Cu(In,Ga)Se $_{\bf 2}$ Solar Cells With Varying Na Content Prepared on Nominally Alkali-Free Glass Substrates

In this paper, Cu(In,Ga)Se₂ (CIGS) thin-film solar cells are prepared on nominally alkali-free glass substrates using an in-line CIGS growth process. As compared with, for example, borosilicate glass or quartz, the glass is engineered to have similar thermal expansion coefficient as soda-lime glass (SLG) but with alkali content close to zero. Na is incorporated in the CIGS material using an ex-situ deposited NaF precursor layer evaporated onto the Mo back contact. Several thicknesses of the NaF layer were tested. The results show that there is a process window, between 15 and 22.5 nm NaF, where the solar cell conversion efficiency is comparable with or exceeding that of SLG references. The effect of an NaF layer that is too thin on the solar cell parameters was mainly lowering the open-circuit voltage, which points to a lower effective dopant concentration in the CIGS layer and is also consistent with presented C - V measurements and modeling results. For excessively thick NaF layers, delamination of the CIGS layer occurred. Additional measurements, such as scanning electron microscopy (SEM), secondary ion mass spectrometry, capacitance-voltage analysis (C - V), time-resolved photoluminescence (TRPL), external quantum efficiency (EQE), current-voltage analysis (J-V), and modeling, are presented, and the results are discussed.