We study poly(3-{2-(2-methoxyethoxy)ethoxymethyl}thiophene-2,5-diyl) (P3MEEMT), a new polythiophene derivative with ethylene glycol-based side chains, as a promising semiconducting polymer ...for accumulation-mode organic electrochemical transistors (OECTs) with figures of merit comparable to those of state-of-the-art materials. By characterizing the OECT performance of P3MEEMT transistors as a function of the anion, we find that large hydrophobic anions lower the threshold voltage. We find that, compared to poly(3-hexylthiophene-2,5-diyl) (P3HT), P3MEEMT has faster anion injection rates, which we attribute to the hydration of the P3MEEMT crystal lattice. We study P3MEEMT-based OECT and organic field-effect transistor (OFET) performance as a function of film crystallinity and show that changing the crystallinity of the polymer by thermal annealing increases the OFET mobility yet decreases the OECT mobility. We attribute this difference to the fact that, unlike OFETs, OECTs operate in aqueous environments. To probe how hydration affects the operation of OECTs, we investigate the role of water in electrochemical doping using electrochemical quartz microbalance (EQCM) gravimetry. We find that steady-state hydration and hydration dynamics under electrochemical bias differ dramatically between the crystalline and amorphous P3MEEMT films. These results suggest that the presence of water reduces the electronic connectivity between the crystalline regions of P3MEEMT, thus lowering the mobility in solution. Overall, our study highlights the importance of the role of polymer hydration and nanoscale morphology in elucidating design principles for OECT operation.
A series of alkyl-substituted indacenodithiophene (alkyl-IDT) semiconducting donor–acceptor polymers were designed by DFT to have varying degrees of backbone planarity and synthesized via direct ...arylation polymerization (DArP). These polymers exhibit weak intermolecular interactions, a glass transition temperature (T g) below room temperature, and low degrees of crystallinity from XRD measurements. Despite this, the field-effect mobilities (μ) of these polymers are relatively high (0.06–0.20 cm2 V–1 s–1) with mobility increasing with increasing backbone planarity. Because of the weak intermolecular interactions, the polymers exhibit low elastic moduli (E f) of less than 450 MPa. The polymer with the most twisted backbone exhibits high ductility with a crack-onset strain (CoS) over 100%. These structure–property relationship studies provide useful guidelines for designing semiconducting polymers with high mobility, low stiffness, and high ductility enabling applications in stretchable electronics.
Although extensive efforts have been devoted to understanding electronic transport in conjugated polymers, little is known about their ionic conduction characteristics in relation to polymer ...chemistry, processing, and morphology. This work presents a combined computational and experimental study on morphology and ion transport in thin-film blends of polythiophene derivatives bearing oligoethylene glycol side-chains and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Using molecular dynamics (MD) simulation, we show that in the amorphous phase, the polythiophene derivative P3MEET bearing oligoethylene glycol side-chains with oxygen directly attached to the thiophene rings possesses lower Li+ ionic conductivity compared to its analog P3MEEMT that has a methyl spacer between the oxygen and the thiophene rings. Structural characterization of P3MEET and P3MEEMT thin film upon blending with LiTFSI indicates that adding LiTFSI expands the side-chain domains of the polymer crystallites and reduces the total degree of crystallinity at the same time. Moreover, LiTFSI is found to infiltrate both the amorphous and crystalline regimes at low concentrations but preferably resides in the amorphous domain at high LiTFSI concentrations. Ionic transport measured by electrochemical impedance spectroscopy in both P3MEET- and P3MEEMT-LiTFSI thin films is found to occur predominately in the amorphous domain, and ionic conductivity in P3MEEMT-LiTFSI is always higher than in P3MEET-LiTFSI samples, consistent with predictions from MD simulations. Our work provides a platform to predict and study the influence of polymer chemistry on the ionic conductivity of conjugated polymers.
The properties of molecularly doped films of conjugated polymers are explored as the crystallinity of the polymer is systematically varied. Solution sequential processing (SqP) was used to introduce ...2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) into poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) while preserving the pristine polymer's degree of crystallinity. X‐ray data suggest that F4TCNQ anions reside primarily in the amorphous regions of the film as well as in the P3HT lamellae between the side chains, but do not π‐stack within the polymer crystallites. Optical spectroscopy shows that the polaron absorption redshifts with increasing polymer crystallinity and increases in cross section. Theoretical modeling suggests that the polaron spectrum is inhomogeneously broadened by the presence of the anions, which reside on average 6–8 Å from the polymer backbone. Electrical measurements show that the conductivity of P3HT films doped by F4TCNQ via SqP can be improved by increasing the polymer crystallinity. AC magnetic field Hall measurements show that the increased conductivity results from improved mobility of the carriers with increasing crystallinity, reaching over 0.1 cm2 V−1 s−1 in the most crystalline P3HT samples. Temperature‐dependent conductivity measurements show that polaron mobility in SqP‐doped P3HT is still dominated by hopping transport, but that more crystalline samples are on the edge of a transition to diffusive transport at room temperature.
This study sequentially dopes conjugated polymer films with controlled crystallinity, finding that dopants do not π‐stack with the polymer chains. The most crystalline films show the highest carrier mobilities and a redshifted absorption with increased cross section due to enhanced polaron delocalization.
