NOVELTY - - A composition (C1) comprises: several discrete copper-tin-zinc-sulfur-selenium (CZTSSe) nanoparticles having specific chemical composition.    USE - As composition useful as nanoparticle ink solution, for forming a thin film coating on substrate; which is useful for fabricating a photovoltaic cell (claimed). The photovoltaic devices include e.g. all-inorganic solar cells, organic-inorganic hybrid solar cells, and photoelectrochemical solar cells.    ADVANTAGE - The composition and method provide synthesizing of multinary chalcogenide CZTSSe nanoparticles containing Cu, Zn, Sn, S, and Se, by simple and scalable synthesis. The compositions comprises several discrete CZTSSe nanoparticles having a chemical composition of the nanoparticles (I), where in contrast to In and Ga in prior art Cu(In,Ga)(S,Se)2 (CIGSSe) nanoparticles, Zn and Sn in CZTSSe have different valences i.e. Zn has valency of 2, and Sn has valency of 4; has specific atomic ratios of the atoms included; and provides non-stoichiometric CZTSSe nanoparticles. Non-stoichiometric CZTSSe nanoparticles may contain molar ratios of element(s) in excess or deficiency relative to a stoichiometric CZTSSe nanoparticle. Thus, a non-stoichiometric CZTSSe nanoparticle may be copper, zinc, tin, or chalcogen rich or poor. The CZTSSe nanoparticles prepared by the method (M2) have suitable optical properties, such as are in the form of thin films synthesized by the method (M2), and can be used as the light absorber in a photovoltaic device (preferably solar cell selected from all-inorganic solar cells (preferably thin film solar cells), organic-inorganic hybrid solar cells (containing mixture of CZTSSe nanoparticles with conducting polymers), and photoelectrochemical solar cells (comprising electrochemical solar cell of a CZTSSe thin film in polyelectrolyte solution).    DETAILED DESCRIPTION - A composition (C1) comprises: several discrete copper-tin-zinc-sulfur-selenium (CZTSSe) nanoparticles, having a chemical composition of formula Cu2ZnSn(SySe1-y)4 (I).    y=0-1.    INDEPENDENT CLAIMS are included for the following:    (1) method (M1) for synthesizing the CZTSSe nanoparticle composition (C1) involving: providing a first set of precursors comprising Cu, Zn, and Sn; providing a second set precursors comprising S, and/or Se; and reacting the first and second sets of precursors in solvent(s) under conditions sufficient for forming the several CZTSSe nanoparticles (I);    (2) method (M2) for forming a thin film formed from the CZTSSe nanoparticles, involving: depositing a layer of the CZTSSe particles (I) on a substrate, and annealing the layer of the particles to form a CZTSSe thin film;    (3) method (M3) for fabricating a photovoltaic cell involving: providing a conductive substrate; forming a thin film of the CZTSSe nanoparticles (I) by the method (M2) on the substrate; and forming a top electrode on the substrate, where at least one of the conductive substrate and top electrode is transparent; and    (4) a composition (C2) comprising: several discrete multinary IB-IIB/IIA-IVA-VIA nanoparticles.