Complex Relationship between Side-Chain Polarity, Conductivity, and Thermal Stability in Molecularly Doped Conjugated Polymers

Abstract

Molecularly doped conjugated polymers with polar side chains are an emerging class of conducting materials exhibiting enhanced and thermally stable conductivity. Here, we study the electronic conductivity (σ) and the corresponding thermal stability of two polythiophene derivatives comprising oligoethylene glycol side chains: one having oxygen attached to the thiophene ring (poly­(3-(methoxyethoxyethoxy)­thiophene) (P3MEET)) and the other having a methylene spacer between the oxygen and the thiophene ring (poly­(3-(methoxyethoxyethoxymethyl)­thiophene) (P3MEEMT)). Thin films were vapor-doped with fluorinated derivatives of tetracyanoquinodimethane (F n TCNQ, n = 4, 2, 1) to determine the role of dopant strength (electron affinity) in maximum achievable σ. Specifically, when vapor doping with F4TCNQ, P3MEET achieved a substantially higher σ of 37.1 ± 10.1 S/cm compared to a σ of 0.82 ± 0.06 S/cm for P3MEEMT. Structural characterization using a combination of X-ray and optical spectroscopy reveals that the higher degree of conformational order of polymer chains in the amorphous domain upon doping with F4TCNQ in P3MEET is a major contributing factor for the higher σ of P3MEET. Additionally, vapor-doped P3MEET exhibited superior thermal stability compared to P3MEEMT, highlighting that the presence of polar side chains alone does not ensure higher thermal stability. Molecular dynamics simulations indicate that the dopant–side-chain nonbond energy is lower in the P3MEET:F4TCNQ mixture, suggesting more favorable dopant–side-chain interaction, which is a factor in improving the thermal stability of a polymer/dopant pair. Our results reveal that additional factors such as polymer ionization energy and side-chain–dopant interaction should be taken into account for the design of thermally stable, highly conductive polymers.

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

“…In the first part of the paper, we study the charge transport and thermoelectric properties of P3MEET thin films vapor doped with FnTCNQ. P3MEET exhibits a maximum σ as high as 27.0 ± 3.7 S/cm upon doping with F4TCNQ, which matches our previous study 23 . Despite having higher EA levels, F2TCNQ/F1TCNQ‐doped P3MEET have a comparable maximum σ at 26.4 ± 3.1 S/cm and 20.1 ± 0.77 S/cm, respectively.…”

“…The IE of P3MEET is found to be 4.92 eV, whereas the EA values of F4TCNQ, F2TCNQ and F1TCNQ are determined to be 5.25, 5.10, and 5.01 eV, respectively (see details of CV measurements in ref. 23). As expected, the EA level decreases with decreasing fluorination level for three dopants.…”

“…As for weaker dopants, Patel et al has shown that the weaker F2TCNQ dopant does not significantly sacrifice σ compared to the stronger F4TCNQ dopant and in turn yields a larger PF 22 . Recently, our group has shown that the values σ are within the same order of magnitude for poly 3‐(methoxyethoxyethoxy) thiophene (P3MEET) when doping with three different fluorinated tetracyanoquinodimethane dopants (FnTCNQ, n = 1, 2, 4), while the values of σ for poly(3‐(methoxyethoxyethoxymethyl)thiophene) (P3MEEMT) decrease significantly with the decreasing fluorination level of the dopants 23 . In this regard, organic dopants such as FnTCNQ are considered to be model dopants since the EA levels of FnTCNQ can be easily tuned by changing the number of fluorine atoms 24 …”

“…To explore this question, in this work we investigate the correlation between σ and α of a polythiophene derivative with a polar, oligoethylene glycol side‐chain, P3MEET (chemical structure shown in Figure 1(A)) vapor‐doped with fluorinated‐derivatives of tetracyanoquinodimethane (FnTCNQ, n = 1, 2, and 4). P3MEET belongs to a new class of p ‐type semiconducting polymers possessing oligoethylene glycol polar side chains which has been increasingly used due to its enhanced processability and thermally stability 21,23,36–41 . Additionally, choosing dopants with varying EA not only enables the study of polymer‐dopant energy mismatch on charge transport properties but can also systematically change the doping efficiency within the crystalline and amorphous domains.…”

Thermoelectric Materials: Current Status and Future Challenges

“…A study by Dong et al revealed that introduction of a methylene spacer between the first side chain oxygen and polymer backbone reduces thermal dopant stability observed through the loss in conductivity at 130 °C when comparing to the same polymer with the EG motif directly bonded to the backbone. (Dong et al, 2020) This has also been observed for another series of EGfunctionalised derivatives of P3HT where the thermal stability at 80 °C decreased with increasing polar co-monomer content. (Finn et al, 2020) EG motifs have also been added to NDI derivatives to improve electrical conductivity of n-type polymers.…”