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Organic Flexible Electronics || Electronic and ionic transport in organic materials and devices
Organic Flexible Electronics || Electronic and ionic transport in organic materials and devices
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سال:
2021
DOI:
10.1016/B978-0-12-818890-3.00003-5
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Electronic and ionic transport in organic materials and devices 3 Paolo Romele, Zsolt M. Kovács-Vajna and Fabrizio Torricelli University of Brescia, Department of Information Engineering, Brescia, Italy 3.1 Charge transport in organic materials Electronic charge transport in organic and polymeric materials impacts the electrical, optical, and thermal properties of organic devices. The wide range of organic materials and fabrication techniques used for the material deposition and device fabrication, provides organic materials with very different morphological structure and properties. Organic devices are obtained by combining several materials including, for example, organic small molecules and polymers, polymer blends, polyelectrolytes, electrolytes, inorganic compounds (typically used as insulators), and metals (typically used as electrodes). This large variety of materials, processes, and device structures results in very different bulk and interface charge transport properties of organic materials and devices. It is quite common that the very same organic semiconductor can display different charge transport properties depending on the solvent, annealing temperature, substrate, interaction with other materials used for the device fabrication, and so on. In addition, depending on the operating conditions of the device (e.g., applied voltage, temperature, and electric field) and device geometries, only a small “window” of the organic material properties can be assessed. By the way of example, organic light emitting diodes operate in space charge limited transport, with a typical charge carrier concentration in the range 10141016 cm23 [1,2], while organic field-effect transistors can operate in both weak and strong accumulation spanning charge concentrations in the range 10161020 cm23 [3,4]. Interestingly, organic ion-gated transistor, comprising both electrolyte-gate and electrochemical transistors, can achieve charge concentrations as high as 1022 cm23 [5,6], making them very interesting for the in; vestigation and understanding of the charge transport at the transition between the semiconducting and conducting regimes [5]. Moreover, in the case of organic electrochemical transistors the charge concentration can be modulated in the whole bulk of the organic polymer by means of ionicelectronic charge interaction and/or electrochemical charge transfer [7]. In organic materials and devices, the energetic and spatial order of the molecules and their interaction with the interface materials are reflected in the density of states (DOS) distributed in the bulk and at the interface. In highly ordered (crystalline) organic materials, the charge carriers are delocalized on several molecules, defining a delocalized band energetically positioned at the lowest unoccupied molecular Organic Flexible Electronics. DOI: https://doi.org/10.1016/B978-0-12-818890-3.00003-5 © 2021 Elsevier Ltd. All rights reserved. 72 Organic Flexible Electronics orbital (LUMO) in the case of electron transport and at the highest occupied molecular orbital in the case of hole transport. Although crystalline organic semiconductors, like rubrene, have been demonstrated, most of the organic semiconductors are polycrystalline or amorphous. By the way of example, the chemical or physical defects in conjugated polymer “brake” the polymer chains into subconjugation subunits. Charge carriers can easily move within a conjugation subunit while an energetic or spatial barrier have to be overcome when passing from one subunit to another. In contrast to highly ordered organic materials where charge carriers hop between closely spaced molecules forming a crystalline stack, in disordered organic materials the charge transport is strictly related to the density of the localized states. Depending on the degree of disorder and on the position of the Fermi energy level, the charge transport in organic semiconductors involves (1) the density of delocalized states only, (2) the density of localized states only, and (3) both the density of delocalized and localized states. The aforementioned cases are described with different charge transport theories. If there is no disorder and/or the Fermi energy level is above the delocalized states (viz. all the traps are filled), charge carriers move like free particles in the delocalized DOS and the Drude model is conceptually appropriate for describing the charge transport [8]. In the case of a medium level of structural disorder (e.g., polycrystalline organic materials) and/or when the Fermi energy level is approaching the delocalized states, both delocalized and localized states have to be considered and the charge transport is described by the multiple trapping and release (MTR) model [9]. The MTR model, originally developed for polycrystalline and amorphous silicon, assumes that most of the charge carriers are trapped into the localized states and released in the delocalized states upon thermal excitation (Fig. 3.1). Charge transport takes place only in delocalized states and only a small fraction of charge carriers contributes to the transport and the conductivity reads: σMTR 5 qμds cds ; (3.1) where q is the elementary charge, μds is the charge carrier mobility in the delocalized states, and cds is the charge concentration in the delocalized states, that can be calculated by solving the Fermi-Dirac integral expression and in the case of electrons and holes reads, respectively: ð nds ðEF Þ 5 gdes ðEÞ ð pds ðEF Þ 5 1 dE 1 1 exp E k2B TEF gdhs ðEÞ exp E 2 EF kB T 1 1 exp (3.2) E 2 EF kB T dE (3.3) Electronic and ionic transport in organic materials and devices 73 Figure 3.1 Schematic view of the MTR transport. A charge carrier (electron or hole) moving in the delocalized states is trapped in a localized stated and then thermally released. MTR, Multiple trapping and release. where gdes(E) and gdhs(E) are the density of delocalized states of electrons and holes, respectively, E is the energy, kB is the Boltzmann constant, T is the temperature, and EF is the Fermi energy level. Finally, the field-effect mobility due to the overall charge concentration can be calculated as: μMTR 5 σMTR ; cds 1 cls (3.4) where cls is the trapped charge concentration in the localized states and can be calculated by replacing in Eqs. (3.2) and (3.3) the gdes(E) and gdhs(E) with the density of localized states gles(E) and glhs(E), respectively. As a consequence, MTR charge transport depends on the shape and energy position of both delocalized and localized DOS. In the most common case of organic devices based on disordered organic materials, charge transport can take place in localized states by means of hopping, viz. thermally assisted tunneling process. The hopping process between two small molecules or subconjugated units of a polymer can be described by considering the equivalent energy site Es and position rs 5 (xs, ys, zs). The charge transport process between two sites i and j is depicted in Fig. 3.2. A charge carrier occupies a site with energy Ei and position ri. Upon phonon absorption, the energy of the charge carrier increases (E 5 Ei 1 ΔE) and because of quantum tunneling there is a probability that the carrier can hop from site i to site j. At the end of this process, the particle has a new energy Ej and position rj. It is worth to note that a carrier may hop 74 Organic Flexible Electronics Figure 3.2 Representation of a hopping process. The two localized states are depicted as potential wells with different energies. The absorption of a phonon with energy ΔE makes the tunneling from the state i to the state j possible. provided that the initial site is occupied and the target site is unoccupied. In order to quantitative describe this hopping process a probability evolution equation, named Master equation, is used [8]: X dfi ðtÞ X 5 υji fj ðtÞ½1 2 fi ðtÞ 2 υij fi ðtÞ 1 2 fj ðtÞ 2 λi fi ðtÞ; dt j6¼i j6¼i (3.5) where fi(t) is the probability that site i is occupied by a carrier at a time t, [1 2 fj(t)] is the probability that site j is unoccupied, ν ij is the transition rate from site i to site j and λi is the decay rate of the excitation at site i. This is a nonlinear equation with a potentially extremely large number of terms. For a given problem, the site locations ri need to be given as well as the decay rates for each site (often one assumes that the recombination is only a small perturbation, and assumes that λi 5 0). The hopping probability is then composed of two factors, the tunnel term and the thermal activation term. Assuming an exponential decay of the wavefunction tails Ψ(r)~exp(2αr), where α is the reverse of the localization length, the transfer integral can be approximated as exp(22αrij). This term takes into account the actual wavefunction overlap of the two sites, which depends not only on the decay length of the wavefunctions but also on the reciprocal orientation of the molecules which can affect the effective overlap of the π orbitals. For upward hops (Ei , Ej), the energetic difference between the two sites is provided by phonons, so, the total hopping probability has to depend