Screening of Coulombic Interactions To Achieve a Higher Power Factor in Conjugated Polymers

Area of Science:

• Materials Science
• Condensed Matter Physics
• Polymer Science

Background:

• Thermoelectric properties of conducting polymers are often limited by molecular disorder and morphology changes induced by counterions.
• Optimizing carrier concentration for thermoelectric performance typically requires morphological control, which is challenging.

Purpose of the Study:

• To explore a new method for enhancing thermoelectric properties in conducting polymers by controlling coulombic interactions between polarons and counterions, independent of morphology.
• To investigate the impact of coulombic screening on charge transport and thermoelectric performance in different conjugated polymers.

Main Methods:

• In situ de-doping thermoelectric experiments were conducted on poly(3-hexylthiophene-2,5-diyl) (P3HT), poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT-C₁₂), and poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno[3,2-b]thiophene)] (OD-PDPP2T-TT).
• Grazing-incidence wide-angle X-ray scattering (GIWAXS) was used to analyze polymer morphologies.
• Boltzmann transport equations were employed to model charge scattering mechanisms.

Main Results:

• A 9-fold enhancement in the thermoelectric power factor was observed in OD-PDPP2T-TT compared to PBTTT-C₁₂.
• The enhancement was attributed to improved coulombic screening of charge carriers, particularly in the optimally doped regime.
• In P3HT and PBTTC-C₁₂, impurity scattering dominated, while OD-PDPP2T-TT exhibited a shift to acoustic phonon limited scattering, enabling more effective counterion screening.

Conclusions:

• Controlling coulombic interactions offers a novel strategy to enhance thermoelectric properties in conducting polymers without relying on morphological regulation.
• The findings provide a deeper understanding of thermoelectric physics in conjugated polymers.
• This work presents a promising pathway for developing high-performance organic thermoelectric materials.