*3.1. Influence of Annealing Temperature on the Properties of Cu2Mg0.2Zn0.8Sn(S,Se)4 Films*

As we all know, the properties of absorbers are easily affected by annealing temperature. For the sake of studying the effect of selenization temperature on the microstructure and photoelectric characteristics of CMZTSSe films during the selenization process, the CMZTSSe samples were annealed for 15 min under Se ambience at different temperatures of 500, 530, and 560 ◦C, hereafter named as A1, A2, and A3, respectively. In addition, the CZTSSe film annealed at a temperature of 530 ◦C for 15 min will be used as a reference, named as sample A.

XRD spectra were always used to evaluate the crystal quality and assess the probable impurity phase during the selenization process. Figure 1 represents the XRD spectra of CMZTSSe samples annealed at different temperatures from 500 to 560 ◦C (samples A1, A2, and A3). The CMZTSSe films at all varied annealing temperatures were in the kesterite phase, with peaks corresponding to the (112), (220), (312), (008), and (332) planes of Cu2ZnSnS4 (CZTS) with the kesterite phase [26,27]. As shown in Figure 1, the characteristic peaks of impurity phases were not found, which indicates that the single-phase CMZTSSe with kesterite structure was formed at all varied annealing temperatures. Figure 1a indicates the XRD patterns for sample A1, annealed at 500 ◦C under atmosphere of Se, and it was found that the peak intensity is weaker. By increasing the annealing temperature to 530 ◦C, as shown in Figure 1b, the peak intensity was found to be enhanced, indicating that the crystal quality of sample A2 is enhanced at 530 ◦C. For sample A3, with the annealing temperature of 560 ◦C, the peak intensity of XRD only slightly changed, as seen from Figure 1c. As seen from the inset, the dominant characteristic peak (112) is observed to be shifted to smaller angles, from 27.20◦ to 26.93◦, with the annealing temperature increasing from 500 to 560 ◦C. This is due to the variety in atomic-lattice distance caused by elemental replacement, where the element Se will take the place of S in the CMZTSSe compound with the increase of annealing temperature. According to the results of XRD, there are no secondary phases in CMZTSSe films annealed at different selenization temperatures from 500 to 560 ◦C.

**Figure 1.** XRD spectra of Cu2Mg0.2Zn0.8Sn(S,Se)4 (CMZTSSe) films annealed at different temperatures (**a**) 500 ◦C, (**b**) 530 ◦C, (**c**) 560 ◦C. Inset: Enlarged view of the corresponding (112) diffraction peak of the CMZTSSe films annealed at different temperatures.

Table 1 shows the values of full width at half-maximum (FWHM), grain size, the a-axis lattice constant (a), and the c-axis lattice constant (c) for the CMZTSSe films annealed at different temperatures, which are obtained from the (112) peak in the XRD profile. It can be seen from Table 1 that, as the annealing temperature increases from 500 to 560 ◦C, the grain size increases first and attains a maximum value at 530 ◦C, and then decreases when the annealing temperature is continuously increased up to 560 ◦C. An opposite changing trend was observed concurrently for the FWHM—when increasing the annealing temperature up to 530 ◦C, the FWHM has a minimum value. Meanwhile, the a-axis lattice constant gradually increases from 5.669 to 5.691 Å as the annealing temperature increases from 500 to 560 ◦C. The c-axis lattice constant also increases from 11.305 to 11.635 Å. The increases in the lattice constants are ascribed to the rise of the Se content in films, where the S (0.184 nm) atoms were replaced by the larger Se (0.198 nm) atoms with the increasing annealing temperature. This may be verified by the EDS results later.


**Table 1.** The full width at half-maximum (FWHM), grain size, a-axis lattice constant (a), and c-axis lattice constant (c) for CMZTSSe films annealed at different temperatures.

It is well known that the discernment of the secondary phases in CZTSSe compounds by XRD patterns is difficult, owing to the nearly overlapped XRD patterns of the secondary ZnS(Se) and Cu2SnS(Se)3 with the kesterite CZTSSe [28]. Therefore, Raman spectroscopy measurement is usually applied as an auxiliary technique to detect possible impurity phases, because it is sensitive to lattice vibrations.