on the number of available phonons with energy ΔE. Assuming the Boltzmann statistic for the phonon distribution, the thermal activation term has the following form: exp(ΔE/ (kBT)). On the contrary, for downward hops (Ei . Ej), no thermal activation is required and only the tunneling term survives. Then the total hopping probability of the jump can be calculated with the Miller-Abrahams hopping rate and reads [10]: Ej 2 Ei 1 Ej 2 Ei υij 5 υ0 exp 2αrij exp 2 ; 2kB T (3.6) where ν 0 is the phonon vibration frequency and it can be intuitively understood as the jump-attempt rate or simply as a normalization factor. The variable range Electronic and ionic transport in organic materials and devices 75 hopping (VRH) charge transport in disordered organic semiconductors is depicted in Fig. 3.3. In this case, the charge carriers move through the localized DOS while the delocalized states are not involved. While the various charge transport models introduced before are based on very different physical scenarios, all of them require the knowledge of the delocalized and/or localized states. Indeed, depending on the amount of localized states, on the relative energy position of the delocalized and localized states, and on the position of the Fermi energy level in the material during the device operation, the charge transfer from one molecule or polymer chain to another can be dominated by VRH or by MTR or by band-like transport. This highlights the key role played by the DOS. The shape of the DOS can be extremely complex, comprising peaks, valleys, and exponential and linear decays. However considering only the energy range that is practically relevant for experimental conditions, it can be approximated with a Gaussian function, an exponential function or a combination of these functions [2,4]. For example, in the case of organic diodes, a VRH transport in a Gaussian DOS is most commonly used, but there are also cases where a Gaussian DOS with a localized exponential tail is observed [11]. VRH or MTR transport in a singleexponential or double-exponential function can be used for the description of organic thin-film transistors [12], while in the case of organic electrochemical transistors the explored DOS can be well approximated by the sum of two or more Gaussian and exponential functions [7,13]. Figure 3.3 Schematic view of the VRH transport. A charge carrier (electron or hole) is moving in the localized states by means of thermally activated quantum tunneling. Each hopping can involve a variation of space and energy of the charge carrier. VRH, Variable range hopping. 76 3.2 Organic Flexible Electronics Ionic transport in mixed ionic-electronic conductors Transport phenomena in mixed conductive polymers can be described by the thermodynamics of irreversible processes [14]. This relies on two fundamental hypotheses: the continuum hypothesis and the local equilibrium hypothesis. The state variables of the system, such as electric potential, pressure and energy, are functions of the position r and time t. The continuum hypothesis states that each volume element dV of the system, with position r in the space, is small enough that infinitesimal calculus can be applied, but still it is macroscopic in the sense that it contains a large number of molecules. The system can be described by means of equilibrium thermodynamic relations, since the changes in the state variables take place over a time scale much larger than that of molecular motion. Thus we can assume local internal equilibrium within the infinitesimal volume element dV. Given these assumptions, the Gibbs energy of the volume element can be defined as: dG 5 X μ~ i ni 5 dU 2 TdS 1 pdV (3.7) i Where μ~ i and ni are the electrochemical potential and number of particles of the specie i, respectively, U is the internal energy, T is the temperature, S is the entropy, and p is the pressure. Dividing Eq. (3.7) by dV, we obtain the fundamental thermodynamic equation that applies to the volume element: g5 X μ~ i ci 5 u 2 Ts 1 p; (3.8) i where g 5 dG/dV is the Gibbs energy density, ci 5 dni/dV is the molar concentration of the specie i, u 5 dU/dV is the internal energy density, and s 5 dS/dV is the entropy density. We consider a binary electrolyte completely dissociated into positively and negatively charged ions. Assuming charge numbers z1 5 2z2 5 1 and given the bulk concentrations c1 and c2 of the cation and anion, respectively, it holds c1z1 1 c2z2 5 0. The internal energy density and the entropy density can be rewritten in terms of molar concentration and electrostatic potential ϕ. More in detail, the energy density results [15]: ε 2 u 5 2 rϕ 1 z1 qc1 ϕ 2 z2 qc2 ϕ: 2 (3.9) Where the first term is the self-energy of the electric field, while the second and the third terms are the electrostatic energies associated to the charged particles in the electric field [16]. Assuming that cations, anions, and solvent molecules have the same size a, the entropic contribution reads [15]: Electronic and ionic transport in organic materials and devices s5 77 kB c1 a3 ln c1 a3 1 c2 a3 ln c2 a3 1 ð1 2 c1 a3 2 c2 a3 Þln 1 2 c1 a3 2 c2 a3 : 3 a (3.10) The first two terms are the entropies of the positive and negative ions, while the last term is the entropy of the solvent molecules. It is worth to note that Eq. (3.10) takes into account the finite size of ions, which results in a maximum ion concentration cmax 5 1/a3. The electrochemical potential for each ionic specie can be obtained as: μ~ 6 5 dg c6 5 z 6 qϕ 1 kB Tln : dc 6 cmax 2 c1 2 c2 (3.11) Diffusion and migration of ionic species are given by gradients in the electrochemical potential, either because of a species concentration difference (a concentration gradient), or because there is a difference of ф (an electric field or potential gradient). In general, a flux of species i will occur to alleviate any electrochemical potential difference. More in detail, under quiescent conditions, the ionic flux can be written as: ! ji 52 Di ci ! r μ~ i ; kB T (3.12) where Di is the ionic diffusion coefficient. Thus combining Eqs. (3.11) and (3.12), we can derive the formulation for the NernstPlanck equation describing ionic fluxes in the binary electrolyte: ! j6 52 ! D6 c6 r! c 6 r! ðc1 1 c2 Þ zq r ϕ 1 kB T 1 kB T : cmax 2 ðc1 1 c2 Þ kB T c6 (3.13) If we assume cmax!N, Eq. (3.13) simplifies to the NernstPlanck equation for diluted systems: ! j6 52 ! D6 c6 r! c 6 zq r ϕ 1 kB T : kB T c6 (3.14) The direct measurement of ionic conduction in a mixed ionic-electronic conductive polymer is not straightforward, because of the difficulty to decouple the ionic and electronic contributions in this kind of analysis. Stavrinidou et al. proposed the moving front experiment as a brilliant approach for the measurement of ion mobility in the prototypical mixed conductive polymer PEDOT:PSS [17]. They took advantage of electrochromism in organic electronic materials, where free electronic carriers create states in the optical gap and change the color of the material [18]. 78 Organic Flexible Electronics Figure 3.4 Experimental setup of the moving front experiment proposed in Ref. [17]. This phenomenon has been used to monitor the doping/dedoping process in the polymer film. Fig. 3.4 shows a schematic representation of the proposed experimental setup. A strip of PEDOT:PSS is contacted to one end by a gold electrode and covered along its length by a passivation layer which acts as an ion barrier. The other end of the strip is in contact with an electrolyte, and an Ag/AgCl electrode is immersed into the electrolyte. By applying a potential between the gold and the Ag/AgCl electrodes, ions drift into the polymer changing its doping state and hence its color. More in detail, when a positive bias is applied at the Ag/AgCl electrode, cations are pushed into the polymer and a dark dedoping front propagates from the electrolyte toward the gold contact at the opposite end. The mobility of the cations is obtained by tracking the propagation velocity of the dedoping front. A potassium mobility of 1.4 3 1023 cm2 V21 s21 was measured, which is higher than the electrophoretic mobility of potassium cations in water at the limit of infinite dilution. This implies first and foremost that the PEDOT:PSS film is highly hydrated, as confirmed by the measurement of a 155% swelling of the film measured by optical profilometry. Furthermore, the morphology of the nanometric-size ion propagation paths through the PSS phase [19] suggests that electroosmotic flow mechanisms could contribute to the increased ion mobility [20]. Interestingly, the ionic mobility in PEDOT:PSS depends on the solvated ion size. H1 ions show larger mobility than K1 ions, which in turn have higher mobility than Na1. It is worth noting that PEDOT:PSS is able to efficiently transport choline (C5H14NO1) [17], which is an organic cation that is essential in nutrient. This implies that the size of the ionic transport paths is larger than the size of the choline ion, and prompts the question of what the maximum size is for the efficient ionic transport in this material. An effective ion transport in mixed conductivity materials is paramount as it determines the maximum operative frequency of the device, with relevant implications for its fields of application. 