Akanksha Paraye1, S. Noyel Victoria2, Arpita Shukla3, R.Manivannan4
1,2,3,4 Department of Chemical Engineering, NIT Raipur (CG), India
Silicon based solar cells are widely used solar cells but it need very thick absorber layer. On the other hand it is very expensive. Therefore, the focus over thin film based solar cells has increased now-a-days. Absorber layers based on cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), copper indium diselenide (CIS) and copper zinc tin sulfide (CZTS) are extensively studied for thin film solar cells. Use of CdTe for thin film solar cells is less preferred due to its toxicity. In comparison with CIGS, thin film solar cells with CZTS absorber layers are preferred since the raw materials are earth abundant, nontoxic and inexpensive. The CZTS materials have high absorption coefficient of 104 cm-1 and have the band gap energy of 1.4-1.5 eV 1. In this work, single step electrodeposition of Cu2ZnSnS4 (CZTS) thin film on gold coated slide using glycine as a complexing agent was carried out. The effect of different annealing time on surface morphology, crystallite size and its element composition was studied. X-ray diffraction (XRD) analysis reveals the kesterite phase of CZTS. By using XRD data and Debye-Scherrer’s formula crystallite size was calculated for different annealing time which shows that with increase in annealing time the crystallite size was also increasing. Deposited CZTS thin film annealed for 30 min. shows the elemental composition near to the desired stoichiometry (Cu:Zn:Sn:S = 2:1:1:4).
Keywords – Thin film, Electrodeposition, CZTS, Annealing time, XRD, Crystallite size.
Enhancement of world population, the consumption of conventional energy and its harmful side effects on environment are also increased. It is expected that demand of energy to be twice that of today’s energy demand i.e. approximately 30 TW. The consumption and demand of this energy for next few years will be very challenging 1. The present energy demand is fulfilled by fossil fuel such as oil, coal and gasses which emits toxic gasses after its consumption and on other hand they are not promising, since there quantity will be not sufficient to fulfill the future energy demand. Therefore, the world is focusing on renewable energy sources to fulfill this energy demand with least side effect on environment. Solar energy is found to be an alternative and promising way to fulfill the present and future energy demand and it is also known as one of the green and clean energy. One of the best applications of solar energy is solar cell or photovoltaic cell. Solar cell is an electric device which converts the solar energy directly into the electricity due to photovoltaic effect which is a physical and chemical phenomenon. Solar cell can be broadly divided into three generations. The first generation solar cells are also known as a conventional, traditional or wafer-based cell which is made of crystalline silicon. Silicon based solar cell are widely in use. But there are many demerits of silicon, such as it have an indirect band gap material, with low absorption coefficient, needs thick absorber layer and they loss their efficiency at higher temperature i.e. in hot sunny days. Second generation solar cells are basically thin film solar cells. Which include amorphous silicon; cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS). Materials used in second generation solar cells as an absorber material are rare, expensive and toxic. The availability of indium (In) and selenium (Se) makes them expensive where as cadmium (Cd) is toxic to environment 2. Third generation solar cells includes a number of thin-film technologies often described as emerging photovoltaic. These solar cells are made of organic based materials. Most of them have not yet been commercially applied and are still in the research or development phase. These solar cells are not promising cells as they loss their efficiency with very short time period. In last ten years, thin film solar cells have attracted the researchers most due to its requirement of less absorber material which reduce the cost of thin films; absorber materials for thin film can be flexibly deposited on substrates such as glass, stainless steel, and plastic, especially suitable for solar building integration 1. Inorganic absorber materials presently in use are copper indium gallium diselenide (CIGS), copper indium diselenide (CIS) and cadmium telluride (CdTe). Elements of these absorber materials also have some demerits such as availability of indium (In) and selenium (Se) makes them expensive and cadmium (Cd) is toxic 3. Absorber materials for thin film technology with desirable requirements such as being inexpensive, environmental friendly with direct band gap are under study. Compared to commercialized absorber materials, CZTS have the benchmark characteristics such as it is composed of only earth abundant, inexpensive and non-toxic elements. Cu2ZnSnS4 (CZTS) is a promising absorber material, with direct band gap of 1.4-1.7 eV and high absorbance coefficient of 104 cm-1 for thin film solar cell, which is expected to become the ideal absorber layer material for next generation thin film solar cell 4. There are various routes for the preparation of CZTS nanoparticles and thin films for the solar cell application. Techniques such as thermal evaporation 5, atom beam sputtering 6, pulsed laser deposition 7, hybrid sputtering 8 etc. is well established routes for the deposition of CZTS thin films. But on other hand they are very expensive and require high energy to operate, with very high vacuum pressure. However, there are other techniques also which demands low energy such as successive ionic layer adsorption and reaction (SILAR) 9, chemical bath deposition 10, spray pyrolysis 11 and electrochemical deposition 12. Electrochemical deposition is one of the well known low energy demand technique as it provides scalable deposition of the desired film at room temperature. In this present work, the electrodeposition of CZTS thin film on gold coated substrate under potentiostatic mode was carried out. Glycine was used as a complexing agent in this study. The deposited sample was then annealed for different time period at constant temperature. Effect of annealing time on its surface morphology, crystallite size and its elemental composition was studied.
