Electrostatic complexation of conjugated polyelectrolytes

Abstract

The development of blends of conjugated polymers is helpful for applications including stretchable electronics, bioelectronics, and electrochemical systems. Electrostatic complex coacervation of oppositely-charged polyelectrolytes has recently emerged as a versatile route to form processable, functional polymer blends of conjugated polyelectrolytes from solution. However, substantial differences in chain stiffness and redox activity among conjugated polyelectrolytes leave the design rules for complex coacervation unresolved. Recent progress towards understanding how electrostatic interactions of conjugated polyelectrolytes can be used to control the properties and processability of blends is presented. The phase behavior of blends of conjugated and non-conjugated polyelectrolytes and the resulting nanostructure in solidified form are discussed. Electrostatic complexation is found to be a powerful tool to tailor the photophysical and electronic transport properties of blends of conjugated polyelectrolytes. The potential space for applications of these processable blends is reviewed by a discussion of recent efforts to form stretchable conductors, battery binders, and bioelectronic devices.

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Wider impact

Conjugated polymers have found applications in thin film electronic devices such as light emitting diodes, solar cells, and thin film transistors. Despite these advances it remains challenging to process them in thick or bulk forms. The emergence of electrostatic complexation as a route to improve the processability of high-solids content polymer blends provides a new opportunity for processing functional conjugated polymers. Electrostatic complexation requires an understanding of the interactions of oppositely-charged polyelectrolytes from the solvated to the solid phase. The phase behavior of blends of polyelectrolytes is complex due to the interplay of interactions between the polyelectrolytes, solvent, and ionic species. The design rules for the formation of electrostatic complexes are beginning to emerge for non-conjugated and conjugated polymers. This review presents a pedagogical review of recent progress in the formation of electrostatic complexes of conjugated polyelectrolytes and their potential applications, such as in battery binders.

Introduction

The majority of semiconducting polymers are designed to be relatively nonpolar, but the addition of ionic groups can lead to new functionalities. Ionic semiconducting polymers are referred to as conjugated polyelectrolytes (CPEs).^1,2 CPEs, unlike most conjugated polymers, are soluble in water or highly polar solvents such as methanol, expanding the range of solvents available for processing. The ionic functionality also offers a handle to modify the electronic properties of CPEs through intermolecular interactions with charged molecules and other polyelectrolytes. Because of these features, CPEs have been studied in applications ranging from biosensing to thin-film electronics.¹

The ionic groups of CPEs provide unique opportunities for controlling their processability and functionality, particularly in polymer blends. Blends of conjugated polymers and insulating polymers have proven useful for applications such as transistors, memory devices, and light-emitting electrochemical devices.^3,4 In favorable cases, conjugated polymers can form aggregated fibrils that can percolate in blends with insulators, giving rise to properties such as stretchability while maintaining their electronic performance.⁵ While blends of two polymers may provide beneficial properties, it is difficult to rationally control the phase separation of chemically dissimilar polymers due to enthalpic interactions between chains that are frequently unfavorable.^3,4 Chemically dissimilar, but oppositely-charged polyelectrolytes, can overcome the tendency for phase separation and offer the potential to control the length scales and structure of polymer blends.^6,7 CPEs therefore provide a natural platform for controlling the structure of polyelectrolyte blends with optoelectronic properties.

Ionic interactions were initially found to be beneficial for processing electrically doped conjugated polymers.⁸ Doped conjugated polymers are distinct from CPEs in the sense that they are neutral polymers that are made ionic by oxidation or reduction of their backbone to create a charge carrier, rather than having an inherent charge due to an ionic moiety. Early work demonstrated that sulfonic acid-based small molecule surfactants dramatically improve the processability of polyaniline (PANI) in its protonated conductive form (Fig. 1).⁸ The widely used conductive polymer blend PEDOT:PSS is formed when a polyelectrolyte, polystyrene sulfonate (PSS), is present during oxidative synthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) (Fig. 1), leading to a water-processable blend.^9,10 The use of PANI and PEDOT:PSS as conductors,¹⁰ interlayers,¹¹ and in organic electrochemical transistors¹² points to the utility of using ionic interactions to control processability. The success of this strategy, however, has yet to be fully realized using the wide variety of structures of conjugated polymers.

Fig. 1 Chemical structures of conjugated polyelectrolytes (CPEs, top) and insulating polyelectrolytes (IPEs, bottom). The acronyms for CPEs are based on the name of the backbone; polymers with specific sidechain structures have varying abbreviations in literature. The CPEs are classified as p-type or n-type depending on the charge carrier that is most typically introduced on the backbone. The counterions that balance the charge of the polyelectrolyte can vary depending on the study.

The interaction of two oppositely-charged polyelectrolytes can lead to electrostatic complexation and is a common pathway to form assemblies in biological systems.^13–15 Electrostatic complexation of CPEs was originally used to sense biological polyelectrolytes in dilute solution,¹⁶ but it can also be leveraged in higher concentration regimes to enhance the processability of blends of CPEs for solid layers. The process of electrostatic complexation of CPEs can be understood using the framework developed for electrically insulating polyelectrolytes (IPEs).⁷ Upon mixing of two solutions of oppositely-charged polyelectrolytes, liquid–liquid phase separation can occur, resulting in a complex coacervate (a relatively dense phase of the two polymer species) and a supernatant (dilute phase) in equilibrium.^7,17 The complex coacervate can be isolated, yielding a processable blend that has a relatively high concentration of polymer (>0.1 M) relative to the dilute solutions (<0.1 M) that are commonly used for thin film electronics.^18–20 Many organic electronic devices, such as transistors or solar cells, can be formed from dilute solutions because they only require films with thicknesses of ∼100 nm, but there are applications that can benefit from the ability to process thicker layers. For example, paste-like semiconducting systems are relevant for mixed conducting battery binders,²¹ for thermoelectrics that require relatively thick legs,²² and for 3D printing of electrically active components.²³ Because of the wide range of chemical structures that can be used to form electrostatic complexes with CPEs, there is a complex design space for controlling their electronic properties and processability.

