Enhancing Fluorescence and Charge Transport in Disordered Organic Semiconductors

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Thomas, Tudor Huw 

High performance optoelectronic applications require simultaneously high mobility (μ) and high quantum efficiency of fluorescence (Φ). While this has been realised for organic small molecule semiconductors, applications such as high efficiency organic photovoltaics and bright organic light-emitting diodes towards electrically driven lasing are hampered by an apparent trade-off between μ and Φ in disordered systems. Recent reports of state-of-the-art device performance often optimise μ and Φ in disordered organic materials separately, and employ multi-layer architectures. In this work, we investigate materials in a class of donor-acceptor polymer materials; the indacenodithiophene-alt-benzothiadiazole family, which demonstrate high μ in spite of a low long-range structural order, to understand the interplay between these two important device figures-of-merit.

In the first section, we evaluate the effect of various tuneable parameters on μ and device performance in organic field-effect transistors. Using chemical modifications to the solubilising side chains, we observe that the substitution of bulky groups leads to a reduction of the hole mobility μh > 2 cm2/Vs to ~ 0.5 cm2/Vs in the benchmark polymer of this family, indacenodithiophene-alt-benzothiadiazole. Crystallographic and exciton-quenching based experiments confirm this observation is closely related to the degree of polymer backbone aggregation, and this leads to a different temperature evolution of the transport behaviour. In order to reliably improve μ in these systems, an elongation of the donor subunit is required. This increases the π-electron density on the donor, and can lead to an improvement in μ where the side chain density is decreasing. This chemical design, leading to a more highly aggregated structural motif is much more potent in determining μ, it seems, than design strategies to further improve the energetic disorder in the joint density of states and the potential barrier to torsion, which may be near optimised in these low-disorder systems.

In the second section, we unpick the precise relationship between the degree of aggregation apparently linking high μ to low Φ. With a prototype system, we compare the photophysics of two indacenodithiophene-alt-benzothiadiazole polymers differing by side chain bulkiness. Despite the aforementioned suppression of μ, we observe an improvement to Φ of <0.02 to ∼0.18 upon backbone separation. This derivative has the highest Φ reported for any polymer with μ exceeding that of amorphous-Si. However, the Φ in the more aggregated derivative is not limited by the formation of non-emissive excitons, but rather by an additional internal conversion pathway which is strongly temperature dependent, and mediated by Raman-active vibrations and close chain coupling. Extending this study, we analyse additional materials in this family with the highest Φμ values reported for conjugated polymers. We find that increasing the energy gap leads to an increase in Φ, and secondary emission pathways via weakly luminescent inter-chain charge transfer species. By solving the rate equations for exciton recombination, we use the radiative rate of inter-chain luminescence as a probe to show strong wavefunction mixing at close-contact points for some polymers, and suggest this as the origin for a superior μ in dithiopheneindenofluorene-alt-benzothiadiazole compared to indacenodithiophene-alt-benzothiadiazole. We demonstrate how low μ can be decoupled from the energy gap (Eg), and propose backbone elongation leading to increased inter-chain wavefunction overlap and a higher Eg as a design rule to increase Φ and μ together.

Finally, we assess the role of low-frequency vibrations in organic semiconductors displaying thermally activated delayed fluorescence (TADF). In the low-aggregation limit where Φ is maximised, we show that non-radiative triplet recombination is strongly related to low frequency torsional motion, and both are reduced in the presence of a rigid polymer host matrix for various TADF materials across different classes. However, we also explore the importance of rotational freedom in determining the oscillator strength, exchange energy, and spin-orbit coupling matrix elements which mediate luminescence in the absence of a rigid host. We demonstrate that suppressing dynamic motion is a powerful tool to modulate the photophysical properties of these emitters, and can lead to improved Φ particularly for low Eg emitters.

Sirringhaus, Henning
PLQE, Polymer, Luminescence, LED, OLED, OFET, Transistor, TADF, photoluminescence
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
iCASE EPSRC Studentship