Conductive Aluminum-doped Zinc Oxide Research Articles

Multifunctional Al-doped ZnO thin films for vertically aligned liquid crystal devices

The integration of ITO-free transparent conductive layers remains a big challenge for development of next generation technologies. Here, we demonstrate the feasibility of transparent and conductive Aluminum-doped Zinc Oxide (AZO) thin films to operate simultaneously as electrode and alignment layer in Liquid Crystal (LC) device configuration. Controlling the growth process of AZO thin films by Atomic Layer Deposition (ALD) technique results of predominantly (100) crystallographic oriented AZO with very high transparency and excellent conductivity. The exceptionally low surface free energy of (100) oriented AZO, along with the topological modification following the mechanical rubbing treatment were used to elucidate the mechanism of LC molecules alignment on the surface of AZO film. The uniform vertical orientation of the nematic LC director was confirmed by polarized optical microscopy and pretilt angle measurements. The competitive electro-optical performance in terms of phase modulation, threshold and saturation voltages and contrast ratio of the assembled LC device reveals the great potential of AZO thin films as transparent electrode and alignment layer for future LC display applications with versatile functionality.

Atomic layer deposition of aluminum-doped zinc oxide onto MoO3 nanorods toward enhanced lithium storage performance

An atomic layer deposition (ALD) of aluminum-doped zinc oxide (AZO) onto MoO3 nanorods (denoted as MoO3@AZO) was developed to enhance the lithium storage performance of MoO3 anode. Thanks to the highly conductive and robust AZO layer, the MoO3@AZO electrode demonstrated high pseudocapacitive contribution (76.2 % at 1.0 mV s−1), enhanced electron/ion transferability, outstanding rate performance (708 mAh g−1 at 2 A g−1) and excellent cycling stability (973 mAh g−1 after 100 cycles at 0.2 A g−1 with a high capacity retention of 92.8 %) when applied as an anode for lithium-ion batteries. Ex-situ transmission electron microscopy analysis revealed that the rod feature of MoO3@AZO was well retained with the protection of the AZO layer even after 100 cycles. Moreover, the ALD-AZO coating may also be commonly used for other metal oxides to enhance their electron conductivity and structural stability.

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.

Enhanced Sol-Gel Route to Obtain a Highly Transparent and Conductive Aluminum-Doped Zinc Oxide Thin Film.

The electrical and optical properties of sol–gel derived aluminum-doped zinc oxide thin films containing 2 at.% Al were investigated considering the modifying effects of (1) increasing the sol H2O content and (2) a thermal treatment procedure with a high-temperature approach followed by an additional heat-treatment step under a reducing atmosphere. According to the results obtained via the TG-DTA analysis, FT-IR spectroscopy, X-ray diffraction technique, and four-point probe resistivity measurements, it is argued that in the modified sample, the sol hydrolysis, decomposition of the deposited gel, and crystallization of grains result in grains of larger crystallite size in the range of 20 to 30 nm and a stronger c-axis preferred orientation with slightly less microstrain. The obtained morphology and grain-boundary characteristics result in improved conductivity considering the resistivity value below 6 mΩ·cm. A detailed investigation of the samples’ optical properties, in terms of analyzing their absorption and dispersion behaviors through UV-Vis-NIR spectroscopy, support our reasoning for the increase of the mobility, and to a lesser extent the concentration of charge carriers, while causing only a slight degradation of optical transmittance down to nearly 80%. Hence, an enhanced performance as a transparent conducting film is claimed for the modified sample by comparing the figure-of-merit values.

Hierarchically Structured Photoelectrodes Utilizing a Protected Transparent Conducting Oxide Grown Via Atomic Layer Deposition

Hierarchically structured materials are often used to improve light absorption and photocarrier collection in thin-film photoelectrodes. However, the structures require a continuous conductive coating to function as an electrode and maintain electrical contact to the entire, tortuous surface while being transparent to visible light to minimize parasitic absorption. The electrochemically robust transparent conducting oxide (TCO) fluorine-doped tin oxide (FTO) can only be grown by line-of-sight physical deposition methods such as RF sputtering or spray pyrolysis, which in general prevents the deposition of uniform thin coatings of FTO on the interior of a hierarchichally structured scaffold. Other TCOs (tin-doped indium oxide (ITO), aluminum-doped zinc oxide (AZO)) can be grown conformally by surface specific methods such as atomic layer deposition (ALD), but are only stable at near-neutral pH, limiting their use in photoelectrochemical energy conversion. Here, we describe a strategy for improving the stability of ALD-deposited AZO films for anodic and cathodic operation in alkaline and acidic electrolytes. The addition of a conformal, nanoscale TiO2 coating preserves a conductive AZO film for more than 24 hours at pH > 13. This is an improvement of several orders of magnitude compared to the lifetime of pure AZO films, which is also observed in acidic (pH < 1) environments. Although the TiO2 coating is not significantly conductive alone, we observed no decrease in the conductivity of the layered AZO/TiO2 film. The TiO2-coated AZO film was used as an electron collector on hierarchically porous scaffolds formed from close-packed inorganic silica spheres. The improved electrochemical stability will be utilized in the construction of ultra-thin film photoelectrodes in hierarchichally-structured systems with earth-abundant semiconductors, such as cupric oxide (CuO) or hematite (α-Fe2O3).