3.3 Ionic-electronic coupling in mixed ionic electronic conductors The bulk ionic/electronic coupling is the core mechanism that makes mixed conductivity organic materials a powerful technology for bioelectronics and energy Electronic and ionic transport in organic materials and devices 79 storage applications. It relies either on capacitive or faradaic coupling between ionic and electronic charges, depending on the adopted material, on the electrolyte and experimental conditions. Capacitive coupling takes advantage of the formation of an electric double layer (EDL) involving ionic and electronic charge carriers, while faradaic coupling is associated to electron transfer between the electrode and the specie dissolved in the electrolyte through electrochemical reactions (Fig. 3.5). The capacitive coupling occurs when an excess or deficiency of electrons is created on the electrode by applying a potential, and it is compensated by ionic charges of the opposite sign provided by the electrolyte, which are attracted at the electrode, originating an EDL. When a mixed ionic-electronic conductor is considered, the EDL involves the whole bulk of the electrode material, with ions penetrating through the ionically conductive paths in the material and electrostatically compensating the electronic charges in the electroactive sites of the polymer. This yields a capacitance that depends on the whole material volume, while in the case of a classic metal electrode the accumulation of ionic and electronic charges involves only the electrode/electrolyte interface and hence it scales with the electrode area. In the case of a metal electrode, the resulting EDL can be described with the GouyChapmanStern (GCS) model [21]. Let us consider an initial interface potential drop at which there is no net charge accumulation. By increasing the electrode potential with respect to the electrolyte potential, the electrode will become positively charged, and charges of opposite sign will be attracted near the electrode surface from the bulk electrolyte. It is worth to note that the electronic charges on the metal plate are confined on the surface, while it may take some significant thickness to accumulate the excess charge needed to counterbalance the electrode charge excess, especially at low concentrations of electrolyte. This “thick” layer of ions is usually referred as diffuse layer. By increasing the applied electrode potential, ions start packing at the intimate interface with the electrode, resulting in a compact layer known as the Helmoltz layer. The Helmoltz layer is separated from the electrode surface by a distance equal to the size of the solvated ion radius a, and the maximum ion packing is given by the finite size of the solvated ions as cmax 5 a23. This is a basic model that gives a simplified view of the ionic/electronic Figure 3.5 Capacitive vs. faradaic coupling. (A) Capacitive coupling results in an EDL at the interface between electronic and ionic charge carriers. (B) Faradaic coupling takes advantage of electron transfer at the interface between the electrode and the electrolyte. EDL, Electric double layer. 80 Organic Flexible Electronics interaction in an EDL at a metal/electrolyte interface, and a mathematical formulation can be found in Ref. [21]. It is worth noting that the GCS model usually fails to capture the complexity of EDL in real applications, in which a quantitative description of the system requires a deep knowledge of the adopted material characteristics and strictly depends on the operative conditions. An extensive analysis of the problem is reported in Ref. [21]. Furthermore, the applicability of the classic EDL model to the capacitive coupling in mixed ionic-electronic conductors is still debated. Considering PEDOT:PSS as a relevant example, it has been demonstrated that the ionic/electronic conversion involves a capacitive phenomenon, with ions penetrating through the ionically conductive PSS phase and electrostatically compensating electric charge carriers in the electrically conductive PEDOT phase [22,23]. It has been shown that the volumetric capacitance in PEDOT:PSS does not show the typical concentration dependence of an EDL [22], and a model describing the capacitive coupling in this widely used polymer is still missing. When faradaic ionic/electronic coupling is involved, a potential difference at the electrode/electrolyte interface yields an energetically favorable transfer of an electron from the electrode to a specie dissolved into the electrolyte (reduction of the dissolved specie) or from the specie to the electrode (oxidation). kR O 1 e2 ! R kO R ! O 1 e2 This electron transfer is usually referred as an electrochemical redox reaction and it involves three fundamental steps. 1. The reactant moves from the bulk electrolyte to the electrode interface: this is termed mass transport. 2. Electron transfer occurs via tunneling between the electrode and reactant close to the electrode. The involved distance is on the nm scale. 3. The product moves away from the electrode and fresh reactant moves to the surface. It follows that faradaic processes are controlled by two factors: the reaction rate of the reactant at the electrode and the mass transport providing fresh reactant for the reaction. Both reduction and oxidation take place simultaneously at the electrode, and the applied potential determines which reaction is occurring at higher rate. This is described by the ButlerVolmer equation, suitably adapted to capture the volumetric response in mixed ionic-electronic conductors: i 5 iO 1 iR 5 FvkO cRð0;tÞ 2 FvkR cOð0;tÞ ; (3.15) where i is the total current resulting from the electron transfer at the interface, iO is the current associated with the oxidation process, F is the Faraday constant, v is the electrode volume, kO is the oxidation reaction rate coefficient, cR(0,t) is the timedependent concentration of the reduced specie at the interface. A similar notation is Electronic and ionic transport in organic materials and devices 81 used for the reduction process current iR. Eq. (3.15) readily shows that the reaction current depends both on the reaction rate, which is determined by the interface potential, and on the concentration of the reactant at the interface. The supply of fresh reactant from the bulk of the electrolyte is described by the ionic transport equations reported in Section 3.2, while the reaction rate coefficient exponentially depends on the applied potential ϕ as: 3 αzF ϕ 2 ϕeq 5; kO 5 Zexp4 RT 2 3 ð1 2 αÞzF ϕ 2 ϕeq 5: kR 5 Zexp4 2 RT (3.16) 2 (3.17) Where the preexponential factor Z can be estimated theoretically, but it also can be treated as an empirically measured parameter, α is called transfer coefficient and is typically found to have a value of 0.5, z is the number of charge of the specie involved in the reaction, R is the ideal gas constant, ϕeq is the potential at which the reaction is at the equilibrium, also known as the Nernst potential, and results: ϕeq 5 ϕo 2 RT c ln R : zF cO (3.18) With cR and co are the bulk concentrations of the reduced and oxidized species. A straightforward method to probe the nature of the ionic/electronic coupling for mixed ionic/electronic materials is the cyclic voltammetry (CV). The typical experimental setup adopts a three-electrodes configuration. The working electrode (WE) is the electrode to be studied, the reference electrode (RE) determines the electrolyte potential and typically an Ag/AgCl electrode or a saturated calomel electrode is used, the counter electrode (CE) is usually a platinum or another inert metal electrode which provides the current required by the WE during the measurement. The WE potential (with respect to the RE potential) is swept at a constant rate in the range of interest, from V1 to V2 and then back to V1 and the WE current is measured. A box-shaped CV is the fingerprint of a capacitive coupling. On the forward scan (from V1 to V2) a current IFW 5 CdV/dt is displayed, where C is the capacitance of the EDL and dV/dt is the sweep rate, on the backward sweep a current IBW 5 2IFW is measured, due to the opposite sign of the sweep rate. A faradaic coupling is usually revealed by current peaks in the CV. A redox peak is due to the two factors governing the reaction kinetic. On the rising side of the peak the increasing potential is exponentially increasing the reaction rate at the interface. With increasing reaction rate at the interface, the supply of fresh reactant from the bulk electrolyte becomes insufficient, the current stops increasing at the highest point of the peak and eventually starts 82 Organic Flexible Electronics decreasing, resulting in a mass-transport-limited process. On the backward sweep the same mechanisms govern the reaction in the opposite direction. Further details on the CV are provided in Section 3.4. It is worth to note that in practical applications capacitive and faradaic mechanisms can be involved at the same time, with one phenomenon or the other dominating the overall process depending on the experimental conditions, and particularly on the applied potentials. Considering, for example, PEDOT:PSS, the capacitive coupling in the typical operating voltage window is displayed by a box-shaped CV, while at larger potential the cyclic voltammogram is dominated by redox currents due to the interaction between the polymer blend and the oxygen dissolved in water [24]. The redox currents become negligible when the oxygen is removed from the electrolyte, stressing once again the importance of the experimental setup on the ionic-electronic coupling. 