Effect of Side-Chain Polarity on Conductivity and Thermal Stability in Molecularly Doped Polythiophene Polymers

The need for 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 such as dopant-side chain interaction and side chain mobility in addition to the role of oxygen atom near the thiophene rings controls the ionization energy and subsequent stabilization of the charge. Overall, the above summarized observations should be accounted for future design of conductive polymers with high thermal stability.