Effect of ZnO buffer layer on AZO film properties and photovoltaic applications

Aluminum-doped ZnO (AZO) transparent conducting films were deposited on glass substrates with and without intrinsic ZnO (i-ZnO) buffer layers by a home made and low cost radio-frequency (RF) magnetron sputtering system at room temperature in pure argon ambient and under a low vacuum level. The films were examined and characterized for electrical, optical, and structural properties for the application of CIGS solar cells. The influence of sputter power, deposition pressure, film thickness and residual pressure on electrical and optical properties of layered films of AZO, i-ZnO and AZO/i-ZnO was investigated. The optimization of coating process parameters (RF power, sputtering pressure, thickness) was carried out. The effects of i-ZnO buffer layer on AZO films were investigated. By inserting thin i-ZnO layers with a thickness not greater than 125 nm under the AZO layers, both the carrier concentration and Hall mobility were increased. The resistivity of these layered films was lower than that of single layered AZO films. The related mechanisms and plasma physics were discussed. Copper indium gallium selenide (CIGS) thin film solar cells were fabricated by incorporating bi-layer ZnO films on CdS/CIGS/Mo/glass substrates. Efficiencies of the order of 7–8% were achieved for the manufactured CIGS solar cells (4–5 cm² in size) without antireflective films. The results demonstrated that RF sputtered layered AZO/i-ZnO films are suitable for application in low cost CIGS solar cells as transparent conductive electrodes.     

Quality enhancement of AZO thin films at various thicknesses by introducing ITO buffer layer

Due to the simultaneously superior optical transmittance and low electrical resistivity, transparent conductive electrodes play a significant role in semiconductor electronics. To enhance the electrical properties of these films, one approach is thickness increment which degrades the optical properties. However, a preferred way to optimize both electrical and optical properties of these layers is to introduce a buffer layer. In this work, the effects of buffer layer and film thickness on the structural, electrical, optical and morphological properties of AZO thin films are investigated. Al-doped zinc oxide (AZO) is prepared at various thicknesses of 100 to 300 nm on the bare and 100 nm-thick indium tin oxide (ITO) coated glass substrates by radio frequency sputtering. Results demonstrate that by introducing ITO as a buffer layer, the average values of sheet resistance and strain within the film are decreased (about 76 and 3.3 times lower than films deposited on bare glasses), respectively. Furthermore, the average transmittance of ITO/AZO bilayer is improved nearly 10% regarding single AZO thin film. This indicates that bilayer thin films show better physical properties rather than conventional monolayer thin films. As the AZO film thickness increases, the interplanar spacing, d₍₀₀₂₎, strain within the film and compressive stress of the film in the hexagonal lattice, decreases indicating the higher yield of AZO crystal. Moreover, with the growth in film thickness, carrier concentration and optical band gap (E_g) of AZO film are increased from 4.62?×?10¹⁹ to 8.21?×?10¹⁹ cm^?3 and from 3.55 to 3.62 eV, respectively due to the Burstein-Moss (BM) effect. The refractive index of AZO thin film is obtained in the range of 2.24–2.26. With the presence of ITO buffer layer, the AZO thin film exhibits a resistivity as low as 6?×?10^?4 Ω cm, a sheet resistance of 15 Ω/sq and a high figure of merit (FOM) of 1.19?×?10⁴ (Ω cm)^?1 at a film thickness of 300 nm. As a result, the quality of AZO thin films deposited on ITO buffer layer is found to be superior regarding those grown on a bare glass substrate. This study has been performed over these two substrates because of their significant usage in the organic light emitting diodes and photovoltaic applications as an enhanced carrier injecting electrodes.     

透明导电AZO/Cu双层薄膜制备及其性能

采用直流磁控溅射方法在玻璃衬底上室温生长了AZO/Cu双层薄膜,Cu层厚度控制在9nm,研究了AZO层厚度对薄膜电学和光学性能的影响。当AZO层厚度为20～80nm时,AZO/Cu双层薄膜具有良好的综合光电性能,方块电阻为12～14Ω/sq,可见光平均透过率为70～75%,品质因子为2×10-3～5×10-3Ω-1。AZO/Cu双层薄膜可以观察到Cu（111）和ZnO（002）的XRD衍射峰。通过退火研究表明,AZO/Cu双层薄膜的光电性能可在400℃下保持稳定,具有良好的热稳定性。本研究制备的透明导电AZO/Cu双层薄膜具有室温制程、综合光电性能良好、结晶性能较好、稳定性高的优点,可以广泛应用于光电器件透明电极及镀膜玻璃等领域。     

Effect of sputtering pressure on the properties of ZnMgO: Ti transparent conducting thin films

ZnMgO: Ti transparent conducting thin films have been deposited on glass substrates by DC-magnetron sputtering, and the effects of sputtering pressure on their properties have been investigated. Electrical resistivity as low as 7.84*10⁻⁴Ω cm was achieved for ZnMgO: Ti thin film. All thin films provided high optical transparencies in the visible region. Thin films deposited at 6 Pa, 8 Pa and 10 Pa showed “blue shift”. These results have not only clarified the effects of the sputtering pressure on the properties of ZnMgO: Ti films, but also revealed the potential of ZnMgO: Ti films in solar cells application. Efficiency of solar cells may be improved by choosing ZnMgO: Ti thin film as transparent electrode.     

