Sodium in Cu(In, Ga)Se2 Solar Cells: To Be or Not to Be Beneficial

In this work, the dual effect of Na in the copper indium gallium diselenide (CIGS) absorber is studied. Na doping exhibits a drastic improvement of the cell efficiency mainly by an increase in the open-circuit current and fill factor. However, when Na content is too high, the Na can deteriorate the cell properties and performance, however, it is still beneficial when compared with a cell without Na inside. Therefore, here electron backscatter diffraction, atom probe tomography, and admittance spectroscopy are used to understand why Na starts to deteriorate the properties when inserted to a too large extent. More exactly, it is showed that the moderate addition of Na strongly decreases the open-circuit voltage (V-OC) deficit, whereas the opposite happens when the Na content is too high. Interestingly, this cell with deteriorated V(OC )value and characterized by a very high Na content exhibits a high density of not only grain boundaries and dislocations but also of Na-rich clusters. These clusters may host a high concentration of deep defects and trap the electrons within the CIGS bulk. At this stage, Na still has its beneficial effects, but the cell properties start to deteriorate compared to the cell with a moderate content of Na, especially when found as clusters in the bulk. In conclusion, this work provides clear statements about the dual effect of Na in the CIGS absorber.