Abstract
We reveal the rather complex interplay of contact-induced re-orientation and interfacial electronic structure – in the presence of Fermi-level pinning – at prototypical molecular heterojunctions comprising copper phthalocyanine (H16CuPc) and its perfluorinated analogue (F16CuPc), by employing ultraviolet photoelectron and X-ray absorption spectroscopy. For both layer sequences, we find that Fermi-level (EF) pinning of the first layer on the conductive polymer substrate modifies the work function encountered by the second layer such that it also becomes EF-pinned, however, at the interface towards the first molecular layer. This results in a charge transfer accompanied by a sheet charge density at the organic/organic interface. While molecules in the bulk of the films exhibit upright orientation, contact formation at the heterojunction results in an interfacial bilayer with lying and co-facial orientation. This interfacial layer is not EF-pinned, but provides for an additional density of states at the interface that is not present in the bulk. With reliable knowledge of the organic heterojunction’s electronic structure we can explain the poor performance of these in photovoltaic cells as well as their valuable function as charge generation layer in electronic devices.
Highlights
We reveal the rather complex interplay of contact-induced re-orientation and interfacial electronic structure – in the presence of Fermi-level pinning – at prototypical molecular heterojunctions comprising copper phthalocyanine (H16CuPc) and its perfluorinated analogue (F16CuPc), by employing ultraviolet photoelectron and X-ray absorption spectroscopy
Starting from the simplest approach to predict the energy levels at electrode/organic and organic/organic heterojunctions, i.e., a constant electrostatic potential throughout the structure – often referred to as vacuum level alignment (Fig. 1a), it rapidly transpired that this situation is rather an exception at real interfaces[1,2,3,4]
For ordered molecular assemblies the molecular orientation with respect to the substrate is important as the ionization energy and electron affinity are orientation-dependent parameters[15]
Summary
For this analysis, only standing molecules need to be involved. The present analysis of the interface between H16CuPc and F16CuPc was motivated to better understand the behaviour of photovoltaic cells containing these materials as donor and acceptor, and their function as charge generation layer. The HOMO-LUMO level gap between the standing molecules of the two Pc’s is only 0.45 eV; even with the lying molecule bilayer as additional barrier the tunnelling process occurs in the reported devices[37]. The reliable correlation of interface morphology and structure with the resulting electronic properties is the necessary starting point for deriving comprehensive structure-property-function relationships
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