It is knonw that the ratio of ordered/disordered domains in thin films has a direct impact on the doping extent and doping kinetics of semiconducting polymers.^[1] By combining ether side chain engineering and uniaxial alignment, we present an effective strategy to finely control the film morphology, leading to unprecedented performance in thermoelectric applications (OTEs) and organic electrochemical transistors (OECTs). In this contribution, we demonstrate in particular the potential of novel single ether side chains to substitute standard alkyl side chains and offer a viable alternative to oligo(ethylene glycol) side chains for the design of high-performing doped materials. Single ether side chains are simple to synthesis and air stable. The enhancement of polarity facilities dopant insertion, while maintaining high thermomechanical cohesion to afford highly oriented films. The resulting films made of PBTTT-⁸O – PBTTT with single ether side chains with the ether function in the 8^th position - delivered record electrical conductivities, reaching 50 000 S cm^‑1 upon chemical doping with F₆TCNNQ for OTEs^[2] and reversible 2 700 S cm^‑1 upon electrochemical doping in OECTs in aqueous KPF₆ electrolyte.^[3] To rationalize these improvements, we studied four polymers bearing single ether side chains with various position of the ether function (x = 3, 5, 8, 11) and compared them to the reference alkyl PBTTT-C₁₂. We found clear dependences of the position of the ether function on the thermo-structural behavior of PBTTT-^xO polymers and their resulting crystallinity index.^[4] We present here how these trends can be exploited to finely tune the doping properties of polymers and their OTE performance. To conclude, we will introduce two new PBTTTs including two and four ether functions along the side chains and investigate how OECT kinetic and transconductance are evolving as compared to single ether side-chains based PBTTTs. This study aims at improving our current understanding on how introducing heteroatoms along side chains impacts polymer organization and dopant accommodation for future generations of polar side chains.

Understanding the factors influencing device switching times is essential for the effective use of organic electrochemical transistors (OECTs) in neuromorphic computing, bioelectronics, and real-time sensing. Current models of OECT operation fail to explain the experimental finding that turn-off times are generally much faster than turn-on times in accumulation mode. In this collaboration, devices containing polythiophene deriatives are studied. We employ operando optical microscopy to visualize the local doping level of the transistor channel. Our results reveal that turn-on occurs in two stages—initial propagation of a doping front, followed by uniform doping—whereas turn-off happens in a single stage. We attribute the faster turn-off to a combination of factors including channel geometry, differing kinetics of doping and dedoping, and carrier-density-dependent mobility. We identify ion transport as the limiting factor in the operational speed of our devices. This study offers valuable insights into the kinetics of OECTs and provides guidelines for engineering faster devices.

The true structure of alternating conjugated polymers – the state-of-the-art materials for many organic electronics applications – often deviates from the idealized picture. Homocoupling defects are in fact inherent to the widely used cross-coupling polymerization methods. Nevertheless, many polymers still perform excellently in the envisaged applications, which raises the question if one should really care about these imperfections.

In our recent work, we have looked at the relevance of chemical precision (and lack thereof) in conjugated polymers covering the entire spectrum from the molecular scale, to the micro- and meso-structure, up to the device level. We have identified, visualized, and quantified the different types of polymerization errors for alkoxylated variants of the benchmark (semi)crystalline polymer PBTTT and we have introduced a general strategy to avoid homocoupling.[1] Through a combination of experiments and supported by simulations, we have shown that these coupling defects hinder fullerene intercalation and limit device performance as compared to the homocoupling-free analogue. This clearly demonstrates that structural defects do matter and should be generally avoided, in particular when the geometrical regularity of the polymer is essential.

In this contribution, our most recent efforts on this topic will be discussed, extending from the benchmark PBTTT polymers to the state-of-the-art accumulation-mode p-type OECT channel material pgBTTT.