Study of optoelectronics and microstructures on the AZO/nano-layer metals/AZO sandwich structures

In this study, growth nano-layer metals (Al, Cu, Ag) and Al-doped ZnO (AZO) thin films are deposited on glass substrates as the transparent conducting oxides (TCOs) to form AZO/nano-layer metals/AZO sandwich structures. The conductivity properties of thin films are enhanced when the average transmittance over the wavelengths 400–800 nm is maintained at higher than 80 %. A radio frequency magnetron sputtering system is used to deposit the metal layers and AZO thin films of different thickness, to form AZO/Al/AZO (ALA), AZO/Cu/AZO (ACA) and AZO/Ag/AZO (AGA) structures. X-ray diffraction and field emission scanning electron microscopy are used to analyze the crystal orientation and structural characteristic. The optical transmission and resistivity are measured by UV–VIS–NIR spectroscopy and Hall effect measurement system, respectively. The results show that when the Ag thickness is maintained at approximately 9 nm, the TCOs thin film has the lowest resistivity of 8.9 × 10^?5 Ω-cm and the highest average transmittance of 81 % over the wavelengths 400–800 nm. The crystalline Ag nano-crystal structures are observed by high-resolution transmission electron microscopy. In addition, the best figure of merit for the AZO/Ag/AZO tri-layer film is 2.7 × 10^?2 (Ω^?1), which is much larger than that for other structures.     

Optimization of ZnO:Al/Ag/ZnO:Al structures for ultra-thin high-performance transparent conductive electrodes

Al-doped ZnO (AZO)/Ag/AZO multilayer coatings (50-70 nm thick) were grown at room temperature on glass substrates with different silver layer thickness, from 3 to 19 nm, by using radio frequency magnetron sputtering. Thermal stability of the compositional, optical and electrical properties of the AZO/Ag/AZO structures were investigated up to 400 °C and as a function of Ag film thickness. An AZO film as thin as 20 nm is an excellent barrier to Ag diffusion. The inclusion of 9.5 nm thin silver layer within the transparent conductive oxide (TCO) material leads to a maximum enhancement of the electro-optical characteristics. The excellent measured properties of low resistance, high transmittance in the visible spectral range and thermal stability allow these ultra-thin AZO/Ag/AZO structures to compete with the 1 μm thick TCO layer currently used in thin film solar cells.     

掺铝氧化锌(AZO)薄膜的应用及研究评述

掺铝氧化锌(AZO)薄膜具有较高的透过率,较小的电阻率等优良的光电性能,使其在太阳能电池电极及各种显示器中具有广阔的应用前景。本文概述了AZO薄膜的结构、光电性能的表征方法,重点介绍了单层AZO膜以及AZO基多层复合薄膜的设计理论基础、影响薄膜性能的因素以及研究现状,除此之外,还对AZO薄膜的应用情况进行了概述,并对AZO薄膜发展前景进行了展望。     

不同退火工艺对AZO膜性能的影响

掺铝氧化锌(AZO)薄膜由于其独特的光学和电学性能而倍受人们的青睐。本文采用射频磁控溅射技术,制备了综合性能优良的AZO薄膜,通过不同退火工艺处理,研究了其对AZO薄膜的组织结构、电学性能及光学性能的影响。氮气环境下的退火使得AZO膜出现了更为明显的蓝移现象,氢气环境下的退火提高了薄膜的导电性。     

溅射功率对AZO透明导电薄膜的影响

采用射频磁控溅射法在玻璃衬底上沉积了AZO透明导电薄膜,并用原子力显微镜观测了薄膜表面形貌,XRD测试了薄膜相结构和单色仪测试了薄膜透射率。结果表明,制备的薄膜具有高度c轴择优取向性,其表面平整,晶粒均匀致密。当溅射功率为180W、溅射气体流量为15sccm、基片温度为200℃时制得的薄膜方阻为10Ω/□,在可见光区平均透射率大于85%。     

Sol–gel and rf sputtered AZO thin films: analysis of oxidation kinetics in harsh environment

