Effects of ZnO buffer layer on the optoelectronic performances of GZO films

Gallium-doped zinc oxide (ZnO:Ga=97:3 wt%, GZO) transparent conducting films have been deposited on glass substrates (Corning 1737F), with and without ZnO buffer layers by radio-frequency (r.f.) magnetron sputtering. The effect of ZnO buffer layer deposition parameters on electrical, structural, morphological and optical properties of GZO films (GZO/ZnO/glass) was investigated. The optimization of coating process parameters (r.f. power, sputtering pressure, thickness, annealing) on ZnO buffer layer with multiple qualities based on the orthogonal array has been studied. The electrical resistivity and the average transmittance of the GZO/ZnO/glass films were improved by annealing in vacuum ambient of the ZnO buffer layer. Findings based on the grey relational analysis show that the lowest electrical resistivity of GZO/ZnO/glass films to be about 9.45×10⁻⁴ Ω cm, and visible range transmittance about 85%.     

Preparation of conducting and transparent indium-doped ZnO thin films by chemical spray

Transparent conducting indium-doped zinc oxide thin films were prepared on soda-lime glass substrates by the spray pyrolysis technique. The dependence of the electrical, structural, morphological and optical properties on the preparation conditions has been studied. Two main variables, substrate temperature and molar concentration, were varied in the ranges of 425–525 °C and 0.05–0.5 M, respectively, in order to obtain films with low electrical resistivity and high optical transparency in the visible region. A minimum resistivity value around of 3×10⁻³ Ω cm was obtained for films deposited from highly concentrated starting solutions, 0.4 and 0.5 M. The values of the free-carrier concentration and the electronic mobility were estimated by Hall effect measurements. X-ray diffraction spectra evidenced a preferential orientation along the (1 0 1) direction. The surface morphology was clearly affected with the molar concentration variation, leading to a smoother appearance as the zinc concentration in the starting solution is increased. Typical optical transmittance values in the order of 85% were obtained in all the films.     

Gallium-doped ZnO thin films deposited by chemical spray

Gallium-doped zinc oxide (ZnO:Ga) thin films were deposited on glass substrates by the spray pyrolysis technique. The effect of the variation of the [Ga]/[Zn] rate in the starting solution, the substrate temperature as well as the post-annealing treatments on the physical properties was examined. The electrical properties of the films show an improvement with the Ga incorporation and the annealing treatment. All the films were found to be polycrystalline and show a (0 0 2) preferential growth, irrespective of the deposition conditions. The films were of n-type conductivity with an electrical resistivity in the order of 8×10⁻³ Ω cm and optical transmittance higher than 80% in the visible region. These results makes chemically sprayed ZnO:Ga potentially applicable as transparent electrode in photovoltaic devices.     

Physical properties of ZnO:F obtained from a fresh and aged solution of zinc acetate and zinc acetylacetonate

Fluorine-doped zinc oxide thin films, ZnO:F, were deposited by the spray pyrolysis technique on sodocalcic glass substrates. Two different zinc precursors were used separately, namely, zinc acetate and zinc pentanedionate. The effect of the zinc precursor type, the aging of the starting solution, the substrate temperature and a vacuum-annealing treatment on the electrical, morphological, structural and optical properties was studied, in order to obtain conductive and transparent ZnO:F thin films. The resistivity values of ZnO:F thin films deposited from aged solutions were lower than those films obtained from fresh solutions. The lowest resistivity values of as-grown films deposited at 500 °C, using a two-day aged starting solution of zinc acetate and zinc pentanedionate, were 1.4×10⁻² and 1.8×10⁻² Ω cm, respectively. After a vacuum annealing treatment performed at 400 °C for 30 min a decrease in the resistivity was obtained, reaching a minimum value of 6.5×10⁻³ Ω cm for films deposited from an aged solution of zinc acetate. The films were polycrystalline, with a (0 0 2) preferential growth orientation in all the cases. Micrographs obtained by SEM show a uniform surface covers by rounded grains. No evident change in the surface morphology was observed with the different precursors used. The transmittance of films in the visible region was higher than 80%.     

