2, Institute of Physics, Photovoltaics Group, Martin-Luther-University Halle-Wittenberg, Halle, , Germany
Currently, the Cu(In,Ga)Se2 (CIGSe) solar-cell devices exhibiting highest conversion efficiencies of close to 23% have been fabricated using polycrystalline absorber layers which contained alkali metals already during growth or into which alkali metals were introduced via a postdeposition treatment (see, e.g., Ref. 1). The beneficial effects of alkali metals on the materials and device properties of CIGSe solar cells are diverse, and it is convenient to divide them into those affecting the bulk properties (such as the net doping density of the CIGSe layer), as well as into those acting at the interfaces to the n-type buffer layer / back contact and at line and planar defects. In various experiments (see, e.g., Refs. 2 and 3), the presence of alkali metals at (random) grain boundaries (GBs) in CIGSe thin films was confirmed. This presence often leads to the premature assumption that alkali metals are responsible for a passivating effect at GBs.
The present work will discuss systematically the impact of various alkali metals, i.e., Na, K, and Rb, on the electrical properties of GBs, i.e., on the barrier heights and on the recombination velocities. Concerning the barrier heights, it can be understood by using the equations provided by Seto  that introducing Na into polycrystalline CIGSe increases the barrier heights at the GBs via increased net doping density. Indeed, this effect was verified by means of scanning probe microscopy analysis. The presence of K and Rb can not be expected to have an impact on the barrier height.
As for the recombination velocity, the values extracted from electron-beam-induced current and cathodoluminescence measurements on devices with Na-free, Na-containing, and Na/Rb-containing CIGSe absorbers exhibited for most of the GBs the same order of magnitude, 103 cm/s, regardless of whether the conversion efficiency of the solar cell was about 10%, 15% or about 22%. Thus, there is not any indication for a substantial passivation effect of any alkali metal at GBs in polycrystalline CIGSe thin films.
 P. Jackson, R. Wuerz, D. Hariskos, E. Lotter, W. Witte, M. Powalla, Effects of heavy alkali elements in Cu(In,Ga)Se2 solar cells with efficiencies up to 22.6%, Phys. Stat. Solidi (RRL) 10 (2016) 583-586.
 O. Cojocaru-Mirédin, T. Schwarz, D. Abou-Ras, Assessment of elemental distributions at line and planar defects in Cu(In,Ga)Se2 thin films by atom probe tomography, Scripta Mater. 148 (2017) 106-114.
 P. Schöppe, S. Schönherr, R. Wuerz, W. Wisniewski, G. Martínez-Criado, M. Ritzer, K. Ritter, C. Ronning, C.S. Schnohr, Rubidium segregation at random grain boundaries in Cu(In,Ga)Se2 absorbers, Nano Energy 42 (2017) 307-313.
 J.Y.W. Seto, The electrical properties of polycrystalline silicon films, J. Appl. Phys. 46 (1975) 5247.