Exciton Dynamics in Perfluoropentacene Single Crystals
The realization of the first bipolar junction transistor in the year 1948 by Bardeen, Brattain and Shockley  sparked off the semiconductor industry, which gradually revolutionized the way we live. Nowadays, semiconductors are the fundamental building blocks of every high-tech electronic device, m...
Optical properties of bulk materials and thin films (for optical properties related to materials treatment, see 81.40.Tv; for optical materials, see 42.70-a; for optical properties of superconductors, see 74.25.Gs; for optical properties of rocks and minerals, see 91.60.Mk)
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|The realization of the first bipolar junction transistor in the year 1948 by Bardeen, Brattain and Shockley  sparked off the semiconductor industry, which gradually revolutionized the way we live. Nowadays, semiconductors are the fundamental building blocks of every high-tech electronic device, most notably the computer which has become an inescapable part of our daily lives. Besides voltage and current control capabilities, semiconductors exhibit intriguing opto-electronic properties; the best known and commercially most successful applications are light emitting diodes (LEDs), laser diodes, charged coupled devices (CCD) and solar cells.[2, 3, 4] Due to the broad variety of material systems, they cover virtually the complete optical spectrum while simultaneously being cost-efficient and easy to miniaturize.
Until the late 90ies, commercially available devices were exclusively based on inorganic semiconductors, primarily on Silicon. However, over the last decade, the class of organic semiconductors has gained an increasing amount of interest, e.g., now one of the most popular smartphone’s display1 is based on OLED2-technology. Flexibility upon stress and deeper color contrasts are typically named as their main advantages over conventional liquid crystal displays (LCD).
While organic semiconductor devices are already well established as light emitters, they are still in research state as light harvesters. In general, organic solar cells offer high photon cross sections in combination with similar flexibility as OLED displays. Additionally, they exhibit the potential for low-cost mass-production, including innovative and versatile procedures such as ink-jet printing.[6, 7] However, two major challenges still exist which need to be addressed before organic solar cells become compatible: the long-term stability and the quantum efficiency. The fast degradation of organic solar cells is caused by oxidation, reduction and thermal instabilities. Research in this field focuses on the synthesis of new organic molecules, thus, it can be assigned to the organic chemistry sector. Quantum efficiencies are determined by the microscopic photon to carrier conversion, i.e., the photovoltaic effect, therefore, it is predominantly a research topic of solid state physics.
This thesis focuses mainly on aspects of the quantum efficiency in the polyacene Perfluoropentacene (PFP) and its underlying decay processes, namely the electronic relaxation dynamics after optical excitation. In particular, the process of singlet exciton fission is analyzed which promises to double the quantum efficiencies, as it converts one singlet exciton into two triplet excitons. Excitons are correlated electron and hole pairs: neutral excitations of the crystal after absorption of a photon. Singlet exciton fission was first proposed in 1968 in order to explain the drastic photoluminescence quench of Tetracene crystals compared to Anthracene crystals. It has gained renewed attention lately, due to its potential application in the growing field of organic solar cells. However, the microscopic understanding is still in its infancy which hampers essential progress in this field; for instance, the influence of the geometrical order of the molecules within the crystal on singlet exciton fission has only been analyzed theoretically. The reason is the lack of single crystal samples allowing for the correlation of molecular packing and electronic dynamics. This issue is resolved in Chapter 5 for the model system of PFP single crystals, where for the first time the singlet exciton fission dynamics are observed along the three crystal axes by polarization-resolved pump-probe spectroscopy. Moreover, the efficient coupling direction is identified as well as the preceding electronic species of the two triplet excitons.
Although spectroscopic analysis on polyacenes date back to the 40ies , lack of computational power and interest lead to the sad state that even interpretations of the linear absorption are still debated today. However, basic knowledge of the linear absorption is essential in order to interpret the non-linear dynamics. Therefore, Chapter 4 serves as a precursor, where the linear absorption of the PFP samples is interpreted using phenomenological models. Here, first indications are given for a dominant coupling direction within the PFP crystal which are then confirmed in Chapter 5. Furthermore, the amount of exciton splitting in PFP is determined, also known as the Davydov-splitting. It is induced by dipole coupling between the two basis molecules of the crystal lattice during excitation.
In Chapter 6 the focus is shifted to inorganic semiconductors. The chapter introduces a fast and convenient method to determine dephasing times of induced coherent exciton polarizations with more precision than a common lineshape analysis of the absorption spectrum. In pump-probe spectroscopy, the transients of the coherent oscillations are exploited to serve as phase indicators for the several excitonic transitions. These transients are observed during the coherent regime before pump and probe pulses perfectly overlap in time. As a proof of principle, the methodology is applied to a set of Germanium quantum well samples and evaluated in respect to their optical quality. In addition, the main dephasing mechanism in Germanium quantum wells is identified.
These three chapters capture the results of the thesis and are preceded by introductory chapters covering basic light-matter interactions and experimental details; they are
succeeded by a conclusion chapter summarizing the essential findings.