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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 the Na content is too high, it can deteriorate the cell properties and performance. Nonetheless, it is still beneficial when compared with a cell without Na inside. Here, we therefore use electron backscatter diffraction, atom probe tomography, and admittance spectroscopy to understand why Na starts to deteriorate the properties when inserted excessively. More exactly, it is shown that a moderate addition of Na strongly decreases the open‐circuit voltage (VOC) deficit, whereas the opposite happens when the Na content is too high. Interestingly, this cell with deteriorated VOC 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 clearly demonstrates the dual effect of Na in the CIGS absorber. [ABSTRACT FROM AUTHOR]

Thin‐film solar cells based on Cu(In,Ga)Se2 (CIGSe) absorber layers reach conversion efficiencies of above 20%. One key to this success is the incorporation of alkali metals, such as Na and K, into the surface and the volume of the CIGSe thin film. The present work discusses the impact of Na and K on the grain‐boundary (GB) properties in CIGSe thin films, i.e., on the barriers for charge carriers, Φb, and on the recombination velocities at the GBs, sGB. First, the physics connected with these two quantities as well as their impact on the device performance are revised, and then the values for the barrier heights and recombination velocities are provided from the literature. The sGB values are measured by means of a cathodoluminescence analysis of Na‐/K‐free CIGSe layers as well as on CIGSe layers on Mo/sapphire substrates, which are submitted to only NaF or only KF postdeposition treatments. Overall, passivating effects on GBs by neither Na nor K can be confirmed. The GB recombination velocities seem to remain on the same order of magnitude, in average about 103–104 cm s−1, irrespective of whether CIGSe thin films are Na‐/K‐free or Na‐/K‐containing. [ABSTRACT FROM AUTHOR]