Room-Temperature Electron Paramagnetic Resonance Study of a Copper-Related Defect in Cu₂ZnSnS₄ Colloidal Nanocrystals

Abstract

We report on electron paramagnetic resonance (EPR) spectroscopy of Cu–Zn–Sn–S (CZTS) nanocrystals (NCs) synthesized by a colloidal chemistry method. The chemical composition of the NCs is characterized by a strong deficit of tin and slight zinc enrichment with respect to the “ideal” stoichiometry of kesterite Cu2ZnSnS4. Nevertheless, X-ray photoemission spectroscopy (XPS) shows that the net cation/anion ratio is very close to 1:1 and the valence states of the constituents correspond to those of Cu2ZnSnS4, in particular, Cu+. The kesterite crystal structure of the NCs is confirmed by X-ray diffraction and Raman spectroscopy. Observing the EPR features for such CZTS NCs characteristic of individual Cu2+ embedded in the crystal lattice can be directly related to cationic disorder and nonstoichiometry. In particular, as a result of Cu–Zn antisites, Sn deficiency, and likely multivalence, a very small fraction of copper ions may occur in the Cu2+ state. The concentration of Cu2+ sites of ∼1017–1018 cm–3 assessed from the EPR spectra is below the sensitivity of XPS; therefore, the latter is dominated by the prevailing Cu+ species. The temperature dependence of the I–V curves reveal a level at about 0.2 eV above the valence band that can be assigned to the CuZn antisite (with copper presumably in the Cu2+ state) and a 14 meV level above the valence band that can correspond to VCu. The new insights, reported here, into the relation between the structure and electronic states of CZTS NCs are important for advancing their applications.

Spectroscopy and Theoretical Modeling of Phonon Vibration Modes and Band Gap Energy of Cu2ZnSn(SxSe1–x)4Bulk Crystals and Thin Films

“…In the last few years, special attention has been paid not only to the change in the content of S to Se atoms − but also of Sn to Ge or Si and − Zn to Cd and other elements − for both films and bulk crystals. As is known, either partial or complete replacement of Cu, Zn, or Sn cations with bigger cations in the Cu–Zn and Cu–Sn planes can increase the formation energy of antisite defects on these planes and, thus, decrease the extent of the disorder. , This allows to deepen understanding of the fundamental properties of this family of compounds and to develop tandem TFSCs able to absorb solar energy in a wider range of wavelengths, increasing their efficiencies . Therefore, an important issue is how to obtain films or bulk crystals with the same composition of elements as the one before synthesis.…”

Optical properties of γ-sensing β-GaLaS3:Er crystal

“…A number of publications reported the influence of γrays irradiation on optical absorption spectra of such materials [10][11][12]. However, only few works are devoted to the analysis of changes in the photoluminescence (PL) intensity under the action of γ-radiation, establishing the mechanism of the PL and the nature of radiation-induced defects at different radiation doses [13][14][15]. This especially applies to the analysis of crystal structures of chalcogenides doped with RE metals.…”