Progress in the fabrication of Cu₂ZnSnS₄ thin film for solar cells

Cu(In, Ga)Se₂(CIGS) is a widely investigated material for thin film solar cells in the last decade. However, it has the concern of short availability of constituent elements (e.g. In, Ga) which limits its development. Research focused on new substitute of light absorber is eagerly needed. As an earth-abundant semiconductor, CZTS(Cu₂ZnSnS₄) has attracted considerable interest recently. CZTS has lots of similarities to CIGS, and is considered to be a promising material in the field of thin film solar cells owning to its fascinating optoelectronic properties, low-cost fabrication, and non-toxic nature. In this article, after introducing the features of CZTS material and the basic structure of CZTS thin film solar cells, different methods for the fabrication of CZTS thin films, as well as the corresponding device performance are discussed. CZTS thin film fabrication methods include vacuum based approaches and non-vacuum based approaches, with the former usually has the advantage of easy control of the chemical composition as well as good reproducibility; and the latter often has merits of low-cost, less-energy consumable, and suitable for large area deposition. Firstly, vacuum based approaches, including sputtering, evaporation and pulsed laser deposition, are discussed. Secondly, according to the different condition of the precursors, three subtypes of non-vacuum based approaches are presented (nanoparticle ink precursors, solution precursors, mixture precursors of solution and nanoparticle ink). Meanwhile, corresponding device efficiencies are summarized. Up to date, the power conversion efficiencies of 12.6% for CZTS based solar cells have been achieved, which is enormously encouraging. The new champion device was fabricated using a recently described hydrazine pure-solution approach, which significantly improves the coating uniformity and film structure. What's more, various film treatment strategies to optimize the CZTS thin film, such as selenization/sulfurization and elemental composition tuning (e.g. S/Se, Zn/Cu), are outlined here; the focus of future research and development are proposed.