Probing Transient Electric Fields in Photoexcited Organic Semiconductor Thin Films and Interfaces by Time-Resolved Second Harmonic Generation

We have probed photoinduced charge separation dynamics in organic semiconductor thin films and at their interfaces by femtosecond time-resolved second harmonic generation (TR-SHG), using the model systems of fullerene (C70) thin films and copper phthalocyanine (CuPc)/C70 interfaces. In neat C70 thin films, the formation of an internal electric field on a ∼10-ps time scale following photoexcitation is attributed to the photo-Dember effect, namely, charge separation resulting from a gradient in excitation density and the differential electron/hole mobility. When an ultrathin film of the electron donor CuPc was deposited on top of the C70 film, an additional interfacial charge separation channel occurring on the ultrafast time scale of ∼0.1 ps was observed. We discuss how SHG fields from different origins can interfere to give the overall transient response.