Figure 2 shows the Raman spectra of CMZTSSe films annealed at different annealing temperatures from 500 to 560 ◦C (samples A1, A2, and A3). The Raman spectrum of sample A1 was fitted using the Gaussian fitting method, and the peaks at 173, 193, and 234 cm−<sup>1</sup> can be observed. The peaks at 173 and 193 cm−<sup>1</sup> conform to the A (A1 and A2) mode Raman vibration peaks of the kesterite CZTSSe phase, as reported in the previous literature [29]. The A modes are pure anion modes which correspond to vibrations of pure chalcogen (S or Se) atoms surrounded by motionless neighboring atoms. The result exhibits a broad peak located at 234 cm−<sup>1</sup> that corresponds to the E vibration mode in connection with Sn–Se bonding in the kesterite CZTSSe phase [29]. In addition, it can be seen that the Raman peaks of all samples conform with the vibration peaks of the kesterite CZTSSe phase. The Raman peaks of the other secondary phases were not discovered. This indicates that all annealed samples consist of a single phase of CZTSSe with kesterite structure. Furthermore, with the variation of annealing temperature from 500 to 560 ◦C, it was found that all Raman peaks were shifted to the lower values, especially for the A1 mode peak. The inset of Figure 2 displays the tracking of the peak position of the A1 vibration mode, and it is obviously noted that the A1 vibration peak shifted from 195.6 to 193.9 cm−<sup>1</sup> as the annealing temperature changed from 500 to 560 ◦C. This phenomenon can be attributed to the increment of selenization temperature from 500 to 560 ◦C, which easily allows larger Se to replace S in the CZTSSe, significantly increasing the lattice constant. It was found from the Raman results that the secondary phases were not discovered, which is consistent with the XRD results. In addition, the peak shift as observed from the XRD and Raman spectra is considered to be due to larger Se taking the site of smaller S in CMZTSSe films with the increase of selenization temperature from 500 to 560 ◦C, which will be better understood from the compositional and morphological studies later.

**Figure 2.** Raman spectra of the CMZTSSe films annealed at different temperatures. Inset: The main Raman peaks of A1 mode for CMZTSSe films annealed at different temperatures.

XPS is sensitive to information about the chemical bonding state and element content. As shown in Figure 3, we identify the elemental composition and valence states of the constituent elements (Cu, Zn, Sn, Se, S, and Mg) in CMZTSSe films annealed at 530 ◦C for 15 min by XPS measurement, and all peaks were corrected by the C1s binding energy (284.8 eV). As seen from Figure 3a, the Cu 2p XPS spectrum consists of a Cu 2p3/<sup>2</sup> peak and a Cu 2p1/<sup>2</sup> peak at 931.6 and 952.5 eV, respectively, with a peak separation of 20.9 eV, which indicates that Cu is in the state of +1 [30]. Figure 3b displays a Zn 2p XPS spectrum, and the peaks presented at 1021.4 and 1044.2 eV are ascribed to Zn 2p3/<sup>2</sup> and Zn 2p1/2, respectively. The binding energy interval between the Zn 2p3/<sup>2</sup> peak and the Zn 2p1/<sup>2</sup> peak is 22.8 eV, indicating the existence of divalent Zn ions [31]. Figure 3c shows the XPS spectrum of Sn 3d, and two peaks attributed to Sn 3d5/<sup>2</sup> and Sn 3d3/<sup>2</sup> can be observed at 485.5 and 494.1 eV. The energy difference of the two Sn 3d peaks is 8.6 eV, suggesting the presence of the Sn4<sup>+</sup> state [32]. Figure 3d displays the Se 3d XPS spectrum, and the S 2p high-resolution spectrum is shown in Figure 3e. The Se 3d XPS spectra can be fitted into two sub-peaks located at 53.3 and 53.8 eV, which can be ascribed to Se 3d3/<sup>2</sup> (green area) and Se 3d1/<sup>2</sup> (purple area), respectively. The above results indicate that Se in the films is likely to exist in the Se2<sup>−</sup> state [33]. It is well known that the S 2p core level and Se 3p core level are almost overlapping, and we used the Gaussian fitting method to fit the XPS spectra into four sub-peaks presented at 159.1, 160.2, 161.3, and 165.8 eV, which are ascribed to Se 2p3/<sup>2</sup> (light green area), S 2p3/<sup>2</sup> (pink area), S 2p1/<sup>2</sup> (deep purple area), and Se 3p1/<sup>2</sup> (grey green area), respectively. The S 2p3/<sup>2</sup> and S 2p1/<sup>2</sup> peaks located at 160.2 and 161.3 eV, respectively, are in the standard reference value range (160–164 eV) [33], which

