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This study developed a novel one-step method for the synthesis of well-dispersed CZTSe nanoparticles from Cu, Zn, Sn, Se elemental powders using a solvent-thermal reflux technique. Polyetheramine was used as a solvent and chelating agent, which provided two NH2 bonding sites at the ends as well as long, continuous epoxy chains in the center, making it ideally suited to the formation of CZTSe for use in ink printing. X-ray diffraction (XRD) and optical Raman spectroscopy were used to identify structural and phase information; transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to confirm the morphology of the film as well as the shape and composition of the particles, and UV-Vis was used to characterize the band gap. We also investigated the composition and morphology of the CZTSe nanoparticles depending on different solvents polyetheramine and isophorondiamine. The polyetheramine-based process produced particles with far more Cu-poor and Zn-rich composition CZTSe phases, a smoother and closed packed morphology, and a more spherical crystal shape. In contrast, the isophorondiamine-based process produced nanoparticles with an irregular sheet-like shape. These results demonstrate that the solvent-thermal reflux method depends mainly on the structure of the chelating agent. The proposed technique is a low-cost and environmentally friendly solution-synthesis method applicable to the large-scale production of CZTSe for photovoltaic devices.
© 2014 Optical Society of America
1. Introduction
Due to global warming, thin-film solar cell has been attracted a lot of interests in roll-to-roll process owing to low-cost and large-area fabrication. Currently, thin-film photovoltaic technology is mainly based on silicon. However, silicon-based thin-film process has disadvantages of low-efficiency and high-cost wafer fabrication. The current high value for solar conversion efficiency devices had been reported by using multi-junction tandem cell approach in III-V semiconductors [1]. Recent works on GaInNAs-containing (III-V-Nitride) quantum wells active regions grown on GaAs template had resulted in superior lasing characteristics with record low threshold devices [2–4], and this progress has recently led to the realization of reports of 43.5% solar conversion efficiency [5]. To further lower down the cost of fabrication, the CuIn1−xGaxSe2 (CIGS)-based or Cu2ZnSnSe4 (CZTSe)-based solar cell represents an exciting approach to address the cost issue in solar devices with relatively moderate efficiency. The chalcopyrite CIGS-based thin film solar cell is very promising. CIGS has high absorption coefficient (>105 cm−1), direct band gap and stability against photo-degradation [6]. The quaternary semiconductor CZTSe is an excellent alternative to CIGS solar cells, due to its nearly optimal direct band gap energy of 1.5 eV and inherent high absorption coefficient (104 cm−1) as well as the low cost and abundance of the constituent elements zinc (Zn) and tin (Sn) and a fabrication process and physical properties similar to those of CIGS [7,8]. Thus, indium-free kesterite-related materials (Cu2ZnSnSe4) have attracted considerable interest among researchers for the fabrication of large-area, low-cost, high-efficiency thin film solar cells. CZTSe is mainly produced using vacuum-based processes, such as magnetron sputtering and co-evaporation; however, this approach [8–10] is relatively complex and expensive due to requirements of high temperature and high vacuum degree environment and difficult to large area production. To overcome these problems, numerous alternative processes have been developed, including non-vacuum processes, such as electrodeposition [11], solution-based processes such as hot–injection [12], and solvothermal processes [13–16] as well as methods based on the synthesis of precursor inks, such as hydrazine based process [17,18], and two-step processes [19–22]. Most solvothermal methods have used organic amine ligands as the solvent due to their low cost, convenient handling, and mild production conditions; however, solvothermal processes require a high atmospheric pressure and a high-temperature environment within a well-sealed autoclave. In addition, this approach often leads to aggregation, which makes it difficult to form densely-packed nanocrystal thin films. A number of researchers have used hydrazine as a reaction solvent and complexing agent for the synthesis of CZTSe nanocrystals or precursors [17,18]. These materials have been used to produce the highest energy-conversion efficiencies; however, hydrazine is highly toxic and unstable, requiring extreme caution during handling and storage. Other solution synthesis methods, such as hot-injection have been proposed for the synthesis of CZTS nanocrystals; however, this approach requires relatively complex devices within a protective atmosphere to enable the rapid injection of metal precursors (Cu, Zn, Sn) into a high-temperature organic reaction medium to ensure the rapid nucleation and growth of CZTSe nanocrystals. This approach can be dangerous and the resulting rapid nucleation leads to poor monodispersion of synthesized particles [12]. Wenchao Liu et al. [23] recently developed a hydrothermal method in which ethylenediamine is combined with distilled water to 80% of its volume with the aim of lowering the reaction temperature. Nonetheless, this approach still requires the transfer of a precursor to a teflon-lined stainless autoclave as in the solvothermal method [23]. Most of similar methods require a thoroughly sealed environment to cope with the lower boiling point and weak solvent, which limit the performance and applicability of these solution synthesis methods.
