Synthesis of CIS (Cu-In-S2) Solar Cell Nanoparticles By Controlling the Concentration of Metal Complexes in an Aqueous Solution
Authors: Takahashi Hideyuki 
Published: February 2020

CuInS2(CIS) is one of the most effective photovoltaic devices with high stability and efficiency, since the bandgap of CIS is 1.5eV while ideal band gap for solar cell is 1.4eV. Moreover, unlike Cu(In,Ga)Se2(CIGS), it does not contain toxic materials, such as Se [1]. However, since vaporizing temperature of four elements (Cu, In, Ga, Se) is extremely different, productivity under the gas phase is relatively low, which read the large amount of waste of resources. 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 homogeneous Copper-Indium (CI) alloy nanoparticles in an aqueous solution [2]. In this method, the concentration ratio of the metal complexes in the solution are calculated by using the critical stability constants, and the reduction rate is also controlled. Synthesized CI nanoparticles were spin coated onto substrate and selenized, consequently it shows the photo voltaic effect with the conversion efficiency of c.a. 2.3%. 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, the condition of Cu-In-complexing reagent(s) system were precisely controlled 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 synthesized. 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.
This work has been supported by the Grant-in-Aid for Scientific Research (B) (18H03416).
[1]Recent Development of Thin Film Compound Semiconductor Photovoltaic Cells, Wada, Publication of CMC, 2007
[2] Aqueous Phase Synthesis of CuIn Alloy Nanoparticles and Their Application for a CIS (CuInSe2)-Based Printable Solar Battery, Hideyuki Takahashi, Hironari Fujiki, Shun Yokoyama, Takayuki Kai, Kazuyuki Tohji, nanomaterials, 2018, 8, 221