Effect of Different Selenium Precursors on Structural Characteristics and Chemical Composition of Cu2ZnSnSe4 Nanocrystals


 In the presented work Cu2ZnSnSe4 nanocrystals have been synthesized by the polyol method. The chemical composition, morphological, structural, and microstructural properties of Cu2ZnSnSe4 nanocrystals depending on synthesis temperature and time as well as precursor composition have been thoroughly investigated using scanning and transmission electron microscopies, energy dispersive X-ray analysis, X-Ray diffraction, FTIR spectroscopy. We compared the properties of nanocrystals synthesized from different precursors containing selenourea or amorphous selenium as a source of selenium and then determined the optimal conditions for nanocrystals synthesis. It was found that the optimal synthesis time for nanocrystals obtained using the first approach was t = (30-45) min, and for the second approach t = 120 min. It was also found that the optimal composition for the synthesis of single-phase Cu2ZnSnSe4 nanocrystals by the second approach is the molar ratio of precursors 2:1.5:1:4 and synthesis temperature T = 280 оС. Cu2ZnSnSe4 nanocrystals synthesized under optimal conditions were used to develop nanoinks for printing solar cell absorbers by 2D and 3D printers.


Introduction
Up to date, research topics related to the development of electronic elements and devices using two-(2D) and three-dimensional (3D) printing techniques are developed rapidly. These techniques are affordable, versatile, economical, and waste-free that can signi cantly simplify the development manufacturing process and reduce production costs [1,2,3]. In addition, there is possibility to apply a wide range of substrates, including exible, because such technologies are low-energy and do not require heating the substrates up to thermal degradation [4][5][6][7][8][9][10].
2D and 3D printing technologies with nanoinks, based on metal NCs allow to develop printed circuit boards [20][21] and use different semiconductor compounds to design passive and active elements of electronic devices [22][23][24][25][26][27][28][29]. However, nowadays the scienti c and technological challenge is to develop the synthesis procedure of semiconductor NCs with controlled properties and the development of the stable and environmentally friendly colloidal solutions with the required surface tension and viscosity which can be used as inks for printing processes [30][31][32].
The modern absorbers of solar cells (SC) contain multicomponent compounds such as Cu 2 ZnSnS 4 , Cu 2 ZnSnSe 4 and Cu 2 ZnSn(S x Se 1-x ) 4 [33][34][35][36][37]. The compounds possess close to optimal energy bandgap (E g ), high light absorption coe cient (~ 10 5 cm -1 ), long lifetime, and high mobility of charge carriers [38]. The kesterite based compounds do not contain rare and environmentally hazardous components, in contrast, the elements are widespread in the Earth's crust and the cost of production is low [39][40]. However, today, certi ed laboratory SE's e ciency based on these materials does not exceed 10-11.3% [41]. Note that absorbers of the most e cient SC (η = 11.1%) were obtained by the vacuum-free method (using a solution based on hydrazine) [42].
However, hydrazine used as a solvent in such process is highly toxic, requires very careful handling and cannot be used in the mass production.
One of the reasons of the low e ciency of SC based on Cu 2 ZnSnS 4 , Cu 2 ZnSnSe 4 , Cu 2 ZnSn(S x Se 1−x ) 4 layers is a narrow range of compound homogeneity which causes the appearance of large number of defects and secondary phases during the thin lms growth, and, as a result, its low structural quality. Therefore, an effective strategy for obtaining thin lms of kesterite compounds with controlled crystal structure and chemical composition, high degree of crystallinity, minimum content of secondary phases, is its processing in three stages: colloidal synthesis of single-phase NCs with the subsequent deposition of its suspension on substrates using non-vacuum methods, for example, spray methods, 2D or 3D printing techniques, and, nally, post-growth thermal annealing of the obtained samples to improve the structural properties [33,43]. As a result, the structural properties of the thin lms, such as phase composition, lattice parameters, and size of coherent scattering regions (CSR), which play an important role in determining the functional characteristics of SCs, mainly depend on the conditions of NCs obtaining, and then the lms, and their post-growth treatments. In such way in the research work [43] SCs with the e ciency of solar energy conversion η = 10.2% were obtained on the basis of Cu 2 ZnSn(S x Se 1−x ) 4 absorbers grown from initial Cu 2 ZnSnSe 4 NCs. At the same time, Cu 2 ZnSn(S x Se 1−x ) 4 lms are now mainly made from Cu 2 ZnSnS 4 nanocrystals [44,45], and not Cu 2 ZnSnSe 4 because the Cu 2 ZnSnS 4 NCs are easier to obtain compared to Cu 2 ZnSnSe 4 NCs. At present, selenourea (NH 2 ) 2 CSe is mainly used as a source of selenium in the synthesis of Cu 2 ZnSnS 4 and Cu 2 ZnSnSe 4 nanocrystals [46][47][48]. Using precursors based on cheaper elemental selenium Cu 2 ZnSnSe 4 , NCs were obtained only by the solvothermal method [49,50], hydrothermal method [51], two-step hot-injection method [52][53], and solventthermal re ux technique [54][55].