Here, we present an overview of the fundamental behavior of CPEs, the polymer physics of electrostatic complexation, and the properties of solid electrostatic complexes of CPEs. We note that this approach differs from work on layer-by-layer assemblies^11,24,25 of CPEs and polyelectrolytes because of the goal of directly mixing the two polymers rather than sequentially processing them into solid assemblies. We also emphasize the connection with the behavior of insulating polyelectrolytes and discuss what is currently unknown in the case of electrostatic complexes containing CPEs.

Structure and properties of conjugated polyelectrolytes

Conjugated polyelectrolytes have a backbone comprising conjugated units with sidechains functionalized with charged groups, most often positioned at the end of a sidechain (Fig. 1).¹ These ionic moieties provide the polymer with solubility in water and highly polar solvents like methanol. The ionic group attached to the CPE can be negatively or positively charged; anionic groups are frequently the conjugate base of an acid, like sulfonate, and cationic groups are frequently alkyl ammonium species.²⁶ We note that conjugated polymers are frequently referred to by acronyms capturing the groups in the backbone, which can be challenging for structures with multiple conjugated moieties in the repeating unit. The structure of CPEs add further complexity given the need to keep track of the ionic groups and structure of the counterion. In this review, we refer to CPEs by their backbone structure and note the structure of the sidechain to remove ambiguity where possible.

CPEs may be homopolymers or co-polymers of monomers with varying sidechains allowing for control of their electrostatic interactions. A polyelectrolyte can be characterized by its charge fraction, f, defined by the average number of charged groups per monomer, which is also equivalent to the number of associated monovalent counterions per monomer.⁷ The number of charges per monomer in the polymer can be controlled by the attachment point to the backbone, such as bridgehead positions in the monomer allowing multiple sidechains per monomer, or by branching of the sidechain itself. Additionally, monomers with charged functionalities can be co-polymerized with neutral monomers to control f (Fig. 2). Semiconducting polymers can also become ionic through electronic doping that oxidizes or reduces the backbone by an electron transfer reaction;²⁷ although doping creates a charged polymer due to the presence of the charge carrier and a charge-balancing counterion, electronically doped polymers are generally not referred to as CPEs in literature. The presence of charge carriers further modifies the number of charges per monomer and leads to an effective charge fraction, f_effective (Fig. 2).^28,29 The design of a CPE for a given application requires consideration of the demands of the electronic properties, which are dominantly defined by the backbone, and electrostatic interactions, which are a combination of the charge fraction and the oxidation state of the backbone.

Fig. 2 (a) The charge fraction, f, of a polyelectrolyte is a function of the number of charged monomers per chain and the number of charged sites on the monomer. The charge fraction can be controlled by formation of (b) homopolymers or (c) copolymers. Electronic doping occurs by oxidation or reduction of the backbone with a dopant changing the overall charge per monomer. For the case of a homopolymer that is doped with a positive charge carrier, the effective charge fraction relative to the undoped CPE will (d) increase for a cationic CPE and (e) decrease for an anionic CPE.

The structure of CPEs leads to the potential for self-doping reactions.^30,31 The introduction of charge carriers into conjugated polymers by reaction with Brønstead acids has long been observed, but the mechanistic details remain elusive. Generally, polymers with an ionization energy (IE) near ∼5.0 eV relative to vacuum, such as P3ATs, PEDOT, and PCPDTBT (Fig. 1), can be doped with strong protonic acids.³² Wudl showed early on that the addition of a sulfonic acid-terminated sidechain to P3ATs could lead to p-type doping (oxidation) of the backbone in polar solvents.³⁰ The self-doping occurs as the strongly acidic polymer is added to a solvent and raises the H⁺ concentration, i.e. lowers the pH in water. The polymer is termed self-doped because it provides its own dopant and the charged sidechain balances the charge carrier on the backbone, leading to net charge neutrality. If instead the same polymer was added to solvent in salt form, e.g. sodium sulfonate terminated sidechains, then no self-doping occurs (without the presence of additional H⁺). Thus, one must not assume that all CPEs are necessarily self-doped. Polymers with high electron affinities have been found to self-dope to become n-type as well; for example, the addition of alkylamine groups to naphthalene diimide-based (PNDI) and diketopyrrolopyrrole-based (PDPP) polymers can lead to self-doping in solvent.^33–37 The exact mechanism of the n-type doping and final products are not fully established in many cases.³⁸ In other cases, the doping of conjugated polymers can occur from spontaneous reactions in aqueous environments³⁹ and the polymers are sometimes referred to as self-doped, although the dopant is from the solvent or the environment rather than the polymer itself.³¹ The mechanism of self-doping of CPEs remains an active area of investigation with the aim of understanding the role of the solvent environment and factors such as aggregation.^31,40

The interaction of the CPE and its counterions needs to be carefully considered to understand their intermolecular interactions and optoelectronic properties in solution. Counterions may be associated in proximity to the backbone of a polyelectrolyte in solution in a process referred to as ion condensation.^17,41 The polyelectrolyte chain can be modeled as a series of connected point charges, in which the balance between electrostatic attraction and thermal energy, captured by the Bjerrum length (l_b), dictates the distance of counterions from the polymer backbone (Fig. 2a). When the spacing between the charged groups on neighboring monomers, a, is smaller than l_b, the counterions tend to condense close to the polymer backbone. The predictions of models for ion condensation in dilute solution vary depending on the structure of the polyelectrolyte, e.g. its flexibility in the absence of ionic interactions measured by the persistence length (l_p) of the polymer chain. The general prediction is that some fraction of ions will be closely associated with the backbone, with the number of ions increasing as the dielectric constant of the solvent decreases (as in less polar solvents).^17,41 Association of counterions also influences the conformation of the polymer chain due to the competition between the electrostatic interactions between charged monomers and the steric interactions of the counterions.⁴¹ The charged groups on each monomer have repulsive electrostatic interactions that tend to stretch the chain out, but the associated counterions can reduce the electrostatic interactions allowing the chain to relax, as long as the counterions are not too bulky. Additionally, for electrically doped CPEs, the number and location of counterions relative to the backbone may affect the extent of the delocalization of the electronic charge carriers.⁴² As the ionic strength of the solution changes with the concentration of the polyelectrolyte or the addition of salt, the chain conformation can further change due to differences in electrostatic interactions.⁴¹ Understanding ion condensation is therefore important for interpreting the phase behavior of solutions of oppositely-charged polyelectrolytes and their optical and electronic properties.

