[2083134X - Materials Science-Poland] Synthesis and Characterization of High-efficiency Low-cost Solar Cell Thin Film

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  © 2019.ThisisanopenaccessarticledistributedundertheCreativeCommonsAttribution-NonCommercial-NoDerivatives4.0License.(    )  Materials Science-Poland, 37(1), 2019, pp. 127-135http://www.materialsscience.pwr.wroc.pl/  DOI: 10.2478/msp-2019-0005 Synthesis and characterization of high-efficiency low-costsolar cell thin film W. C HRISTOPHER  I MMANUEL 1, ∗ , S. P AUL  M ARY  D EBORRAH 2 , S.S.R. I NBANATHAN 2 ,D. N ITHYAA  S REE 1 Department of Physics, EGS Pillay Engineering College (Autonomous), Nagapattinam, Tamil Nadu, India 2 Research Department of Physics American College, Madurai, Tamil Nadu, IndiaPolycrystalline chalcogenide semiconductors play a vital role in solar cell applications due to their outstanding electricaland optical properties. Among the chalcogenide semi-conductors, CdZnS is one kind of such important material for applica-tions in various modern solid state devices such as solar cells, light emitting diode, detector etc. Due to their applications innumerous electro-optic devices, group II-VI semiconductors have been studied extensively. In recent years, major attentionhas been given to the study of electrical and optical properties of CdZnS thin films. In this work, Cd 1 − x Zn x S thin films wereprepared by chemical bath deposition technique. Phase purity and surface morphology properties were analyzed using fieldemission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD) studies. Chemical composition was studied us-ing energy dispersive spectrophotometry (EDS). Optical band gap property was investigated using UV-Spectroscopy. Electricalconductivity studies were performed by two probe method and thermoelectric power setup (TEP) to determine the type of thematerial. This work reports the effect of Zn on structural, electrical, microstructural and optical properties of these films.Keywords:  semiconductors; Cd  1 −  x  Zn  x S; electrodeposition; FESEM; XRD; EDS  1. Introduction Sulfides of zinc and cadmium have been usedsuccessfully in various optoelectronic devices. Thegrowth of ternary semiconductor thin films havebeen studied very extensively in the recent years,since these films play an important role in the fabri-cation of solar cells due to their favorable electricaland optical properties [1]. They are of a great tech- nological interest in heterojunction solar cells andin photoconductive devices due to the fact that cad-mium zinc sulfides (CdZnS) thin films have a widebandgap. The II-VI compounds due to their photo-conductivity, found their use in broad applicationssuchasphotovoltaicsolarenergyandthinfilmtran-sistor electronics [2]. Wide range of polycrystallinesemiconducting materials have been studied for thephotoconductivity in the visible light. Photodecayand photoresponse properties of photoconductivematerials and photovoltaic structures were exam-ined [3]. In this work an attempt has been made ∗ E-mail: christ.phy@gmail.com to establish the effect of Zn on structural, elec-trical, micro structural and optical properties of Cd 1 − x Zn x S films. 2. Experimental Physical properties of electrodepositedCd 1 − x Zn x S films are dependent on the depositionparameters such as the bath temperature, relativeconcentrations of kind of reactants in the solution,pH value and type of substrate. Electrodepositiontechnique was employed to deposit thin filmsof Cd 1 − x Zn x S on glass substrates. The startingmaterials, cadmium sulfate and thiourea usedwere of analytical grade. For the deposition of Cd 1 − x Zn x S thin film, a well cleaned glass substratewas immersed vertically in the solution. The tem-perature of bath was maintained at 70 °C for 3 h.Triethanolamine (TEA) was used as a complexingagent. Ammonia solution was used to maintain thepH of the bath at 10. Finally, the coated substrateswere washed with distilled water and annealedat a temperature of 225 °C. The quality of thin  128 W. C HRISTOPHER  I MMANUEL  et al. films generally depends on the parameters suchas deposition time and temperature of deposition.The deposition parameters employed in this studyare collected in the Table 1. 2.1. Structural properties 2.1.1. X-ray diffraction The phase purity of the film was analyzed withX-ray diffraction (Miniflex, Rigaku, Japan) usingCuK α   radiation with a wavelength of 1.542 Å.The structural characterization is very impor-tant for explaining structural, microstructural andelectrical properties of Cd 1 − x Zn x S thin films. TheX-ray diffraction patterns were recorded from20° to 80° as shown in Fig. 1. The XRD analysisrevealed that all the films show nanocrystalline na-ture with cubic phase of Cd 1 − x Zn x S. The XRD pat-terns confirm the formation of ternary system al-loy Cd 1 − x Zn x S with Zn content x  =  0.2, 0.4, 0.6and 0.8, respectively. The compositions were fur-ther confirmed using EDS analysis. The presenceof sharp peaks indicates crystalline nature of thethin films, from Fig. 1. The observed peaks corre-spond to the planes (1 0 0), (0 0 2), (1 0 1), (1 0 2),(1 1 0) and (1 1 0) which was found by matchingwith standard JCPDS data of CdZnS. [7]The average crystallite size of Cd 1 − x Zn x S thinfilm samples were calculated by using the Scherrerformula:  D = 0 . 