Size and shape control in the synthesis of CIGS nanocrystals

During the last decade, the colloidal synthesis of especially Cd – and Pb chalcogenide nanocrystals has been well-developed. Monodisperse sols of nanocrystals over a large size range have become available and are widely used as building blocks for electronic and optical devices. However, the issue of toxicity remains a major obstacle towards large-scale integration of semiconductor nanocrystals in applications. I-III-VI materials (Cu(In,Ga)(S,Se)2 (CIGS) and related compounds) are interesting candidates towards greener chemistry and offer the possibility of band gap engineering both by using quantum confinement and altering material composition. Moreover, CIGS is a well-known absorber material for high efficiency thin-film photovoltaics, and the use of nanocrystals as precursor inks is an interesting route to decrease the production cost of CIGS solar cells. Up to now, the synthesis of CIGS nanocrystals is mostly developed on an empirical basis. In this work, our aim is to understand the relation between reaction parameters (temperature, concentration, reaction time,..) on one hand and nanocrystal properties (size, shape, composition) on the other hand. We demonstrate that by altering reaction parameters in a rational way (1), CuInS2 and CuGaS2 nanocrystals with sizes from 5 to 20 nm can be obtained which vary in shape from quasi-spheres to flat hexagonal prisms. The ability to control the size and shape of CIGS nanocrystals is of particular importance for the deposition of crack-free thin films by, e.g., inkjet printing that can be incorporated in thin film solar cells. (1) S. Abé, R. Čapek, B. De Geyter, Z. Hens (2012), "Tuning the Postfocused Size of Colloidal Nanocrystals by the Reaction Rate: From Theory to Application", ACS Nano, 6, 1: 42-53