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"Conjugated polymer nanoparticles for photovoltaic and bio-imaging applications"

Conjugated polymers are very interesting for many applications due to their outstanding optical and opto-electronic properties. However, most conjugated polymers are highly hydrophobic and need organic solvents to dissolve and process them. Using a large amount of these solvents for industrial processing, has a dramatic impact on the environment and is hazardous to human health. Therefore researchers are looking for green solvent alternatives that can be used to process conjugated polymers. A promising approach is the use of conjugated polymer nanoparticles (NPs) synthesized as water-based dispersions. Some research groups already reported on the synthesis of these conjugated polymer NPs. However, only limited literature is available on low bandgap conjugated polymer nanoparticles. Low bandgap polymers, a class of conjugated polymers having a bandgap below 2eV, often suffer from solubility issues and are sometimes not soluble in any of the low boiling solvents. This renders particle formation using these polymers quite tricky. Therefore, in this work, for the first time, a new synthesis method was developed for the fabrication of low bandgap polymer nanoparticles. This method takes the previously reported miniemulsion technique one step further, as a high boiling solvent can be used now for particle formation. The particles were obtained by slow evaporation of the solvent using temperatures (60 °C) well below the glass transition temperature of the polymers. The opto-electronic and photophysical properties of the applied conjugated polymers were studied in detail toward applications in organic solar cells on the one hand and bio-imaging on the other hand. Usually, when devices are made from these polymers, they are dissolved in an organic solvent in order to deposit the active layer. By providing conjugated polymers in the form of water-based nanoparticle dispersions, the morphology formation and layer deposition are separated into two different processes. In this way, large surfaces can be coated in an eco-friendly way. Due to their very interesting optical properties, such as high fluorescence brightness and good photostability, conjugated polymers are not only interesting for opto-electronic devices, but also for biomedical applications, including bio-imaging. Generally, these polymers cannot be used for this application as they only dissolve in (toxic) organic solvents. However, making these conjugated polymers in the form of nanoparticles dispersed in water offers a solution to this problem and gives these polymers the unique opportunity to be used in bio-imaging applications.

This PhD thesis consists of four different parts. The first two parts focus on the use of conjugated polymer nanoparticles for eco-friendly fabrication of organic solar cells, whereas the last two parts deal with conjugated polymer nanoparticles for bio-imaging....