Implications of Annealing Temperature and Dopant Size on Structure and Electrochemical Properties of PBTTT

Conducting polymers (CPs) are used in a variety of electronic applications, including as bio-compatible transistors, platforms for neuromorphic computing, and as electrode materials for energy harvesting and storage. Unlike molecular counterparts, the electronic properties of CPs are intimately tied to the ordering and microstructure of the polymer itself. In this study, we investigate how the ordering phase, as selected through annealing temperatures, of a well-characterized, ridged polymer, poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene]; PBTTT, affect electronic properties, energetics, extent of doping, and structural changes at nanometer length scales. PBTTT’s well-studied rigid structure, driven by its thienophene unit on the backbone, promotes favorable pi-pi interactions and enhanced charge mobility, making it an ideal platform to study these effects.

We hypothesize that larger ions will face more difficulty entering ordered regions during doping compared to smaller ions, leading to size-dependent changes in these regions. Dopant size is also expected to influence electrochemical parameters, such as redox peak position and onset potential. Annealing temperature plays a crucial role in selecting the ordering phase assumed by PBTTT. Lower annealing temperatures (50°C), which is below the phase transition temperature, results in reduced order, requiring higher potentials to begin the doping process. Higher temperatures (250°C), which are above the second phase transition temperature, lead to enhanced molecular interactions, likely hindering dopant incorporation.

Utilizing operando GIWAXS in combination with spectroelectrochemistry and voltammetry, we provide new insights into the relationship between counterion incorporation, microstructural evolution, and electrochemical behavior. We find that the doped states of PBTTT are not stable once potential control is removed, making operando analysis critical for capturing transient structural dynamics. These techniques allow us to observe how structure changes during doping, particularly under operando conditions, allowing for more accurate and novel correlations with variations in electrochemical properties, conductivity, and mobility.

Our results demonstrate distinct structural and electrochemical differences across dopant sizes and annealing temperatures, as revealed by operando GIWAXS, spectroelectrochemistry, and electrochemical measurements. These findings contribute to a deeper understanding of the role of microstructure in determining the electrochemical properties of PBTTT, with implications for optimizing the performance of doped semiconducting polymers for incorporation into electrode applications.