RF magnetron sputtered ZnO:Al thin films on glass substrates: A study of damp heat stability on their optical and electrical properties

ZnO:Al thin films were deposited on glass substrates by RF magnetron sputtering from a powder compacted ceramic target. Structural, electrical and optical properties of the films with different thickness were characterized. The damp heat stability of ZnO:Al thin film was investigated for its application in thin-film solar cells. After the 1000 h damp heat treatment in harsh conditions of 85% relative humidity at 85 °C for all samples, a degradation of electrical properties was observed, while the transmissions of the films were almost unchanged. Thick films with a relative large grain size could form compact structure to resist the corrosion by oxygen and water molecules.     

High quality textured ZnO:Al surfaces obtained by a two-step wet-chemical etching method for applications in thin film silicon solar cells

ZnO:Al films deposited at 250 °C on Corning glass by radio frequency magnetron sputtering were studied for their use as front contact for thin film silicon solar cells. For this purpose, a two-step etching method combining different concentrations of diluted hydrochloric acid (from 0.1% to 3%) with different etching times was developed. Its influence on morphological, electrical and optical properties of the etched films was evaluated. This new etching method led to more uniform textured surfaces, where the electrical properties remained unchangeable after the etching process, and with adapted light scattering properties similar to those exhibited by commercial substrates.     

RF sputter deposition of the high-quality intrinsic and n-type ZnO window layers for Cu(In,Ga)Se2-based solar cell applications

Transparent ZnO films were prepared by rf magnetron sputtering, and their electrical, optical, and structural properties were investigated under various sputtering conditions. Aluminum-doped n-type(n-ZnO) and undoped intrinsic-ZnO (i-ZnO) layers were deposited on a glass substrate by incorporating different targets in the same reaction chamber. The n-ZnO films were strongly affected by argon ambient pressure and substrate temperature, and films deposited at 2 mTorr and 100°C showed superior properties in resistivity, transmission, and figure of merit (FOM). The sheet resistance of ZnO film was less dependent on film thickness when the substrate was heated during deposition. These positive effects of elevated substrate temperature are presumably attributed to the rearrangement of the sputtered atoms by the heat energy. Also, the films are electrically uniform through the 5 cm×5 cm substrate. The maximum deviation in sheet resistance is less than 10%. All of the films showed strong (0 0 2) diffraction peak near 2θ =34°. The undoped i-ZnO films deposited in the mixture of argon and oxygen gases showed high transmission properties in the visible range, independent of the Ar/O₂ ratio, while resistivity rose with increased oxygen partial pressure. The Cu(In,Ga)Se₂ solar cells, incorporating bi-layer ZnO films (n-ZnO/i-ZnO) as window layer, were finally fabricated. The fabricated solar cells showed 14.48% solar efficiency under AM 1.5 conditions (100 mW/cm²).     

Chemically sprayed ZnO:F thin films deposited from diluted solutions: Effect of the time of aging on physical characteristics

Transparent and conductive fluorine-doped zinc oxide (ZnO:F) thin films were deposited on glass substrates by the chemical-spray technique starting from a diluted solution of zinc acetate and hydrofluoric acid. The effect of the aging time of the starting solution on the electrical, structural, morphological and optical characteristics of ZnO:F thin films was observed and analyzed. The resistivity of the ZnO:F thin films decreases as a more aged solution is used, reaching a saturation value of 6×10⁻² Ω cm. X-ray diffraction reveals that the films are polycrystalline in nature with a (1 0 0) preferential growth in almost all the cases. High-resolution scanning electron microscopy clearly reveals that the films are composed of nanoparticles of spherical shape, whose average diameter is in the order of 15 nm that matches well with the crystallite size calculated from X-ray diffraction. This result shows that fluorine incorporation effectively inhibits grain growth. This, in turn, produces a porous structure. Also, the increase in the time of aging enhances slightly the transmittance of the films.     

