http://jurnal.untirta.ac.id/index.php/Gravity
ISSN: 244-515x; e-ISSN: 2528-1976 Vol. 9, No. 1, February 2023, Page 72-81
Review of synthesis Cu
2ZnSnS
4using ball-milling method as thin film solar cell absorber layer
Maghfirani Aulia Rachman, Eka Cahya Prima*, Andhy Setiawan
1Solar Energy Materials Laboratory, Universitas Pendidikan Indonesia, Indonesia
*E-mail: ekacahyaprima@upi.edu
(Received: 10 September 2021; Accepted: 15 February 2023; Published: 27 February 2023)
ABSTRAK
CZTS (Cu2ZnSnS4) is a readily available, cost-effective, and non-toxic material that uses as a material for absorbing coatings on solar cells. This study aims to examine making CZTS using the Ball-milling method. This method uses a cylindrical grinding machine to reduce coarse materials into more delicate fabrics. The CZTS synthesis process using ball-milling produces better results than other non-sputtering methods. In addition, the synthesis of CZTS using ball-milling can increase the yield of better solar cells. However, in ball-milling, there are still shortcomings, one of which is the presence of heterogeneous elements after the synthesis.
Keywords: Ball milling, CZTS, solar cell, synthesis
DOI: 10.30870/gravity.v9i1.12348
INTRODUCTION
Of the various renewable energy sources, solar energy is the best alternative to use as an alternative to fossil fuels, where solar energy is easy to reach in today's society (Chapin, Fuller,
& Pearson, 1954; Green, 1982; Shockley & Queisser, 1961). CZTS (Cu2ZnSnS4) is a solar cell material that is easy to obtain, cost-effective and non-toxic, can be used as a material for absorbent coatings on solar cells. CZTS (Cu2ZnSnS4) is a solar cell material that is easy to obtain, cost-effective, non-toxic, and can use as a material for absorbing coatings on solar cells (Panatarani et al., 2020; Prawira, Prima, Refantero, Setyo, & Panatarani, 2020; Prima, Wong, Ibrahim, & Yuliarto, 2021). CZTS is a crystalline semiconductor compound (Ravindiran &
Praveenkumar, 2018). CZTS has a bandgap value of 1.45-1.5 eV with an absorption coefficient value above 104 cm-1. The use of CZTS has an efficiency that can reach 12.6% (Wang et al., 2014).
The development of the use of CZTS has existed since 1996 using the sulfurization technique. In his research using CZTS, the efficiency value was 0.66% (Katagiri et al., 2009).
However, until 2006 using CZTS, the efficiency value increased to 5.7% (Guo et al., 2010;
Katagiri et al., 2009). CZTS can be synthesized by several methods, including sputtering, thermal evaporation, pulsed laser deposition, sulfurization, electron beam evaporation, sol-gel sulfurization method, and more (Babu, Kumar, Bhaskar, & Vanjari, 2010; Ma et al., 2020;
Ojeda-Durán et al., 2020; Schubert et al., 2011; Sun et al., 2012; T. Tanaka et al., 2005).
Another method that can use is the synthesis of CZTS using ball-milling (Gu, Yin, et al., 2019b; Li et al., 2015; Pareek, Balasubramaniam, & Sharma, 2016; Pulgarín-Agudelo et al., 2017; Ricardo et al., 2013; Singh et al., 2018; Song, Teymur, Zhou, Ngaboyamahina, & Mitzi, 2021; Yao, Wang, Wang, Wang, & Zhang, 2014; Zhang, Fu, Zheng, & Wang, 2019). Ball mill is one type of grinding machine in the form of a cylinder that functions and is used for crushing hard materials into delicate materials. According to Gu and Lin (2019); Gu, Yin, et al. (2019a), ball-milling is more effective because it can make CZTS on an industrial scale. In addition, by using ball-milling, the solid content in the precursor is more than the pure precursor solution;
the precursor solution with ball-milling is environmentally friendly. However, the drawback of this synthesis using ball-milling is the lack of elemental homogeneity, or some elements still exist that are difficult to refine.
In the research of Ricardo et al. (2013), ball-milling is used to mix the powder precursors used and grind the precursor grains to produce nano-sized precursors and obtain homogeneous and well-dispersed results. This study aims to examine the process of making CZTS using the Ball-milling method so that the best CZTS crystals produce to develop non-vacuum CZTS synthesis.
RESEARCH METHODS
Before entering the ball-milling method, there are several non-sputtering methods that previous researchers have carried out. In the research of Ennaoui et al. (2009), CZTS is synthesis using Atotech Deutschland GmbH on Mo glass. In his research, three electrodes were used: Ag/AgCl, inert anode, and Mo working electrode. These materials will be synthesized on a 3mm glass substrate with an area of 10×10 cm2, and these materials are immersed in a solution containing sodium-pyrophosphate.