3.4 Mixed conduction in supercapacitors Supercapacitors (SCs) are energy-storage devices that gained a lot of attention owing to their unique features like high power, long cycle life, and environment friendly nature [25,26]. SCs bridge the gap between batteries and capacitors to form fast-charging energy-storage devices of intermediate specific energy (Fig. 3.6) [27]. They are a promising technology in fields requiring fast-storage capability, midrange energy density, and enhanced cycling stability. SCs have potential application in renewable energy storage, regenerative breaking in hybrid electric vehicles, and power supply in portable devices. Figure 3.6 Ragone plot for energy-storage devices. Supercapacitors show intermediate performances between batteries and capacitors in terms of power density and energy density. Electronic and ionic transport in organic materials and devices 83 A traditional capacitor consists of two metal plates separated by an insulator. Charge of opposite sign (Q) is accumulated on the metal plates by applying a potential difference (V) and reads: Q 5 CV; (3.19) where C is the capacitance, which, in turn, depends on the capacitor geometries and on the insulator properties as: C5 εA ; d (3.20) where A is the metal plates area, d is the insulator thickness, and ε is the insulator dielectric constant. Typical capacitances per unit area Cs 5 C/A are in the order of 1 μF cm22. The credit for the beginning of capacitor technology goes to the invention of the Leyden Jar (174546) which was made of a glass jar with metal foils covering the inside and the outside surfaces. The metal foils served as electrodes and the jar acted as the dielectric. In the 1920s came the first electrolytic capacitor, where higher capacitances are achieved by incorporating an electrolyte in the capacitor structure [25]. Interestingly, it took advantage of the formation of an EDL at the interface between the electrode and the electrolyte. In an EDL, ionic charges provided by an electrolyte are accumulated at the interface with an electrode and compensated by electronic charges. In this charge configuration the surroundings of the ionic charge separating ionic and electronic charges act as a subnanometric thick insulator, allowing to achieve capacitances in the order of 10100 μF cm22. In 1957 the first SC consisting of two porous carbon electrodes separated by an electrolyte was patented by General Electric [28]. The porous carbon electrodes yielded a larger effective area for the electrode/electrolyte interface, resulting in increased capacitance with respect to the traditional electrolytic capacitor. When traditional conductive materials are used as electrodes, the ionic-electronic charge interaction takes place at the planar interface between the electrode and the electrolyte. In modern SCs, conductive polymer showing mixed ionic-electronic conduction are used as electrodes, allowing the ionic charges to penetrate the bulk of the electrode and extending the ionicelectronic charge interaction from 2D to 3D. This results in higher equivalent capacitances per unit area, dramatically increasing the charge-storage capability of this class of devices. When mixed conductive polymers are used as electrodes, the charge storage can be obtained through a purely electrostatic phenomena (electric double layer capacitors, EDLCs) or a pseudocapacitive (PC) phenomena involving faradaic reactions at the electrode-electrolyte interface. EDLCs usually show faster dynamics, while PC SCs allow larger energy storage [26,27]. The main figures of merit of a SC are the cell-specific capacitance (F g21), specific energy (W h g21), specific power (W g21), and the cycle life. Other important parameters are the operating voltage, the temperature stability, the reliability and 84 Organic Flexible Electronics Table 3.1 Performances comparison between capacitors, supercapacitors, and batteries. Characteristic Capacitor 21 Supercapacitor 1 Battery Max. specific energy (W h kg21) 10 10 103 Max. specific power (W kg21) 107 105 103 Charge time (s) 10261023 1102 103104 Discharge time (s) 10261023 1102 103105 Cycle life (cycles) Almost infinite .500,000 About 1000 Operative voltage determinants Dielectric thickness and composition Electrode and electrolyte stability window Thermodynamics of phase reaction Charge storage determinants Electrode area and dielectric Electrode structure and composition, electrolyte composition Thermodynamics and active mass Source: Adapted from W. Raza, F. Ali, N. Raza, Y. Luo, K. Kim, J. Yang, et al., Recent advancements in supercapacitor technology, Nano Energy 52 (2018) 441473 [26]; A.G. Pandolfo, A.F. Hollenkamp, Carbon properties and their role in supercapacitors, J. Power Sources 157 (1) (2006) 1127 [29]. safety of the device. Table 3.1 shows the performances of SCs compared to capacitors and batteries. The main techniques used to evaluate the SCs performances are the CV, the galvanostatic charge/discharge (GCD), and the electrochemical impedance spectroscopy (EIS). The CV is a DC method where the properties of the SCs electrode/ electrolyte system are characterized as a function of the voltage. It is usually performed in a three electrodes configuration as shown in Fig. 3.7A, where the SC electrode is the WE, a nonpolarizable electrode is the RE, and a polarizable electrode is used as the CE. The voltage applied to the WE (VI) is swept in the range of interest from V1 to V2 at a rate dVI/dt, while the RE fixes the electrolyte voltage to a reference level and the CE supplies the current required by the electrochemical processes at the WE. The shape of the CV gives information on the charge-storage mechanism at the electrode. Redox peaks are displayed in the case of PCs, while a rectangle-shaped CV is typical for EDLCs (Fig. 3.7B and C). The capacitance of the electrode-electrolyte system can be calculated as: C5 Qtot ; 2jV1 2 V2 j (3.21) Electronic and ionic transport in organic materials and devices 85 Figure 3.7 The cyclic voltammetry. (A) Schematic experimental setup for the measurement of a cyclic voltammetry. A1 ensures that the RE voltage is fixed to a reference level, while the CE voltage is set to the voltage required to supply IO. A voltage source inputs VI to A2. The generated voltage is replicated at the WE, and IO 5 (VO 2 VI)/R1. The reference electrode is usually a nonpolarizable electrode such as an Ag/AgCl electrode or a saturated calomel electrode. A nonpolarizable electrode is ideally able to exchange an infinite faradaic current density with the electrolyte by applying an infinitesimal voltage between the electrolyte and the electrode itself. This ensures that the input polarization current required by A1 can be provided by the RE with a negligible voltage drop at the RE/electrolyte interface, resulting in a stable reference voltage level. The counter electrode is usually a polarizable electrode, made of an electrochemically inert material like platinum or gold. Considering an ideally polarizable electrode, no redox current is exchanged with the electrolyte even at large applied voltages. Thus the current required by the WE is provided with a capacitive current by the CE, avoiding the formation of redox reactions products that could interfere with the processes at the WE/electrolyte interface. (B) Example of a CV plot in the case of faradaic reactions at the WE. The typical redox peaks are displayed. (C) Example of a CV plot in the case of EDLC, with the typical rectangle-shaped characteristic. CE, Counter electrode; CV, cyclic voltammetry; EDLC, electric double layer capacitor; RE, reference electrode. 86 Organic Flexible Electronics where Qtot is the total charge stored and it is calculated as: Ð Qtot 5 V2 V1 IO dV 1 Ð V1 V2 IO dV dVI =dt : (3.22) The specific capacitance is: Cs 5 C ; m (3.23) where m is the electrode mass. The cycle life of the SC can be evaluated by monitoring the capacitance over repeated measurement cycles. Figure 3.8 (A) Schematic setup for the GCD technique. The TE is grounded, the NPE is polarized with a constant current II, while the VME measures the potential drop at the TE interface. An Ag/AgCl pellet can be used for the NPE, while the VME is usually an electrochemically inert electrode, like platinum or gold. (B) GCD plot in the case of PC mechanisms. The VO-t plot shows nonlinear charge and discharge characteristics. (C) GCD plot in the case of an EDLC. The VO-t plot shows linear charge and discharge characteristics. EDLC, Electric double layer capacitor; GCD, galvanostatic charge/discharge; TE, tested electrode; VME, voltage measurement electrode. Electronic and ionic transport in organic materials and devices 87 The GCD is a DC technique where the electrode/electrolyte system is characterized by charging the electrode with a constant current II and measuring the voltage drop VO at the interface. A schematic representation of the setup is shown in Fig. 3.8A. A constant current is applied at a nonpolarizable electrode (NPE) and charges the tested electrode (TE) which is polarized at a reference voltage, while the voltage measurement electrode (VME) measures the potential drop at the TE. The shape of the GCD curve allows to identify the charge-storage process: in the case of an EDLC a linear increase and decrease of the interface potential is displayed over the time t, while in the case of PC mechanisms a deviation from the linear behavior is shown (Fig. 3.8B and C). The electrode capacitance can be estimated as: C5 II : dVI =dt (3.24) And again, the specific capacitance is obtained as Cs 5 C/m, where m is the electrode mass. The