Low temperature Si doped ZnO thin films for transparent conducting oxides

Si doped zinc oxide (SZO, Si 3%) thin films are grown at low substrate temperature (T≤150 °C) under oxygen atmosphere, using pulsed laser deposition (PLD). Si addition leads to film amorphization and higher densification. Hall effect measurements indicate a resistivity of 7.9×10⁻⁴ Ω cm for SZO thin films deposited at 100 °C under optimized 1.0 Pa oxygen pressure. This value is in good agreement with optical resistivity simulated from the transmittance spectra. XPS measurements suggest more than one oxygen environment, and a Si oxidation state lying in between 2 and 3 only. As a matter of fact, the values of both measured and simulated carrier numbers are smaller than the ones expected, assuming that all Si cations in the ZnO matrix are at the 4⁺ oxidation state. Finally, the differences in the electrical and optical properties of SZO thin films deposited both on glass and PET substrates confirm the strong dependency of the electronic properties to the film crystallinity and stoichiometry in relationship with the substrate nature.     

Industrial high-rate (∼14 nm/s) deposition of low resistive and transparent ZnOx:Al films on glass

Aluminum doped ZnO_x (ZnO_x:Al) films have been deposited on glass in an in-line industrial-type reactor by a metalorganic chemical vapor deposition process at atmospheric pressure. Tertiary-butanol has been used as oxidant for diethylzinc and trimethylaluminium as dopant gas. ZnO_x:Al films can be grown at very high deposition rates of ∼14 nm/s for a substrate speed from 150 to 500 mm/min. The electrical, structural (crystallinity and morphology) and optical properties of the deposited films have been characterized by using Hall, four point probe, X-ray diffraction, atomic force microscope and spectrophotometer, respectively. All the films have c-axis, (002) preferential orientation and good crystalline quality. ZnO_x:Al films are highly conductive (R<9 Ω/sq, for a film thickness above 1300 nm) and transparent in the visible range (>80%). These results show that ZnO_x:Al films with good electrical and optical properties can be grown with a high throughput industrial CVD process at atmospheric pressure. First p-i-n a-Si:H solar cells have been deposited on this material, with initial efficiency approaching 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.     

Low temperature deposition of high open-circuit voltage (>1.0 V) p-i-n type amorphous silicon solar cells

P-i-n type hydrogenated amorphous silicon (a-Si:H) solar cells were deposited by the radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) process at a low substrate temperature of 125 °C, which is compatible with low-cost poly (ethylene terephthalate) (PET) plastic substrates. Wide band gap (E_opt>1.88 eV) intrinsic a-Si:H films were achieved before the onset of the microcrystalline regime by changing the hydrogen dilution ratios. On the other hand, the structural, optical and electrical properties of p-type hydrogenated amorphous silicon carbide (p-a-SiC:H) window layers have been optimized at 125 °C. High quality p-a-SiC:H film with high optical band gap (E₀₄=2.02 eV) and high conductivity (σ_d=1.0×10⁻⁷ S/cm) was deposited at ‘low-power regime’ under low silane flow rates and high H₂ dilution conditions. With the combination of wide band gap p-a-SiC:H window layers and intrinsic a-Si:H layers, a high V_oc of 1.01 V (efficiency=5.51%, FF=0.72, J_sc=7.58 mA/cm²) was obtained for single junction a-Si:H p-i-n solar cell at a low temperature of 125 °C. Finally, flexible a-Si:H solar cell on PET substrate with efficiency of 4.60% (V_oc=0.98 V, FF=0.69, J_sc=6.82 mA/cm²) was obtained.     

