Characterization of CuInS2 thin films for solar cells prepared by spray pyrolysis

We have made a study of the chemical composition, the electrical, the optical and the structural properties of polycrystalline CuInS₂ thin films prepared by spray pyrolysis to be used for thin film solar cells. These films were deposited starting from aqueous solutions with different chemical compositions ([Cu]/[In] and [S]/[Cu] ratios) and at different substrate temperatures. In all cases, the material is p-type with grains preferentially oriented in the (112) direction of the sphalerite structure. The electro-optical properties show a very strong dependence on the [Cu]/[In] ratio in the solution. Films with copper excess have smaller resistivity and better crystallinity than those which are stoichiometric or have indium excess. The results obtained in this work show the possibility of having CuInS₂ thin films with a wide range of resistivity, a fact that could be important for making solar cells based on this material.     

Pathways to thin absorbers in CdTe solar cells

We present approaches to reduce the absorber thickness of CdTe solar cells. The investigations were done with CdTe absorber films deposited by the close-space sublimation (CSS) technique. Using these CdTe films, complete solar cells were produced in our own laboratory. The absorber thickness as the crucial parameter was varied between 1 and 11 µm in these experiments. It is analyzed how process steps following the CdTe layer deposition influence the structure of the absorber films as well as the solar cell properties. Three ways of back contact formation are compared. These include (i) the wet chemical etching of the CdTe surface, (ii) a plasma etching step, and (iii) the vacuum deposition of a thin intermediate copper layer. In the latter case, voids and shunts related to preferential etching at grain boundaries are avoided admitting the use of thinner absorber films. Thus, the solar-cell efficiencies were increased from below 9% to more than 10% while the CdTe film thickness was reduced from 11 µm to less than 4 µm.     

Lead antimony sulfides as potential solar absorbers for thin film solar cells

In an effort to prepare thin films of novel semiconductor materials that contain only cost effective, abundant, and relatively less-toxic materials, lead antimony sulfides films have been prepared. Herein, we report the thin film preparation of semseyite (Pb₉Sb₈S₂₁) via annealing of precursor films under sulfur vapor. Pb/Sb alloy precursor films suffered substantial changes in stoichiometry and produced rough films, whereas precursor films composed of multilayers of PbS and amorphous (Sb,S) produced smooth and compact phase-pure films composed of fine grains. Optical measurements indicated a direct band gap of 1.93 eV and a strong absorption coefficient of 1.0 × 10⁵ cm^− 1.     

CuInS2 thin films obtained by spray pyrolysis for photovoltaic applications

Copper indium disulphide, CuInS₂, is a promising absorber material for thin film photovoltaic which has recently attracted considerable attention due to its suitability to reach high efficiency solar cells by using low-cost techniques. In this work CuInS₂ thin films have been deposited by chemical spray pyrolysis onto glass substrates at ambient atmosphere, using different composition solutions at various substrate temperatures. Structural, chemical composition and optical properties of CIS films were analysed by X-ray diffraction, energy dispersive X-ray spectroscopy and optical spectroscopy. Sprayed CIS films are polycrystalline with a chalcopyrite structure with a preferential orientation along the <112> direction and no remains of oxides were found after spraying in suitable conditions. X-ray microanalysis shows that a chemical composition near to stochiometry can be obtained. An optical gap of about 1.51 eV was found for sprayed CIS thin films.     

Thin-film solar cells

The rapid progress that is being made with inorganic thin-film photovoltaic (PV) technologies, both in the laboratory and in industry, is reviewed. While amorphous silicon based PV modules have been around for more than 20 years, recent industrial developments include the first polycrystalline silicon thin-film solar cells on glass and the first tandem solar cells based on stacks of amorphous and microcrystalline silicon films (“micromorph cells”). Significant thin-film PV production levels are also being set up for cadmium telluride and copper indium diselenide.     

Zinc oxide thin films by the spray pyrolysis method

Undoped and In-doped ZnO thin films have been prepared on glass substrates from solutions of Zn(CH₃CO₂)₂2H₂O in a mixture of deionized water and isopropyl alcohol by spray pyrolysis. Their optical, morphological and structural qualities have been studied and the effect of the preparation conditions discussed. It was shown that the main factors determining the parameters of ZnO films are the growth temperature and the indium concentration. The growth temperatures of 625–675 K, indium doping levels of 1–1.5 at.% and precursor concentrations of 0.1–0.2 mol 1⁻¹ are preferable to achieve ZnO films with optical and structural qualities as required for solar cell applications.     

