The surface potential (SP) undulation on the surfaces of tris(8-hydroxyquinolinato) aluminum (III) (Alq3) films has been investigated with Kelvin probe force microscopy (KFM) and scanning near-field optical microscope (SNOM)-KFM. The SP undulation observed on the amorphous Alq3 films with thicknesses of up to 300 nm showed a cloud-like morphology of 200–300 nm in lateral size. The temporal change of SP undulation was traced through cyclic measurement with KFM observation with intermittent photoexposure, as well as in situ localized photoexcitation with SNOM-KFM. We concluded that the origin of the SP undulation is the nonuniform distribution of charged traps and drift mobility in the Alq3 films.
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