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Addressing the Effects of Counterion Size on the Properties of Electrochemically Doped Conjugated Polymers

CP 114
Speaker(s) / Presenter(s):
Kyle Baustert

Conjugated polymers (CPs) are an emerging class of materials that are attractive for many applications due to their low cost, flexibility, and ease of processing. Both chemical and electrochemical doping of CPs is done to alter important properties such as their electrical conductivities and optical absorbances. This ability to tune their properties allows for their use in devices such as organic electrochemical transistors (OECTs) for biosensing and bioelectronics, electrochromic devices for color changing windows, as well as wearable thermoelectrics to use body heat to power wearable electronics. There are several factors that can influence the properties of doped CPs including doping level, film morphology, and the dopant or counterion used in the process. One disputed topic is what influence the ion size within the electrolyte has on the properties of electrochemically doped CPs. These ions balance the positive or negative charges on the backbones of doped CPs and their size can influence their interactions with the charge carriers and the overall morphologies of the films, therefore influencing their optical and electronic properties. This dissertation focuses primarily on understanding the influence of anion size on the properties of doped CPs as a function of their respective doping level. 

            The work presented here first focuses on the electrochemical doping ability of two benchmark CPs regioregular and regiorandom poly(3-hexylthiophene), rr- and rra-P3HT respectively, in electrolytes with anions of different sizes. Films of rr-P3HT are semicrystalline while those for rra-P3HT are amorphous allowing us to probe anion size as a function of CP morphology. Measured oxidation potentials and UV-vis data suggest that the larger anions are positioned further from the backbone of the polymer with two distinct polaron/bipolaron transitions apparent for rr-P3HT and only one for rra-P3HT. Further investigation into these materials as a function of doping level shows two distinct regimes that are important to consider. In the low doping regime, Coulombic interactions with the CP backbone and counterion largely contribute to the film’s properties with the larger anion having a higher electrical conductivity and lower Seebeck coefficient than the smaller and more Coulombically bound anion. Alternatively, in the high doping regime all charges are essentially “free” and CP morphology has the largest impact on film properties. In this case, the larger anion is more disruptive to the film morphology granting a lower electrical conductivity but higher Seebeck coefficient. At all measured doping levels, the films with the larger anions display higher thermoelectric power factors, proving that counterion size is an important consideration for these devices.