Synthesis and Size Control of CuIn Alloy Nanoparticles in Aqueous Solution and Its Application for CuInSe2 Solar Cell

Since CIS (CuInSe₂) solar cell, one of the attractive solar cell materials, shows the maximum efficiency reached up to 20.9%, CIS/CIGS solar cell is considered to comparable with that of Si solar cells. However, it is also true that these cell are deposited in high vacuum condition, which lead the loss of natural resources and also energy. So, various researches were vigorously researched of chemical reduction method since it is ecological and also economical technique. Ye et al.^[1] synthesized Cu-In alloy particles by using chemical reduction method in organic solvent and the Cu-In alloy precursor films were transformed to CuInSe₂films by selenization. The resulting cells demonstrate a maximum energy conversion efficiency of 3.92 %. However, total cost of this method is not low since organic solvent should be used.

On the other hand, until now, we reported that the synthesis of uniform and well crystallized alloy nanoparticles, such as Pd₂₀Te₇ and Bi₂Te₃by restrict controlling the homogenization of metallic complexes in the aqueous solution under room temperature [2]. In this method, reduction rate of the metal species can be controlled under the room temperature, nevertheless ternary, or more, alloy nano materials with uniform and well crystallized structure was not synthesized until now.

　　Therefore, in this study, CIS solar cell was tried to construct by using Cu-In (CI) alloy precursors prepared in aqueous phase by our method.

     CI alloy nanoparticles were synthesized and size controlled by chemical reduction method under ambient conditions using metal chlorides as metal sources and sodium borohydride as a reducing agent. Synthesized CI nanoparticles (0.4 g ) was dispersed in 4 mL of various solution to make a sample dispersion. CI alloy nanoparticles dispersion was coated onto Mo-sputtered glass substrates by spin coat to make CI alloy precursor films. The desiccated thin films were selenized at temperatures ranging from 300 ºC to 575 ºC for 60min.

By obeying the calculation results and also electrochemical potential measurement, various copper complexes was successfully restricted to single species, and relationship between stability constants of complex reagents and reduction potential were summarized. As a results, CI nanoparticles can be successfully synthesized. XRD mesurments shows that synthesized materials were single phase of CuInSe alloy at selenization at 575 ºC. The CuInSe₂ thin film solar cells synthesized by this method exhibited a photovoltaic conversion efficiency of 0.5 % under AM 1.5 G illumination. However, many pinhole, which lead defective contact of electrode and causes a decrease in efficiency, was formed on the CuInSe₂thin film surface at this treatment condition. Therefore, in order to form pinhole-free films by spin coat, particle size of Cu-In ally nanoparticles precursor was controlled to have narrow particle size distribution. As a result, diameter of Cu-In particles become 30-70 nm. By using these size and distribution controlled CI nanoparticles, photovoltaic conversion efficiency was increased to 1.5 %. Other results will reported in our presentation.

This work has been supported by the Grant-in-Aid for Scientific Research (B) (No. 26281054).

 [1] Ye Seul Lim, et.,al, J. Phys. Chem. C, 2013, 117 (23), pp 11930–11940, [2] H. Takahashi et.,al, Applied Catalysis A: General 392 (2011) 80–85.