Pressure effect on structural and electrical properties of AZO thin films annealed in N2/H2 atmosphere

Transparent and conductive aluminum-doped zinc oxide (AZO) thin films were developed using a sol-gel method and annealed in 95 N2/5 H2 forming gas. The pressure effects of forming gas on the structural and electrical properties, and the underlying mechanisms were investigated. XPS analysis reveals that the fraction of the chemisorbed oxygen in the annealed film in 0.01 MPa 95 N2/5 H2 did not exhibit any apparent change compared to the non-annealed films, while the oxygen vacancy content exhibited a remarkable increase. When pressure goes up to 0.03 MPa, significant changes of both the oxygen vacancy concentration and the chemisorbed oxygen content were observed in the annealed films. At 0.03 MPa, the films displayed a perfect orientation along the c-axis and exhibited the lowest resistivity of 1.6 × 10−3 Ω·cm. Our study shows that it is possible to optimize the oxygen distribution and improve the electrical property of AZO film by adjustment of the forming gas pressure.

Rapid, Selective, Ambient Growth and Optimization of Copper Benzene-1,3,5-Tricarboxylate (Cu–BTC) Metal–Organic Framework Thin Films on a Conductive Metal Oxide

Metal–organic frameworks (MOFs) are useful for thin film-based device integration as they show high adsorption capacity for selected gases. However, optimized device integration of MOFs for various platforms requires high-quality, well controlled thin film growth techniques that are flexible, scalable, and manufacturable. For this reason, there is a critical need for the ability to grow MOFs in the form of dense and uniform thin films efficiently on a wide range of substrates. In this work, copper benzene-1,3,5-tricarboxylate (Cu–BTC) MOF thin films are rapidly produced at room temperature by using a conductive aluminum-doped zinc oxide (AZO) as a seed layer to template Cu–BTC growth, with growth only occurring on the AZO layer via a hydroxy double salt intermediate. The formation pathway of the Cu–BTC films was investigated in detail because of the significant importance of improving the Cu–BTC film growth from the perspective of optimized device integration. We demonstrate that the structure of the resu...

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.

Solution-processed composite electrodes composed of silver nanowires and aluminum-doped zinc oxide nanoparticles for thin-film solar cells applications

Transparent conducting electrodes based on silver nanowires (AgNWs) have arisen great interest due to their excellent photoelectric properties compared with the conventional sputtered indium tin oxide (ITO) electrodes. However, the practical applications of these electrodes are limited due to their coarse surface morphology, weak mechanical adhesion and high junction resistivity as well as poor thermal and chemical stability. In this work, we develop a new processing technique to resolve all above limitations by embedding random AgNWs networks in solution-processed conductive aluminum-doped zinc oxide nanoparticles (AZO-NPs) matrices via a spin-coating process at low temperature (150°C). Our results show that coating the AZO-NPs on the surface of AgNWs networks effectively fill the holes between AgNWs and tightened the AgNWs junctions, resulting in ultralow surface roughness and excellent wire-substrate mechanical adhesion. Surrounding the AgNWs by conductive AZO-NPs could provide more electron transport channels between AgNWs so as to improve the wire-wire electrical conductivity. The resulting composite electrodes demonstrated an excellent performance with low sheet resistivity < 28Ω/sq, high optical transmittance > 91% at 550nm, excellent thermal stability at temperature as high as 270°C, good chemical corrosion resistance, and retained high electrical conductivity showing less 1% deviation in sheet resistivity after 300 continuous convex and concave bending cycle. Kesterite pure sulfide Cu2ZnSnS4 thin film solar cells using such composite electrodes as top contact show a best conversion efficiency of 7.45%, achieving a 11.5% increase compared with conventionally sputtered ITO-based CZTS thin film solar cells due to higher short circuit current density and fill factor.

