Image from Energy
Environ. Sci., 2015, 8, 2634-2637 DOI: 10.1039/C5EE01204J
Photograph:
Reproduced by permission of Gerrit Boschloo and The Royal Society of Chemistry
(October 9, 2015) A
group of researchers at Uppsala University has discovered a "zombie solar
cell" that continues to generate electricity with unexpected effectiveness
although the liquid transferring charges between the electrodes has dried out.
The results were recently published in the journal Energy and Environmental
Science.
The discovery was made by Gerrit Boschloo’s group at the
Department of Chemistry-Ångström Laboratory, Division of Physical Chemistry.
When the researchers tested old dye-sensitized solar cells, also known as
Grätzel cells, these were still active, despite the fact that the electrolyte
conducting electricity between the minus and plus poles had evaporated.
“The dried-out solar cells worked in some cases even better
than when they were liquid-filled and alive. The power conversion efficiency of
specific cells had increased to 8 percent, which is a record for dye-sensitized
solar cells with a solid hole conductor. Our post-doc Marina Freitag who
produced and studied the solar cells named them ’zombie solar cells’ since they
were alive although they should be dead,” laughs Gerrit Boschloo.
In a Grätzel cell, an electrically conductive liquid
facilitates a flow of electrons with the use of substances that can give away
or take up electrons, a so called redox couple. But when this liquid dried out
in "zombie solar cell", a solid hole conducting structure was
created, continuing to transport positive charge.
However, this only occurs with certain copper based redox
couples. Gerrit Boschloo also points out that dye-sensitized solar cells with
solid hole conductors have been developed before, but that the high efficiency
of this "zombie cell" had taken the researchers by surprise. To
ensure the result, the project was repeated under controlled conditions.
“But it turned out to be quite difficult to produce the cell
in the way we usually produce solid state solar cells. The best option was to
instead make a liquid-based cell and letting it dry out slowly in order to
achieve the right structure.”