Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 3(7), 69-74, July (2013) Res. J. Chem. Sci. International Science Congress Association 69 Synthesis of Silver Sulphide Nanoparticles by Modified Chemical Route For Solar Cell ApplicationsJadhav U.M., Patel S.N. and Patil R.S.1*PSGVPM’S Arts, Science and Commerce College Shahada, Dist- Nandurbar-425409 MS INDIA SPDM Arts SBB & SHD Commerce and SMA Science College, Shirpur, Dhule-425405, MS, INDIAAvailable online at: www.isca.in Received 22nd June 2013, revised 28th June 2013, accepted 16th July 2013Abstract Modified chemical bath deposition technique has been implemented for the deposition of silver sulphide thin (CBD) method onto the glass and fluorine doped tin oxide (FTO) coated glass substrate from an aqueous alkaline bath at room temperature (300K). The deposition bath consists of silver nitrate, thiourea and ammonia. The preparative parameters such as ion concentration, deposition time, pH of solution, were optimized for Ag2S thin films. The as deposited films on glass substrate were studied for their structural, surface morphological, optical and electrical properties using techniques such as X-ray diffraction, atomic force microscopy (AFM), Scanning electron microscopy (SEM), optical absorption and dc two probe method. The photoelectrochemical (PEC) investigations of AgS films were carried out by using chemical cell configuration n- Ag2S /1M/NaOH-NaS-S/Pt. and the nanocrystalline films were found to be photoactive in polysulphide solution. The dynamic current–voltage (I–V) characteristic was examined at room temperature. The photovoltaic output characteristics were used to calculate the fill factor (FF) and power conversion efficiency (). The photovoltaic conversion efficiency of the thin film was found to be 0.002% with n-type conductivity. Keywords: Silver sulphide, chemical synthesis, nanocrystalline thin films, PEC properties.X-ray diffraction, morphological, optical properties and Band gap. IntroductionSilver sulfide (AgS) is an important inorganic compound which has been studied for its numerous applications. Silver sulphide (AgS) belongs to I-VI compound semiconductor materials with monoclinic crystal structure. Silver sulphide appears to be a promising material for conversion of solar energy into electrical energy as its band gap is in between 1-2 eV. The semiconductor silver sulfide has photoelectric and thermoelectric properties1,2. Silver sulphide (AgS) has been used in IR detectors, photoconductors, photovoltaic cells, electrochemical storage cells4,5, etc. Various techniques such as chemical bath deposition6,7 (CBD), spray pyrolysis deposition (SPD), successive ionic layer adsorption and reaction (SILAR), molecular beam epitaxy (MBE)10, thermal evaporation11, sol-gel and ion implantation techniques12, gamma irradiation 13 etc. have been reported for the preparation of silver sulphide thin films. Among these chemical deposition processes are economically effective and has been industrially exploited to large area deposition. Chemical deposition techniques are most important methods for the growth of the films due to their versatility for depositing very large no of elements and compounds at relatively low temperature14. Chemical methods are chemical bath deposition or modified chemical bath deposition method may be used. In chemical deposition method, thin films are deposited on the substrate from aqueous solution by chemical reaction under appropriate conditions. Among them, chemical bath deposition (CBD) is well known as a low temperature aqueous technique for depositing large area of semiconductor thin films. Many researchers have deposited AgS thin film by CBD. In CBD method the precipitation and film deposition takes place when ionic product exceeds the solubility product which produces wasteful unavoidable and uncontrollable bulk precipitate15. To overcome the difficulty, we have used the modified chemical bath deposition method for the synthesis of AgS thin film. Modified chemical bath deposition method (MCBD)16,17 is simple, less expensive and useful for large area deposition of any composition, easily controlled film thickness method at atomic level. This method does require sophisticated instruments and conductive substrate. The MCBD method is suitable for growing thin multilayer structure due to low temperature since diffusion of ion is slow. The growth of the film can be easily controlled through various parameters including concentration of bath temperature, immersions time and immersion cycles, etc. Semiconducting nanocrystalline thin films have attracted the attention of the scientists as non-conventional energy resources for solar energy conversion in recent years18,19. Recent investigations are carried out in the field of energy sources to meet the challenges of diminishing