Al-doped ZnO (AZO) thin film, which possess the advantages of low cost, low sheet resistance and high transmittance, are one of the most promising candidates to replace indium tin oxide films as the transparent electrode. However, oxidation causes a substantial increase in the sheet resistance of AZO film after exposing in ambient and especially, damp heat environment. In this work, we compare structural, optical, electrical properties and environmental stability between films prepared by two different methods: sol–gel and rf sputtering. Experimental results indicate that the properties of film can be affected by different deposition method. From the X-ray diffraction analysis, all films have hexagonal wurtzite crystal structure with different preferable orientation in two different methods. Optical transmittance spectra of the AZO films exhibited transparency higher than about 80 % within the visible wavelength region and the optical band gap (E_g) of these films was increased in sputtered film, probably due to the increase of carrier concentration. The better environmental stability was found in AZO film prepared by sputtering method. Improved surface morphology and enhancement of crystal orientation (110) was considered for this improvement.     

绒面AZO干法制备及性能研究

利用射频溅射方法,制得AZO透明导电膜,并用离子束刻蚀制备绒面,得到绒面AZO透明导电膜。比较刻蚀前后光电性能及表面形貌,发现透过率稍有下降,在可见光波段透过率在80%以上;电阻率略有上升,但仍保持在10-3?·cm数量级,最低为2.91×10-3?·cm;刻蚀后薄膜表面形貌变化较大,大多数薄膜表面呈现"坑状"结构,横向尺寸在0.5?1.0μm,开口角在120°左右,表面粗糙度从7.29nm上升到36.64nm。薄膜具有较好的表面微结构,在作太阳能电池前电极方面有较好的应用前景。     

Preparation of Al-doped ZnO thin films as transparent conductive substrate in dye-sensitized solar cell

We have prepared aluminum-doped Zinc oxide (AZO) thin films on glass substrates by rf magnetron sputtering technique using ZnO ceramic target in pure argon gas with different aluminum concentrations. The bandgap of the ZnO films slightly widens with increase in Al content and the lowest sheet resistance of AZO films with Al concentration of 4.3 at.% was obtained. The effects of post-annealing treatment on structural, electrical and optical properties of the AZO thin films were investigated. Using AZO thin film with 4.3 at.% Al as the transparent substrate, a titanium dioxide based dye-sensitized solar cell was constructed and a solar to electrical energy conversion efficiency of 2.9% was achieved under AM 1.5 solar simulated sunlight.     

Studies on aluminum-doped ZnO films for transparent electrode and antireflection coating of β-FeSi2 optoelectronic devices

β-FeSi₂ can be used for various optoelectronic devices owing to its superior material features including high optical absorption coefficient and direct band gap of about 0.8 eV. Due to its high refractive index (>5.6), however, suitable antireflection coating (ARC) is necessary for practical device applications. In order to increase the effective areas of optoelectronic devices, transparent electrodes should be also developed. In this work, Al-doped ZnO (AZO) films were fabricated by sputtering on β-FeSi₂ thin films and were found suitable for both transparent electrodes and ARC films. Choosing optimum substrate temperature and sputtering rate, high quality AZO films were formed. The conductivity of AZO films was as high as 3×10³ S/cm and ohmic contact was easily achieved between AZO and β-FeSi₂ films, indicating AZO film as an ideal transparent electrode for β-FeSi₂. The transmittance of 400-nm-thick AZO films was >80% and >70% in the wavelength ranges 400-1400 and 1400-1600 nm, respectively. By changing the thickness of AZO film, the central wavelength of minimum reflectance was adjusted to 1550 nm where the total reflectance of AZO/β-FeSi₂/Si structure was reduced below 2%.     

Effects of sputtering pressure and Al buffer layer thickness on properties of AZO films grown by rf magnetron sputtering

Transparent and conductive Al-doped (2 wt.%) zinc oxide (AZO) films were deposited on inexpensive soda-lime glass substrates by using rf magnetron sputtering at room temperature. This study analyzed the effects of argon sputtering pressure, which varied in the range from 0.46 to 2.0 Pa, on the morphological, electrical and optical properties of AZO films. The only (0 0 2) diffraction peak of the film were observed at 2θ～34.45°, exhibiting that the AZO films had hexagonal ZnO wurtzite structure, and a preferred orientation with the c-axis perpendicular to the substrate. By applying a very thin aluminum buffer layer with the thickness of 2 nm, findings show that the electrical resistivity was 9.46 × 10⁻⁴ Ω-cm, and the average optical transmittance in the visible part of the spectra was approximately 81%. Furthermore, as for 10 nm thick buffer layer, the electrical resistivity was lower, but the transmittance was decreased.     

