Enhancing Fluorescence and Charge Transport in Disordered Organic Semiconductors
High performance optoelectronic applications require simultaneously high mobility ($\mu$) and high quantum efficiency of fluorescence ($\Phi$). 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 $\mu$ and $\Phi$ in disordered systems. Recent reports of state-of-the-art device performance often optimise $\mu$ and $\Phi$ 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-$\textit{alt}$-benzothiadiazole family, which demonstrate high $\mu$ 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 $\mu$ 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 $\mu_h$ > 2 cm$^{2}$/Vs to ~ 0.5 cm$^{2}$/Vs in the benchmark polymer of this family, indacenodithiophene-$\textit{alt}$-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 $\mu$ in these systems, an elongation of the donor subunit is required. This increases the $\pi$-electron density on the donor, and can lead to an improvement in $\mu$ where the side chain density is decreasing. This chemical design, leading to a more highly aggregated structural motif is much more potent in determining $\mu$, 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 $\mu$ to low $\Phi$. With a prototype system, we compare the photophysics of two indacenodithiophene-$\textit{alt}$-benzothiadiazole polymers differing by side chain bulkiness. Despite the aforementioned suppression of $\mu$, we observe an improvement to $\Phi$ of $< 0.02$ to $\sim 0.18$ upon backbone separation. This derivative has the highest $\Phi$ reported for any polymer with $\mu$ exceeding that of amorphous-Si. However, the $\Phi$ 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 $\Phi \cdot \mu$ values reported for conjugated polymers. We find that increasing the energy gap leads to an increase in $\Phi$, 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 $\mu$ in dithiopheneindenofluorene-$\textit{alt}$-benzothiadiazole compared to indacenodithiophene-$\textit{alt}$-benzothiadiazole. We demonstrate how low $\mu$ can be decoupled from the energy gap ($E_g$), and propose backbone elongation leading to increased inter-chain wavefunction overlap and a higher $E_g$ as a design rule to increase $\Phi$ and $\mu$ 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 $\Phi$ 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 $\Phi$ particularly for low $E_g$ emitters.
PLQE, Polymer, Luminescence, LED, OLED, OFET, Transistor, TADF, photoluminescence
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