Metastable Behavior on Cesium Fluoride‐Treated Cu(In1–x,Gax)Se2 Solar Cells

Metastable behavior of cesium fluoride (CsF)‐treated Cu(In_1–x,Ga_x)Se₂ (CIGS) solar cells is investigated under heat‐light soaking (HLS) and heat‐soaking (HS) treatments. HLS increases open‐circuit voltage, fill factor, efficiency, and net carrier concentration and decreases short‐circuit current density, whereas heat‐soaking treatment acts oppositely. The performance of a CsF postdeposition treatment to CIGS thin film in selenium vapor, and closer to stoichiometry copper content, did not mitigate the open‐circuit voltage improvement after HLS. These results argue the traditional concept of the V_Se–V_Cu divacancy complex for the total beneficial effect of HLS in alkali‐treated CIGS solar cells. The metastable behavior observed in the CsF‐treated devices due to the HLS and HS treatments is explained by the specific behavior of alkali‐containing new compounds at the surface and/or the migration of alkali metals at the surface and bulk regions.