Fabrication and opto-electric properties of ITO/ZnO bilayer films on polyethersulfone substrates by ion beam-assisted evaporation

Transparent conducting oxides bilayer films stacked by one 130-nm-thick indium tin oxide (ITO) top layer and one 75-nm-thick zinc oxide (ZnO) buffer layer were grown onto polyethersulfone (PES) substrates by ion beam-assisted evaporation. The effects of ion energy and ZnO buffer layers on the structural and opto-electric properties of ITO films were initially investigated. The as-deposited ZnO buffer layers show wurtzite (0 0 2) preferred orientation on the PES substrates with ion beam assistance. The results of X-ray diffraction reveal a marked increase in the crystallinity of the ITO films which use ZnO as a buffer layer material. A drop of ∼60% in electrical resistivity of the ITO film on the PES can be achieved by using ZnO buffer layer. The transmittance of the ITO/ZnO bilayer was not deteriorated due to the insertion of ZnO layer. The lowest electrical resistivity of 6.552×10⁻⁴ Ω-cm associated with the transmittance of ∼80% at the wavelength of 550 nm can be obtained for the ITO film on the ZnO-coated PES at ion energy of 60 eV. The ITO films on the ZnO-buffered PES with moderate control of ion energy have a promising future for the application of the contact layers for flexible solar cells.     

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.     

Epi-n-IZO thin films/1 0 0 Si, GaAs and InP by L-MBE––a novel feasibility study for SIS type solar cells

High quality epitaxial indium zinc oxide (heavily indium oxide doped) (epi-n-IZO) thin films were optimized by laser-molecular beam epitaxy (L-MBE) i.e., pulsed laser deposition (PLD) technique for fabricating novel iso- and hetero-semiconductor–insulator–semiconductor (SIS) type solar cells using Johnson Matthey “specpure”- grade 90% In₂O₃ mixed 10% ZnO (as commercial indium tin oxide (ITO) composition) pellets. The effects of substrate temperatures, substrates and heavy indium oxide incorporation on IZO thin film growth, opto-electronic properties with 1 0 0 silicon (Si), gallium arsenide (GaAs) and indium phosphide (InP) wafers were studied. As well as the feasibility of developing some novel models of iso- and hetero-SIS type solar cells using epi-IZO thin films as transparent conducting oxides (TCOs) and 1 0 0 oriented Si, GaAs and InP wafers as base substrates was also studied simultaneously. The optimized films were highly oriented, uniform, single crystalline approachment, nano-crystalline, anti-reflective (AR) and epitaxially lattice matched with 1 0 0 Si, GaAs and InP wafers without any buffer layers. The optical transmission T (max) 95% is broader and absolute rivals that of other TCOs such as ITO. The highest conductivity observed is σ=0.47×10³ Ω⁻¹ cm⁻¹ (n-type), carrier density n=0.168×10²⁰ cm⁻³ and mobility μ=123 cm²/V s. From opto-electronic characterizations, the solar cell characteristics and feasibilities of fabricating respective epi-n-TCO/1 0 0 wafer SIS type solar cells were confirmed. Also, the essential parameters of these cells were calculated and tabulated. We hope that these data be helpful either as a scientific or technical basis in semiconductor processing.     

In doped CdO films: Electrical,optical, structural and surface properties

Recently, there has been a lot of work on the production and investigation of the physical properties of Transparent Conducting Oxide (TCO) materials which have common application area in photovoltaic solar cells and some optoelectronic devices. In this work, CdO film which is a material belongs to TCO family has been produced by Ultrasonic Spray Pyrolysis technique on microscope glass substrates at the substrate temperature of 250 ± 5 °C. Electrical, optical, structural and surface properties of undoped and In doped (at 1.3 and 5%) CdO films and the effect of In doping percentage on the physical properties of CdO films have been investigated. It has been determined that electrical conductivity of CdO film is high and this value has been decreased by In doping. After the optical investigations, it has been observed that the transmittances of the films are about 30% and decreased dramatically by In doping. XRD investigations showed that, films have polycrystalline structure and good crystallinity levels. It has been found that In element hasn't got an important effect on the morphology of the films after the examination of surface micrographs. It has been determined that Cd and O elements are present in the solid film by using EDS. After all investigations, it has been concluded that In doping has an important effect on the electrical, optical, structural and surface properties of CdO films.     