Recently, two-step processes have been developed using spin-coating with ternary Cu-Zn-Sn solution for the deposition of precursor layers prior to temperature-controlled selenization. This approach is a simple, low-cost, and low-temperature process capable of large-area deposition with good scale up potential; however, the film morphology phase composition are difficult to control, requiring additional organic binders to improve film properties [24,25]. This has made it necessary to develop new synthesis methods with alternative solvents to advance the synthesis of high-quality nanocrystals in order to address the challenge of fabricating low-cost, high-efficiency solar cells. The reaction solvent plays an important role in solution process methods. Solvents based on amine groups can act as reactant, solvent, or binder, depending on the chemical structure and polarity. Thus, amine bonding, synthesis environment, and temperature have considerable effect on the morphology of CZTSe nanoparticles. Previously, we reported the synthesis of CZTSe nanoparticles using isophorondiamine as solvent [26]. However, there is in common problems of incomplete reaction and poor binding resulting in particles with a Cu-rich, Zn-poor composition, which is precisely the opposite of the optimal configuration with regard to film morphology and coating ability.
In this paper, we first developed a facile solution synthesis method (hereafter referred to as a solvent-thermal reflux process) using novel organic solvent polyetherammine and various elemental metal powders as starting materials. The proposed method involves a one-step process beginning with the addition of raw elements to form the CZTSe nano-ink without the need for the injection of hot ink material. To enable low-cost, large-area coating, two-step process involving the synthesis of a precursor ink, followed by coating and then selenization. Polyetheramine contains an epoxy group in the middle, which provides sufficient viscosity for uniform particles and excellent morphology applicable to large-area printing without the need for additional organic binders, having well potential for two step process. The strong chelating ability and high-solubility of the starting material can be attributed to double NH2 bonding and hydrogen bonding provided by the polyetheramine. This means that using polyetheramine enables a rapid, thorough reaction without the need for high temperatures or high atmospheric pressure. In this study, we began by implementing the proposed solvent-thermal reflux method in the synthesis of pure CZTSe nano-ink ideally-suited to large area thin-film solar cell applications. We also investigated the composition and morphology of inks produced using different solvent polyetheramine or isophorondiamine. The structural, compositional and morphology information were characterized and solvent chemical structure dependent properties such as phase composition and film morphology have been discussed.
2. Experimental
This study synthesized well dispersed, pure Cu2ZnSnSe4 nano-ink using a facile solution route in a solvent-thermal reflux process. Elemental metal sources (Kurt Lesker, purity in 99.99%) and elemental selenium (Kurt Lesker, purity in 99.99%), Cu powder (1.0 mmol), Zn powder (0.5 mmol), Sn powder (0.5 mmol), and Se powder (2.0 mmol) all were dissolved in 130 ml of polyetheramine (CH3CH(NH2)CH2[OCH2CH(CH3)]6.1NH2) or isophorondiamine in three-necked flasks, followed by constant stirring and heating to 230°C over a period of 30 hours under nitrogen atmosphere. Furthermore, to make reaction more efficiently, bringing the polyetheramine to its boiling point accelerated the reaction and resulted in a more complete reaction. A cold condenser was connected externally to cool the CZTSe solution ink to room temperature after the reaction, where upon 150 ml of alcohol, dilute water, and acetone were added to precipitate the as-synthesized nanocrystals. To characterize CZTSe nanoparticles in polyetheramine, the structural properties of the synthesized CZTS powder samples were studied using X-ray diffraction (XRD) with Cu/K radiation to charactrize the phase and crystallinity of the as-prepared samples. The voltage and current were held at 40 kV and 40 mA, respectively. The formation of CZTS phase was further confilrmed by Raman measurement at 633 nm. Energy-dispersive X-ray spectroscopy (EDS) was used to analyze the chemical composition and proportions of the elements; transmission electron microscopy (TEM) was used to investigate the size, shape, and structure of the CZTSe nanocrystals. The morphology and micro-structure were characterized using a JEOL JSM-7001 F (at 10 KV) field emission scanning electron microscopy (FESEM). UV–Vis absorption spectra were measured to evaluate the optical properties of the as-synthesized CZTSe nanocrystals. Second, differences in the film morphology and composition of CZTSe synthesized in polyetheramine or isophorondiamine were characterized using field emission scanning electron microscopy (FESEM) and an energy-dispersive X-ray spectroscope (EDS).
