The Formation and Impact of Sodium rich Secondary Phases in the Absorber layer of Cu(In,Ga)Se2 Based Solar Cells

We report the formation of sodium rich amorphous secondary phases in the absorber layer of Cu(In,Ga)Se2 solar cells and discuss their potential impact on cell performance. We find that increasing CIGS growth temperatures results in the formation of progressively larger sodium rich, Cu poor amorphous secondary phases at the CIGSMo interface, clearly distinguishable from CIGS in compositional analysis of the CIGS backside and in cross section. We observe a sharp decline in performance (Voc) when penetration depth of these phases into the CIGS absorber layer exceeds 0.5 microns. We further observe a negative correlation between the penetration depth of secondary phases into CIGS and activation energy, indicating sensitivity of device performance to changes in microstructure in the back portion of the absorber layer.