We report a systematic time-dependent investigation of Cu(In-0.7, Ga-0.3)Se-2 nanocrystals (CIGSe NCs) by the hot-injection process at low reaction temperature. The reaction time was varied from 0 min to 16 h, to study the reaction mechanism and growth process of CIGSe NCs. Structural analysis showed the formation of beta-CuSe phase at 0 min and mixed phase of beta-CuSe and CIGSe, when the reaction time was between 5 min and 4 h. The size of NCs was increased from 3 to 38 nm as the reaction time increases. The composition of Cu/(In + Ga) was decreased while Ga/(In + Ga) increased as a function of reaction time. Based on the results, the possible formation mechanism of CIGSe NCs was proposed. The sequence of CIGSe formation is initiated from binary beta-CuSe and then the mixture of beta-CuSe and CIGSe, which subsequently transferred to single phase CIGSe by gradual incorporation of In3+ and Ga3+ ions into the beta-CuSe crystal lattice. Formation of single phase CIGSe with the optimum bandgap of 1.24 eV and targeted composition was obtained at 8 h. Additionally, thin films were prepared by drop casting of NCs ink. Single phase CIGSe NCs films showed significant improvement in the morphological, electrical and optoelectronic properties than other films, indicating their potential applicability in low-cost thin film solar cells. (C) 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.