Correlating conductivity and Seebeck coefficient to doping within crystalline and amorphous domains in poly(3‐(methoxyethoxyethoxy)thiophene)

Abstract

Molecular doping of conjugated polymers (CPs) plays a vital role in optimizing organic electronic and energy applications. For the case of organic thermoelectrics, it is commonly believed that doping CPs with a strong dopant could result in higher conductivity (σ) and thus better power factor (PF). Herein, by investigating thermoelectric performance of a polar side‐chain bearing CP, poly(3‐(methoxyethoxyethoxy)thiophene) (P3MEET), vapor doped with fluorinated‐derivative of tetracyanoquinodimethane FnTCNQ (n = 1, 2, 4), we show that using strong dopants can in fact have detrimental effects on the thermoelectric performance of CPs. Despite possessing higher electron affinity, doping P3MEET with F4TCNQ only results in a σ (27.0 S/cm) comparable to samples doped with other two weaker dopants F2TCNQ and F1TCNQ (26.4 and 20.1 S/cm). Interestingly, F4TCNQ‐doped samples display a marked reduction in the Seebeck coefficient (α) compared to F1TCNQ‐ and F2TCNQ‐doped samples from 42 to 13 μV/K, leading to an undesirable suppression of the PF. Structural characterizations coupled with Kang‐Snyder modeling of the α–σ relation show that the reduction of α in F4TCNQ‐doped P3MEET samples originates from the generation of low mobility carrier within P3MEET's amorphous domain. Our results demonstrate that factors such as dopant distribution and doping efficiency within the crystalline and amorphous domains of CPs should play a crucial role in advancing rational design for organic thermoelectrics.

Probing the evolution of conductivity and structural changes in vapor-F4TCNQ doped P3HT

“…27 This phenomenon was shown to also exist in polythiophenes with polar sidechains by Ma et al , where P3MEET shows a lower conjugation length than P3HT, but similarly stiffens in the presence of dopant. 46…”

Organic Materials for Electronic and Thermoelectric Applications

“…Materials that exhibit the best thermoelectric properties are semiconductors with a figure of merit zT exceeding 1. Achieving higher values of the figure of merit is possible with doping, and the same procedure is also conducted in polymers for the purpose of obtaining higher conductivities [8], [9]. However, reaching values above 3 is a highly desirable target and a challenge that has been approached but not yet reached.…”

“…Some dopants, such as dodecaborane-based ones [56], electrostatically shield counterions from carriers by effectively intercalating into the lamellae, far from polarons on the polymer backbone. Thus, the importance of choosing the right dopant [9] for a certain host is crucial to obtaining good conductivity.…”

Modeling the Interplay of Conformational and Electronic Structure in Conjugated Polyelectrolytes

“…The physical properties of CPEs arise from a complex interplay of multiscale chemical and morphological features, making CPE design a challenge that could be facilitated by computational modeling. Previous studies of CPEs and doped conjugated polymers have separated the conductivity of ions and electrons, revealing nonmonotonic changes in conductivities as a function of dopant concentration, , alterations to backbone chemistry, side chains, − solvent, copolymer chemistry, and multiscale morphology. , However, because of the significant cost and time requirements for experimental characterizations of CPEs, only a small portion of the design space can be explored, and many questions remain regarding the fundamental polymer physics and electronic structure of CPEs. Furthermore, the growing diversity of backbone chemistries, side chains, solvents, salts, dopants, and deposition methods , further complicates the exploration of the synthetic and morphological design space.…”