capacitance extracted over repeated measurements gives information on the cycle life. The CV and GCD methods give overall information on the charge storage over the operative range of the SC. Nevertheless, the extracted capacitance yields a global picture of a complex and highly nonlinear system, where, for instance, the capacitance of the electrode depends on the voltage at the electrode/electrolyte interface. Moreover, the same electrode/electrolyte system can behave as an EDLC with no faradaic mechanism involved over a certain voltage range, while PC behavior can be displayed outside of this voltage window. EIS is a powerful AC technique that can give precise information on the electrode and electrolyte properties at a fixed applied voltage. It is performed with a three electrodes configuration as shown in Fig. 3.9A. The RE is a nonpolarizable electrode and it determines the electrolyte potential, the characterized electrode (WE) is biased at a fixed DC voltage and a small amplitude AC signal (usually 6 5 mV) is applied, while a polarizable CE provides the current required by the WE. A sweep on the frequency of the AC signal is performed and the impedance of the electrode/electrolyte system is measured at each frequency. The information is usually displayed with a Bode plot, showing the magnitude and phase of the impedance as a function of the frequency, or a Nyquist plot, which shows the real part and imaginary part of the impedance on the x and y axes of the plot, respectively (Fig. 3.9B and C). The EIS measurement can be fitted by using the Randles circuit [30] (Fig. 3.10A). It consist of a resistance (Rs) in series with the parallel of a capacitor (Cp) and the series of a resistance (Rp) and a Warburg element (W). Fig. 3.10B shows the frequency-dependent impedance formulation for each component of the Randles circuit. Rs is the electrolyte resistance and describes the ion transport through the electrolyte, Cp accounts for the ionic/electronic charge accumulation at the WE, Rp describes the faradaic charge transport at the WE/electrolyte interface and the Warburg element accounts for mass transport limited faradaic processes. In practical cases, inhomogeneities on the nano- or microscale or adsorption 88 Organic Flexible Electronics Figure 3.9 (A) Schematic of the experimental setup for the EIS measurement. A1 ensures that the RE voltage is fixed to a reference level, while the CE voltage is set to the voltage required to supply IO. A voltage source inputs VI to the A2. The generated voltage is replicated at the WE, and IO 5 (VOVI)/R1. The output signal is filtered through a bandpass filter and only the frequency equal to the input signal frequency is kept. The impedance at the target frequency ω is Z(ω) 5 VI(ω)/[(VOF(ω)VI(ω))/R1]. The reference electrode is usually an Ag/AgCl electrode or a saturated calomel electrode, the counter electrode is usually made of an electrochemically inert material, like platinum or gold. (B) Typical Bode plot (impedance modulus |Z| and phase) of a Randles circuit. In the low-frequency regime the impedance is dominated by the parallel resistance, resulting in a flat |Z| and phase close to 0. In the medium frequency range the impedance is dominated by the parallel capacitance, |Z| decreases with 20 dB dec21 and the phase approaches π/2. In the high-frequency range the series resistance dominates the overall impedance, yielding a flat |Z| and phase close to 0. (C) Corresponding Nyquist plot. The modulus of the impedance is given by the distance of the characteristic from the origin of the plot, while the phase is given by the angle between Re (Z) and the vector connecting the origin and the characteristic. The high-frequency information, where the characteristic is dominated by the series resistor, is displayed near to the origin of the plot, with Re(Z) . 0 Ω and Im(Z) 5 0 Ω. With decreasing frequency, the characteristics move farther from the origin, showing capacitive behavior, with increasing impedance modulus and phase. Further decreasing the frequency, the characteristic is dominated by the parallel resistor, resulting in a decreasing phase and increasing modulus, with Im(Z) approaching 0 Ω. CE, Counter electrode; EIS, electrochemical impedance spectroscopy; RE, reference electrode; WE, working electrode. Electronic and ionic transport in organic materials and devices 89 Figure 3.10 (A) The Randles circuit and (B) the Randles circuit elements. Figure 3.11 Structure of polymers used for the fabrication of supercapacitor’s electrodes. (A) Polyaniline (PANI). (B) Polypyrrole (PPy). (C) Poly(3,4-ethylenedioxythiophene). phenomena at the WE/electrolyte interface yield a nonideal capacitance for the Cp element [3133]. This is usually modeled by replacing the Cp with a constant phase element (CPE). The α parameter of the CPE (see Fig. 3.10B) ranges from 0 to 1 (usually α . 0.6), with values closer to 1 describing an almost ideal capacitive phenomena. The Q parameter for the CPE is the equivalent to the C parameter of the ideal capacitor. The γ parameter in ZW is usually assumed equal to 0.5, but in practical cases it could range from 0 to 0.5. The performances of a SC mostly depend on the electrodes and electrolyte materials. Typical conductive polymers for SCs electrodes are polyaniline (PANI), polypyrrole (PPy), and derivatives of polythiophene. PANI and PPy have been studied extensively as SC electrode materials. Their structure is illustrated in Fig. 3.11. They show high electrical conductivity in doped states (B100 2 10,000 S m21), low environmental impact, low cost, and compatibility with large-area fabrication techniques [34]. Their excellent specific capacitance typically ranges between 500 and 3400 F g21, depending on preparation conditions [35]. This value is larger than that of conventional carbon-based electrodes (B100 2 200 F g21) and comparable to PC metal oxides [36]. However both PANI and PPy suffer from structural 90 Organic Flexible Electronics breakdown and thus fast capacitance decay due to the swelling and shrinking as a result of ion doping and dedoping. Apparently, most PANI- and PPy-based electrode lose more than 50% of the initial capacitance after cycling for 1000 times. Therefore cycling instability is a major issue for practical applications of conductive polymer electrodes. A number of methods have been proposed to improve the cycling stability of PANI and PPy, such as polymer blending with carbon nanotubes or polymer encapsulation in carbonaceous shell, which yielded a capacitance retention up to 95% after 10,000 cycles [34]. Among polythiophene derivatives, poly (3,4-ethylenedioxythiophene) (PEDOT, structure in Fig. 3.11) has gained large interest due to commercial availability, good electrical conductivity, excellent electrochemical and thermal stability, and biocompatibility [37]. On the negative side, because of its large molecular weight, it has a relatively low specific capacitance. PEDOT thin film with a specific capacitance of 180 F g21 have been demonstrated, but severe polymer degradation reduced the cycle life of the electrodes. Mixing PEDOT with carbon nanotubes yields higher mechanical stability and hence a longer cycle life, with specific capacitance up to 160 F g21 [38]. The electrolyte plays a fundamental role in the SC structure, since it impacts both the capacitance and the charge/discharge velocity. Furthermore, easy handling and no leakage are required. Aqueous electrolytes, organic electrolytes, and ionic liquids (ILs) are generally used in SCs. Aqueous electrolytes show good conductivity, at least 1 order of magnitude higher than that of ILs and organic electrolytes. This results in a low electrolyte resistance and hence fast charge and discharge kinetics. The size of the hydrated ions plays a fundamental role on the specific capacitance, with smaller size yielding larger capacitances. Aqueous electrolytes can be modified by adding different gels to create a mechanically stable electrolytic system, resulting in easier handling and no leakage. Wang et al. [39] fabricated a novel gel by dipping polyacrylamide gel (PAM-G) into an aqueous solution of 6 M KOH for about 60 hours. The resulting system provided excellent cycling stability and high specific capacitance (196 F g21 at GCD 1 A g21), probably due to the homogeneous three-dimensional microporous structure of the PAM-G, which facilitated the ionic transportation. However aqueous electrolytes are not an optimal choice for commercial electrochemical SCs because of their small electric potential windows. This is one key reason why commercial SCs generally use organic electrolytes, with a potential window in the range of 2.52.8 V. Typically, the organic electrolytes used for SCs are conductive salts [40], such as tetraethylammonium tetrafluoroborate (TEABF4) dissolved in propylene carbonate or ACN. Nevertheless, organic electrolytes suffer from low conductivity, small specific capacitance, and high cost. They are volatile, toxic, and flammable [41]. Moreover, they require complex assembly and purification procedures under controlled environment to eliminate impurities (i.e., moisture) that could degrade their performances. The organic electrolytes used in SCs at the commercial level are not suitable for temperatures above 70 C because of their low ignition and detonation temperatures, while the effectiveness of aqueous electrolytes is limited by the boiling temperature of water and cannot be practically used above 80 C. Ionic liquids like 1-ethyl-3methylimidazolium acetate (EMIM Ac) are potential candidates for use at even Electronic and ionic transport in organic materials and devices 91 very high temperatures because of their high chemical and thermal stabilities, with wide operating voltage ranges, negligible vapor pressure, and nonflammability [41]. 