Flux-Boosted Sulfide Crystal Growth: Growth of CuInS2 Crystals by

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Flux-Boosted Sulfide Crystal Growth: Growth of CuInS2 Crystals by NaCl−InCl3 Evaporation Masaaki Kurihara,† Fumitaka Hayashi,‡ Kosuke Shimizu,‡ Hajime Wagata,‡ Toshiyuki Hirano,† Yasuhiro Nakajima,† Kunio Yubuta,¶ Shuji Oishi,‡ and Katsuya Teshima*,‡,∥ †

Renewable Energy Materials Development Group, Energy & Environment R&D Center, Corporate Research & Development, Asahi Kasei Corporation, 1-3-1 Yakoh, Kawasaki-ku, Kawasaki 210-0863, Japan ‡ Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, Wakasato, Nagano 380-8553, Japan ¶ Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan ∥ Center for Energy and Environmental Science, Shinshu University, Wakasato, Nagano 380-8553, Japan S Supporting Information *

ABSTRACT: Copper−indium−gallium−sulfide−selenide (CIGSSe) is used in photovoltaic cells and photocathodes, because of its tunable optoelectronic properties, but the fabrication of CIGSSe samples usually requires a multistage process under vacuum. Herein we used a flux growth technique for the sulfide system and achieved efficient flux growth of idiomorphic copper−indium−sulfide CuInS2 crystals of size ∼5 μm from a NaCl−InCl3 flux under mild conditions at ambient pressures. We first examined the flux growth conditions such as holding temperature, solute concentration, and holding time for growing highly crystalline CuInS2 crystals. A moderate holding temperature (∼550 °C) and high solute concentration (∼70 mol %) yielded idiomorphic pure CuInS2 crystals. High-resolution transmission electron microscopy showed clear electron diffraction spots, indicating that the resultant CuInS2 crystals had a highly crystalline, intrinsic tetragonal crystal structure. Thermogravimetry-differential thermal analysis showed that the CuInS2 crystals grew efficiently during flux evaporation at 550 °C, at which the flux evaporation degree reached ∼81%. The CuInS2 crystal growth mode is discussed based on the characterization results.

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INTRODUCTION Chalcopyrite-type compounds, copper−indium−gallium−sulfide−selenides (CIGSSe), receive much attention because of their high chemical stabilities and unique optoelectronic properties (high absorption coefficients and tunable band gap values), which can be controlled based on the chemical compositions.1 The performances of CIGSSe materials as lightabsorbers in thin-film solar cells are better than those of other promising candidate materials such as CdTe and amorphous silicon, and provide high-performance photovoltaic (PV) devices.2 The solar energy conversion efficiency of CIGSSebased PV cells can reach 21.7%.3 CIGSSe materials can also be used as photocathodes for water reduction to produce solar hydrogen. Gunawan and co-workers reported that Pt- and In2S3-modified CuInS2 gave a highly efficient H2 evolution performance (applied bias photon-to-current efficiency, 1.97% at 0.28 V versus reversible hydrogen electrode).4 One of the obstacles to widespread commercialization of CIGSSe materials is the difficulty in controlling the chemical composition of CIGSSe over large device areas.5 CIGSSe crystal layers are generally fabricated on devices via a multistep evaporation process in which alternating In, Ga, and Se layers are deposited, followed by reaction with Cu and a S/Se source in a vacuum © XXXX American Chemical Society

chamber. This multistep process is advantageous to construction of the integrated structure, which improves the energy conversion efficiency of the CIGSSe photovoltaic devices. However, such processes require the special apparatus in a high vacuum; therefore, it is ill suited to rapid fabrication of the CIGSSe crystal layer with large areas and alternative CIGSSe fabrication methods are needed. With regard to flux growth of CuInS2 crystals, there have been no reports on growth of micrometer-sized crystals. To realize rapid and cost-efficient fabrication processes, many processes for the CIGSSe have been studied, such as solutionbased nanoparticle coating,6−10 sol−gel coating,11−13 and spray pyrolysis.14,15 Such approaches are useful for the fabrication of large-area CIGS layers, but the crystallinity of resulting crystals is not sufficient. Crystal engineering at interfaces is a key technology in the photovoltaic field because submicrometers thin layers without grain boundaries are crucial to this application. Received: August 9, 2015 Revised: January 15, 2016

