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We fabricated CZTSSe thin films using optimized SLG-Mo/Zn/Cu/Sn (MZCT) as a stacked structure and described the phenomenon of Zn elemental volatilization using the MZCT stacked structure.

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Using an appropriate SeS2/Se weight ratio, band gap grading was realized. By increasing the value of VOC through band gap grading in the depletion region, a record VOC deficit of 0.576 V and an efficiency of 12.3% were obtained.

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For kesterite copper zinc tin sulfide/selenide (CZTSSe) solar cells to enter the market, in addition to efficiency improvements, the technological capability to produce flexible and large-area modules with homogeneous properties is necessary. Here, we report a greater than 10% efficiency for a cell area of approximately 0.5 cm 2 and a greater than 8% efficiency for a cell area larger than 2 cm 2 of certified flexible CZTSSe solar cells. By designing a thin and multi-layered precursor structure, the formation of defects and defect clusters, particularly tin-related donor defects, is controlled, and the open circuit voltage value is enhanced. Using statistical analysis, we verify that the cell-to-cell and within-cell uniformity characteristics are improved. This study reports the highest efficiency so far for flexible CZTSSe solar cells with small and large areas. These results also present methods for improving the efficiency and enlarging the cell area.

Cu2ZnSnS4 (CZTS)‐based materials have a useful band gap and a high absorption coefficient; however, their power conversion efficiency is low compared with that of CdTe and Cu(In,Ga)Se2‐based solar cells. Two of the factors that strongly affect CZTS solar cell characteristics are the MoS2 layer and the presence of defects. In this study, Mo back‐contact layers were annealed to control MoS2 layer formation and the Na content in the Mo layer before the absorber precursor layer was deposited. The increase in oxygen content in the Mo layer suppressed MoS2 layer formation. In addition, the increase in Na diffusion during the initial stage of the absorber precursor deposition decreased the defect density in the absorber layer and in the absorber–buffer interface. These results were verified through measurements of the external quantum efficiency, the temperature dependence of the open‐circuit voltage (VOC), and admittance spectra. The current densities (JSC) and VOC, as well as the power conversion efficiencies, improved as the annealing temperature of the Mo layer increased, which suggests that CZTS solar cell characteristics can be improved by suppressing MoS2 layer formation and increasing Na content in the Mo layer before deposition of the absorber precursor layer. Copyright © 2014 John Wiley & Sons, Ltd.