Incorporation of alkali metals in chalcogenide solar cells

Since 1993 it has been recognized that for solar cells based on the chalcogenide absorber material Cu(In,Ga)Se2 (CIGSe) the incorporation of Na is crucial to obtain the highest values of power conversion efficiency. Since then, many reports have investigated the effects of Na in different chalcogenide solar cell materials. In the present review, we discuss the various sodium incorporation strategies for chalcogenide solar cells that have been reported. We briefly discuss the origin and the different interpretations of the positive effects that the presence of sodium provides in CIGSe and Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. On the contrary, at the current stage of development of CdTe solar cells, Na is unwanted instead of incorporated. We review the various incorporation methods that have been reported: (a) diffusion from substrates like soda-lime glass, specialty glasses and sodium doped molybdenum (MoNa) layers; (b) external strategies by deposition of Na-containing compounds before, during and after the absorber growth; (c) other non-conventional methods. For each method, we present a literature review and critically present benefits and weak points. Finally, we provide an overview of characterization methods capable of directly probing the presence of sodium.