Comparative atom probe study of Cu(In,Ga)Se₂ thin-film solar cells deposited on soda-lime glass and mild steel substrates

We report on a comparative study of Cu(In,Ga)Se₂ solar cells deposited on soda-lime glass and mild steel substrates, using atom probe tomography in conjunction with secondary ion mass spectrometry, x-ray fluorescence, current density-voltage, and external quantum efficiency measurements. Cu(In,Ga)Se₂ films deposited on soda-lime glass substrates and on steel substrates with a NaF precursor layer on top of the Mo back contact contain a significant amount of Na impurities and yield an enhanced open circuit voltage and fill factor. Using atom probe tomography, Na atoms are found to be segregated at grain boundaries and clustered in both bulk and grain boundaries. The atom probe data indicate that Na_Cu point defects are most likely formed at grain boundaries, reducing the number of compensating In_Cu point defects and thus contributing to an enhanced cell efficiency. However, for steel substrates the positive effect of Na on the cell performance is counterbalanced by the incorporation of Fe impurities into the Cu(In,Ga)Se₂ film. Fe atoms are homogeneously distributed inside the grains suggesting that Fe introduces point defects in the bulk