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Fabrication of Cu2ZnSnS4 thin films using a Cu-Zn-Sn-O amorphous precursor and supercritical fluid sulfurization
Accepted Manuscript Fabrication of Cu2ZnSnS4 thin films using a Cu-Zn-Sn-O amorphous precursor and supercritical fluid sulfurization
Yuta Nakayasu, Takaaki Tomai, Nobuto Oka, Kanako Shojiki, Shigeyuki Kuboya, Ryuji Katayama, Liwen Sang, Masatomo Sumiya, Itaru Honma PII: DOI: Reference:
S0040-6090(17)30557-6 doi: 10.1016/j.tsf.2017.07.063 TSF 36129
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
10 February 2017 23 July 2017 25 July 2017
Please cite this article as: Yuta Nakayasu, Takaaki Tomai, Nobuto Oka, Kanako Shojiki, Shigeyuki Kuboya, Ryuji Katayama, Liwen Sang, Masatomo Sumiya, Itaru Honma , Fabrication of Cu2ZnSnS4 thin films using a Cu-Zn-Sn-O amorphous precursor and supercritical fluid sulfurization, Thin Solid Films (2017), doi: 10.1016/j.tsf.2017.07.063
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ACCEPTED MANUSCRIPT
Fabrication of Cu2ZnSnS4 thin films using a Cu-Zn-Sn-O amorphous precursor and
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supercritical fluid sulfurization Yuta Nakayasu1, Takaaki Tomai1,*, Nobuto Oka1, Kanako Shojiki2, Shigeyuki
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
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Kuboya2, Ryuji Katayama3, Liwen Sang4, Masatomo Sumiya 4 and Itaru Honma1
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2-1-1, Katahira, Aoba-ku, Sendai, Miyagi, 980-8577, Japan Institute for Materials Research (IMR), Tohoku University, 2-1-1 Katahira, Aoba-ku,
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Sendai, Miyagi, 980-8577, Japan
Department of Electrical, Electronic and Information Engineering, Graduate
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School of engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871,
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Japan
National Institute for Materials Science (NIMS), 1-1, Namiki, Tsukuba, Ibaraki,
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305-0044, Japan
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* Corresponding author: Takaaki Tomai
[email protected]
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Abstract A supercritical ethanol (scEtOH) environment, in which sulfur is highly soluble, can lead to safety improvements and cost reductions in the sulfurization process. In this study, the feasibility of the sulfurization process in scEtOH for the preparation of
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Cu2ZnSnS4 (CZTS) thin films at low temperature was verified with the purpose of creating a sustainable and cost-effective process for fabricating metal-sulfide solar
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cells. We found that to promote atomic diffusion of sulfur and achieve sulfurization
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at a low temperature, the presence of defects in the amorphous oxide thin film was preferable. Moreover, in thin film fabricated by sulfurization under ethanol, the
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crystal size was strongly affected by ethanol density. The grain size increased up to
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about 1 µm as the ethanol density increased, and grain growth was remarkable, particularly in the high-density conditions of over 3.0 mol/L. Finally, we fabricated a crystalline CZTS thin film, which exhibited structural and optical properties
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comparable to those of a film fabricated using conventional vapor-phase
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sulfurization. For the prepared disordered-kesterite CZTS thin film with a Cu-poor and Zn-rich composition, photoluminescence measurements confirmed that the
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donor or acceptor defects engage in the emission of about 1.24 eV at 5 K, and
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UV-Vis measurement revealed a bandgap of 1.38 eV at room temperature.
Keywords:
CZTS thin film
Supercritical fluid sulfurization
Cu-Zn-Sn-O precursor
Ethanol density
Grain size
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Energy devices
Solar cells
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ACCEPTED MANUSCRIPT 1. Introduction Since sustainable and cost-effective fabrication processes are the most important requirements for manufacturing energy devices, many researchers have focused their efforts on addressing these requirements [1]. Recently, metal -sulfide
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semiconductors (CuInS2, Cu2ZnSnS4, MoS2, SnS) have been considered as promising materials for next-generation solar cells, fuel cells, and secondary
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batteries. One of the most promising types of commercial solar cells is thin -film
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solar cells, which rely on direct-bandgap chalcogenide semiconductors such as Cu(In,Ga)Se2 (CIGS), and CdTe. Because of their high absorption coefficients, the
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material cost for thin absorption layers can be lower than that for crystalline Si solar cells. For next-generation solar cells, Cu2ZnSnS4 (CZTS), in which the indium and
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selenium of CIGS are replaced by the elements zinc and tin, have attracted considerable attention [2, 3]. The improvement in conversion efficiency is very rapid,
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and in 2013, H. Hiroi et al. achieved 9.2% conversion efficiency for CZTS [4].
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Various fabrication processes for CZTS thin films have been proposed [5]. The highest-efficiency CZTS cell was fabricated using a vapor-phase sulfurization
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process [4, 6]. This process is used commercially to fabricate CIGS cells, and is also applicable for producing CZTS thin films. In general, this process uses H 2S as a
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sulfur source. Crystalline alloys or metal chalcogenides are used as precursors. Sulfurization occurs via a chemical reaction in which sulfur radicals, formed by the thermal decomposition of H 2S, react with the precursors. Although this process produces metal sulfide films with good crystallinity, there are several disadvantages: 1) Process risks: H2S is a highly poisonous and hazardous gas. It is not environmentally friendly, and requires expensive equipment for handling and processing the exhaust gases to prevent unwanted emissions. 2) High-temperature process: A process temperature of more than 550 °C leads to high operating costs 4
ACCEPTED MANUSCRIPT and the desorption of volatile compounds such as SnS and Sn. These compounds easily evaporate at temperatures greater than 400 °C [7]. In addition, when a flexible substrate such as polyimide is used in a solar cell, all processes should be conducted at less than 500 °C. Although the use of less hazardous substances and lower temperatures is
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preferable for an environmentally friendly and more effective sulfurization process,
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it is difficult to solve these problems simultaneously. For example, elemental sulfur (S8) has been evaluated as a safer alternative sulfur source to H 2S, but complete
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sulfurization of metal precursor films requires higher processing temperatures
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(~570 °C) [8, 9] to compensate for the low vapor pressure of sulfur. A low vapor pressure leads to an insufficient supply of highly reactive sulfur sources, such as
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lower weight sulfur molecules (S 2, sulfur radicals).
To achieve a sufficient supply of reactive sulfur sources from stable
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elemental sulfur and sulfurization at less than 400 °C, our study focused on a
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supercritical fluid (SCF). Above the critical temperature and pressure, SCF has properties intermediate between those of a gas and liquid. SCF has some advantages as a chemical reaction field because they have gas-like high diffusivity with
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liquid-like high density and high solubility. SCF reaction media can therefore
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accelerate chemical reaction rates because of their high density and the highly diffuse supply of solute, compared to a gas-phase chemical reaction field. In previous studies, we have proposed new selenization, sulfurization, and
simultaneous chalcogenization processes for the manufacture of Cu 2ZnSnSe4, Cu2ZnSnS4, and Cu2ZnSn(S,Se)4, respectively, at 400 °C. These processes use stable solid feedstocks (SeO 2 and S8), Cu-Zn-Sn-O precursors that are like amorphous materials, and a supercritical ethanol (scEtOH) reaction medium [10]. We found that SCF readily dissolved sulfur and provided a sufficient supply of sulfur to the 5
ACCEPTED MANUSCRIPT precursor, even at 400 °C. However, the question remains whether this new environmentally friendly and cost-effective fabrication process can be an alternative to the conventional vapor-phase chalcogenization process employed in commercial thin film chalcogenide solar cells. In a previous study on SCF chalcogenization [10], an
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unoptimized precursor film with carbon contamination reduced the structural and
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optical quality of the fabricated thin films, so the SCF chalcogenization could not be evaluated independently of the precursor film fabrication process. The carbon
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contamination originated from the organometallic complex solution, which is used in
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the spin-coating process to fabricate the precursor films. This residual carbon prevented crystal growth.
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In this study, to verify the feasibility of preparing CZTS thin films at a low temperature by sulfurization in scEtOH, we eliminated carbon contamination by
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using a Cu-Zn-Sn-O amorphous precursor. We also examined grain size and grain
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growth as a function of ethanol density. Finally, we fabricated an optimized crystalline Cu2ZnSnS4 (CZTS) thin film, which exhibited structural and optical
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sulfurization.
