Berlin 2012 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 38: [DS] Organic electronics and photovoltaics: electronic properties I (jointly with CPP, HL, O)

O 38.2: Vortrag

Mittwoch, 28. März 2012, 09:45–10:00, H 2032

Intermolecular hybridization governs molecular electrical doping — •Ingo Salzmann¹, Georg Heimel¹, Steffen Duhm², Martin Oehzelt³, Patrick Pingel⁴, Benjamin George³, Alexander Schnegg³, Klaus Lips³, Ralf-Peter Blum¹, Antje Vollmer³, and Norbert Koch^1,3 — ¹Humboldt Universität zu Berlin, Germany — ²Chiba University, Japan — ³Helmholtz Zentrum Berlin, Germany — ⁴Universität Potsdam, Germany

Strong molecular acceptors in organic semiconductor (OSC) films are typically used for molecular electrical p-type doping assuming electron transfer from OSC to dopant. Positive polarons in the fundamental OSC gap are thus expected and could be observable in ultraviolet photoelectron spectroscopy (UPS) as emission feature at, or close to the Fermi Energy (E_F) with reduced ionization energy (IE), which, however, has not been observed to date. We present a study on the prototypical OSC/p-dopant pair pentacene (PEN) and tetrafluoro-tetracyano-quinodimethane (F4-TCNQ) up to 1:1 ratio [1]. The IE of 1:1 mixed (amorphous) films is increased and larger than possible for pure PEN, while all states occur well below E_F in UPS. We suggest OSC/dopant frontier molecular orbital hybridization forming a doubly occupied bonding and an empty anti-bonding supramolecular hybrid orbital with reduced fundamental gap, which is in fact found by optical absorption measurements and corroborated by density-functional theory (DFT) calculations. Based on similar results for various OSCs, common doping-related observations are discussed within this framework. [1] I. Salzmann, G. Heimel et al., Phys. Rev. Lett., in press.