Effect of sodium-doping on the performance of CZTS absorb layer: Single and bifacial sodium-incorporation method

In current work, sodium doped CZTS films are deposited onto Mo-coated stainless-steel foil for fabrication of flexible solar cells. New strategies for sodium incorporation of Cu₂ZnSnS₄ (CZTS) thin films are introduced and investigated. In detail, bilayer NaF film was deposited at the bottom and on the top of CZTS films to obtain a uniformly doped CZTS film. For comparison, CZTS films were also doped with a single layer NaF from bottom and top, respectively. Secondary ion mass spectroscopy (SIMS) is applied to investigate the distribution of different elements along the depth, X-ray diffraction and Raman spectroscopy are used to identify the crystallinity and secondary phases. Finally, solar cells are assembled based on the prepared CZTS films with an efficiency of 2.3%. Although the obtained devices show a far lower efficiency compared with other reports, current research provides a feasible path for sodium incorporation when fabricating CZTS films on Na-free substrates.