The Gerrit Boschloo’s group at the Department of Chemistry-Ångström Laboratory, Division of Physical Chemistry, Uppsala University, has just discovered "zombie solar cells". The so-called Grätzel cells, which are old dye-sensitised solar cells, 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%, 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.
Dye-sensitised solar cells work as follows:
The dye absorbs light and injects an electron into the TiO2. It gets an electron back from the hole conductor. Electrons in TiO2 are collected and perform work outside the cell. They return to the cell at the counter electrode, where they are taken up by the hole conductor.
However, this only occurs with certain copper based redox couples. Gerrit Boschloo also points out that dye-sensitised 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. However, the production of such solar cells, in comparison with the production of solid state solar cells, has been proved to be very difficult. Therefore, the best option was to make a liquid-based cell and, by letting it dry out slowly, achieve the best structure. The next challenge is to be able to seal these liquid cells properly and affordably, in order to avoid leakages and corrosions in the surrounding material, and reach proper stability.
Gerrit Boschloo envisages that this potential improvement in the development of photovoltaic technology could change the way photovoltaic modules are produced:
Gerrit Boschloo’s research group is collaborating with two chemistry groups at KTH since 2005. They are responsible for the custom-made dye which is part of the Grätzel solar cell. The collaboration also involves physicists from Uppsala University and experts in the field of industrial manufacturing from Swerea IVF. After the discovery just over a year ago, the researchers filed a patent application for the "zombie solar cell" through their own company Dyenamo.