Structural, electrical and optical properties of ZnO:Al films deposited on flexible organic substrates for solar cell applications

Aluminum doped ZnO thin films (ZnO:Al) were deposited on glass and poly carbonate (PC) substrate by r.f. magnetron sputtering. In addition, the electrical, optical properties of the films prepared at various sputtering powers were investigated. The XRD measurements revealed that all of the obtained films were polycrystalline with the hexagonal structure and had a preferred orientation with the c-axis perpendicular to the substrate. The ZnO:Al films were increasingly dark gray colored as the sputter power increased, resulting in the loss of transmittance. High quality films with the resistivity as low as 9.7 × 10^− 4 Ω-cm and transmittance over 90% have been obtained by suitably controlling the r.f. power.     

Study of in situ CdCl2 treatment on CSS deposited CdTe films and CdS/CdTe solar cells

Effect of in situ CdCl₂ treatment on the morphological, structural and electrical properties of CdTe films as well as on solar cell characteristics of CdS/CdTe junction has been investigated. XRD measurements show that the presence of CdCl₂ vapours induces 111 oriented growth in the CdTe films. CdCl₂ concentration required for this oriented growth is found to be directly proportional to the substrate temperature. SEM measurements show enhanced grain growth in the presence of CdCl₂. Spectral response of the CdCl₂ treated CdS/CdTe solar cells shows an enhanced CdS diffusion in to the CdTe, which results in an improved spectral response in UV range and a consequent reduction in the interface states density. A drastic reduction in the deep levels due to the CdCl₂ treatment, as seen in the photo-capacitance studies, has results in CdS/CdTe solar cells having efficiency >8%.     

The use of aluminium doped ZnO as transparent conductive oxide for CdS/CdTe solar cells

CdTe/CdS and CdTe/ZnO thin film solar cells were grown with a high vacuum evaporation based low temperature process (≤ 420 °C). Aluminium doped zinc oxide (AZO) was used as transparent conducting oxide (TCO) material. AZO exhibited excellent stability during the solar cell processing, and no significant change in electrical conductivity or transparency was observed. The current density loss due to absorption in the 1 μm thick AZO layer with 5 Ω per square sheet resistance was found to be 1.2 mA/cm². We investigated the influence of an intrinsic ZnO layer (i:ZnO) in combination with various CdS thicknesses. The i:ZnO layer was found to significantly increase the open circuit voltage of the solar cells with very thin CdS layer. Increasing thickness of the i:ZnO layer leads to UV absorption losses, narrowing of the depletion layer width and hence reduced collection efficiency in the long wavelength (685-830 nm) part. With AZO/i:ZnO bi-layer TCO we could achieve cell efficiencies of 15.6% on glass and 12.4% on the flexible polyimide film.     

Rapid thermal-plasma annealing of ZnO:Al films for silicon thin-film solar cells

Rapid thermal annealing of sputter-deposited ZnO and Al-doped ZnO (AZO) films with and without an amorphous silicon (a-Si) capping layer was investigated using a radio-frequency (rf) argon thermal plasma jet at atmospheric pressure. The resistivity of bare ZnO films on glass decreased drastically from 10⁶ to 10³ Ω·cm at maximum surface temperatures T_max above 650 °C, whereas the resistivity increased from 10^− 4 to 10^− 3-10^− 2 Ω·cm for bare AZO films. On the other hand, the resistivity of AZO films with a 30-nm-thick a-Si capping layer remained below 10^− 4 Ω·cm, even after TPJ annealing at a T_max of 825 °C. X-ray diffraction and X-ray photoemission electron studies revealed that the film crystallization of both AZO and a-Si layers was promoted without the formation of an intermixing layer. Additionally, the crystallization of phosphorous- and boron-doped a-Si layers at the sample surface was promoted, compared to that of intrinsic a-Si under identical plasma annealing conditions. The role of the a-Si capping layer on sputter-deposited AZO and ZnO films during TPJ annealing is demonstrated. The effects of the mixing of phosphorous and boron impurities in a-Si:H during TPJ annealing of flat and textured AZOs are also discussed.     