Effect of the substrate temperature and acidity of the spray solution on the physical properties of F-doped ZnO thin films deposited by chemical spray

F-doped ZnO thin films were prepared by using the spray pyrolysis technique. The dependence of the electrical, optical, structural and morphological properties on the substrate temperature and spray solution acidity was studied. Additionally, aging of the spray solution presents a clear effect on the resistivity of ZnO thin films. The best films obtained show a resistivity, mobility and carrier concentration of the order of 1.5×10⁻² Ω cm, 6 cm²/V s and 2×10¹⁹ cm⁻³, respectively. Wurtzite hexagonal structure, with a preferential growth along the [0 0 2] direction for all substrate temperatures and acidities used, was obtained. From scanning electron microscopy and atomic force microscopy analysis, it was determined that the grain size of the films decreases and its homogeneity increases when the acidity of the starting solution is increased. High optical transmittances, in the order of 90%, were obtained in all the cases.     

Effect of the pH on the physical properties of ZnO : In thin films deposited by spray pyrolysis

The optical, electrical and structural properties of polycrystalline ZnO : In thin films grown from starting solutions with different pH values are reported. The films were deposited on glass substrates by spray pyrolysis at temperatures below 500°C. Zinc acetate dissolved in an aqueous solution was used as a precursor. Different amounts of acetic acid were added to change the pH of the solution to two different values. The average optical transmission of the films was above 85% and the film thickness was about 0.65 μm. The resistivity of the films decreased as the pH of the solution decreased. Concentration and mobility values, measured from Hall effect, were slightly lower for pH=3.2 grown films. pH=3.8 grown films showed textured surfaces with the texture changing from rounded to rough and angular uniform grains as the substrate temperature increased. pH=3.2 grown films had smooth surfaces. X-ray diffraction spectra showed a 0 0 2 preferential orientation for pH=3.2 grown films, and 0 0 2 and 1 0 1 orientations for pH=3.8 grown films. These results have been discussed on the basis of the chemical species involved in the solution. A two-dimensional predominance zone diagram was constructed in order to define the thermodynamic distribution of chemical species in both systems.     

Thickness dependence of ZnO:In thin films doped with different indium compounds and deposited by chemical spray

Indium-doped zinc oxide thin films were deposited on glass substrates by the chemical spray technique. Hydrated zinc 2,4-pentanedionate was used as zinc source. Four different indium compounds were separately used as dopants (indium nitrate, indium sulfate, indium acetate, and indium chloride). The effect of the thickness on the electrical, structural, morphological and optical characteristics of ZnO:In thin films was studied. Electrical resistivity values corresponding to good conductive electrodes were obtained irrespective of the indium compound used, although a decrease in electrical resistivity is found in all the cases as the film thickness increases, reaching a saturation value of 2×10⁻³ Ω cm. All films were polycrystalline with a wurtzite phase, and exhibited a (1 0 1) preferential orientation almost irrespective of neither the doping source nor thickness. The surface morphology was analyzed by scanning electron microscopy and a strong dependence on the indium compound and thickness was found, since grain geometry variations from rounded to rod-like forms were observed. As the film thickness increases, the film transmittance and the band-gap values decrease. Band-gap energy values were in the range of 3.21 to 3.44 eV.     

