Organic semiconductor materials are already being employed today in solar cells and organic LEDs (OLEDs) amongst others. Until now, however, little was known about how the doping molecules are integrated into the chemical structure of organic semiconductors. The Molecular Systems Joint Research Team of the Helmholtz-Zentrum Berlin and Humboldt-Universität zu Berlin at BESSY II have now analysed this with surprising results. The molecules are not necessarily uniformly dispersed in the host lattice, as it is usual with inorganic semiconductors, but instead form what are known as co-crystallites.