Synthesis of Cu 2 ZnSnS 4 (CZTS) Ink by an Easy Hydrothermal Method

. The quadrilateral p-type semiconductor Cu 2 ZnSnS 4 (CZTS) with a direct bandgap of 1.4 to 1.5 eV and a 14 high absorption coefficient in the visible light range, is considered an excellent absorbent layer in the 15 production of solar cells. The application of Cu 2 ZnSnS 4 film absorbent materials is promising in the field 16 of low-cost solar cell production. In this paper, a simple, efficient, controllable, and inexpensive 17 solvothermal method is used to make the CZTS nanoparticles from zinc acetate, copper acetate, tin 18 chloride, thiourea, and hexadecyl amine solvent. The ink was prepared from the CZTS powder and 19 applied by the doctor Blade technique on soda-lime glass. The X-ray diffraction (XRD) and Raman 20 spectroscopy analysis showed that the CZTS synthesis nanoparticles had a pure Kesterite structure. The 21 thermo-gravimetric analysis showed about 12% of the loss weight of CZTS nanoparticles using field 22 emission scanning electron microscopy, energy dispersive spectroscopy, dynamic light scattering, and 23 zeta potential indicated that the synthesized nanoparticles had a strong absorption in the range of 125-477 24 nm with an average particle size of 300 nm and plate shape. The energy bandgap of CZTS nanoparticles 25 was measured to be 1.49 eV using UV-Vis spectroscopy.


Introduction 1
In the last decade, copper-based quaternary semiconductor material Cu2ZnSnS4 (CZTS) was 2 widely considered as a most promising absorber layer candidate for low-cost thin-film solar 3 cells, owing to its high absorption coefficient (>10 4 cm -1 ) and optimal bandgap (1.0-1.5 eV) 4 which are ideal properties for the application under solar irradiation. [1][2][3][4] .The compounds such as 5 copper-zinc, tin-sulfur accompanied by non-toxic and friendly environmental solvents have 6 many photovoltaic technology applications. 5,6 The features of these compounds make them 7 suitable for widespread and low-cost applications in the thin-film solar cells. 7 Furthermore, all high-performance solar cells using non-volatile methods. 6,7 One of the crucial issues in 15 fabricating high absorbance coefficient thin-films finds the suitable ink and preparation methods 16 due to non-toxic solvents and elements. Nowadays, most materials and solvents used in solar 17 cells are not suitable from this point of view. 8,9 The function of a solar cell is highly sensitive to 18 optical and electrical properties, which greatly depends on the crystalline structure and the 19 absorbent material's composition. Therefore, to achieve the desired stoichiometry, understanding 20 and optimizing the growth and formation of phases in photovoltaic materials is highly important. 21 CZTS, as a quadruple combination, often involves dual and triple phases, which lead to difficult 22 stoichiometry control. Therefore, it is necessary to have good control over the synthesis 23 3 parameters to obtain the desired phase materials. 10 Among different morphologies, nanocrystals 1 (NCs) provide the bandgap engineering in which there are several electron-holes series per 2 photon, and provide opportunities for using in high-performance photovoltaic devices. The NCs 3 powders can be synthesized through a wet chemical route for preparing the nanoparticle inks. 11 4 Although the nonvacuum methods of manufacturing are attractive in terms of their low complex 5 fabrication route, cost ,and scalability, the need for using toxic solvents and organic/metal 6 solutions containing large amounts of pollutants to reduce the causes of cracks during the 7 dissolution process is challenging. 12-15 Consequently, determining the desired properties of 8 CZTS nanocrystals for inkjet printers requires designing new nanoparticle synthesis methods.

9
The ability to control the crystallinity, morphology, and size of the CZTS nanocrystals provides 10 an opportunity to examine further these properties' effects on the formation and synthesis 11 behaviors. Among various process methods that lead to the formation of developed oxide 12 nanoparticles, numerous strategies were implemented including using pre-made nano-reactors 13 derived from surfactants, 16  process was also used to synthesize CZTS. 29  The chemicals used in this study to synthesize the chemical composition of Cu2ZnSnS4 included: was placed in an ultrasonic bath and finally in an autoclave container (internal Teflon liner). In a 20 way, 50% of the container (50 ml) was filled). The autoclave was sealed for 4 hours at 200°C.

21
Then, it was slowly cooled in room temperature. The resulting precipitate was filtered and 22 washed several times with distilled water and pure ethanol to remove organic matter and dry in 23 5 the air. To produce the ink, the final obtained powder was added to alpha-terpineol (Aldrich, 1 96%), and the doctor blade technique applied the ink on a 20 ×10 mm 2 soda-lime glass. The 2 film was dried on a hot plate at 150°C. The flow chart of process is shown in Fig. 1. 3 The CZTS ink with a coating thickness of 2 µm was applied to the substrate surface in two steps.