退火气氛对Al-F共掺杂ZnO薄膜结构和光电性能的影响

用射频磁控溅射法制备了Al-F共掺杂ZnO（ZnO（Al,F））透明导电薄膜,研究了不同退火气氛对ZnO（Al,F）薄膜的结构、电学和光学特性的影响。结果表明:在真空和还原性气氛中退火后的薄膜透光率呈现"蓝移"趋势,在空气中退火处理后的薄膜透光率则表现为"红移";在真空中,400℃×60min的退火处理,使ZnO（Al,F）薄膜的电阻率降低至1.41×10^-3Ω·cm,透光率则上升到93%以上,有效提高了薄膜的光电特性;所有退火气氛下,薄膜均具有（002）单一择优取向的多晶六方纤锌矿结构;薄膜的晶粒尺寸为25～30nm。     

Solid-phase crystallization of amorphous silicon on ZnO:Al for thin-film solar cells

The suitability of ZnO:Al thin films for polycrystalline silicon (poly-Si) thin-film solar cell fabrication was investigated. The electrical and optical properties of 700 -nm-thick ZnO:Al films on glass were analyzed after typical annealing steps occurring during poly-Si film preparation. If the ZnO:Al layer is covered by a 30 nm thin silicon film, the initial sheet resistance of ZnO:Al drops from 4.2 to 2.2 Ω after 22 h annealing at 600 °C and only slightly increases for a 200 s heat treatment at 900 °C. A thin-film solar cell concept consisting of poly-Si films on ZnO:Al coated glass is introduced. First solar cell results will be presented using absorber layers either prepared by solid-phase crystallization (SPC) or by direct deposition at 600 °C.     

The structural and electrical properties of Ga-doped ZnO and Ga, B-codoped ZnO thin films: The effects of additional boron impurity

Transparent conducting films of Ga-doped ZnO (GZO) and Ga-, B-codoped ZnO (GZOB) were deposited by dc magnetron sputtering. The dependence of the electrical and structural properties on the type of doping (Ga-doping or Ga-, B-codoping) and substrate temperature were investigated. Microstructural analysis suggests that the substrate temperature and the type of doping modify the microstructure and surface morphology of thin films. GZOB films grown at 200 °C showed a dense structure without columns, a low-resistivity value of 4.2×10⁻⁴ Ω cm, and a visible transmission of 90% with a thickness of 200 nm. In addition, the thermal stability of resistivity of GZOB films was greater than one of GZO films.     

Effects of substrate temperature on the properties of facing-target sputtered Al-doped ZnO films

ZnO:Al films (Al 2.5 wt%) were deposited using a DC facing targets magnetron sputtering via two ZnO targets mixed with Al₂O₃. The structural, electrical and optical properties of the deposited films were strongly influenced by substrate temperature. Films with better texture, higher transmission, lower resistivity and larger carrier concentration were obtained for the samples fabricated at higher substrate temperature. The optimal condition for deposition of ZnO:Al film with the lowest resistivity of 3.18×10⁻⁴ Ω cm, the highest carrier concentration of 4.58×10²⁰ cm⁻³, and a transmission toward 85% in the visible range was obtained at 200 °C. This film proposes a promising future for the application of the practical window and contact layers for solar cells.     

Deposition of transparent and conductive Al-doped ZnO thin films for photovoltaic solar cells

The effect of the substrate temperature on the optoelectronic properties of ZnO-based thin films prepared by rf magnetron sputtering has been studied. Three different targets (Zn/Al 98/2 at%, ZnO:Al 98/2 at% and ZnO:Al₂O₃ 98/2 wt%) have been investigated in order to compare resulting samples and try to reduce the substrate temperature down to room temperature. From the ZnO:Al₂O₃ target, transparent conductive zinc oxide has been obtained at 25°C with the average optical transmission in the 400–800 nm wavelength range, T = 80–90% and resistivity, = 3−5 × 10⁻³ Ωcm. In Al:Zn0 layers, the spatial distribution of the electrical properties across the substrate placed parallel to the target has been improved by depositing at high substrate temperatures, above 200°C. Besides, owing to diffusion processes of CuInSe₂ and CdS take place at 200°C, an AI:ZnO/CdS/CuInSe₂ polycrystalline solar cell made with the Al:ZnO deposited at 25°C as the transparent conductive oxide, has shown a more efficient photovoltaic response, η = 6.8%, than the one measured when the aluminium-doped zinc oxide has been prepared at 200°C, η = 1.8%.     