The impact of ionic groups on the conformation of conjugated backbones has not been extensively studied for CPEs, but existing studies show system-specific results. Here, we will consider cases where the conjugated backbone is not electrically doped, which would stiffen the backbone due to delocalization of the carrier independently of the counterions. The majority of CPEs have structures with a pendant charge group at the end of a sidechain with varying lengths (Fig. 1); the charge fraction along with the length and conformational flexibility of the backbone and sidechains, will therefore contribute to the average distance of separation between charged groups (Fig. 3a). Because of the molecular structure of CPEs, it is difficult to predict how electrostatics will modify their conformation(s) in dilute solution.⁴³ Small angle neutron scattering (SANS) of dilute solutions of P3ATs terminated with carboxylate groups shows signatures of aggregation, making it difficult to assess the exact role of electrostatics.⁴⁴ Aggregation also occurs in cationic CPEs based on poly(fluorene-alt-thiophene) (PFT) that have two ionic groups per monomer, resulting in cylindrical micellular structures.^45,46 While aggregation is commonly observed, it can depend on the choice of counterion and backbone structure. A poly(3-hexylthiophene) (P3HT) analog terminated with butylimidazolium cations and Br⁻ counterions was found to have worm-like chains by SANS and the persistence length across charge fractions was unaffected in mixtures of polar solvents (Fig. 3b).⁴⁷ For fully sulfonated analogs of P3HT with bulky tetrabutylammonium counterions, SANS across a range of molecular weights (MW) indicated that the chains were fully extended as cylinders in water with minimal aggregation (Fig. 3c).⁴⁸ The difference between these two studies suggests that the chemical identity of the ionic functional group and structure of the counterion plays a significant role in the conformation of the conjugated backbone, as observed for interactions of CPEs with surfactants.⁴⁹ At this point, it is difficult to make general conclusions about the expectations for the impact of ionic groups on the chain conformation(s) of CPEs in solvent, leaving an area ripe for detailed experimental investigation and the development of predictive computational models.⁴³

Fig. 3 (a) The conformation of polyelectrolytes in solution can be characterized by the persistence length, l_p, which is affected by factors including: the distance between ionic groups on the polyelectrolyte, a, the dielectric constant of the solution, ε, and the ionic strength of the solution. The dielectric constant sets the Bjerrum length, l_b, where the electrostatic interaction of oppositely-charged point charges equals the thermal energy (k_bT). (b) The persistence lengths of a cationic P3HT-based CPE and a PCPBTBT-based CPE were found to be independent of charge fraction, f, in polar solvents. Reproduced from ref. 47 with permission from American Chemical Society, copyright 2019. (c) Anionic sulfonated P3HT CPEs with varying degree of polymerization (n = 13, n = 30) were found by SANS to have a rod-like structure in D₂O with a size that depends on the length of the polymer chain. Reproduced from ref. 48.

Phase behavior of conjugated polyelectrolytes

Blends of conjugated polymers and polyelectrolytes have frequently been formed during synthesis (Fig. 4a). For example, PEDOT:PSS is synthesized by polymerization of EDOT monomers in the presence of PSS, leading to a processable blend in water.^9,10,50 Similarly, PANI can be polymerized in the presence of insulating polyelectrolytes.^51,52 The structure and molecular weight of the insulating polyelectrolyte has a strong influence on the resulting properties of these blends, suggesting that there is a templating effect of the IPE due to its chain conformation in solution. In detailed studies, the synthetic procedure allows for control of the ratio of monomers of the conducting polymer to those of the insulating polyelectrolyte.^10,53 In contrast, PEDOT:PSS is frequently obtained from commercial sources, and its composition may be poorly understood. For example, treatments such as solvent rinses are done to remove PSS from solid films to improve their electrical properties.⁵⁴ Given the complex interplay between the polymerization of the conjugated polymer and its interactions with the polyelectrolyte, studying the phase behavior of these systems in solvents remains challenging.⁵⁵

Fig. 4 Schematic of strategies for blending conjugated and insulating components: (a) templated polymerization is a process where neutral monomers are reacted in the presence of another polyelectrolyte to form the conjugated polymer, resulting in a dilute solution or a stable dilute dispersion in water or a polar solvent. (b) Electrostatic complexation is a process where two previously synthesized polyelectrolytes in water or polar solvent are blended. Association of the oppositely-charged polyelectrolytes can lead to phase separation into a polymer-dense coacervate phase and a salt-rich, polymer-dilute supernatant phase.

We,^20,56,57 and others,^48,58–61 have recently begun to investigate the phase behavior of electrostatic complexation of CPEs with insulating polyelectrolytes (IPEs) or other CPEs. Complex coacervation occurs upon mixing of two well-defined polyelectrolytes in solvent after they are synthesized, allowing for characterization of the MW of both species.¹⁷ In solvent, the two oppositely-charged polyelectrolytes may segregate through liquid–liquid phase separation, resulting in a coacervate, a relatively dense liquid-like phase of the two polymer species, and a dilute supernatant phase in equilibrium (Fig. 4b).^7,18 The two polyelectrolytes may also form a solid-like phase referred to as a precipitate.¹⁸ The electrostatic interactions between the polyelectrolyte components overcome the tendency of the dissimilar polymers to phase separate due to unfavorable enthalpic interactions between monomers, resulting in a macroscopically homogeneous blend. The thermodynamics of complex coacervation is multi-dimensional due to the chemical species present, including two dissimilar polyelectrolytes, their counterions, solvent system (neat or a mixture), and potentially additional salt.⁷ An additional complexity is the challenge of controlling of the molecular weight of the CPE because many synthetic routes may limit the range of accessible chain lengths.^1,62

