A team at BESSY II 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. The study has now been published in the journal PCCP, and its illustration selected for the cover.
Transition-metal complexes – that is a cumbersome word for a class of molecules with important properties: An element from the group of transition metals sits 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.
Why not Iron?
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.
Image: 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.
Credit: T. Splettstoesser/HZB