Structural and optical properties of textured ZnO:Al films on glass substrates prepared by in-line rf magnetron sputtering

Textured ZnO:Al films with excellent light scattering properties as a front electrode of silicon thin film solar cells were prepared on glass substrates by an in-line rf magnetron sputtering, followed by a wet-etching process to modify the surface morphologies of the films. Deposition parameters and wet etching conditions of the films were controlled precisely to obtain the optimized surface features. All as-deposited films show a strong preferred orientation in the [0 0 1] direction under our experimental conditions. The microstructure of the films was significantly affected by working pressure and film compactness was reduced with increasing working pressure, while the effect of a substrate temperature on the microstructure is less pronounced. A low resistivity of 4.25×10⁻⁴ Ω cm and high optical transmittance of above 80% in a visible range were obtained in the films deposited at 1.5 mTorr and 100 °C. After wet etching process, the surface morphologies of the films were changed dramatically depending on the microstructure and film compactness of the initial films. By controlling the surface feature, the haze factor and angular resolved distribution of the textured ZnO:Al films were improved remarkably when compared with commercial SnO₂:F films. The textured ZnO:Al and SnO₂:F films were applied as substrates for a silicon thin film solar cells with tandem structure of a-Si:H/μc-Si:H. Compared with the solar cells with the SnO₂:F films, a significant enhancement in the short-circuit current density of the μc-Si:H bottom cell was achieved, which is due to the improved light scattering on the highly textured ZnO:Al film surfaces in the long wavelength above 600 nm.     

Optical transmittance and photoconductivity studies on ZnO : Al thin films prepared by the sol–gel technique

Polycrystalline ZnO : Al thin films have been prepared by the (Sol–gel) chemical deposition method. The ZnO : Al thin films are very transparent (90%) in the near UV, VIS and IR regions. The films are oriented along the c-axis ([0 0 2] direction) in the hexagonal structure. It is known that pure ZnO thin films are not chemically stable in corrosive media, but aluminium stabilizes the ZnO system and increases its electrical conductivity. Finally, the ZnO : Al thin films are reasonably stable under storage in air and in reactive atmospheres like O₂, H₂O, H₂ or in weak acids. Dark- and photo-conductivity of the ZnO : Al films are very high (1–100 Ω⁻¹ cm⁻¹), so that they can be used as transparent conductors in solar cells or in electrochromic devices.     

Highly transparent and conductive Zn0.85Mg0.15O:Al thin films prepared by pulsed laser deposition

Zn_1−xMg_xO:Al thin films have been prepared on glass substrates by pulsed laser deposition (PLD). The effect of substrate temperature has been investigated from room temperature to 500 °C by analyzing the structural, optical and electrical properties. The best sample deposited at 250 °C shows the lowest room-temperature resistivity of 5.16×10⁻⁴ Ω cm, and optical transmittance higher than 80% in the visible region. It is observed that the optical band gap decreases from 3.92 to 3.68 eV when the substrate temperature increases from 100 to 500 °C. The probable mechanism is discussed.     

Investigations on ZnxCd1−xO thin films obtained by spray pyrolysis

Zn_xCd_1−xO thin films were prepared on glass substrates by spray pyrolysis technique. The precursor solutions were obtained by varying the concentration of Zn(NO₃)₂·6H₂O and Cd(NO₃)₂·4H₂O in bi-distilled water. The structural properties have been studied using X-ray diffraction spectra. All the structures include the basic compounds, i.e. ZnO and CdO. The orientation and the crystalline phases of the deposited films were specified. With the addition of Zn to the precursor solution, we can observe the preferential orientation of the CdO in the [2 0 0] direction. The electrical measurements were performed using method of four contacts. Thin films transmittances, in the 1.5–4.3 eV range, for different compositions have been measured and the optical gaps have been determined. The variations are explained considering the gaps of the two pure films. The influence of increased Cd concentration in the films on the structural, electrical and optical properties is investigated in this study.