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

Abstract

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 (VCu-Se), related to the LS effect without the additional formation of deep-level defect states into the CIGS bandgap. Moreover, VCu-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.