Pinning the energy levels at organic interfaces.

Organic electronic devices like light emitting diodes or photovoltaic cells comprise multiple thin layers of organic molecules or polymers with interfaces that facilitate different functionalities such as charge injection, transport, or charge separation/recombination. It then comes as no surprise that interface properties have been the subject of extensive research, with the alignment of energy levels at all-organic and organic-inorganic interfaces being the most prominent topic. In recent years we have developed a model for the formation of interface potential steps based upon electron transfer across interfaces between donors and acceptors and global charge equilibration across a multilayer. The model explains hitherto puzzling observations, such as why a potential step at a particular organic-organic interface sometimes seem to depend on the order of the individual layers in the multilayer, or why it depends on the substrate on which the multilayer is deposited.

The parameters in the model are obtained from first-principles calculations and the calculated potential profiles are in quantitative agreement with the results obtained from photo-electron spectroscopy. At metal-organic interfaces, two regimes can generally be distinguished. In the absence of electron transfer across the interface, the vacuum levels line up which yields the Schottky-Mott limit. If electron transfer occurs, the Fermi level is pinned by the organic layer at energy levels for holes or electrons that are characteristic of the particular organic material. Computational modeling provides a microscopic understanding of these regimes. For instance, ordering of molecules and their corresponding dipoles at the interface gives a displacement with respect to the Schottky-Mott limit.

Linda Lindell, Deniz Çakır, Geert Brocks, Mats Fahman, Slawomir Braun, Role of intrinsic molecular dipole in energy level alignment at organic interfaces, Applied Physics Letters 102, 223301 (2013).

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Figure 1. (a) Work function ORG|SUB of NTCDA (yellow) and Alq3 (bluegreen) layers as a function of the work function of the substrate SUB. The pinning levels for electrons in NTCDA (EICT) and for holes in Alq3 (EICT+) are indicated. The dashed line gives the Schottky-Mott limit of vacuum level alignment. The displacement of the Alq3 line with respect to this limit is interpreted in terms of an ordering of Alq3 molecules at the interface, leading to a dipole layer and a potential step, as shown schematically in (b).