Continuous production of the quaternary nanocrystals in a flow reactor

Alexey Shavel

Catalonia Institute for Energy Research, IREC, Functional Nanomaterials, Barcelona, Spain

Copper-based quaternary chalcogenide, CZTS (Cu2ZnSnS4) is a new promising materials for the solar cell application. The advantage of this material is a low cost and toxicity compared to conventional photovoltaic materials, a high optical absorption coefficient and appropriate direct band gap (1.49 eV). In organometallic way, the solution of the nanometers sizes nanoparticles can be prepared, forming so called 'inorganic ink' which allows direct printing or drop-casting conductive film of the absorber layer of the photovoltaic device.
The classical colloidal synthesis procedure based on the decomposition of organometallic precursors is well established and widely used. However, these synthetic routes have been usually developed and optimized for the production of small amounts of material, which are insufficient for most practical applications. This synthetic strategy is also too labor-intensive to provide enough amount of materials by a multiple small-scale batch production. The obvious up-scaling procedure derived from classical colloidal synthesis routes is to maintain the synthetic conditions and concentrations optimized in small batches, but increase the total volume of precursors. This simple up-scaling technique inevitably results in a degradation of the product homogeneity due to the reduced thermal and compositional uniformity of large volumes of solution, especially in the need of a hot injection step.
In the present work, a successful route for the continuous production of Cu2ZnSnS4 (CZTS) nanoparticles is presented. The preparation procedure allowed a simple and efficient control of the nanoparticles composition in a wide range. The route was used for the preparation of several grams of CZTS nanoparticles, which were used for the thermoelectric characterization of this material in a nanocrystalline form. Single particle HRTEM-EDX analysis confirmed all four elements to be within each nanocrystal, but demonstrated a compositional distribution among nanoparticles within each sample. The nanoparticle composition distribution was minimized at the highest reaction temperatures used.