Influence of working pressure on ZnO:Al films from tube targets for silicon thin film solar cells

Mid-frequency magnetron sputtering of aluminum doped zinc oxide films (ZnO:Al) from tube ceramic targets has been investigated for silicon based thin film solar cell applications. The influence of working pressure on structural, electrical, and optical properties of sputtered ZnO:Al films was studied. ZnO:Al thin films with a minimum resistivity of 3.4 × 10⁻⁴ Ω cm, high mobility of 50 cm²/Vs, and high optical transmission close to 90% in visible spectrum region were achieved. The surface texture of ZnO:Al films after a chemical etching step was investigated. A gradual increase in feature sizes (diameter and depth) was observed with increasing sputter pressure. Silicon based thin film solar cells were prepared using the etched ZnO:Al films as front contacts. Energy conversion efficiencies of up to 10.2% were obtained for amorphous/microcrystalline silicon tandem solar cells.     

磁控溅射法制备的CdS:Al薄膜的性质研究

采用Al和CdS双靶共溅射的方法, 调控Al和CdS源的沉积速率, 制备出不同Al掺杂浓度的CdS:Al薄膜。通过XRD、SEM、AFM、紫外-可见透射光谱分析、常温霍尔测试对CdS: Al薄膜的结构、形貌、光学和电学性质进行表征。XRD结果表明, 不同Al掺杂浓度的CdS:Al薄膜均为六方纤锌矿结构的多晶薄膜, 并且在(002)方向择优生长。SEM和AFM结果表明, CdS:Al薄膜的表面均匀致密, 表面粗糙度随着Al掺杂浓度的增加略有增加。紫外-可见透射光谱分析表明, CdS:Al薄膜禁带宽度在2.42~2.46 eV 之间, 随着Al掺杂浓度的增加而略微减小。常温霍尔测试结果证明, 掺Al对CdS薄膜的电学性质影响显著, 掺Al原子浓度3.8%以上的CdS薄膜, 载流子浓度增加了3个数量级, 电阻率下降了3个数量级。掺Al后的CdS薄膜n型更强, 有利于与CdTe形成更强的内建场, 从而提高太阳电池效率。用溅射方法制备的CdS:Al薄膜的性质适合用作CdTe薄膜太阳电池的窗口层。     

Sputtering of ZnO:Al films from dual tube targets with tilted magnetrons

Aluminum doped zinc oxide (ZnO:Al) films were deposited by mid-frequency sputtering rotating tube targets at high discharge powers in a double cathode system. The magnetrons located inside the tube targets were tilted by ± 30°, leading to different racetrack orientations. Deposition rate and electrical properties of statically deposited films were investigated. Different properties of ZnO:Al films show lateral variations corresponding to the racetrack positions, which shift according to the tilt angles of double magnetrons. The highest average static deposition rate and the corresponding dynamic value were up to 360 nm/min and 111 nm m/min, respectively, for magnetrons tilted towards the center of the cathodes. The material properties of the ZnO:Al film prepared in dynamic mode were found to behave like the superpositions of properties of static films at different positions. Upon wet chemical etching in diluted hydrochloric acid (HCl), the surfaces of sputtered ZnO:Al films became rough, and three typical surface structures were observed and identified on statically deposited ZnO:Al films. The related plasma physics, growth and chemical etching mechanisms were discussed.     

Al掺杂量对ZnO:Al薄膜微观结构和光电性能的影响

采用溶胶-凝胶法在玻璃基片上制备出不同Al掺杂量的ZnO:Al(ZAO)薄膜.系统研究了Al掺杂量对薄膜微结构和光电性能的影响.结果表明:溶胶-凝胶法制备的薄膜具有完好C轴择优取向,在可见光区的透射率均大于85%;随着Al掺杂量的增加,薄膜的平均颗粒尺寸减小,表面电阻率先降低后升高,薄膜的光学带隙宽变宽.在5%H2 95%N2气氛退火可显著降低薄膜电阻率,掺Al量为2%的薄膜具有最低电阻率5.5×10-3Ω·Cm.     

