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.