Solar Energy Is The Indeterminate Future

Sensitizers are dark-colored compounds that absorb light and convert its energy into electrical energy by releasing electrons and "injecting" them into the semiconductor. So far, the sensitizers used in dye-sensitized solar cells have either a relatively short lifespan or require the use of very rare and expensive metals. Therefore, the holy grail of photovoltaic research is the development of sensitizers that use iron-a metal that is both environmentally friendly and the most abundant transition metal on earth.

For many years, experts believe that iron compounds are not suitable for these applications because their excited state lifetime after absorbing light is too short to be used for energy production. About seven years ago, this situation changed with the discovery of a new type of iron compound with N-heterocyclic carbene (NHC).

A research team led by Professor Edwin Constable and Professor Catherine Housecroft of the Department of Chemistry at the University of Basel has been studying these compounds for many years. The team led by the project leader, Dr. Mariia Becker, is now reporting in the professional journal Dalton Transactions the results of their use of sensitizers based on the new NHC family.

A little vinegar and fat

"We knew that we had to develop materials that adhere to the surface of semiconductors, the properties of which also allowed to optimize the arrangement of functional light-absorbing components on the surface," Becker explained.

The researchers responded to these challenges in a two-pronged approach: First, they incorporated carboxylic acid groups (as found in vinegar) into iron compounds in order to bind them to the semiconductor surface. Second, they added long carbon chains to make the surface layer more fluid and easier to fix, thereby making the compound "greasy."

These dye-sensitized solar cell prototypes only achieved an overall efficiency of 1%, while the efficiency of today's commercial solar cells has reached about 20%. "Nevertheless, the results represent a milestone and will encourage further research on these new materials," Becker firmly believes.