Abstract
This thesis aims to gain more understanding of the defect system and the special electrical features of CZTSSe solar cells, which are believed to be the breadcrumbs to make the trustworthy strategy to improve the cell performance of kesterite solar cells. We point out that the special features (collapse of JSC and FF at low temperature) of CZTSSe solar cells are closely related to the defect system and we also find the possible existence of deep n-type defect in CZTSSe solar cells. To control the defect system of kesterite CZTSSe solar cells, we investigate a new compound – (Ag,Cu)2ZnSnSe4 to suppress the formation of CuZn antisite defects, which exist in kesterite CZTSSe in a large amount and are believed to be the reason for low VOC. We prove that the CuZn antisite defects are indeed suppressed by Ag substitution and find the importance of surface passivation in Ag-alloyed kesterite solar cells. With the effective suppression of CuZn and adequate surface passivation, the cell performance can be further improved. Moreover, the Ag substitution can also control the valence band maximum, which enables us to design a better band structure for the absorber layer. This study paves a new avenue for the development of kesterite solar cells and is promising for breaking the bottleneck of kesterite solar cells in the near future.