On Exciton Vibration and Exciton Photon Interactions in Organic Semiconductors

What are the physical mechanisms that underlie the efficient generation and transfer of energy at the nanoscale? Nature seems to know the answer to this question, having optimised the process of photosynthesis in plants over millions of years of evolution. It is conceivable that humans could mimic this process using synthetic materials, and organic semiconductors have attracted a lot of attention in this respect.

Once an organic semiconductor absorbs light, bound pairs of electrons with positively charged holes, termed `excitons', are formed. Excitons behave as fundamental energy carriers, hence understanding the physics behind their efficient generation and transfer is critical to realising the potential of organic semiconductors for light-harvesting and other applications, such as LEDs and transistors. However, this problem is extremely challenging since excitons can interact very strongly with photons. Moreover, simultaneously with the exciton motion, organic molecules canvibrate in hundreds of possible ways, having a very strong effect on energy transfer.

The description of these complex phenomena is often beyond the reach of standard quantum mechanical methods which rely on the assumption of weak interactions between excitons, photons and vibrations. In this thesis, Antonios Alvertis addresses this problem through the development and application of a variety of different theoretical methods to the description of these strong interactions, providing pedagogical explanations of the underlying physics. A comprehensive introduction to organic semiconductors is followed by a review of the background theory that is employed to approach the relevant research questions, and the theoretical results are presented in close connection with experiment, yielding valuable insights for experimentalists and theoreticians alike.

Description of out-of-equilibrium exciton dynamics beyond the Born-Oppenheimer approximation Rigorous comparison of theory and experiment interpretation Description of strong exciton-phonon coupling beyond perturbation theory

Auteur

Antonios Alvertis grew up in Athens, Greece, where he also studied for his undergraduate degree at the National and Kapodistrian University of Athens. In 2014 he was awarded a scholarship from the German Academic Exchange Service (DAAD) to study for a M.Sc. degree in Organic and Molecular Electronics at the TU Dresden. In 2016, after receiving a scholarship from the Engineering and Physical Sciences Research Council of the United Kingdom, he moved to the University of Cambridge to undertake an M.Phil. in Scientific Computing and subsequently a Ph.D. in Physics at the Cavendish Laboratory. During his time in Cambridge, he worked on solar-energy harvesting in organic semiconductors, and specifically towards achieving a better theoretical understanding of the properties of fundamental energy carriers called "excitons", and their interactions with light and the vibrational motion of these materials.

Antonios was officially awarded his Ph.D. in April of 2021, also receiving the Cavendish Ph.D. prize in Computational Physics in recognition of the impact of his research. At the time of writing, he is a postdoctoral researcher at the Cavendish Laboratory, and a visitor at UC Berkeley in the USA, having been awarded a research-exchange fellowship from the Cambridge-based Winton Programme for the Physics of Sustainability. His current research aims to contribute to a more unified understanding of the exciton physics of diverse materials beyond organic semiconductors, including low-dimensional systems.

When Antonios is not thinking about excitons, he can often be found reading one of the books of Jack London, Leo Tolstoy or Fyodor Dostoevsky. Sometimes he can also be spotted running along the river Cam, or attempting to bake a cake.

Contenu
Introduction.- Organic Semiconductors and Their Properties.- The Time-Dependent Quantum Mechanical Problem.- Modelling of the Electronic and Vibrational Structure.- First Principles Modelling of Exciton-photon Interactions.