Preparation and properties of CdS thin films prepared on cold substrate as a window layer for solar cells

CdS films, usually, prepared on hot substrate at temperature range from 180 to 220°C. The electrical properties of the films are dependent on many parameters such as film thickness, deposition rate, film structure and substrate temperature. To control all these parameters to get film resistivity suitable for manufacturing solar cells, it needs a lot of precautions. CdS prepared on cold substrate could be the solution for this problem. Evaporation of CdS film at constant evaporation rate, then annealed in open air up to 600°C according to the film thickness. The resultant film have been studied. The properties of the films were comparable to the films prepared by the other methods with less control complexity.     

Determination of the interdiffusion coefficient for the CdS/CdTe heteroestructure by AES sputter depth profiling

Double heterostructures were prepared by depositing CdTe films on stainless steel (ss) substrates by the close spaced sublimation (CSS) method. The CdTe films were treated with a saturated solution of CdCl₂ in methanol, dried in air and annealed at 400 °C. CdS layer of 0.2 μm was deposited on the CdTe film by the chemical bath method. The CdS/CdTe system was treated with saturated solution of CdCl₂ in methanol and annealed for 30 min in air at different temperatures from 300 to 400 °C. The samples were characterized by scanning electron microscopy (SEM) and Auger electron spectroscopy (AES). The main effect of the temperature is to change the surface morphology of the CdS film from polycrystalline to an amorphous texture. By AES depth profiling the diffusion processes of the constituent element of the film was studied.     

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%.     

CdTe/CdS Solar cells on flexible molybdenum substrates

Development of CdTe/CdS solar cells on flexible metallic substrates is highly interesting due to the light weight and flexible nature of the solar modules. We have deposited CdTe films onto flexible molybdenum substrates using close-spaced sublimation technique and the CdTe/CdS junction was developed by depositing a thin layer of CdS onto the CdTe substrate from a chemical bath. The devices were characterized by Current–voltage (I–V) and photocurrent spectroscopy techniques. Prior to the deposition of the transparent conducting layer, the devices were annealed in air at different temperatures and found that the devices annealed at 400°C have better photovoltaic parameters. The efficiency of a typical device under 60 mW cm⁻² illumination was estimated as 3.5%.     

Highly textured ZnO thin films doped with indium prepared by the pyrosol method

Indium doped ZnO thin films have been prepared on heated Corning 7059 glass by the pyrosol spray method. It was found that indium doping has an important role in grain growth at high substrate temperature. Indium also was used to improve the electrical properties, acting as an N type dopant, and we obtained highly conductive ZnO:In thin films with a resistivity of 3.0 × 10⁻³ Ω cm. At substrate temperatures from 425°C to 475°C, the deposited ZnO:In thin films have clear hexagonal crystallites and, therefore, a highly textured surface showing optical haze phenomena due to the crystallites. The haze ratio of ZnO:ln thin films can be controlled from 10% to 50% at the wavelength of 550 nm by varying the substrate temperature from 375°C to 475°C.     

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).     

High-Ts amorphous top cells for increased top cell currents in micromorph tandem cells

In the present paper, the authors discuss the application of amorphous p–i–n solar cells containing i-layers which are deposited at high substrate temperatures as top cells in amorphous silicon/microcrystalline silicon tandem (“micromorph”) solar cells. Increasing the substrate temperature for the deposition of intrinsic a-Si : H results in a reduced optical gap. The optical absorption is enhanced and thereby the current generation. A high-current generation within a relatively thin amorphous top cell is very interesting in the context of micromorph tandem cells, where the amorphous top cell should contribute a current of at least 13 mA/cm² for a total cell current density of 26 mA/cm². A detailed study of the intrinsic material deposited by VHF-GD at 70 MHz at substrate temperatures between 220°C and 360°C is presented, including samples deposited from hydrogen-diluted silane plasmas. The stability of the films against light soaking is investigated employing the μ₀τ₀ parameter, which has been shown to be directly correlated to the cell performance. The paper discusses in detail the technological problems arising from the insertion of i-layers deposited at high substrate temperatures into solar cells. These problems are specially pronounced in the case of cells in the p–i–n (superstrate) structure. The authors demonstrate that an appropriate interface layer at the p/i-interface can largely reduce the detrimental effects of i-layer deposition at high temperatures. Finally, the application of such optimized high-temperature amorphous cells as top cells in micromorph tandem cells is discussed. Current densities of 13 mA/cm² in the top cell are available with a top cell i-layer thickness of only 250 nm.     