Advanced in situ and operando characterization techniques are required for the continued development and advancement of organic mixed ionic-electronic conductors (OMIECs). The key benefit of OMIECs is that ionic-electronic coupling allows the modulation of nearly all materials properties through the electrochemical control of charge density. Included is the ability to electrochemically control electronic conductivity, color, surface energy, volumetric swelling, moduli, miscibility, catalytic activity, thermal conductivity, emissivity, and more. This enables numerous applications that are inaccessible with traditional electronic materials. However, the massively tunable ionic and electronic charge density means that OMIECs do not possess a single discrete microstructure but a range of ion (and solvent) swollen structures with widely varying composition that depends on environment and applied electrochemical potential. This greatly limits the insight gained from traditional dry ex situ characterization that probe states that can have little in common with the structure, composition, and environment of OMIECs in functioning devices and applications. Therefore, in situ and operando characterization in device/application relevant conditions is crucial for understanding the complex structure-property relationships that ultimately dictate OMIEC performance. 

Synchrotron X-rays present a powerful tool for in situ/operando characterization. Their unparalleled brilliance and coherence allow for time resolved measurements of microstructure, composition, and dynamics using grazing incidence wide angle X-ray scattering (GIWAXS), X-ray fluorescence (XRF), and X-ray photon correlation spectroscopy (XPCS), respectively. Amongst other insights, in situ GIWAXS reveals the structural sources of enhanced mixed conducting properties, operando XRF reveals complicated interfacial dominated proton and metal ion transport, and XPCS reveals long time scale domain coarsening. This helps explain the underlying phenomena that dictate OMIEC structure-property relationships, device performance, and materials stability. The limits of these synchrotron measurements and their potential for future improvement will be considered. Complementary in situ/operando techniques will be addressed, as well as their multi-modal possibilities. Overall, in situ synchrotron techniques are just beginning to unravel the structural complexity of OMIECs, yet already they provide concrete directions for the rational design and processing of improved OMIEC materials.

1
Electroactive polymer content: a tool to design the side chains of next-generation OMIECs
Olivier Bardagot,a,b‡* Brandon T. DiTullio,c‡ Austin L. Jones,c Justin Speregen,c John R.
Reynolds,c Natalie Banerji,b
‡ These authors contributed equally to this work.
a Institute of Chemistry and Processes for Energy Environment and Health (ICPEES), CNRS
University of Strasbourg, UMR 7515, 25 rue de Becquerel, Strasbourg Cedex 02, 67087,
France
bDepartment of Chemistry, Biochemistry and Pharmaceutical Sciences (DCBP), University of
Bern, 3012 Bern, Switzerland
c School of Chemistry and Biochemistry, School of Materials Science and Engineering,
Center for Organic Photonics and Electronics, Georgia Tech Polymer Network, Georgia
Institute of Technology, Atlanta, Georgia, 30332, USA
*Corresponding author email: olivier.bardagot@cnrs.fr
The development of organic electrochemical transistors (OECTs)-based applications relies on
the rational design of high-performing organic mixed ionic-electronic conductors (OMIECs).[1]
In this talk, we present two series of ProDOT-based copolymers with linear or branched
oligo(ethylene oxy) (OE) side chains.[2,3] We demonstrate that:
1. We also develop the newly introduced concept of “electroactive polymer content” as an
important parameter to design high-performance OMIECs. Our result and literature
review suggest that a balance mass of backbone and side chain, resulting in an
electroactive polymer content of 40-50%, may be ideal for OE-substituted
polythiophenes.
2. By increasing the electroactive polymer content from 34% to 49%, the pulsing stability
increases with IDS retention boosted from 1.8% up to 99% after 200 ON/OFF cycles.
3. Optimizing the electroactive polymer content to 49% allows to enhance the OECT
signal amplification; as shown by the normalized transconductance of 453 ± 70 S cm-1
achieved for PE2-OE4 (with 4 OE units) in a saline aqueous electrolyte.
With this talk, we would like to encourage the OMIEC community to (i) calculate the
electroactive polymer content of an OMIEC when designing the length of the side chains and
(ii) consider ProDOT-based polymers as highly interesting OECT channel materials.

