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, Cu2−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 Eg 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. Low‐cost photovoltaics: Surfactant‐free water‐soluble Cu2ZnSnS4 nanocrystals are synthesized and characterized. The hydrothermal method employed is easy and inexpensive, and the environmentally friendly p‐type semiconductor particles (nanocrystal ink) are stable for weeks in water. The high optical absorption (see picture), ease of annealing, and electrical properties suggest the Cu2ZnSnS4 nanocrystals could be a low‐cost efficient absorbing material for use in solid‐state solar cells.