Photovoltaics: Highly Relevant Basic Research

In their laser lab, Dominik Thiel and Phillip Greißel are investigating the interactions between molecules and light to enhance solar cell efficiency.

As we continue to rely heavily on fossil fuels like crude oil and coal for our energy needs, we are simultaneously increasing the levels of carbon dioxide in the atmosphere, contributing to global warming. To combat climate change, a shift in our energy production methods is essential. Photovoltaic systems, which convert sunlight into electricity, play a crucial role in this transition. Chemists Phillip Greißel and Dominik Thiel from Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) are dedicated to optimizing this process. Their research may pave the way for a new generation of solar cells that significantly outperform current models.

We spoke with the scientists about their work.

Mr. Greißel, Mr. Thiel, current solar cells convert at most one quarter of solar energy into electrical energy. What accounts for this inefficiency?

Dominik Thiel: There are several factors at play. A significant portion of the energy that hits the solar cell is lost as heat. To understand this, it’s essential to recognize that solar cells are composed of semiconducting materials like silicon, which typically have poor electrical conductivity. These materials lack freely movable charge carriers necessary for electricity flow, as the available electrons are primarily engaged in atomic bonds within the solar cell.

And this changes when light interacts with the cell?

Dominik Thiel: Exactly. When a photon strikes a semiconductor atom, it can excite one of the electrons. This excitation allows the electron to jump from the valence band to the conduction band, creating an electrical charge that can be harnessed to power devices. However, electrons need a specific minimum energy to make this jump across the band gap. If a photon doesn’t possess enough energy, it cannot effectively move the electron.

Is it true that the energy of light depends on its color?

Phillip Greißel: Yes, that’s correct. The energy of light varies with color: red light has less energy than yellow, while yellow has less energy than blue. For instance, if a solar cell requires yellow light to cross its band gap, red light will be ineffective because its energy is too low. More intense red light means more photons, but not more energy per photon. Thus, red photons do not contribute to electrical charge generation.

So, in this scenario, red photons are essentially wasted?

Dominik Thiel: Precisely. A blue photon, on the other hand, has enough energy to cross the band gap, but this excess energy is often lost as heat. Even if a high-energy photon could theoretically excite two electrons, it typically only excites one, with the remaining energy dissipated as heat. This energy loss limits the maximum efficiency of traditional solar cells to around 33 percent, with commercially available cells averaging around 22 percent. However, there are theoretical approaches that could potentially achieve efficiencies of up to 45 percent.

What do these approaches entail?

Phillip Greißel: That’s the focus of our research in Prof. Dr. Dirk Guldi’s group at the FAU Profile Center Solar. We aim to harness the surplus energy from high-energy photons to generate two free charge carriers instead of just one. This method, known as “singlet fission,” involves converting a high-energy singlet excited electronic state into two lower-energy excited states. Stabilizing these new excited states is vital for generating free charge carriers efficiently.

Which molecules have you investigated in this context?

Dominik Thiel: In our latest study, we utilized a hexamer, a compound made up of six identical molecules. This structure allows for the rapid achievement of two lower-energy excited states, while diffusion enables them to separate, increasing stability. Balancing the goals of rapid formation and high stability is crucial for effectively utilizing high-energy light to generate free charge carriers.

How do you determine which compounds are suitable for your research?

Dominik Thiel: We collaborate with researchers in theoretical chemistry who can predict the necessary characteristics for molecules to conduct singlet fission when exposed to specific light energies. Based on these insights, we work with organic chemistry teams to synthesize the molecules. Successful synthesis leads to testing and further optimization in collaboration with theorists.

Phillip Greißel: This interdisciplinary collaboration is truly exciting. It allows us to develop molecules with the properties we aim for, which is especially rewarding given the significance of finding sustainable energy solutions.

When can we expect to see a solar cell based on your compounds?

Phillip Greißel: It will not be available in the near future; we are still in the basic research phase. To create a functional solar cell, we would need to integrate our compounds with compatible semiconductors. Currently, our molecules work only with a few exotic semiconductor materials. However, we believe that the principles we are exploring could be adapted for use in traditional silicon solar cells. Some of our colleagues have already demonstrated that our singlet fission molecules can convert light into electrical energy in a laboratory setting, but there is still a long way to go before they are market-ready.

What are your next steps?

Dominik Thiel: I am nearing the end of my doctoral studies and contemplating my career path—whether to remain in academia or transition to the research and development sector in industry. I envision myself in a role that bridges both realms, possibly at a research institute closely linked with industry.

Phillip Greißel: I aim to stay in academia, particularly focused on renewable energy, but I’m also open to exploring other fields. Fortunately, chemistry is a diverse discipline, which I find particularly appealing. My current interest lies in understanding how molecules react to light, especially through the innovative laser optic methods we employ to examine our compounds.

For further information, please contact:
– Dominik Thiel, Chair of Physical Chemistry: dominik.dt.thiel@fau.de
– Phillip Greißel, Chair of Physical Chemistry: phillip.greissel@fau.de