The electrochemical doping/dedoping kinetics, and the organic electrochemical transistor (OECT) performance of a series of polythiophene homopolymers with ethylene glycol units in their side chains ...using both kosmotropic and chaotropic anion solutions were studied. We compare their performance to a reference polymer, the polythiophene derivative with diethylene glycol side chains, poly(3-{2-(2-methoxyethoxy)ethoxymethyl}thiophene-2,5-diyl) (P3MEEMT). We find larger OECT material figure of merit,
μC
*, where
μ
is the carrier mobility and
C
* is the volumetric capacitance, and faster doping kinetics with more oxygen atoms on the side chains, and if the oxygen atom is farther from the polythiophene backbone. Replacing the oxygen atom close to the polythiophene backbone with an alkyl unit increases the film π-stacking crystallinity (higher electronic conductivity in the undoped film) but sacrifices the available doping sites (lower volumetric capacitance
C
* in OECT). We show that this variation in
C
* is the dominant factor in changing the
μC
* product for this family of polymers. With more oxygen atoms on the side chain, or with the oxygen atom farther from the polymer backbone, we observe both more passive swelling and higher
C
*. In addition, we show that, compared to the doping speed, the dedoping speed, as measured
via
spectroelectrochemistry, is both generally faster and less dependent on ion species or side chain oxygen content. Last, through OECT, electrochemical impedance spectroscopy (EIS) and spectroelectrochemistry measurements, we show that the chaotropic anion PF
6
−
facilitates higher doping levels, faster doping kinetics, and lower doping thresholds compared to the kosmotropic anion Cl
−
, although the exact differences depend on the polymer side chains. Our results highlight the importance of balancing
μ
and
C
* when designing molecular structures for OECT active layers.
We find larger
μC
* and faster doping kinetics with more oxygen atoms on the side chain, and if the oxygen atom is farther from the polymer backbone. We show that this variation in
C
* is the dominant factor in changing the
μC
* for these polymers.
We find that conjugated polymers can undergo reversible structural phase transitions during electrochemical oxidation and ion injection. We study ...poly2,5-bis(thiophenyl)-1,4-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)benzene (PB2T-TEG), a conjugated polymer with glycolated side chains. Using grazing incidence wide-angle X-ray scattering (GIWAXS), we show that, in contrast to previously known polymers, this polymer switches between two structurally distinct crystalline phases associated with electrochemical oxidation/reduction in an aqueous electrolyte. Importantly, we show that this unique phase change behavior has important physical consequences for ion-polaron pair transport. Notably, using moving front experiments visualized by both optical microscopy and super-resolution photoinduced force microscopy (PiFM), we show that a laterally propagating ion-polaron pair front in PB2T-TEG exhibits non-Fickian transport, retaining a sharp step-edge profile, in stark contrast to the Fickian diffusion more commonly observed in polymers like P3MEEMT. This structural phase transition is reminiscent of those accompanying ion uptake in inorganic materials like LiFePO4. We propose that the engineering of similar properties in future conjugated polymers may enable the realization of new materials with superior performance in electrochemical energy storage or neuromorphic memory applications.
The field of stretchable electronics has recently gained significant interest from the academic community, with a focus on producing materials that demonstrate reliable electrical performance with ...improved response to mechanical deformation. This review highlights the recent progress in understanding the relationships between the mechanical behavior and electrical performance of such devices. Potential solutions can take the form of intrinsically elastic polymers, polymer semiconductor/elastomer blends and alternative engineering-oriented approaches, which are discussed herein. Trends and design strategies are beginning to manifest in this early stage of the stretchable electronics field. The development of stretchable electrical systems can provide unique applications of organic electronics.
Mixed ion/electron conducting polymers have recently received significant interest from a number of research communities, spanning from biological to mechanical. Their ability to conduct ions and ...electrons in the same material enables their use in a wide range of electrochemical devices. This functionality can be used to improve performance of more traditional devices or enable completely novel ones. Herein the use of blended polymers, block copolymers, and homopolymers as mixed conducting polymer systems is discussed, with special emphasis on connecting polymer structure and morphology to mixed conduction performance. Following this discussion, the outlook for the future of this field is presented.
A review highlighting the implications of morphology on the mixed conduction performance of polymers.
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.
Poly(indacenodithiophene-benzothiadiazole) has received significant interest because of its exceptional hole mobility despite its near-amorphous thin-film morphology and brittleness at low M n. In ...comparison, poly(indacenodithiophene-benzopyrollodione) (PIDTBPD) has a lower hole mobility but is exceptionally ductile at similar M n. Herein, we synthesize random indacenodithiophene (IDT) copolymers with varying amounts of incorporated benzothiadiazole and benzopyrollodione (BPD), which introduces varied degrees of backbone twist to each respective polymer system. This allows us to elucidate how the BPD monomer introduction leads to conformational and morphological changes that influence the crack onset strain (CoS) and hole mobility of these near-amorphous IDT copolymers and the rates by which each material property responds to sequentially larger BPD incorporation. Results of density functional theory calculations suggest that BPD introduction does not lead to significant differences in backbone linearity between the studied polymers, and grazing incidence wide-angle X-ray scattering demonstrates that the degree of crystallinity within thin films is not significantly altered. It does, however, lead to a more varied circular distribution of the hexadecyl side chains around the polymer backbone. With increasing BPD incorporation, a crossover point between CoS and hole mobility emerges. At this crossover point, a random copolymer with 30% BPD introduction displays increased CoS and an average hole mobility value equal to that of the PIDTBPD system, suggesting that hole mobility is more sensitive to torsion along the polymer backbone, while the response of the CoS is relatively delayed. The data also suggest that the increase in CoS with increasing BPD content does not arise because of differences in rigidity but because the more circular distribution of the side chains makes polymer chains with sufficient BPD content better able to flow.
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