In addition to using non-vacuum electroplating techniques in synthesizing CZTS, some researchers use the sol-gel sulfurization method (Akhavan et al., 2012; Ashfaq et al., 2019;
Buwarda, 2019; Ferdaous et al., 2019; Kahraman, Çetinkaya, Çetinkara, & Güder, 2014; Patel
& Gohel, 2018; K. Tanaka, Fukui, Moritake, & Uchiki, 2011). For example, in the research of K. Tanaka et al. (2011) the substrates used were Mo and SGL. The materials used are copper (II) acetate monohydrate, zinc (II) acetate dihydrate, and tin (II) chloride dihydrate, which are dissolved in 2-methoxy ethanol. So that the substrate used is not eroded when given the sol-gel solution, the substrate must be coated with a lower concentration of a sol-gel solution. For the stable sol-gel solution, deionized water and ammonium acetate were added for the lower concentration of a sol-gel solution and mono ethanol solution to stabilize the sol-gel solution
with a higher concentration. Then annealing was carried out in N2 and H2S atmosphere at 500C for 1 hour.
Several studies using ball-milling as a non-sputtering synthesis method:
a. Synthesis of Cu2(Zn, Fe)SnS4 (Azanza Ricardo et al., 2013)
In his research, researchers added Fe doping to CZTS so that the structure of the CZTS was better than before. The materials used for the precursor of CZTS are Cu powder (<75μm, 99%), Zn powder (purum, 99%), Fe powder (99%), Sn powder (purists, 99%), and Sulfur flux (purum, 99.5%), ethanol (99.8%), α-terpineol (96%), and ethyl cellulose. CZTS synthesis using ball milling (80 mL stainless steel container, 25 balls with 12mm diameter). The relative ratio used is between ω = 540 rpm and = 300 rpm. Then after that, it was heated on argon flux with a temperature of 550C and 2 hours.
b. Synthesis of Cu2Zn1-xCdxSnS4 (Gu, Yin, Han, Zhou, Tai, Zhang, Zhou, et al., 2019) In his research, researchers synthesized CZTS and CZCTS with sulfur and without the addition of sulfur. The materials used are CdCl2•2.5H2O, Zn powder, Cu powder, S powder, SnCl2•2H2O. The solvent used is ethanol. These materials synthesis using a ball mill using 80.0 g balls of zirconium oxide at a speed of 300 rpm and 12 hours (including a total rest time of 2 hours).
c. Synthesis of Cu2ZnSnS4 (Gu, Yin, Han, Zhou, Tai, Zhang, Li, et al., 2019)
The materials used are Cu powder, Zn powder, SnCl2•2H2O, S powder. The solvent used is ethanol. Ball-milling was carried out using 80.0 g zirconium oxide balls, speed of 300 rpm, for 60 hours (including a total break time of 20 hours). In this research, the parameter calculated is the time of heating by adding a high boiling point solvent (HBPS) into the prepared precursor.
In addition to the synthesis of CZTS, ball-milling is used to synthesize other materials such as TiO2, which will operate as an electron transport layer in perovskite solar cells.
d. Synthesis of TiO2 (Singh et al., 2018)
The material used is TiO2 powder (97%) with the solvent used IPA. The materials were put into a container and mounted on a mill and 600 g of micro zirconia beads. Milling was carried out at 2000 rpm for 360 minutes.
RESULTS AND DISCUSSION
Using a non-vacuum electroplating technique method (Ennaoui et al., 2009), the resulting stern based on the results of characterization using EDX, namely in the form of large grains, pores on the surface of the substrate also show a reasonably large pore density. Furthermore, the reaction between the metal materials used has not yet been formed to completion. The results obtained using the sol-gel method after XRD's characterization show the absence of a secondary phase. The content of Zn and Sn in the synthesized CZTS decreased due to evaporation caused by the lack of the second phase in the CZTS thin film (K. Tanaka, Oonuki, Moritake, & Uchiki, 2009).
From the ball-milling results that were carried out for 3 hours, the powder from CZTS became smaller in size, but the Zn Sn fibers are not wholly defined by the ball-milling (Ricardo et al., 2013). However, after heating, the pure powder can be dissolved. Based on the XRD
results that researchers have carried out, the ball-milled materials are not optimal. Therefore, the researchers suggest further increasing the materials that are not milling because they correlate with the progressive milling process. The researcher explained that this was because of wearing out the container used for the ball mill, so the researcher recommended optimizing the heat treatment carried out so that the CZTS could shape in a smaller size and the heat treatment, there was no trace of metal residue.
Figure 1. XRD results. Effect of progressive grinding (left) and heat treatment on ground powder (right) (Azanza Ricardo et al., 2013).
Based on the SEM-EDAX and XPS results in Figure 1, we can say that the effects of this milling did not contain Fe content from the ball-mill container used. However, the result of this Fe is Fe metal which the researcher added.
In the research of (Gu, Yin, Han, Zhou, Tai, Zhang, Zhou, et al., 2019) using this ball- mill, solar cells have an efficiency of 7.5%. The researchers milled CZTS/CZCTS with and without the addition of sulfur, CZTS/CZCTS without the addition of sulfur synthesized with this ball-mill had a significant effect on the results. The researchers also suggested doing further research by changing the ratio of the sulfur used.
