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NANOSYSTEMS: PHYSICS, CHEMISTRY, MATHEMATICS, 2016, 7 (3), P. 523-527
Chemosynthesis, characterization and PEC performance of CdZn(SSe)2 thin films by arrested precipitation technique (APT)
S.K. Jagadale, D.B. Shinde, R.M. Mane, K.V. Khot, V.B. Ghanwat, P.N. Bhosale*, R.K. Mane** Materials Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur-416004, India *p_n_bhosale@rediffmail.com, **rmane.1970@gmail.com
DOI 10.17586/2220-8054-2016-7-3-523-527
In the present work, we have used simple, cost effective arrested precipitation technique (APT) to deposit CdZn(SSe)2 thin films. Preparative conditions were optimized during the initial stage of experimentation to obtain good quality CdZn(SSe)2 thin films. The as-deposited film was studied for its structural, morphological, optical, and compositional analysis by XRD, SEM, UV-Vis-NIR spectrophotometer and EDSanalysis techniques respectively. XRD analysis revealed that the film was polycrystalline in nature and exhibit hexagonal crystal structure. The SEM micrograph shows the formation of spherical surface morphology. EDS results confirm the presence of Cd, Zn, S and Se elements in the synthesized thin film. The band gap value of thin film was calculated from the absorption spectra which is found to be 1.8 eV. From J-V measurements, photo-conversion efficiency is found to be 0.07%.
Keywords: arrested precipitation technique, thin films, XRD, photoconversion efficiency.
Received: 4 February 2016 Revised: 10 April 2016
1. Introduction
The II-VI compounds are becoming interesting and important because of their major applications in solar cells [1] and opto-electronic devices [2]. A variety of techniques presently used for the synthesis of cadmium and zinc chalcogenide semiconductor thin films [3-8]. In all these methods, APT is self organized, cost effective, suitable for large area deposition [9] and is presently usedby us to prepare CdZn(SSe)2 thin films. In the present investigation, we propose the synthesis, growth mechanism, optostructural, morphological, compositional and photoelectrical properties of quaternary CdZn(SSe)2 thin films by arrested precipitation technique (APT).
2. Experimental details
All the chemicals used in the present investigation are of AR grade and used as received without further purification. The cadmium sulfate hydrate (CdSO43H2O), zinc sulfate hydrate (ZnSO47H2O), thiourea (NH2-CS-NH2), and sodium selenosulfite (Na2SeSO3), were used as precursors for Cd2+,Zn2+, S2-, Se2-ions. Ammonia is used to maintain pH of reaction bath and triethanolamine (TEA) was used as a complexing agent. For measuring the PEC performance, sulfide/polysulfide redox electrolyte is used. The solution of sodium selenosulfite was prepared by refluxing selenium metal powder with Na2SO3 at 90 °C for 9 h. Commercial glass slides and FTO were used as the substrate for thin film deposition. In the APT method, metal ions of the precursors are arrested using a stable organic complexing agent (in this case triethanolamine) in alkaline medium. All the preparative parameters such as pH of reaction bath, precursor concentration and temperature are optimized initially to 10.8 ± 0.2, 0.1 M and 55 ± 2 °C respectively, to obtain good quality thin films.
The optical absorbance was measured using UV-Visible NIR-spectrophotometer (Hitachi model 330, Japan) in the wavelength range 300 - 1100 nm. The structural analysis is doneby X-ray diffraction (XRD) analysis[Brukers AXS Analytical Instruments. Model D2 PHASER] with Cu Ka target for the 2d ranging from 10 ° to 100 °.The compositional analysis of deposited thin film was determined by energy dispersive X-ray analysis (EDS) attached to scanning electron microscope (SEM). (JEOL-JSM-6360A). J-V measurements (PEC) were recorded on semiconductor characterization instrument (SCS-4200 Keithley, Germany) using a two electrode configuration.
3. Results and discussion
3.1. Reaction mechanism for thin film formation
In the present investigation, we have successfully deposited CdZn(SSe)2 thin films by arrested precipitation technique. APT is based on Ostwald ripening law [9]. The mechanism is given below. In an alkaline medium, Cd-TEA and Zn-TEA complexes slowly release Cd2+ and Zn2+-ions at pH 10.8 ± 0.2.
(NH4)4[Cd2N(CH2 - CH2 - 0)3] + 6H2O ^ Cd2+ + [2N (CH2 - CH2 - OH)s] + 4NH4OH + 2OH- (1)
(NH4)4[Zn2N(CH2 - CH2 - O)3] + 6H2O ^ Zn2+ + [2N (CH2 - CH2 - OH)3] + 4NH4OH + 2OH- (2) Na2SeSO3 and thiourea dissociates in an alkaline medium to produce Se2- and S2-ions respectively
Na2SeSO3 + OH- ^ Na2SO4 + HSe-
HSe- + OH- ^ Se2- + H2O (3)
(NH2)2C=S + OH- ^ (NH2)2C=O + HS- + H2O
HS- + OH- ^ S2- + H2O (4)
The overall reaction is ...
