Structure-Property Interrelations of Polymers for Photonics and Electronics

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Balzer, Alex
Stingelin, Natalie
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In the first chapter of this thesis, the production and characterization of inorganic-organic hybrid materials is discussed. Inorganic-organic hybrids allow for the modification of physical properties and the addition of new features in polymer-based systems, and thus have received great interest. This thesis demonstrates that titanium oxide hydrates are highly versatile inorganic ‘fillers’ in polymer-based hybrids. Using the hydrate allows for the creation of versatile, low-temperature, sol-gel processed hybrids. This thesis shows that these hydrates increase the refractive index of hybrid materials with no or minimal optical loss due to the formation of a covalently crosslinked network. The high transparency in the visible and near-infrared range also proves useful for producing low-loss distributed Bragg Reflectors (DBRs) to aid and improve light and heat management. In particular, a visible range reflecting DBR can improve efficiencies of semi-transparent solar cells while maintaining transparency to chlorophyll absorption bands. This allows for charge generation in greenhouse applications. Additionally, shifting the stopband of the DBRs to the near-infrared range can reduce energy consumption in buildings by reflecting the heat from solar irradiation while maintaining visible transparency. Ultimately, we show a general understanding for this hybrid’s unique structure which can be expanded to new hybrids based on other metal oxide hydrates. In the second chapter of this thesis, structure-property interrelations of semiconducting polymers are established using fast scanning calorimetry. The field of polymer-based electronics has witnessed major developments in the past few years that have led to systems of vastly improved charge transport- and energy-harvesting properties. This progress can be predominantly attributed to synthetic efforts in the form of the creation of new materials, which often comprise backbones of a significantly more rigid nature than the first-generation polymer semiconductors and most bulk commodity plastics. Moreover, many semiconducting polymers frequently lack significant long-range order, but it is hypothesized that they may exhibit liquid-crystalline-like behavior because of their hairy-rod nature. To understand the polymer phase behavior, how it relates to chemical design and how it dictates important optoelectronic features, we use fast scanning calorimetry to identify the glass transition and possible liquid-crystalline-like transitions, as well as side-chain softening regimes, using physical aging signatures and isothermal annealing measurements. We show that the intrinsic UV-stability of semiconducting polymers can be tracked through increases of glass transitions through crosslinking. Additionally, the side-chain ordering and liquid-crystalline ordering of hairy-rod polymers are sensitive to backbone planarity and molecular weight, which affect optoelectronic and mechanical properties. The glass transition temperature for propylenedioxythiophene-based polymers is also shown to be affected by side-chain lengths, but independent of polarity and sterics. This approach allows us to gain insights on the role of polymer assembly and solid-state structure on energetic disorder and photophysical characteristics towards the delivery of important structure-property interrelations for design of fourth generation semiconducting plastics for organic solar cells, plastic electronics, wearable sensors, and beyond.
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