Modification in the chemical bath deposition apparatus, growth and characterization of CdS semiconducting thin films for photovoltaic applications

In this paper, growth and characterization of CdS thin films by Chemical Bath Deposition (CBD) technique using the reaction between CdCl₂, (NH₂)₂CS and NH₃ in an aqueous solution has been reported. The parameters actively involved in the process of deposition have been identified. A commonly available CBD system has been sucessfully modified to obtain the precious control over the pH of the solution at 90°C during the deposition and studies have been made to understand the fundamental parameters like concentrations of the solution, pH and temperature of the solution involved in the chemical bath deposition of CdS. It is confirmed that the pH of the solution plays a vital role in the quality of the CBD–CdS films. Structural, optical and electrical properties have been analysed for the as-deposited and annealed films. XRD studies on the CBD–CdS films reveal that the change in Cadmium ion concentration in the bath results in the change in crystallization from cubic phase with (1 1 1) predominant orientation to a hexagonal phase with (0 0 2) predominant orientation. The structural changes due to varying cadmium ion concentration in the bath affects the optical and electrical properties. Optimum electrical resistivity, band gap and refractive index value are observed for the annealed films deposited from 0.8 M cadmium ion concentration. The films are suitable for solar cell fabrication. Further on, annealing the samples at 350°C in H₂ for 30 min resulted in an increased diffraction intensity as well as shifts in the peak towards lower scattering angles due to enlarged CdS unit cell. This in turn brought about an increase in the lattice parameters and narrowing in the band-gap values. The results are compared with the analysis of previous work.     

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.     

Hydrogenation effect on structural,electrical and optical properties of CdS thin films for solar cell

Adsorption effects would be expected to be of considerable importance with thin films because of the changes in electron location accompanying adsorption. The effects of hydrogenation on structural, optical and electrical properties of the CdS thin films have been reported. GIXRD patterns shows that films have polycrystalline nature with a hexagonal structure. The optical band gap increased after hydrogenation of the film. The variation of conductivity of CdS films have been investigated depending upon the applied voltage at room temperature. The resistivity increased after hydrogenation of the films. Hydrogenated thin films can be used in solar cells because hydrogen plays an important role to modify the physical properties.     

CdS films from two different low cost techniques for solar cell applications – a comparative study

Abstract

CdS films with good structural and optical properties were fabricated by employing a simple and inexpensive spray pyrolysis technique using a perfume atomizer. The films were deposited at a relatively low substrate temperature (280°C) compared with conventional spray pyrolysis technique. The microstructure, surface morphology and optical properties of the sprayed CdS films prepared from starting solutions having different S/Cd ratios (1 : 1, 3 : 1and 5 : 1) were investigated and compared with that of the chemical bath deposited films. It was observed that, for the sprayed CdS films, the preferred orientation changes from (002) plane to (101) plane when S/Cd ratio in the starting solution increases, whereas in the case of their chemical bath deposited counterparts only the degree of preferred orientation increases with the increase in S/Cd ratio. The sprayed CdS films showed good visible transmittance (>85%) and wide optical band gap (2·42 eV) suitable for solar cell applications.     

Pulse plated CdSxTe1−x films and their properties

CdS_xTe_1−x films were deposited on titanium and conducting glass substrates at room temperature using 0.25 M cadmium sulphate, the concentration of sodium thiosulphate and TeO₂ dissolved in sodium hydroxide was varied in the range of 0.01-0.05 M. The as deposited films exhibited hexagonal structure irrespective of the composition. The FWHM maximum of the x-ray diffraction peaks were found to decrease with increase of duty cycle. The optical energy gap values are in the range of 1.54-2.32 eV for films of different composition, it is observed that the band gap shifts towards CdS side as the concentration of CdS in the films increase. XPS studies indicated the formation of CdSTe solid solution. The grain size increases from 11.54 to 99.40 nm as the value of x increases from 0.2 to 0.8. The surface roughness is found to increase from 0.22 to 2.50 nm as the value of ‘x’ increases from 0.2 to 0.8. The resistivity is found to vary from 53 to 8 ohm cm as the ‘x’ value decreases from 1 to 0.     

