(642c) The Role of Side-Chain Polarity on Conductivity and Thermal Stability in Molecularly Doped Conjugated Polymers

The ability to achieve both high and thermally stable electronic conductivity in doped organic semiconductors is a derisible property for many optoelectronic and energy harvesting applications. Here, we investigate the electronic conductivity (σ) and the corresponding thermal stability of two polythiophene derivatives comprising oligoethylene glycol side-chains with an oxygen directly attached to the thiophene rings (P3MEET) and its analog P3MEEMT that has a methyl spacer between the oxygen and the thiophene rings (P3MEEMT). To modulate the electronic conductivity, thin films were vapor-doped with fluorinated-derivatives of tetracyanoquinodimethane (FnTCNQ, n = 4, 2, 1) to determine the role of dopant strength (electron affinity) on maximum achievable σ. The values of σ are within the same order of magnitude for all three fluorinated dopants while σ of P3MEEMT decreases significantly with the decreasing fluorination level of dopants. Specifically, when vapor doping with F4TCNQ, P3MEET exhibits substantially higher σ of 37.1 ± 10.1 S/cm compared to σ of 0.82 ± 0.06 S/cm for P3MEEMT. Structural characterization using a combination of X-ray and optical spectroscopies reveals that the higher degree of conformational order of polymer chains in the amorphous domain, which facilitates faster charge carrier mobility, is a major contributing factor for the higher σ of P3MEET. Lastly, vapor-doped P3MEET exhibited superior thermal stability compared to P3MEEMT, which showcases that in contrast to previous belief, the presence of polar side chains alone does not permit higher thermal stability. In fact, the description is more nuanced comprising multiple factors factor such as dopant-side chain interaction and side chain mobility in addition the role of the role oxygen atom near the thiophene rings controlling the ionization energy and subsequent stabilization of the hole charge carrier. Overall, the above summarized observations should be accounted for future design of conductive polymers with high thermally stability.

*Data from this presentation is published in Chem. Mater. 2021, 33, 2, 741–753 (https://doi.org/10.1021/acs.chemmater.0c04153). This work was supported by NSF DMREF Award number 1922259.