Effect of potassium hydrogen phthalate (C8H5KO4) on the one-step electrodeposition of single-phase CuInS2 thin films from acidic solution

The effect of potassium hydrogen phthalate (C8H5KO4) as a special additive on the one-step electrodeposition of single-phase CuInS2 thin films from acidic solution (pH 2.5) was investigated in detail. The XRD, SEM and UV–vis–NIR characterization confirms that the addition of an adequate concentration of C8H5KO4 (23 mM) to the electrolytic bath containing 12.5 mM Cu2+, 10 mM In3+, 40 mM S2O32− and 100 mM LiCl can contribute greatly to the controllable growth of pure chalcopyrite CuInS2 films with uniform surfaces and an ideal band gap of approximately 1.54 eV. Complexation studies of C8H5KO4 with Cu2+ and In3+ in electrolytic solutions indicated that C8H5KO4 can complex Cu2+ more strongly than In3+ and move the electrode potentials of Cu2+ and In3+ near each other as determined by polarization analysis. Furthermore, the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) analysis performed in a series of solution systems revealed a three-step reaction mechanism for CuInS2 deposition and considerable adsorption of C8H5O4− and Cu(C8H5O4)+ to the cathode surface. This deposition shows that the synergetic effects of complexation and adsorption originated from the additive on the Cu2+ electro-reduction, thus promoting the co-deposition of copper, indium and sulfur in the form of single-phase CuInS2.