CuInS₂(CIS) is one of the most effective photovoltaic devices for developing high stability and low cost solar cell. Bandgap of CIS is 1.5eV while ideal band gap for solar cell is 1.4eV, so CIS is suitable as solar cell materials. Moreover, unlike Cu(In,Ga)Se₂(CIGS), it does not contain toxic materials, such as Se [1]. Until now, CIS solar cells have been synthesized by utilizing gas phase method. However, the gas phase evaporation process needs high energy consumption, high initial cost, and large amount loss of resources. For spreading the solar cells greatly, it is required to develop a method which can resolve the above problems, so it is necessary to synthesize CIS solar cells in liquid method.

Here, we have already developed a synthesizing method of single phase and homogeneous Copper-Indium (CI) alloy nanoparticles in an aqueous solution. In this method, the concentration ratio of the metal complexes in the solution are calculated by using the critical stability constants, and the reduction reaction rate is also controlled. Synthesized CI nanoparticles were spin coated onto substrate and selenized, consequently it shows the photo voltaic effect. Thus, printable solar cell can be formed by using this method. However, CIS₂ nanoparticles were not synthesized until now, because of ligand exchange reaction between the complexing agents and Cu and In.

Therefore, in this study, to prevent these undesirable reaction, we tried to control the condition of Cu-In-complexing reagent(s) system precisely by using the calculation. By using these calculation results, CIS₂ particles were tried to synthesized in an aqueous solution

Results of calculation indicated that iminodiacetic acid (IDA) was appropriate complexing reagent. In the ternary system of Cu-In-IDA, it was calculated that the Copper comlexes and Indium complexes can be restricted to homogenized species, [(Cu²⁺)(IDA)] (pH3) and [(In³⁺)(IDA)₂] (pH3) under the condition that the total molar concentration of Copper and Indium ions were 5 mM, respectively.

Based on this calculation, a solution under the above condition was prepared, and both metal complexes were tried to co-sulfurize. As a results, particles with amorphous like morphology were obtained. Therefore, in order to improve the crystallinity, the particles were heated for 30 min at 500 °C under N₂ atmosphere. XRD results of heat treated samples indicated that several peaks with a dominant CIS₂ peak were observed. Thus, it can be said that CIS₂ nanoparticles were successfully synthesized in an aqueous solution, without undesirable ligand exchange reaction during reaction.

These results indicated that homogenize metal complexes system of Cu-In-IDA are essential for the synthesis of CIS₂ particles.

In our presentation, detailed and another results will be introduced.

[1]Recent Development of Thin Film Compound Semiconductor Photovoltaic Cells, Wada, Publication of CMC, 2007