Some materials scientists have been on a quest to build energy transformation and storage devices, such as solar cells, using Earth-abundant materials. These materials could help lower the costs of such devices.
Now, researchers in Sweden have designed an iron-based light-harvesting complex that collects solar energy and transmits it to semiconductor particles with exceptional efficiency (Nat. Chem. 2015, DOI: 10.1038/nchem.2365). Those processes are the first steps in producing electric power in the widely studied family of photovoltaic devices known as dye-sensitized solar cells (DSSCs).
DSSCs and closely related perovskite solar cells attract major research attention because they have the potential to help meet rapidly growing global energy needs by inexpensively tapping the nearly limitless power of the sun.
Shining sunlight on a DSSC excites electrons in a layer of light-absorbing molecules. These molecules, which are referred to as dyes or sensitizers, inject the excited electrons into semiconductor particles such as TiO2, to which the molecules are anchored. From there, the electrons migrate to an electrode to produce electric current.
Owing to their attractive electronic properties, ruthenium-based compounds, including ones with polypyridine ligands, are among the most successful sensitizers used in DSSCs. But ruthenium is relatively rare, expensive, and toxic. So researchers have been looking for suitable substitutes, especially ones based on iron, which is inexpensive and widely available.
But the iron compounds studied until now, which include Fe-polypyridines, suffer from other shortcomings. For example, the compounds produce low yields of excited electrons—referred to as injection yields—and the lifetimes of the excited states have been too short to inject the electrons into TiO2.
A team of researchers led by Villy Sundström and Kenneth Wärnmark of Lund University has come up with an iron-based N-heterocyclic carbene sensitizer that bypasses those problems. The complex has an excited-state lifetime that is roughly 1,000 times as long as those of Fe-polypyridyl complexes. The Fe(II) compound also boasts an injection yield of 92%. Ruthenium complexes have a yield near 100%.
As the team evaluated various ligands for the complex, they compared carboxy-functionalized and unfunctionalized forms of the complex and probed the compounds with a host of spectroscopy techniques. They found that the carboxyl groups substantially extended the lifetime of the excited state responsible for charge transfer—doubling it when the molecules were in solution and quadrupling it when the compounds were immobilized on a solid. The researchers also found that the complex injects the electrons from a longer-lived low energy state rather than from a shorter-lived high energy state.
The breakthrough in Fe(II) complexes will probably spark a renewed interest in molecular sensitizers, says Elena Galoppini, a Rutgers University chemist who studies functionalized semiconductors. “This result, which culminates the efforts by many groups toward tuning the excited-state properties of Fe(II) complexes, may finally lead to Earth-abundant sensitizers for solar devices.”