Enhanced sulfide chemisorption by conductive Al-doped ZnO decorated carbon nanoflakes for advanced Li–S batteries

Lithium–sulfur batteries have attracted significant attention recently due to their high theoretical capacity, energy density and cost effectiveness. However, sulfur cathodes suffer from issues such as shuttle effects, uncontrollable deposition of lithium sulfides species, and volume expansion of sulfur, which result in rapid capacity fading and low Coulombic efficiency. In recent years, metal-oxide nanostructures have been widely used in Li–S batteries, owing to their effective inhibition of the shuttle effect and controlled deposition of lithium sulfide. However, the nonconductive metal-oxides used in Li–S batteries suffer from extra diffusion process, which slows down the electrochemical reaction kinetics. Herein, we report the synthesis of carbon nanoflakes decorated with conductive aluminium-doped zinc oxide (AZO@C) nanoparticles, through a facile biotemplating method using kapok fibers as both the template and carbon source. A sulfur cathode based on the AZO@C nanocomposites shows better electrochemical performance than those of cathodes based on ZnO and Al2O3 with poor conductivity, with a stable capacity of 927 mAh·g−1 at 0.1C (1C = 1,675 mA·g−1) after 100 cycles. A reversible capacity of 544 mAh·g−1 after 300 cycles was obtained even after increasing the current density to 0.5C, with a 0.039% capacity decay per cycle under a sulfur loading of 3.3 mg·cm−2. Moreover, a capacity of 466 mAh·g−1 after 100 cycles at 0.5C could still be obtained when the sulfur loading was increased to 6.96 mg·cm−2. The excellent electrochemical performance of the AZO@C/S composite can be attributed to its high conductivity of the polar AZO host, which suppresses the shuttle effect while simultaneously improving the redox kinetics in the reciprocal transformation of lithium sulfide species.

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.

Island‐Like AZO/Al2O3 Bilayer Channel Structure for Thin Film Transistors

Aluminum‐doped zinc oxide (AZO) has been regarded as a potential and promising material for thin‐film transistors (TFTs) owing to its low cost and nontoxicity. Generally, the high conductivity of AZO should be suitably reduced before using in TFTs as a semiconductor. Traditional ways to reduce the conductivity of AZO is by increasing doping content of aluminum (&gt;5 at%). Herein, a novel approach is first reported to convert conductive AZO into a semiconductor by rationally designing a bilayer channel structure that consists of an AZO island film and a thin Al2O3 film, serving as “electron donor” and “electron bridge,” respectively. Consequently, TFTs with island‐like AZO/Al2O3 channel material present the saturation mobility of 8.04 cm2 V−1 s−1, on/off current of 1.6 × 107, and subthreshold swing of 0.83 V dec−1. Moreover, the underlying mechanism for the excellent electrical properties of obtained TFTs is that the electron conduction in AZO/Al2O3‐TFTs is proved to be dominated by the percolation conduction due to the effective control of carriers concentration. This method proposed in this report sheds light on the conversion of conductive oxide film to semiconductor layer for TFTs.

Highly conductive AZO thin films obtained by rationally optimizing substrate temperature and oxygen partial pressure

ABSTRACTIn this work, an optimal procedure was proposed to prepare aluminum-doped zinc oxide (AZO) thin films with a sheet resistance of 17.03 Ω/sq by rationally optimizing oxygen partial pressure and substrate temperature. The results showed that increased temperature has an improving trend in the resistivity of AZO thin films while elevated oxygen partial pressure exhibited a deteriorating trend. With rising of substrate temperature in an oxygen-rich atmosphere, oxygen adsorption phenomenon gets stronger. Adsorbed oxygen atoms or molecules suppress the formation of oxygen vacancy and trap electrons, which is responsible for the increase of resistivity. Besides, atomic force microscope analysis indicated that surface roughness of AZO thin films decreases as increase of substrate temperature.

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.

Influence of deposition pressure on properties of ZnO: Al films fabricated by RF magnetron sputtering

Transparent conductive aluminum doped zinc oxide (ZnO:Al, AZO) films were prepared on glass substrates by rf (radio frequency) magnetron sputtering from ZnO: 3wt% Al2O3 ceramic target. The effect of argon gas pressure (PAr) was investigated with small variations to understand the influence on the electrical, optical and structural properties of the films. Structural examinations using X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that the ZnO:Al thin films were (002) oriented. The resistivity values were measured by four-point probe with the lowest resistivity of 5.76×10-4 Ω·cm (sheet resistance=9.6 Ω/sq. for a thickness=600 nm) obtained at the PAr of 0.3 Pa. The transmittance was achieved from ultraviolet-visible (UV-VIS) spectrophotometer, 84% higher than that in the visible region for all AZO thin films. The properties of deposited thin films showed a significant dependence on the PAr.

