CODEN (USA): IJCRGG, ISSN: 0974-4290, ISSN(Online):2455-9555 Vol.11 No.01, pp 50-55, 2018
Abstract : Nanocrystalline cadmium sulfoselenide (CdS1-xSex; x=0.2, 0.5)thin films are successfully deposited onto a Flourine doped Tin Oxide (FTO:SnO2 ; 18-20 Ωcm-2) coated glass substrates under optimized growth conditions by chemical bath deposition technique (CBD). The as-deposited thin films were yellow-orange in colour and were vacuum annealed at a temperature of 250 oC for 5hour. The X-Ray diffraction (XRD) analysis reveals that the films are polycrystalline with hexagonal phase. The crystallite size of the CdS0.8Se0.2 and CdS0.5Se0.5 thin films were found to be 39.1nm and 42.2 nm respectively.The band gap of the films found to be decreased slightly with increase in composition of selenium. The surface micrographs of the films obtained from the field emission scanning electron microscope (FESEM) shows that both the films exhibitflakes-like morphology with spherical shaped nano clusters. CdS0.5Se0.5 films are relatively uniform, compact, well covered,well adherent to the substrate than compared to CdS0.8Se0.2 thin films.The surface wettability test showed hydrophilic and hydrophobic nature of CdS0.8Se0.2 and CdS0.5Se0.5 thin films respectively. The Photoelectrochemical (PEC) analysis reveals that thin film of CdS0.5Se0.5 is more photoactive than CdS0.8Se0.2 thin film. Keywords : Thin films, cadmium sulfoselenide, chemical bath deposition, flakes-like morphology, PE.
In recent times II-VImetal chalcogenides1 and alloys have received considerable attention of researchers due to the significant progress in the deposition techniques as well as their wide applications in the optoelectronic devices used in the UV-VIS and near IR regions of electromagnetic spectrum. Cadmiumsulfoselenide(CdSSe) is a ternary alloy metal chalcogenide has the band gap lies between that of CdS and CdSe. Its band gap can be tuned almost covering the entire visible range of electromagnetic spectrum. This shows that CdSSe is a promising candidate to achieve wavelength tunable emission. CdSSe is a highly photosensitive material and finds its applicability in various practical applications such as in optical filter, photovoltaic, photoelectrochemical devices, photoconductors and other optoelectronic devices2-3.CdS1-xSex thin films are deposited by various methods such as screen printing, spray pyrolysis, epitaxial growth, chemical bath deposition(CBD)4-7etc. Among all the methods, chemical bath deposition is a simple, convenient, inexpensive technique and can be used for large area deposition. In the present work CdS1-xSex is depositedon fluorine doped tin oxide (FTO:SnO2, 18-20Ωcm-2)coated glass substrates using chemical bath deposition method and a comparative study between CdS0.8Se0.2 and CdS0.5Se0.5 has been made in order to understand the effect of selenium composition on structural, morphological, optical, photoelectrochemical properties of the CdS1xSexthin films by modulating the chemical composition(x=0.2, 0.5).
In the experiment for the synthesis of CdS0.8Se0.2 thin films, the chemical bath with 0.5M, 20 mL cadmium sulfate (3CdSO4.8H2O) was taken in a 50 mL beaker. 30% liquid ammonia was added to it slowly drop by drop till pH 11.80 was obtained. Initially white precipitate is seen, the precipitate disappears upon excess addition of ammonia. 0.5M, 16 mL of thiourea (CS(NH2)2) was added to it and stirred well. Freshly prepared 0.5M, 4mL sodium selenosulphite (Na2SeSO3) solution was added to the previous solution and stirred well. Precleaned FTO substrates (50cmx0.3cmx1cm) are immersed into the chemical bath in the vertical position using a substrate holder. The chemical bath was then placed in constant temperature water bath maintained at a temperature of 75oC. The contents of the bath are stirred slowly at a uniform rate using a magnetic stirrer. After 90 minutes the substrates were removed from the bath washed with double distilled water (DDW) and dried in air. Similar procedure was employed to deposit CdS0.5Se0.5thin films by using 0.5M, 10 mL CS(NH2)2) and 0.5M, 10mL Na2SeSO3 solution at fixed volume and concentration of cadmium sulfate under the same growth conditions. The as deposited films of CdS1-xSex were yellow-orange in colour. The films were then vacuum annealed at a temperature of 250oC for 5hour.CdSSe-1 and CdSSe-2 are the sample nomenclature for CdS0.8Se0.2and CdS0.5Se0.5 samples respectively.