Previously, we optimized the conditions to synthesize Cu 2 ZnSnS 4 and ZnO NCs using the polyol method [56][57][58][59][60]. As a result, we obtained single-phase nanocrystals of the kesterite (CZTS) and wurtzite (ZnO) phases with a controlled sizes. In this work, we optimized the process of Cu 2 ZnSnSe 4 NCs synthesis using two different sources of selenium in the precursor: selenourea and elemental selenium and then we investigated the morphological, structural, and microstructural properties depending on the synthesis temperature, time, and precursors combination.

Material And Methods
Cu 2 ZnSnSe 4 NCs were synthesized by the polyol method in the reaction media of diethylene glycol (DEG) and triethylene glycol (TEG) using two different selenium precursors. According to the rst approach, initially, the solution of polyvinylpyrrolidone (PVP) in the mass ratio 1:1 to the nished product was prepared. Then, a mixture of salts CuCl 2 ⋅2H 2 O, Zn(CH 3 COO) 2 ⋅2H 2 O, SnCl 2 ⋅2H 2 O in the different molar ratios Cu : Zn was added to the resulting solution and stirred with magnetic stirrer heated to 65°C for 1 h. The synthesis of the NCs at the temperature of the reaction medium was carried out separately with the exposure interval of τ = 0, 15, 30, 60 min. The synthesis procedure more detail described in [57].
According to the second approach, a mixture of CuCl 2 ⋅2H 2 O, Zn(CH 3 COO) 2 ⋅2H 2 O and SnCl 2 ⋅2H 2 O salts as well as amorphous selenium, grounded in an agate mortar, were placed in a 50 ml three-necked ask. Then, 12 ml of TEG was added to that with subsequent heating as described in the rst approach. In contrast to the rst approach and literature review, where selenourea is used as a source of selenium, i.e. (NH 2 ) 2 CSe compound [61][62][63], the use of elemental selenium allowed us to signi cantly reduce the cost of synthesis of the nal compound. After reaching a temperature of 120°C, the air was pumped out of the ask and inert argon gas was charged. The mixture was kept at this temperature for 30 min and quickly heated to the synthesis temperatures Т = 220, 240, 250, 260, 270, 280 °С for 120 min. In the process of Cu 2 ZnSnSe 4 NCs synthesis, samples with an exposure interval τ = 30, 60, 90, and 120 min were obtained at the temperature of the reaction medium of 260 °С. The initial molar ratio of elements in the precursors used corresponded to the stoichiometric composition of the compound (2:1:1:4) and the amount of zinc was exceeded.
In the rst and the second approaches, after syntheses, the mixture was cooled down to room temperature and the synthesized product was separated from the organic component (DEG, TEG) by centrifugation. Residues of polyol compounds were washed using ethanol with vigorous shaking followed by centrifugation. The washed Cu 2 ZnSnSe 4 samples were dried at 60 °С for 12 h.
X-ray diffractometer DRON 4-07 was used to determine the structural and microstructural properties of the synthesized NCs. The analysis was made in Niltered K α radiation of a copper anode. The obtained diffractograms were normalized to the peak intensity (112) of the tetragonal phase of the Cu 2 ZnSnSe 4 compound. The phase analysis was performed by comparing the diffraction angles from the test samples and diffraction angles according to the reference data of JCPDS (Cu 2 ZnSnSe 4 -№ 00-052-0868 [64]).
