On the magnitude of the persistent photoconductivity (PPC) effect in CIGS layers with and without sodium

In this contribution we analyze the influence of the presence of sodium on the magnitude of the persistent photoconductivity effect (PPC) in Cu(In,Ga)Se₂ layers. Results of DLCP measurements on CIGS samples with and without sodium are presented. By solving the occupation equations we show that the V_Se - related model alone is not sufficient to explain large differences between the samples with and without sodium. We propose that the decrease of the of the magnitude of the PPC effect in Na-free samples by a factor of 10 is caused by very deep donor level located in the range of 0.2 eV - 0.3 eV above the valence band. We show a numerical example of the influence of a deep donor on the magnitude of the PPC effect.