The present invention provides a thin compact crystallized layer with large grains of an absorber material essentially consisting of Cu, Zn and Sn sulphide(s), preferably CZTS, with less defects and preferably with improved composition homogeneity and/or reduced content of secondary phases, by producing a method for double annealing, in determined atmospheres, of thin layers of particles of a so-called absorber material based on copper, zinc and tin sulphide, preferably on CZTS, deposited on a substrate covered with molybdenum (Mo), said thin annealed absorber layer deposited on said Mo substrate imparting improved photovoltaic performances to a photovoltaic device which comprises them.