Insertion of the AGS layer at the CIGSe/ITO interface: A way to reduce the formation of the GaOx interfacial phase in CIGSe solar cells

Semitransparent (ST) Cu(In,Ga)Se2 (CIGSe) solar cells based on the ultrathin absorbers with a bandgap of ~1.5 eV are prepared on the tin-doped indium oxide (ITO) back electrode. In a solar cell with absorber thickness ≤300 nm, incorporation of a sulfurized-AgGa (AGS) layer between the CIGSe layer and the ITO back contact was found to improve the device efficiency. This article explores the effect of the AGS layer on the CIGSe structure and CIGSe/ITO interface. Meanwhile, an ordered defect compound formation is observed in the AGS interlayered-CIGSe absorbers, the origin of which is the decrease in the (Ag+Cu)/(Ga+In) ratio arising from diffusion of In from the ITO to the absorber and the Ag/Ga ratio of 0.60 in the AGS layer. It is found that the AGS layer can significantly reduce the formation of the GaOx phase at the CIGSe/ITO interface. However, the increase in the deposition time of the CIGSe layer in the AGS interlayered-absorbers resulted in an increase in the GaOx interfacial (IF) layer thickness. The roll-over phenomenon in the illuminated J-V curves of devices based on AGS interlayered-absorbers became increasingly pronounced when the thickness of GaOx IF layer increased. The SCAPS (solar cell capacitance simulator) simulation results indicate that the reason for the roll-over effect in the illuminated J-V curves of the solar cells with AGS layers is the GaOx IF phase (n-type in nature). In light of the present results, possibilities to further enhance the conversion efficiency of ST CIGSe solar cells are also discussed.