Organic electrochemical transistors (OECTs) are being widely studied due to their numerous applications such as organic bioelectronics, neuromorphic systems, sensors, etc. OECTs offer the advantages of transporting both electrons and ions due to the organic mixed ionic-electronic conductors (OMIECs) that allow ions to penetrate the channel throughout its volume. However, the design rules to fully optimize the OECTs are still unclear. It is possible to “tune” the performance, that is, the transconductance (g_m), of OECTs according to the Bernard model for variations in channel length, thickness and width. On the other hand, the effects due to the morphology/structure of the channel are always present, but their influence is not yet clear. The effect of gate-voltage-dependent resistance, known as contact resistance (R_C), is critical to performance. This parasitic R_C, obtained by transmission-line method (TLM), is present at the interface between the source/drain electrodes and the channel. Here, the results of the doping effect of Lithium bis(trifluoromethanesulfonyl)imide (LiTFSi) on R_C, more specifically in the source-drain/channel interfaces, are analyzed. Unlike organic field-effect transistors (OFETs) in which molecular contact doping (a dopant layer is deposited between the channel and the metallic contacts) or OECTs using source/drain-electrode surface modification, LiTFSi is used in ultra-low quantities. This ultra-low LiTFSi content is defined by the ratio between the number of LiTFSi molecules and the number of olygo(ethylene-glycol) (OEG). By introducing LiTFSi into the bulk of the channel, the R_C is enhanced by several orders or magnitude. Our results showed that an ultra-low presence of LiTFSi improves (reduces)  R_C by ~ 1-2 orders of magnitude. R_C improvement has been observed in p-type (p(g2T-T)) and n-type (p(gNDI-gT2)) OECTs and can be reduced to very low values of approximately 0.002 Ω∙cm during operation. Both polymers present OEG chains that facilitate the transport of Li⁺ ions through the bulk of the channel and could explain the R_C improvement. On the other hand, in some devices, the performance (transconductance) increases significantly up to 3-4 times due to low R_C values.

In the last decades, organic semiconductors have attracted significant attention due to their biocompatibility, mechanical flexibility, solution processability, and lightweight nature. A recent achievement brought the engineering of side chains in polymer films to enhance ion intercalation, as demonstrated by the newly developed polymer P(g₃2T-T). This derivative of P3HT, featuring oligoether instead of aliphatic side chains, emerges as a promising candidate for chemical doping because of the affinity of its side chains to dopant anions as well as potential applications in bioelectronic devices. Chemical doping of the P(g₃2T-T) has been found to achieve an up to fourfold increase in macroscopic conductivity compared to P3HT. However, this enhanced conductivity is highly dependent on specific doping conditions. While general principles of doping are now well-understood, the precise effects of various doping parameters on the charge transport properties of doped thin films still need to be investigated.

Our study employs in-situ absorbance spectroscopy as well as THz spectroscopy to investigate the effects of differing dopant and electrolyte concentrations on the conductivity of chemically doped P(g₃2T-T) films. We reveal that the early kinetics of the doping process have a significant influence on the final thin film conductivity. The results demonstrate that the bipolaron-to-polaron ratio is a rather weak indicator of favorable charge transport properties and that instead the bipolaron formation rate is a superior predictor. We presume that the bipolaron formation rate is entangled with the swelling of the polymer film, the intercalation of the dopant anions, and thus the packing of the polymer film as well as its charge transport properties. We explore two distinct doping methods—immersed doping and anion exchange doping—and use two different dopants, F₄TCNQ and Magic Blue, to rule out the impacts of specific doping techniques. Especially for the anion exchange doping method, our results offer for the first time an explanation for the dependence of the electrolyte concentration on the film conductivity. Moreover, we report an excellent restoration of long-range charge transport in chemically doped P(g₃2T-T), compared to the short range (100 nm) derived from THz spectroscopy. Overall, our results shed light on the underlying principles of chemical doping and the revealed dependencies on specific doping conditions can help facilitate the development of the next generation of organic semiconducting polymers that can be employed at the interface between biological and electronic systems.

The key component of an organic electrochemical transistor (OECT) is the channel material which is made of an organic mixed ionic-electronic conductor (OMIEC). Its role is to conduct both electronic and ionic charges within the entire volume of the channel and to modulate its redox states and conductivity through interactions with ions and solvent molecules of the electrolytes. Conjugated polymers quickly appear as an ideal platform for mixed conductance. The electronic charge transport is occurring along the conjugated backbone and the ionic transport is allowed through the bulk.