formal fossil fuels. It is necessary to find the easiest way of conversion of solar energy into electrical energy, which is abundant, nonpolluting and inexhaustible source of energy. One of the most efficient solar to electrical energy conversion devices is the photo electrochemical (PEC) cells. Many workers have reported PEC characteristics of silver sulphide semiconducting thin films. Many binary as well as ternary and Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(7), 69-74, July (2013) Res. J. Chem. Sci. International Science Congress Association 70 quaternary materials applied for solar energy conversion20,21. Properties of nanocrystlline materials are different and often superior to those of conventional coarse-grained materials and also amorphous alloys of the same configuration20-25. In the present investigation, nanocrystalline AgS thin films have been deposited using MCBD method at room temperature. The MCBD method for metal chalcogenide thin film preparation is currently attracting considerable attention, as it is relatively less expensive, simple. A variety of substrates such as insulators, semiconductors or metals can be used, since it is a low temperature process, avoids oxidation or corrosion of metallic substrates. The structural, compositional, surface morphological and optical characterizations were carried out for the confirmation of nanocrystalline nature. The nanocrystalline AgS thin films deposited onto FTO substrate were used as photoelectrode in PEC cells. The current voltage (I-V) and photovoltaic power output characteristics of AgS thin films deposited onto FTO substrate were studied. Material and Methods Preparation of thin film: Loba analytical reagent grade silver nitrate and thiourea were used for the deposition of nanocrystlline AgS thin films. The AgS films were grown on ultrasonically cleaned corning glass substrate (25mm X 75mm X 1mm)from an alkaline bath. In the synthesis of AgS thin film the cationic precursor was 0.1M silver sulphate solution complexed with ammonia solution at pH=10 for silver ion and anionic precursor was 0.5M thiourea solution for sulphide ion. Direct exchange of cationic and anionic ions was modified by means of rinsing in double distilled water. Each MCBD cycle consist of i. adsorption of Ag+ ions from silver sulphate solution for 20s, ii. rinsing with double distilled water for 10s, iii. reaction with sulphide precursor solution from thiourea solution for 20s and finally iv. rinsing with double distilled water for 10 s. This process was repeated for 30 times to get desired film thickness. The substrates were taken out, washed with distilled water and dried in open air at room temperature. A film obtained was uniform, well adherent and blackish gray in color. The films on the glass substrates were used to study structural, compositional, surface morphological and optical properties. For PEC study, AgS films deposited onto the FTO coated glass substrates were used. Characterizations of thin film: The as-deposited thin films of AgS were characterized for structural, optical and electrical properties. The AgS film thickness was measured by weight difference method. The structural characterization of films was carried out using a Philips (PW-3710) X-ray diffractometer in the scanning range 20–80(2) using Cu K radiations with wavelength 1.5405A°. The semi-quantitative elemental composition was determined by EDAX technique. The surface morphology was studied by scanning electron microscopy (SEM, JOEL-JSM-5600) model. The three dimensional surface morphology of the film was recorded using atomic force microscopy by Quesant Instrument corporation; Q-Scope 250. The optical absorption spectrum of the film was recorded in the range 350–800 nm by means of Shimadzu UV-3101PC spectrophotometer. Photo electrochemical characterization of AgS thin film: The photoelectrochemical cell was devised by employing AgS thin film as photoelectrode, platinum as counter electrode and polysulphide (1M NaOH + 1M NaS + 1M S) as an electrolyte. The distance between photoelectrode and counter electrode was 0.5 cm. The photocurrent was measured under 35 mW/cm light intensity. The current-voltage characteristics were determined in dark and under illumination. The fill factor and power conversion efficiency of the cell was calculated from the power output characteristics. The spectral response of the cell was recorded in the voltage range from - 200 mV to + 400 mV under dark and light illumination using standard AM 1.5 conditions with 100 mW/cm for structure FTO / AgS / Electrolyte / Platinum.Results and Discussion Optimisation of preparative parameters: The preparative parameters for silver sulphide thin films deposited using MCBD were optimized. In the present study, for the concentration of anionic precursor (thiourea) was arbitrarily kept constant as 0.5 M (pH ~ 10) and the concentration of cationic precursor (silver nitrate) was optimized. For cationic precursor, initially 20 ml silver nitrate solution was taken in a 50 ml capacity beaker, and ammonia solution was added to it, which acts as a complexing agent. The pH of the cationic precursor solution was maintained at 10 by adding ammonia solution. Making several trials for different concentration, immersion cycle, and immersion time, preparative conditions for AgS thin films were optimized. Table. 