退火处理对N掺杂Al:ZnO薄膜结构及性能的影响

采用射频磁控溅射方法, 常温条件下以N₂作为N掺杂源, 在玻璃基底制备了N掺杂Al:ZnO薄膜。在真空氛围下对样品进行了不同温度的退火处理15 min。通过X射线衍射、霍尔效应测试、紫外-可见光谱和X射线光电子能谱 (XPS) 仪分析了退火对样品结构和光电性能的影响。结果表明真空400℃退火15 min时成功制备出性能优异的p型ZnO薄膜, 其空穴载流子浓度为3.738×10²⁰cm^-3, 电阻率为1.299×10^-2Ω·cm, 样品可见光透射率达到了85%以上。XPS分析说明No受主缺陷的含量大于 (N₂)_o施主缺陷导致薄膜实现了p型转变。     

Studies on the effect of hydrogen doping during deposition of Al:ZnO films using RF magnetron sputtering

Aluminum doped ZnO (ZnO:Al) films were deposited using rf magnetron sputtering in the presence of hydrogen gas in the chamber. A comparative study of the films deposited with and without hydrogen was performed. The XPS studies indicated that the decrease in resistivity of ZnO:Al films with the introduction of hydrogen gas is attributed to the reduced adsorption of oxygen species in the film grain boundaries. The average percentage transmission in the visible region of the films was around 92–95% and band gap was found to be about in the range of 3.15–3.17 eV. The lowest resistivity of 1.8 × 10⁻⁴ Ω cm was achieved for the ZnO:Al film deposited with hydrogen.     

利用脉冲直流磁控溅射制备ZnO:Al薄膜的研究

利用中频脉冲直流磁控溅射法制备了平面ZnO:Al(AZO)透明导电薄膜,研究了沉积压力、衬底温度和溅射功率对AZO薄膜光电性能、薄膜稳定性的影响.结果表明:在较低沉积压力、衬底温度及溅射功率下,可获得具有低电阻率、高透过率、高稳定性的AZO薄膜.     

Comparison of ZnO:Al,ITO and carbon nanotube transparent conductive layers in flexible solar cells applications

In the paper the mechanical, optical and electrical parameters of transparent conductive layers (TCLs) made of carbon nanotubes and metal conductive oxides are explored and compared. All investigated materials are deposited on transparent, flexible polymer foils used for solar cell applications. Obtained results are compared with available parameters of rigid transparent conductive oxides (TCOs) as well as literature reports about Indium–Tin Oxide (ITO) on flexible substrates. Presented paper is a report from the preliminary stage of a new flexible solar cell construction.     

ZnO:Al透明导电薄膜的研制

介绍用直流平面磁控溅射方法制备掺铝的氧化锌透明导电薄膜并研究了其特性，阐述了金属氧化物透明导电薄膜研究的发展情况及其应用前景，并讨论了氧化锌掺铝薄膜的优点。介绍了ZnO∶Al薄膜的制备情况：靶的制备及薄膜的制备过程。测量了薄膜的光电特性，包括透射比、折射率、消光系数、方块电阻、电阻率、载流子浓度和迁移率等参数，并分析了各种实验条件对薄膜性能的影响。     

A detailed study of the series resistance effect on CdS/CdTe solar cells with Cu/Mo back contact

CdS/CdTe thin film solar cells with an area of 1 cm² were obtained and studied in detail. A ZnO buffer layer was deposited by reactive RF-sputtering on commercial ITO substrates. The CdS layer was grown on ZnO also by using RF-sputtering and CdTe thin film was deposited by conventional CSS technique. The chlorination of the solar cells is performed into Freon atmosphere at 400 °C. The CdTe thin film surface was chemically etched by using Br-Methanol solution. The back contact was deposited using RF-sputtering from a pure Cu and Mo targets. The procedure developed in this work led us to make systematically solar cells with good efficiency. However, the series resistance has a high value for an area of 1 cm² (22 Ω cm²). In order to make more detailed study, the solar cell with an area of 1 cm² was divided in a 3 × 3 matrix. A good homogeneity in cell properties is observed and the efficiency increases to more than 11%, fundamentally through decreasing series resistance.     

Nitrogen ion irradiation effect on enhancing photocatalytic performance of CdTe/ZnO heterostructures

To deal with the increasingly deteriorating environment problems, more and more harsh requirements are put forward for photocatalysis application. Building semiconductor heterostructures has been proven to be an efficient way to enhance photocatalytic performance. A kind of CdTe/ZnO heterostructures were synthesized by a hydrothermal and successive ionic layer absorption and reaction (SILAR) method and achieved obviously efficient photocatalytic performance. Moreover, after the N ion irradiation treatment, the photocatalytic activity was further enhanced, which can be ascribed to the introduction of oxygen vacancy defects. The photocatalytic performance enhancement mechanism by coupling constructing heterostructures and ion irradiation are further studied to give us an overall understanding on ZnO nanowires.     

