Investigating the Nucleation and Growth of Quaternary Cu2ZnSnS4 Nanocrystals

Abstract

Among inorganic semiconductors, ternary and quaternary chalcogenides have attracted interest as light absorbers in photovoltaic applications. Cu 2 ZnSnS 4 (CZTS) has drown considerable attention as it has band‐gap suitable for solar‐harvesting applications, it shows p‐type conductivity and a high absorption coefficient. Moreover it only consists of inexpensive, non‐toxic and earth‐abundant materials. Synthesis by wet‐chemical methods are promising alternatives to physical deposition processes, as more easily implemented and cheaper. One of the challenges in the synthesis of colloidal CZTS nanocrystals is the control of internal structure and composition, which influence significantly their optoelectronic properties [1]. In this presentation we show the evidence of cation ordering in CZTS structure thanks to STEM HAADF imaging and we analyze nanocrystals homogeneity and composition by STEM EDX. CZTS nanocrystals were syntesized following an heating‐up method [2]. The first stage of the synthesis consists in a 30 minutes pre‐heating at 110°C of the organometallic precursors mixed in oleylamine. Then, CZTS nCs are obtained by increasing the mixing temperature up to 280°C and keeping it constant for one hour. The presence of long‐chained organic ligands passivating the surface of nanocrystals is fundamental for avoiding agglomeration in solution phase, it allows a slow and controlled growth; nevertheless it is detrimental for application in devices and for electron microscopy studies, in particular in spectroscopy (where contamination is critical). By drop‐casting the sample on graphene membranes, we could test the influence of several purification strategies. Thanks to the low‐contrast support we could image the unwanted parasitic residuals. In particular we proved the efficiency of solvent/antisolvent chloroform/aceton + acetic acid dispersion cycles [3]. HRTEM characterization was performed ex‐situ. HRTEM and STEM‐HAADF images were used to measure size dispersion of the nanocrystals. HRSTEM‐HAADF is sensible to chemical contrast, the signal being dependent on the atomic number Z; it is then possible to observe the sites occupied by the heavier atoms (Sn) in the structure, and distinguish then between kesterite (space group I‐4) or stannite (space group I‐42m) and pre‐mixed Cu‐Au (PMCA, space group P‐42m) structures, which show different characteristic “bright” motifs. The latter (PCMA) structure was the one found when nanocrystals were showing the good direction for phase identification (111). HRSTEM‐HAADF experimental images were compared with simulated ones obtained by multislice method and thermal diffusion scattering approximation [4]. STEM‐EDX was carried out on a dedicated FEI Themis with SuperX detector, in order to ensure chemical homogeneity between nanocrystals and inside a single crystal. Spectra were analyzed and quantified using Bruker Esprit 1.9 software.An overview of the nucleation and growing process was obtained by in‐situ Wide‐Angle X‐ray Scattering (WAXS) and Small‐Angle X‐ray Scattering (SAXS), performed on the ID01 beamline at the European Synchrotron Radiation Facility.