Water Phase Synthesis Method of CuInS₂ Nanoparticles and Its Application for Photovoltaic Device

CIS (CuInS₂) is one of the most effective photovoltaic devices with high stability and efficiency, because of its suitable bandgap (1.5eV). Moreover, it does not contain toxic materials, such as Se, within its components. However, productivity and utilization of natural resources under the gas phase synthesis method is relatively low, since vaporizing temperature of elements is extremely different. Thus, it is required to develop a synthesis method of CIS solar cell materials within liquid phase.

Here, we have already developed a synthesizing method of single phase and homogenous CI (CuIn) alloy nanoparticles in aqueous solution. In this method, the concentration of the metal complexes in the solution are calculated by using the critical stability constants, and the reduction reaction rate of both metal ions is also controlled. Synthesized CI nanoparticles were spin coted onto Mo/glass substrate and selenized, consequently it shows the photo voltaic effect. Thus, printable solar cells can be formed by using this method. The synthesizing method contains the selenization which is a sort of evaporation method, so it is not liquid process completely. To synthesize CIS solar cells without the selenization, we tried to sulfurize CI nanoparticles but CIS particles were not synthesized. It is difficult to sulfurize CI nanoparticles because they are stable and hard to react with sulfur ions, so it is necessary to synthesize from copper ions and indium ions to CuInS₂ particles directly.

However, nobody can synthesize CuInS₂ particles in aqueous solution because there is a difference between the sulfurization reaction rate of copper and indium ions. If the sulfurization reaction rate of both metal ions is different, they are sulfurized independently to form their own sulfides, such as Cu₂S or CuS and In₂S₃. It is necessary to control and approximate the sulfurization reaction rate of both metal ions for synthesis of CuInS₂ particles. Here, in the sulfurization process, the sulfurization reaction rate is correlation with potential energy such as gibbs free energy (ΔG) or activation energy (E_a). Hence, CuInS₂ particles can be synthesized under the condition that the ΔG or E_a in the sulfurization reaction of copper and indium ions are approximated.

To synthesize CuInS₂ particles in aqueous solution, we tried to approximate the ΔG of the sulfurization reaction of copper and indium ions.

By calculating ΔG_Cu2S and ΔG_In2S3, we obtained the correlation of K_In and K_Cu and we were able to select appropriate complexing agents, so we tried to synthesize CuInS₂ particles by using these reagents.

XRD data indicates that several peaks with a dominant CuInS₂ peak were observed. From the above results, it can be said that the synthesis of the precursor of CuInS₂ particles in aqueous solution was successful because the E_a of the sulfurization process are approximated in both metal ions.

In addition, it was found that the ΔG of copper and indium ions are approximated in the sulfurization system (Cu-In-IDA), so it is necessary to control and approximate both the ΔG and E_a for synthesis of CuInS₂ particles in aqueous solution.

Although the precursor of CuInS₂ were synthesized in aqueous solution, the size and shape of the particles were not uniform. Therefore, for preventing degradation of conversion efficiency, it is needed to develop a technique for homogenizing the shape and the size of the particles.

Detailed and another results will be introduced in our presentation. This work has been supported by the Grant-in-Aid for Scientific Research (B) (18H03416).