Microstructure Evolution during Selenization of Cu₂ZnSnS₄ Colloidal Nanocrystal Coatings

Annealing colloidal nanocrystal coatings in a selenium-containing environment to form polycrystalline thin films of the earth-abundant solar absorber copper zinc tin sulfoselenide (CZTSSe) is an attractive approach for making solar cells. We used a closed selenization system to investigate how coatings comprising copper zinc tin sulfide (CZTS) nanocrystals evolve into polycrystalline CZTSSe thin films and studied the effects of selenium vapor pressure, annealing temperature, and heating rate. These studies revealed two different types of microstructures and two different grain growth mechanisms depending on whether the CZTS nanocrystals are exposed to selenium vapor only or to both selenium vapor and liquid selenium. Coatings annealed in the presence of selenium vapor form a microstructure comprising micron-size CZTSSe grains on top of a nanocrystalline, carbon-rich, CZTSSe layer. The film microstructure is controlled by concurrent normal and abnormal grain growth, and the grain size distribution is bimodal, similar to that observed when CZTS nanocrystal coatings are annealed in sulfur vapor. The size of the abnormal crystals increases with selenium pressure and temperature to as large as 4 μm after annealing at 700 °C in 450 Torr of selenium. Carbon, initially present on nanocrystals as dispersion stabilizing ligands, segregates to the region between the CZTSSe grains and the substrate instead of desorbing from the coating as volatile reaction products such as CSe₂. Experiments suggest that carbon segregation occurs due to the tendency for CSe₂ to polymerize and form (CSe_2-x)_n. Coatings annealed in the presence of liquid selenium exhibit neither the bimodal grain size distribution nor the carbon-rich layer between CZTSSe grains and the substrate. In the presence of liquid selenium, the CZTS nanocrystals selenize, grow, and coarsen to ∼1 μm in size, forming compact CZTSSe films through liquid phase sintering, a mechanism wherein both grain size coarsening and film densification are mediated by the presence of a liquid phase.