Fig-1 shows the XRD patterns for CdSSe-1 and CdSSe-2 samples. The XRD pattern reveals that both the thin films are polycrystalline and are in hexagonal phase[JCPDS data sheet No.00-040-0837]. The grain size(D) was calculated using scherrer’s equation for (002) plane. The structural parameters are calculated and tabulated in Table-1. With the increase of selenium composition from x=0.2 to 0.5, the grain size was found to increase from 39.1nm to 42.2nm and a slight decrease in dislocation density and microstrain was observed8-9 . This shows the improvement in crystallinity of CdSSe-2 films.
Fig-1: XRD patterns for (A) CdSSe-1(B) CdSSe-2 Table-1: Calculated values of structural parameters of CdS1-xSex
The UV-Visible spectroscopy was used to study the optical properties of CdSSe-1 and CdSSe-2 thin films using UV-VIS spectrophotometer in the wavelength range 300-900nm at room temperature.
Fig-2 shows the plot of (αhν)2versus hν for the CdSSe-1 and CdSSe-2 sample. The band gap was found to decrease from 2.32 to 2.28eV with the increase of x from 0.2 to 0.510 .
The surface morphology of CdS1-xSex thin film was carried out using FESEM. Fig-3 shows the FESEM micrograph of both CdSSe-1 and CdSSe-2 thin film exhibiting nanoflakes-like morphology11. CdSSe-1 film has voids, non uniform surface morphology with the presence of spherical shaped nano clusters. However the micrograph of CdSSe-2 film shows comparatively uniform surface morphology, the film is compact, well covered and adherent to the substrate.
Fig-3: FESEM micrographs of (A) CdSSe-1 (B) CdSSe-2
The surface wettability study of CdS1-xSexfilms was carried out by measuring water contact angle to theCdS1-xSex thin film. Fig-4 shows the captured water drop image on the CdSSe-1 and CdSSe-2 samples. The mean contact angle for CdSSe-1 is 75 while, it is 95 for CdSSe-2 sample. This shows that the surface of CdSSe-1 sample is hydrophilic and that of CdSSe-2 is hydrophobic in nature12.Hence the adsorption is stronger for CdSSe-2 sample than CdSSe-1 sample. The enhancement in the contact angle for CdSSe-2 sample indicates the decrease in the surface energy of the sample.
Fig-4: Captured waterdrop image(A) CdSSe-1 (B) CdSSe-2
In the photoelectrochemical study of the CdS1-xSex thin film, a photoelectrochemical cell was fabricated with CdS1-xSex thin film having area 1cm2 as photoelectrode and graphite as counter electrode respectively. 1M polysulfide electrolyte (1M (NaOH) +1M (Na2S) + 1M (S)) was used as an aqueous redox electrolyte in the PEC cell.
The PEC performance of CdS1-xSexphotoelectrode was examined by measuring current density(J) – voltage (V) characteristics in dark as well under illumination shown in Fig-5. The measurements were performed under the input power of 50mWcm-2. The fill factor (FF) and power conversion efficiency (ɳ) are calculated and tabulated in the Table-2.
It is observed that with the increase of selenium composition there is an improvement in the PEC performance13 . This might be due to improved crystallinity, surface morphology along with decrease in microstrain, dislocation density and grain boundary resistance.
Nanocrystalline CdS1-x Sex thin films with the measurable changes in structural and optical properties are obtained using chemical bath deposition technique by modifying the chemical composition.The XRD studiesreveal the presence of polycrystalline with hexagonal structure. The increase in grain size, slight improvement in crystallinity, the reduction in dislocation density as well as microstrain was observed with the increase of selenium composition.The decrease in band gap, relatively better and uniform surface morphology, enhancement in wetting characteristics with the increase of selenium composition was observed.The slight improvement in the PEC performance of CdS0.5Se0.5 compared toCdS0.8Se0.2 confirms that CdS0.5Se0.5 is more photoactive than CdS0.8Se0.2 thin film.
7. Chaudhari J.B., Deshpande N.G., Gudage Y.G., Ramphal Sharma, Studies on growth and characterization of ternary CdS1−xSex alloy thin films deposited by chemical bath deposition technique,Appl. Surf. Sci., 2008, 254, 6810–6816.
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