means that S exists in the form of S2<sup>−</sup>. Figure 3f displays the XPS high-resolution spectrum of Mg 1s, and the peak can be observed at 1303.6 eV, which indicates that divalent Mg2<sup>+</sup> exists in our study [19]. The results of the XPS analysis show that the constituent elements (Cu, Zn, Sn, Se, S, and Mg) exist in the forms of Cu1<sup>+</sup>, Zn2<sup>+</sup>, Mg2+, Sn4+, Se2−, and S2<sup>−</sup> in CMZTSSe.

**Figure 3.** X-ray photoelectron spectroscopy (XPS) spectra of CMZTSSe films annealed at 530 ◦C for 15 min: (**a**) Cu, (**b**) Zn, (**c**) Sn, (**d**) Se, (**e**) S, and (**f**) Mg.

Figure 4a–d shows the scanning electron microscopy (SEM) images of the CZTSSe film annealed at 530 ◦C for 15 min (sample A) and the CMZTSSe films annealed under atmosphere of Se for 15 min at temperatures of 500, 530, and 560 ◦C (samples A1, A2, and A3). Figure 4a shows the surface SEM images of sample A. As we can see from Figure 4a, irregular and small grain sizes of 0.5–0.8 μm were observed. In addition, it was clearly seen that the surface morphology of Cu2ZnSn(S,Se)4 films is very rough. Figure 4b shows the SEM of sample A1, and it is clearly seen that the film consists of nanograins, with the grain size being even smaller than that of sample A, and the surface morphology is also rough. As shown in Figure 4c, the crystal quality was improved for sample A2, and the grain size of the film reached 1.0–2.5 μm while the surface morphology became smooth and compact. When the selenization temperature was increased to 560 ◦C, the grain size of sample A3 slightly reduced to 1.0–1.5 μm and displayed a rough morphology, as displayed in Figure 4d. The results indicate that the proper selenization temperature is 530 ◦C, and that this is beneficial to promote the grain growth. Excessively high or low temperatures will cause the deterioration of film quality. When the selenization temperature is 530 ◦C, not only does the grain size of CMZTSSe film reach its maximum, but the surface morphology of the film also becomes smooth and compact.

As we all know, the photoelectric properties of CZTSSe-based films need to rely heavily on the stoichiometric ratios of Cu, Zn, Sn, S, and Se in CZTSSe-based films [34]. Table 2 displays the EDS results of CMZTSSe films annealed at different temperatures from 500 to 560 ◦C (samples A1, A2, and A3). According to the EDS results, we confirmed the existence of Cu, Zn, Mg, Sn, S, and Se elements in samples A1, A2, and A3. It was found that the atomic percentages of Se increased from 35.87% to 45.20% and S evidently decreased from 11.46% to 3.16% with increasing selenization temperature from 500 to 560 ◦C, indicating that Se will partially replace S in the CMZTSSe compound. The compositions of the other elements (Cu, Sn, and Mg) in all three samples were found to be nearly the same, while the atomic percentages of Zn decreased from 11.30% to 8.16%, indicating that Zn loss happened when the selenization temperature changed from 500 to 560 ◦C. In the precursors, the ratios of Cu/(Zn + Mg + Sn) and Mg/(Mg + Zn) were about 0.82 and 0.2, respectively. The ratios of Mg/(Mg + Zn) in all the films were close to 0.2, but the ratios of Cu/(Zn + Mg + Sn) in all films became significantly larger. This

may be due to the decrease of Zn content as the annealing temperature increases. As mentioned before, Se/(S + Se) > 50% is highly suitable for the fabrication of high-efficiency solar cells [35]. The percentage of Se/(S + Se) is 75.79% for A1, and the percentages of Se/(S + Se) increase to 82.22% and 93.47% for A2 and A3, respectively, indicating that A1, A2, and A3 samples are suitable for the fabrication of efficient solar cell devices. Sample A2 has an appropriate Se/(S + Se) ratio and the grain size became larger compared to samples A1 and A3. Therefore, sample A2 is more suitable to fabricate the high-efficiency solar cells. Figure 5 summarizes the elements composition analysis of CMZTSSe films according to Table 2. As shown from Figure 5, when increasing the selenization temperature from 500 to 560 ◦C, the proportion of Se increases gradually while the content of S decreases, while the atomic percentages of Cu, Zn, Sn, and Mg remain relatively constant compared with those of S and Se.