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properties comparable to those of a film fabricated using conventional vapor-phase
2. Experimental Procedure 2.1 Comparison of precursors in SCF sulfurization To fabricate films, a metal alloy precursor and an amorphous oxide precursor were deposited on an unheated Mo-coated glass substrate by RF magnetron sputtering with a power of 50 W, using a Cu-Zn-Sn target (φ: 2 inch, thickness: 3mm) and a sintered Cu-Zn-Sn-O target (φ: 2 inch, thickness: 3mm), respectively. The sputtering chamber was evacuated to a pressure of less than 1.0 × 6
ACCEPTED MANUSCRIPT 10-4 Pa. The total gas pressure of the Ar sputtering gas was maintained at 1.0 Pa during deposition. To sulfurize the precursor films, scEtOH [Tc (Critical Temperature) = 241 °C, Pc (Critical Pressure) = 6.14 MPa, ρc (Critical Density) = 0.276 g/cm 3 (6.0 mol/L)] was utilized as the chemical reaction medium. Precursor films were placed
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in reactor vessels containing 50 mmol/L of elemental sulfur (S 8) with ethanol in a
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glove box filled with Ar gas (H 2O and O2 less than 1 ppm). Typically a yellow solid, S8 has a low vapor pressure (~0.1 Pa at 20°C). The SCF sulfurization process for
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each precursor was carried out in a 10-mL batch-type Hastelloy reactor at 400 °C
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and 6.0 mol/L of ethanol density for 40 min. After heating, the reactor was submerged into a water bath to terminate the reaction. The treated films were washed
2.2 Crystal growth effect by SCF
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with deionized water and dried under vacuum.
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In order to investigate the role of SCF, the SCF sulfurization process for the
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amorphous oxide precursor was carried out in the reactor. The reaction time dependence of sulfurization was carried out for 10, 25, 40 and 70 min, at 400 °C and 6.0 mol/L of ethanol density.
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The ethanol density dependence of sulfurization was carried out at 6.0, 3.0,
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1.5, 0.75, 0.38, and 0 mol/L for 25 min at 400 °C. Based on a previous report showing a phase diagram of pure ethanol, ethanol is in the gaseous phase at ethanol densities of 0.75 and 0.38 mol/L at 400 °C, and is in the SCF condition at densities of 6.0, 3.0, and 1.5 mol/L at the same temperature [11]. 2.3 Measurement of structural and optical properties of CZTS thin film In order to obtain better optical properties, an optimized amorphous oxide precursor was prepared so as to adjust to the Cu-poor and Zn-rich composition as shown in [12] after the RF sputtering. To fabricate the optimized amorphous oxide 7
ACCEPTED MANUSCRIPT precursor, a new sintered Cu-Zn-Sn oxide target (Cu:Zn:Sn = 2.00:1.60:1.00 [at.], φ: 2 inch, thickness: 3mm) was used for sputtering in a similar condition to the formation of the amorphous oxide precursor. The SCF sulfurization process was carried out in the reactor at 400 °C and 3.0 mol/L of ethanol density for 40 min. These conditions were chosen after experiments were performed to find the optimum
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reaction time and ethanol density.
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The crystalline structure of the films was analyzed in the range of θ–2θ by X-ray diffraction (XRD) (BRUKER AXS D8 ADVANCE) using Cu-Kα radiation
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and Raman spectroscopy. The grain size and thickness of thin films were observed
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by surface scanning electron microscopy (SEM). The defect levels were estimated using photoluminescence measurements. The absorption spectra and bandgap were
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obtained using a UV–vis absorption spectrometer (JASCO UVN-6700 spectrophotometer). To measure the absorption spectra, the Cu 2ZnSnS4 films were
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lifted from the Mo-coated glass using a strong adhesive tape. Current-voltage
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measurements (OTENTO-SUN III, BUNKOUKEIKI Co.) were conducted to estimate the photoconductivity.
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3. Results and Discussion
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3.1 Comparison of precursors in SCF sulfurization In order to verify the requirement of an amorphous oxide precursor, we
compared thin films after SCF sulfurization using the metal alloy precursor and the amorphous oxide precursor. The SEM images and XRD patterns of thin films deposited by using the Cu-Zn-Sn target are shown in Fig. 1(a,b). Before SCF sulfurization, grain sizes of a few hundred nanometers were observed, and the XRD patterns of Sn and some metal alloys were confirmed. After SCF sulfurization, the peak patterns derived from CZTS at 28.6°and 47.5° appeared, but metal alloy and 8
ACCEPTED MANUSCRIPT metal peaks such as Cu 3Sn, Cu6Sn5 and Sn [8] remained. The SEM image and XRD patterns of the thin film deposited by using the sintered Cu-Zn-Sn-O target are shown in Fig. 1 (c, d). Before SCF sulfurization, the flat surface and absence of XRD peaks (except Mo) indicate the formation of an amorphous condition. It is believed that the increase of temperature during
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sputtering does not affect the crystallization of Cu-Zn-Sn-O and a structural defect,
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such as O, generated by the highly energetic ion bombardments, remained. After SCF sulfurization, strong XRD peaks confirmed the presence of CZTS at 28.6°,
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33.1°, 47.5°, and 56.4°, and oxygen was not detected by EDX. There were no
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different phases observed when using the metal alloy precursor. These results indicate that the amorphous oxide precursor was preferable for the complete
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sulfurization at 400 °C.
Previous studies have compared metal oxide precursors to metal alloy or
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metal sulfide precursors [13,14]. During the sulfurization process, Sn is lost because
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of its high vapor pressure and then the loss of Sn prevents the crystal growth of CZTS, but this loss can be reduced by using SnO 2 because of its stability against evaporation, unlike non-oxide precursors. On the other hand, using a Cu-Zn-Sn-O
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amorphous precursor with vapor-phase sulfurization at 550 °C, Ishio et al. [15]
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succeeded in fabricating a CZTS thin film whose surface morphology was smoother than that fabricated from a Cu-Zn-Sn alloy precursor. The presence of defects in an amorphous film promotes the fast elemental diffusion in the film, resulting in the favorable conversion of oxygen and sulfur. However, by simply applying amorphous oxide, the process temperature in sulfurization requires much more than 400 °C due to low vapor pressure of sulfur at 400 °C. Supercritical ethanol can dissolve liquid sulfur and the sulfur is possible to acquire high diffusivity. Therefore, the effect of supercritical ethanol sulfur should play an important role for low-temperature 9
ACCEPTED MANUSCRIPT sulfurization. 3.2 Crystal growth effects by SCF We investigated the reaction time dependence of SCF sulfurization as shown in Fig. 2 (a-c). Among all of the reaction times, the films after sulfurization only show XRD peaks corresponding to CZTS at 28.6°, 47.5°, and 56.4°. The full
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width half maximum (FWHM) of the strongest peak derived from CZTS gradually
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decreased as the reaction time increased and the value reached a minimum and stable at 40 min. EDX analysis shows that the molar ratio of sulfur reached a stable
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composition at 40 min. Higher-ratio copper at 10 min shows that the formation of
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Cu2S. As shown in reference [16], in general sulfurization process, copper diffuses to the top and then Cu 2S is formed in the upper part at the beginning. After that, by
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keeping on supplying sulfur with a sufficiently high temperature, each element diffuses throughout the film and CZTS is formed. Similarly, in our study, it is
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expected that the diffused copper reacts to sulfur and then Cu 2S is formed in the
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upper part at 10 min, it is considered that sulfurization is incomplete and Cu 2S formed on the surface was detected. In order to elucidate the role of SCF for low-temperature sulfurization, the
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effects of ethanol density on the crystal growth of the thin films was investigated.