Titanium dioxide thin films prepared by chemical vapor deposition

Titanium dioxide thin films were prepared by a low-temperature atmospheric-pressure chemical vapor deposition method in air. The source gases were generated by heating isopropyl alcohol solutions of titanium tetraacetylacetonate and bis(acetylacetonate) titanium diisopropoxide. On glass and silicon (100) substrates, an amorphous film can be obtained at a substrate temperature in the range 200–500°C, and a polycrystalline anatase film can be obtained at a substrate temperature above 500°C.     

The effect of substrate temperature on P-CVD deposited a-SiGe : H films

Undoped a-SiGe : H films were deposited by the RF plasma chemical vapor deposition method. Films deposited at different substrate temperatures ranging between 100°C and 300°C were studied for their optoelectronic and structural properties. Structural defects like vacancies and microvoids were studied by positron lifetime spectroscopy (PLTS) at room temperature. Optoelectronic properties of the films were correlated with the PLTS measurements. The observations show a decrease in the deposition rate with substrate temperature. Good optoelectronic properties and proper structural relaxation have been obtained with a decrease in microvoid concentration.     

Crystallographic analysis of high quality poly-Si thin films deposited by atmospheric pressure chemical vapor deposition

Crystallinity of thin film polycrystalline silicon (poly-Si) grown by atmospheric pressure chemical vapor deposition has been investigated by X-ray diffraction measurement and Raman spectroscopy. Poly-Si films deposited at high temperatures of 850–1050°C preferred to 2 2 0 direction. By Raman spectroscopy, the broad peak of around 480–500 cm⁻¹ belonged to microcrystalline Si (μc-Si) phase was observed even for the poly-Si deposited at 950°C. After high-temperature annealing (1050°C) 3 3 1 direction of poly-Si increased. This result indicates that the μc-Si phase at grain boundary became poly-Si phase preferred to 3 3 1 direction by high-temperature annealing. Effective diffusion length of poly-Si films deposited at 1000°C was estimated to be 11.9–13.5 μm and 10.2–12.9 μm before and after annealing, respectively.     

Co-deposition of TiO2/CdS films electrode for photo-electrochemical cells

A densely packed TiO₂ thin film onto an indium doped–tin oxide (ITO) substrate was synthesized at room temperature by chemical deposition and a CdS thin film was deposited onto the pre-deposited TiO₂ film by a doctor blade route (powder of CdS was obtained from chemical deposition). TiO₂/CdS film was annealed at 300 °C for 1 h in air for crystallinity improvement. The first grown TiO₂ film was nanocrystalline, whereas the CdS film was polycrystalline as evidenced by X-ray diffraction (XRD) and selected area electron diffraction (SAED). Scanning electron microscopy (SEM) images show formation of mono-dispersed CdS spherical grains onto compact, densely packed spherical nanocrystalline grains of TiO₂. The TiO₂/CdS bilayer film was used in a photo-electrochemical cell as a working electrode, and a platinum electrode as a counter electrode (0.1 M lithium iodide electrolyte) under 80 mW/cm² light illumination intensity.     

