TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The solution is formed in a hermetically-sealed reaction vessel and the forming step further comprises a degassing process to produce a first reactant. The degassing process comprises: holding the solution at a vacuum level not exceeding 0.05 bar and heating the solution to a first temperature sufficient to boil the organic solvent under the vacuum level; or then introducing an atmosphere comprising at least one of a noble gas, nitrogen gas or carbon dioxide gas into the reaction vessel to attain a pressure of at least 0.08 bar for at least two seconds. In the wet-chemical synthesis method, after the atmosphere is introduced into the reaction vessel to attain the pressure of at least 0.08 bar for at least two seconds, the degassing process further comprises repeating steps of holding the reaction vessel at the vacuum level not exceeding 0.05 bar and then reintroducing the atmosphere into the reaction vessel to attain a pressure of at least 0.08 bar for at least two seconds, until the first reactant forms. The wet-chemical synthesis method further comprises: heating the first reactant within the reaction vessel to greater than 200 degrees C and then introducing a sulfur solution into the reactor vessel to form a product solution, where the sulfur solution being introduced into the reaction vessel at a temperature below the temperature of the first reactant; washing the product solution in a solvent and then treating the product solution to an antisolvent to yield a dry precipitate comprising nanoparticles and less than 20 wt.% solvents; dispersing the nanoparticles in a solvent to form a nanoparticle solution; applying the nanoparticle solution to a substrate; drying the nanoparticle solution to form a nanoparticle coating on the substrate; then annealing the nanoparticle coating in an atmosphere containing a chalcogen to produce an absorber layer of the group I-III-VI2 semiconductor material; coiling the substrate into a roll after annealing; applying at least one buffer layer over the absorber layer, where the buffer layer is an amorphous n-type material having a higher bandgap than the absorber layer; applying at least one transparent electrically conductive layer over the absorber layer; and forming front electrically conductive contacts on the substrate. The annealing step comprises heating the substrate and the nanoparticle coating on it to a temperature of 200-800 degrees C for at least 4 minutes in an atmosphere at a pressure of greater than 0.05 bar, where the atmosphere containing elemental selenium vapor and/or at least one gaseous selenium compound. Preferred Components: The group I precursor is present in the solution at less than 100% of the stoichiometric ratio of the group I element in the group I-III-VI2 semiconductor material. The group III element comprises first and second group III elements, the first group III element is present in the solution at greater than 100% of the stoichiometric ratio of the first group III element in the group I-III-VI2 semiconductor material, and the second group III element is present in the solution at less than 110% of the stoichiometric ratio of the second group III element in the group I-III-VI2 semiconductor material. The group I element is copper. The group III element comprises first and second group III elements, where the first group III element is at least one comprising indium and zinc, and the second group III element is at least one comprising gallium and tin. The group I-III-VI2 semiconductor material is copper indium gallium selenide/sulfide. The organic solvent is at least one comprising alkylamine, fatty acid and trioctylphosphine. The group I and group III precursors are halide, chalcogenide, nitrate, sulfate, citrate, acetate, and acetylacetonate compounds of the group I and group III elements. The sulfur solution contains elemental sulfur dissolved in a solvent to achieve a sulfur concentration of 1-4 M and the sulfur solution introduces sulfur into the reaction vessel at at least 80% of stoichiometric for sulfur in the group 1-111-VI2 semiconductor material. The nanoparticle solution is applied to a surface region of the substrate comprising a contact layer, where the contact layer is an electrically and thermally conductive layer of a metal that forms an ohmic junction (105) with the absorber layer. The chalcogen is at least one of elemental tellurium, selenium or sulfur. The absorber layer consists of semiconducting photoabsorptive nanocrystals of the group I-III-VI2 semiconductor material. The substrate is a flexible substrate and dispensed from a roll prior to annealing. The transparent electrically-conductive layer is applied by photo-electroplating bath deposition while illuminated at an intensity of at least 5 lumen. The photovoltaic or opto-electronic device further comprises: a contact layer between the substrate and absorber layers, where the contact layer being an electrically and thermally conductive layer of a metal that forms an ohmic junction with the absorber layer; at least a first buffer layer overlying the absorber layer, where the first buffer layer consisting of an amorphous n-type material having a higher bandgap than the absorber layer; a second buffer layer overlying the first buffer layer, where the second buffer layer consisting of an intrinsic n-type material; a transparent electrically-conductive layer overlying the absorber layer; and front electrically conductive contacts electrically contacting the transparent electrically-conductive layer. The metal of the contact layer is molybdenum, tungsten, titanium or copper. The amorphous n-type material of the first buffer layer is cadmium, zinc, or tin sulfide. The intrinsic n-type material of the second buffer layer is a metal oxide. The nanocrystals are densely packed and each nanocrystal has a height that is greater than 2% of a thickness of the absorber layer. EXAMPLE - No suitable example given.