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Optical studies on indicators of order and disorder in films and crytals of a bulky substituted polythiophene

Fanuel Mugwanga Keheze. Inauguraldissertation 2018

The optical and electronic properties of semiconducting polymers have attracted a lot of interest in both academic and industrial research. Over the past few decades, there has been a growing interest due to the attractive properties that polymeric materials offer for electronic applications and devices. Conjugated polymers are projected to be the next generation of optoelectronic materials with the advantages of portability, solution processability, flexibility, and cost- effectiveness. The greatest setback for the conjugated polymers has been the low charge mobilities and relatively large band gap that hinder their applications in electronic devices unlike their inorganic counterparts. To address this shortcoming, many studies have been done with the aim of understanding the correlation between morphology and electronic structure. Crystallization allows generating polymer chains with highly planarized backbones, embedded in structures exhibiting long-range order. Based on this, it is paramount to get a deeper understanding of relationships between the morphology of conjugated polymer, structural changes induced during crystallization, and the optoelectronic properties of the resultant structures. It has been established that the type of ordered structures depends on the processing conditions and the resulting morphology may exhibit variable amounts of grain boundaries, lattice disorder, and amorphous (disordered) regions. The simultaneous existence of both ordered and disordered regions within a sample makes it difficult to quantify and interpret the electronic behavior of such materials. In order to understand the correlation between ordered structures and the corresponding optical properties, we have studied the formation of spherulitic crystals of poly (3-(2, 5-dioctylphenyl) thiophene) (PDOPT) with bulky side groups prepared by isothermal crystallization, which enabled us to control the crystallization kinetics (by holding the temperature constant, we can control the transport of molecules to and from the growth front during crystallization). PDOPT was completely melted at a temperature far above the nominal melting temperature, followed by isothermal crystallization at temperature TC that was 12 - 20 °C below the nominal melting temperature. By applying the above procedure, at a lower degree of super cooling allowed for relatively slow rate of growth, promoting higher molecular order. In the present study, we used spatially resolved optical spectroscopy to quantify the differences in the degree of order in films and crystals of PDOPT. Firstly, localized spectroscopic measurements enabled us to spatially distinguish between disordered and ordered regions in films and spherulitic crystals and therefore to identify the contribution of these regions in absorption and emission spectra. A Gaussian multi-peak analysis of the emission spectra (at room temperature) exhibited that the peak positions (the energy of the emitted photons), were always about the same, independent of degree of order of the samples under investigation. However, the emission intensity changed significantly between as cast films and spherulitic crystals. Interestingly, for spherulitic crystals, the intensity of the 0-0 electronic transition peak reduced as compared to film, thus a tendency of the transition being less with increasing level of order. These relative changes in intensity of emission spectra between the samples provided a possibility to quantify the degree of structural order in the conjugated system. Furthermore, we compared absorption and photoluminescence (PL) measurements from large spherulitic crystals and the same region after being rapidly recrystallized from the molten state, which allowed identifying characteristic features of ordered and less ordered regions. In addition, based on temperature-dependent absorbance and PL measurements, we followed insitumelting and recrystallization processes, i.e., transitions between ordered and disordered phases. A multi-peak analysis of absorption and PL spectra based on a modified Franck-Condon progression showed changes in, e.g., the relative intensities of each peak, the excitonic bandwidth, and the vibronic energy as a function of temperature. Most importantly, at the phase transition temperature, a clear change in the positions of the peaks (i.e., their wavelengths, corresponding to the energy of the emitted photons) was detected. In particular, the relative absorption and PL intensities depended sensitively on the extent of order within the PDOPT samples. Furthermore, based on a comparison with calorimetric measurements, we have confirmed correlations between changes in the relative absorbance and PL intensities with variations in levels of order occurring during melting and recrystallization processes.

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