On Exciton–Vibration and Exciton–Photon Interactions in Organic Semiconductors (Springer Theses)

✍ Scribed by Antonios M. Alvertis

• Publisher: Springer
• Year: 2021
• Tongue: English
• Leaves: 213
• Category: Library

✦ Synopsis

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.

✦ Table of Contents

Supervisor’s Foreword
References
Abstract
Acknowledgements
Contents
1 Introduction
References
Part I Background
2 Organic Semiconductors and Their Properties
2.1 The Chemistry of Carbon and -conjugation
2.2 The Exciton State
2.2.1 Formation of Bound Electron-Hole Pairs
2.2.2 Exciton Spin
2.2.3 The Role of Spin for Exciton-Photon Interactions
2.2.4 Singlet Exciton Fission
2.3 Vibrational Motion in Organic Semiconductors
2.4 Differences and Similarities to Inorganic Semiconductors
References
3 The Time-Dependent Quantum Mechanical Problem
3.1 Density Matrix Formalism
3.2 The System and Its Environment
3.3 Approximate Quantum Master Equations
3.4 Discussion of the Approximations
References
4 Modelling of the Electronic and Vibrational Structure
4.1 The Many-Body Hamiltonian and the Born-Oppenheimer Approximation
4.2 Ground State Properties: Density Functional Theory (DFT)
4.2.1 Fundamentals
4.2.2 Approximations Within DFT
4.2.3 Applications
4.3 Excited State Properties: Model Hamiltonians and the PPP Method
4.4 Excited State Properties: Many-Body Perturbation Theory
4.4.1 Green's Functions and the Dyson Equation
4.4.2 Quasiparticles: The GW Approximation
4.4.3 Excitons: The Bethe-Salpeter Equation
References
Part II Results
5 First Principles Modelling of Exciton-Photon Interactions
5.1 Introduction
5.2 Theoretical Background
5.3 Application to Disorder-Free PDA
5.4 Static Disorder: The Effect of Finite Chain Length
5.5 Dynamic Disorder: The Effect of Vibrations
5.6 Conclusions
References
6 Impact of Exciton Delocalisation on Exciton-Vibration Interactions
6.1 Introduction
6.2 Methodology
6.3 Exciton-Phonon Coupling
6.4 Exciton Temperature Dependence
6.5 The Effect of Nuclear Quantum Fluctuations on Exciton Energies
6.6 Exciton Pressure Dependence
6.7 Conclusions
References
7 Interplay of Vibrational Relaxation and Charge Transfer
7.1 Introduction
7.2 Methodology
7.2.1 Tensor Networks
7.2.2 Studied System
7.3 Results
7.3.1 Band-Edge Excitation
7.3.2 Incoherent Excitation
7.3.3 Coherent Excitation
7.4 Conclusions
References
8 Molecular Movie of Ultrafast Singlet Exciton Fission
8.1 Introduction
8.2 Methodology
8.3 Results
8.3.1 Vibrationally Coherent Singlet Fission
8.3.2 Identification of Transferred Coherence
8.3.3 Simulation of Full Quantum Dynamics
8.3.4 Benchmarking Against Vibronic Spectroscopy
8.3.5 Character of Coupling and Tuning Modes
8.3.6 Coordinated Interplay of Coupling and Tuning Modes
8.4 Conclusions
References
9 Controlling the Coherent Versus Incoherent Character of Singlet Fission
9.1 Introduction
9.2 Molecular Structure and Coupling
9.3 Exciton States and State Mixing
9.4 Multiple Emissive Species
9.5 CT-Mediated Singlet Fission
9.6 Singlet Fission Yield Variation
9.7 Modelling of Vibrational and Solvent Effects
9.8 Coherent and Incoherent Singlet Fission
9.9 Conclusions
References
10 Conclusions and Outlook
References