Impact of Sequential Chemical Doping on the Thin Film Mechanical Properties of Conjugated Polymers

Conjugated polymer (CP) films with nanometer-scale thickness exhibit unique properties distinct from their bulk counterparts, which is an important consideration for their end application as thin film devices. In the realm of organic electronic devices, enabling high electrical conductance properties of CPs often necessitates doping. However, the impact of doping on intrinsic polymer mechanical properties, such as the elastic modulus, in ultrathin films at device-relevant thicknesses is not well understood and has not been directly measured. In this study, we quantified the effect of doping on the mechanical properties of poly(3-alkylthiophenes) (P3ATs) using pseudofree-standing tensile testing. We observed modulation of the mechanical properties of ultrathin CP films through sequential doping of P3ATs thin films (60-80 nm thick) with the molecular dopant F4TCNQ. Our findings reveal that, despite the ease of doping all P3ATs with F4TCNQ, the resulting changes in mechanical properties are highly dependent on the side-chain lengths of the P3ATs. Specifically, the elastic modulus of rubbery P3ATs with side-chain lengths of six carbons or more (e.g., P3HT and P3OT) increases significantly─by one to two times─upon F4TCNQ doping, while the modulus of the glassy poly(3-butylthiophene-2,5-diyl) (P3BT) remains nearly unchanged. Such a phenomenon is linked to the changes in the glass transition temperature (T_g) of the doped film, where the rise of T_g results in a large change in the modulus for P3HT samples. However, the P3BT remained in a glassy state before and after doping, exhibiting a minimal change in its mechanical properties. These insights into the mechanical behavior of doped ultrathin CP films are crucial for the design and optimization of flexible electronic devices.