Preparation of monolithic Cu(In_0.7Ga_0.3)Se ₂ nanopowders and subsequent fabrication of sintered CIGS films

Cu(In,Ga)Se₂ (CIGS) is a compound semiconductor and is one of the most attractive light-absorbing materials for use in thin film solar cells. Among the various approaches to prepare CIGS thin films, the powder process offers an extremely simple and materials-efficient method. Here, we report the mechano-chemical preparation of CIGS compound powders suitable for fabrication of CIGS films by a powder process. We found that the CIGS phase was formed from the elemental powders of Cu, In, and Se and liquid Ga using high energy milling process with a milling time as short as 40 min at 200 rpm due to a self-accelerating exothermic reaction. The morphology and size of the CIGS powders changed with a function of the milling speed (100-300 rpm), leading to an optimal condition of milling at 200 rpm for 120 min. We also found that it was difficult to obtain a monolithic phase of the CIGS powders without severe particle aggregation by mechano-chemical milling alone. Therefore, in combination with the milling, subsequent heat-treatment at 300 °C was performed, which successfully provided monolithic CIGS nanopowders suitable for powder process. When a thin film was fabricated from the monolithic CIGS nanopowders, a highly dense film with large crystalline grains was obtained. The CIGS film preserved its chemical composition of CuIn_0.7Ga _0.3Se₂ after sintering as evidenced by Raman spectroscopy, EDS and SAED pattern of transmission electron microscopy. The film was also found suitable for a light absorbing layer of CIGS solar cells with its band gap energy of 1.14 eV evaluated by transmittance spectroscopy.