Conductive polymer PEDOT:PSS back contact for CdTe solar cell

Two types of superstrate glass/ITO/CdS/CdTe PV structures were prepared by high vacuum evaporation technique with (i) activation of CdS layer and CdS/CdTe bi-layer structure step-by-step and (ii) activation of CdS/CdTe bi-layer structure. The activation was performed by annealing the structures with CdCl₂ in air at 400 °C for 15 min. Main conditions for CdS and CdTe thin films deposition and following treatment were selected from the literature data with the purpose to prepare and compare complete CdTe solar cells with standard p + Cu_xTe back contact and conductive polymer poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid (PEDOT:PSS) back contact. Obtained layers and structures were characterized using the XRD, SEM and I-V methods. Both the methods of activation treatment give comparable results from the point of view PV properties of complete solar cells. It was found that highly conductive PEDOT:PSS intermediate layer can significantly improve the back contact characteristics of CdTe. However these hybrid structures need to be further optimized to compete successfully with conventional inorganic back contacts in complete CdTe solar cells.     

Reactive sputtering of ZnO/ZnO:Al contacts for chalcopyrite solar modules

Reactive sputtering is an option to further reduce costs associated with the deposition of the transparent front contact for chalcopyrite-based solar modules. Our approach here is to develop a proof of concept for a ZnO window, where the i-ZnO and the doped ZnO:Al are both sputtered in a reactive process. It is shown, that on cell level the device performance is preserved when replacing the standard RF processes with the fully reactive process. It is also shown, that the series resistance of module test structures increases with reactively sputtered ZnO. This finding, as well as a reduced damp heat stability of mini modules without encapsulation are tentatively assigned to an increased contact resistance and corrosion at the Molybdenum/ZnO interface within the interconnects.     

衬底温度和氢气退火对ZnO:Al薄膜性能的影响

采用射频磁控溅射法在石英玻璃衬底上制备了性能良好的透明导电ZnO:Al薄膜,并研究了衬底温度和氢气退火对薄膜结构和光电性能的影响。结果表明,衬底加热可以改善薄膜结晶质量和c轴择优取向,减小内应力,并提高其电学性能。经稀释氢气退火后,500℃沉积的薄膜电阻率由9.4×10-4Ω.cm减小到5.1×10-4Ω.cm,迁移率由16.4cm2.V-1.s-1增大到23.3 cm2.V-1.s-1,载流子浓度由4.1×1020cm-3提高到5.2×1020cm-3,薄膜的可见光区平均透射率仍达85%以上。禁带宽度随着衬底温度的升高和氢气退火而展宽。     

Optical properties of down-shifting barium borate glass for CdTe solar cells

CdTe thin film solar cells have a poor response in the ultraviolet and blue spectral range, mainly due to absorption and thermalization losses in the CdS buffer layer. To overcome this efficiency drop in the short wavelength range trivalent rare-earth doped barium borate glass is investigated for its potential as frequency down-shifting cover glass on top of the cell. The glass is doped with either Tb³⁺ or Eu³⁺ up to a level of 2.5 at.% leading to strong absorption in the ultraviolet/blue spectral range. Tb³⁺ shows intense emission bands in the green spectral range while Eu³⁺ emits in the orange/red spectral range. Based on rare-earth absorption and luminescence quantum efficiency the possible gain in short-circuit current density is calculated.     

Etching process optimization using NH4Cl aqueous solution to texture ZnO:Al films for efficient light trapping in flexible thin film solar cells

0.5 μm-thick aluminum-doped zinc oxide (ZnO:Al) films were deposited at 100 °C on polyethylene terephthalate substrates by Radio Frequency magnetron sputtering. The as-deposited films were compact and dense, showing grain sizes of 32.0 ± 6.4 nm and resistivities of (8.5 ± 0.7) × 10^− 4 Ω cm. The average transmittance in the visible wavelength range of the structure ZnO:Al/PET was around 77%. The capability of a novel two-step chemical etching using diluted NH₄Cl aqueous solution to achieve efficient textured surfaces for light trapping was analyzed. The results indicated that both the aqueous solution and the etching method resulted appropriated to obtain etched surfaces with a surface roughness of 32 ± 5 nm, haze factors at 500 nm of 9% and light scattering at angles up to 50°. To validate all these results, a commercially ITO coated PET substrate was used for comparison.