Improvement of Efficiency in Kesterite Solar Cells by Intentionally Inserting a Thin MoS₂ Layer into the Back...

A Mo(S,Se)₂ interfacial layer is formed inevitably and uncontrollably between the Mo electrode and Cu₂ZnSn(S,Se)₄ (CZTSSe) absorber during the selenization process, which significantly influences the performance of CZTSSe solar cells. In this work, an ultrathin MoS₂ layer is intentionally inserted into Mo/CZTSSe to reduce the recombination and thus optimize the interface quality. It is revealed that the absorber exhibits a continuous and compact morphology with bigger grains and remarkably without pinholes across the surface or cross-sectional regions after MoS₂ modification. Benefitting from this, the shunt resistance (R_Sh) of the device increased evidently from ∼395 to ∼634 Ω·cm², and simultaneously, the reverse saturation current density (J₀) realized an effective depression. As a result, the power conversion efficiency (PCE) of the MoS₂-modified device reaches 9.64% via the optimization of the thickness of the MoS₂ layer, indicating performance improvements with respect to the MoS₂-free case. Furthermore, the main contribution to the performance improvement is derived and analyzed in detail from the increased R_Sh, decreased J₀, and diode ideality factor. Our results suggest that the Mo/CZTSSe interface quality and performance of CZTSSe solar cells can be modulated and improved by appropriately designing and optimizing the thickness of the inserted MoS₂ layer.