4
The films were heated at 300, 350, 400, 450, 500, and 550 °C for 20 minutes in a tubular furnace 5 (laboratory tunnel furnace with quartz chamber) using reduced conditions of 95% N2. The heat 6 treatment was performed in the presence of pure sulfur and tin to prevent the presence of vapors 7 and increase the grain size. The CZTS film formation process mechanism (reaction and growth) 8 can be divided into three stages (equation (1)): After stirring and mixing the above mixture in ethanol, the color was initially blue to green-blue.  The free ions of Cu 2+ and Sn 4+ are preferable to Zn 2+ ions to join Tu in the final solution. After  Fig. 2 showed that the first weight loss of CZTS occurred at a relatively low temperature (240°C) 2 with a significant amount of volatile substances (about 12% of the weight). This weight loss 3 corresponds to thiourea decomposition. 6 It is worth noting that the complete elimination of 4 volatile substances with a higher weight loss occurs at 600°C in the same samples. Where β is the line broadening at half of the maximum intensity, and λ is the X-ray wavelength 18 (for λ =1.5418 cm -1 , k=0.9). Raman spectrum was performed using a Jobin-Yvon HR800 Raman The DLS of samples were measured based on time-dependent spatial oscillation fluctuations, and 16 these data were used to calculate the emission factor and particle size. The relation between the 17 particle size and its emission factor is defined in the Stokes-Einstein equation (5): Where, D is the penetration coefficient, T is the absolute temperature, is liquid viscosity, K is 19 the Boltzmann constant, and d is the particle size. To obtain reliable information on particle size, for non-spherical, flat, rigid particles and is approximate for others. Zeta potential using Zeta 1 PALS Zeta Potential Analyzer, Brookhaven Instruments Corporation, US) using a low and high 2 electric field, E = 137 Vcm -1 and 274 Vcm -1 , using palladium electrodes and Zeta potential 3 amounts measured from two fields which were tested at least 10 times and constant.  Further, the average crystallite particle size of samples calculated by Scherrer (Fig. 4). According  The X-ray diffraction analysis, it is possible to fully match the peaks of CZTS polymorphs and 1 secondary phases (such as CTS and SnxSy); therefore, the results of XRD analysis measurements 2 are insufficient to describe the separation of kesterite and CTS phase. Raman spectroscopy is a 3 common study method to investigate the structures of chalcogenide and kesterite. The Raman 4 spectra of synthesized CZTS sample films are shown in Fig. 5. 5 All films generally have a strong peak at 336 cm -1 and a fairly weak peak at 287 cm -1 . There is 6 no definite peak in the samples synthesized at 300 and 350 degrees. Comparing the samples' 7 shape indicates that the intensity of the peaks slowly increases with increasing temperature. 8 Raman results indicate that the ink prepared by the solvothermal method had a pure kesterite 9 phase which is compatible with XRD peaks. It is worth noting that there is a trace of secondary 10 phases in two of the samples (A and B). In other samples, more peaks at 303 and 224 cm -1 are 11 detected which is related to tetragonal CTS and SnxSy phases, relatively. 33

12
To better describe these volatile compounds' identity, FTIR analysis was performed at intervals 13 of 500 to 4000 cm -1 and showed different peaks with the most intense peaks at 3330, 1629, and 14 1415 cm -1 (Fig. 5). The strong peak absorbed in 2938 and 2369 cm -1 is due to the S-H bond. The 15 peaks at 1629 and 1415 cm -1 are due to the C = S and NH2 bond, respectively. Moreover, the 16 removal of organic compounds shown; the results were following the TGA curve. 34

17
FESEM was used to analyze the surface morphology of CZTS samples. As can be observed from 18 Fig. 7, the CZTS films consist of agglomerated and porous form interconnected spherical grains.

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By increasing the temperature, CZTS particles grew plate shape which became isolated. The sample heated at 550°C, the particles turned into a curved petal. It can be seen that the length of 23 particles has increased from 125 to 477 nm by increasing temperature which showed in Fig. 7. 1 The CZTS fine grains were connected to form the dangling bonds and chain-similar matrix.
2 Also, the results are matched with the XRD and Raman spectroscopy results discussed above.

3
Due to the XRD, Raman Spectra, and FESEM analysis results, sample E, structurally, and 4 morphologically showed better properties and studies.

5
The EDS analysis indicate the situation of (Cu-Zn-S-Sn), (Al-Ca-Si), and (Au) elements, which 6 can be referred to as the CZTS layer, the substrate, and the coating, respectively. In sample E, the 7 composition proportion of CZTS layer is Cu/Zn/Sn/S =2.8/1.5/0.95/5.5. 8 The quantities results show that the chemical composition of sample heated at 500ºC involved 9 zinc -abundant and tinneedy (Fig. 8). According to studies by Bandres 35 and Chen et al., 36 the 10 best result to use in a solar cell can be achieved in zinc-rich and the copper-poor conditions. Zinc 11 is critical in the final performance of device.

12
The UV-Vis Transmittance spectrums of samples are drawn in Fig. 9. The size range of CZTS ν is the frequency of radiation (Hz),

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A is an appropriate constant, and Eg is the bandgap (eV).

20
The sample bandgap is in the range of 1.49 to 1.62 eV. According to the graph (hν) versus (αhν) 2 21 as shown in Fig. 9, the bandgap energy in the synthesized sample at 500°C has the best value