Impact of\nSequential Chemical Doping on the Thin Film\nMechanical Properties of Conjugated Polymers

Conjugated polymer (CP) films with nanometer-scale thickness\nexhibit\nunique properties distinct from their bulk counterparts, which is\nan important consideration for their end application as thin film\ndevices. In the realm of organic electronic devices, enabling high\nelectrical conductance properties of CPs often necessitates doping.\nHowever, the impact of doping on intrinsic polymer mechanical properties,\nsuch as the elastic modulus, in ultrathin films at device-relevant\nthicknesses is not well understood and has not been directly measured.\nIn this study, we quantified the effect of doping on the mechanical\nproperties of poly(3-alkylthiophenes) (P3ATs) using pseudofree-standing\ntensile testing. We observed modulation of the mechanical properties\nof ultrathin CP films through sequential doping of P3ATs thin films\n(60–80 nm thick) with the molecular dopant F4TCNQ. Our findings\nreveal that, despite the ease of doping all P3ATs with F4TCNQ, the\nresulting changes in mechanical properties are highly dependent on\nthe side-chain lengths of the P3ATs. Specifically, the elastic modulus\nof rubbery P3ATs with side-chain lengths of six carbons or more (e.g.,\nP3HT and P3OT) increases significantlyby one to two timesupon\nF4TCNQ doping, while the modulus of the glassy poly(3-butylthiophene-2,5-diyl)\n(P3BT) remains nearly unchanged. Such a phenomenon is linked to the\nchanges in the glass transition temperature (<i>T</i><sub>g</sub>) of the doped film, where the rise of <i>T</i><sub>g</sub> results in a large change in the modulus for P3HT samples.\nHowever, the P3BT remained in a glassy state before and after doping,\nexhibiting a minimal change in its mechanical properties. These insights\ninto the mechanical behavior of doped ultrathin CP films are crucial\nfor the design and optimization of flexible electronic devices.