General behavior

The phase behavior of electrostatic complexes of polyelectrolytes is dependent on the addition of salt, the composition of the solvent, and the polymer concentration in the blend. Given the large number of components, a common strategy is to examine phase diagrams of polyelectrolytes in terms of total polymer concentration versus the total amount of salt added to the solution (Fig. 5). We note that in this approach, the salt concentration typically only refers to the added ions and not the counterions from the two polyelectrolytes. As solutions of the polycation and polyanion are mixed, the polyelectrolytes may associate to form a polymer-dense coacervate phase and expel counterions into the polymer-dilute supernatant phase. The driving force for the phase separation is highly dependent on the entropy of complexation, which is related to the release of the counterions associated with the polyelectrolytes into the solvent and the reorganization of the solvent around the resulting ionic species.^7,17,63 The partitioning of the salt and polymer present into the coacervate and supernatant phases is indicated by tie lines across the binodal region in the phase diagram. When additional salt is introduced, the ion pairing between the oppositely-charged polyelectrolytes can be displaced by the salt, leading to reduction and eventual dissolution of the coacervate phase. A simple way to understand the phase behavior is that the entropy gained from the release of the counterions from the polyelectrolytes to solvent is reduced as the additional salt concentration increases. Fundamentally, the resistance to dissolution by the additional salt measures the stability of the coacervate phase and depends sensitively on the identity of the added salt, as well as the solvent-polymer, solvent-ion, and ion-polymer interactions. The phase behavior of CPEs is not as well understood as for IPEs, leading to detailed studies of the interplay of these factors.

Fig. 5 Schematic of a model binodal phase diagram of polyelectrolyte coacervation as a function of added salt and total polymer concentration. Different states are represented by the circles where the color indicates the relative polymer concentration. The dark blue tie line shows spontaneous phase separation, and the light blue line indicates the salt-induced dissolution transition.

Influence of salt

Studies of the addition of salt to complexes of CPEs and IPEs have revealed key features of phase behavior and the subsequent influence on electronic properties. In a study of an alkylsulfonate-functionalized PEDOT with an imidazolium-based IPE (PImPAM), the phase diagram showed complexation over a wide range of compositions (Fig. 6a).²⁰ The addition of salt broadened the window where only solution and coacervate were present and prevented the formation of precipitates, thus providing a means to control processability. The structure of the added salt frequently influences the phase behavior of polyelectrolyte blends. Salts in water can be described using a framework by Hofmeister to be chaotropes, which disrupt the local interactions of water, or kosmotropes, which induce local structuring. In coacervation of non-conjugated polyelectrolytes, chaotropic ions appear to partition more to the dense phase than kosmotropic salts.⁶⁴ A study of a cationic poly(fluorene-alt-phenyl) (PFP) CPE and PSS in water found that larger ions, in the series Li⁺, K⁺, and Cs⁺, increasingly partition respectively into the dense polymer-rich phase consistent with results from conventional polyelectrolytes (Fig. 6b).⁶⁵ Simultaneously, as salt was added (salt concentration increased in the mixture), the water content in the dense phase decreased, which suggests that the ions can displace water. Using time-resolved photoluminescence (PL) anisotropy measurements that are sensitive to the conformation and dynamics of CPE chains, the complex coacervate phase was found to be spatially heterogeneous with depolarization times that depend on the ion type, suggesting additional influence of the identity of the salt on the properties of the CPE chains in complexes. Furthermore, compared to simple inorganic salts, the salt-induced dissolution transition has been shown to occur at lower concentrations for organic salts. This trend was observed for a range of organic ions regardless of their chemical structure, indicating that the hydrophobic nature of the ion plays an important role in the phase behavior (Fig. 6c).

Fig. 6 (a) The experimental phase diagram of a sulfonated-PEDOT and PImPAM as a function of added KBr and total polymer concentration shows a broad window of the 2-phase region. Optical microscopy shows characteristics of the complex coacervate and precipitate phases (Scale bar: 100 µm). Reproduced from ref. 20 with permission from the American Chemical Society, copyright 2021. (b) Fluorescence microscopy of PFP:PSS shows that the size of the salt cation affects the phase behavior and the concentration of salt in the dense phase. Reproduced from ref. 65 with permission from the American Chemical Society, copyright 2021. (c) The addition of inorganic and organic salt yields different salt resistance for the resulting dense (coacervate) phase for PFPI:NaPCPT (Scale bar: 50 µm) reproduced from ref. 61 with permission from Wiley-VCH GmbH, copyright 2022.

Influence of solvent

Because CPEs are soluble in polar solvents, complexation can be examined in non-aqueous solvents or solvent mixtures with water. The backbone of CPEs is generally hydrophobic, leading to the expectation that solvent quality will be a factor controlling the physical state of polyelectrolyte complexes. Indeed, the addition of an organic solvent (THF) to aqueous solutions converted solid-like CPE:IPE precipitates into fluidlike complex coacervates, attributed to interactions of the organic solvent with the poly(thiophene)-based CPE.⁵⁷ In this case, the coacervate region developed with added organic solvent rather than added salt as typically observed in hydrophobic systems.⁵⁷ This result contrasts with work on hydrophilic polypeptide⁶⁶ and functionalized polystyrene⁶⁷ systems, where co-solvents systematically weakened salt resistance of complexes, suggesting that aqueous solvent mixtures can reduce the stability of strongly interacting polyelectrolyte pairs. For comparison, a study of IPE:IPE complexation in trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP) found that the solvent dielectric constant substantially influences salt resistance and phase behavior, with the lower-polarity HFIP system showing higher salt resistance than TFE.⁶⁸ Given that the identity of the counterion and solvent interactions are difficult to separate, further studies of the phase behavior of CPE:IPE blends across solvents and salts will help to reveal the interplay of these factors.