2. MATERIAL & METHODS
Gold coated microscopic slide having thickness of 100Å (Sigma-Aldrich, 99.99% Au) as a substrate was used for electrodeposition of CZTS. Precursors of analytical grades (Merck) were used for the deposition of CZTS thin film. Copper sulfate pentahydrate (CuSO4.5H2O) was used as a copper precursor, zinc sulfate heptahydrate (ZnSO4.7H2O) was used as a zinc precursor, tin chloride (SnCl2) was used as a tin precursor and sodium thiosulfate (Na2S2O3) was used as a sulfur precursor. Glycine as a complexing agent was used in this study to narrow down the cathodic potential. Electrolyte was prepared by using deionized water having concentration 10mM, 20mM, 5mM, 80mM and 100mM of Cu, Zn, Sn, S and glycine respectively. HNO3 or KOH solutions were used to adjust the pH at 2.5. CZTS deposition was performed under potentiostate mode.
Deposition studies of CZTS were performed in an electrochemical workstation, CHI 660E, CH instruments, USA. Experiment for CZTS deposition was carried out using three electrode configurations (counter, reference and working). Three electrodes were platinum wire and Ag/AgCl as a counter and reference electrodes respectively. The working electrode was prepared by using gold coated microscopic slide, having dimensions of 10mm × 10mm × 1.1mm. Cyclic voltametry (CV) run for the electrolyte having all precursors with complexing agent was carried out to find out the suitable deposition potential of CZTS. CV run was carried out with scan rate of 0.5 v/sec. Deposition of CZTS was done for 20 mins under potentiostatic mode. Obtained deposition was then washed using deionized water, dried and annealed at 300°C for different time period (30 min, 45 min, 60 min & 1h: 15min.) in inert environment. Scanning electron microscope (SEM) (Zeiss Evo-Model EVO 18) was used to study the morphology of the deposits. Elemental composition (EDX) (INCA 250 EDS with X-MAX 20mm Detector) was also done for elemental composition. X-ray diffraction (XRD) (PANalytical 3 kW X’pert) was used to analyze the crystalline nature of the deposits. The energy band gap was calculated by using absorption spectra obtained by Shimadzu-UV-1800, UV-visible spectrophotometer (UV-vis).
3. RESULTS & DISCUSSION
3.1 Cyclic Voltammetry
The cyclic voltagrams obtained for CZTS in presence and absence of complexing agent are shown in Fig. 1. Elements copper, zinc, tin and sulfide have different deposition/reduction potentials due to which it’s very hard to co-electrodeposite from a single electrolyte 13. In order to narrow down this reduction potential gap between all the elements glycine was used as a complexing agent to an electrolyte and reduction potential was optimized 14. Effect of complexing agent can be clearly seen by obtained cyclic voltagrams shown in fig.1. After addition of glycine as a complexing agent the reduction potential shifted more near to the negative direction and an optimal reduction potential for CZTS+glycine was found to be -0.86 V (Vs Ag/AgCl).