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DOI: 10.1021/acs.cgd.5b01142 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Flux methods are simple and useful processes for preparing multicomponent crystals at temperatures below the melting points of the solutes.16,17 A specific flux can promote the growth of crystal facets without defects, which have lower energies of formation than other crystal facets.17 Wire-, sheet-, and plate-type crystals have been fabricated using this approach. In particular, chloride-based flux growth has the following three advantages. First, various micrometer-sized photocatalytic crystals can be grown from chloride-based fluxes.18 Second, a chloride flux is suitable for the growth of nonoxide crystals such as sulfides and nitrides, because no oxygen species are present. Third, a chloride flux is cost-effective and environmentally benign compared with other fluxes such as sulfide-based fluxes. In the present study, we selected CuInS2 as a model CIGSSe sulfide, and achieved efficient flux growth of CuInS2 crystals of controlled particle size (∼5 μm) from a NaCl−InCl3 flux under mild condition (∼550 °C). CuInS2 crystals are cubic, and the congruent melting point of CuInS2 is ∼1100 °C. The present study is divided into two parts. First, crystal growth of CuInS2 was studied as a function of holding temperature and solute concentration, to explore the optimum growth conditions. Second, the growth mode of the resultant highly crystalline CuInS2 crystals from the NaCl−InCl3 flux was investigated in detail.

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Figure 1. FE-SEM images of CuInS2 crystals grown at holding temperature of (a) 350, (b) 440, and (c, d) 550 °C. Holding times were 1 (a−c) and 10 h (d), respectively. Images a−d correspond to runs 1−4 in Table S1.

Figure 2 shows the XRD patterns of the CuInS2 crystals grown at 350, 440, 550, and 800 °C, with those of CuInS2,

EXPERIMENTAL SECTION

Flux Growth of CuInS2 Sulfide Crystal. Reagent-grade Cu2S (Wako) and In2S3 (Kojundo Chemical Laboratory) were used as the starting materials; InCl3 (Wako) and NaCl (Wako) were used as the flux. The growth conditions are summarized in Table S1. Different holding temperatures (350, 440, 550, and 800 °C) and holding times (1 and 10 h) were used to grow CuInS2 crystals. The heating and cooling rates were 300 and 200 °C h−1, respectively, and the atmosphere was nitrogen. All crystals were grown in a platinum crucible. The resulting products were washed with hot water and dried at 65 °C. Characterization. X-ray diffraction (XRD) patterns were obtained using a MiniflexII diffractometer (Rigaku) with monochromated Cu Kα radiation (λ = 0.15418 nm, 30 kV, 20 mA). Field-emission scanning electron microscopy (FE-SEM) images were obtained using a JSM-7600F instrument (JEOL). High-resolution transmission electron microscopy (HR-TEM; TOPCON, EM-002B) at 200 kV was used to evaluate the CuInS2 crystallinity. Thermogravimetry (TG) and differential thermal analysis (DTA) were performed using a Thermo plus EVOII (Rigaku) detector.

Figure 2. XRD patterns of CuInS2 crystals grown at (a) 350, (b) 440, (c) 550, and (d) 800 °C, and those of (e) CuInS2 (ICDD PDF 00047-1372), (f) In2S3 (ICDD PDF 00-025-0390), and (g) Si (ICDD PDF 00-005-0565). Holding time, 1 h. Patterns a−d correspond to runs 1−3 and 5 in Table S1.

RESULTS AND DISCUSSION Influence of Growth Conditions on Flux Growth of CuInS2 Crystal. First, we explored the effect of holding temperature on the particle morphology of the CuInS2 crystal. Figure 1 shows FE-SEM images of CuInS2 crystals grown at holding temperatures of 350, 440, and 550 °C, with a solute concentration of 20 mol % and holding times of 1 or 10 h. The flux growth conditions are summarized in Table S1, runs 1−4. Holding at 350 °C for 1 h yielded poorly grown and collapsed particles (Figure 1a). Increasing the temperature to 440 and 550 °C increased the particle size and improved the particle morphology (Figure 1b and 1c). At 550 °C, increasing the holding time to 10 h led to the formation of large, octahedral particles of size ∼15 μm (Figure 1d), indicating efficient crystal growth. However, a long holding time was ineffective for close packing of CuInS2 on the substrate because of the larger particle sizes.