1 shows optimized preparative parameters for AgS thin films on glass substrate. The variation of film thickness against concentration of silver nitrate for 30 deposition cycles is shown in figure 1(a). The AgS film formation was started from concentration 0.025 M of silver nitrate. Initially the film thickness increases with concentration, reaches maximum at concentration 0.1 M. After this AgS film thickness was decreased, this may be due to formation of outer porous layer as film peeled off from the glass substrate26. Figure 1(b) shows variation of AgS film thickness with number of immersion cycles for optimized concentration of silver nitrate (0.1M) and thiourea (0.5M). The AgS film had a maximum terminal thickness of 135 nm at 30 immersion cycles afterwards, AgS film thickness was decreased due to the peeling off the outer powdery layer. Film formation reaction mechanism: The reactions for the AgS film formation can be written as follows: i. Formation of a complex AgNO3 + 4NH3 [Ag (NH)]NO3, ii.Dissociation of thiourea in a alkaline medium as CS (NH + OH CH + HO + HS Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(7), 69-74, July (2013) Res. J. Chem. Sci. International Science Congress Association 71 iii. Formation of Sulphide ion HS + OH S2- + HO, iv. Formation of AgS film 2 [Ag (NH)] + S2- AgS + 2NH3. Figure-1a Variation of AgS thin film thickness with concentration Figure-1b Variation of AgS thin film thickness with deposition cycle Table-1 Optimized preparative parameters forAgS thin films deposited on glass substrate Sr. No. Parameters Precursors Silver nitrate Thiourea 1 Concentration (M) 0.1 0.5 2 Complexing agent Ammonia Ammonia 3 pH 5 10 4 Immersion time (sec) 20 20 5 Rinsing time (sec) 10 10 6 Number of deposition cycles 30 30 7 Deposition temperature (K) 300 300 Structural studies: The XRD pattern of AgS thin film onto the glass substrate is shown in Fig. 2. In the diffraction pattern, five dominant peaks at 24.90, 36.33, 40.55, 43.43 and 44.32corresponding to (110), (120), (031), (200) and (102) planes are seen with polycrystalline monoclinic (acantite) crystal structure .The average crystallite size of was calculated by using well known Debye-Scherrer’s formula. D = 0.9cosWhere D is the crystallite size, is the broadening of diffraction line measured at half of its maximum intensity (FWHM) and is the X-ray wavelength (1.5406Å).The plot of ( 2 versus , which is linear at the absorption edge, confirming that the material has a direct band gap. The calculated average crystallite size was 21 nm. Figure-2XRD pattern of AgS thin film The quantitative analysis of AgS film deposited on glass substrate was carried out using EDAX techniqueat room temperature to study the stoichiometry of the film. The EDAX was recorded in the energy region 0-20 keV. The presence of EDAX peaks for Ag and S are conformed from analysis and the presence of other elements like Si and O from the glass substrate. A typical EDAX pattern is shown in figure 3. Surface morphological studies: The SEM micrograph is a convenient method to study the the microstructure of thin film. Figure 4 shows the surface morphology of the AgS thin films deposited by MCBD at magnification 30,000 X. The scale bar length is 0.5 µm. Films are found to be nanocrystalline. AgS thin film deposited on to glass substrate with 135 nm thickness was used for study of surface morphology. It is observed from the images that the as-deposited AgS film is homogeneous, well covered the substrate without any voids and crakes. The SEM image shows that surface is smooth and well covered with AgS film. It can be seen that nanoparticles are grouped and form large grain. