October 9, 2015

"Zombie solar cells" discovered at Uppsala University

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.”

journal reference >>