3.5 Mixed conduction in bioelectronics The emerging field of bioelectronics provides a bidirectional connection between biology and electronics. Bioelectronic devices can be used to specifically regulate the physiology and the processes of cells, tissues, and organs through electrochemical stimulation. Conversely, bioelectronics enables high-precision monitoring of biological phenomena and physiological signals. Typical examples of such devices are electrodes for electrophysiology recording, biosensors for the monitoring of biomarkers, and drug-delivery iontronic devices enabling targeted pharmaceutical treatment. While electronic signals are carried through electrons, signaling in biological systems relies on the transport of ions. Thus mixed ionic-electronic conduction in organic materials provides a seamless interface between biology and electronics. Mixed conduction devices combine low-voltage operation due to efficient ionic/electronic coupling with the benefits typical of organic technologies, such as biological compatibility, large-area deposition with simple and low-cost techniques, chemically tunable properties, mechanical flexibility, and softness [4245]. Soft electronic materials with low Young’s module are suitable to operate in contact with biological tissues with minimum risk of adverse reaction, while mechanical flexibility enables comfortable wearable devices [46]. Large area electronics allow the monitoring of complex and dynamic structures such as organs with high spatial resolution. Low cost is highly desirable in the case of bioelectronics, where the risk of contamination and infection requires the use of disposable devices. Furthermore, devices can be fabricated on ultrathin, flexible, and biocompatible substrates [47]. Biocompatibility of the employed materials is paramount when it comes to bioelectronics and it is strictly dependent on the involved cells type. PPy is noncytotoxic for a wide range of cell types in vitro, it supports cell adhesion and growth, and yields low inflammatory response in vivo, and similarly encouraging results are reported for PEDOT:PSS and PANI [48]. Mixed conductors like PEDOT:PSS show long-term stability in physiological environment [49], which is fundamental for long-term implants for electrophysiology. Fueled by this unique benefits combination, mixed ionic-electronic organic conductors are a promising platform for the next-generation bioelectronics. 3.5.1 The organic electrochemical transistor The organic electrochemical transistor (OECT) is a groundbreaking device with huge potential for the biological to electrical information transduction in bioelectronic applications. It relies on mixed conduction to provide intrinsic ionic-electronic conversion together with local signal amplification and low voltage operation. An OECT comprises an organic semiconductor channel connected to source and drain 92 Organic Flexible Electronics metal electrodes. An electrolyte solution is in contact with the semiconductor, and a gate electrode is immersed into the solution (Fig. 3.12A). Typical electrical characteristics (transfer characteristics) of an OECT based on a p-type semiconductor are shown in Fig. 3.12B. By applying a positive gate voltage (VG) cations drift into the polymer, reduce the hole concentration and lower the drain current (ID). Analogously, when a negative VG is applied previously injected cations drift out of the polymer while anions drift into the polymer, the hole concentration increases and this results in a larger ID. The magnitude of the drain current increases by applying a more negative voltage to the drain electrode. Bernards and Malliaras described the operation of OECTs through a simple model, providing the drain current as a function of the geometrical and physical parameters and of the bias conditions [50,51]. The model reads: ID 5 2 Wd μCv Vt 2VG;eff 2L ID 5 Wd V2 μCv Vt 2 VG;eff VD 2 D L 2 when VG 2 Vt . VD ; when VG 2 Vt # VD ; (3.25) (3.26) where W, d, and L are the channel width, thickness, and length, respectively. Cv is the volumetric capacitance accounting for the electrostatic interaction between ionic and electronic charges in the transistor channel. μ is the electronic carrier mobility in the channel and Vt is the pinch off voltage. VG,eff accounts for the effective gating voltage and reads: VG;eff 5 βVG 1 Vo ; (3.27) Figure 3.12 (A) Schematic structure of an OECT. (B) Typical transfer characteristics of a ptype OECT. OECT, Organic electrochemical transistor. Source: Adapted from P. Romele, M. Ghittorelli, Z.M. Kovács-Vajna, F. Torricelli, Ion buffering and interface charge enable high performance electronics with organic electrochemical transistors, Nat. Commun. 10 (2019) 3044. Electronic and ionic transport in organic materials and devices 93 where β is a parameter accounting for the gating efficiency in the OECT and it is equal to 1 in the case that a nonpolarizable gate electrode is used (e.g., Ag/AgCl electrode), while β 5 CG/(CG 1 CCH), with CG the gate capacitance, when a polarizable gate electrode is used (viz. gold). Vo is a voltage parameter accounting for the work function differences in the gate-electrolyte-channel structure. Importantly, the model captured the key feature of OECTs, that is, the volumetric response due to the ion penetration into the transistor channel. OECTs can be used to sense biological events, which usually involve potential variations due to ionic currents. The potential variation is captured by the gate voltage (dVG) and results in a drain current modulation (dID). Thus the OECT transconductance, defined as gm 5 dID/dVG, gives the sensitivity of the device to a biological event. When the OECT is integrated in an amplifying structure (e.g., an inverter [22]), the gm parameter, combined with the output resistance ro 5 dID/dVD of the device, allows to amplify the input signal by a factor that is proportional to the product gmro. As a result, the OECT is able to provide simultaneous detection and local amplification of a biological event, enabling biosensing with increased signal-to-noise ratio (SNR). OECTs usually show an extremely large transconductance normalized to the channel geometries gmL/(Wd), in the order of 10 mS cm21, resulting from the ionic-electronic interaction through the bulk of the polymeric channel [52]. As shown in Eqs. (3.25) and (3.26), the transconductance of an OECT depends on the channel geometry, on the applied bias and on the product of the material dependent parameters μ and Cv. Thus the μCv product is a device-based figure of merit that enables to determine the performances of a material when integrated in OECTs [53]. The μCv product can be directly estimated by the measurement of gm and considering the device geometries and biasing conditions. Decoupling the μ and Cv contributions provides understanding on why one material outperforms another. The μ parameter depends on the electronic charge transport properties, and it can be extracted by measuring ID as a function of time while applying a constant IG [50]. Considering an OECT operated in the linear regime, it holds: ID 5 Wd Q qμp0 1 2 Vd ; L qp0 WdL (3.28) where q is the elementary charge, p0 is the intrinsic hole concentration in the Ð t polymer, and Q is the ionic charge injected into the channel and is equal to Q 5 t0 IG dt. By considering a constant IG, it follows from Eq. (3.28): μ5 L2 dID IG VD dt (3.29) The volumetric capacitance, depending on the ionic and electronic charge storage capability of the polymer, can thus be determined knowing gm and μ. It can also be measured through EIS as follows. The experimental setup is the same as reported in Section 3.4, with the OECT source and drain shortened together and the 94 Organic Flexible Electronics channel acting as the WE, a nonpolarizable electrode as the RE and a polarizable electrode as the CE. The gate electrode is intentionally excluded from the measurement (i.e., not connected), since we are probing a property of the OECT channel. The typical bode plot for an OECT is shown in Fig. 3.13. In the low-frequency regime the impedance depends on the frequency and a 220 dB dec21 variation with 2 π/2 phase is displayed, while Z is constant in the high-frequency regime, showing a 0 phase. The spectra can be described by means of a capacitance C, given by the ionic/electronic charge accumulation on the channel volumetric capacitance, in series with a resistance RS accounting for the ionic transport in the electrolyte. By reproducing the data with this simple model, the device volumetric capacitance can be derived as Cv 5 C/(WtL). PEDOT:PSS is the most commonly employed material for the fabrication of OECTs [54]. Among the reasons for its widespread use are the excellent electric and ionic conductivity as well as its aqueous stability, easy processability, biocompatibility, and importantly, commercial availability. PEDOT:PSS OECTs fabricated with the procedure reported in Ref. [52] show Cv is equal to 40 F cm23, with a μ 5 2 cm2 V21 s21, for an overall record μCv product equal to 80 F cm21 V21 s21 [53]. The increasing interest in the field of OECTs for bioelectronics resulted in a large effort in