Two new materials Tor the light-emitting layer in organic light emitting devices, N'N'Bis(2,6-xylyl)perylene-3,4:9,10-bis (dicarboximide) (DPP-PTCDI), and N,N'-Dimethylperylene-3,4:9,10-bis(dicarboximide) (DMe-PTCDI) art compared by an spectroscopic and opto-electronic study. Measurements of the absorbance-spectra, the I-V-characteristic, photovoltage- and electroluminescence-spectra were done. Both materials could be identified as n-type materials. DPP-PTCDI is much more efficient as a light emitting material than DMe-PTCDI due to a hampered dimerization for the DPP-PTCDI-molecules. The influence of the size of the functional group on the aggregation and therewith on the quantum-efficiency for fluorescence and electroluminescence is discussed.