Optimizing the thickness of sputtering-Zn(O, S) buffer layer for all-dry Cd-free CIGS solar cells

Recently, research on the Zn(O, S) thin films as an alternative buffer layer for CIGS thin film solar cells has gained increasing popularity. The replacement of CdS and i-ZnO films by the sputtered Zn(O, S) films not only overcomes the shortcomings of using the heavy metal (Cd) and a non-vacuum production process, but also simplifies the in-line process in the industrial level. In this paper, the effects of varying the Zn(O, S) films thickness on its material properties and the corresponding device performance are investigated. At the same time, the wxAMPS simulation software is applied to studying the impact of thickness of Zn(O, S) films on the energy band structure, the built-in electric field distribution, and the generation/recombination rate of carriers. It's found that defect density in the bulk of Zn(O, S) and at the interface of CIGS/Zn(O, S) will increase with a thinner film, which further enhances interface recombination. Moreover, a deteriorated device performance is observed as a result of a very thick film, which is likely caused by the light-loss and a weaker electric field. The optimum thickness is determined based on the results of the experiment and simulation. Finally, insights are provided on reducing the defect density at the heterojunction interface in order to improve the performance of Cd-free device prepared by all-dry processes.