3. Results and discussions
This study used XRD to identify the crystal structure and phase of the synthesized CZTSe nanoparticles. Figure 1 presents the XRD patterns, showing (112), (204), (312) peaks, characteristic of CZTSe nanoparticles with a tetragonal structure (JCPDS file: No. 52-0868). No other second phases were observed, demonstrating the purity of the synthesized CZTSe nanoparticles. According to the FWHM of peak XRD patterns, the calculated average grain size, obtained using the Scherrer equation, was 28 nm. The peak intensity was high and the FWHM of the (112) peak was very narrow, indicating good crystallinity without the need for additional heat treatment [8,24].
Raman spectroscopy was applied to further confirm the above structure-related information and distinguish CZTSe phase from CTSe phase. Figure 2 presents two main peaks at 171 cm−1, 190 cm−1, and a broad band in the range of 230-235 cm−1, which match the characteristic of CZTSe phase, thereby supporting the XRD data in which only a single quaternary CZTSe phase was observed. The 190 cm−1 peak (representing the A1 mode of CZTSe) was sharp. There is no 180 cm−1 peak, indicating a complete absence of CTSe phase [8,24]. According to XRD and Raman results, the proposed solvent-thermal reflux method in conjunction with polyetheramine improved the phase formation of CZTSe through improved reactivity and binding ability.
TEM microscopy was used to investigate the dimensions, composition, and crystal structure of as-synthesized nanoparticles. Figures 3(a)-3(c) presents a TEM image, high resolution image, and SAED image, respectively. The average size ranged between a relatively narrow 20 and 24 nm, showing particles of homogeneous shape and dispersion. SAED patterns display three concentric rings, typical of tetragonal CZTSe for the planes of (112), (204), (312), thereby supporting the XRD results. HRTEM showed good crystallinity and a lattice spacing of 0.33 nm corresponding to the (112) CZTSe peak. EDS analysis of CZTSe nanoparticles presented the following atomic ratios: Cu:Zn:Sn:Se = 21.04:12.72:10.04:56.20, Cu/(Zn + Sn) = 0.92, Zn/Sn = 1.26. According to the EDS results, the CZTSe crystal revealed a distinct Cu-poor and Zn-rich composition. Compared to our previous experiments using isophorondiamine [26], the use of polyetheramine as a solvent and binder proved highly effective in the synthesis of well-dispersed ink, resulting in a closely-packed film with a Cu-poor and Zn-rich composition without the need for additional composition engineering or annealing treatment.
As shown in Fig. 4, UV-Vis spectroscopy was used to measure the absorption spectrum versus wavelength in order to obtain information regarding the band gap of the as-synthesized CZTSe nanoparticles. The band gaps shown in Figs. 4(a)-4(c) were as estimated at 1.42 eV, 1.5eV, 1.55eV for different solvents: polyetheramine, isophorondiamine and oleylamine, respectively, by extrapolating the linear region of a plot of the absorbance squared versus energy. The observed band gap corresponds well with that reported in previous studies [25]. In addition, the differences of band gaps are similar to the results reported by Yan-Fang Du et al. [27]. They confirmed that ethylenediamine solvent produced a faster reaction rate, completely reaction and greatly improved average composition due to its stronger chelating ability. Similarly, polyetheramine has a superior chelating ability over than isophorondiamine and oleylamine due to its double NH2 bonding. In contrast, isophorondiamine and oleylamine have only one NH2 bonding. As a result, isophorondiamine and oleylamine have larger band gap than polyetheramine. To make deeper insight into the chemical structure effect on optical properties, Juyeon Chang studied about the effect of number of amine groups on solvent stability and phase reaction. We conclude that solvent reactivity plays a main factor in phase formation of CZTSe nanoparticles [28]. The band gaps of CZTSe nanoparticles ranged from 1.0 to 1.5 eV were good agreement with the studies of A. H. Reshak [29].
Fig. 4 (a) polyetheramine, (b) isophorondiamine (c) oleylamine of UV–Vis absorption spectrum of as-synthesized nano-particles: the inset presents (αhν)2 vs. energy.
