Aqueous Phase Synthesis of CuIn Alloy Nanoparticles and Their Application for a CIS (CuInSe₂)-Based Printable Solar Battery

To apply CuInSe₂ (CIS)-based printable solar batteries; an aqueous phase synthesis method of Cu-In (CI) alloy nanoparticles is studied. Metal complexes in the original solution are restricted to homogenized species by utilizing calculations. For example; [(Cu²⁺)(ASP^2-)₂] [ASP: the "body (C₄H₅O₄N)" of aspartic acid (C₄H₇O₄N)] is predominant in the pH 6-13 region (C_ASP/C_Cu > 6); while In complexes can be restricted to [(In³⁺)(OH^-)(EDTA^4-)] (pH 10-12; C_EDTA/C_In = 2) and/or [(In³⁺)(ASP^2-)₂] (pH 7-9; C_ASP/C_In = 5). These results indicate that the added amount of complex reagents should be determined by calculations and not the stoichiometric ratio. The reduction potential of homogenized metal complex is measured by cyclic voltammetry (CV) measurements and evaluated by Nernst's equation using the overall stability constants. CuIn alloy nanoparticles with a small amount of byproduct (In nanoparticles) are successfully synthesized. The CI precursor films are spin-coated onto the substrate using a 2-propanol dispersion. Then the films are converted into CIS solar cells; which show a maximum conversion efficiency of 2.30%. The relationship between the open circuit potential; short circuit current density; and fill factor indicate that smoothing of the CIS films and improving the crystallinity and thickness increase the solar cell conversion efficiency.