In2S3, and Si as references. At 350 °C, the main phase was CuInS2 (ICDD PDF 00-047-1372), but unidentified peaks resulting from impurity phases were also present (Figure 2a). The grown crystals are analyzed by EDX analysis and the result is shown in Figure S1 and Table S2. The chemical compositions of crystals at all the points 1−6 in Figure S1 were roughly the same as that of CuInS2. The impurity phase was not identified by the EDX analysis. An increase in the reaction temperature to 440, 550, and 800 °C suppressed formation of byproducts, leading to a single phase of CuInS2 (Figure 2b−d). The intensity ratio of (1 1 2) peak at 27.9° to (0 2 4) peak at 46.3° was varied with increasing the holding temperature. The observed high ratio in Figure 2c could be attributed to the efficient crystal growth as a result of flux evaporation as explained later. The XRD patterns of the CuInS2 crystals depended little on the holding time under the present conditions (data not shown).

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DOI: 10.1021/acs.cgd.5b01142 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Next, the flux growth of CuInS2 crystals at 550 °C was studied as a function of solute concentration to achieve efficient growth of CuInS2 crystals; the holding time was 10 h. Figure 3

was studied using TEM and TG-DTA analyses. Figure 5a and 5b show HR-TEM image and the corresponding selected-area

Figure 5. (a) High-resolution TEM image and (b) selected-area electron diffraction pattern of CuInS2 crystal grown at 550 °C and 70 mol % solute concentration. Holding time, 10 h. Expected electron diffraction spots from CuInS2, taken with the incident beam along the [11̅0] direction, is shown in panel c.

Figure 3. FE-SEM images of CuInS2 crystals grown at 550 °C as a function of solute concentrations of (a) 10, (b) 50, (c) 70, and (d) 90 mol %. Holding time = 10 h.

shows FE-SEM images of CuInS2 crystals grown at solute concentrations of 10−90 mol %. Idiomorphic, finely packed particles were clearly formed at 50 and 70 mol % (Figure 3b and 3c). In contrast, at 10 and 90 mol %, spherical or undeveloped particles were obtained (Figure 3a and 3d). This solute concentration dependence indicates the importance of achieving a balance between dissolution and recrystallization during holding at 550 °C, as discussed in the next section. Figure 4 shows the XRD patterns of CuInS2 crystals grown at 550 °C and solute concentrations of 10−90 mol %. The

electron diffraction pattern of a CuInS2 crystal grown at 550 °C and 70 mol %. The same fringe pattern extends continuously throughout the observed region. A clear lattice image with continuous lattice fringes indicates that there were almost no defects on the crystal in this range. The periodicity of the fringes from inside to outside the crystal was 0.32 nm, which corresponds to d112 of CuInS2 (0.3195 nm). The highly ordered diffraction spots, taken with the incident beam along the [110̅ ] direction, indicate that the CuInS2 crystals are single crystals and have high crystallinity in this range (Figure 5b). The absence of satellite spots shows that there were no intergrowth, grain boundaries, and crystalline impurities. These results show that the CuInS2 crystal was predominantly a single crystal, with a d spacing. Assignment of the diffraction spots for CuInS2 are summarized in Figure 5c, and are consistent with those observed in Figure 5b. Figure 6a shows the TG-DTA profile of the solute mixture (Cu2S and In2S3) and NaCl−InCl3 flux. The weight loss along the y axis indicates the evaporation degrees of the flux based on the initial flux content. The TG-DTA profile shows the following three features. First, there are a sharp weight decrement peak and an endothermic peak at ∼100 and ∼120 °C, respectively. Both these peaks are attributed to desorption of water molecules from the NaCl−InCl3 flux. Second, there is an endothermic peak at around 270 °C. This peak is due to the eutectic temperature point in NaCl−InCl3 system, in which most of the NaCl and InCl3 fluxes melt together above the temperature. A small endothermic peak at ∼400 °C could be attributed to the dissolution/liquidus of precursors/intermediates for the CuInS2. Third, there is a large weight decrement peak at around 550 °C. This is attributed to evaporation of the NaCl−InCl3 flux, based on its melting point. Figure 6b shows the evaporation degrees of the NaCl−InCl3 flux as a function of holding temperature, with a holding time of 10 h. The evaporation degree increased linearly with increasing temperature and reached ∼81% at 550 °C. Above 550 °C, the evaporation degree was almost saturated. A comparison of the TG-DTA results with the XRD and FE-SEM results indicates that CuInS2 crystals grew effectively at around 550 °C,