Undoped and Cr-doped TiO2 thin films obtained by spray pyrolysis

Undoped and chromium doped titanium oxide thin films were fabricated by spray pyrolysis by using a solution of titanium tetrachloride and ethyl alcohol. The films have been deposited on heated glass substrates at 373 K. After annealing for 90 min at 723 K, the initially amorphous films became polycrystalline with a predominant anatase structure and average crystallite sizes depending on dopant (Cr) concentration. The repartition of chromium impurities in the matrix of titanium oxide films, analyzed by electron paramagnetic resonance and X-ray photoelectron spectroscopy showed that the entrance of chromium into the anatase structure is mainly achieved by substitution. A decrease in unit cell parameters ratio (c/a) with the increase of chromium content sustains this assertion. The wetting properties of the titanium oxide films were evaluated from contact angle measurements between de-ionized water and films surface during- and post-irradiation with UV light. The correlation between the concentration of the dopant, film structure, surface morphology and wettability characteristics is discussed.     

Deposition and annealing effect on lanthanum sulfide thin films by spray pyrolysis

Lanthanum sulfide thin films were prepared on glass substrates from aqueous medium using spray pyrolysis technique. The effect of preparative parameters such as substrate temperature and solution concentration on the films was studied. The lanthanum sulfide films were annealed in air at 300 °C for 2 h. The films were characterized by X-ray diffraction (XRD), optical microscopy, optical absorption, electrical resistivity and thermo-emf measurement techniques. The XRD studies revealed that the as deposited films are amorphous, while annealed films are polycrystalline. The optical band gap of the as deposited film is decreased from 2.5 to 2.2 eV after annealing due to improvement in crystallinity. The electrical resistivity is of the order of 10⁴-10⁵ Ω cm and showed semiconducting behaviour. Thermo-emf measurement revealed that the conductivity of lanthanum sulfide is p-type.     

Structural, optical, and electrical properties of tin sulfide thin films grown by spray pyrolysis

Tin sulfide (SnS) thin films have been prepared by spray pyrolysis (SP) technique using tin chloride and N, N-dimethylthiourea as precursor compounds. Thin films prepared at different temperatures have been characterized using several techniques. X-ray diffraction studies have shown that substrate temperature (T_s) affects the crystalline structure of the deposited material as well as the optoelectronic properties. The calculated optical band gap (E_g) value for films deposited at T_s = 320-396 °C was 1.70 eV (SnS). Additional phases of SnS₂ at 455 °C and SnO₂ at 488 °C were formed. The measured electrical resistivity value for SnS films was ∼ 1 × 10⁴ Ω-cm.     

ZnO:In deposition by spray pyrolysis — Influence of the growth conditions on the electrical and optical properties

Indium-doped zinc oxide (ZnO:In) was spray-deposited on glass at a substrate temperature of 370 °C. The spray was generated by means of an ultrasonic nebulizer and using air as carrier gas. To the 0.2 mol/l zinc acetate solution indium acetate has been added as dopant, the [In]/[Zn] ratio was changed between 1 and 5 at.%. The mobility and resistivity have an optimum at an [In]/[Zn] ratio of 3 at.% with values of 2.9 mΩ cm and 12.5 cm²/Vs at a layer thickness of 1.3 μm. The application of a nucleation layer of 100 nm intrinsic ZnO (i-ZnO) on the glass substrate and the subsequent deposition of ZnO:In lead to a layer with significant improvement of the optical properties; the ultraviolet to visible (UV-VIS) light transmittance increased to above 80% in the visible light region. The new layer configuration caused a change in the layer morphology, which is visualised by Scanning Electron Microscope photographs, Energy Dispersive X-ray and X-ray Diffraction measurements. The electrical properties remained unchanged.     

N silicon solar cells emitters realized using phosphoric acid as doping source in a spray process

The spray technique is used to realize the n⁺ emitter from phosphoric acid H₃PO₄ as a doping source. Emulsions have been prepared using several organic solvents. It was found that H₃PO₄:2-butanol mixture provides the most uniform deposited layer. The sheet resistance and the n⁺ profile were measured with a four point probe and the Hall profiling, respectively. The variety of emitters obtained are characterized by a sheet resistance ranging from 10 to 86 Ω/□ and a junction depth of about 0.2 to 0.7 μm which can be adequate for emitters in a polycrystalline silicon solar cell process.     

