NOVELTY - Preparation of copper-indium-selenium (Cu-In-Se) i.e. CIS compound involves: producing several first composite particles having an indium selenide outer layer physically coupled to at least a part of a copper selenide seed particle surface or several second composite particles having a copper selenide outer layer physically coupled to at least a part of an indium selenide seed particle surface; and preparing CIS compound by thermally treating composite particles selected from the first composite particles and/or the second composite particles.    USE - In the preparation of copper-indium-selenium (Cu-In-Se) i.e. CIS compound useful for preparing CIS compound thin film; in composite particle; in solar cells (claimed) and other electronic devices; as an absorption layer of solar cells.    ADVANTAGE - The method prevents loss of selenium, which inevitably requires selenium environment and improves dispersion, coupling and reaction uniformity for the formation of CIS compound. The CIS compound prepared by the method has crystal size of 0.5-1.5 microns which is greatly increased in comparison to the conventional art. The solar cell containing the CIS compound and having absorption layer with great crystal size shows excellent efficiency.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) method (M1) for preparing CIS (Cu-In-Se) compound thin film involving: a) producing several first composite particles having an indium selenide outer layer physically coupled to at least a part of a copper selenide seed particle surface or several second composite particles having a copper selenide outer layer physically coupled to at least a part of an indium selenide seed particle surface; b) coating a paste including the composite particles on a substrate; and c) forming a thin film composed of a CIS compound by thermally treating the resultant material of the step (b);    (2) a composite particle comprising: a copper selenide seed particle; and an indium selenide outer layer physically coupled to at least a part of a surface of the copper selenide seed particle;    (3) a composite particle comprising: an indium selenide seed particle; and a copper selenide outer layer physically coupled to at least a part of a surface of the indium selenide seed particle; and    (4) a solar cell (10) in which a substrate (11), a conductive layer (12), an absorption layer (13), a buffer layer (14) and a transparent window layer (15) are subsequently formed, where the absorption layer is the CIS compound thin film prepared by the method (M1).    DESCRIPTION OF DRAWING(S) - The figure shows schematic sectional view showing solar cell to which a solar cell absorption layer is applied.    solar cell (10)    substrate (11)    conductive layer (12)    p-type absorption layer (13)    n-type buffer layer (14)    n-type transparent window layer (15)    finger patterns. (16)