Substrate temperature and thickness dependence of properties of boron and gallium co-doped ZnO films

Transparent conducting oxide films of boron and gallium co-doped ZnO (BGZO) were prepared on glass substrates by radio frequency magnetron sputtering at room temperature and 200°C, respectively. The dependence of structural, electrical and optical properties on the thickness and substrate temperature were investigated. All films demonstrated c-axis preferred orientation and showed highly transparent in the visible wavelength region. With the increase thickness and substrate temperature, the grain size of BGZO films increased and the full width at half maximum decreased, the carrier mobility increased and resistivity decreased, which indicated that the crystallinity and conductivity of films were improved. The research also found that the optical band gap (E_g) of BGZO thin films decreased with increased substrate temperature and thickness.     

Preparation and characterization of thin films of electrodeposited CdTe semiconductors

Thin films of CdTe semiconductors were prepared by electrodeposition technique in aqueous solutions. The deposition mechanism was investigated by cyclic voltammetry. The potential regions for the formation of the n-CdTe and p-CdTe films were determined. The structure, composition and morphology characteristics of as-deposited thin films of CdTe grown on SnO₂/glass and CdS/SnO₂/glass were investigated by XRD, EDAX and SEM techniques. The optical properties were measured to determine the absorption coefficient and band gap values. The as-deposited CdTe films grown on SnO₂/glass contained free Te while those grown on CdS/SnO₂/Glass did not contain this phase. The CdTe has the cubic structure with strong (111) orientation. The EDAX analysis showed a nearly stiochiometric Cd:Te ratio. The band gap has a value of 1.48 eV, which is in a good accordance with those reported in the literature. The effect of annealing at 350 and 400°C after CdCl₂ treatment on the structure and morphology was also examined.     

Photoluminescence studies of CdS thin films annealed in CdCl2 atmosphere

We have grown CdS films by the Close Spaced Vapor Transport technique under specific conditions: substrate temperature (T_s): 450 °C, source temperature (T_so): 725 °C, argon pressure in the chamber (P_Ar): 100, 200 and 500 mT, deposition time (t_d): 100 s. The films were studied by measuring the luminescence properties at different temperatures in the range 10–300 K. The room-temperature PL spectrum of the as-grown CdS films showed a very broad band centered at 2.26 eV and a shoulder in the low-energy side at 1.80 eV. After CdCl₂ thermal annealing at 300 K, the spectrum showed better PL characteristics: a strong band in the low-energy side at 1.67 eV and a band in the high-energy side at 2.47 eV. The analysis at lower temperatures showed that the high-energy band becomes most intense and shifts to higher energies reaching a value of 2.54 eV, very close to the energy band gap at 10 K. The low-energy band becomes broader and centered around 1.9 eV. Analysis of the PL intensity as a function of temperature in an Arrhenius representation, allows applying a theoretical model for the quenching of the PL intensity.     

Synthesis,characterization, and photoelectrochemical study of Cd1−xZnxS solid solution thin films deposited by spray pyrolysis for water splitting

A series of Cd_1−xZn_xS thin films were deposited onto indium-doped tin oxide (ITO) coated glass substrates by ultrasonic spray pyrolysis CdCl₂, ZnCl₂, and CS(NH₂)₂ aqueous solutions. The XRD patterns revealed that these films processed a wurtzite structure and a series of solid solutions of CdS and ZnS formed. The lattice constants decreased as the x value increased. From the transmittance and reflectance, the optical band gap was estimated to be between 2.45 eV and 3.72 eV, and the band gap increased as the x value increased according to a near linear relationship with the x value. The Mott-Schottky tests revealed that the flat potential shifted negatively as the x value increased. The photo responses agreed with the optical absorption of these films quite well. The current–potential measurements under chopped Xe lamp light irradiation show that the CdS deposited at 300 °C had best photoresponse. Its photoelectrochemical efficiency was estimated to be about 0.95% under 0.73 V bias from two electrodes current–potential tests.     

PH值对化学沉积制备CdS薄膜性质的影响

实验研究了pH值对化学沉积制备CdS薄膜性质的影响。表明：在柠檬酸钠作为络合剂的体系中随着溶液中氨水浓度的提高CdS薄膜会发生相变，从立方相变为六方相，即当氨水浓度为0．31M时，得到立方相的CdS薄膜；而当氨水的浓度大于0．51M时，得到六方相的CdS薄膜。氨水浓度的提高也使得CdS薄膜的形貌有了很大的改善，且制备得到的CdS薄膜从富CA变为富S，但是薄膜仍是n型。氨水浓度对CdS薄膜的光学性质也有很大的影响，随着氨水浓度的提高所得到的CdS薄膜的禁带宽度增大。     

Optical and photoconductive properties of SnS thin films prepared by electron beam evaporation