Porous photocatalytically active ZnO films obtained from ethylcellulose modified solutions by spray pyrolysis

Nanocrystalline porous ZnO films are deposited onto alumina foil substrates by polymer-modified spray pyrolysis of zinc nitrate and zinc acetate solutions. The dependence of the concentration of added ethylcellulose and the type of zinc precursor on both the photocatalytic properties and films morphologies is investigated. It is established that the addition of ethylcellulose as a modifier in the spray solution leads to the formation of a porous structure with crystallites sizes about 15 nm, when zinc acetate is used as precursor. These films show better photocatalytic activity for degradation of Malachite Green (MG) dye than the films obtained from zinc nitrate modified solution. The zinc nitrate films exhibit weaker activity for degradation of MG regardless of their smaller crystallite size (8–12 nm). This can be explained with their lower porosity than that of polymer-modified zinc acetate films. It is established that 450 °C is the limit temperature of treatment for the preparation of ZnO films with good photocatalytic activities. This photocatalytic activity in films shows a drop in the comparison to the films treated at lower temperature, due to decreasing of the pore number and deterioration of the surface morphology.     

Characterization of superstrate type CuInS2 solar cells deposited by spray pyrolysis method

CuInS₂ films were deposited on glass/FTO/TiO₂/In₂S₃ air ambient air at 300 °C by spray pyrolysis, resulting in superstrate-structured solar cells. The crystallinity of the spray-deposited CuInS₂ films was generally good. The CuInS₂ films with a thickness of below 2 μm showed only one layer and good adhesion. On the other hand, the CuInS₂ films with a thickness of more than 3 μm were formed with several layers, and were easily peeled off during deposition. The band gap value of CuInS₂ samples was around 1.3 eV. The performance of the best cell obtained was V_oc = 0.37 V, J_sc = 11.2 mA/cm², FF = 0.35, and had an efficiency = 1.7%. For large size solar cells (2 × 2 cm²), the effect of In₂S₃ film thickness on the cell performance was significant. In order to characterize the spray-deposited CuInS₂ films, the results of EPMA, XRD, XPS, and UV-vis absorption spectra have been discussed.     

Influences of deposition and post-annealing temperatures on properties of TiO2 blocking layer prepared by spray pyrolysis for solid-state dye-sensitized solar cells

Influences of the temperature (T_s) for spray pyrolysis deposition of TiO₂ blocking layer (BL) using titanium diisopropoxide bis(acetylacetonate) (TAA) as a precursor and the temperature (T_p) for post-annealing of the BL films on the resulting BL film morphology and photovoltaic performance of solid-state dye-sensitized solar cells (SDSC) are investigated. A T_s ranging from 300 to 400 °C is found preferable for the formation of BL films with smooth surface and dense grain packing. A T_s lower than 300 °C results in insufficient decomposition of the TAA precursor and is unable to form smooth BL films, while a T_s over 400 °C leads to loosely packed grain in BL films. Power conversion efficiency (PCE) of ~ 4.0% is obtained for SDSC devices with BL films prepared at a T_s in the range of 300-450 °C. High temperature post-annealing (T_p = 500-550 °C) of the BL films prepared at a low T_s, such as 300 °C, can improve the PCE up to 4.6%. The improvements are considered due to the higher purity, increased crystallinity, and retained high grain packing density of the post-annealed BL films, which facilitate charge transport and suppress charge recombination.     

Initial stages of deposition and film formation during spray pyrolysis — Nickel oxide, cerium gadolinium oxide and mixtures thereof

The formation of nickel oxide (NiO), cerium gadolinium oxide (CGO) and NiO-CGO thin films by air blast spray pyrolysis was studied at two scales. First, single droplets of precursor were deposited on sapphire substrates and the morphology of the formed residue was studied as a function of the substrate surface temperature, type of metal salt, salt saturation, and organic solvent in the precursor. Second, the synthesis of continuous films from repetitively deposited droplets and crack formation in the films were studied as a function of substrate temperature and salt decomposition kinetics. Nitrates, acetates, perchlorates and chlorides of nickel, cerium and gadolinium were the metal salts used, and mixtures of ethanol or water with di-, tri- and tetraethylene glycol were used as solvents.Regular ring- or disc-shaped deposits were formed from single droplets that evaporated without boiling and were mainly observed with metal acetate- and chloride-based spray solutions or at low substrate temperatures. Disc-shaped residues were obtained for saturated salt solutions and changed to rings with diminishing rim thickness with decreasing salt saturation. The formation of bubbles in the droplet from boiling or salt decomposition during evaporation resulted in the distortion of the circular shape and was predominantly observed for metal nitrate-based precursors and at high substrate surface temperatures.Continuous, dense and crack-free films of CGO and NiO-CGO with thicknesses up to 500 and 800 nm, respectively, were prepared from metal nitrate/chloride mixtures in a tetraethylene glycol-based solvent. The maximum crack-free thickness decreased with decreasing deposition temperature and was correlated to the metal salt decomposition kinetics.     