Fig.1 Cyclic voltagrams in presence and absence of complexing agent.
In order to analyze the effect of annealing time on surface morphology (Fig.2) and elemental composition (Table 1) of the CZTS thin films, SEM measurement was carried out. Morphological study for all the samples annealed for different time duration shows agglomeration of particles in nature and this helps in preventing recombination for solar cell applications 14. Sample annealed for 30 min shows small and spherical particles in their shape. Sample annealed for 45 min showed big size of particles compared to sample annealed for 30 min 15 and rod type features on the surface can be seen which are highlighted with red circles. Similarly, samples annealed for 60 min and 1h: 15 min particles got flat in shape.
Fig. 2 Effect of different annealing time on surface morphology.
Effect of different annealing time on elemental composition was also carried out. It can be seen that sample annealed for 30 min shows closer to desired stoichiometry (Cu:Zn:Sn:S = 2:1:1:4).
Table 1. Effect of different annealing time on elemental composition (Atomic %).
The X-ray diffraction patterns of annealed samples for different annealing time are shown in Fig. 3. The XRD spectra recorded for sample annealed at 300°C for 30 and 45 minutes shows amorphous in nature. Small diffraction peaks was observed at 28.6° which corresponds to (1 1 2) crystal plane of kesterite CZTS COD (Crystallography Open Database) 96-900-4751. It is observed that with increase in annealing time the intensity (1 1 2) of diffraction peak becomes sharp, which indicates that the crystalline nature of CZTS thin film is also improved with increase in annealing time. The diffraction peak positions are not shifted with annealing time, suggesting that CZTS phase is stable and its formation is independent of annealing time. XRD spectra obtained for all the samples also shows that no secondary phases are formed.
Fig. 3 X-ray diffraction patterns of CZTS thin films annealed for different annealing time.
Effect of annealing time on crystallite size (Table 2) in nanometer was calculated by using Debye-Scherre’s formula 16.
d = crystallite size (nm).
? = the incident X-ray wavelength (Å).
? = broadening of diffraction line full width half maximum (Radians).
? = diffraction angle of the prominent peak.
It is observed from XRD spectra that with increase in annealing time the crystalline nature of CZTS thin film is also improved. The calculated crystallite size for all the samples annealed for different time is shown in table 2. The calculated values shows increase in crystallite size 12.932 nm,13.772 nm, 16.142 nm 16.109 nm with respect to annealing time 30min, 45 min, 60 min, and 1h:15 min respectively. Similarly the effect of annealing temperature on crystallite size has been reported 17.
Table 2. Effect of different annealing time on crystallite size (d).
The absorbance spectra for sample annealed for 30 min is shown in fig.4. It was found that the sample shows a good absorbance in the entire visible spectrum, thus suitable for solar cell application.
Fig. 4 Absorbance spectra of deposited CZTS annealed at 300°C for 30 min.
The band gap of the sample was calculated from absorbance data by using the Tauc relation 14
? = absorption coefficient.
E = photon energy in eV.
Eg = band gap energy in eV.
n = constant which is assigned values 1/2, 3/2 and 3 for direct allowed, indirect forbidden and indirect allowed materials respectively.
The Tauc plot for sample annealed at 300°C for 30 min (gave closer stoichiometry) is shown in Fig.5. It is observed that the band gap for the sample is 1.65 eV, which is an optimal band gap for the solar cell applications 1-12.
Fig. 5 Tauc plot of CZTS annealed at 300°C for 30 min.
Single-step electrodepositon of CZTS on gold substrate using glycine as a complexing agent was successfully done. XRD result shows that with increase in annealing time, crystallite size was observed to be increased too. The sample annealed for 30 min at 300°C shows closer to the desired stoichiometry. EDAX result also shown that the sulfur concentration was decreased with increase in annealing time. Sample annealed for 30 min at 300°C shows an optimal band gap of 1.65 eV.
Authors would like to thank the Department of Science and Technology – Solar Energy Research Initiative (DST-SERI) for financing the research work under the Grant No. SEDST/TMC/SERI/2K12/57 and DST-SERB under the Grant No. SR/FTP/ETA-412/2013 for the analytical facilities.
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