Iron-Complex Compounds for Solar Cells

The illustration shows a molecule with an iron atom at its centre, bound to 4 CN groups and a bipyridine molecule. The highest occupied iron orbital is shown as a green-red cloud. As soon as a cyan group is present, the outer iron orbitals are observed to delocalize so that electrons are also densely present around the nitrogen atoms. (Source: T. Splettstoesser, HZB)

A team at BESSY II in Berlin has inves­tigated how various iron-complex compounds process energy from incident light. They were able to show why certain compounds have the potential to convert light into electrical energy. The results are important for the development of organic solar cells. Transition-metal complexes have an element from the group of transition metals sitting in the centre. The outer electrons of the transition-metal atom are located in clover­leaf-like extended d-orbitals that can be easily influenced by external excitation. Some transition-metal complexes act as catalysts to acce­lerate certain chemical reactions, and others can even convert sunlight into elec­tricity. The well-known dye solar cell developed by Michael Graetzel (EPFL) in the 1990s is based on a ruthenium complex.

However, it has not yet been possible to replace the rare and expensive transition metal ruthenium with a less expensive element, such as iron. This is asto­nishing, because the same number of electrons is found on extended outer d-orbitals of iron. However, exci­tation with light from the visible region does not release long-lived charge carriers in most of the iron complex compounds inves­tigated so far. A team at BESSY II has now investigated this question in more detail. The group headed by Alexander Föhlisch has systema­tically irradiated different iron-complex compounds in solution using soft X-ray light.

They were able to measure how much energy of this light was absorbed by the molecules using resonant inelastic X-ray scat­tering. They investigated complexes in which the iron atom was surrounded either by bipyridine molecules or cyan groups (CN), as well as mixed forms in which the iron centre is bound to one bipyridine and four cyan groups each. The measurements showed that the mixed forms, which had hardly been inves­tigated so far, are parti­cularly interes­ting: in the case where iron is surrounded by three bipyridine molecules or six cyan groups (CN), optical excitation leads to only short-term release of charge carriers, or to none at all.

The situation changes only once two of the cyano groups are replaced by a bipyridine molecule. “Then we can see with the soft X-ray exci­tation how the iron 3d-orbitals delo­calize onto the cyan groups, while at the same time the bipyridine molecule can take up the charge carrier”, explains Raphael Jay, whose doctoral work is in this field. The results show that inex­pensive tran­sition-metal complexes could also be suitable for use in solar cells – if they are surrounded by suitable molecule groups. So there is still a rich field here for material develop­ment. (Source: HZB)

Reference: R. M. Jay et al.: The nature of frontier orbitals under systematic ligand exchange in (pseudo-)octahedral Fe(II) complexes, 20, 27745 (2018); DOI: 10.1039/C8CP04341H

Link: Methods and Instrumentation for Synchrotron Radiation Research, Helmholtz Zentrum Berlin für Materialien und Energie HZB, Berlin, Germany

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