The surface morphologies of CZTSe thin films synthesized in polyetheramine and isophorondiamine were compared using field emission scanning electron microscopy (FE-SEM) in Fig. 5. Figure 5(a) reveals the flat morphology and dense, uniformly sized grains without aggregation; Fig. 5(b) reveals enlarged graph, showing that CZTSe film composed of uniform and completely grown grains. These results indicate that polyetheramine pursuit grain growth and able to form larger CZTSe particles. In contrast, as shown in Figs. 5(c)-5(d), the isophorondiamine-based solution resulted in a very rough surface morphology with greater porosity and lack of uniformity in grain size. These results are similar to those reported by Pawar et al. [30] regarding complexing agents on film properties. Tri-sodium was used as complexing agent. According to the report by Pawar et al., the film prepared without complexing agent showed large cracks and large planar structures. Whereas those prepared using complexing agent, the surface of the films exhibited slightly porous and uniform grown particles. From this, we can conclude that the addition of certain complexing agents can greatly improve the morphology of the resulting films, particularly with regard to uniformity and distribution.
Fig. 5 (a) SEM top view of as-coated CZTSe particles synthesized in polyetheramine, (b) enlarged SEM micrograph of CZTSe particles synthesized in polyetheramine (c) enlarged SEM micrograph CZTSe film synthesized in IPDA (d) enlarged SEM micrograph of CZTSe grain particles synthesized in IPDA.
This considerable difference in surface morphology may be due to differences in the chemical structures of oleylamine, polyetheramine, and isophorondiamine, as shown in Fig. 6. The structure of the solvents in polyetheramine provided strong chelating ability through the two NH2 bonding sites, additional good coating properties and film uniformity resulting from the long-chain epoxy group. However, the structure in oleylamine only has a single NH2 bonding site and that in isophorondiamine has the benzene structure of isophorondiamine which is relatively bulky. These make the two solvents difficult in forming reactants from metal complexes and complexing ability resulting in poor surface morphology. Thus, the structural properties of the solvent, such as amine group and molecule size, are critical to reactivity and chelating ability. The ability to form metal-solvent chelate complexes is an important determining factor in the synthesis of CZTSe [31]. Furthermore, we observed a clearly differentiated grain shape in Figs. 5(b) and 5(d), respectively. Figure 5(b) shows a uniform and spherical shape. In contrast, Fig. 5(d) exhibits a nonuniform and irregular sheet structure. The difference in the shape of CZTSe nanoparticles may be due to the superior dissolution of metal ions in polyetheramine. Compared to isophorondiamine, polyetheramine exhibts a stronger chelating ability and binding effects which resulting in the formation of large quantities of complexes. These enhance a rapid reaction and binding together spontaneously and produce spherical-shape nanoparticles. In contrast, isophorondiamine present a weaker chelating ability and low quantities of metal-complexes which resulting in irregular growth and sheet-shape nanoparticles [27].
4. Conclusion
This study succeeded in the synthesis of well-dispersed CZTSe nanoparticles using polyetheramine as a solvent. The proposed solution method is referred to as a solvent-thermal reflux method. This process produced pure phase CZTSe nanoparticles approximately 20-24 nm in size with a band gap of 1.42 eV and good crystallinity without the need for additional heat treatment. We also compared these results with those previously obtained using isophorondiamine, which revealed that the shape and film morphology of the CZTSe nanoparticles was greatly affected by the selection of solvent used in this non-vacuum solution method. This alternative solvent proved highly beneficial with regard to uniformity in size and the shape distribution of CZTSe nanoparticles, due to the two NH2 bonding sites of the polyetheramine at the ends as well as the large molecular epoxy group well-suited to the chemical coordination of metal species. These contribute to Cu-poor and Zn-rich trend in composition. The result represented that polyetheramine is a suitable solvent for highly soluble, high-viscosity ink, ideal for coating a large area of film. This research provides a simple, low-cost, method for the mass production of CZTSe photovoltaic devices.
Acknowledgments
The author would like to thank the Bureau of Energy, Ministry of Economic Affairs of Taiwan, R.O.C. for the financial support under Contract 101-D0204-6 and the LED Lighting Research Center of NCKU for the assistance of device characterization and the National Science Council of Taiwan, R. O. C., under Contract No. NSC 101-2622-E-024-002-CC3, and NSC 101-3114-E009-002-CC2.
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