Karl Leo (born 10 July 1960 in Freiburg im Breisgau, Baden-Württemberg, Germany) is a German physicist.
Leo studied physics at the Albert-Ludwigs-Universität Freiburg and obtained the Diplomphysiker degree with a thesis on solar cells under supervision of Adolf Goetzberger at the Fraunhofer-Institut für Solare Energiesysteme. In 1986 he joined theMax-Planck-Institut für Festkörperforschung in Stuttgart for a PhD under the guidance of Hans Queisser.He then joined AT&T Bell Laboratories in Holmdel (New Jersey) as a postdoctoral research associate. In 1991 he joined the RWTH Aachen as an assistant professor and obtained the Habilitation degree. In 1993 he joined the Technische Universitaet Dresden as a professor of optoelectronics. Since 2002 he has been also with the Fraunhofer-Institut für Photonische Mikrosysteme, currently as director.
Leo works in the field of semiconductor optics and the physics of thin organic films. In 1992 he discovered Bloch oscillations in a semiconductor superlattice. His work on organic semiconductors led to Organic Light Emitting Diodes with the highest power efficiencies reported and to Organic Solar Cells with leading efficiency values.[1] In 2002 he won the Leibniz award, which is Germany's most prestigious scientific award.[2] In 2021, he was awarded the European Inventor Award in the 'Lifetime Achievement' category.[3]