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The results of the ethanol density dependence of sulfurization are shown in Fig. 3 and 4. SEM observations of the film surfaces after sulfurization are shown in Fig. 3 (a–f). For the sample fabricated in the condition without ethanol, the SEM image of the surface (Fig. 3 (a)) was different from the other conditions, and the XRD patterns (Fig. 4 (a)) show the presence of phases different from CZTS that could not be identified. On the other hand, from the SEM images and XRD patterns for the sample fabricated with ethanol, shown in Fig. 3 (b-f) and Fig. 4 (a), at all reaction conditions we confirmed the formation of CZTS without unidentifiable phases. 10
ACCEPTED MANUSCRIPT At low ethanol densities, only small grains were observed in Fig. 3 (b–d). At high ethanol densities, large grains were observed in Fig. 3 (e, f). FWHM values of the strongest XRD peak derived from CZTS (at around 28.6°) shown in Fig. 4 (b) gradually decreased as the ethanol density increased. This tendency corresponds to the increase in the grain size, as shown in Fig. 3 (b-f). Particularly in the
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high-density conditions of over 3.0 mol/L, the grain size remarkably increased up to
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about 1 µm. The FWHM of CZTS hardly changed over the range 3.0–6.0 mol/L of ethanol density.
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It is known that scEtOH has a high solubility and a high reducing power. As
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shown in Fig. 4 (c), oxygen is almost undetectable when the ethanol density is contained. This means that even at the lower density, EtOH reduced the oxide
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precursor. In contrast, the values of the FWHM gradually decreased as the ethanol density increased. This implies that crystal growth is involved in solubility, not in the
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reduction of the oxide precursor.
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We exhibit the composition difference at each ethanol density in Fig. 4(c). As mentioned earlier, in an imcomplete sulfurization, Cu 2S is formed in the upper part. Under 1.5 mol/L or less, it is considered that sulfurization is incomplete and
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Cu2S formed on the surface was detected. Lower-sulfur ratio at 1.5 mol/L indicates
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that the crystal growth of Cu2S proceeded only in the upper part of the film due to the insufficient S supply for the complete sulfurization of a whole film. Conversely, we suppose, in the range between 0 and 0.75 mol/L, sulfur just adhered on the surface and the formation of Cu 2S did not sufficiently proceed, thus the sulfur ratios was relatively highr than one of 1.5 mol/L. Over 3.0 mol/L, it is indicated that the complete formation of CZTS thin film was achieved due to sufficient S supply by a high-density ethanol. A reason why CZTS films with good crystallinity can be fabricated at less 11
ACCEPTED MANUSCRIPT than 400 °C is proposed as follows: The melting point and boiling point of sulfur are 115 °C and 444 °C, respectively. Sulfur gas pressure at 400 °C is 0.049 MPa. Sulfur consists of seven different molecules, S2, S3, S4, S5, S6, S7, and S8 in a previous report [17]. According to this report, the fraction of S 2, S3, S4, and S5 with high reactivity is only 2.3% at 400 °C and the majority of sulfur (97.7%) is composed of
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S6, S7, and S8, having a low reactivity in the gas phase. To form the CZTS at 400 °C,
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low reactive liquid sulfur and S 6, S7, and S8 must be activated for sulfurization. In the process described in this study, liquid sulfur can be dissolved into SCF and used
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for sulfurization. We believe that the solvation effect of SCF facilitates the
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decomposition of high molecular-sized sulfur. The scEtOH is inserted into the S-S bonds and decomposes them, resulting in an increase in the density of highly
achieved at 400 °C after 25 min.
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reactive species. Based on these factors, it is estimated that CZTS formation was
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3.3 Optical properties of CZTS film
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Finally, we conducted the measurement of optical properties to prove that the SCF process can be an alternative method for the fabrication of optical-grade CZTS film. Based on the results of 3.2, the reaction time and ethanol density was set
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to 40 min and 3.0 mol/L, rescpectively. The XRD patterns of the optimized
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amorphous oxide precursor films and the film after sulfurization are shown in Fig. 5 (a). The films after sulfurization only show peaks corresponding to the Cu 2ZnSnS4 structure (except Mo), i.e., (112), (200)/(004), (220)/(204), and (312)/(116) at 28.6°, 33.1°, 47.5°, and 56.4°, respectively. It is noted that CZTS has several crystal structures (kesterite, stannite, and disordered kesterite), which cannot be distinguished using only XRD. CZTS with an ideal composition for solar cells has a disordered kesterite structure. The Raman spectra of Cu 2ZnSnS4 shown in Fig. 5 (b) is consistent with that obtained in previous studies [18, 19], and other binary and 12
ACCEPTED MANUSCRIPT ternary compounds were not detected. The peaks at 285, 332, and 374 cm –1 show the disordered kesterite structure. Normally, the kesterite structure shows an A1 mode at 338 cm–1. The strong peak at 332 cm–1 is related to the presence of local structural inhomogeneities within the disordered cation sublattice antisites. In general, these necessarily occur in Cu-poor conditions, and are of two types: Zn Cu-type and
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CuZn-type [18, 20]. From these results, it was confirmed that a disordered
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kesterite-structured Cu2ZnSnS4 film can be formed from the precursor film. It is known that in the case of direct-bandgap semiconductors such as CZTS,
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the bandgap can be estimated by plotting (Ahν) 2 (A: absorbance, h: Planck’s
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constant, and ν: frequency) against the photon energy. The energy at which the extrapolated line of (Ahν) 2 versus hν reaches (Ahν) 2 = 0 can be regarded as the
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bandgap. Fig. 6 shows the plot of (Ahν) 2 as a function of hν for the absorption spectra of CZTS fabricated by using the optimized amorphous oxide precursor.
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Normally, the bandgap of the CZTS thin film is around 1.5 eV, but in the Zn-rich and
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Cu-poor state, the occurrence of Zn Cu-type and CuZn-type antisites is significant, which causes an increase in the valence band maximum and a decrease in the conduction band minimum, resulting in a decrease to about 1.4 eV in the bandgap
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[21]. The estimated bandgap energy of the fabricated CZTS film was around 1.38 eV,
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corresponding to the reported value for the Zn-rich and Cu-poor CZTS [21]. The photoluminescence (PL) was measured, as shown in Fig. 7(a, b). In the
CZTS system, band edge luminescence was not observed, even in a single crystal [22]. In previous reports in which PL emission for CZTS was around 1.25 eV at low temperature, the emission was attributed to the band-to-tail transition by highly doped donors and acceptors [23] or to donor-acceptor pair (DAP) transition by shallow donors and acceptors [23-25]. The PL spectra of CZTS at 5 K are shown for different values of excitation power in Fig. 7(a). The maximum intensity obtained 13
ACCEPTED MANUSCRIPT was at approximately 1.24 eV in the red circle of Fig. 7(b). The PL peak position showed blue shift with increasing excitation power, which is a common feature of DAP transition [24, 25] and band-to-tail transition by potential fluctuation [23, 26]. As for the intensity, when the PL integrated intensity follows the power law: Intensity ∝ Pγ (where P is the excitation power and γ is an adjustable parameter), a
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value of γ < 1 means that donor or acceptor defects engage in the emission for CZTS
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[23,24,27,28]. As shown in the green quadrilateral of Fig. 7 (b), a value of γ = 0.96 was obtained, indicating the observed PL emission is derived from the donors and
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acceptors in CZTS as reported in previous studies.
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Moreover, the photoconductivity measurement of the CZTS film was carried out at room temperature. As shown in Fig. 8, the photoconductivity of CZTS was 5
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times larger than the dark conductivity. This is due to the increase of photo-generated carriers in the CZTS semiconductor. These optical property results
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indicate that by using an SCF sulfurization process at 400 °C, a CZTS thin film can
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be produced that has very similar structural and optical properties as those of a fi lm fabricated using conventional vapor-phase sulfurization. In this study, the composition analysis by EDX or inductively coupled
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plasma (ICP) for the optical-grade CZTS film was not carried out. Generally, in
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EDX analysis, it is difficult to quantitatively evaluate a composition for a sample with high surface roughness and multielement compounds like as our sample. In addition, in general sulfurization process for CZTS fabrication, ZnS is left in the bottom[28], thus ICP analysis cannot show the accurate composition value. In contrast, structural and optical properties show that our sample has the Cu-poor and Zn-rich composition. Furthermore, from the result of cross-sectional EDX for the optimized Cu-Zn-Sn oxide precursor with little surface roughness showed a composition of Cu/(Zn+Sn):0.88 and Zn/Sn:1.46 and the solution left in the reactor 14
ACCEPTED MANUSCRIPT after the reaction were analyzed by ICP, and as a result, some elution of copper was confirmed, but little elution of tin and zinc was confirmed. Therefore, although more detailed composition analysis is required, it was determined that our CZTS film has Cu-poor and Zn-rich composition from the
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results of structure and optical properties.