Figure 4. Variations in XRD patterns of CuInS2 crystals with increasing solute concentrations to (a) 10, (b) 50, (c) 70, and (d) 90 mol % at 550 °C. Holding time, 10 h. Samples e−g are CuInS2 (ICDD PDF 00-047-1372), In2S3 (ICDD PDF 00-025-0390), and Si (ICDD PDF 00-005-0565), respectively. In the data of samples b and c, the diffraction peaks of Si used as a reference were observed.

holding time was 10 h. Each pattern shows single-phase CuInS2 crystals, irrespective of the solute concentration. These results show that the optimum conditions for idiomorphic CuInS2 crystal growth are as follows: holding temperature, 550 °C; solute concentration, 50−70 mol %, holding time, 10 h. Mode of CuInS2 Crystal Growth from NaCl−InCl3. The growth mode of idiomorphic CuInS2 crystals from NaCl−InCl3 C

DOI: 10.1021/acs.cgd.5b01142 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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by the XRD results in Figure 2. It should be noted that most of the Cu2S and In2S3 was transformed into CuInS2 crystals at 350 °C. At this middle stage, some dissolved CuSx and InSx species are present in the flux, and the reaction between the two species also yields CuInS2 crystals. At the same time, parts of the resulting CuInS2 crystals, especially their surfaces, can dissolve and precipitate, leading to anisotropic growth of specific facets. In the final stage of crystal growth, the evaporation of NaCl− InCl3 facilitates precipitation of CuInS2 crystals at 550 °C together with dissolution of CuInS2 in the flux. Such repeated dissolution−precipitation processes combined with the reaction shown in eq 1 enable the growth of idiomorphic CuInS2 crystals. This type of growth mode at high solute concentrations is similar to that of perovskite-type SrSnO321 and garnet-type Li5La3Ta2O12,22 but is rare in flux growth of oxides and sulfides. The combined effect of dissolution−precipitation and the chemical reaction result in the growth of idiomorphic CuInS2 crystals at high solute concentrations.

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CONCLUSIONS We developed a new solution-based approach to the growth of idiomorphic CuInS2 sulfide crystals from a NaCl−InCl3 flux. Experiments to determine the temperature and holding time dependences indicated that mild holding temperature (∼550 °C) and high solute concentration (50−70 mol %) yielded highly crystalline CuInS2 crystals of size ∼5 μm. HR-TEM images with clear electron diffraction spots showed that each resulting CuInS2 crystal was a single crystal. TG-DTA showed that the CuInS2 crystals were grown during flux evaporation at around ∼550 °C; about 80% of the NaCl−InCl3 flux was evaporated during holding. The characterization results showed that the growth mode of CuInS2 crystals from a NaCl−InCl3 flux is based on a combination of dissolution−precipitation and the chemical reaction shown in eq 1. This type of crystal growth is rare in the flux growth of sulfides and oxides. The present approach provides new routes to the fabrication of CIGSSe-based materials and will be useful in practical fabrication processes such as roll-to-roll systems using fluxcoating techniques.

Figure 6. (a) TG-DTA profile of mixture of solute (Cu2S and In2S3) and NaCl−InCl3 flux, and (b) evaporation degree of 1:1 NaCl−InCl3 flux at holding temperatures of 350, 440, 550, and 800 °C. Holding time = 10 h.

especially upon flux evaporation. The acceleration of CuInS2 crystal formation by flux evaporation is similar to the cases for ruby19 and sodium titanate20 crystals from molybdenum-based fluxes. We investigated the formation mode of idiomorphic CuInS2 crystals from a high-concentration NaCl−InCl3 flux. Figure 7 shows a schematic diagram of a possible CuInS2 formation mechanism. At or below 270 °C, the reaction shown in eq 1 does not occur at this early stage because the NaCl-InCl3 flux is still solid-state. Cu 2S + In2S3 → 2CuInS2

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ASSOCIATED CONTENT

S Supporting Information *

(1)

An increase in the holding temperature facilitates the solidstate reaction of Cu2S with In2S3 at or above 350 °C, as shown

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01142.

Figure 7. Schematic diagram of possible growth mode for idiomorphic CuInS2 crystals from NaCl−InCl3 flux. D

DOI: 10.1021/acs.cgd.5b01142 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Crystal growth conditions (PDF)

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by a JSPS Grant-in-Aid for Scientific Research (A) 25249089. REFERENCES

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DOI: 10.1021/acs.cgd.5b01142 Cryst. Growth Des. XXXX, XXX, XXX−XXX