by Nikolaus Hößle

Mr. Grätzel, you have developed the solar cell type dye-sensitized solar cell so decisively that one speaks of the Grätzel cell. When did you start to deal with dye-sensitized solar cells? What was the landscape of solar energy research like at this time and how did it change in recent years?

We started our research on dye solar cells in the eighties. Solar energy research was still in its early stages at this time. Since then it has developed rapidly and became an important part of future renewable energy sources.

Some researchers have tried to imitate photosynthesis in the laboratory as a model for electricity generation. How did you experience your breakthrough and what did you do differently?

I had already been interested in photosynthesis as a student and when the first gasoline crisis broke out in the 70s and it was only allowed to refuel at some days of the week, I started thinking about how much oil reserves actually were there. What can be used as a substitute for oil? Hydrogen was a dream product, because hydrogen supplies electrical energy. When it is burned in a fuel cell, the only byproduct is water. I was therefore particularly interested in photocatalytic hydrogen formation. This was at a time when a Japanese group was studying water splitting via photoelectrochemical cells by using titanium dioxide. The conversion efficiency of sunlight was low because titanium dioxide only absorbs in the UV range and sunlight contains very few UV photons. Attempts have been made to increase the yield by sensitizing titanium dioxide electrodes with dyes. However, these also resulted in only very low conversion efficiencies.  

At that time we had embarked on a new research path. We were the first group to investigate charge transfer processes on colloidal titanium dioxide nanoparticles. Those particles disperse in water, producing a transparent solution. The processes triggered by light excitation of the particles could be studied very well with laser spectroscopy. We then applied dye to the surface of the particles and analyzed how this sensitization process works in detail. We discovered that the light-induced charge separation of such colloidal particles lasts very long. We have made use of this effect for photovoltaics by coating an electrically conductive glass plate, which acted as a current collector, with the particles. This was a completely new concept, a new idea that actually emerged from basic research. If you had conceptualized it that way before, you would probably have said: this cannot work! But it worked so well that we could increase efficiency by many orders of magnitude. That was the most surprising thing at that time. This was our breakthrough in the 1980s.

When the German solar industry was in a crisis in 2012, did you also experience that? Or did that not affect you at all as a researcher in dye-sensitized solar cells? Can a branch of research in a country like Germany persist despite the lack of industry?

Yes, of course we felt the effects of it, both during and after the subprime crisis in America in 2012. The PV industry collapsed, e.g. all silicon companies in Germany collapsed. This was a very difficult time for us, too. But we survived. We were able to show that our cells are very efficient in diffuse daylight, for example, and that they can be produced in a simple way. This also attracted the industry, which at that time started with the production of flexible cells.   

From our visit at your laboratory I know that dye solar cells are not in competition with silicon cells. Can you explain the most important differences in applications and the potential of dye solar cells?

While silicon cells are mainly used on roofs and solar farms, the application of dye cells is in the field of energy glasses, i.e. glass elements that generate electrical current under exposure to light, and the supply of energy for electronic devices by converting ambient light. Efficiencies of over 30% are achieved here.

Why are your solar cells so effective even in diffuse light? It is often claimed that the efficiency is the HP of the solar cells. But how important is it really, especially for the dye cell?

It’s because we can precisely match the spectral sensitivity of the cell to the wavelength distribution of the diffuse light by selecting the dye. Of course, efficiency plays an important role. But often the aesthetic aspects are also important for the customer, for example when you think of energy glasses.

How does your future vision of the dye-sensitized solar cell look like?

We are quite optimistic on this regard. Research is progressing well and the dye cell by now has found markets where it is able to compete. Next, applications will focus on the power-giving glass elements (energy glasses) and the use of ambient light to supply energy to electronic systems.

Which hopes, wishes and expectations do you have for a university project with the title ‘the power of… where design meets solar energy’?

I see a lot of possibilities in terms of design. In Denmark, for example, there was an architect who integrated our solar cell into a tabletop. And we also have a design school here in Lausanne (ECAL). There was a competition that set the task: what do you do with the dye solar cell? The students were asked to develop creative ideas on how to use the dye cells in a wide range of applications. Some of these projects were then given awards and there was also an exhibition with the winning projects touring around the world. That was about 10 years ago. It was still a bit rudimentary at that time, but it was an excellent approach. You can now take advantage of much more experience.

Photo: Roland Herzog

Michael Grätzel is a Swiss chemist and professor at the École Polytechnique Fédérale in Lausanne. He is the inventor and namesake of the famous “Grätzel cell”, a solar cell using organic dyes which absorb light energy and exceeds the efficiency of silicon cells in diffuse light. Michael Grätzel has received numerous awards and prizes and ranks among the most cited scientists in the world.