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4. Conclusions
This study demonstrated the advantages of combining SCF sulfurization
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with a Cu-Zn-Sn-O amorphous precursor for low-temperature sulfurization. It was
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shown that SCF sulfurization was complete when using an amorphous oxide precursor at 400 °C. However, very little SCF sulfurization took place when using a
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Cu-Zn-Sn alloy precursor. The amorphous metal oxide contains many point defects. Hence, elemental diffusion in amorphous metal oxides can be faster than that in the
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alloy precursor.
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The ethanol density dependence on the grain size was established from the SEM images and the FWHM of the strongest peak derived from metal sulfides. The grain size increased as the ethanol density increased, and this phenomenon was
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particularly noticeable in the high-density conditions of over 3.0 mol/L. Crystal
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growth is believed to be caused by an increase in the sulfur solubility in ethanol as the ethanol density increases. By combining SCF sulfurization with a Cu-Zn-Sn-O amorphous precursor,
the formation of CZTS films consisting of Cu-poor and Zn-rich disordered kesterite were confirmed by XRD and Raman spectra. PL emission of about 1.24 eV at 5 K caused by the donor or acceptor defects indicated a bandgap of 1.38 eV at room temperature. The optical properties, determined by UV–vis absorption, PL measurement, and photoconductivity, were consistent with previous reports. 15
ACCEPTED MANUSCRIPT In short, the combination of SCF sulfurization and the use of a Cu-Zn-Sn-O amorphous precursor enabled the fabrication of large-grain CZTS films at 400 °C. The SCF sulfurization process using a metal oxide precursor can be applied to other
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energy devices based on metal-chalcogenide materials.
Acknowledgements
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This work was financially supported by JSPS KAKENHI (Grant Number
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25630353 and 15J051979), the Inter-Graduate School Doctoral Degree Program on
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Science for global safety and the Tohoku University Division for Interdisciplinary Advanced Research and Education. We thank the Analytical Research Core for
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Advanced Materials in Tohoku University for property measurements
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(cross-sectional EDX, Operator: Dr. Makoto Nagasako).
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12) H. Katagiri, K. Jimbo, W. S. Maw et al. Development of CZTS-based thin film solar cells. Thin Solid Films. 517 (2009) 2455–2460. 13) G. Chen, C. Yuan, J. Liu et al. Fabrication of Cu2ZnSnS4 thin films using oxides nanoparticles ink for solar cell. J. Power Sources. 276 (2015) 145–152. 14) T. Washio, T. Shinji, S. Tajima et al. 6% efficiency Cu 2ZnSnS4-based thin film solar cells using oxide precursors by open atmosphere type CVD. J. Mater. Chem. 22 (2012) 4021–4024. 17
ACCEPTED MANUSCRIPT 15) R. Ishio, K. Fukushima, T. Minemoto, Improvement of Cu2ZnSnS4 thin film morphology using Cu-Zn-Sn-O precursor fabricated by sputtering. Curr. Appl. Phys. 13 (2013) 1861–1864. 16) A. Fairbrother, X. Fontane´, V. Izquierdo-Roca et al. On The Formation
Stacks. Solar Cells. Sol. Energy. 112 (2013) 97–105.
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Mechanisms of Zn-rich Cu2ZnSnS4 Films Prepared by Sulfurization of Metallic
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17) H. Rau, T. R. N. Kutty, J.R.F. Guedes De Carvalho, Thermodynamics of sulphur vapour J. Chem. Thermodynamics. 5 (1973) 833–844.
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18) M. Y. Valakh, O. F. Kolomys, S.S. Ponomaryov et al. Raman scattering and
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disorder effect in Cu2ZnSnS4. Phys. Status Solidi RRL. 7 (2013) 258–261. 19) R. Caballero, E. Garcia-Llamas, JM Merino et al. Non-stoichiometry effect and
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disorder in Cu2ZnSnS4 thin films obtained by flash evaporation: Raman scattering investigation. Acta Mater. 65 (2014) 412–417.
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20) T. Washio, H. Nozaki, T. Fukano et al. Analysis of lattice site occupancy in
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kesterite structure of Cu2ZnSnS4 films using synchrotron radiation X-ray diffraction. J. Appl. Phys. 110 (2011) 074511. 21) S. Chen, J.H. Yang, X. G. Gong et al. Intrinsic point defects and complexes in
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245204.
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the quaternary kesterite semiconductor Cu2ZnSnS4. Phys. Rev. B 81 (2010)
22) M. Grossberg, J. Krustok, J. Raudoja et al. Photoluminescence and Raman study of Cu2ZnSn(SexS1− x)4 monograins for photovoltaic applications. Thin Solid Films. 519 (2011) 7403-7406. 23) J. P. Leitao, N. M. Santos, P. A. Fernandes et al. Photoluminescence and electrical study of fluctuating potentials in Cu2ZnSnS4-based thin films. Phys. Rev. B. 84 (2011) 024120.
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ACCEPTED MANUSCRIPT 24) M. Tanaka, Y. Miyamoto, H. Uchiki et al. Donor-acceptor pair recombination luminescence from Cu 2ZnSnS4 bulk single crystals. Phys. Stat. Sol. (a). 203 (2006) 2891–2896. 25) L. Yin, G. Cheng, Y. Feng et al. Limitation factors for the performance of kesterite Cu2ZnSnS4 thin film solar cells studied by defect characterization. RSC
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Adv. 5 (2015) 40369–40374.
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26) M. J. Romero, H. Du, G. Teeter et al. Comparative study of the luminescence and intrinsic point defects in the kesterite Cu2ZnSnS4 and chalcopyrite Cu(In,Ga)Se2
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thin films used in photovoltaic applications. Phys. Rev. B. 84 (2011) 165324.
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27) T. Schumidt, K. Lischka, W. Zulehner, Excitation-power dependence of the near-band-edge photoluminescence of semiconductors. Phys. Rev. B. 45 (1992)
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8989–8994.
28) K. Tanaka, T. Shinji, H. Uchiki, Photoluminescence from Cu 2ZnSnS4 thin films
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with different compositions fabricated by a sputtering-sulfurization method. Sol.
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Energy Mater. Sol. Cells. 126 (2014), 143–148.
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(b) CZTS (112)
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Fig. 1 SEM images in two types of precursors and XRD patterns before and after
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SCF sulfurization for each precursor. (a) SEM image of precursor deposited using Cu-Zn-Sn alloy target
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(b) XRD patterns before and after SCF sulfurization for (a) (c) SEM image of precursor deposited using sintered Cu-Zn-Sn-O target (d) XRD patterns before and after SCF sulfurization for (c)
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Fig. 2 XRD patterns, FWHM of the strongest peak of CZTS (around at 28.6
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Fig. 3 SEM images after sulfurization for various ethanol densities for CZTS film
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(a) Only Ar (b) 0.38 mol/L (c) 0.75 mol/L (d) 1.5 mol/L (e) 3 mol/L (f) 6 mol/L of
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Fig. 4 XRD patterns, FWHM of the strongest peak of metal sulfides (around at 28.6
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degrees) and surface EDX of thin film fabricated using the amorphous oxide
(a) XRD patterns
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(■CZTS (112) □CZTS (200/004) ●CZTS (220/204) ○CZTS (312/116) /▲Mo (110))
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CZTS (312/116) /▲Mo (110)) (b) Raman shift of CZTS film fabricated using the
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Fig. 6 Plot of (Ahν)2 as a function of hν (A: absorbance, h: Planck’s constant, and ν: frequency) for CZTS fabricated using the optimized amorphous oxide precursor. Data for the range 880 and 900 nm (graph missing) was removed because the
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detector was changed at 900 nm and there was a large deviation in the value.
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169 mW 259 mW 346 mW 438 mW 529 mW 621 mW 716 mW 813 mW 911 mW 9 mW 10
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Fig. 7 (a) Plot of PL peak energy of CZTS film fabricated using the optimized
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amorphous oxide precursor as a function of PL exciton power at 5 K (b) Plot of the
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Research Highlights ►An amorphous metal-oxide precursor is utilized for fabrication of Cu2ZnSnS4 film. ►Sulfurization is conducted under supercritical ethanol having reducibility. ►The crystal growth proceeds in a high ethanol density region. ►The structural and optical properties derived from Cu 2ZnSnS4 are shown.