Improving efficiency of organic light-emitting diodes fabricated utilizing AZO/Ag/AZO multilayer electrode

Multilayer transparent electrode based on Al-doped zinc oxide (AZO)/Ag/Al-doped zinc oxide (AZO) was fabricated by sputtering, and a green organic light-emitting diode (OLED) device utilizing AZO/Ag/AZO as anode was fabricated. The AZO/Ag/AZO multilayer film exhibited superior square resistance and optical transmittance to those of commercial indium tin oxide (ITO). In comparison with the green OLEDs based on ITO and pure AZO anode, the green OLED based on AZO/Ag/AZO showed the highest light-emitting efficiency. The results indicate that AZO/Ag/AZO multilayer electrodes are a promising low-cost, low-toxic and low-temperature processing electrode scheme for OLED application.     

薄膜厚度和工作压强对室温制备AZO薄膜性能的影响

采用射频磁控溅射法在室温下、普通玻璃基片上制备了AZO透明导电薄膜。用X射线衍射仪、原子力显微镜、紫外-可见分光光度计和四探针测量了不同薄膜厚度和不同工作压强下所得样品的结构、电学和光学性能,结果表明,所制备的AZO薄膜均具有六角纤锌矿结构,沿c轴择优取向生长;在可见光范围内,薄膜平均透过率约为80%;随着薄膜厚度的增加和工作压强的降低,薄膜的电阻率呈下降趋势;得到的薄膜最低方块电阻为7.5Ω/□。     

Preparation of aluminum doped zinc oxide films and the study of their microstructure, electrical and optical properties

Aluminum doped zinc oxide (AZO) polycrystalline thin films were prepared by sol-gel dip-coating process on optical glass substrates. Zinc acetate solutions of 0.5 M in isopropanol stabilized by diethanolamine and doped with a concentrated solution of aluminum nitrate in ethanol were used. The content of aluminum in the sol was varied from 1 to 3 at.%. Crystalline ZnO thin films were obtained following an annealing process at temperatures between 300 °C and 500 °C for 1 h. The coatings have been characterized by X-ray diffraction, UV-Visible spectrophotometry, scanning electron microscopy, and electrical resistance measurement. The ZnO:Al thin films are transparent (∼ 90%) in near ultraviolet and visible regions. With the annealing temperature increasing from 300 °C to 500 °C, the film was oriented more preferentially along the (0 0 2) direction, the grain size of the film increased, the transmittance also became higher and the electrical resistivity decreased. The X-ray diffraction analysis revealed single-phase ZnO hexagonal wurtzite structure. The best conductors were obtained for the AZO films containing 1 at.% of Al, annealed at 500 °C, 780 nm film thickness.     

氢化及沉积温度对掺铝ZnO透明导电薄膜性能影响

采用射频磁控溅射方法在玻璃衬底上制备了掺铝ZnO透明导电薄膜(AZO)。为了降低AZO薄膜的电阻率, 采用在溅射气氛中通入一定比例H₂的方法对AZO薄膜进行氢化处理, 并研究了溅射气氛中H₂含量及衬底温度对AZO薄膜氢化效果的影响。结果表明: 在低温条件下, 氢化处理能有效降低AZO薄膜的电阻率; 在衬底温度为100℃的低温条件下, 通过调节溅射气氛中H₂的比例, 制备了电阻率为6.0×10^-4 Ω·cm的高质量氢化AZO薄膜, 该电阻值低于同等条件下未氢化AZO薄膜电阻值的1/3; 但随着衬底温度的升高, 氢化处理对薄膜电学性能的改善效果逐渐减弱。     

AZO透明导电薄膜的制备技术及应用进展

概述了国内外AZO透明导电薄膜的多种制备技术和开发应用进展。详细介绍了磁控溅射、溶胶-凝胶、脉冲激光沉积、真空蒸镀、化学气相沉积等工艺在AZO薄膜制备中的研究现状,并且在对AZO膜与ITO膜性能比较的基础上,指出AZO薄膜的产业化前景好。     

AZO薄膜用于GaN基LED透明电极的性能研究

采用脉冲激光沉积法制备了Al掺杂ZnO(AZO)薄膜, 研究了不同沉积氧压下薄膜的光电性能。当沉积压强为0.1 Pa时, AZO薄膜光电性能最优。将该薄膜用于GaN基LED透明电极作为电流扩展层, 在20 mA正向电流下观察到了520 nm处很强的芯片发光峰, 但芯片工作电压较高, 约为10 V, 芯片亮度随正向电流的增大而增强。二次离子质谱测试表明, AZO薄膜与GaN层界面处两种材料导电性能的变化以及钝化层的形成是导致芯片工作电压偏高的原因。