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Formation and Optical Characterization of Single Crystals of Poly(3-Hexylthiophene) (P3HT), a Model Conjugated Polymer

John O. Agumba. Inaugural-Dissertation zur Erlangung des Doktorgrads der Fakultät für Mathematik und Physik der Universität Freiburg, 2016

Conjugated polymers pose the next generation of conducting materials with the advantages of portability, solution processability, flexibility and costeffectiveness. The optoelectronic energy gaps and the charge carrier mobilities are the key metrics by which these materials are gauged and they are the decisive factors that determine their performance in devices such as organic photovoltaics, organic field effect transistors and organic light emitting diodes among others. On one hand, given the reported impact of the microstructure on the named metrics, controlled ordering of molecules within this special class of polymeric materials assumes paramount importance. On the other hand, the precise knowledge on how these metrics are affected by external stimuli (operating environment) is desirable.

This thesis addresses on one hand the fundamental investigation of the correlations between the morphology and microstructure formation approaches of a model conjugated polymer, poly (3-hexylthiophene) (P3HT) and the corresponding optical features of the resultant structures at ambient temperature. On the other hand, temperature  dependence of optical properties of P3HT single crystals grown controllably by the self self-seeding technique have been mapped in order to unravel new insights into the role of crystallization processes on thermal and optical properties of the crystalline objects in comparison to their unordered counterparts. More specifically, different crystallization approaches have been used and the optical properties of the product crystals studied using optical microscopy, UV-Vis absorption spectroscopy and photoluminescence spectroscopy. By comparing the spectral line shapes from the well-ordered crystals to those of the unordered bulk, solutions and thin films, the role of the adopted crystal formation methods (crystallization) on their optical properties have then been systematically investigated both at room temperature and at elevated temperatures.

We report a strong correlation between the crystal processing conditions, ranging from solvent type, polymer-substrate interaction, temperature and concentration on the crystal microstructure, and the corresponding optical properties. At room temperature, crystals self-seeded from solutions and thin films have shown high birefringence anisotropy and appearance of absorption bands with peaks at a wavelength of ca. 675nm that are much red-shifted with respect to the spectra from solution and spin cast film samples, suggesting a close structural-optical correlation. Interestingly, single crystals have shown clear reversibility both in birefringence properties between 30°C and 270°C and in UV-Vis absorption and photoluminescence (PL) spectra between 30°C and ca. 290°C , the latter being a temperature range much higher than that measured for thin films (ca. 220°C) and solutions (ca. 80°C). Intriguingly, maximum absorption and PL peaks from the crystals still remained red-shifted as compared to that of thin films even at 290°C confirming the memory of crystalline order and a proof that the crystals do not melt even at such high temperatures. Interestigly, temperature dependent PL spectra measurements have provided feasible means to elucidate the nature of the emissive species and the melt transitions in P3HT single crystals in which multiple vibrational replicas yielding two emitting species between 30°C and 220°C and three emitting species between 240°C and 290°C have been unveiled.

The possibly lowered energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO gap) as witnessed in the red-shift, i.e. Δλ ≈ 75nm (wavelength of the peak position at ca. 675 nm) of their lowest energy absorption peak make the crystals ideal for use in low band gap devices. Furthermore, their temperature dependent behavior confirms that they are thermally and optically stable structures that are effective for use in high temperature range devices such as temperature sensors. From the investigations as reported in this thesis, we can deduce that the here studied approaches of the crystals growth based on external stimuli such as temperature can be used to tune the optical properties (optical band gap) of the conjugated polymer crystals.

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