Cu2ZnSnS4 and Cu2ZnSnSe4 (CZTS
and CZTSe, respectively) and their mixed chalcogenide phase
Cu2ZnSnS
x
Se4–x (CZTSS(e)) are benign and cheap photovoltaic absorber materials
that represent a valuable alternative to the more expensive chalcogenide
systems: i.e., Cu(In,Ga)SS(e)2 (CIGSS(e)). One of the main
challenges related to the fabrication of CZTS(e) layers is the control
over both the crystalline phase (tetragonal, cubic, or hexagonal)
and the formation of binary (MS, M = Cu(II), Zn(II), Sn(II); M′2–x
S, M′= Cu(I), x = 0, 0.2; M″S2, M″ = Sn(IV)) and ternary
products (CTS phases, Cu2SnS3, Cu3SnS4) that hinder the performance of the corresponding
devices. In the present work, we rationalize the formation pathway
of the CZTS phase through binary and ternary products when salt precursors
with chloride and acetate as counteranions, respectively, are employed.
The results show that the counteranions have a remarkable influence
on the formation pathway of CZTS nanoparticles. The use of chloride
precursors leads to the predominant formation of CTSs ternary phases
(Cu2SnS3, Cu3SnS4), whereas
the formation of the CZTS phase is not observed even for higher temperature
and longer reaction time (250 °C, 24 h). In the case of acetates
the copresence of CZTS as the main product, together with binary and
ternary phases, is observed in the early stages of the reaction even
at lower temperature and shorter reaction time (200 °C, 2 h),
while when the reaction time and temperature are increased, only the
CZTS phase is observed. In addition to a careful microstructural characterization
of the as-synthesized materials by Raman spectroscopy, X-ray diffraction
(XRD), Energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron
spectroscopy (XPS), and high-resolution transmission electron microscopy
(HRTEM), we shed light on the reactivity among the metal precursors,
the organic ligand oleylamine, and the sulfur precursor carbon disulfide
(CS2) by 13C nuclear magnetic resonance (13C NMR) and investigate in depth the effect on particle surfaces
by Fourier transform infrared spectroscopy (FTIR), thermogravimetric
analysis (TGA), and XPS. A rationale for the formation pathway of
CZTS nanoparticles is proposed and supported by experimental evidence.
