Room-Temperature Electron Paramagnetic Resonance Study Of A Copper-Related Defect In Cu2znsns4 Colloidal Nanocrystals

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, Cut. 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 similar to 10(17)-10(18) 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 Cu2+, antisite (with copper presumably in the Cu2+ state) and a 14 meV level above the valence band that can correspond to V-Cu. The new insights, reported here, into the relation between the structure and electronic states of CZTS NCs are important for advancing their applications.