Modeling Thermo-Mechanical Stress of Flexible CIGS Solar Cells

Copper indium gallium diselenide (CIGS) thin-film solar cells are fabricated through several deposition and annealing processes at high temperatures, which can generate significant thermal residual stress to solar cells. Moreover, since CIGS solar cells with flexible substrates are rollable and bendable, they are susceptible to mechanical stresses during these processes. In addition, partial shading (hotspot) can exert high heat on the CIGS solar cell. In this paper, we investigate the thermo-mechanical stress of each active layer of CIGS solar cells due to annealing, external bending, and hotspot using finite element method (FEM). We found that average stress of each active layer decreases and maximum stress in the cell increases when interface crack is introduced between cadmium sulfide (CdS)/CIGS. Our overarching goal is to quantify the relationship between the fabrication/operating process and the reliability of CIGS solar cells (energy release rate and internal stresses), which could improve the reliability of flexible solar cells. It is also found that lowering the annealing temperature can reduce the stresses in the cells and lowering CIGS thickness can reduce the delamination probability of CdS/CIGS interface. Finally, we investigate the effect of the crack length of the CdS/CIGS interface on the electrical performance of CIGS solar cells through FEM simulations. We found that as the crack size between CdS and CIGS layers increases, short-circuit current density decreases, while open-circuit voltage remains almost constant.