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).

Ion beam fabrication of aluminum-doped zinc oxide layer for high-performance liquid crystals alignment.

In this paper, a 1.8 keV ion beam (IB) sputtered thin layer of aluminum-doped zinc oxide (AZO) with columnar AZO bumps covering the surface working as an alignment layer for the homogeneous alignment of liquid crystals (LC) is investigated. Bumpy AZO alignment layers in twisted nematic (TN) cells generated larger LC pre-tilt angles and thus enabled accelerated switching of LC, and the highly conductive bumpy AZO thin layers allowed super-fast release of accumulated charges, and led to low residual DC performance. These results indicate the promising applications of AZO bumps layer as alignment layer in LC devices.

Photoelectrochemical investigation of bare transparent conducting oxides for water oxidation

Bare tin-doped indium oxide (ITO) and fluorine-doped tin oxide (FTO), which are widely used transparent conducting metal oxide as anode materials for various electrochemical applications, are not used as anode materials for photoelectrochemical (PEC) oxidation of water as those required high potential to obtain efficient oxidation of water. To reduce the oxidation potential, tedious modification with photoelectrocatalyst is required. A straightforward alternative of using the modified electrode would be established if any bare metal oxide could be used as an efficient electrode material for water oxidation. Here, we report for the first time that commercially available bare gallium doped zinc oxide (GZO) has great potential to be utilized as an anode material for efficient PEC water oxidation at low potential under UV–Vis light. The higher performance of GZO was revealed by comparing the PEC behavior of other commercially available and frequently employed transparent conductive metal oxide electrodes, namely, ITO, FTO, and aluminum-doped zinc oxide. The GZO and AZO show the highest and second highest water oxidation signal, respectively, among the tested electrodes. Moreover, the GZO shows higher stability as compared to AZO under applied experimental conditions. ITO electrode also exhibited a good stability but its efficacy towards water oxidation was the lowest. The efficiency of the GZO toward PEC water oxidation at 0.6 V is ca. 36.5 times higher than that obtained at ITO.

Microstructure and electrical property of aluminum doped zinc oxide ceramics by isolating current under spark plasma sintering

The present paper reports a method for sintering conductive aluminum doped zinc oxide (AZO) ceramics by isolating the current from sample during SPS. It is shown that isolating the current from AZO ceramics during SPS has a significant influence on the grain growth and dislocation defects which dominate the electrical property of ceramics. With isolating the current from AZO ceramics during SPS, the grain size of AZO and a secondary phase ZnAl2O4 is obvious greater than that of samples prepared with current passing through, especially for the sintering temperature of 1200°C. Furthermore, less dislocation defects in AZO ceramics is observed by isolating the current from AZO ceramics. Hall mobility of aluminum doped zinc oxide ceramics has been greatly improved reaching the value of 107.3cm2V−1s−1 attributed by the large grain size of ZnO and secondary phase as well as less dislocation defects. The superior electronic transport property with such high vale of Hall mobility ultimately results in a fairly low resistivity of 6.7×10−4Ωcm compared to 1.3×10−3Ωcm reported in the literature for AZO fabricated by spark plasma sintering.

On the growth of conductive aluminum doped zinc oxide on 001 strontium titanate single crystals

Aluminum doped zinc oxide (AZO) thin films were obtained by pulsed laser deposition on (001) SrTiO3 (STO) on a range of substrate temperatures during ablation between 300°C and 600°C. A hexagonal system lying on a cubic one should be difficult to be obtained in epitaxial form. The geometrical selection of the AZO growth on (001) STO is not giving a unique preferential orientation. Two orientations, c-axis (along [001]) and 110, have been observed experimentally with different ratios at different substrate temperature. Discussions are made with respect to the temperature dependence of lattice mismatch between the two cases and the cubic surface of the substrate, and to the substrate surface morphology and terminating atomic layer composition. The 110 AZO is the main phase at deposition temperature of 550°C, while for other substrate temperatures the 001 is the preferential orientation. The conductive character of 110 AZO thin film have been inferred from both ellipsometry spectra and current–voltage measurements. Excepting the samples deposited at 300°C, the lowest resistivity is recorded for the samples with 110 AZO as the main phase.