could offer a new material platform for such applications as sensors and thermoelectric devices.

Growth and characterization of indium phosphide nanowires on transparent conductive ZnO:Al films

The epitaxial growth of indium phosphide nanowires (InP NWs) on transparent conductive aluminum-doped zinc oxide (ZnO:Al) thin films is proposed and demonstrated. ZnO:Al thin films were prepared on quartz substrates by radio frequency magnetron sputtering, then InP NWs were grown on them by plasma enhanced metal organic chemical vapor deposition with gold catalyst. Microstructure and optical properties of InP nanowires on ZnO:Al thin films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectric spectroscopy (XPS), photoluminescence and Raman spectroscopy at room temperature. SEM shows that randomly oriented and intersecting InP nanowires were grown to form a network on ZnO:Al thin films. Both wurtzite (WZ) and zincblende (ZB) structures coexist in the random orientation InP NWs on ZnO:Al thin film had been proved by XRD analysis. XPS result indicates Zn diffusion exists in the InP NWs on ZnO:Al. The photoluminescence spectra of InP nanowires with Zn diffusion present an emission at 915 nm. Zn diffusion also bring effect on Raman spectra of InP NWs, leading to more Raman-shift and larger relative intensity ratio of TO/LO.

Optical and electrical studies of transparent conductive AZO and ITO sputtered thin films for CIGS photovoltaics

AbstractDue to the many advantages of aluminum doped zinc oxide (AZO) as an alternative to indium tin oxide (ITO), the optical and electrical properties of AZO thin films deposited on large substrates by D.C. pulsed magnetron sputtering have been investigated in view of future industrial applications. In order to evaluate the effectiveness of AZO thin films as transparent conductive oxide (TCO), their properties have been compared with those of standard ITO grown with the same technique. The effect of the thickness on the optical properties revealed higher transmittance values for AZO (T&gt;86%) than for ITO (T&gt;83%) films. The best resistivity values obtained for AZO were about 9.7×10–4 Ωcm versus 5.09×10–4 Ωcm measured for ITO films. Moreover, it was observed that the film resistivity is significantly influenced by the working pressure in the case of AZO films while the ITO resistivity appears almost unaffected. Finally, since both the optical and electrical properties are influenced by the film thickness, it was crucial to determine the proper AZO thickness in order to obtain the optimum of electrical and optical properties for the photovoltaic applications. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Preparation and Hydrophobic Processing of Aluminum-Doped Zinc Oxide Films

In this research, transparent and conductive aluminum-doped zinc oxide (AZO) films were prepared on glass substrates by metal-organic chemical vapor deposition (MOCVD). A nanostructured hydrophobic layer of fluorocarbon (FC) compounds was formed on the films by low-temperature dielectric barrier discharge plasma enhanced chemical vapor deposition (DBD-PECVD), as hydrophobic processing. Scanning electron microscopy (SEM) and contact angle analyzer were used to characterize and analyze the surface morphology, structure and hydrophobicity of these samples. The results indicate that the hydrophobicity of transparent conductive AZO films was enhanced by the deposition of the FC film.

Impedance Control of Reactive Sputtering Process in Mid-Frequency Mode with Dual Cathodes to Deposit Al-Doped ZnO Films

Aluminum-doped zinc oxide (AZO) films were deposited on glass substrates at 300°C by reactive mid-frequency (mf, 50 kHz) magnetron sputtering using dual magnetron cathodes with aluminum–zinc alloy targets. In order to keep the very high deposition rate stable in the transition region of the reactive sputtering system, the reactive gas (O2) flow was controlled using the “discharge impedance feedback system”, where the discharge current value was used to control the O2 flow. The highest deposition rate for the transparent conductive AZO films achieved by this dual magnetron sputtering (DMS) system was 242 nm/min, which was higher by one order of magnitude than that achieved by the conventional reactive sputtering system. The lowest resistivity of the AZO film obtained by such a high deposition rate was 4.4×10-4 Ωcm. The structure and electrical properties of the films varied systematically by controlling the discharge current in the transition region using this system.

Al-Doped ZnO Films Deposited by Reactive Magnetron Sputtering in Mid-Frequency Mode with Dual Cathodes

Aluminum-doped zinc oxide (AZO) films were deposited on heated (300°C) glass substrates by reactive mid-frequency (mf, 50 kHz) magnetron sputtering using dual magnetron cathodes with aluminum–zinc alloy targets. In order to keep the very high deposition rate, reactive gas control using plasma emission was carried out to stabilize the discharge in the “transition region” of the reactive sputtering system. The highest deposition rate for the transparent conductive AZO films by this dual magnetron sputtering (DMS) system was 290 nm/min, which was higher than that for the conventional reactive sputtering system by one order of magnitude. The lowest resistivity of the AZO film was 3.9×10-4 Ω cm. The structure and electrical properties of the films could be controlled systematically by the reactive gas control system in the transition region.

Preparation and properties of transparent conductive aluminum-doped zinc oxide thin films by sol–gel process

Highly conductive and transparent aluminum-doped zinc oxide thin films have been prepared from the solution of zinc acetate and aluminum nitrate in ethanol by the sol–gel process. The effect of changing the aluminum-to-zinc ratio from 0 to 5 at. % and annealing temperature from 0 °C to 700 °C in air has been investigated. The resistivities of thin films were measured as a function of annealing temperature and also as a function of aluminum dopant concentration in the solution. As-deposited films have high resistivity and high optical transmission. Annealing of the as-deposited films in air leads to a substantial reduction in resistivity. The films have a minimum value of resistivity of 1.5×10−4 Ω cm for 0.8 at. % aluminum-doped zinc oxide and a maximum transmission of about 91% when deposited on glass substrates. X-ray diffraction measurements employing Cu Kα radiation were performed to determine the crystallinity of the ZnO:Al films which showed that the films were polycrystalline with a hexagonal structure when annealed at 500 °C.

Effects of aluminum content and substrate temperature on the structural and electrical properties of aluminum-doped ZnO films prepared by ultrasonic spray pyrolysis

Transparent conductive aluminum-doped zinc oxide (AZO) films were prepared by an ultrasonic spray pyrolysis method. A vertical type hot wall furnace was used as a reactor in the deposition system Zinc acetate dissolved in methanol was selected as a precursor. The substrate temperature was varied from 180 °C to 240 °C. Aluminum (Al) was doped into ZnO films by incorporating anhydrous aluminum chloride AlCl3 in the zinc acetate CH3CO2)2Zn solution. The proportion of the Al in the starting solution was varied from 0 wt % to 3.0 wt %. The crystallographic properties and surface morphologies of the films were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The resistivity of the films was measured by the Van der Pauw method, and the mobility and carrier concentration were obtained through Hall effect measurements. Transmittance was measured in the visible region. The effects of substrate temperature and aluminum content in the starting solution on the structural and electrical properties of the AZO films are discussed.