NOVELTY - Fabrication (M1) of coated nanoparticles involves obtaining core nanoparticles containing at least one elements from group IB and/or IIIA and/or VIA and coating the core nanoparticles with at least one layer of metal from group IB, IIIA or an element from group VIA in a controlled fashion such that the resulting coated nanoparticles have a desired stoichiometric ratio of elements.    USE - For forming ink, paste or paint (claimed) in solution-deposition of an absorber layer in photovoltaic cell.    ADVANTAGE - The method provides low-cost, highly tunable, reproducible and rapid synthesis of a nanoparticulate Cu-In and Cu-Ga material for use as an ink, paste, or paint in solution-deposited absorber layers for solar cells. Coating the nanoparticles allows for precisely tuned stoichiometry, and/or phase, and/or size, and/or shape, e.g., for Cu/In or Cu/Ga nanoparticles.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) forming (M2), an absorber layer for a photovoltaic device, involving obtaining core nanoparticles containing at least one element from group IB and/or IIIA; and coating the core nanoparticles with at least one layer of metal from group IB, IIIA or an element from group VIA in a controlled fashion such that the resulting film of coated nanoparticles have a desired stoichiometric ratio of elements forming an ink, paste or paint containing the nanoparticles; forming a thin film of the ink, paste or paint; annealing the thin film; and incorporating at least one element of group VIA into the film to form the absorber layer;    (2) composition of matter comprising a powder or solution containing coated nanoparticles made of core nanoparticles, coated with at least one layer containing at least one element of group IB, IIIA or VIA, where each core nanoparticle contains an element of group IB, IIIA or VIA, at least one of the layers contains an element is different from at least one of the group IB, IIIA or VIA elements in the core nanoparticle;    (3) coating (M3) nanoparticles, involving placing nanoparticles on a stage, vibrating the stage to fluidize the nanoparticles and coating the nanoparticles fluidized by vibration of the stage using atomic layer deposition; and    (4) an optoelectronic device comprising an electrode and a transparent electrode, an absorber layer disposed between the electrode and the transparent electrode, where the absorber layer contains elements of groups IB, IIIA and VIB; and a window layer disposed between the absorber layer and the transparent electrode, where the absorber layer is formed.