**Figure 4.** SEM images of CZTSSe annealed at (**a**) 530 ◦C and CMZTSSe films annealed at (**b**) 500; (**c**) 530; and (**d**) 560 ◦C.

**Figure 5.** Energy-dispersive X-ray spectroscopy (EDS) composition analyses of CMZTSSe films annealed at different temperatures.

**Table 2.** EDS results of the CMZTSSe films annealed at different temperatures from 500 to 560 ◦C.


In order to research the influence of selenization temperature on the optical bandgaps of CMZTSSe films (samples A1, A2, and A3), we studied the optical absorption measurements of the CMZTSSe films annealed at different selenization temperatures by an UV-vis-NIR spectrophotometer. Figure 6 displays the (α*h*υ) 2–*h*υ plots of CMZTSSe films. We use the solids band theory to express the relation between the absorption coefficient (α) and the photon energy (*h*υ) as follows [36]:

$$(\alpha \text{h} \upsilon) = B(\text{h} \upsilon - E\_{\text{g}})^{\text{n}} \tag{1}$$

where *h*, *B*, υ, and *E*<sup>g</sup> are Plank's constant, a constant, photon frequency, and optical bandgap, respectively. The values of n can employ 3, 2, 3/2, and 1/2, when transitions are indirect unallowed, indirect allowed, direct unallowed, and direct allowed, respectively [37]. The values of n can employ 1/2 for direct bandgaps of semiconductor CZTSSe [36]. By using the Equation (1) and the data in Figure 6, the bandgaps of CMZTSSe are evaluated to be 1.04, 1.02, and 0.93 eV for samples A1, A2, and A3, respectively, as shown in the illustration of Figure 6. It was found that the bandgap values of samples A1, A2, and A3 gradually decrease with increasing selenization temperature, which can be attributed to the changes of crystal lattice and disparities in electronegativities owing to alloying and modified atomic structures through Se taking the site of S.

**Figure 6.** The plot of (α*h*υ) <sup>2</sup> vs. *h*υ for the absorption spectra. Inset: Bandgap variation as a function of the selenization temperature.

The electrical properties of the absorbing layer are also important factors affecting the efficiency of solar cells. Table 3 displays the electrical characteristics of CMZTSSe annealed at different temperatures (samples A1, A2, and A3) by the Vander Paw method at room temperature. It was found that the CMZTSSe films annealed at different temperatures behave with p-type semiconductor characteristics. When the selenization temperature is increased from 500 to 530 ◦C, the resistivity first decreases from 6.18 <sup>×</sup> 100 to 2.85 <sup>×</sup> <sup>10</sup>−<sup>1</sup> <sup>Ω</sup>·cm, then increases to 1.10 <sup>×</sup> 102 <sup>Ω</sup>·cm at the selenization temperature of 560 ◦C. Obviously, when the selenization temperature is 530 ◦C, the resistivity is optimal. Simultaneously, it is clear that the corresponding carrier concentration shows a best value of 6.47 <sup>×</sup> 1018 cm−<sup>3</sup> at the selenization temperature of 530 ◦C. In addition, the mobility reduced from 1.09 <sup>×</sup> 100 cm2V−1s−<sup>1</sup> (500 ◦C) to 3.31 <sup>×</sup> <sup>10</sup>−<sup>1</sup> cm2V−1s−<sup>1</sup> (530 ◦C), but increased to 1.04 <sup>×</sup> <sup>10</sup>−<sup>1</sup> cm2V−1s−<sup>1</sup> at the selenization temperature of 560 ◦C. We analyzed the reasons for the change of CMZTSSe electrical properties, combined with the characterization of SEM. It was concluded that the defects at the surfaces of the absorbing layers are passivated, and owing to the crystal quality of CMZTSSe films improving with the selenization temperature increasing from 500 to 530 ◦C, the resistivity and carrier concentration achieve the best values with sample A2. It is obvious that the deterioration of resistivity and carrier concentration is owing to the deterioration of crystal quality when the selenization temperature further increases from 530 to 560 ◦C.


**Table 3.** Electrical properties of the CMZTSSe films annealed at different temperatures from 500 to 560 ◦C.
