Colloidal Nanocrystals of Wurtzite‐Type Cu₂ZnSnS₄: Facile Noninjection Synthesis and Formation Mechanism

Abstract

Cu 2 ZnSnS 4 (CZTS), a quarternary chalcogenide p-type semiconductor, is currently receiving considerable attention as absorber materials for low-cost photovoltaics due to its high absorption coefficient, optimal band gap, and naturally abundant and nontoxic elemental components.[1] The growing technological interest in this material has motivated the study of the nature of its crystal structures.[2] The groundstate crystal structure of CZTS is the kesterite form (space group I4 ), which is a tetragonal superstructure derived from the binary II-VI cubic zinc-blende (ZB) lattice. Several other ZB-related structural modifications of CZTS have been considered, and these include the stannite structure (space group I4 2m). This structure type differs from the kesterite form only in the ordering of Cu + and Zn 2 + ions. In a recent theoretical study, Chen et al. have shown that there are two low-energy structural configurations of CZTS that are not based on the ZB unit cell.[2f] Instead, these two predicted structures are derivatives of the binary II-VI hexagonal wurtzite (WZ) structure and are conveniently referred to as WZ-kesterite and WZ-stannite, owing to their structural relationship with the ZB-derived kesterite and stannite polytypes, respectively. The WZ-kesterite form is a monoclinic (pseudo-orthorhombic) superstructure of the WZ unit cell and has the space group Pc. The WZ-stannite phase, on the other hand, is described by an orthorhombic supercell with space group Pmn2 1 . In bulk form, WZ-type superstructures have long been reported for a number of quarternary chalcogenides such as Cu 2 MGeS 4 (in which M = Mn, Zn, Cd).[3] Bulk CZTS with a WZ-derived phase, however, has yet to be synthesized.In nanocrystalline form, CZTS materials have been colloidally prepared by means of the hot-injection synthetic strategy involving the reaction of the Cu, Zn, and Sn precursors with elemental sulfur in oleylamine at high-temperature conditions.[4] The nanocrystals produced by this method are quite polydisperse in shape and size, and adopt the thermodynamically more stable ZB-derived tetragonal phase. Very recently, Lu et al. have employed the hot-injection technique and used dodecanethiol as the sulfur source in preparing CZTS nanoprisms and nanoplates that are 20-50 nm in size.[5] X-ray diffraction (XRD) measurements revealed that these nanocrystals possess a WZ-related crystal structure. Their proposed structure is based on the hexagonal WZZnS unit cell described by the space group P6 3 mc, in which the metal cations are randomly distributed in the cation sites (i.e., cation-disordered). However, the possibility that their WZ-type CZTS exhibits the theoretically predicted lower-energy cation-ordered WZ-kesterite and WZ-stannite structures has not been considered.Herein, we provide a facile noninjection synthetic route to preparing monodisperse anisotropic CZTS nanocrystals that adopt a WZ-type crystal structure. The noninjection or "heating up" approach to colloidal nanocrystals is better in terms of synthetic reprodu...

Phase-transition-driven growth of compound semiconductor crystals from ordered metastable nanorods

“…1a for a larger angle range). In the literature nanocrystals with orthorhombic wurtz-stannite and pseudo-orthorhombic wurtz-kesterite structures are reported [27][28][29][30][31] which produce similar diffraction signals due to a similar hexagonal anion sublattice, but differ in the ordering of the cations [30]. In our measurements, additional reflexes that indicate presence of wurtz-stannite or wurtz-kesterite instead of hexagonal wurtzite structure are not observed.…”

Chalcogenide Quaternary Cu2FeSnS4 Nanocrystals for Solar Cells: Explosive Character of Mechanochemical Synthesis and Environmental Challenge

“…The research into earth-abundant, environmentally sustainable and cheap analogues is therefore fully justified. The main focus is on kesterite Cu 2 ZnSnS 4 (CZTS) quaternary crystals, which emerged as potential candidates [5][6][7][8][9][10][11][12].…”

Green synthesis of wurtzite copper zinc tin sulfide nanocones for improved solar photovoltaic utilization

“…In general, CZTS nanocrystals are mostly in the kesterite or stannite phase (Shavel et al 2010). It has been reported by Regulacio et al that in colloidal nanocrystal synthesis, the presence of organic surfactants or capping ligands is known to strongly influence the crystallographic phase, morphology and growth of the nanocrystals (Regulacio et al 2012). Thus, the formation of metastable phases can be induced by proper selection of surfactants.…”

“…Thus, the formation of metastable phases can be induced by proper selection of surfactants. CZTS nanocrystals having wurtzite (WZ) phase has also been synthesized by various methods such as noninjection (Regulacio et al 2012), hot-injection (Singh et al 2012) and hydrothermal method (Jiang et al 2012). These processes require inert gas protection (nitrogen or argon), stepwise heating and relatively high temperature.…”