Dependence of the Magnitude of Persistent Photoconductivity on Sodium Content in Cu(In,Ga)Se2 Solar Cells and Thin Films

The persistent photoconductivity (PPC), i.e., the metastable increase in hole concentration due to light soaking in Cu-poor Cu(In
_0.7
,Ga
_0.3
)Se
₂
solar cells is observed to be proportional to the Na concentration in the absorber layer. Hole concentration in cells and conductivity in thin films follow a similar trend, suggesting that the effect originates from the absorber layer and is somehow linked to sodium concentration. Our results suggest that the large lattice relaxation model of an amphoteric V
_Se
-V
_Cu
vacancy complex being responsible for PPC cannot itself explain the Na-PPC relation. Therefore. either the passivation of the grain boundary barriers by sodium or a sodium-related modification of a defect complex needs to be included in the model.