the synthesis of new and improved materials for the device channel fabrication. The best performances are displayed by p(g2T-TT) [55], a p-type OECT material which shows Cv 5 240 F cm23 and μ 5 1 cm2 V21 s21, resulting in μCv 5 240 F cm21 V21 s21. N-type materials with stability in water have been recently proposed for the fabrication of OECTs. BBL was proposed by Berggren’s group [56], showing a Cv 5 930 F cm23, μ 5 7 3 1024 cm2 V21 s21 and μCv 5 0.65 F cm21 V21 s21, while McCulloch’s group synthesized p(gNDI-gT2) with Cv 5 400 F cm23, μ 5 3 3 1024 cm2 V21 s21, and μCv 5 0.12 F cm21 V21 s21 [57]. Figure 3.13 Typical EIS bode plot for an OECT. The low frequency regime is dominated by the volumetric capacitance, while at high frequency the electrolyte resistance is the major contribution. EIS, Electrochemical impedance spectroscopy; OECT, organic electrochemical transistor. Electronic and ionic transport in organic materials and devices 95 It is worth to note that the μCv product gives information on the steady-state performances of the polymer, while the transient behavior suitability of the polymer, as well as other properties like the biocompatibility, have to be evaluated on a case-by-case basis according to the considered application. Owing to their intrinsic ionic/electronic transduction and local amplification, OECTs have been successfully used in bioelectronics as electrodes or biosensors, and the investigation of their fundamental properties is enabling the design of OECT-based circuits, opening new opportunities for the future bioelectronic applications. 3.5.2 Electrodes Electrodes are used in electrophysiology both for the monitoring of biological processes and for the stimulation of cells and tissues. High-precision monitoring of electrophysiological signals with high spatial and temporal resolution is an invaluable tool for diagnostic purposes and for advancing our understanding of physiology. The first stage in the recording of vital signals is performed by electrodes, translating the biological information into an electronic signal, which can be eventually processed and stored. The ionic-electronic transduction can happen either through capacitive or faradaic processes, depending on the electrode material and on the electrolyte. When a capacitive mechanism is involved, the sensed potential results in an accumulation of ions and electronic charge carriers at the electrolyte/electrode interface and no chemical reaction is involved. This phenomenon can be modeled as an EDL, with a capacitance in the order of 10 μF cm22. On the other hand, in a faradaic mechanism the sensed potential results in charge transfer across the electrolyte/electrode interface which can be described by means of the ButlerVolmer equation [58]. Since redox processes with electron transfer are involved, it could cause variations in the chemical environment, potentially creating chemical species or variations in the pH that could damage the biological tissues or the electrode [48]. Biological signals are usually weak, and the resulting electronic signal can range from the μV (e.g., electroencephalography) to the mV (e.g., electrocardiography, ECG) [59], with frequencies up to 1 KHz [60]. Thus the electrodes design is of paramount importance to obtain high-quality recordings. Metallic wires or discs are usually adopted as electrodes. The miniaturization and integration of electrodes in arrays allow to simultaneously record multiple signals with high spatial resolution, and the small size results in less invasive recording procedures. However as the area of the interaction between the electrode and the biological medium decreases, so does the ionic-electronic coupling, viz. the electrode impedance increases. A high electrode impedance is usually associated to higher noise voltage [60,61], dramatically lowering the recording quality of small electrophysiological signals. Low impedance can be achieved with small surface electrode by using mixed conductivity organic materials such as PEDOT:PSS. In this class of materials, the ionicelectronic coupling develops through the whole bulk of the material, resulting in lower impedances with respect to metal electrodes with the same area [62]. 96 Organic Flexible Electronics PEDOT:PSS microelectrode arrays (MEAs) have been successfully used for in vitro recording of the activity of hippocampal cell cultures treated with several drugs [60] and of stimulus-locked cognitive responses to audiovisual stimuli with high spatial specificity [63]. Hippocampal cells activity has been measured in vivo with a nonpenetrating MEA allowing for high-SNR, long-term monitoring without the risk of damaging the tissues [64]. In vivo monitoring of nerve signals in mice has been demonstrated with PEDOT:PSS-based microelectrodes, showing improved miniaturization features and enhanced mechanical stability with respect to standard technologies [65] (Fig. 3.14). PEDOT:PSS was also integrated in tattoo electronics for electromyography allowing for low cost, noninvasive, and conformal devices with performances comparable to the Ag/AgCl commercial standard, providing inherently low skin/electrode impedance without the use of electrolyte gels as required by traditional electrodes [66]. When the biosignal to be measured is extremely weak, amplification near to the signal source allows to achieve higher SNR. OECTs can be used to monitor electrophysiological signals providing local amplification, resulting in improved Figure 3.14 Chronic recording of rat sympathetic nerve activity (RSNA). (A) Scheme showing the PEDOT:PSS-based electrode connected to the renal nerve. The elaboration and transmission circuitry is located on the back under the skin. (B) Optical micrograph of electrode located below renal nerve before sealing. (C) Trace of the recorded signals: renal sympathetic nerve activity (RSNA), arterial pressure (AP), and heart rate (HR). (D) Detail of RSNA (black) and AP (red) trace. (E) Cross-correlation of RSNA spiking activity and AP. Source: Adapted from F. Decataldo, T. Cramer, D. Martelli, et al. Stretchable low impedance electrodes for bioelectronic recording from small peripheral nerves, Sci. Rep. 9 (2019) 10598. Electronic and ionic transport in organic materials and devices 97 recording quality. In vivo recording of mice brain activity has been demonstrated with PEDOT:PSS OECTs, showing improved SNR with respect to the same signal measured with a PEDOT:PSS surface electrode and a penetrating metal electrode [67]. ECG and electrooculography have been measured with OECTs thank to the large amplification and bandwidth [59]. OECTs integrated on stretchable and conformable substrates have been used for in vivo recording of cardiac signals in rats showing blood compatibility, allowing for long-term ECG signal monitoring [68]. In addition to monitoring electrophysiological signals, electrodes can be used to stimulate cells or tissues. In vivo electrical stimulation can replace lost physiological functionality or treat diseases and injuries. A variety of clinical examples of electrical stimulation have been proposed, including a cochlear implant to restore hearing, deep brain stimulation to alleviate symptoms of neurological disorders such as dystonia and Parkinson’s disease, and spinal cord stimulation for pain management [48]. Stimulation treatments are investigated for many other pathologies in both clinical trials as well as preclinical studies [69] and their healthcare impact will likely continue to expand over the next decades. Also in the case of electrical stimulation, the ionic-electronic coupling can be capacitive or faradaic. Since faradaic reactions can generate undesired variations in the biological environment, capacitive-coupled operation is highly desirable. In order to operate in capacitive regime, it is fundamental to apply low voltages to the electrode, generally below 1 V. To achieve low-voltage operation and small electrode area for site-specific stimulation, a high capacitance per unit area is required. Mixed ionic-electronic conductors provide bulk ionic-electronic coupling, allowing to obtain high capacitances per unit area by increasing the electrode thickness. PEDOT:PSS-coated electrodes have been used for in vivo stimulation of the auditory system in rats [70]. The reduced size of the electrodes allowed to precisely stimulate the target neurons, while the flexibility and conformability of the device allowed for improved stimulation due to closer contact to the site. The performances of PEDOT:PSS electrodes have been tested in vivo for intracortical microstimulation applications, showing better results than iridium oxide electrodes [71]. Although numerous hurdles have to be overcome for safe and reliable recording/ stimulation in patients, the integration of flexible, biocompatible, and high performances electrodes based on mixed ionic-electronic conductive materials has broad applicability in the field of diagnostics as well as in the understanding of physiologic activity, and therapeutic stimulation for the treatment of diseases [72]. 