AlkaCIGS - Effects of Alkali elements in Cu(In,Ga)Se2 thin films and solar cells

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Alkali elements play an important role in polycrystalline Cu(In,Ga)Se2 (CIGS) thin-film solar cells. The positive effect of sodium (Na) on CIGS solar cell performance was found by accident by using soda lime glass instead of sodium free substrates. Later potassium (K) appeared as a further booster for cell efficiency. Currentliy, the heavier alkali elements rubidium (Rb) and cesium (Cs) have been identified as further candidates to increase the performance. Many experiments have been done to find out if Na mainly acts at the grain boundaries or in the bulk of the grains or both. Atomic Probe Tomography (APT) measurements revealed that Na is inside the grains as well as in the grain boundaries, where it strongly segregates. But so far it is unclear whether passivation of grain boundaries is the dominating effect of Na or if the increase of p doping in the grain bulk by Na is the main effect. The effect of Na and other alkali elements on the defect physics of CIGS is also unknown, since in most former experiments CIGS layers grown in the presence of Na were compared to layers grown without Na. Therefore, the effect of Na on the formation of defects during the CIGS layer growth could not be separated from its effect on opto-electronic properties. By post deposition treatment of CIGS layers with alkali elements after CIGS growth is finished, as to be realized in this project, the effect of alkalis on defect physics and opto-electronic properties of CIGS layers and solar cells can be clearly identified. The aim of the project is to better understand the positive effect of sodium and other alkali metals on the CIGS layer and solar cell properties and to investigate the interaction between the different alkali elements. This includes a task to distinguish between grain boundary, grain bulk and interface effects. The better fundamental understanding of the effects of alkali elements in CIGS could help to further improve cell efficiency.