Electric field-induced second harmonic generation imaging of organic thin-film devices*

The application of an electric field is an effective mechanism for breaking the symmetry in otherwise symmetric molecular systems. Electric-field induced second harmonic generation (EFISHG) provides a tool for investigating chromophore orientation in nonlinear optical (NLO) materials and understanding transport at the semiconductor-dielectric interface in thin film transistors. A microscopic imaging platform in the reflection geometry has been developed for measuring second harmonic generation (SHG) and EFISHG signals from organic thin-film devices. The polymer poly(vinylidene fluoride) (PVDF), with its low glass transition temperature, provides a medium for orienting embedded NLO molecules under the application of external fields. Malonitrile based NLO chromophores were mixed with PVDF to form thin films. An in-plane electric field upwards of 6×10⁵ V/m yields a strong EFISHG signal. The SHG signal is seen to increase with the magnitude of the applied in-plane field and decreases upon the application of a vertical electric field. We further apply this technique to small molecule based organic transistors, where the application of a source-drain potential breaks the symmetry and allows the investigation of carrier transport using an EFISHG signal.