Influence of molecular structure

Electrostatic complexation can also be used to form complexes of two oppositely-charged CPEs, allowing for the intimate association of polymers that would normally phase separate. Complexes of oppositely-charged CPEs demonstrate good stability even at relatively high added salt concentrations. The added salt size, more than identity, was shown to affect the rheological properties and exciton behavior, as discussed further in the next section. In a study of complexes of PCPDTBT CPEs functionalized with alkylsulfonate or alkylpyridinium groups, the phase behavior was found to show two-phase regions and also a single soft-solid phase, depending on the total concentration of the anionic and cationic CPEs.⁵⁶ Interestingly, blends of anionic and cationic polyalkylthiophene-based CPEs have been reported not to show obvious signs of liquid–liquid phase separation or precipitation in water.⁴⁸ In this study, the solutions were relatively dilute, therefore, it is possible that coacervation could still occur in other parts of the phase diagram. The ability to form complexes of oppositely-charged CPEs opens up new opportunities to form assemblies relevant for the study of light harvesting with conjugated polymers.^59,61

Optical properties of conjugated polyelectrolyte complexes in solution

Electrostatic interactions have been recognized as a pathway to control the optical properties of CPEs. The optical absorption of CPEs generally resembles that of the parent neutral polymer in both solutions and thin films.^49,69,70 The small spectral shifts observed are consistent with the remote position and separation of the electronic levels of the sidechain and backbone and can be attributed to changes in the refractive index of the medium. Like their parent neutral form, the optical absorption and photoluminescence of CPEs are highly sensitive to chain conformation and interchain interactions.¹ The interaction of CPEs and IPEs in solvent can further modify both their chain conformation and interchain interactions, such as aggregation, thereby changing their optical properties. For example, dilute complexes of an alkylsulfonate-poly(p-phenylene vinylene) (PPV) with a cationic polyelectrolyte in aqueous solution were found to improve the ability to sense the presence of cationic and anionic analytes through changes in the photoluminescence (PL) of the PPV chains.^16,71 The variable, sensitive, and tunable optoelectronic properties of CPEs have been leveraged to develop innovative biosensors,² which are usually examined under more dilute conditions than the concentration range used in electrostatic complexation.

Electrostatic complexation allows for intimate intermolecular electronic interactions between CPEs with different backbones, providing new avenues to examine energy transfer upon optical excitation. For example, complexes of a cationic polyfluorene-based CPE (PFPI) donor and varying anionic CPE acceptors form electrostatic complexes with effective exciton energy transfer.^{59–61,65,72} By using PFPI as a donor and a polythiophene-based CPE (PTA) as an acceptor (Fig. 1a), it has been shown that the PL of the polythiophene can be enhanced in the dense phase upon complexation (Fig. 7a).⁶⁰ This result is surprising because of the tendency to observe PL quenching in aggregates of conjugated polymers, and the enhancement was attributed to planarization of the backbones of the CPEs evidenced by changes in the vibronic progression in the PL of the complexes. The structure of complexes of PFPI with carboxylic acid-functionalized polythiophenes was further found to evolve as a function of temperature from disordered to ladder-like aggregates based on shifts in the PL spectra.⁵⁹ By mixing PFP as an electron donor with an anionic cyclopentadithiophene-based (PCPT) CPE as an acceptor, complex coacervation occurs and allows for the observation of efficient exciton energy transfer in a dense liquid phase. This pair had phase behavior such that the addition of salt increased the polymer concentration in the dense phase and the use of an inorganic or organic salt was shown to tune the mechanical modulus of the coacervate phase.⁶¹ Examination of PL demonstrated efficient energy transfer between the two CPEs with an increase in the exciton lifetime on the acceptor in the dense phase, relative to dilute solution (Fig. 7b). The coacervate phase, therefore, provides a unique strategy to control the formation of assemblies of conjugated polymers that would normally phase separate. Consequently, electrostatic complexation provides a unique route to examine fundamental polymer chain conformation and charge transfer processes relevant to organic solar cells and light-emitting devices that rely on heterojunctions of conjugated polymers.

Fig. 7 (a) Schematic of a complex of the energy donor PFPI and energy acceptor PTAK showing the energy flow in the electrostatic complex upon excitation primarily of PFPI and emission dominantly from PTAK. The PL spectra of PTAK in the complexes (right panel) show a vibronic progression indicating stiffening of the backbone relative to the neat CPEs (left panel). Reproduced from ref. 60 with permission from the American Chemical Society, copyright 2016. (b) For complexes of PFPI and PCPT, the average photoluminescence lifetime varies as a function of the identity of added salt, increasing lifetime correlated to increasing salt. Reproduced from ref. 61 with permission from Wiley-VCH GmbH, copyright 2022.

Electronic properties of solid conjugated polyelectrolyte complexes

The route used to form polyelectrolyte complexes in solvated form has a strong influence on how they are processed into solid forms. For cases where electrostatic complexes are formed directly from synthesis, the resulting solution is typically dilute (in polymer) and can be cast into thin (≈100 nm) films using methods like spin coating. In contrast, after the complex coacervation of two polyelectrolytes, the resulting dense phase is polymer-rich (≈20–50 wt% polymer). The highly viscous dense phase can be isolated from the dilute supernatant phase and processed into solid form through methods such as blade coating or extrusion printing. Because of the high concentration of polymer, electrostatic complexation allows thick films (≈1–1000 µm) to be formed readily. In a different approach, precipitates formed by complexation can be made more processable by swelling with water and salt; the resulting phase is referred to as saloplastic because the salt acts as a plasticizer.^19,73,74 The polymer complexes formed from different routes may or may not have comparable solvent and ionic composition, so it is important to note processing conditions when comparing studies.

The properties of solidified blends are known to be affected by the detailed structure of the insulating polyelectrolytes when conjugated polymers are synthesized in the presence of a polyelectrolyte. For example, the degree of sulfonation of polystyrene, along with the identity of the counterion, have been shown to strongly impact the work function and hole injection behavior of PEDOT:PSS.¹⁰ The polydispersity of PSS has been shown to modify the electrical conductivity of PEDOT:PSS with and without solvent additives (Fig. 8).⁵³ Both the charge fraction of PSS and the volume fraction of PEDOT can affect the final electrical conductivity of PEDOT:PSS; the differences were attributed to conformational effects from PSS.¹⁰ Similarly, the molecular weight and polydispersity of PAAMPSA, a polyacid that can dope PANI (Fig. 1), have been shown to have a substantial impact on the electrical conductivity of PANI:PAMPSA blends (Fig. 8). These effects are likely due to the conformation of the polyelectrolytes, which are known to vary depending on MW and the charge fraction.^17,41 The differences in the interactions of the polyelectrolytes in solution leads to templating of the nanostructure of the resulting blend, impacting the final electrical properties. Due to challenges in characterizing the MW of PEDOT and PANI in these blends, it is difficult to completely separate the role of electrostatic interactions from modifications of the chain length of the CPE, but the templating effect is clearly observed across systems. Additionally, the chemical structure of the IPE can also modify the electrical conductivity of PEDOT:IPE blends. It has been found that across anionic polyelectrolytes only particular anionic groups with styrenic backbones, such as PSS and PSTFSI (Fig. 1), lead to high electrical conductivity with PEDOT.⁵⁰ While charge transport through insulating regions has been implicated as a bottleneck for transport in PEDOT:PSS, it is difficult to assess the role of microstructure on transport given that PEDOT may form dispersed aggregates in addition to mixing intimately with the polyelectrolyte.⁷⁵ Because these complexes are formed directly during synthesis, it is not clear if these observations can be directly translated to results of complexation from directly mixing CPEs and IPEs, but one expects similar factors to affect the final properties of solid complexes.