Transparent conductive Ga-doped ZnO/Cu multilayers prepared on polymer substrates at room temperature

In order to improve the transparency and durability of the Cu films deposited on polycarbonate (PC) substrates, a Ga-doped ZnO (GZO) layer could be deposited directly onto the Cu film. Compared with a single-layered GZO film, the GZO/Cu multilayer coatings have much lower sheet resistance and much thinner thickness. In our work, GZO/Cu multilayers were deposited by magnetron sputtering on PC substrates at room temperature. The structural, electrical, and optical properties of multilayers were investigated at various thicknesses of the Cu and GZO layers. As the Cu layer thickness increases, the resistivity decreases. As the GZO layer thickness increases, the resistivity increases. And we obtained that the transmittance of the GZO/Cu multilayer coatings was higher than that of the single Cu layer. The lowest resistivity of 5.7×10⁻⁵ Ω cm with a carrier concentration of 3.25×10²² cm⁻³ was obtained at the optimum Cu (12 nm) and GZO (10 nm) layer thickness. The best figure of merit φ_TC is 4.66×10⁻³ Ω⁻¹ for the GZO(30 nm)/Cu(12 nm) multilayer.     

Preparation and characterization of spray-deposited Cu2ZnSnS4 thin films

Thin films of Cu₂ZnSnS₄ (CZTS), a potential candidate for absorber layer in thin film heterojunction solar cell, have been successfully deposited by spray pyrolysis technique on soda-lime glass substrates. The effect of substrate temperature on the growth of CZTS films is investigated. X-ray diffraction studies reveal that polycrystalline CZTS films with better crystallinity could be obtained for substrate temperatures in the range 643-683 K. The lattice parameters are found to be a=0.542 and c=1.085 nm. The optical band gap of films deposited at various substrate temperatures is found to lie between 1.40 and 1.45 eV. The average optical absorption coefficient is found to be >10⁴ cm⁻¹.     

Plasma-induced TCO texture of ZnO:Ga back contacts on silicon thin film solar cells

This paper considers texturing of ZnO:Ga (GZO) films used as back contacts in amorphous silicon (a-Si) thin film solar cells. GZO thin films are first prepared by conventional methods. The as-deposited GZO surface properties are modified so that their use as back contacts on a-Si solar cells is enhanced. Texturing is performed by simple dry plasma etching in a CVD process chamber,at power=100 W, substrate temperature=190 °C (temperature is held at 190 °C because thin film solar cells are damaged above 200 °C), pressure=400 Pa and process gas H₂ flow=700 sccm. Conventional a-Si solar cells are fabricated with and without GZO back contact surface treatment. Comparison of the with/without texturing GZO films shows that plasma etching increases optical scattering reflectance and reflection haze. SEM and TEM are used to evaluate the morphological treatment-induced changes in the films. Comparison of the a-Si solar cells with/without texturing shows that the plasma treatment increases both the short-circuit current density and fill factor. Consequently, a-Si solar cell efficiency is relatively improved by 4.6%.     

Large area (9 × 9 cm) electrostatically sprayed nanocrystalline zincite thin films for hydrogen production application

Synthesis of single phase nanocrystalline zinc ferrite (zincite) thin films that provides high surface area for active redox reactions is reported. Electrostatic spray method is employed for obtaining these ferrite films onto a stainless steel substrate of 9 × 9 cm² area using zinc acetate and iron nitrate precursor solutions, mixed in 1:1.5 proportions in triply-distilled water. Single zinc ferrite phase in films, confirmed from the X-ray diffraction pattern, reveals the aggregation of several nanometer-sized spherical grains. About 4 nm average roughness is obtained from the 3D AFM image. Zinc ferrite film deposited onto a glass substrate shows both direct (2.96 eV) and indirect (1.92 eV) band gap energies. Hydrophilic behavior that might facilitate easy reaction kinetics in a water splitting process is noticed. Finally, 450 mL/h hydrogen production rate is confirmed when zinc ferrite film electrode was used as an anode in presence of Pt mesh as a cathode.     

