Non-toxic ethanol based particulate inks for low temperature processed Cu₂ZnSn(S,Se)₄ solar cells without S/Se treatment

Solution processing for kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) thin-film solar cells has the potential to provide economical and large-scale implementation of highly efficient photovoltaics. However, to date, post-sulfurization/selenization at high temperatures (500-550 °C) or highly toxic solvents have generally been involved in most fabrication routes, resulting in problems such as phase separation, chalcogen loss (or Sn loss), and safety issues. In this paper, an ethanol solvent-based precursor ink is newly designed for fabricating a CZTSSe absorber layer at much lower temperatures (as low as 400 °C) without a post-sulfurization/selenization process. The precursor ink is composed of a commercially available powder mixture of easily decomposable copper(II) formate, Zn, Sn, S, and Se, which allow us to fabricate a CZTSSe film with large grains (1-1.5 μm) and a reasonable band gap (~1.1 eV). It is found that the formation of a homogeneous CZTSSe phase could be effectively promoted by rapid annealing, suggesting that the ramping rate is a critical factor to obtaining high quality absorber layer. Our CZTSSe device (rapidly annealed at 400 °C) exhibits an efficiency of 4.67% under AM 1.5 illumination.