**4. Conclusions**

We synthesized a CZTS nanoparticle ink by a wet ball milling method together with centrifugation treatments based on only nontoxic solvents. The ink was then used to deposit CZTS precursor films by a spin-coating method, which led to extremely flat surfaces with high-uniformity. The precursor was annealed at a high temperature of 600 ◦C under a sulfur atmosphere and the grain size increased to approximately 1 μm from the original size of less than 100 nm. Both the composition and crystallinity of the CZTS film were markedly improved by annealing. The absence of secondary phase formation during the annealing process was confirmed by XRD and Raman analysis. A solar cell efficiency of 6.2% (*V*oc = 633.3 mV, *J*sc = 17.6 mA/cm2, and FF = 55.8%) with an area of 0.2 cm2 was achieved using annealed CZTS film as the light absorbing layer. To improve solar cell performance, it is necessary to increase grain size, improve crystallinity, and reduce defects in the film. Because the fabrication process of CZTS features a complex growth mechanism, the formation of secondary phases should be checked to confirm film quality, which directly affects solar cell performance.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/3/336/s1, Figure S1: (a) Surface and (b) cross-section of an annealed CZTS film using centrifugation condition: 6000 rpm for 20 min. (c) Surface Morphology of an annealed CZTS film fabricated with CZTS ink using centrifugation condition: 1500 rpm for 10 min. The annealing was conducted at a temperature of 600 ◦C in S rich atmosphere, Figure S2: J-V curve of CZTS solar cells for (a) centrifugation 6000 rpm for 20 min; (b) 1500 rpm for 10 min, Table S1: Solar cell performance of CZTS solar cells.

**Author Contributions:** Conceptualization, X.Z., part of the characterization: E.F.; funding acquisition, Y.W.; draft review and editing, C.Z.

**Acknowledgments:** Part of the work was financially supported by Grant for special Research Project of Zhongshan Institute (funding No. 417YKQ10). This work was also financially supported by National Science Foundation of China under grants of (61302044, 61671140). We thank Edanz Group for editing a draft of this manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders contribute in collecting results and editing the manuscript.
