Investigation of single phase $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnS}{\mathrm{n}}_x\mathrm{S}{\mathrm{b}}_{1-x}{\mathrm{S}}_4$ compounds processed by mechanochemical synthesis

The copper zinc tin sulfide (CZTS) compound is a promising candidate as an alternative absorber material for thin-film solar cells. In this study, we investigate the direct formation of $\mathrm{C}{\mathrm{u}}_{1.92}\mathrm{ZnS}{\mathrm{n}}_x\left(\mathrm{S}{\mathrm{b}}_{1-x}\right){\mathrm{S}}_4$ compounds $\text{[CZT}\left(A\right)\text{S]}$, with $x=1$, 0.85, 0.70, and 0.50, via a mechanochemical synthesis (MCS) approach, starting from powders of the corresponding metals, zinc sulfide, and sulfur. The thermal stability of the $\text{CZT}\left(A\right)\text{S}$ compounds was evaluated in detail by in situ synchrotron high-energy x-ray diffraction measurements up to 700 °C. The $\text{CZT}\left(A\right)\text{S}$ compounds prepared via MCS revealed a sphalerite-type crystal structure with strong structural stability over the studied temperature range. The contribution of the MCS to the formation of such a structure at room temperature is analyzed in detail. Additionally, this study provides insights into the MCS of CZTS-based compounds: the possibility of a large-scale substitution of Sn by Sb and the production of single phase $\text{CZT}\left(A\right)\text{S}$ with a Cu-poor/Zn-poor composition. A slight increase in the band gap from 1.45 to 1.49–1.51 eV was observed with the incorporation of Sb, indicating that these novel compounds can be further explored for thin-film solar cells.

Figure 1

Crystal structures reported in the literature for CZTS [32]: (a) Kesterite, (b) stannite, (c) disordered kesterite, (d) PMCA (primitive mixed CuAu-like), and (e) sphalerite.

Figure 2

(a,b), (e,f) Typical SEM images showing the morphology of the mechanochemical synthesized $\text{CZT}\left(A\right)\text{S}$ powders here exemplified, respectively, for the CZTS ± and CZTAS 50/50 powders; (c) and (g) typical SEM/BSE images showing the microstructure of the mechanochemical synthesized $\text{CZT}\left(A\right)\text{S}$ powders here exemplified, respectively, for the CZTS and CZTAS (50/50) powders; (d) and (h) typical EDS, elemental composition (in at. %) and composition ratios for the metallic elements.

Figure 3

Typical particle size distribution curves of the mechanochemical synthesized $\text{CZT}\left(A\right)\text{S}$ powders. (a) CZTS, (b) CZTAS (85/15), (c) CZTAS (70/30), and (d) CZTAS (50/50).

Figure 4

(a) 1D x-ray diffractograms taken at different temperatures for the CZTS, CZTAS (85/15), CZTAS (70/30), and CZTAS (50/50) samples. (b) 1D x-ray diffractograms for the CZTS and CZTS 50/50 samples taken at 30 and 700 °C. (c) Lattice parameter dependence on temperature. (d) Lattice parameter and crystallite size dependence on the Sn/Sb substitution based on the XRD data at 30 °C. In (a) and (b), the 2θ axis scale reflects the wavelength (0.14235 Å) of the high-energy x-ray monochromated incident beam.

Figure 5

(a) Raman spectra of the mechanochemical synthesized $\text{CZT}\left(A\right)\text{S}$ powder samples and (b) Raman spectra of the CZTS sample after MCS and MCS + heat treatment.

Figure 6

Tauc plots for the mechanochemical synthesized powder samples: (a) CZTS, (b) CZTAS (85/15), (c) CZTAS (70/30), and (d) CZTAS (50/50). The dotted lines indicate the linear part used for estimating the optical band gap.