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Yuta Nakayasu, Takaaki Tomai, Nobuto Oka, Kanako Shojiki, Shigeyuki Kuboya, Ryuji Katayama, Liwen Sang, Masatomo Sumiya, Itaru Honma PII: DOI: Reference:
S0040-6090(17)30557-6 doi: 10.1016/j.tsf.2017.07.063 TSF 36129
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
10 February 2017 23 July 2017 25 July 2017
Please cite this article as: Yuta Nakayasu, Takaaki Tomai, Nobuto Oka, Kanako Shojiki, Shigeyuki Kuboya, Ryuji Katayama, Liwen Sang, Masatomo Sumiya, Itaru Honma , Fabrication of Cu2ZnSnS4 thin films using a Cu-Zn-Sn-O amorphous precursor and supercritical fluid sulfurization, Thin Solid Films (2017), doi: 10.1016/j.tsf.2017.07.063
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Fabrication of Cu2ZnSnS4 thin films using a Cu-Zn-Sn-O amorphous precursor and
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supercritical fluid sulfurization Yuta Nakayasu1, Takaaki Tomai1,*, Nobuto Oka1, Kanako Shojiki2, Shigeyuki
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
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Kuboya2, Ryuji Katayama3, Liwen Sang4, Masatomo Sumiya 4 and Itaru Honma1
2
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2-1-1, Katahira, Aoba-ku, Sendai, Miyagi, 980-8577, Japan Institute for Materials Research (IMR), Tohoku University, 2-1-1 Katahira, Aoba-ku,
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Sendai, Miyagi, 980-8577, Japan
Department of Electrical, Electronic and Information Engineering, Graduate
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School of engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871,
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Japan
National Institute for Materials Science (NIMS), 1-1, Namiki, Tsukuba, Ibaraki,
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305-0044, Japan
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* Corresponding author: Takaaki Tomai
[email protected]
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Abstract A supercritical ethanol (scEtOH) environment, in which sulfur is highly soluble, can lead to safety improvements and cost reductions in the sulfurization process. In this study, the feasibility of the sulfurization process in scEtOH for the preparation of
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Cu2ZnSnS4 (CZTS) thin films at low temperature was verified with the purpose of creating a sustainable and cost-effective process for fabricating metal-sulfide solar
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cells. We found that to promote atomic diffusion of sulfur and achieve sulfurization
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at a low temperature, the presence of defects in the amorphous oxide thin film was preferable. Moreover, in thin film fabricated by sulfurization under ethanol, the
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crystal size was strongly affected by ethanol density. The grain size increased up to
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about 1 µm as the ethanol density increased, and grain growth was remarkable, particularly in the high-density conditions of over 3.0 mol/L. Finally, we fabricated a crystalline CZTS thin film, which exhibited structural and optical properties
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comparable to those of a film fabricated using conventional vapor-phase
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sulfurization. For the prepared disordered-kesterite CZTS thin film with a Cu-poor and Zn-rich composition, photoluminescence measurements confirmed that the
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donor or acceptor defects engage in the emission of about 1.24 eV at 5 K, and
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UV-Vis measurement revealed a bandgap of 1.38 eV at room temperature.
Keywords:
CZTS thin film
Supercritical fluid sulfurization
Cu-Zn-Sn-O precursor
Ethanol density
Grain size
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Energy devices
Solar cells
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ACCEPTED MANUSCRIPT 1. Introduction Since sustainable and cost-effective fabrication processes are the most important requirements for manufacturing energy devices, many researchers have focused their efforts on addressing these requirements [1]. Recently, metal -sulfide
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semiconductors (CuInS2, Cu2ZnSnS4, MoS2, SnS) have been considered as promising materials for next-generation solar cells, fuel cells, and secondary
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batteries. One of the most promising types of commercial solar cells is thin -film
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solar cells, which rely on direct-bandgap chalcogenide semiconductors such as Cu(In,Ga)Se2 (CIGS), and CdTe. Because of their high absorption coefficients, the
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material cost for thin absorption layers can be lower than that for crystalline Si solar cells. For next-generation solar cells, Cu2ZnSnS4 (CZTS), in which the indium and
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selenium of CIGS are replaced by the elements zinc and tin, have attracted considerable attention [2, 3]. The improvement in conversion efficiency is very rapid,
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and in 2013, H. Hiroi et al. achieved 9.2% conversion efficiency for CZTS [4].
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Various fabrication processes for CZTS thin films have been proposed [5]. The highest-efficiency CZTS cell was fabricated using a vapor-phase sulfurization
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process [4, 6]. This process is used commercially to fabricate CIGS cells, and is also applicable for producing CZTS thin films. In general, this process uses H 2S as a
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sulfur source. Crystalline alloys or metal chalcogenides are used as precursors. Sulfurization occurs via a chemical reaction in which sulfur radicals, formed by the thermal decomposition of H 2S, react with the precursors. Although this process produces metal sulfide films with good crystallinity, there are several disadvantages: 1) Process risks: H2S is a highly poisonous and hazardous gas. It is not environmentally friendly, and requires expensive equipment for handling and processing the exhaust gases to prevent unwanted emissions. 2) High-temperature process: A process temperature of more than 550 °C leads to high operating costs 4
ACCEPTED MANUSCRIPT and the desorption of volatile compounds such as SnS and Sn. These compounds easily evaporate at temperatures greater than 400 °C [7]. In addition, when a flexible substrate such as polyimide is used in a solar cell, all processes should be conducted at less than 500 °C. Although the use of less hazardous substances and lower temperatures is
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preferable for an environmentally friendly and more effective sulfurization process,
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it is difficult to solve these problems simultaneously. For example, elemental sulfur (S8) has been evaluated as a safer alternative sulfur source to H 2S, but complete
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sulfurization of metal precursor films requires higher processing temperatures
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(~570 °C) [8, 9] to compensate for the low vapor pressure of sulfur. A low vapor pressure leads to an insufficient supply of highly reactive sulfur sources, such as
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lower weight sulfur molecules (S 2, sulfur radicals).
To achieve a sufficient supply of reactive sulfur sources from stable
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elemental sulfur and sulfurization at less than 400 °C, our study focused on a
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supercritical fluid (SCF). Above the critical temperature and pressure, SCF has properties intermediate between those of a gas and liquid. SCF has some advantages as a chemical reaction field because they have gas-like high diffusivity with
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liquid-like high density and high solubility. SCF reaction media can therefore
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accelerate chemical reaction rates because of their high density and the highly diffuse supply of solute, compared to a gas-phase chemical reaction field. In previous studies, we have proposed new selenization, sulfurization, and
simultaneous chalcogenization processes for the manufacture of Cu 2ZnSnSe4, Cu2ZnSnS4, and Cu2ZnSn(S,Se)4, respectively, at 400 °C. These processes use stable solid feedstocks (SeO 2 and S8), Cu-Zn-Sn-O precursors that are like amorphous materials, and a supercritical ethanol (scEtOH) reaction medium [10]. We found that SCF readily dissolved sulfur and provided a sufficient supply of sulfur to the 5
ACCEPTED MANUSCRIPT precursor, even at 400 °C. However, the question remains whether this new environmentally friendly and cost-effective fabrication process can be an alternative to the conventional vapor-phase chalcogenization process employed in commercial thin film chalcogenide solar cells. In a previous study on SCF chalcogenization [10], an
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unoptimized precursor film with carbon contamination reduced the structural and
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optical quality of the fabricated thin films, so the SCF chalcogenization could not be evaluated independently of the precursor film fabrication process. The carbon
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contamination originated from the organometallic complex solution, which is used in
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the spin-coating process to fabricate the precursor films. This residual carbon prevented crystal growth.
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In this study, to verify the feasibility of preparing CZTS thin films at a low temperature by sulfurization in scEtOH, we eliminated carbon contamination by
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using a Cu-Zn-Sn-O amorphous precursor. We also examined grain size and grain
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growth as a function of ethanol density. Finally, we fabricated an optimized crystalline Cu2ZnSnS4 (CZTS) thin film, which exhibited structural and optical
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sulfurization.