In his research, the IV curve value obtained is the most considerable current in the CZCTS sample without S, which is approximately 23 mA/cm2 with a voltage value of 0.55 V. While the highest voltage value is in the CZTS sample without S, which is 0.62 V. with a current of 16.2 mA/cm2. IPCE results show an increase in Jsc, Voc, FF, and PCE values. This result happens, influenced by the large grains obtained, the importance of the conduction band alignment of the absorbent/buffer layer, and others. Without the addition of S, this also affects the results of IPCE, as shown in Figure 2.
Figure 2. I-V curve. b) IPCE spectrum (Gu, Yin, Han, Zhou, Tai, Zhang, Zhou, et al., 2019).
Based on the SEM results in Figure 3, the research of (Gu, Yin, Han, Zhou, Tai, Zhang, Zhou, et al., 2019) explained that with the addition of S, the film's surface is uneven for the size of the material used; there are small and large powders. Whereas without the addition of S, the surface of the film obtained is neater, the material on the surface of the film has large grains, which are the same size. In the XRD results, a higher peak was obtained for the sample without the addition of S.
Figure 3. SEM results. a) CZTS with powder S, b) CZTS without powder S, c) CZCTS with powder S, d) CZCTS without powder S (Gu, Yin, Han, Zhou, Tai, Zhang, Zhou, et al., 2019).
Figure 4. a) XRD pattern, b) Steady-state of CZTS/CZCTS PL films with and without S, c) CZTS/CZCTS bandgap values without the addition of S (Gu, Yin, Han, Zhou, Tai, Zhang, Zhou, et
al., 2019).
In his research (Gu, Yin, Han, Zhou, Tai, Zhang, Li, et al., 2019) performed the addition of HBPS to eliminate the presence of heterogeneous grain growth after ball milling, reducing the surface roughness of the CZTS fil. In his research, the researcher then heated the results of the CZTS synthesis using ball-milling. The addition of HBPS was very positive, whereas after heating with the addition of HBPS, the film formed was more even. Hence, it was possible to
apply thin and transparent ETL.
Figure 5. a) A top-view SEM image and (c) a cross-sectional image of a CZTS film processed by typical ball milling after 30 min of annealing; b) top view SEM image and (d) 5000 r.p.m cross- section image. HBPS-treated ball milling CZTS films after 90 min annealing. Top-view SEM image of
e) CZTS film processed by typical ball milling after 90 min annealing, f) 3000 r.p.m. Ball-milling processed CZTS films treated with HBPS after 30 minutes of annealing, g-j) processed CZTS films
treated with HBPS at 2000 r.p.m., 3000 r.p.m., 4000 r.p.m. and 7000 rpm after 90 minutes of annealing. Set annealing conditions at 530°C in 5% H2S/N2 atmosphere (Gu, Yin, Han, Zhou, Tai,
Zhang, Li, et al., 2019).
Based on the SEM results in Figure 5, as well as the XRD results that have been carried out, the researcher explained that the CZTS treatment, which was the process by ball-milling treated with HBPS at 5000 rpm, got the best results, where the sample with 5000 rpm produced the highest intensity. Furthermore, based on the IPCE results, the result can say that the addition of the HBPS significantly affects the increase in the IPCE results, where can apply the addition of this HBPS to the manufacture of a ZnCdS buffer layer too.
From the research results of Singh et al. (2018), after grinding for 6 hours, the suspension has a milky color; wherein the independent layer on top of the zirconia beads is visible. The top layer of TiO2 NP is separated; the residue from zirconia beads settles to the bottom of the chamber due to its higher density. The final concentration of ground TiO2 paste (G-TiO2 NPs) was 3wt%.
Figure 6. (a) TEM results of pure an-TiO2 powder; inset: appropriate diffraction pattern. (b) TEM results of ground TiO2 (NP G-TiO2). (c) G-TiO2 NP selective area diffraction pattern with orientation
(101). (d) HRTEM results of G-TiO2 NPs; inset: corresponding FFT pattern (Singh et al., 2018).
Based on the results of SEM Figure 6 in the research, milling using ball-milling makes the crystals of TiO2 more delicate. The bandgap energy obtained for the G-TiO2 and S-TiO2
samples is 3.13 eV and 3.01 eV, respectively.
From several studies that have been described previously, the use of this ball-mill as a synthesis method does not require a significant initial investment. This ball mill is also used to refine the structure of the precursor used. In some studies, using ball-milling can increase the solar cell's efficiency.
CONCLUSION
Ball-milling is one method that can use without a significant initial investment. Synthesis of CZTS using ball-milling can increase the yield of better solar cells. However, in ball-milling, there are still shortcomings, one of which is the presence of heterogeneous elements after the synthesis. Nevertheless, the ball milling method can prove again that ball milling is effective;
it is possible to conduct research focusing on the length of milling time as a parameter or the magnitude of the speed at which CZTS is milled.
ACKNOWLEDGEMENT
This research was funded by the Higher Education Excellence Research Grant (No.
272/UN40.LP/PT.01.03/2021), Ministry of Education and Culture of the Republic of Indonesia 2021. This work was also partially supported by Penelitian Unggulan UPI 2023.
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