Synthesis, Characterization, and Growth Mechanism of n-Type CuInS₂ Colloidal Particles

We report on the growth of CuInS₂ n-type semiconductive particles, prepared using a modified Czekelius’s colloidal method, as well as their chemical and electrochemical properties. Solid state Raman spectroscopy revealed two crystalline phases: chalcopyrite and the so-called copper−gold phase. Increasing the annealing temperature of the particles favors the formation of the chalcopyrite phase. As shown by XPS, EDX, and ICP-AES, an excess of indium was obtained, which was greater at the surface (CuIn_1.45S_1.9 at 450 °C) than in the bulk (CuIn_1.04S_1.74 at 450 °C). UV−visible measurements showed that the n-type CuInS₂ possesses a direct bandgap energy of 1.45 eV. Two organic redox couples in nonaqueous media were used to perform the capacitance measurements carried out by EIS on a CuInS₂ film: 5-mercapto-1-methyltetrazolate (T⁻)/di-5-(1-methyltetrazole) disulfide (T₂) and 5-trifluoromethyl-2-mercapto-1,3,4-thiadiazolate (G⁻)/5,5′-bis(2-trifluoromethyl-1,3,4-thiadiazole) disulfide (G₂). Fermi levels of −3.95 eV and −3.64 eV and majority charge carrier densities of 4.1 × 10¹⁸ and 1.8 × 10¹⁸ cm⁻³ were determined, respectively, using these redox couples. On the basis of the CuInS₂/electrolyte energy level diagrams, the G⁻/G₂ redox couple is expected to lead to a more efficient device (greater photocurrent and photovoltage). In situ Raman spectroscopy measurements showed that the reactivity of copper with hexamethyldisilathiane is faster than with indium. This explains the excess of indium at the surface of the CuInS₂ particles, as well as its n-type semiconductivity.