The influence of organic additives on the morphologic and crystalline properties of SnO2 obtained by spray pyrolysis deposition

The paper presents a comparative analysis of the SnO₂ thin layers, obtained via spray pyrolysis deposition (SPD), using polymeric precursors with different compositions and concentrations. The changes in the crystalline structure (X-ray diffraction), morphology and surface energy (atomic force microscopy and contact angle) and electric (electrical conductivity) properties of the layers were investigated. The photocatalytic efficiency of the samples was tested considering these layers' future applications in removing organic pollutants.     

Evaluation of different deposition conditions on thin films deposited by electrostatic spray deposition using a uniformity test

Copper indium disulphide films were produced by electrostatic spray deposition using a water/alcohol solution of copper chloride (CuCl₂), indium chloride (InCl₃) and thiourea (CS(NH₂)₂) sprayed onto SnO₂:F coated glass substrates. The influence of various deposition parameters, namely substrate temperature (380-450 °C), applied voltage (12-18 kV), solution concentration (0.21-0.49 M), flow rate (25-200 μl/min) and needle-substrate distance (40-70 mm) were investigated. particle image velocimetry measurements were made of the spray cone and correlated with the film uniformity. The film uniformity was measured using an optically based test developed in-house. Results show that the highest concentrated spray solution and lowest deposition temperature produce non-uniform films. In contrast, a needle-substrate distance of 50 mm, and the lowest applied voltage and flow rate resulted in the most uniform films.     

Fabrication and characterization of Cu2ZnSnS4 thin films deposited by spray pyrolysis technique

We have investigated synthesis conditions and some properties of sprayed Cu₂ZnSnS₄ (CZTS) thin films in order to determine the best preparation conditions for the realization of CZTS based photovoltaic solar cells. The thin films are made by means of spraying of aqueous solutions containing copper chloride, zinc chloride, tin chloride and thiourea on heated glass substrates at various temperatures. In order to optimize the synthesis conditions of the CZTS films, two series of experiments are performed. In the first series the sprayed duration was fixed at 30 min and in the second it is fixed at 60 min. In each series, the substrate temperature was changed from 553°K to 633°K. The X-ray diffraction shows, on one hand, that the best crystallinity was obtained for 613°K as substrate temperature and 60 min as sprayed duration. On the other hand, these CZTS films exhibit the kesterite structure with preferential orientation along the [112] direction. Atomical Force Microscopy was used to determine the grain sizes and the roughness of these CZTS thin film. After the annealing treatment, we estimated the optical band-gap energy of the CZTS thin film exhibiting the best crystallinity as 1.5 eV which is quite close to the optimum value for a solar cell.     

Effect of temperature on structural, optical and photoluminescence properties of antimony (Sb) doped polycrystalline CuInS2 thin films prepared by spray pyrolysis

Copper indium disulphide (CuInS₂) is an absorber material for solar cell and photovoltaic applications. By suitably doping CuInS₂ thin films with dopants such as Zn, Cd, Na, Bi, Sn, N, P and As its structural, optical, photoluminescence properties and electrical conductivities could be controlled and modified. In this work, Sb (0.01 mole (M)) doped CuInS₂ thin films are grown in the temperature range 300-400 °C on heated glass substrates. It is observed that the film growth temperature, the ion ratio (Cu/In = 1.25) and Sb-doping affects the structural, optical and photoluminescence properties of sprayed CuInS₂ films.The XRD patterns confirm that the Sb-doping suppresses the growth of CuInS₂ polycrystalline thin films along (1 1 2) preferred plane and in other characteristic planes. The EDAX results confirm the presence of Cu, In, S and Sb. About 60% of light transmission occurs in the wavelength range 350-1100 nm. The absorption coefficient (α) is found to be in the order of 10⁵ cm⁻¹. The band gap energy increases as the temperature increases from 300-400 °C (1.35-1.40 eV). SEM photographs depict that large sized crystals of Sb-doped CuInS₂ (1 μm) are formed on the surface of the films. Well defined sharp blue and green band emissions are exhibited by Sb-doped CuInS₂ thin films. Defects-related photoluminescence emissions are discussed. These Sb-doped CuInS₂ thin films are prepared by the cost effective method of spray pyrolysis from the aqueous solutions of CuCl₂, InCl₃, SC(NH₂)₂ and SbCl₃ on heated glass substrates.     

Thin film CdZnS/CuInSe2 solar cells by spray pyrolysis

Cd_1−xZn_xS/CuInSe₂ solar cells having efficiencies in the range of 2·3% were fabricated by spray pyrolysis. The best cell had the following parameters:V _oc = 305 mV,J _sc = 32 mA/cm², FF = 0·32 area = 0·4 cm² and efficiency = 3·149%.V _oc versus temperature measurements showed that the electron affinity difference was 0·22 eV. Forward dark current versus voltage curves were plotted and a possible current mechanism occurring in these cells has been proposed.