Degradation studies of transparent conducting oxide: a substrate for microcrystalline silicon thin film solar cells

Aluminium doped ZnO films have been developed by RF-magnetron sputtering at 350 °C substrate temperature on glass substrate and commercially available SnO₂-coated glass substrate. The developed ZnO and SnO₂/ZnO films can be used as the substrates of microcrystalline silicon based solar cell. The electrical, optical properties and surface morphologies of ZnO film and SnO₂/ZnO bi-layer films have been investigated and they are compared with the commercially available SnO_2-coated glass substrate. The resistivities of ZnO and SnO₂ films are comparable (10⁻⁴ Ω-cm). Surface morphologies of different transparent conducting oxide coated substrates before and after H-plasma exposure were studied by scanning electron microscopy. The optical transmission of ZnO, SnO₂/ZnO and SnO₂ films are comparable and varies from 85 to 90% in the visible region. The optical transmission reduces drastically to less than 20% in SnO₂ films and for ZnO film it remains almost unchanged after H-plasma exposure. For SnO₂/ZnO film transmission decreases slightly but remains considerably high (80%). The performance of microcrystalline silicon solar cells fabricated on different transparent conducting oxides as substrates (ZnO/glass, SnO₂/glass and ZnO/SnO₂/glass double layer) is investigated in detail.     

Characterization of ZnO films obtained by ultrasonic spray pyrolysis technique

In this work, ZnO film was deposited onto microscope glass substrates at 300 ± 5 °C by ultrasonic spray pyrolysis technique to investigate its application potential in photovoltaic solar cells. Optical, surface, structural and electrical properties of the film were investigated. Transmittance and absorbance spectra were taken to examine the optical properties, and band gap was calculated by optical method. Scanning electron microscopy micrographs and X-ray diffraction pattern were used to investigate the surface and structural properties of the film, respectively. Temperature dependent current measurements were performed by two-probe method to analyze electrical properties, and electrical conductivity at room temperature and activation energy values were calculated. After all investigations, application potential of ZnO film in photovoltaic cells as transparent conducting oxide contact was searched.     

Some physical properties of Cd1−xSnxS films used as window layer in heterojunction solar cells

CdS has been proved to be an ideal material for use as the window layer for heterojunction solar cells especially with n-CdS/p-CdTe. CdS, Cd_0.9Sn_0.1S and Cd_0.8Sn_0.2S films were deposited onto glass substrates at 300 °C substrate temperature by using ultrasonic spray pyrolysis technique (USP). The effect of Sn concentration on some structural, optical and electrical properties of the films was presented. The crystal structure and orientation of the films were investigated by X-ray diffraction (XRD) patterns. XRD patterns showed that films have polycrystalline nature with a hexagonal structure. The grain size of the films decreased with increasing x values. The optical band gap values were obtained from optical absorption spectra of the films. The optical band gap values of the films were found to be between 2.44 and 2.45 eV. The variations of conductivity of Cd_1−xSn_xS (0 ≤ x ≤ 0.2) films have been investigated depending on applied voltage in dark and under illumination. The resistivity significantly decreased with increasing tin concentration and under illumination.     

Effects of deposition temperature on characteristics of Ga-doped ZnO film prepared by highly efficient cylindrical rotating magnetron sputtering for organic solar cells

We report the characteristics of Ga-doped zinc oxide (GZO) films prepared by a highly efficient cylindrical rotating magnetron sputtering (CRMS) system as a function of substrate temperature for use as a transparent conducting electrode in bulk hetero-junction organic solar cells (OSCs). Using a rotating cylindrical GZO target, low sheet resistance (∼11.67 Ω/square) and highly transparent (90%) GZO films were deposited with high usage (∼80%) of the cylindrical GZO target. High usage of the cylindrical GZO target in the CRMS system indicates that CRMS is a promising deposition technique to prepare cost-efficient GZO electrodes for low cost OSCs. Resistivity and optical transmittance of the CRMS-grown GZO film were mainly affected by substrate temperature because the grain size and activation of the Ga dopant were critically dependent on the substrate temperature. In addition, the performance of OSC fabricated on GZO electrode sputtered at 230 °C (11.67 Ω/square) is better than OSC fabricated on as-deposited GZO electrode (29.20 Ω/square). OSCs fabricated on the GZO electrode sputtered at 230 °C showed an open circuit voltage of 0.558 V, short circuit current of 8.987 m A/cm², fill factor of 0.628 and power conversion efficiency of 3.149%.     