Fig. 8 (a) Increasing the molecular weight of PAAMPSA was found to decrease the electrical conductivity of blends with PANI. Reproduced from ref. 51 (b) The electrical conductivity of PEDOT:PSS depends on the molecular weight of PSS with smaller differences in blends formed without solvent additives and larger changes in performance with additives commonly used to fabricate organic electrochemical transistors (ethylene glycol (EG), (3-glycidyloxypropyl)trimethoxysilane (GOPS), and dodecylbenzenesulfonic acid (DBSA)) Reproduced from ref. 53.

The utility of blends of CPEs and IPEs can also be seen in applications of films made through layer-by-layer (LBL) deposition.^{11,25,76–80} In such systems, composite films are cast by sequentially depositing oppositely-charged polyelectrolytes, resulting in a stratified structure of layers with thicknesses comparable to single polymer chains.⁸¹ For example, water-soluble, alkoxysulfonate-functionalized PEDOT (PEDOT-S) was synthesized both chemically and electrochemically for layer-by-layer deposition with poly(allylamine hydrochloride) (PAH) (Fig. 9a).¹¹ The resulting polyelectrolyte LBL films of PEDOT-S/PAH showed superior and reproducible electrochemical properties compared to electrochemically deposited films of PEDOT-S and were favored in applications requiring the use of aqueous-based electrolytes (Fig. 9b). The viscoelastic behavior of such polyelectrolyte multilayers under water resembles very soft hydrogels and are comparable to adsorbed protein layers.⁸² Such assemblies have proven useful as injection layers in organic solar cells and LEDs, electrochromics, and in supercapacitors, in part because of their resistance to removal by solvent.^24,77,78 A drawback of layer-by-layer approaches is the speed of the sequential deposition of the polymers and the difficulty in controlling the exact stoichiometry of the two polyelectrolytes during the deposition process.

Fig. 9 (a) A schematic of layer-by-layer (LBL) deposition of PEDOT-S and PAH. (b) Spectroelectrochemistry reveals that LBL films of PEDOT-S and PAH (right) exhibit a simple transition between two states with an isobestic point as a function of applied potential while a monolayer film of PEDOT-S (left) shows evidence of structural evolution as a function of potential (left). Reproduced from ref. 11 with permission from the American Chemical Society, copyright 2005.

The solution route to complexation offers the ability to control the nanostructure of the resulting complexes by controlling the charge fraction of each component. The interactions between polyelectrolytes can be thought of as a balance between interactions of the analogous neutral chains and electrostatic interactions. A number of theoretical predictions have been made for the structure of the polymer dense phase as a function of charge fraction, with a tendency of high charge fraction blends being homogeneous and a transition to a nano/microphase separated structure at low charge fraction.^83,84 The length scale for the phase separated structure correlates with the chain size of the constituent polyelectrolytes and therefore tends to be at the 10-nm scale. An important consideration is whether the equilibrium structure of the solvent-swollen complex will match that of the dry, solid complex. For P3AT-based CPEs with model IPEs, the theoretical predictions are in reasonable agreement with observations of microemulsion-like structures, evidenced by small angle X-ray scattering (SAXS), occurring at low charge fraction and homogeneous blends at high charge fraction (>75%) (Fig. 10a).⁸⁵ High charge fraction blends of sulfonated-PEDOTs and linear IPEs were observed to be homogeneous by SAXS also agreeing with these predictions (Fig. 10b).^20,28 There has not been an investigation to determine the phase behavior of complexes of a CPE across a broad range of IPEs to determine the limits of control of the nanostructure, but it is apparent that high charge fraction blends generally lead to more homogeneous structures with intimate mixing of the two polyelectrolytes.

Fig. 10 (a) Small angle X-ray scattering of complexes of P3AT-based CPEs and an imidazolium-functionalized PAA-based IPE show the emergence of microphase separation at charge fractions of 75% and below. Both the CPE and IPE are random co-polymers of the two monomers. Reproduced from ref. 87 with permission from the American Chemical Society, copyright 2023. (b) Lack of microphase separation is observed in complexes of PEDOT-based CPEs and PMETAC with a high charge fraction. Reproduced from ref. 28 with permission from the American Chemical Society, copyright 2025.

Electrostatic complexation also affects the local interactions of the polymer chains in CPE:IPE complexes. The optical absorption of solid CPE:IPE complexes has been shown to shift with respect to the neat CPE, with the possibility of blue or red shifts depending on the CPE.^29,86–88 The shift of the spectrum depends on whether the CPE is amorphous or structurally ordered in its neat state, given the expectation of reduced aggregation of CPE chains in complexes. For example, the UV-vis spectrum of a cationic imidazolium-functionalized P3AT with 100% charge fraction complexed with sodium PSS showed a red-shifted spectrum relative to the amorphous, neat CPE.⁸⁶ In contrast, the optical absorption of the complex of a 50% charge fraction variant was blue-shifted relative to the neat film, which had signs of ordered aggregates. The absorption of both complexes was essentially identical, indicating that both CPEs had a similar conjugation length in complexes with PSS. Both aggregation and stiffening of the backbone could lead to red-shifts in the optical absorption spectrum. Analysis of the vibronic structure of the PL spectra of a series of sulfonated-P3ATs complexed with a polyionic liquid indicated that the CPE chain stiffens upon complexation, resulting in chain planarization.⁸⁷ The interactions of the CPE and IPE therefore provides a route to control the optical properties of CPEs in solid-state blends.