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(7), 69-74, July (2013) Res. J. Chem. Sci. International Science Congress Association 72 Figure- 3 The EDAX spectrum of AgS thin film Figure-4 The SEM pattern of AgS thin film The atomic force miscroscpopy is one of the primary method and foremost tools for imaging, measuring, and manipulating matter at the nanoscale. Fig. 5(a) shows 2D and Fig. 5(b)shows 3D atomic force microscopy images of AgS thin film. The AFM images clearly show the layer like morphology of AgS nanoparticles. The surface roughness of the AgS thin film is 96.35 nm. Optical studies: The optical absorption spectrum of as-deposited AgS film has been recorded at room temperature without considering losses due to reflection and transmission. The absorption spectra of AgS thin film deposited on glass substrate was studied in the wavelength range 350– 800 nm. This spectrum reveals that film has high absorbance (104 cm-1). Figure 6 shows the optical absorption curves for AgS thin film. The figure shows that absorbance decays exponentially with an increase in wavelength. The long absorption tail confirms the formation of AgS thin film. The absorption coefficient for direct band gap is determined by using the relation, ) = A ( h-Eg 1/2where, is the absorption coefficient, h is photon energy, A is constant which is related to the effective masses with the valence and conduction bands, E is separation between valance and conduction bands. The band gap were determined from the intersect of straight line portion of ( versus h graph. The straight line nature of the graph suggests the AgS is ditect band gap material27. The as-deposited AgS thin film shows the optical band gap 2.09 eV. Figure- 5(a) Figure- 5 (b) The 2D and 3D image of AFM image of AgS thin film respectively Photoelctrochemical characterization: Type of conductivity: The PEC cell with AgS as photoelectrode can be represented as n-AgS/polysulphide/platinum. The PEC cell gives some dark voltage V, and dark current I. The polarity of the dark voltage is positive towards the AgS electrode. The origin of this dark voltage is attributed to the difference between two half cell potentials in the PEC cell and can be written as AgS E = Eplatinum – EAg2S Where Eplatinum and EAg2S are the half cell potentials when dipped in the polysulphide electrolyte. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(7), 69-74, July (2013) Res. J. Chem. Sci. International Science Congress Association 73 Figure- 6 Absorbance spectra of AgS thin film on glass substrate PEC Solar Cell Studies: The AgS thin films were deposited onto FTO coated glass substrate using optimized conditions by CBD method. The basic requirements of a good thin film electrode for PEC cells are low resistivity and large crystallite size or grain size. The large crystallite/grain size leads to reduction of grain boundary area of thin film with important consequence for efficient energy conversion. The efficiency of the PEC system is controlled by light intensity, spectral quality, properties of electrolyte, cell configuration etc. The photoelectrode used in the solar cell should satisfy the requirements as, the band gap energy should match with span of solar spectrum, stable throughout the process, should be direct band gap type with high absorption coefficient, charge carrier should have high mobility and life time. The as-deposited AgS thin films were used in the PEC cells as a photoelectrode and their performance as tested by using AgS thin films grown onto FTO coated glass substrate as photoelectrode (anode), the polysulphide as an electrolyte and Platinum (Pt.) as a counter electrode (cathode). The I-V characteristics of the PEC cell formed with AgS thin film electrode in dark and light illumination of 35 mW/cm are shown in figure 7. The non-symmetric nature of the I-V curve in the forward and reverse bias shows the rectification property of the semiconductor electrolyte junction. After illumination, shifting of the I-V curve in the fourth quadrant of the graph suggests that the electrons are the majority carriers, confirming the n-type conductivity of AgS and act as generator for electricity. Increase in the current under illumination indicated that the AgS film is photoactive. The power conversion efficiency ( %), fill factor (FF %), short circuit current (Isc), high series resistance Rs and low shunt resistance Rsh calculated from I-V Curve are listed in the table 2. Table- 2Photovoltaic parameters for AgS thin film Sr. No Photovoltaic parameters Values 1 I sc (mA/cm 2 ) 0.080 2 FF 0.085 3 V oc (V) 0.325 4 (%) 0.002 5 R s () 291 6 R sh () 709 The low efficiency in this investigation may be due to high series resistance Rs and low shunt resistance Rsh of the cell and interface states, which are responsible for the recombination mechanism. Figure- 7 Current-Voltage (I-V) output characteristic of AgS polysulphide/Pt. PEC cell ConclusionIn this work the silver sulphide thin film on glass and FTO coated glass substrate have been successfully deposited using, a simple, cost effective, entirely reproducible modified chemical bath deposition (MCBD) method. Good quality AgS thin films were deposited. XRD study showed that the grains are small. The films are nanocrystalline having monoclinic crystal structure. The SEM study reveals that a uniform distribution of particles on substrate. The films are found to have high absorbance. 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