Metastable defect curing by alkaline earth metal in chalcogenide thin-film solar cells

This study investigates the use of an alkaline earth metal precursor (MgF₂) to enhance the performance of chalcogenide-based Cu(In,Ga)Se₂ (CIGS) solar cells with a chemically bath deposited-Zn(O,S) (CBD-Zn(O,S)) buffer layer via post-deposited treatment (PDT). The optimal substrate temperature and layer thickness are 570 °C and 5 nm, and the light soaking (LS) treatment does not be required in this condition. The morphological properties and chemical reaction at the p-n junction of CIGS/CBD-Zn(O,S) are examined as a function of MgF₂ PDT layer thickness. As the MgF₂ PDT layer thickness increases, the CIGS surface becomes rough with vigorously agglomerated Cu clusters owing to the substantially high substrate temperature, which increases the incorporation of In-Se bonds and the oxygenation rate of MgF₂. Density functional theory (DFT) clarifies the improved cell efficiency without the need for LS treatment (MgF₂ PDT, 5 nm) by calculating the defect-related electronic behavior. The MgF₂ phase effectively passivates metastable defect Cu-Se vacancy defects (V_Cu-Se), related to the LS effect without the additional formation of deep-level defect states into the CIGS bandgap. Moreover, V_Cu-Se states exert the most influence on the LS effect, and the control of defect states in the CIGS layer (not the buffer layer) is crucial for cell efficiency.