The electrical properties of solidified electrostatic complexes show behavior that is a function of both local and longer-range structural features. In an electrostatic complex, the volume fraction of the conjugated polymer is determined by the insulating polyelectrolyte. With linear polyelectrolytes, the volume fraction of the CPE is ≈50% for simple 1 : 1 complexes. One would therefore expect the electrical conductivity upon doping to be lower than that of a neat material. In complexes where the neat CPE has a high electrical conductivity, the expected decrease has been observed. In contrast, for some CPEs with a relatively low MW, the electrical conductivity of a doped complex can be higher than that of the neat material.^86,87 For conventional conjugated polymers, the carrier mobility is a function of MW because the higher MW chains provide molecular ties between aggregated domains that may not occur at lower MW.^89,90 Many CPEs have relatively low MW, e.g. ∼10 kDa, and they can be estimated to have electrical conductivities near ∼10⁻³ S cm⁻¹ at high carrier concentration (∼10²⁰ cm⁻³), assuming they have carrier mobilities similar to neutral polymers. In contrast, electrostatic complexes of low MW polythiophene-based CPEs have been found to have relatively high electrical conductivities ∼10⁻¹ to 1 S cm⁻¹ upon doping, suggesting that the interaction of the CPE and IPE can improve the ability of carriers to hop along and between CPE chains (Fig. 11).^86,87,91 A detailed study of temperature-dependent electrical conductivity along a series of complexes of sulfonated P3AT-based CPEs with varying charge fraction showed that the activation energy for transport in the complexes was generally lower than that for the neat CPE, and the high temperature limit of the conductivity was higher for the complexes.⁸⁷ Thus, the electrical properties of CPE-based complexes can be distinct from those of the neat CPE, and design rules to control the behavior of complexes are under investigation.

Fig. 11 The electrical conductivity of HTFSI-doped electrostatic complexes of P3HT-based CPEs and PSS were found to be higher than that of the doped CPE films. Reproduced from ref. 91 with permission from the American Chemical Society, copyright 2023.

Applications for electrostatic complexes

Electrostatic complexation of CPEs has been studied more frequently in solution, but there are a variety of applications where the possibility of using the dense phase to form solids would provide benefits. For example, the rheological behavior of electrostatic complexes provides a route to create formulations for direct-ink writing relevant for 3D printing.^23,92–95 The ability to compatibilize polymers that would otherwise phase separate allows for the introduction of complexes with multiple functional properties.^29,96 Electrostatic complexes also allow for the transport of both electronic charge carriers and ionic species relevant for applications like batteries and bioelectronics, where mixed conduction is essential.^97,98 Here, we discuss a few of the initial studies into the applications that benefit from the properties of electrostatic complexes of CPEs.

Stretchable conductors

The ability to compatibilize polymers with vastly different architectures and chemistries using electrostatic complexation offers new routes to form mechanically stretchable conductors. While in many electrostatic complexes, the IPE is a conventional linear polymer, it is also possible to form CPE:IPE complexes where the IPE has a very different architecture. Bottlebrush polymers are effectively “polymers of polymers” leading to a structure that mitigates entanglements and can lead to supersoft behavior, that is a low mechanical modulus (<10 kPa) without the presence of solvents.⁹⁹ Ionic groups can be added to the end of macromolecular sidechains along the backbone of the bottlebrush, leading to bottlebrush polyelectrolytes.¹⁰⁰ Both a sulfonated P3HT⁹⁶ and a sulfonated PCPDTBT²⁹ were shown to form compatibilized blends with a BPE with a norbornene-based backbone with imidazolium-functionalized poly (4-methyl caprolactone) sidechains (PMCL-IM) (Fig. 1). The resulting solid complexes are supersoft and stretchable due to the ionic crosslinks between chains (Fig. 12). Despite the relatively high-volume percentage of the BPE in complexes with CPEs, the CPE chains can still percolate through the blend to form pathways for electrical conductivity. Complexes of P3HT-S and BPE yielded electronic conductivities up to 0.3 S cm⁻¹ after doping and also had a low tensile modulus (0.2 MPa). Such demonstrations show the potential to use electrostatic complexation to form solids with multifunctional properties.

Fig. 12 Electrostatic complexes of a sulfonated P3HT-based CPE and a BPE have tensile moduli ≈700 kPa (undoped) and ≈200 kPa (doped); the undoped complexes show significantly higher strain tolerance. In their doped state, the complexes show stretchability and recovery below the yield point. The change in resistance shows a smaller change than expected for a homogeneous elastic conductor. Reproduced from ref. 96 with permission from the American Chemical Society, copyright 2023.

Battery binders

A key feature of CPE-based complexes is their ability to form mixed ionic-electronic conductors when doped with additional salt. Because of their ability to host mixed conduction, electrostatic complexes of CPEs and IPEs have been studied for applications in lithium-ion batteries.^21,86 The polar electrolytes used in most batteries tend to dissolve CPEs and many IPEs. However, P3AT-based CPE:IPE electrostatic complexes have shown resistance to dissolution by the polar electrolyte solutions used for lithium-ion batteries (Fig. 12). Additionally, the ionic environment of the complex allows for the dissociation of added salt despite the relatively low dielectric constant of the polymers (in their neutral form). At charge fractions of >0.5, the concentration of charged groups in a typical CPE:IPE complex is ∼1 M, comparable to that of a concentrated electrolyte. These charged groups are constrained by the attachment to the polymer backbone and do not contribute to the overall ionic conductivity upon the addition of salt. The ionic conductivity of complexes of cationic P3ATs with added salt (LiTFSI) depends on the diffuseness of the charge density of the pendant ion because of differences in their electrostatic interactions with Li⁺.⁹¹ The Li⁺ transference numbers, the portion of the ionic conductivity attributed to Li⁺ ions, are 0.17–0.26 in the dry state for P3ATs with imidazolium groups and comparable to well-performing ion conductors. Comparison of the ionic conductivities of a series of complexes of CPEs with added salt show reduced, matched, or improved values compared to the constituent CPE indicating that the complex leads to a distinct environment.^86,88,91

