PhD defence by Martin Cross

Supervisors

• Principal supervisor: Peter Uhd Jepsen, DTU Electro, Denmark
• Co-supervisor: Assistant Professor Edmund Kelleher, DTU Electro, Denmark

Assessment committee

• Professor Ole Bang, DTU Electro, Denmark (chair)
• Associate Professor Jeremy Johnson, Department of Chemistry and Biochemistry, Bingham University, Utah, USA.
• Professor, Research Director Eric Freysz, CNRS, University of Bordeaux, France

Master of the Ceremony

• Associate Professor Binbin Zhou, DTU Electro, Denmark

Abstract:

This project aims to uncover the dynamic properties of materials at terahertz (THz) frequencies, which operate on incredibly fast timescales measured in trillionths of a second, or picoseconds. These frequencies are particularly interesting as they encompass the fundamental motions of atomic nuclei within molecules and solids, including rotations and vibrations. These motions hold significant importance as they are closely linked to the properties exhibited by the materials themselves.

For instance, in novel perovskite-based photovoltaic (PV) materials used in solar panels, these fundamental nuclear motions are responsible for extending the lifespan of electricity carriers compared to traditional silicon-based PVs. This prolonged carrier lifetime has the potential to enable perovskite PVs to surpass the efficiency limits observed in silicon PVs, thereby maximizing the conversion of solar energy into electricity.

To achieve this breakthrough, a thorough understanding of how these fundamental nuclear motions lead to long-lived carriers is crucial. Consequently, the development of an instrument capable of probing such motions using terahertz-frequency light pulses has been undertaken. Significantly, this experimental setup employs two consecutive pulses to disentangle interconnected processes, such as the interactions between carriers and nuclei in perovskites.

The initial investigations focus on studying zinc telluride, a simpler material where two distinct processes have been successfully distinguished. Intriguingly, these processes are strongly coupled. In the first process, the terahertz pulses cause vibrations in the material, leading to the emission of new light. Subsequently, the generated light interacts with the vibrations once again, resulting in the production of a second type of light that was not initially possible.

By developing the tools to study the intricate nature of these coupled processes, this research holds the potential to uncover fundamental insights into the relationship between nuclear motions and carrier dynamics. Ultimately, such knowledge could pave the way for groundbreaking advancements in the design and optimization of solar cells, bringing us closer to surpassing existing efficiency limits and revolutionizing renewable energy conversion.