Investigation of the role of sodium in Cu₂ZnSnS₄ films and the resulting phase evolution during sulfurization

Kesterite Cu₂ZnSnS₄ (CZTS) is one of the most attractive materials in future photovoltaics. However, copper could accumulate at the surface of CZTS films causing the combination of carriers and short circuiting of solar cells. One possible solution is sodium doping. In this paper, in order to investigate the role of sodium in CZTS phase distribution and segregation, a series of experimental works were carried out with Na-containing and Na-free samples. From the analyses of X-ray photoelectron spectra (XPS) and Raman spectra, sodium was found to accumulate near the surface and interface, where Zn-rich regions were also detected. To explain this phenomenon, we deduced that sodium plays an important role in Zn accumulation and contributes to the surface and interface modifications. Based on this deduction, we conducted a detailed phase reaction process on samples containing a 40 nm NaF layer annealed at various temperatures. The phase evolution route was confirmed and the results gave us a better understanding of the role of sodium in CZTS evolution.