Regensburg 2013 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 65: Poster: Organic electronics and photovoltaics (CPP; jointly with HL, O)

HL 65.17: Poster

Wednesday, March 13, 2013, 16:30–18:30, Poster C

Preferential orientation of molecular dipoles in amorphous vacuum deposited films studied by surface potential measurements — •Christian Weigel, Sami Hamwi, and Wolfgang Kowalsky — Institut fuer Hochfrequenztechnik, TU Braunschweig

Amorphous thin films of semiconducting small molecules are commonly used in organic optoelectronic devices to avoid detrimental effects of grain boundaries in polycrystalline thin films. Studies of surface potential development with film thickness have shown that sublimation of common OLED molecules such as Tris(8-hydroxyquinolinato) aluminum (Alq3) or 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBi) results in amorphous films with a preferential orientation of the molecular dipoles. Kelvin probe measurements provide a sensitive tool to detect a shift in the average molecular dipole orientation by as little as 5 degrees from the isotropic case. The detected shift in surface potential is characteristic for each molecules and is found in the order of several volts per 100 nm, comparable to operating voltages of organic electronic devices. The recorded increase in surface potential is largely independent of material or preparation of the substrate upon which the amorphous film is grown (barring the different interface dipoles between substrate and film), so that anisotropy at the organic film to vacuum interface drives the preferential orientation. Upon optical excitation, the measured potential vanishes, presumably due to mobile charges screening the dipole field.