All pro le lines were processed to the following procedures: background extraction, emission smoothing, separation K α double that were performed using standard diffractometer software.
The lattice parameters a and c of the tetragonal unit cell were calculated using the following equations [65][66]: where 2θ is Bragg angle; λ is the wavelength of X-ray radiation; (hkl) are Miller indices. The re ection from the crystallographic plane (112) was used for the calculations. The unit volume of materials was calculated using formula [65][66][67]. Also, we use UnitCell program to calculate the lattice parameters of the material [65][66][67].
The average size of CSR and the level of microdeformations (ε) in the obtained samples were calculated by the physical broadening of the diffraction peaks (112), (204), (312) using the following equations [65][66][67][68]: where λ is the wavelength; β is the value of the physical expansion of the corresponding diffraction maximum; θ is the diffraction angle.
The morphological properties of the synthesized NCs were studied using a transmission electron microscope TEM-125K. The study of NCs elemental composition was performed using a scanning electron microscope SEO-SEM Inspect S50-B, which is equipped with an energy-dispersive spectrometer AZtecOne with X-MaxN20 detector (Oxford Instruments plc).
The methodology for studying the structural, microstructural, chemical properties of the nanoparticulate products is described in detail in our previous works [57,59,[66][67][68].

Results And Discussion
The electron microscopic images of Cu 2 ZnSnSe 4 NCs synthesized at the different growth times using the rst and second approaches are shown in Fig. 1. It was found that, the NCs average size synthesized by the rst method increased from (5.0 ± 3.0) nm (τ = 30 min) to (12.0 ± 3.0) nm (τ. = 60 min). NCs sizes obtained by the second method was two to three times larger than obtained by the rst method. The crystals showed different shapes, from quasi-spherical to pyramidal forms. During the synthesis process, NCs collected in conglomerates which sizes (50-80) nm (Fig. 1).
To determine the optimal conditions for the obtained NCs, we investigated the dependence of the phase composition on temperature, synthesis time and precursor composition. of the tetragonal phase. The secondary phases in the material were not detected using the XRD method, because its concentration did not exceed the detectable ability of the method (3-5% by weight) [69]. Diffractograms of Cu 2 ZnSnSe 4 NCs synthesized using the rst approach were characterized by the following features: with increasing synthesis time, the intensity of diffraction peaks increased signi cantly (for example, for the plane (112) and synthesis time 0 and 60 min, this value increased doubled), and their half-width decreased.
In the case of syntheses using elemental selenium (the second approach), the exposure interval of the reaction medium at different temperatures was 120 min. The initial molar ratio of the precursors used corresponded to the stoichiometric composition of the compound and was CuCl 2 : Zn(CH 3 COO) 2 : SnCl 2 : Se = 2:1:1:4. Diffractograms obtained from the synthesized NCs are shown in As can be seen, at a synthesis temperature of 220°C the diffraction patterns from the NCs shows a large number of lines, indicating that there is a mixture of different phases (curve 1). According to [69], in the case of zinc-depleted material, the Cu 2 SnSе 3 , Cu 2 ZnSn 3 Sе 8 , Cu 2 Sе and Cu 4 SnS 4 compounds can be formed. These compounds are characterized by the existence of several polymorphic modi cations. But the main feature that complicates a more accurate interpretation of the results of X-ray phase analysis is that these phases have spectra very similar to the diffraction spectra of the Cu 2 ZnSnSe 4 compound of the tetragonal phase [70][71][72][73].
It is established that with increasing synthesis temperature the number of peaks in the diffraction patterns decreases (curves [2][3] and at temperatures T ≥ 260°C in the reaction medium, a product or mixture of products with almost the same position of peaks and their intensity ratio (curves 4-6) were formed. To determine the optimal conditions for the synthesis of NCs at a temperature of 260 °С, when material single-phase was formed, samples were taken at the different exposure times of the reaction mixture. The corresponding diffractograms from the samples synthesized over time τ = 30, 60, 90, and 120 min are shown in Fig. 3b. The results obtained indicate that optimal interval of NCs synthesis was 120 min (curve 4).