Thin films of SnS have been prepared by electron beam evaporation. The films represent Herzbergite orthorhombic structure, established by their XRD patterns. The band gap energy and type of optical transitions were determined from transmission spectra and an optical band gap of E_g(tr)=1.23 eV for indirect transitions and E_g(tr)=1.38 eV for direct transitions were estimated. Using the dependence of photoconductivity from wavelength, a band gap of E_g(ph)=1.2 eV was determined as well. A thermal band gap of E_g(T)=1.29 eV was evaluated from the temperature dependence of the dark resistivity, and admixture level with activation energies (0.25 and 0.36 eV) were found. Roughness of the surface of SnS thin films was evaluated using atomic force microscopy.     

Preparation and evaluation of Cu2ZnSnS4 thin films by sulfurization of E---B evaporated precursors

By sulfurization of E---B evaporated precursors, CZTS(Cu₂ZnSnS₄) films could be prepared successfully. This semiconductor does not consist of any rare-metal such as In. The X-ray diffraction pattern of CZTS thin films showed that these films had a stannite structure. This study estimated the optical band gap energy as 1.45 eV. The optical absorption coefficient was in the order of 10⁴cm⁻¹. The resistivity was in the the order of 10⁴ Ω cm and the conduction type was p-type. Fabricated solar cells, Al/ZnO/CdS/CZTS/Mo/Soda Lime Glass, showed an open-circuit voltage up to 400 mV.     

Synthesis and electro-optic properties of nanosized-boron doped cadmium oxide thin films for solar cell applications

Boron doped CdO thin films were prepared by sol–gel dip coating technique. Atomic force microscopy results indicate that the boron doped CdO films have the nanostructure. The influence of the boron doping on the film growth is resulted in a change of grain size. The optical band gap of the CdO films was significantly changed by boron dopant. The refractive index dispersion of the films obeys the single oscillator model. The dispersion parameters, oscillator and dispersion energy were changed by boron dopant. The optical absorption results show that the optical band gap of the CdO film can be engineered over a wide range of 2.27–2.45 eV by introducing B dopant. For solar cell applications of the CdO film, a p-Si/1% B doped n-CdO heterojunction solar cell was fabricated and the solar cell shows the best values of open circuit voltage, V_oc = 0.37 and short circuit current density, J_sc = 0.81 mA/cm² under AM1.5 illumination, despite the fact that V_oc and J_sc are lower than those reported in the literature without using frontal grid contacts and or post-deposition annealing. It is evaluated that this work is useful as a basis search for synthesis of the nanosized-boron doped cadmium oxide thin films for solar cell applications and more competitive p-Si/n-CdO based solar cells.     

Physical properties of chemical bath deposited CdS thin films

Cadmium sulfide films of different thicknesses were deposited by chemical bath deposition (CBD) from a bath containing cadmium chloride, ammonium chloride, ammonium hydroxide and thiourea. The XRD patterns show that the films have a hexagonal phase with a preferential (0 0 2) orientation. The photoluminescence spectra show a defect structure, characteristics of the CdS films obtained by CBD. The electrical behavior in dark and under illumination, the optical properties and the band gap value reported in this work is in agreement with that reported in the literature.     

Structural and optical characterization of hot wall deposited CdSexTe1−x films

CdSe_0.3Te_0.7 alloy was prepared from the individual components and its composition and structural analysis were done. Films were prepared by hot wall deposition technique using 0.15 m length tube under a vacuum of 5×10⁻⁵ Torr on well cleaned glass substrates. The composition, structural, morphological, and optical properties of hot wall deposited films were investigated. The XRD analysis revealed that the films are like amorphous in nature for lower thicknesses but with increasing thickness a more preferred orientation along (1 0 1) direction was observed. The crystallite size (D), dislocation density (δ) and strain () were evaluated. From the EDX composition analysis, the individual concentrations of Se and Te in the films were estimated. An analysis of optical measurements shows that all the films have fairly good transparency above 850 nm. The optical band gap was found to be around 1.55 eV and decreases with increasing thickness. Also comparison of band gap with corresponding values for CdSe and CdTe are made.     

