Preparation of precursor particles by cryogenic mechanical milling for the deposition of CuInS2 thin films

Cryogenic milling process is more effective in generating nano- and micro-size particles than conventional high-energy mechanical milling in terms of their size and processing time needed as the cryogenic temperature lowers the activation energy for elastic fracture and minimizes agglomeration among particles. Following the previous success in fabricating uniform chalcopyrite CuInSe2 nanoparticles using the cryogenic milling process, this study has deposited CuInS2 thin films through a particle-based, non-vacuum sequential process that includes the preparation of precursor particles using a cryogenic milling process. The paste formulated with the particles was coated on Mo-deposited glass substrate and subsequently sulfurized under S/H2/Ar atmosphere. The chalcopyrite CuInS2 thin films were successfully made after sulfurization at the substrate temperature of 550 °C in 30 min. Optimization of pre-baking step prior to sulfurization was critical in achieving dense morphology of the films and the residual oxygen in the nanoparticles was efficiently removed by its reduction reaction with H2 during the sulfurization process. While the cryogenic milling process can effectively produce particles in a short period of time, preventing agglomeration, further study is necessary to understand the effect of structural and morphological properties of the starting materials on the final outcome from the cryogenic milling process.