3.5.3 Iontronic delivery devices As opposed to traditional electronic devices, where signals are carried by electrons, in iontronic devices ions are the main information carriers. During the last decade, a variety of iontronic devices based on organic materials have been developed, from the iontronic resistor, also referred as organic electronic ion pump (OEIP), to the ionic transistor [73]. This class of devices finds fundamental application in the delivery of ions and drugs to a target biosystem at specific location and time. An 98 Organic Flexible Electronics OEIP is composed of an ionically conductive and electrically insulating ionexchange membrane (IEM) connecting an ion source reservoir and a target outlet (Fig. 3.15). The ionic transport from the source to the target is accomplished by applying an electric potential between the reservoir and the target electrolyte, resulting in an electric field over the IEM. Similarly to an electronic resistor, the ionic current in OEIPs is proportional to the applied voltage and to the IEM ionic resistivity, which, in turn, depends on the material morphology and geometries. Diffusive “leakage” ionic currents can occur due to the charged molecules concentration mismatch between the source and the target reservoir. An effective way to reduce such diffusive currents in OEIPs is to design long and narrow IEM channels, nevertheless leading to high ionic resistances. When an electric field is applied to the IEM, the resulting ionic current consists mostly of counterions (majority charge carriers) transported in the forward direction, but also a small contribution from coions (minority charge carriers) arises. The coions contribution to the overall current is determined by the IEM selectivity, which depends on the material adopted. In conventional drug delivery systems such as microfluidics or microelectromechanical systems, the biomolecule is delivered together with a solvent, which can interfere with the target biological environment. In contrast with conventional approaches, the electrophoretic ion transport in iontronic devices is not flow based, meaning that only the ionic specie is delivered to the target. Furthermore, iontronic devices can rely on well-established solid state manufacturing techniques, allowing for easy miniaturization and integration with electronic conditioning circuits. As a result, iontronic devices are characterized by precise control of the delivered drug amount as well as high spatial resolution and speed comparable to that of biological events. A drawback for this technology is that only charged species can be delivered and the molecule size can in principle be a limitation. The iontronic device electrodes can be made of a conducting polymer, while the source reservoir can be an electrolyte solution containing the ionic specie to be delivered. In principle, a solid state OEIP can be obtained by replacing the Figure 3.15 Schematic representation of an organic electronic ion pump. Electronic and ionic transport in organic materials and devices 99 electrically conductive polymer electrode and the liquid reservoir with a mixed ionic-electronic conductor, simultaneously providing electric connectivity and ionic reservoir. Furthermore, the mixed conductivity electrode yields lower impedance per unit area, significantly improving the drug-delivery capacity. The most frequently used electrode material is PEDOT:PSS, providing both large electrical conductivity and ionic storage capacitance. IEMs are usually based on polyanions or polycations, resulting in a cation-exchange membrane or anion exchange membrane, respectively. Several different materials have been used for IEM in iontronic components, from linear polymers to advanced tailor-made hyperbranched (dendrolyte) materials [73]. The selectivity and resistivity of the IEM depend both on the transported ionic specie and on the chemical and geometric microstructure of the adopted membrane material. A high fixed charge concentration allows to achieve selectivity through Donnan exclusion, and the ion mobility depends on the pore size of the hydrated membrane. Linear IEM materials efficiently transport small ions such as K1, Na1, Cl2, and Ca21 and small molecules with molar mass lower than 200 g mol21, such as γ-aminobutyric acid, glutamate, aspartate, and acetylcholine. When larger molecules are involved, dendrolyte-based IEMs with large, welldefined porous structures can be used. OEIPs have been used in vitro for the first time to induce physiological signaling events in neuronal cells [74]. K 1 was delivered in high quantities in the neuron environment, which in turn activated voltage-operated Ca21 channels. The localized action of the OEIP allowed to activate an equivalent Ca21 response to that attainable by manual addition by delivering significantly lower K 1 concentration. The miniaturization of the device enabled the individual stimulation of single neuronal cells. The neurotransmitters glutamate, aspartate, and γ-amino butyric acid were successfully delivered in vivo to the cochlea of guinea pigs, showing the possibility to target specific hair cells inside the cochlea [75]. Chemical stimulation and electrical sensing functions at the same site were successfully performed by integrating a PEDOT:PSS recording electrode and an OEIP on the same device [76]. The electrode recorded epileptiform discharges induced in mouse hippocampal preparation. The inhibitory neurotransmitter, γ-aminobutyric acid (GABA), was then actively delivered through the recording electrodes via OEIP. GABA delivery stopped epileptiform activity, recorded simultaneously and locally. This multifunctional “neural pixel” paves the way to implantable therapeutic devices with automated feedback, where the drug release is activated by locally recorded signals. Moving from iontronic devices to circuits, controlled chemical delivery was demonstrated with independent release of several charged compounds in the milliseconds time scale [77]. Bipolar membranes structures with diode-like characteristics allowed to selectively switch off delivery paths with high on/off ionic current ratio. Iontronics prospectively opens for new therapeutic procedures, as well as a new paradigm of functional circuits. The integration of iontronics and electronics could enable innovative tools for bioelectronic applications. The development of new materials specifically designed for use in iontronic devices, a deep understanding of the device physics and improved microfabrication techniques could eventually enhance the performances and the possibilities achievable with this novel class of devices. 100 3.6 Organic Flexible Electronics Conclusions Mixed conduction organic materials exploiting ionic and electronic transport show a huge potential for the future of bioelectronics and charge-storage devices. The disruptive market of electric cars, as well as the pervasiveness of electronic devices and the need to store renewable energy for power grids are urgently demanding for charge-storage devices providing high-power and energy density, high durability and safety [78]. State-of-the art SCs are a promising technology, usually showing high-power density, long cycling life and safe operation. However their practical application is limited by their poor-energy density, which is still far from that of batteries [79]. The energy density of SCs is limited by a relatively low capacitance and operating voltage window. Mixed conduction materials are able to provide superior specific capacitances by exploiting a volumetric nanoscale ionic-electronic interaction, but their voltage window and safety are still hampered by the choice of the electrolyte. Furthermore, mechanical stability issues may arise due to the penetration of ions into the material, leading to swelling and cracking. The development of new materials will drive the integration of improved mixed conductor-based SCs. The tuning of the electronic and ionic conductivity could result in increased energy density without compromising the power-density performance. The development of new solid state electrolytes could broaden the operative voltage window and improve the safety of the devices, and the engineering of the electrodes mechanical properties could eventually lead to high durability SCs. The world of bioelectronics opens an even wider and more dynamic scenario. The aim is to provide a ubiquitous and smooth link between biology and electronics, with potential applications in the fields of personalized medicine, continuous health monitoring, early-stage diagnosis, treatment of neurological diseases, and targeted drug delivery. But we can also push our imagination further toward even more futuristic applications, such as plant-based biohybrid systems [80] or advanced brain-machine interfaces efficiently connecting humans and computers [72]. Mixed conduction organic materials are the ideal platform for this scope, inherently bridging the world of biology and electronics. The open challenges here are diverse and each application has its own requirements. They involve not only the development of new and improved functional materials, but also the progress of microfabrication techniques and the integration of biocircuits with an application aware design that must be able to meet the demands of the biological world, with its complexity and delicateness. The understanding of the ionic and electronic transport mechanisms and their coupling in mixed conduction materials should guide the development of enhanced materials and technologies, allowing to identify the physics under the weak points of current technologies. The wide variety and the versatility of organic materials make this a hard challenge, because very different electronic and ionic charge transport mechanisms can be involved when considering different materials. 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