Complexes of CPEs have been examined as binders in model cathodes for lithium-ion batteries.^21,86 To form a cathode (or anode) of a battery, a redox-active inorganic compound is blended with a carbon-based conductor, such as carbon black, and an electrically insulating polymer binder, such as poly(vinylidene fluoride) (PVDF). The binder provides processability and helps to promote the movement of ionic species along with electrical charge in the electrode. In the assembled battery, the binder also provides mechanical stability to maintain the structure of the electrode in the presence of electrolyte. The composite electrode is usually slurry-casted onto the current collector and thus processability at high-solids loading is required to form homogeneous electrodes. Lithium iron phosphate (LFP), an alternative to cobalt-based materials, suffers from poor electrical conductivity that can slow the insertion of Li⁺ ions into the cathode. A current solution is to coat particles of LFP with conductive carbon to improve the performance with conventional binders. The addition of a mixed-conducting binder can further help to minimize the impedance of the electrode and to improve the rate capability and cycle life of the battery. Binders for LFP cathodes formed with electrostatic complexes of P3AT-based CPEs exhibited improved electrode kinetics, higher rate capability, and increased cycle life relative to analogous coin cell batteries with PVDF.^21,86 The complex binders have ionic conductivities of the order of 10⁻⁴ S cm⁻¹ when swollen with an electrolyte at room temperature, helping to improve the overall electrode kinetics (Fig. 13). The ability to modify the chemistry and electrochemical properties of the CPE provides a platform to leverage these properties for emerging battery electrodes.

Fig. 13 (a) Battery cathodes are composites comprising an active material for the reversible storage of lithium ions, a polymeric binder for mechanical support, and carbon additives. The cathode is swollen with liquid electrolyte. (b) Neat CPEs are frequently soluble in electrolytes, but electrostatic complexes can be stable. The photograph shows vials with 1M LiPF₆ in 1:1 v:v ethylene carbonate:dimethyl carbonate dissolving, from left to right, P3HT-TMA⁺Br⁻, P3HT-Im⁺Br⁻, and P3HT-co-P3HT⁺Br⁻ (top; chemical structures in Fig. 11), but not their respective complexes with NaPSS (bottom). (c) Rate capability data for coin cell batteries made with cathodes of LFP/carbon/binder (85 : 6 : 9 wt%) during symmetric galvanostatic charge/discharge using PVDF or P3HT-Im⁺:PSS⁻. C/n indicates the charge rate for the cycle; 1C indicates full charging (or discharging) in 1 hour while C/2 indicates a rate of 2 hours and 2C indicates a rate of 0.5 hours. Reproduced from ref. 86 with permission from the American Chemical Society, copyright 2023.

Bioelectronics

The development of design rules for electrostatic complexes that exhibit mixed ionic-electronic conduction is likely to aid the development of bioelectronic devices.¹⁰¹ For example, blends of sulfonated P3ATs and sulfonated PEDOTs (PEDOT-S) have been found to be useful to tune the threshold voltage that sets the onset of conduction in organic electrochemical transistors (OECTs) and their switching times (Fig. 14).¹⁰² The ability to tune the threshold voltage helps to enable operation near physiological potentials, improving signal detection without causing harmful electrochemical reactions.³⁹ Whether these polymers were phase-separated in the resulting solid film was not investigated in detail, but the results suggest that the ability to control composition through electrostatic complexation will provide new routes to modify the properties of OECTs. Blends of CPEs and IPEs have been formed in other routes, resulting in gels relevant for bioelectronics. Hydrogels of carboxylic acid-terminated polythiophenes with PDADMAC show salt-tunable mechanical moduli and resistance to dissolution in polar solvents, while still exhibiting mixed ion–electron conduction.¹⁰³ In another example, the electronic properties of blends of PEDOT-S with an amine-rich gelatin polyelectrolyte have been examined.¹⁰⁴ The addition of a PEDOT co-monomer with an activated ester allowing a coupling reaction stabilized the blends compared to PEDOT-S alone, but the question of electrostatic stabilization between the anionic CPE and the cationic gelatin was not investigated.¹⁰⁴ Similarly, PEDOT-S was found to help aid infiltration into collagen with additional stabilization by a reaction to form a covalent link with a co-monomer (Fig. 14a).¹⁰⁵ Recently, composites of biological cells and PEDOT-S have been formed by infiltration and studied for biophotovoltaic applications (Fig. 14b).¹⁰⁶ The emerging understanding of electrostatic complexation of CPEs and IPEs should help to rationally design blends for the multifaceted environment required in the operation of bioelectronics.

Fig. 14 (a) Collagen is a charged biopolymer that is useful as a scaffold for growth of biological cells. PEDOT-S has been found to show better infiltration into collagen structures in comparison to PEDOT:PSS, likely due to electrostatic interactions. Reproduced from ref. 105 with permission from Wiley-VCH GmbH, copyright 2024. (b) Biological cells have complex structures with charged surfaces. Interactions with water soluble CPEs allow for the formation of composites of biological cells and PEDOT-S. Reproduced from ref. 106 with permission from Springer-Nature, copyright 2025.

Outlook

Conjugated polyelectrolytes have proven to hold unique properties for applications across organic electronics and biosensing. The opportunities and design rules for using CPEs in electrostatic blends continue to emerge. Building better ties between insights from the polymer physics of conventional polyelectrolytes and these functional polymers will bring new control of their electronic, mechanical, and optical properties. The expansion of chemistries to synthesize CPEs with varying architectures and ionic groups will open new avenues for exploration of electrostatic interactions in different solvent environments. CPEs will also help reveal the fundamental behavior of polyelectrolyte complexes by providing an avenue to explore the role of chain structure and flexibility upon complexation. Given the interest in materials systems for mixed ionic-electronic conduction across applications, electrostatic complexation will continue to be explored as a route to form processable materials with unique combinations of properties and performance.

Conflicts of interest

There are no conflicts to declare.

Data availability

This article is a review and does not contain new data.

Acknowledgements

We acknowledge support from U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-SC0016390 for research related to mixed ion–electron transport. We acknowledge support from the National Science Foundation through the Materials Research Science and Engineering Center (MRSEC) at UC Santa Barbara: NSF DMR-2308708 (IRG-1) for research related to compatibilization of polyelectrolytes.

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