Aqueous Phase Synthesis of Cu-in-Ga Photovoltaic Nanoparticles for the CIGS Printable Solar Cell Application
Authors: Takahashi Hideyuki , Yokoyama Shun , Tohji Kazuyuki 
Published: February 2020

It is well known that CIGS (Cu(In,Ga)Se2) is one of the most effective photovoltaic devices for developing low cost and high efficiency solar cell. Bandgap of these can be adjusted from 1.04eV to 1.67eV by controlling the ratio of Ga in materials [1]. Moreover, it is ecological and economical solar cell since it needs only 1-2µm thickness because of its high optical absorption coefficient (105cm-1) [2]. Until now, CIGS solar cell, synthesized by utilizing gas phase method, achieved high conversion efficiency above 20%, nevertheless gas phase evaporation process consume high energy and natural resources. Therefore the cost of CIGS is still high and is not be widely used [3].
To solve this problem, we reported completely new approach to synthesize low cost CIGS solar cell [4]. This process consists of three steps; 1) synthesis of CI and/or CIG nanoparticles by utilizing aqueous phase reduction method, 2) print of these precursor nano-materials on a substrate, 3) selenization/sulfurization of these to form CIS/CIGS solar cell. As a result, CIS solar cell with the conversion efficiency of c.a. 3% was successfully synthesized. To increase the conversion efficiency, Ga should be incorporated into CI nanoparticles during aqueous phase synthesis procedure. However, co-reduction of Ga with Cu and/or In cannot be achieved until now, because of the reduction potential difference between Ga and Cu-In, and of low melting point of Ga (29.76℃).
Therefore, in this study, aqueous phase synthesis procedure of Cu-In-Ga nanoparticles were developed.
To reduce the Ga ion in aqueous phase, Ga complex species should be homogenized. So, appropriate complexing reagent, concentration of metals and/or complexing reagent, pH of solution were expected by utilizing calculation using critical stability constants. Reduction potential of homogenized Ga complex was measured by CV (cyclic voltammetry) analysis method. Co-deposition of Ga with Cu and In under expected condition were also progressed, and synthesized particles were evaluated by SEM-EDX, TEM-EDX and XRD.
Results of XRD and SEM/EDX measurement demonstrated that Ga can be doped into CI nanoparticles, and doping ratio of Ga into CI nanoparticles were depended on the reduction rate and also reduction method. For example, maximum Ga concentration in CIG nanoparticles were reached c.a. 20% when the molar ratio of reducing agent against to metals reached to ten. Moreover, these results shows that reduction procedure was seriously affected to the doping ratio of Ga.
Ratio of synthesized CIG particles were close to the target composition, by controlling solution condition and reaction field. In addition, large scale synthesis enough to make CIG precursor film were successfully achieved. By applying PVP as a dispersing agent, generation of aggregate particles was suppressed, and therefore smooth CIG film was successfully fabricated.
Another detailed results will be presented in our session.
[1] Appl. Phys. A, 74 (2002), pp. 659–664, [2] Basic technic of CIGS solar cell, Nakata, Nikkankougyoshinbunsha, 2010, [3] Phys. Status Solidi RRL, 9 (2015), pp. 28–31, [4] For example, 228th ECS meetings, Z01-1846 (2015)