Synthesis of n-type CuInS₂ Particles Using N-methylimidazole, Characterization and Growth Mechanism

We report on the growth of CuInS 2 n-type semiconductive particles, prepared using N -methylimidazole, as a solvent and/or a complexing agent, as well as their chemical and electrochemical properties. XPS, EDX and ICP-AES have shown that an excess of indium was obtained, which was greater at the surface (CuIn 1.19 S 1.7 at 500 °C) than in the bulk (CuIn 1.07 S 1.9 at 500 °C). Solid state Raman spectroscopy revealed two crystalline phases: chalcopyrite and the so-called copper−gold phase, and by increasing the annealing temperature of the particles, the formation of the chalcopyrite phase is favored. UV−visible measurements showed that the n-type CuInS 2 possesses a direct bandgap energy of 1.55 eV. To perform the capacitance measurements on a CuInS 2 film by EIS, we used two organic redox couples in nonaqueous media: 5-mercapto-1-methyltetrazolate (T − )/di-5-(1-methyltetrazole) disulfide (T 2 ), and 5-trifluoromethyl-2-mercapto-1,3,4-thiadiazolate (G − )/5,5′-bis(2-trifluoromethyl-1,3,4-thiadiazole) disulfide (G 2 ). Using these redox couples, we determined Fermi levels of −4.51 eV and −4.53 eV, and majority charge carrier densities of 2.8 × 10 18 and 9.6 × 10 18 cm −3, respectively. According to the energy level diagram of the CuInS 2 /electrolyte interface, the G − /G 2 redox couple is expected to lead to a more efficient device. The present work shows that the complexation of the metal ions and the negative charge on sulfur anions play a key role in the mechanism of formation of CuInS 2 particles. In situ Raman spectroscopy measurements showed that an indium−sulfur precursor is formed prior to the formation of CuInS 2 particles. Indeed, if an indium−sulfur precursor is formed prior to the reaction of sulfur with copper, a much better control of the n-type CuInS 2 properties is obtained. This explains the excess of indium at the surface of the CuInS 2 particles, as well as its n-type semiconductivity.