(99d) Microstructure Development in Cu2znsn(SxSe1-x)4 Thin Films during Annealing of Colloidal Nanocrystal Coatings

Copper zinc tin sulfoselenide is a promising material for thin film solar cells because it comprises inexpensive and abundant elements. Thin polycrystalline films of Cu₂ZnSn(S_xSe_1-x)₄, or CZTSSe, may be formed by annealing nanocrystal coatings cast from colloidal dispersions in a selenium and/or sulfur atmosphere. This is a promising low-cost and high-throughput route for fabrication of CZTSSe-based thin film solar cells. During annealing, the nanocrystal films can transform into polycrystalline thin films with micrometer size grains. While benchmark efficiencies of CZTSSe solar cells have risen sharply in the last two decades, continued improvements in the solar cell performance and cost reduction require a fundamental understanding of the factors that influence the film microstructure. We have been investigating the microstructure evolution of polycrystalline CZTSSe thin films through systematic control of parameters such as chalcogen (sulfur or selenium) vapor pressure, annealing temperature, and heating rate in a closed system. In this talk, we will contrast microstructure development in CZTSSe films formed by annealing CZTS nanocrystal coatings in selenium vapor with microstructure development in CZTS films formed by annealing CZTS nanocrystal coatings in sulfur vapor. There are significant differences in the mechanisms of microstructure development despite the similar approaches. These differences are due to differences in thermodynamic phase equilibria of sulfur and selenium as well as due to differences in chemical reactions of sulfur and selenium with the carbon containing ligands that surround the nancrystals: these ligands are used to stabilize the nanocrystal dispersions and often end up in the coatings and films even after annealing. For example, annealing CZTS nanocrystal coatings at high selenium pressure (e.g., 450 Torr) leads to the formation of a bilayer microstructure containing a microcrystalline CZTSSe layer on top of a nanocrystalline layer that is rich in carbon. In contrast, films annealed with sulfur do not show a distinct carbon-rich layer and most of the carbon volatilizes from the film during annealing. We suggest that carbon remains in the film due to the formation of a non-volatile carbon selenide polymer, the presence of which inhibits nanocrystal sintering and leads to the formation of a large-crystal top layer. The microstructure of films annealed in the presence of selenium liquid is influenced by liquid phase sintering. These films exhibit monomodal coarsening and densification into a compact film without the formation of a bilayer structure.