Illumination of a Conjugated Polymer with Visible Light to Control Crystallization and Enhance Photoluminescence

Due to their unique properties, conjugated polymers are interesting for scientific explorations and industrial applications. Polymeric semiconducting materials can be easily processed at low temperatures from solution. Thus, devices based on conjugated polymers can be cost-effective, allow for a high degree of mechanical flexibility and provide tunable optical and electrical properties. Absorption and emission of light as well as the mobility of charge carriers can be modified through chemical engineering of the monomeric units and the architecture of the polymer prior the processing of devices and films. Properties can be adjusted by synthesizing polymers with control of many parameters, including backbone units, side chains, monomers, molecular weight, and polymer configuration. Additionally, control of morphology and molecular order of the processed films and devices significantly affect optical and electronic properties of conjugated polymers. Emission and absorption behavior of conjugated polymers, including polythiophene-based polymers, has been shown to be influenced by the changes in molecular order (crystallinity) and chain conformations. In the here presented thesis, we present for two polythiophene-based polymers that illumination of conjugated polymers with light of appropriate wavelength can also affect properties like nucleation density and growth rate of spherulitic crystals. In addition, we demonstrate for a bulky substituted polythiophene crystallized under defined conditions that illumination also impacts the emission probability.

In this dissertation, we used optical microscopy to study the influence of illumination with white light on the crystallization behavior in films of molten poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT) and poly(3-hexylthiophene) (P3HT). These two polymers were used for the following reasons. PDOPT is a unique conjugated polymer with sterically demanding side groups, causing low melting and crystallization temperatures. Results are compared to a widely studied and used polymer, that is, P3HT. In both polymers, we observed that illumination induced a reduction in nucleation density and crystal growth rate. PDOPT was crystallized under illumination at different temperatures. Interestingly, the amount of reduction in nucleation density and crystal growth rate was more pronounced at increased light intensity. Melting samples previously crystallized under illumination and recrystallized in the dark showed full reversibility of the crystallization behavior. This observation confirmed that these illumination-induced changes in crystallization behavior were not permanent and were not caused, e.g., by degradation of the polymer. We tentatively suggest that absorption of photons by the polymer induced chain stiffening, possibly causing a reduction in polymer diffusivity, which, in turn, slowed down crystal growth rate and reduced the nucleation probability. Besides PDOPT and P3HT, we expect that many other conjugated polymers will show a similar impact of illumination on crystallization.

The second part is devoted to exploration of the impact of illumination on the emission probability of a conjugated polymer. To this end, we measured absorbance and photoluminescence (PL) spectra for films of PDOPT as a function of time (𝑡) of illumination at various temperature (𝑇). While having no detectable influence on the absorbance of this conjugated polymer, our experiments clearly demonstrated that illumination of PDOPT caused a substantial increase in PL intensity, that is, an increase in emission probability. Without illumination, we consistently observed a decrease in PL intensity with time we attributed to ageing effects and an improvement in crystalline order. An increase in PL intensity was only evident when the sample was illuminated. Interestingly, the changes in emission probability occur in time scale of minutes to hours which is much slower than the time scale of absorption and the emission of photons. The influence of illumination on changes in PL intensity was qualitatively similar for slowly and rapidly crystallized PDOPT, that is, the degree of crystallinity was not decisive for the observation. The rate of the increase in PL intensity depended clearly on the power of the illumination from the light source. As a function of the illumination time, the change in PL intensity was non-monotonic and depended on sample temperature. We speculate that changes in polymer interactions caused by excited electronic states might have induced slow changes in polymer conformations.

The observations presented in this thesis revealed the role of illumination by white light in controlling nucleation and growth rate of crystals in polythiophene-based conjugated polymers. Furthermore, we demonstrated that continuous illumination of PDOPT caused a substantial enhancement of the emission probability. We tentatively proposed that both phenomena are related to changes of chain conformations induced  by absorption of photons.