news of the week OCTOBER 19, 2015 EDITED BY CRAIG BETTENHAUSEN & MITCH ANDRÉ GARCIA
IRON COMPLEX HARVESTS SOLAR ENERGY
tutes, 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 PHOTOVOLTAICS: New sensitizer “This result, which yields—and the lifetimes of the excitcould lead to low-cost dyeculminates the efforts ed states have been too short to inject sensitized solar cells by many groups toward the electrons into TiO2. tuning the excited A team of researchers led by Villy Sundström and Kenneth Wärnmark state properties of OME MATERIALS SCIENTISTS have been of Lund University has come up with Fe(II) complexes, may on a quest to build energy transformation and an iron-based N-heterocyclic carbene finally lead to Earthstorage devices, such as solar cells, using Earthsensitizer that bypasses those probabundant sensitizers abundant materials. These materials could help lower lems. The complex has an excitedfor solar devices.” the costs of such devices. state lifetime that is roughly 1,000 –ELENA GALOPPINI, RUTGERS UNIVERSITY Now, researchers in Sweden have designed an irontimes as long as those of Fe-polypyribased light-harvesting complex that collects solar endyl complexes. The Fe(II) compound ergy and transmits it to semiconductor particles with also boasts an injection yield of 92%. Ruthenium comexceptional efficiency (Nat. Chem. 2015, DOI: 10.1038/ plexes have a yield near 100%. nchem.2365). Those processes are the first steps in proAs the team evaluated various ligands for the comducing electric power in the widely plex, they compared carboxystudied family of photovoltaic functionalized and unfunctionaldevices known as dye-sensitized ized forms of the complex and Sunlight CO2H solar cells (DSSCs). probed the compounds with a DSSCs and closely related host of spectroscopy techniques. perovskite solar cells attract maThey found that the carboxyl jor research attention because groups substantially extended N N N CH3 they have the potential to help the lifetime of the excited state N H 3C N Fe meet rapidly growing global enresponsible for charge transfer— N CH H 3C N 3 ergy needs by inexpensively tapdoubling it when the molecules N N N ping the nearly limitless power of were in solution and quadrupling the sun. it when the compounds were Shining sunlight on a DSSC immobilized on a solid. The e– excites electrons in a layer of researchers also found that the O O light-absorbing molecules. These complex injects the electrons molecules, which are referred to from a longer-lived low energy as dyes or sensitizers, inject the state rather than from a shorterexcited electrons into semiconlived high energy state. ductor particles such as TiO2, to The breakthrough in Fe(II) TiO2 which the molecules are anchored. complexes will probably spark From there, the electrons migrate a renewed interest in molecular to an electrode to produce electric sensitizers, says Elena Galoppini, current. a Rutgers University chemist Owing to their attractive elecwho studies functionalized tronic properties, rutheniumsemiconductors. “This result, A SHOT OF ELECTRONS based compounds, including ones which culminates the efforts Absorption of sunlight by this with polypyridine ligands, are by many groups toward tuning Fe-heterocyclic sensitizer among the most successful sensithe excited-state properties of complex causes excited tizers used in DSSCs. But rutheFe(II) complexes, may finally electrons to be injected nium is relatively rare, expensive, lead to Earth-abundant sensitizefciently into attached TiO2 and toxic. So researchers have ers for solar devices.”—MITCH particles. been looking for suitable substiJACOBY KENNETH WÄRNMARK/LUND U
S
CEN.ACS.ORG
5
OCTOBER 19, 2015