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properties comparable to those of a film fabricated using conventional vapor-phase
2. Experimental Procedure 2.1 Comparison of precursors in SCF sulfurization To fabricate films, a metal alloy precursor and an amorphous oxide precursor were deposited on an unheated Mo-coated glass substrate by RF magnetron sputtering with a power of 50 W, using a Cu-Zn-Sn target (φ: 2 inch, thickness: 3mm) and a sintered Cu-Zn-Sn-O target (φ: 2 inch, thickness: 3mm), respectively. The sputtering chamber was evacuated to a pressure of less than 1.0 × 6
ACCEPTED MANUSCRIPT 10-4 Pa. The total gas pressure of the Ar sputtering gas was maintained at 1.0 Pa during deposition. To sulfurize the precursor films, scEtOH [Tc (Critical Temperature) = 241 °C, Pc (Critical Pressure) = 6.14 MPa, ρc (Critical Density) = 0.276 g/cm 3 (6.0 mol/L)] was utilized as the chemical reaction medium. Precursor films were placed
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in reactor vessels containing 50 mmol/L of elemental sulfur (S 8) with ethanol in a
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glove box filled with Ar gas (H 2O and O2 less than 1 ppm). Typically a yellow solid, S8 has a low vapor pressure (~0.1 Pa at 20°C). The SCF sulfurization process for
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each precursor was carried out in a 10-mL batch-type Hastelloy reactor at 400 °C
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and 6.0 mol/L of ethanol density for 40 min. After heating, the reactor was submerged into a water bath to terminate the reaction. The treated films were washed
2.2 Crystal growth effect by SCF
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with deionized water and dried under vacuum.
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In order to investigate the role of SCF, the SCF sulfurization process for the
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amorphous oxide precursor was carried out in the reactor. The reaction time dependence of sulfurization was carried out for 10, 25, 40 and 70 min, at 400 °C and 6.0 mol/L of ethanol density.
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The ethanol density dependence of sulfurization was carried out at 6.0, 3.0,
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1.5, 0.75, 0.38, and 0 mol/L for 25 min at 400 °C. Based on a previous report showing a phase diagram of pure ethanol, ethanol is in the gaseous phase at ethanol densities of 0.75 and 0.38 mol/L at 400 °C, and is in the SCF condition at densities of 6.0, 3.0, and 1.5 mol/L at the same temperature [11]. 2.3 Measurement of structural and optical properties of CZTS thin film In order to obtain better optical properties, an optimized amorphous oxide precursor was prepared so as to adjust to the Cu-poor and Zn-rich composition as shown in [12] after the RF sputtering. To fabricate the optimized amorphous oxide 7
ACCEPTED MANUSCRIPT precursor, a new sintered Cu-Zn-Sn oxide target (Cu:Zn:Sn = 2.00:1.60:1.00 [at.], φ: 2 inch, thickness: 3mm) was used for sputtering in a similar condition to the formation of the amorphous oxide precursor. The SCF sulfurization process was carried out in the reactor at 400 °C and 3.0 mol/L of ethanol density for 40 min. These conditions were chosen after experiments were performed to find the optimum
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reaction time and ethanol density.
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The crystalline structure of the films was analyzed in the range of θ–2θ by X-ray diffraction (XRD) (BRUKER AXS D8 ADVANCE) using Cu-Kα radiation
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and Raman spectroscopy. The grain size and thickness of thin films were observed
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by surface scanning electron microscopy (SEM). The defect levels were estimated using photoluminescence measurements. The absorption spectra and bandgap were
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obtained using a UV–vis absorption spectrometer (JASCO UVN-6700 spectrophotometer). To measure the absorption spectra, the Cu 2ZnSnS4 films were
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lifted from the Mo-coated glass using a strong adhesive tape. Current-voltage
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measurements (OTENTO-SUN III, BUNKOUKEIKI Co.) were conducted to estimate the photoconductivity.
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3. Results and Discussion
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3.1 Comparison of precursors in SCF sulfurization In order to verify the requirement of an amorphous oxide precursor, we
compared thin films after SCF sulfurization using the metal alloy precursor and the amorphous oxide precursor. The SEM images and XRD patterns of thin films deposited by using the Cu-Zn-Sn target are shown in Fig. 1(a,b). Before SCF sulfurization, grain sizes of a few hundred nanometers were observed, and the XRD patterns of Sn and some metal alloys were confirmed. After SCF sulfurization, the peak patterns derived from CZTS at 28.6°and 47.5° appeared, but metal alloy and 8
ACCEPTED MANUSCRIPT metal peaks such as Cu 3Sn, Cu6Sn5 and Sn [8] remained. The SEM image and XRD patterns of the thin film deposited by using the sintered Cu-Zn-Sn-O target are shown in Fig. 1 (c, d). Before SCF sulfurization, the flat surface and absence of XRD peaks (except Mo) indicate the formation of an amorphous condition. It is believed that the increase of temperature during
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sputtering does not affect the crystallization of Cu-Zn-Sn-O and a structural defect,
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such as O, generated by the highly energetic ion bombardments, remained. After SCF sulfurization, strong XRD peaks confirmed the presence of CZTS at 28.6°,
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33.1°, 47.5°, and 56.4°, and oxygen was not detected by EDX. There were no
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different phases observed when using the metal alloy precursor. These results indicate that the amorphous oxide precursor was preferable for the complete
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sulfurization at 400 °C.
Previous studies have compared metal oxide precursors to metal alloy or
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metal sulfide precursors [13,14]. During the sulfurization process, Sn is lost because
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of its high vapor pressure and then the loss of Sn prevents the crystal growth of CZTS, but this loss can be reduced by using SnO 2 because of its stability against evaporation, unlike non-oxide precursors. On the other hand, using a Cu-Zn-Sn-O
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amorphous precursor with vapor-phase sulfurization at 550 °C, Ishio et al. [15]
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succeeded in fabricating a CZTS thin film whose surface morphology was smoother than that fabricated from a Cu-Zn-Sn alloy precursor. The presence of defects in an amorphous film promotes the fast elemental diffusion in the film, resulting in the favorable conversion of oxygen and sulfur. However, by simply applying amorphous oxide, the process temperature in sulfurization requires much more than 400 °C due to low vapor pressure of sulfur at 400 °C. Supercritical ethanol can dissolve liquid sulfur and the sulfur is possible to acquire high diffusivity. Therefore, the effect of supercritical ethanol sulfur should play an important role for low-temperature 9
ACCEPTED MANUSCRIPT sulfurization. 3.2 Crystal growth effects by SCF We investigated the reaction time dependence of SCF sulfurization as shown in Fig. 2 (a-c). Among all of the reaction times, the films after sulfurization only show XRD peaks corresponding to CZTS at 28.6°, 47.5°, and 56.4°. The full
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width half maximum (FWHM) of the strongest peak derived from CZTS gradually
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decreased as the reaction time increased and the value reached a minimum and stable at 40 min. EDX analysis shows that the molar ratio of sulfur reached a stable
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composition at 40 min. Higher-ratio copper at 10 min shows that the formation of
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Cu2S. As shown in reference [16], in general sulfurization process, copper diffuses to the top and then Cu 2S is formed in the upper part at the beginning. After that, by
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keeping on supplying sulfur with a sufficiently high temperature, each element diffuses throughout the film and CZTS is formed. Similarly, in our study, it is
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expected that the diffused copper reacts to sulfur and then Cu 2S is formed in the
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upper part at 10 min, it is considered that sulfurization is incomplete and Cu 2S formed on the surface was detected. In order to elucidate the role of SCF for low-temperature sulfurization, the
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effects of ethanol density on the crystal growth of the thin films was investigated.
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The results of the ethanol density dependence of sulfurization are shown in Fig. 3 and 4. SEM observations of the film surfaces after sulfurization are shown in Fig. 3 (a–f). For the sample fabricated in the condition without ethanol, the SEM image of the surface (Fig. 3 (a)) was different from the other conditions, and the XRD patterns (Fig. 4 (a)) show the presence of phases different from CZTS that could not be identified. On the other hand, from the SEM images and XRD patterns for the sample fabricated with ethanol, shown in Fig. 3 (b-f) and Fig. 4 (a), at all reaction conditions we confirmed the formation of CZTS without unidentifiable phases. 10
ACCEPTED MANUSCRIPT At low ethanol densities, only small grains were observed in Fig. 3 (b–d). At high ethanol densities, large grains were observed in Fig. 3 (e, f). FWHM values of the strongest XRD peak derived from CZTS (at around 28.6°) shown in Fig. 4 (b) gradually decreased as the ethanol density increased. This tendency corresponds to the increase in the grain size, as shown in Fig. 3 (b-f). Particularly in the
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high-density conditions of over 3.0 mol/L, the grain size remarkably increased up to
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about 1 µm. The FWHM of CZTS hardly changed over the range 3.0–6.0 mol/L of ethanol density.