Electrochromic properties of heat-treated thin films of CeO2–TiO2–ZrO2 prepared by sol–gel route

CeO₂–TiO₂–ZrO₂ thin films were prepared using the sol–gel process and deposited on glass and ITO-coated glass substrates via dip-coating technique. The samples were heat treated between 100 and 500 °C. The heat treatment effects on the electrochromic performances of the films were determined by means of cyclic voltammetry measurements. The structural behavior of the film was characterized by atomic force microscopy and X-ray diffraction. Refractive index, extinction coefficient, and thickness of the films were determined in the 350–1000 nm wavelength, using nkd spectrophotometry analysis.Heat treatment temperature affects the electrochromic, optical, and structural properties of the film. The charge density of the samples increased from 8.8 to 14.8 mC/cm², with increasing heat-treatment temperatures from 100 to 500 °C. It was determined that the highest ratio between anodic and cathodic charge takes place with increase of temperature up to 500 °C.     

Accelerated aging tests of chromium containing amorphous hydrogenated carbon coatings for solar collectors

Chromium containing amorphous hydrogenated carbon films (a-C : H/Cr) have been prepared by simultaneous rf plasma activated chemical vapour deposition of methane and magnetron sputtering of a chromium target. During deposition the substrates were heated (up to 300°C) and DC biased (−200 and −600 V) in order to obtain films with high chemical stability. Constant temperature tests were performed at 250°C in air with coatings deposited on silicon substrates. The degradation of the coatings was monitored by Raman spectroscopy and reflectance and transmission measurements. The main degradation mechanisms are discussed and the relevant parameters which improve the durability of the coatings are presented. Furthermore, the durability of solar selective, multilayered coatings which were deposited on copper sheets was investigated. Based on accelerated aging tests at different temperature loads in air (at 220°C, 250°C and 300°C) and in a humid environment (80°C sample temperature in humid air with 85°C and 95% relative humidity) the service lifetime in a flat plate collector is predicted to amount to more than 25 years.     

Preparation of Cu(In,Ga)Se2 thin films from Cu–Se/In–Ga–Se precursors for high-efficiency solar cells

Improved preparation process of a device quality Cu(In,Ga)Se₂ (CIGS) thin film was proposed for production of CIGS solar cells. In–Ga–Se layer were deposited on Mo-coated soda-lime glass, and then the layer was exposed to Cu and Se fluxes to form Cu–Se/In–Ga–Se precursor film at substrate temperature of over 200°C. The precursor film was annealed in Se flux at substrate temperature of over 500°C to obtain high-quality CIGS film. The solar cell with a MgF₂/ITO/ZnO/CdS/CIGS/Mo/glass structure showed an efficiency of 17.5% (V_oc=0.634 V, J_sc=36.4 mA/cm², FF=0.756).     

Low-temperature growth of highly crystallized transparent conductive fluorine-doped tin oxide films by intermittent spray pyrolysis deposition

Following the procedure by Sawada et al. (Thin Solid Films 409 (2002) 46), high-quality SnO₂:F films were grown on glass substrates at relatively low temperatures of 325–340°C by intermittent spray pyrolysis deposition using a perfume atomizer for cosmetics use. Even though the substrate temperature is low, as-deposited films show a high optical transmittance of 92% in the visible range, a low electric resistivity of 5.8×10⁻⁴ Ω cm and a high Hall mobility of 28 cm²/V s. The F/Sn atomic ratio (0.0074) in the films is low in comparison with the value (0.5) in the sprayed solution. The carrier density in the film is approximately equal to the F-ion density, suggesting that most of the F-ions effectively function as active dopants. Films’ transmittance and resistivity show little change after a 450°C 60 min heat treatment in the atmosphere, evidencing a high heat resistance. The SnO₂:F films obtained in this work remove the difficulty to improve the figure of merit at low synthesis temperatures.     

