‘Chemistry often appears in the news with a negative aftertaste. Wrongly! It is so much more than pfas, plastic pollution or the nitrogen crisis.” Tessel Bouwens (30) has just spent 45 minutes talking about her research into molecular machines in solar cells, and she really needs to get this off her chest. “Chemistry has mainly improved life, it has enormously developed nutrition and materials. And I think chemistry will also be very important in finding solutions to problems we are currently facing, such as the energy crisis.”
Bouwens is certainly committed to that. She received her PhD from the University of Amsterdam in September 2021 on a molecular ‘shuttle bus’ that ensures that electrons in an innovative type of solar cells arrive at their destination, thus making the solar cells more efficient. She is now working on similar ideas as a postdoc at the University of Cambridge, in the hope of getting artificial photosynthesis off the ground.
Dye-sensitized solar cells, as the innovative solar cells are called, consist of different materials than the silicon cells used in standard solar panels. A top layer of dye is the functional part. When sunlight shines on the pigment, electrons flow and electricity is generated. The solar cells can be made in all kinds of colors – Bouwens’ were orange – which makes them suitable for integration into buildings. They can also be transparent, making windows a possible surface for generating energy. Also nice: they already work in low light.
Electrons that do not move properly
“Unfortunately, the efficiency of these solar cells is much lower than that of silicon solar cells,” says Bouwens. “There are two types dye-sensitized solar cells, one achieves about 15 percent return and the other 2.5 percent. That’s really dramatic. So I focused on this type.”
The problem in this solar cell is that the electrons do not move in the right direction. Electrons have to move from A to B, but they often do not arrive at B. They turn around halfway through, go back to where they came from and produce no power.
“Many people, including my professor at the UvA, have been trying to solve this problem for some time,” says Bouwens. In the meantime, she had become interested in molecules that can move, so-called molecular machines, during a course she took for her master’s degree. “That’s how I came across rotaxanes, a molecular wire with a ring around it that can move along the wire. We need that for those solar cells, I thought, because we want to move those electrons from A to B. My professor also thought it was a good idea. I have applied for my own grant to conduct this research.”
Dissolved molecular rings
The solar cell that Bouwens designed consists of a glass plate with a semiconductor against which a layer of pigment molecules is attached to a molecular wire. Below this is a layer of liquid electrolyte in which the molecular rings are dissolved. Below this is the electrode to which the electrons from the pigment move via the molecular machine.
“The chemical properties of the molecules ensure that an electron likes to sit on a ring,” Bouwens explains. “And the ring wants to hang on the molecular thread that is attached to the pigment. They then form a supramolecular bond; the molecules do feel each other, but the bond is not that strong and can therefore easily come loose. That is also the intention. Once the ring is in place on the wire, the electron can hop over and the connection is then automatically broken, ‘firing’ the ring. The electron has then left place A. A new ring will be placed where the ring was. So there is no way back. The electron has to go to point B.”
“It was an ambitious idea, I had to know enough about both solar cells and molecular machines. Sometimes it was not possible to make a solar cell, sometimes it was not possible to get the pigment right. It wasn’t until the end of my PhD that everything came together. And then I still had to prove that it worked. We were unable to measure this properly at the UvA. Fortunately, the University of Twente had new equipment that made this possible. Without their help I would not have been able to provide the evidence.”
New grant applied for
There is more in store. “I still have many ideas that I want to implement. Ultimately, you can also use these solar cells for chemical synthesis, where a certain molecule is converted into another molecule. Plants do something similar in photosynthesis; under the influence of light they convert simple molecules into complex molecules. After my PhD, I applied for a new grant and I am now in Cambridge for two years with a research group that is working on this.”
“I really enjoy doing research. With chemistry you make something that no one has ever made before. But working in academia also involves difficult things, such as temporary grants. My grant here expires in six months, I am now in several places in Europe looking at how I can continue. I don’t know yet where I will live next year.”