To determine the chemical composition of the obtained material, its chemical analysis was performed using the EDX method. It is known that the theoretical atomic ratio of the components of the Cu 2 ZnSnSe 4 compound must be С Cu : С Zn : С Sn : С Sе = 25.0 : 12.5 : 12.5 : 50.0 (Table 1). Accordingly, the content of the components of the compound in the precursor was selected. However, determining the composition of the obtained samples indicates that the entry of different elements into the NCs occurs at different speeds. So, at the synthesis temperature T = 260 °С and the time of 30 min zinc atoms are not embedded from the reaction medium into the compound (Table 1). In this case, the CuSe and Cu 2 SnSe 3 phases are formed in the sediment in a ratio of 2:1, which is con rmed by X-ray diffractometry data. With increasing growth time of NCs to 120 min, the zinc content in the synthesized product increases to 2 at.%. It was found that with increasing synthesis temperature, the amount of zinc in the samples gradually increases from 3.8 to 8.2 at.% for temperatures of 270 °С and 280 °С, respectively. That is, increasing the synthesis temperature promotes to the entry of zinc atoms into the nal product. Since the experimentally obtained ratio of the constituent atoms in the material is nonstoichiometric, it can be assumed that the obtained product still consists of several phases.
To obtain a single-phase product during synthesis at a temperature of 280 °С and a time of 120 min the concentration of zinc in the reaction medium was increased from 1.5 to 1.8 times. At the same time, the content of other components of the compound in the precursor remained unchanged. The results of the X-ray phase analysis showed that the spectra from the samples are similar to the previously obtained spectra (Fig. 4), which are characteristic to single-phase samples.

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Analysis of the results obtained by the EDAX method showed ( Table 1) that the optimal composition for the synthesis of the Cu 2 ZnSnSe 4 compound is a molar ratio of 2:1.5:1:4. In this case, the formed product was the closest (С Cu = 29.0 at.%, С Zn = 12,1 at.%, С Sn = 12.7 at.%, С Se = 46.2 at.%) to the stoichiometric composition (С Cu = 25.0 at.%, С Zn = 12,5 at.%, С Sn = 12.5 at.%, С Se = 50.0 at.%). With a further increase in the proportion of zinc in the reaction mixture to 1.8, there is an increase in the content of copper and tin in the samples to 32.7 at.% and 13.2 at.%, respectively, with a simultaneous decrease in the nal synthesis product, namely zinc and selenium to 11.0 at.% and 43.2 at.%, respectively (Table. 1).
To establish the in uence of growth time on the structural and substructural characteristics of Cu 2 ZnSnSe 4 NCs, we determined the sizes of coherent scattering regions (L), the level of microdeformations (ε), and the lattice parameters (а, с, с/a V unit. ). The corresponding calculated values are given in Table 2. There are also reference values of the crystal lattice constants of the studied compounds. The lattice parameters of semiconductor materials are a characteristic that is extremely sensitive to the introduction of impurities, changes in stoichiometry, oxidation, so the exact de nition of these values allows us to study the relevant processes. Table 2 shows the obtained values of Cu 2 ZnSnSe 4 NCs lattice parameters calculated using relations (1) for re ections from the crystallographic plane (112) and the program UnitCell. It was found that the values of the lattice parameters found by two different methods differed quite signi cantly in magnitude. The general form of the dependences a, c, c / 2a, V unit from T, τ and precursors composition was very similar. Since the results obtained using the UnitCell program are based on the analysis of all re ections presented on the samples diffractograms, they are more accurate. In this regard, only this data will be discussed.
Analysis of the results of diffractometric studies shows that the lattice parameters of the material of nanocrystals synthesized by the rst method vary in the following range: a UС = (0.56605-0.56777) nm, с UС = (1.12621-1.13345) nm, c UС /2a UС = (0.99245-1.0009). At the start, with an increase of the synthesis time, the value increases to the maximum (a UС = 0.56777 nm, at τ = 30 min), and then decreases to the minimum (a UС =0.56605 nm, τ = 60 min). Closest to the reference data (a UС = 0.56930 nm) had material of NCs synthesized at τ = 45 min (а UС = 0.56695 nm).
At the same time, the value of the lattice parameter с with an increase in the NCs synthesis time increases, practically reaching at τ = 60 min (с UС = 1.13298 nm) values typical for stoichiometric materials (c UС = 1.13330 nm) [68][69][70]. A similar increase is observed for the ratio of these quantities c UС /2a UС , which at synthesis times τ = (45-60) min begins to exceed the reference data values.