Cadmium sulphide prepared from cadmium oxide thin films

Undoped and doped CdS thin films have conveniently been prepared by conversion of cadmium oxide thin films using a chalcogen vapour. Dopants can be incorporated during the oxide formation stage, and the effects of three different dopants on the carrier transport properties are compared. Four X-ray diffraction peaks of the thin film polycrystalline material could be associated with the hexagonal (wurtzite) phase, two of the peaks are also consistent with the cubic (zincblende) phase. The reaction time required for the quantitative conversion of CdO to CdS decreased as the temperature increased, and quantitative conversion occurred in 30 min at 400°C to give a Cd/S atomic ratio of 1.01. The absorption spectrum indicated a direct transition with a band gap of 2.42 eV. The surface of the sample appeared uniform and densely packed with crystallites that are substantially smaller than 1 μm. This preparation technique can conveniently provide CdS films having a carrier concentration over the whole range of 10¹¹ to 10¹⁷ cm⁻³. Likewise, the resistivities can be conveniently controlled with values ranging from 5 Ω cm to 5 × 10⁷ Ω cm. In addition, this material has been prepared by several other procedures described in the literature, and the properties of the films are compared.     

Structural, optical and photoelectrochemical studies of the cadmium sulphide films grown by chemical bath deposition

Cadmium sulphide films have been grown by chemical bath deposition using an aqueous medium. Growth rate has been studied on the basis of the film thickness variation with deposition period. Structural investigation using XRD has shown the presence of polycrystalline deposits of wurtzite structure. The optical absorbance has revealed that the band gap of the CdS layers has been 2.39 eV. The CdS films have been employed in electrochemical photovoltaic cells of configuration glass/ITO/CdS//S₂S²⁻₂//Ni to test their photoactivity. The solar cell parameters have been correlated with the growth parameters.     

Electrochromic characteristics of fibrous reticulated WO3 thin films prepared by pulsed spray pyrolysis technique

The electrochromic (EC) behavior of fibrous reticulated WO₃ films prepared from ammonium tungstate precursor by pulsed spray pyrolysis method was investigated. All the films were prepared using identical technological parameters and a thorough investigation of the electrochromic properties of the films deposited at 300 °C is reported. The structural properties were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrochromic and optical properties were measured using cyclic voltammetry and ultraviolet (UV)-visible spectrophotometry. The films are amorphous and have a fibrous reticulate-like morphology having micron-size circular rings. The films show high transparency in the visible range and the optical band gap energy is about 3.1 eV. Electrical measurements show that the resistivity monotonically decreases as temperature increases, which indicates thermal hopping transport. The activation energy for hopping transport is of the order 4×10⁻⁴ eV. The electrochromic coloration efficiency (CE) is found to be 34 cm²/C at 630 nm.     

Properties of cadmium sulfide thin films deposited by chemical bath deposition with ultrasonication

Ultrasonic agitation was applied during the chemical bath deposition of CdS thin films. Ultrasonication resulted in a dramatic difference in surface morphology, growth rate, and optical properties of CdS films. There were virtually no colloidal particles adsorbed on the surface. The surface roughness measured by atomic force microscopy was reduced by a factor of two. Band gap energy increased to 2.39 eV from 2.37 eV. X-ray patterns showed that the preferred orientation changed from hexagonal(002)/cubic(111) to hexagonal(101). Optical transmission improved in the wavelength range longer than 520 nm. The chemical reaction for CdS formation started at a lower temperature under ultrasonication, and dense films were obtained even when the chemical composition of the aqueous solution deviated far from the optimum conditions.     

The effects of inclusion of iodine in CdTe thin films on material properties and solar cell performance

CdTe thin films were potentiostatically electrodeposited from a non-aqueous electrolytic bath containing ethylene glycol. In order to dope the CdTe using an n-type dopant according to a proposed new model, varying concentrations of iodine were added into the electrolytic bath. The resulting materials were studied for structural, morphological, optical and electrical properties. Structural analysis indicated the formation of CdTe layers without other possible phases at a concentration of 0.05 M of iodine in the bath. Optical absorption measurements have yielded a direct band gap value of 1.42±0.03 eV without showing any noticeable changes of the energy gap. Inclusion of iodine in CdTe layers have increased the electrical conductivity by a factor of 5, indicating positive n-type doping effects. The diodes of FTO/CdS/CdTe/Au structures showed improved current–voltage characteristics indicating the presence of a high potential barrier of 1.20 eV with low ideality factors around 1.40. These results demonstrate a considerable reduction of active recombination and generation centres from the structure. Although the fill factors observed are low, for the studied batches in this project, remarkable improvement of short-circuit current densities over 40 mA cm⁻² were observed together with open circuit voltage values in the range 500–700 mV. Capacitance–voltage measurements indicate a formation of a fully depleted device, desirable for photovoltaic conversion.