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It is known that scEtOH has a high solubility and a high reducing power. As
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shown in Fig. 4 (c), oxygen is almost undetectable when the ethanol density is contained. This means that even at the lower density, EtOH reduced the oxide
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precursor. In contrast, the values of the FWHM gradually decreased as the ethanol density increased. This implies that crystal growth is involved in solubility, not in the
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reduction of the oxide precursor.
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We exhibit the composition difference at each ethanol density in Fig. 4(c). As mentioned earlier, in an imcomplete sulfurization, Cu 2S is formed in the upper part. Under 1.5 mol/L or less, it is considered that sulfurization is incomplete and
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Cu2S formed on the surface was detected. Lower-sulfur ratio at 1.5 mol/L indicates
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that the crystal growth of Cu2S proceeded only in the upper part of the film due to the insufficient S supply for the complete sulfurization of a whole film. Conversely, we suppose, in the range between 0 and 0.75 mol/L, sulfur just adhered on the surface and the formation of Cu 2S did not sufficiently proceed, thus the sulfur ratios was relatively highr than one of 1.5 mol/L. Over 3.0 mol/L, it is indicated that the complete formation of CZTS thin film was achieved due to sufficient S supply by a high-density ethanol. A reason why CZTS films with good crystallinity can be fabricated at less 11
ACCEPTED MANUSCRIPT than 400 °C is proposed as follows: The melting point and boiling point of sulfur are 115 °C and 444 °C, respectively. Sulfur gas pressure at 400 °C is 0.049 MPa. Sulfur consists of seven different molecules, S2, S3, S4, S5, S6, S7, and S8 in a previous report [17]. According to this report, the fraction of S 2, S3, S4, and S5 with high reactivity is only 2.3% at 400 °C and the majority of sulfur (97.7%) is composed of
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S6, S7, and S8, having a low reactivity in the gas phase. To form the CZTS at 400 °C,
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low reactive liquid sulfur and S 6, S7, and S8 must be activated for sulfurization. In the process described in this study, liquid sulfur can be dissolved into SCF and used
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for sulfurization. We believe that the solvation effect of SCF facilitates the
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decomposition of high molecular-sized sulfur. The scEtOH is inserted into the S-S bonds and decomposes them, resulting in an increase in the density of highly
achieved at 400 °C after 25 min.
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reactive species. Based on these factors, it is estimated that CZTS formation was
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3.3 Optical properties of CZTS film
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Finally, we conducted the measurement of optical properties to prove that the SCF process can be an alternative method for the fabrication of optical-grade CZTS film. Based on the results of 3.2, the reaction time and ethanol density was set
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to 40 min and 3.0 mol/L, rescpectively. The XRD patterns of the optimized
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amorphous oxide precursor films and the film after sulfurization are shown in Fig. 5 (a). The films after sulfurization only show peaks corresponding to the Cu 2ZnSnS4 structure (except Mo), i.e., (112), (200)/(004), (220)/(204), and (312)/(116) at 28.6°, 33.1°, 47.5°, and 56.4°, respectively. It is noted that CZTS has several crystal structures (kesterite, stannite, and disordered kesterite), which cannot be distinguished using only XRD. CZTS with an ideal composition for solar cells has a disordered kesterite structure. The Raman spectra of Cu 2ZnSnS4 shown in Fig. 5 (b) is consistent with that obtained in previous studies [18, 19], and other binary and 12
ACCEPTED MANUSCRIPT ternary compounds were not detected. The peaks at 285, 332, and 374 cm –1 show the disordered kesterite structure. Normally, the kesterite structure shows an A1 mode at 338 cm–1. The strong peak at 332 cm–1 is related to the presence of local structural inhomogeneities within the disordered cation sublattice antisites. In general, these necessarily occur in Cu-poor conditions, and are of two types: Zn Cu-type and
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CuZn-type [18, 20]. From these results, it was confirmed that a disordered
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kesterite-structured Cu2ZnSnS4 film can be formed from the precursor film. It is known that in the case of direct-bandgap semiconductors such as CZTS,
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the bandgap can be estimated by plotting (Ahν) 2 (A: absorbance, h: Planck’s
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constant, and ν: frequency) against the photon energy. The energy at which the extrapolated line of (Ahν) 2 versus hν reaches (Ahν) 2 = 0 can be regarded as the
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bandgap. Fig. 6 shows the plot of (Ahν) 2 as a function of hν for the absorption spectra of CZTS fabricated by using the optimized amorphous oxide precursor.
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Normally, the bandgap of the CZTS thin film is around 1.5 eV, but in the Zn-rich and
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Cu-poor state, the occurrence of Zn Cu-type and CuZn-type antisites is significant, which causes an increase in the valence band maximum and a decrease in the conduction band minimum, resulting in a decrease to about 1.4 eV in the bandgap
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[21]. The estimated bandgap energy of the fabricated CZTS film was around 1.38 eV,
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corresponding to the reported value for the Zn-rich and Cu-poor CZTS [21]. The photoluminescence (PL) was measured, as shown in Fig. 7(a, b). In the
CZTS system, band edge luminescence was not observed, even in a single crystal [22]. In previous reports in which PL emission for CZTS was around 1.25 eV at low temperature, the emission was attributed to the band-to-tail transition by highly doped donors and acceptors [23] or to donor-acceptor pair (DAP) transition by shallow donors and acceptors [23-25]. The PL spectra of CZTS at 5 K are shown for different values of excitation power in Fig. 7(a). The maximum intensity obtained 13
ACCEPTED MANUSCRIPT was at approximately 1.24 eV in the red circle of Fig. 7(b). The PL peak position showed blue shift with increasing excitation power, which is a common feature of DAP transition [24, 25] and band-to-tail transition by potential fluctuation [23, 26]. As for the intensity, when the PL integrated intensity follows the power law: Intensity ∝ Pγ (where P is the excitation power and γ is an adjustable parameter), a
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value of γ < 1 means that donor or acceptor defects engage in the emission for CZTS
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[23,24,27,28]. As shown in the green quadrilateral of Fig. 7 (b), a value of γ = 0.96 was obtained, indicating the observed PL emission is derived from the donors and
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acceptors in CZTS as reported in previous studies.
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Moreover, the photoconductivity measurement of the CZTS film was carried out at room temperature. As shown in Fig. 8, the photoconductivity of CZTS was 5
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times larger than the dark conductivity. This is due to the increase of photo-generated carriers in the CZTS semiconductor. These optical property results
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indicate that by using an SCF sulfurization process at 400 °C, a CZTS thin film can
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be produced that has very similar structural and optical properties as those of a fi lm fabricated using conventional vapor-phase sulfurization. In this study, the composition analysis by EDX or inductively coupled
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plasma (ICP) for the optical-grade CZTS film was not carried out. Generally, in
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EDX analysis, it is difficult to quantitatively evaluate a composition for a sample with high surface roughness and multielement compounds like as our sample. In addition, in general sulfurization process for CZTS fabrication, ZnS is left in the bottom[28], thus ICP analysis cannot show the accurate composition value. In contrast, structural and optical properties show that our sample has the Cu-poor and Zn-rich composition. Furthermore, from the result of cross-sectional EDX for the optimized Cu-Zn-Sn oxide precursor with little surface roughness showed a composition of Cu/(Zn+Sn):0.88 and Zn/Sn:1.46 and the solution left in the reactor 14
ACCEPTED MANUSCRIPT after the reaction were analyzed by ICP, and as a result, some elution of copper was confirmed, but little elution of tin and zinc was confirmed. Therefore, although more detailed composition analysis is required, it was determined that our CZTS film has Cu-poor and Zn-rich composition from the
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results of structure and optical properties.
