Atomic-scale distribution of impurities in CuInSe2-based thin-film solar cells

Atom Probe Tomography was employed to investigate the distribution of impurities, in particular sodium and oxygen, in a CuInSe2-based thin-film solar cell. It could be shown that sodium, oxygen, and silicon diffuse from the soda lime glass substrate into the CuInSe2 film and accumulate at the grain boundaries. Highly dilute concentrations of sodium and oxygen were measured in the bulk. Selenium was found to be depleted at the grain boundaries. These observations could be confirmed by complementary energy dispersive X-ray spectroscopy studies. Our results support the model proposed by Kronik et al. (1998) [1], which explains the enhanced photovoltaic efficiency of sodium containing CuInSe2 solar cells by the passivation of selenium vacancies at grain boundaries.

Research highlights
► Atom Probe Tomography was employed to investigate the elemental distribution in the CIS absorber layer.
► Na and O atoms were found to be distributed homogeneously in the CuInSe2 grains.
► Correlated enrichments of Na and O atoms were detected, presumably at a grain boundary.
► These enrichments are accompanied by Se depletion in agreement with the “defect chemical model” proposed by Kronik.
► The “Na effect” in our CIS-based solar cell can be explained by the passivation of VSe (detrimental donor defects) by O, where Na acts as a catalyst.