While many efforts have been made to understand how to design materials with high charge mobilities, such structure-property relationships are still lacking when these polymers are interacting with ions and solvent molecules. By using a combined classical/quantum approach, we have simulated the charge and ionic transports in different OMiEC candidates to get a better understanding of this mixed conductance and to propose design criteria for new materials.

We focused our attention on the IDTBT polymer, a state-of-the-art p-type polymer, for which we studied the impact of the incorporation of glycol side chains on both the charge transport properties and ion diffusion. While our simulations suggest that glycolated side chains do not alter the number and quality of interachain contacts for both dry and swollen films, experimental evidence points to disappointing OECT performances [1]. The latter could be rationalized theoretically by larger electrostatic energetic disorder due to the presence of polar atoms close to the conjugated backbones. Our calculations therefore suggest designing polar side chains by incorporating the hydrophilic segment between two apolar regions and using semicrystalline materials instead of near-amorphous materials.

pgBTTT and p(g2T-TT) are two interesting materials which demonstrate that subtle changes of the chemical structure of the glycolated PBTTT, i.e., regioisomers, can strongly influence their properties [2]. Interestingly, it has been shown that pgBTTT is more efficient in OECT than p(g2T-TT). By simulating the ion insertion process in crystalline stacks of both polymers, our calculations pointed out differences in the planarity of the conjugated backbones and, more importantly, the localization of the ions in the swollen films; rationalizing their different behaviors.

Finally, our latest results on the insertion of ions within BBL polymers, a state-of-the-art n-type polymer for OECT, will be presented.

The advancement of electrochemical device technology is intricately linked to the structural nuances of polymers. This comprehensive research investigates the influence of ethylene glycol side chains on the performance characteristics of pgBTTT, a regiochemically defined organic electrochemical transistor. A series of pgBTTT polymers were synthesized with varying concentrations of hydrophilic ethylene glycol side chains, ranging from 50% to 100%. The outcomes of this study indicate a significant positive correlation between the proportion of ethylene glycol side chains and the volumetric capacitance of these polymers, with a notable deviation at a 90% side chain concentration.[1] [2]

Building on these findings, the study delved into the realm of blending methodologies as a strategy to further enhance the volumetric capacitance of pgBTTT polymers. Two distinct blending techniques were employed: one involving a blend of pgBTTT with pBTTT(OR)₂, and another comprising a blend of pgBTTT with pgBTTT-OEG-OR. The blend incorporating pBTTT(OR)₂ exhibited a volumetric capacitance that aligned with the trends observed in varying side chain percentages. Contrastingly, the blend with pgBTTT-OEG-OR demonstrated a consistent volumetric capacitance across various ratios.[3] Both blending strategies yielded superior volumetric capacitance compared to the copolymers, particularly at a concentration of 90% ethylene glycol side chains.[4]

A pivotal aspect of this research involved assessing the impact of these blending techniques on the kinetics of the doping and dedoping processes. This inquiry revealed that blends, especially those with matching side chain ratios, displayed markedly improved kinetic efficiency in both doping and dedoping procedures.[5] This discovery opens new pathways for enhancing the performance and efficiency of organic electrochemical transistors and mixed conductors.

In conclusion, the study provides critical insights into the manipulation of polymer structures, specifically through the adjustment of ethylene glycol side chains and strategic blending. These modifications are key to advancing the design and functionality of high-performance electrochemical devices. The findings serve as a valuable foundation for future endeavors in optimizing organic electrochemical transistors and mixed conductors for enhanced efficiency and kinetics.

Organic electrochemical transistors (OECTs) prove to be effective devices in various applications such as neuromorphic functionalities, bioelectronics, and sensors. Analyzing these mixed ionic-electronic devices is often complex due to the coupling of hole transport along the channel with ion insertion from the electrolyte. The transient response is a fundamental feature for operating the transistor in synapsis emulation.