Cd2++Zn2+ + Se2- + S2- ^ CdZn(SSe)2. (5)
3.2. Optical study
The optical absorption was measured at room temperature for the CdZn(SSe)2 thin films at wavelengths ranging from 300 - 1100 nm. The linear nature of the optical absorption plot confirms the direct allowed transition and is obtained by using formula given in [9].
4.5-,
cdzn(sse), |
4.0- [ HlZnjSSrlJ j
««p.
3.5- 1
■5 3.0- s i
g 2.5-
|,o- U>
a 1.5. 1 » 1» 154 175 200 225 7» 175 >4
Ph«lon enfqt) |hu.eV)
1.0.
0,50.0-
400 600 800 1000
Wavelength (nm) '
Fig. 1. Optical absorption plot and Inset: band gap energy plot of synthesized thin film
3.3. X-ray diffraction study
Figure 2 shows X-ray diffraction pattern of CdZn(SSe)2 thin film. Four peaks observed at diffraction angle of 24.92 (100), 26.20 (100), 41.92 (110), and 71.87 (105) corresponding to 3.568, 3.396, 2.149 and 1.312 'd' values respectively. These values are matched with standard JCPDS data (card No. 80-006, 35-1469 and 77-2307). XRD data demonstrate a polycrystalline phase and a hexagonal crystal structure of CdZn(SSe)2 thin film. The crystallite size, calculated by using the Debye Scherer formula, was found to be 20 nm.
3.4. SEM /EDAX studies
Figure 3 shows SEM micrograph ofsurface morphology of CdZn(SSe)2 thin film which exhibits well adherent, smooth and uniform distribution of nanosphere, which cover the entire substrate surface. The average grain size is calculated by standard scale bar method, and was found to be ^120 nm. The EDS spectrum for CdZn(SSe)2 film is shown in Fig. 4. The EDS spectrum indicates the present peaks for the Cd, Zn, S and Se elements in the synthesized thin film. The spectrum also shows peak for platinum, which is used for coating the sample during analysis.
Fig. 2. X-ray diffraction pattern of CdZn(SSe)2 thin film
Fig. 3. SEM micrographs of CdZn(SSe)2 thin film
Fig. 4. EDS spectrumof CdZn(SSe)2 thin film
3.5. Photoelectrochemical performance
The PEC performance of deposited CdZn(SSe)2 thin film was verified using the standard two electrode configuration, both in dark and under light illumination of a 500 W tungsten filament lamp having a light intensity of 30 mW cm-2, in sulphide/polysulphide redox electrolyte. The current density-voltage (J-V) characteristics ofglass/FTO/CdZn(SSe)2/electrolyte/graphite were measured. J-V characteristic curve of CdZn(SSe)2 thin film in dark display diode-like rectifying characteristics. Upon illumination, the magnitude of the open circuit voltage (Voc) increases with negative polarity towards the CdZn(SSe)2 electrode, indicating cathodic behavior and which confirms that CdZn(SSe)2 thin film is p-type. The output parameters of the PEC solar cell, i.e. light conversion efficiency (n%) and fill factor (FF), were calculated from eqn (6) and (7), respectively:
FF
Jsc XVn,
n% =
JscXVo, p.
J in
XFFX100 j
(6)
(7)
where Jsc is the short-circuit current density and Voc is the open circuit voltage. Jmax and Vmax are the maximum current density and the maximum voltage, and Pin is the input light intensity (30 mW cm-2). From the J-V measurements, the obtained values for Jsc, Voc and FF for the sample are 0.1917 mA cm-2, 457.2 mV and 0.27 respectively. The resultant conversion efficiency for the synthesized thin film is 0.07 %.
Fig. 5. J-V measurement curve of CdZn(SSe)2 thin films
JmaxX Vmax
4. Conclusions
The arrested precipitation technique is found to be the most convenient method for the deposition of metal chalcogenide thin films. Optical band gap energy value was found to be 1.87 eV. The XRD study revealed the polycrystalline nature with purehexagonal crystal structure of the CdZn(SSe)2 thin film. The SEM micrograph shows that the spherical grain structure of the surface without any pinholes.EDS results confirm the presence of Cd, Zn, S and Se elements in the synthesized thin film. The J-V measurement curve showed the efficiency of CdZn(SSe)2 film to be 0.07 %. This revealed that CdZn(SSe)2 thin films deposited by APT technique, showed potential as candidates for solar cell applications.
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