Converter device for converting energy from electromagnetic radiation, in particular concentrated solar energy, in electrical power, comprising a thermionic emitter separated by an inter-electrode space from a thermionic collector, the thermionic emitter and the thermionic collector being provided with electrical connection means (109, 110, 113, 114, 500) configured to be connectable to a first external electrical load for supplying electrical power, the converter device being characterised in that it further comprises an absorber of electromagnetic radiation, configured to transform electromagnetic radiation energy to thermal energy, having an outer surface configured to be exposed to electromagnetic radiation and an inner surface integrally coupled to the thermionic emitter, the outer surface being provided with a sub-micrometer periodic surface structure, the thermionic emitter being monolithically integrated on said inner surface of the absorber, the absorber being made of a material selected from the group comprising or consisting of: - ceramic materials selected from the group comprising or consisting of: hafnium carbide (HfC)-based ceramic material comprising molybdenum silicide (MoSi 2) in a volume percentage ranging from 2% to 40%, silicon carbide (SiC)-based ceramic materials comprising molybdenum silicide in a volume percentage ranging from 20% to 40%, aluminium nitride -based ceramic materials containing additives and further containing a volume percentage of silicon carbide ranging from 5% to 25% and a volume percentage of molybdenum silicide ranging from 20% to 40%, tantalum carbide (TaC)-based ceramic material comprising molybdenum silicide (M O Si 2) in a volume percentage ranging from 2% to 40%, the thermionic emitter being made of a material selected from the group comprising or consisting of: - thin film diamond of thickness lower than 25 micrometers deposited through chemical vapour deposition, - thin film diamond of thickness lowerthan 25 micrometers deposited through CVD having a n-type doping, - thin film diamond of thickness lowerthan 25 micrometers deposited through CVD having surface chemical termination with monoatomic hydrogen and/or depositions of zirconium and/or caesium, - thin film diamond of thickness lowerthan 25 micrometers deposited through CVD having a n-type doping and having surface chemical termination with monoatomic hydrogen and/or depositions of zirconium and/or caesium, - thin film ceramic materials of thickness lower than 25 micrometers selected from the group comprising or consisting of: titanium nitride (TiN)-based ceramic materials, molybdenum silicide-based ceramic materials, carbide-based ceramic materials, boride-based ceramic materials, - refractory metals in thin film form with thickness not larger than 1 micrometer. - ceramic materials selected from the group comprising or consisting of: hafnium carbide (HfC)-based ceramic material comprising molybdenum silicide (MoSi 2) in a volume percentage ranging from 2% to 40%, silicon carbide (SiC)-based ceramic materials comprising molybdenum silicide in a volume percentage ranging from 20% to 40%, aluminium nitride -based ceramic materials containing additives and further containing a volume percentage of silicon carbide ranging from 5% to 25% and a volume percentage of molybdenum silicide ranging from 20% to 40%, tantalum carbide (TaC)-based ceramic material comprising molybdenum silicide (M O Si 2) in a volume percentage ranging from 2% to 40%, hafnium carbide (HfC)-based ceramic material comprising molybdenum silicide (MoSi 2) in a volume percentage ranging from 2% to 40%, silicon carbide (SiC)-based ceramic materials comprising molybdenum silicide in a volume percentage ranging from 20% to 40%, aluminium nitride -based ceramic materials containing additives and further containing a volume percentage of silicon carbide ranging from 5% to 25% and a volume percentage of molybdenum silicide ranging from 20% to 40%, tantalum carbide (TaC)-based ceramic material comprising molybdenum silicide (M O Si 2) in a volume percentage ranging from 2% to 40%, - thin film diamond of thickness lower than 25 micrometers deposited through chemical vapour deposition, - thin film diamond of thickness lowerthan 25 micrometers deposited through CVD having a n-type doping, - thin film diamond of thickness lowerthan 25 micrometers deposited through CVD having surface chemical termination with monoatomic hydrogen and/or depositions of zirconium and/or caesium, - thin film diamond of thickness lowerthan 25 micrometers deposited through CVD having a n-type doping and having surface chemical termination with monoatomic hydrogen and/or depositions of zirconium and/or caesium, - thin film ceramic materials of thickness lower than 25 micrometers selected from the group comprising or consisting of: titanium nitride (TiN)-based ceramic materials, molybdenum silicide-based ceramic materials, carbide-based ceramic materials, boride-based ceramic materials, titanium nitride (TiN)-based ceramic materials, molybdenum silicide-based ceramic materials, carbide-based ceramic materials, boride-based ceramic materials, - refractory metals in thin film form with thickness not larger than 1 micrometer.