9 λ β   cos θ   (1)where, D is average crystallite size,  λ  isX-ray wavelength (1.542 Å),  β  is FWHM of thepeak,  θ  is diffraction peak position. The averagecrystallite size of the samples is presented in Table. 2.1.2. Field emission scanning electron micro-scope analysis The surface morphology of the prepared filmswas analyzed using a field emission scanning elec-tron microscope coupled with energy dispersiveX-ray analysis (EDS) (FE-SEM, JEOL JED 6300).FE-SEM images of Cd 1 − x Zn x S thin films arepresented in Fig. 2. The FE-SEM images resolvethe nanoparticles associated with the film at high (a)(b)(c)(d) Fig. 1. X-ray diffraction patterns of Cd 1 − x Zn x S thinfilms with (a) 0.2 % Zn, (b) 0.4 % Zn, (c) 0.6 %Zn and (d) 0.8 % Zn. magnification of ×15000. Fig. 2 shows the hierar-chical formation [19] of the particle for Cd 1 − x Zn x Sthin film. Fig. 2d shows agglomerations of thegrains. Grain size has been tabulated in Table 3. Itis observed that the grain size decreases with anincrease in Zn content. The films show a fiber-likemorphology as Zn content increases, which may beuseful for gas sensing applications.  Synthesis and characterization of high-efficiency low-cost solar cell thin film  129 Table 1. Deposition parameters of Cd 1 − x Zn x S thin films.Deposition parameter Optimum value/itemDeposition time 70 minpH 10Concentration of precursor cadmium sulfate,zinc sulfate, thiourea 0.1 MSolvent Deionized waterZn content [wt.%] 0.2, 0.4, 0.6, 0.8Deposition temperature 70 °C (a) (b)(c) (d) Fig. 2. FE-SEM images of Cd 1 − x Zn x S thin film for sample S1 (a), S2 (b), S3 (c) and S4 (d) 2.1.3. Quantitative elemental analysis (EDS) The quantitative elemental composition of Cd 1 − x Zn x S thin film was analyzed using an energydispersive spectrometer. Fig. 3 shows that the pre-pared Cd 1 − x Zn x S thin film is nonstoichiometric innature.Table 2 shows that Cd 1 − x Zn x S thin films arenonstoichiometric. 2.2. Optical properties study using UV-spectroscopy Optical absorption studies of hierarchicalCd 1 − x Zn x S thin films were carried out in the wave-  130 W. C HRISTOPHER  I MMANUEL  et al. Table 2. Quantitative elemental analysis for as-prepared Cd 1 − x Zn x S thin films.Element ObservedS1 S2 S3 S4wt.% at % wt.% at.% wt.% at.% wt.% at.%Cd 40.80 33.65 46.65 31.87 48.90 30.68 47.65 29.88S 18.96 32.80 14.29 32.44 15.21 32.89 13.29 33.61Zn 40.24 33.55 38.66 35.69 35.89 36.43 39.06 36.51Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00Fig. 3. EDAX of Cd 1 − x Zn x S thin film sample (S3). length  λ  range of 300 nm to 600 nm at roomtemperature, using UV-Vis-2450 spectrophotome-ter. The change in absorbance with the wavelength λ  is shown in Fig. 4. The band gap energies of thesamples were calculated from the absorption edgesof the spectra [8].The slope drawn from the start of an absorp-tion edge (the onset of absorbance) and horizon-tal tangent drawn on absorption minimum intercepteach other at some point shown in Fig. 4. The ef-fect of Zn content on the band gap Eg value of theCd 1 − x Zn x S films have been studied. To obtain theband gap value, the absorption coefficient  α   wascalculated from the absorption data. Optical bandgap energies of the samples were observed to beslightly varying from 3.55 eV to 3.71 eV [9]. It iswell known that a momentous increase in the bandgap energy is possible when the size of crystallitesreaches the size of the quantum dots. It can be seen Fig. 4. Variation of absorbance with the wavelength forsamples S1, S2, S3 and S4. that the band gap varies with Zn content in a non-linear way [13]. 2.3. Variation of crystallite size, grain sizeand dislocation density with Zn content The details of the composition and calculatedcrystal sizes varying with Zn content are given inTable 3. It is clear from Table 3, that the grain size decreases from 39 nm to 31 nm with an increasein Zn content in the thin films and optical bandgap energy increases with an increase in Zn con-tent [4, 8–10]. This may be due to the enhancement in the crystallinity with an increase in Zn contentwhich leads to minimum imperfection. A slight in-crease in the optical band gap energy of the filmswith increasing Zn percentage can be attributed tothe increase in the crystallite and grain size.
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