We show a transmission line model that unites vertical ion diffusion and horizontal electronic transport for the analysis of the time-dependent current response of OECTs. We show the reduction of the general model to simple time-dependent equations for the average ionic/hole concentration inside the organic film, which produces a Bernards-Malliaras conservation equation coupled with a diffusion equation. We provide a basic classification of the transient response to a voltage pulse, and the correspondent hysteresis effects of the transfer curves, and the impedance and admittance responses. The shape of transients is basically related to the main control phenomenon, either the vertical diffusion of ions during doping and dedoping, or the equilibration of electronic current along the channel length.

Conducting polymers have a number of unique properties relative to conventional photoelectrode materials.  The mixed electrical-ionic transport properties present a complex polymer/electrolyte interphase that if understood, could provide control over local environments afforded through synthesis, long-lived charge carrier lifetimes, and flexible, low-cost, and scalable thin film formats which circumvent the shortcomings of inorganic materials (surface states, grain boundaries, challenges in processing, and mechanically unstable platforms).

Creating organic semiconductor-based photoelectrodes is not as simple as interfacing optimized organic photovoltaic materials with an electrolyte. Durable and high performing organic photoelectrodes require balancing the photojunction properties with charge transport, attention to catalytic attachment, and a strong emphasis on mitigating parasitic chemical reactions and resistances.

The Center for Soft PhotoElectroChemical Systems (SPECS) is an Energy Frontier Research Center focused on the basic science questions that underpin the development of low-cost, robust energy conversion and energy storage technologies based on new organic polymer (plastic) electronic materials. These materials are predicted to fill a critical position in the U.S. energy portfolio, providing for next-generation fuel-forming platforms (energy conversion) and batteries (energy storage) that cannot currently be achieved with conventional (hard) inorganic materials. A number of emerging in situ/operando spectroelectrochemical and scanning electrochemical cell microscopy approaches will be discussed for this exciting new area of energy conversion.

Organic mixed ion-electron conducting materials (OMIECs) find use in several exciting applications due to their ability to transduce ionic and electronic signals. Some recent work has leveraged an anti-ambipolar behavior, whereby extensive electrochemical doping lowers the conductivity rather than increasing it further. The underlying mechanisms governing this phenomenon are unknown but highly significant for further device exploration and optimization. Device-relevant material characterization is not straightforward, however, since OMIECs operate in a complex environment including an electrolyte containing positively and negatively charged ions. Given their nature of mixed conductivity, OMIECs are expected to show strongly different material characteristics ex-situ, which markedly calls for in-operando characterization.

Here, we apply several in-operando characterization techniques to state-of-the-art n-type ladder-type OMIECs and uncover previously undetected ion-backbone interactions. Through in-operando infra-red (IR) spectroscopy we find these ion-backbone interactions are crucially important to explain OECT and anti-ambipolar characteristics. Furthermore, we find that the same cation-backbone interactions instigate changes in the film morphology, whereby the choice of electrolyte dictates the nature of the changes.

Our results provide insight into the electrochemical doping mechanism of these polymers and uncover structure-property relations governing their device functions. These insights enable targeted optimization of both polymer structure and employed electrolyte, as well as provide instruments for possible new applications.

Organic mixed conductors have the ability to transport both ions and electronic charge carriers. Many devices which leverage this property have been developed, including biosensors,[1] energy storage materials,[2] light emitting electrochemical cells,[3] organic electrochemical transistors,[4] and neuromorphic computing elements,[5] to name a few. In addition, the problem is connected to the earliest days of organic electronics research through measurements via standard electrochemical methods.[6] More broadly, the problem of mixed conduction sits at the crossroads of polymer physics and activity-based approaches to electrolyte equilibria. I will present results that demonstrate the fundamental mechanisms operating in organic electrochemical transistors,[7] along with key structure-property relationships that govern ion dynamics.[8] Approaching the problem from a polymer thermodynamics perspective also provides insight into thin film swelling behavior[9] and interfacial stability.[10] The perspective treats doped polymer films as solid state solutions, describing local field interactions as the correct first order effect. Of course, pi-conjugated polymers are not uniform homogeneous materials, and modifications to the perspective that take polymer morphology into account will be discussed. Lastly, we will present results that compare organic electrochemical materials with activated carbon and demonstrate that they do not resemble each other in any meaningful way beyond lumped circuit element models.