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

We report on the growth of CuInS2 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 (CuIn1.19S1.7 at 500 degrees C) than in the bulk (CuIn1.07S1.9 at 500 degrees 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 CuInS2 possesses a direct bandgap energy of 1.55 eV. To perform the capacitance measurements on a CuInS2 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 x 10(18) and 9.6 x 10(18) cm(-3), respectively. According to the energy level diagram of the CuInS2/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 CuInS2 particles. In situ Raman spectroscopy measurements showed that an indium sulfur precursor is formed prior to the formation of CuInS2 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 CuInS2 properties is obtained. This explains the excess of indium at the surface of the CuInS2 particles, as well as its n-type semiconductivity.