"Zombie solar cells" promise innovation for photovoltaic technology

“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:

 “The final purpose is of course energy production in the form of solar cells on the roof. We know this works for solid state photovoltaics. Grätzel solar cells work very well even if the light source is not optimal - you can acquire kilowatt hours even if it’s cloudy. To get there is however a long road, it takes a lot more research. But if we can make these ‘zombie solar cells’ very affordable, you can imagine them almost anywhere, for example charging mobiles or running computers.”

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.