Simultaneously calculated unit cell volume of the compound (V unit(UС) = (0.3626-0.3638) nm 3 ) turned out to be less than the reference (V unit(UС) = 0.3673 nm 3 ) overall range of synthesis times.
For NCs synthesized by the second method, there is a tendency to increase such lattice parameters as а UС , с UС , V unit(UС) with an increase of the synthesis temperature and zinc content in precursor at same time relationship c UС /2a UС decreases ( Table 2).
The results of determining the size of СSR L and the level of microdeformations ε in Cu 2 ZnSnSe 4 NCs, obtained by the physical broadening of the diffraction peaks (112), (204), (312) are shown in Table 2. The CSR size of NCs synthesized by the second method turned out to be much larger than the by the rst and vary in range L (112) = (16.54-24.23) nm. There was also a tendency for nanocrystals CSR sizes to increase with an increase of the synthesis temperature and the amount of zinc in the precursor. A similar process was observed for L in the directions [204] and [312]. As in the rst case, the CSR sizes were close to spherical. It should be noted that the CSR sizes practically coincided with the NCs sizes determined using transmission electron microscopy (Fig. 2). This suggests that nanocrystals usually consist of one single CSR.  Table 2). The same effect was caused by an increase in temperature and an increase in zinc content in the precursor during the synthesis of NCs by method two. It should be noted that when we use amorphous selenium as the Se source, the resulting NCs had larger CSR sizes and had a signi cantly (2-4 times) lower level of microdeformations (ε (112) = (4.07⋅10 − 3 -8.16⋅10 − 3 )) compared to those obtained using selenourea (ε (112) = (1.55⋅10 − 3 -2.27⋅10 − 3 )). Since both the CSR boundaries and microdeformations in the material are due to the presence of dislocations, increasing L and decreasing ε should lead to a decrease in the density of dislocations in lms, which will subsequently be created using synthesized NCs [71][72][73]. Figure 5 shows the FTIR spectra from NCs, taken in the mode of full re ection of light at room temperature. FTIR absorption spectra were measured to identify the chemical bonds in the investigated samples [74]. As can be seen from the gure, the absorption bands are observed in the spectra at the following frequencies: 3741 cm − 1 , 3421 cm − 1 , 2558 cm − 1 , 2164 cm − 1 , 2037 cm − 1 , 1979 cm − 1 , 1647 cm − 1 , 1165 cm − 1 , 1068 cm − 1 . It should be noted that the Cu 2 ZnSnSe 4 kesterite phase's active IR oscillations were not detected in the measured range due to their location at lower wavelengths (50-400 cm − 1 ) [75]. Therefore, the primary attention was paid to identifying residues of organic surfactants and solvents used in the synthesis of NCs.
As the spectra analysis shows, the absorption bands at 3741 cm − 1 and 1647 cm − 1 can be associated with the valence vibrations of O -H bonds and the H -O -H oscillation group, respectively [76]. The appearance of these bands indicates the presence of water residues in NCs. The peak at a frequency of 1068 cm − 1 belongs to the CO 2 carbonate group, which was formed in connection with carbon dioxide in the air [77]. At the same time, the band at 2558 cm − 1 belong to the S-H oscillation group. The most pronounced absorption bands at the frequencies of 2164 cm − 1 , 2037 cm − 1 , 1979 cm − 1 , belong to С-H group, due to the use of organic matter in the synthesis of NCs. The peak at 1165 cm − 1 is associated with stretched С-ОH bonds. Thus, the studied samples mainly contain residues of substances used in their synthesis. Conclusion 1. This work proposes a new procedure for the polyol synthesis of NCs of the Cu 2 ZnSnSe 4 compound, where amorphous selenium is used as a Se source. The optimal synthesis conditions are selected: its temperature (Т = 280 0 С), time (τ = 120 min) and the molar ratio of the components in the precursor (2:1.5:1:4) in which the crystals had a single-phase structure of the kesterite type and the composition (С Cu = 29.0 at.%, С Zn = 12.1 at.%, С Sn = 12.7 at.%, С Se = 46.2 at.%) close to stoichiometric.
The use of elemental selenium for synthesis made it possible to reduce the cost of obtaining the compound signi cantly.