An apparatus is described that converts electromagnetic radiation into electric current, of the type comprising a plurality of junctions between a first element (11; 111) of material with conduction through carriers of positive charge and a second element (12; 112) of a material with conduction through carriers of negative charge, said plurality of junctions defining a plurality of couples (10; 111, 112) comprising a hot junction (19) and a cold junction (18), said plurality of couples (10; 110) receiving a thermal flow (FT) generated by a radiation (R) incident in the direction of a region (172) including the hot junction (19), said apparatus (20; 30; 70; 170; 190) including a device (25; 75; 175) to guide said radiation (R) towards said plurality of couples (10; 111, 112). According to the invention, said first element (11; 111) of material with conduction through carriers of positive charge and/or said second element (12; 112) of material with conduction through carriers of negative charge comprise nanostructured materials and in that said device to guide said radiation (R) comprises a plurality of guiding elements (25; 75) that guide said radiation (R) onto faces (19; 39; 119) exposed to said radiation (R) of the respective first element (11) and second element (12) of the couples (10).