Optical and electrochemical properties of V2O5:Ta Sol–Gel thin films

Vanadium and tantalum-doped vanadium pentoxide, V₂O₅ and V₂O₅:Ta thin films (2.5 and 5 mol% of Ta) were prepared using sol–gel dip-coating technique.The coating solutions were prepared by reacting vanadium (V) oxytripropoxide and tantalum ethoxide (V) as precursors using anhydrous isopropyl alcohol as solvent.The films were deposited on a transparent glass substrate with ITO conducting film by dip-coating technique, with a withdrawal of 20 cm/min from the vanadium–tantalum solution and heat treated at 300 °C for 1 h. The resulting films were characterized by cyclic voltammetry, optical spectroscopy and by X-ray diffraction analysis (XRD). XRD data show that the films thermally treated at 300 °C were crystalline.A charge density of 70 mC/cm² was obtained for the film with 5 mol% of Ta, with an excellent stability up to 1500 cycles.     

Stability of thin film solar cells having less-hydrogenated amorphous silicon i-layers

In this study, highly stabilized hydrogenated amorphous silicon films and their solar cells were developed. The films were fabricated using the triode deposition system, where a mesh was installed between the cathode and the anode (substrate) in a plasma-enhanced chemical vapor deposition system. At a substrate temperature of 250 °C, the hydrogen concentration of the resulting film (Si–H=4.0 at%, Si–H₂<1×10²⁰ cm⁻³) was significantly less than that of conventionally prepared films. The films were used to develop the i-layers of solar cells that exhibited a significantly low degradation ratio of 7.96%.     

Electrical and optical properties of CdTe films prepared by vacuum evaporation with close spacing between source and substrate

CdTe films for solar cell application were prepared at various growth temperatures by a vacuum evaporation system having close spacing between the source and substrate. The CdTe thin films had a cubic structure highly oriented with the (1 1 1) direction perpendicular to the substrate surface, regardless of the growth temperature. The crystallites are of random shape and reach up to about 2 μm in size with increasing growth temperature. The higher growth temperature contributed to the reduction of dark resistivity from 6×10⁷ Ω cm at room temperature to 5.4×10⁶ Ω cm at 300°C. The photovoltaic properties of the CdS/CdTe solar cell were considerably improved with the increase in the growth temperature, which was caused by the increase of (1 1 1) texture and grain size in CdTe films.     

Galvanic deposition of cadmium sulfide thin films

A technique is presented for the deposition of high quality cadmium sulfide (CdS) thin films onto SnO₂ substrates by a galvanic method. Single phase films were deposited in a bath of cadmium chloride and sodium thiosulfate at pH = 4 and TEMPERATURE = 85°C at a growth rate of 1 nm/min. In the pH range of 3 to 4, the deposition rate is sensitive to cadmium chloride concentration. At higher pH the deposition rate is very low while at lower pH mixed phase films were obtained and homogeneous US formation occurred in the bath. The structural and optical properties of the US films are also presented and are comparable to films deposited by other methods. CdS/CdTe solar cells with efficiencies over 8% were fabricated using evaporated CdTe to demonstrate the utility of US films deposited by this simple technique. The galvanic deposition technique is useful in laboratory settings with limited deposition hardware and limited chemical waste disposal facilities.     

Preparation of selective absorbers based on CuMn spinels by dip-coating method

CuMn-spinel thin films were prepared on aluminum substrates by the so-called dip-coating method. The layers were deposited from alcoholic solutions based on nitrate precursors and subsequently sintered in air at 500 °C. Reflectance spectra in the NIR–vis–UV interval were measured for samples with different composition and thickness. The absorber quality of the films was checked by calculating the solar absorptance. The films displaying the best reflectance spectra and the highest solar absorptances (α_s>0.87) were deposited from solutions containing molar ratio Cu/Mn=1. The analysis of composition showed that Cu/Mn ratio in the film was very close to the ratio in the dip-in solution and supported the formation of a spinel-like material of stoichiometry Cu_1.5Mn_1.5O₄. Solar absorptance was dramatically improved when a SiO₂ antireflective layer was deposited onto the spinel. By optimizing film thickness of both CuMn-spinel and SiO₂ layers optical parameter values as good as α_s=0.94 and ε_T(1 0 0)=0.06 were achieved.