Properties of fluorine doped ZnO thin films deposited by magnetron sputtering

Fluorine doped ZnO (FZO) films were deposited on Corning glass by radio frequency (rf) magnetron sputtering of pure ZnO target in CF₄ containing gas mixtures, and the compositional, electrical, optical, and structural properties of the as-grown films as well as the vacuum-annealed films were investigated. The fluorine content in FZO films increased with increasing CF₄ content in sputter gas. FZO films deposited at elevated temperature of 150 °C had considerably lower fluorine content and showed a poorer electrical properties than the films deposited at room temperature. Despite high fluorine contents in the films, for all the FZO films, the carrier concentration remained below 2×10²⁰ cm⁻³, leading to fairly low doping efficiency level. Vacuum-annealing of the FZO films deposited at room temperature resulted in substantial increase of Hall mobilities, reaching as high as 43 cm²/Vs. This was attributed partly to the removing of oxygen vacancies and/or the forming chemical bonds with interstitial zinc atoms by fluorine interstitials and partly to the passivation effect of excess fluorine atoms by filling in the dangling bonds at the grain boundaries. For all the films with thickness of around 300 nm, the optical transmissions in visible were higher than 80%, and increased with increasing fluorine content up to 85% for the film with highest fluorine content.     

Effect of Mg content on structural, electrical and optical properties of Zn1−xMgxO nanocomposite thin films

We report on the growth of Zn_1−xMg_xO (ZMO) thin films on quartz substrate using pulsed laser deposition (PLD) technique. The influence of varying Mg composition on structural, electrical and optical properties of ZMO films has been systematically investigated. Increase in Mg content (in the range 0.0?x?1.0), reflects the structural phase transition from wurtzite via mixed phase region to cubic one. X-ray diffraction (XRD) studies indicate the hexagonal wurtzite phase at Mg composition ranging from 0% to 30%; mixture of wurtzite and cubic phases for 40% and single cubic phase at Mg content greater than 50%. The variation of the cation-anion bond length to Mg content shows that the lattice constant of the hexagonal ZMO decreases with corresponding increase in Mg content, which result in the structure gradually deviating from the wurtzite structure. The optical measurements reveal a blue shift in absorption edge and increase in transmittance from 75% to 96% with increase in Mg content. Tuning of the band gap has been obtained from 3.41 to 6.58 eV with corresponding increase in Mg content from x=0.0 to 1.0, which demonstrates that the films are useful for window layer of solar cells that improve the overall efficiency by decreasing the absorption loss.     

Solid-phase crystallized Si films on glass substrates for thin film solar cells

We investigate the potential of solid-phase crystallized Si films on glass for use in polycrystalline Si thin film solar cells. Low-pressure chemical vapour deposition serves to form amorphous Si films on borosilicate, SiO₂-coated borosilicate, aluminosilicate glass and fused silica substrates. The films are crystallized at temperatures of around 600°C. Using transmission electron microscopy we determine the grain size in the crystallized films. The average grain size strongly depends on the substrate type, increases with the deposition rate of the amorphous film and is independent of the film thickness. The grain size distribution in our films is log-normal. Films crystallized on SiO₂-coated borosilicate glass have an average grain size up to 2.3 μm, while the area weighted average grain size peaks at 4 μm. Since thin crystalline Si solar cells only require a film thickness of several micron, our films seem to be suitable for application to such devices.     

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.