Development and Properties of Surfactant‐Free Water‐Dispersible Cu2ZnSnS4 Nanocrystals: A Material for Low‐Cost Photovoltaics

A simple, yet novel hydrothermal method has been developed to synthesize surfactant‐free Cu₂ZnSnS₄ nanocrystal ink in water. The environmentally friendly, 2–4 nm ultrafine particles are stable in water for several weeks. Detailed X‐ray diffraction (XRD) and high‐resolution transmission electron microscopy revealed the formation of single‐crystalline‐kesterite‐phase Cu₂ZnSnS₄. Elemental mapping by scanning electron microscopy/energy dispersive spectrometry corroborated the presence of all four elements in a stoichiometric ratio with minor sulfur deficiency. Finally, Raman spectroscopy ruled out the possible presence of impurities of ZnS, Cu₂SnS₃, SnS, SnS₂, Cu_2?xS, or Sn₂S₃, which often interfere with the XRD and optical spectra of Cu₂ZnSnS₄. X‐ray photoelectron spectroscopic studies of the as‐synthesized samples confirmed that the oxidation states of the four elements match those of the bulk sample. Optical absorption analyses of thin film and solution samples showed high absorption efficiency (>10⁴ cm^?1) across the visible and near‐infrared spectral regions and a band gap E_g of 1.75 eV for the as‐synthesized sample. A non‐ohmic asymmetric rectifying response was observed in the I–V measurement at room temperature. The nonlinearity was more pronounced for this p‐type semiconductor when the resistance was measured against temperature in the range 180–400 K, which was detected in the hot‐point probe measurement.