CdS/ZnS Bilayer Thin Films Used As Buffer Layer in 10%-Efficient Cu₂ZnSnSe₄Solar Cells

This work deals with the soda-lime glass/Mo/Cu2ZnSnSe4/CdS/ZnS/i-ZnO/ITO solar cells. CdS/ZnS bilayers were synthesized by chemical bath deposition (CBD) method as buffer layers for Cu2ZnSnSe4 (CZTSe) solar cells. The depositions were carried out by varying the deposition time of the CdS film, while keeping the deposition time of the ZnS film constant. The devices went from 7.2% efficiency in a reference device using CdS to 10% in a device including a thin film of ZnS. All devices were processed without any additional annealing treatment on CdS/ZnS layers. J-V, EQE, SEM, Raman, and C-V characterizations were performed to investigate the properties of the solar cells as a function of the thickness of the CdS layer and to shed light on the origin of influence of the ZnS layer to the device performance. Moreover, the influence of physical properties of the buffer bilayers on the electrical parameters of the solar cells are discussed by means of numerical simulation.