Synthesis of CIGS(Cu(In,Ga)Se2) Photovoltaic Materials in an Aqueous Solution for the Printable Solar Cell Application
Authors: Takagi Masaki , Yokoyama Shun , Takahashi Hideyuki , Tohji Kazuyuki 
Published: February 2020

Cu(In,Ga)Se2(CIGS) is one of the most effective photovoltaic devices for developing low cost and high efficiency solar cell. Bandgap of CIGS can be adjusted in the range from 1.04eV(Cu1.0In1.0Ga0.0Se2.0) to 1.67eV(Cu1.0In0.0Ga1.0Se2.0) by controlling the ratio of Ga in CIGS, while ideal band gap for solar cell is 1.40eV [1]. Moreover, CIGS solar cell needs only 1-2µm thickness, since it has a large optical absorption coefficient (105cm-1) [2]. Until now, CIGS solar cell, synthesized by utilizing gas phase method, achieved high conversion efficiency above 20% [3]. It is well known that gas phase evaporation process produces high quality films, however, this process needs high energy consumption and large amount loss of natural resources.
Therefore, we reported completely new approach to synthesis low cost CIGS solar cell [4]. This process consists of three steps; 1) fabrication of CI and/or CIG nanoparticles by utilizing aqueous phase reduction method (low energy and resources consumption method), 2) print of these nanoparticles on a substrate, 3) selenization 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, CIGS with the bandgap of 1.40eV should be developed. Thus, Ga should be incorporated into nanoparticles during aqueous phase CI 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, to synthesize Cu-In-Ga nanoparticles in an aqueous solution, relationship between the condition of Ga complexes in an aqueous phase and its reduction potentials was evaluated.
Results of calculation shows that Ga complexes in the Ga-Cl-OH-aspartic acid system can be restricted to homogenized species, [(Ga3+)(Asp2-)] (pH5) and [(Ga3+)(OH-)4] (pH9), by controlling the pH and concentration of complex reagent. Reduction potential of these species became apparent from cyclic voltammetry (CV) measurement. However, it also became clear that Ga complexes cannot be reduced by reducing agent (NaBH4), under Ga unary system. On the other hand, in a Cu-In-Ga ternary system, it became apparent that Ga can be co-reduced with Cu and/or In, and Ga incorporation ratio was varied by controlling pH and reaction temperature. SEM-EDX, XRD and TEM-EDX results expected that Ga was incorporated in Cu2In to form Cu2In0.95Ga0.25.
In order to produce CIGS solar cells, it is necessary to synthesize CIG nanoparticles with the Ga/In+Ga ratio over 30%, while synthesized CIG nanoparticles (Cu2.0In0.95Ga0.25 phase and/or Cu1.0In1.1Ga0.022 phase) has the Ga/In+Ga ratio under 12%. So it is needed to increase the Ga incorporation ratio. To increase the Ga incorporation ratio, concentration of reducing agent (NaBH4) and Ga complex were selected as parameters, because these parameters have a potential of accelerating the reaction of Ga reduction process.
The ratio of Ga/In+Ga in CIG particles increased to 14% with decreasing the concentration of Cu,In,Ga ion to 0.059M and with increasing the concentration of NaBH4 to 0.91M (i.e. the ratio of (NaBH4 [M]) /(Cu,In,Ga ion [M]) was 40 times larger than the method mentioned). On the other hand, the ratio of Ga/In+Ga in CIG particles increased to 18% with increasing the concentration of Ga ion to 0.68mM and decreasing the concentration of In ion to 0.80mM (i.e. the ratio of (Ga ion [mM])/(In+Ga ion [mM]) in aqueous solution was 2.9 times larger than above method). From these results, it became clear that the ratio of (Ga ion [mM])/(In+Ga ion [mM]) in aqueous solution was seriously affected to the ratio of Ga/In+Ga ratio in CIG particles. Another results will be released 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) This work has been supported by the Grant-in-Aid for Scientific Research (B) (No. 26281054).