Incorporation of Na in Cu(In,Ga)Se$_{\bf 2}$ Thin-Film Solar Cells: A Statistical Comparison Between Na From Soda-Lime Glass and From a Precursor Layer of NaF

The presence of Na in Cu(In,Ga)Se₂ layers increases the electrical performance of this type of thin-film solar cell. A detailed comparison of incorporating Na in the CIGS layer by two different methods is performed by evaluating several hundred devices fabricated under similar conditions. The first method is based on the conventionally used Na diffusion from the soda-lime glass substrate, whereas the second method is based on a NaF precursor layer deposited on a Mo-coated alkali-free glass substrate. The sample where Na is introduced by using a NaF precursor layer shows an orientation weighted toward (2 0 4)/(2 2 0) and a net acceptor concentration of 3.4 × 10¹⁶ cm^-3, while SLG shows a (1 1 2) orientation with a 2.9 × 10¹⁶ cm^-3 acceptor concentration. Both sample types show close identical elemental depth profiles, morphology, and electrical performance.