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4. Conclusions
This study demonstrated the advantages of combining SCF sulfurization
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with a Cu-Zn-Sn-O amorphous precursor for low-temperature sulfurization. It was
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shown that SCF sulfurization was complete when using an amorphous oxide precursor at 400 °C. However, very little SCF sulfurization took place when using a
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Cu-Zn-Sn alloy precursor. The amorphous metal oxide contains many point defects. Hence, elemental diffusion in amorphous metal oxides can be faster than that in the
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alloy precursor.
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The ethanol density dependence on the grain size was established from the SEM images and the FWHM of the strongest peak derived from metal sulfides. The grain size increased as the ethanol density increased, and this phenomenon was
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particularly noticeable in the high-density conditions of over 3.0 mol/L. Crystal
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growth is believed to be caused by an increase in the sulfur solubility in ethanol as the ethanol density increases. By combining SCF sulfurization with a Cu-Zn-Sn-O amorphous precursor,
the formation of CZTS films consisting of Cu-poor and Zn-rich disordered kesterite were confirmed by XRD and Raman spectra. PL emission of about 1.24 eV at 5 K caused by the donor or acceptor defects indicated a bandgap of 1.38 eV at room temperature. The optical properties, determined by UV–vis absorption, PL measurement, and photoconductivity, were consistent with previous reports. 15
ACCEPTED MANUSCRIPT In short, the combination of SCF sulfurization and the use of a Cu-Zn-Sn-O amorphous precursor enabled the fabrication of large-grain CZTS films at 400 °C. The SCF sulfurization process using a metal oxide precursor can be applied to other
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energy devices based on metal-chalcogenide materials.
Acknowledgements
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This work was financially supported by JSPS KAKENHI (Grant Number
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25630353 and 15J051979), the Inter-Graduate School Doctoral Degree Program on
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Science for global safety and the Tohoku University Division for Interdisciplinary Advanced Research and Education. We thank the Analytical Research Core for
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Advanced Materials in Tohoku University for property measurements
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(cross-sectional EDX, Operator: Dr. Makoto Nagasako).
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ACCEPTED MANUSCRIPT 5) X. Song, X. Ji, M. Li et al. A review on development prospect of CZTS based thin film solar cells. Int. J. Photoenergy. 2014 (2014) 613173. 6) T. Nakada, H. Ohbo, T. Watanabe et al. Improved Cu(In,Ga)(S,Se) 2 thin film solar cells by surface sulfurization. Sol. Energy Mater. Sol. Cells. 49 (1997) 285–290.
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(b) CZTS (112)
Intensity (arb.unit)
1 μm
After
Cu3Sn CZTS (220/204)
Cu6Sn5
Sn
25
30
Cu3Sn
Mo (110)
Cu6Sn5
35
Sn
40
Alloy precursor
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45
50
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Cu-Kα 2θ (deg)
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(112) CZTS (200/004)
30
35
40
45
CZTS (312/116)
Amorphous precursor
50
55
Cu-Kα 2θ (deg)
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After
CZTS (220/204)
Mo (110)
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Intensity (arb.unit)
1 μm
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(d) CZTS
(c)
Fig. 1 SEM images in two types of precursors and XRD patterns before and after
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SCF sulfurization for each precursor. (a) SEM image of precursor deposited using Cu-Zn-Sn alloy target
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(b) XRD patterns before and after SCF sulfurization for (a) (c) SEM image of precursor deposited using sintered Cu-Zn-Sn-O target (d) XRD patterns before and after SCF sulfurization for (c)
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40 min 25 min 10 min
FWHM (deg)
Intensity (arb. unit)
70 min
100
(b)
0.16
0.15
30
35
40
45
50
55
60
Cu
80
Sn
70
Zn
60
O
50 40 30 20 10
Precursor 0.14
25
S
(c)
90
Composition [%]
0.17
(a)
0 0
10
Cu-Kα 2θ (deg)
20
30
40
50
60
70
0
Reaction time (min)
10
20
30
40
50
60
70
Reaction time (min)
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Fig. 2 XRD patterns, FWHM of the strongest peak of CZTS (around at 28.6
precursor after sulfurization for each reaction time.
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(a) XRD patterns
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degrees) and surface EDX of thin film fabricated using the amorphous oxide
(■CZTS (112) □CZTS (200/004) ●CZTS (220/204) ○CZTS (312/116) /▲Mo
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(110))
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(b) FWHM of the strongest peak of CZTS (around at 28.6 degrees)
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(c) Surface EDX
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(a)
(c)
(b)
5 μm
5 μm
(e)
(f)
5 μm
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5 μm
5 μm
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(d)
5 μm
Fig. 3 SEM images after sulfurization for various ethanol densities for CZTS film
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fabricated using the amorphous oxide precursor.
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(a) Only Ar (b) 0.38 mol/L (c) 0.75 mol/L (d) 1.5 mol/L (e) 3 mol/L (f) 6 mol/L of
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ethanol density
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0.26
(a)
6 mmol/L
(b)
1.5 mmol/L 0.75 mmol/L 0.38 mmol/L
FWHM (deg)
3 mmol/L
0.22 0.20
0.18 0.16
Ar 0.14
Precursor 30
35
40
45
50
55
60
Cu-Kα 2θ (deg)
S Cu
80
Sn
70
Zn
60
O
50 40 30
20 10 0
0.12 25
(c)
90
Composition [%]
Intensity (arb. unit)
0.24
0
1
2
3
4
5
6
0
1
2
3
4
5
6
EtOH-density [mol/L]
EtOH density [mol/L]
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Fig. 4 XRD patterns, FWHM of the strongest peak of metal sulfides (around at 28.6
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precursor after sulfurization for each ethanol density.
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degrees) and surface EDX of thin film fabricated using the amorphous oxide
(a) XRD patterns
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(■CZTS (112) □CZTS (200/004) ●CZTS (220/204) ○CZTS (312/116) /▲Mo (110))
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(b) FWHM of the strongest peak of CZTS (around at 28.6 degrees)
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(c) Surface EDX
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(b)
(a)
332 cm-1
Intensity (arb. unit)
Intensity (arb.unit)
CZTS
285 cm-1
25
30
35
40
45
50
55
150
60
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Precursor
200
Cu-Kα 2θ (deg)
250
300
374 cm-1
350
400
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Raman shift (cm -1)
Fig. 5 (a) XRD patterns (■CZTS (112) □CZTS (200/004) ●CZTS (220/204) ○
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CZTS (312/116) /▲Mo (110)) (b) Raman shift of CZTS film fabricated using the
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optimized amorphous oxide precursor (excitation wavelength: 532 nm).
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15
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(Ahν)2 [eV2]
20
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0.6 0.8 1 1.2 1.4 1.6 1.8
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hν [eV]
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Fig. 6 Plot of (Ahν)2 as a function of hν (A: absorbance, h: Planck’s constant, and ν: frequency) for CZTS fabricated using the optimized amorphous oxide precursor. Data for the range 880 and 900 nm (graph missing) was removed because the
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detector was changed at 900 nm and there was a large deviation in the value.
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169 mW 259 mW 346 mW 438 mW 529 mW 621 mW 716 mW 813 mW 911 mW 9 mW 10
950
1000 1050 Wavelength [nm]
1100
1 0.9
1.25
(b)
0.8
Intensity = 0.02 P 0.96
0.7
R² = 1.00
1.24 1.23
0.6 0.5
1.22
0.4 0.3
1.21
0.2 0.1
PL Peak Energy [eV]
(a)
Integrated PL Intensity (P/Pmax) [-]
Intensity [arb. unit]
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1.20 5
15
25
35
45
55
Excitation power [mW]
65
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Fig. 7 (a) Plot of PL peak energy of CZTS film fabricated using the optimized
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amorphous oxide precursor as a function of PL exciton power at 5 K (b) Plot of the
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PL peak intensity as a function of PL exciton power at 5 K (Excitation wavelength:
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532 nm).
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0.2
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0
-0.2
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Current (mA)
0.4
-0.6 -1
-0.5
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-0.4
Light Dark
0
0.5
1
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Voltage (V)
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Fig. 8 I-V characteristics of the Cu2ZnSnS4 fabricated by using the optimized amorphous oxide precursor.
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Research Highlights ►An amorphous metal-oxide precursor is utilized for fabrication of Cu2ZnSnS4 film. ►Sulfurization is conducted under supercritical ethanol having reducibility. ►The crystal growth proceeds in a high ethanol density region. ►The structural and optical properties derived from Cu 2ZnSnS4 are shown.
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