Internal electric fields in vacuum-evaporated organic films as studied by electroabsorption spectroscopy

Electroabsorption (EA) spectroscopy (Stark effect spectroscopy) has been applied to provide information on the internal electric fields occurring in thin films of organic compounds sandwiched between two semitransparent aluminium electrodes (Al/organic film/Al/glass substrate) under ambient conditions. A wide range of molecular materials, commonly used in organic light-emitting diodes, has been investigated. The analysis of the (1ω)EA signals measured on the first harmonic of the applied sinusoidal electric field frequency (ω) indicates a sample asymmetry and the presence of internal electric fields (Fi) in organic films with estimated values of Fi between 4 × 104 and 3 × 105 V/cm. The possible origin of internal electric fields is discussed. The observed features of Fi can be consistently explained by the asymmetrical distribution of space charge in the organic film associated with different defect (of structural and/or impurity origin) characteristics on the substrate and non-substrate side of the sample.