The present invention relates to a method for preparing an organic semiconductor layer in a vacuum chamber at a pressure of 10−5 to 10−9 mbar comprising a step of sublimating a composition from a single vacuum thermal evaporation source arranged in the vacuum chamber, wherein the composition comprises a physical mixture of (a) a first organic aromatic matrix compound having a molecular weight ≥400 and ≤1,000: and (b) a first alkali organic complex having a molecular weight of ≥100 and ≤400.