Sulfur-Treated ITO Back Contact for Enhanced Performance of Semitransparent Ultrathin Cu(In,Ga)Se₂Solar Cells

Ultrathin Cu(In,Ga)Se₂(CIGSe) solar cells can be fabricated using various back contact materials. Among those back contact materials, transparent conductive oxides have enabled a lot of applications in semitransparent ultrathin (STUT) CIGSe solar cells. Especially, tin-doped indium oxide (ITO) has been used as a back contact material to fabricate STUT CIGSe solar cells because of its high transparency and conductivity. However, the CIGSe absorber layer deposited on ITO substrates is known to form an undesirable GaO_xlayer at high processing temperatures owing to Ga diffusion. To improve the back contact properties at the CIGSe and ITO interface, sulfur treatment of the ITO back electrode was conducted. Its rear-side interface analysis indicated that the GaO_xinterfacial layer changed to a mixed interfacial layer upon undergoing sulfur treatment, wherein the GaS_xand GaO_xphases were formed at the CIGSe/ITO rear-side interface. A fundamental investigation of the material properties implied that GaO_xand GaS_xhave different energy band alignments at the rear-side interface and that GaS_xcan be more helpful for hole extraction than the GaO_xphase. After fabrication of the STUT CIGSe solar cell, the power conversion efficiency with an optimal sulfur-treatment time of 5 min increased to 9.0% while that of the control solar cell on bare ITO (i.e., without sulfur treatment) remained at 7.0%. The results suggested that the composition and characteristics of the interfacial layer had a substantial impact on the photovoltaic properties of the STUT CIGSe solar cells.