Regensburg 2013 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 54: Organic Electronics and Photovoltaics (jointly with CPP, DS, HL)

O 54.4: Vortrag

Mittwoch, 13. März 2013, 16:45–17:00, H33

Overcoming the limitations of work-function modifications induced by adsorption of self-assembled monolayers — •Oliver T. Hofmann, Yong Xu, Patrick Rinke, and Matthias Scheffler — Fritz-Haber Institut der MPG, Berlin, Germany

Controlling the work function of electrodes critically determines charge-injection barriers and is of high importance for organic electronic devices. Such control can be easily achieved by adsorbing dipolar self-assembled monolayers. Despite the common application of this method, its limitations remain largely unexplored. It has, however, been demonstrated that charge-transfer occurs as soon as the molecular LUMO comes into resonance with the Fermi-energy when the molecular dipole moment is systematically increased by adding multiple repeat units. This Fermi-level pinning limits the achievable work-function modification. In turn, we argue that molecules with negative electron affinities never reach this limit and can reduce the work-function in principle all the way down to zero. As a proof of concept, we study the interaction between the ZnO(1010) surface and pyridine using hybrid density functional theory with a variable fraction of exact exchange and a correction scheme for screened van-der-Waals forces. In agreement with experimental observations, we find an adsorption-induced work-function reduction of up to -2.9 eV. For a hypothetical ultra-dense pyridine monolayer the work-function reduction could even reach -4.3 eV for this surface.