Electroabsorption Study of Internal Electric Fields in Vacuum-Evaporated Layers of Organic Materials

The Stark effect in opfical absorption of some organic compounds has been analyzed to yield information on the internal electric fields (IEF) occurring in their vacuum-evaporated thin Myers. IEF strengths in excess 10 5 V/cm have been observed with films of Epindolidione (EPI: C 18 H 12 N 2 O 2 ) and linear trans - Quinacridone (QAC: C 20 H 12 N 2 O 2 ) at room temperature. The lower values of IEF have been obtained for QAC, where, in contrast to EPI, the IEF are directed from the non-substrate to substrate side of the samples. No external field dependence of IEF was detected. However, the exact value of IEF depends on the preparation procedure of the layers. The hypothesis has been confirmed that formation of IEF involves a charge generation process on non-identified generation centres associated with defects showing an asymmetry in their characteristics on the substrate and non-substrate side of the samples.