Acceptor Backbone Cationization via B ← N Functionalization Enables High‐Performance Porous n‐Type Organic Mixed Ionic‐Electronic Conductors for Biosensors

ABSTRACT

The advancement of high‐performance n‐type organic mixed ionic‐electronic conductors (OMIECs) is pivotal to advancing organic electrochemical transistors (OECTs) for next‐generation bioelectronics. While current design strategies predominantly center on non‐ionic conjugated polymers, their inherently hydrophobic backbones lead to suboptimal ionic transport characteristics. To address this challenge, we introduce an ionic acceptor design strategy of backbone cationization via B ← N functionalization within bipyridine and bipyrazine frameworks. Leveraging these cationic acceptors, we synthesized two n‐type ionic polymer OMIECs, PBPyBF₃, and PBPzBF₃, which exhibit low‐lying LUMO levels as low as −4.0 eV, elevated backbone torsional barriers, and ordered microstructures. Most critically, backbone cationization via B ← N coordination significantly enhances hydrophilicity by inducing a distinct porous film morphology, facilitated by hydrogen bonding with processing solvents. This structural evolution translates to a dramatically improved volumetric capacitance of 581 F cm⁻³. Consequently, OECTs based on cationic polymer PBPzBF₃ exhibit an exceptional normalized transconductance of 38.9 S cm⁻¹ and figure of merit of 215.9 F cm⁻¹ V⁻¹ s⁻¹, ranking among the highest values reported for n‐type OMIECs to date. Notably, electrocardiogram sensors integrated with PBPzBF₃‐OECTs exhibit high signal‐to‐noise ratios and superior sensitivity. This work establishes fundamental structure‐property relationships governing ion‐electron coupled transport in conjugated polymers.

A strategy of acceptor backbone cationization via B ← N Functionalization is proposed. Thus, two n‐type cationic polymeric OMIECs are synthesized with low‐lying LUMOs and a distinctive porous film morphology. These structural features translate into a markedly enhanced transconductance of 38.9 S cm⁻¹ and figure of merit of 215.9 F cm⁻¹ V⁻¹ s⁻¹, placing them among the highest‐performing n‐type OMIECs reported to date.

1. Introduction

The development of bioelectronics has forged a critical interface between biological systems and electronic systems. Developing electronic devices capable of detecting and recording electrochemical signals, biomolecular alterations, or physiological processes in vivo is pivotal for the continued advancement of bioelectronics [1, 2, 3]. Organic electrochemical transistors (OECTs) are a versatile class of organic electronic devices that modulate their electrical conductivity via ionic doping/dedoping in an electrolyte, enabling signal switching or amplification [4]. They exhibit substantial application potential in fields including biosensing, flexible medical diagnostic devices, and neuromorphic computing [5, 6, 7, 8]. The active layer material of OECTs is typically an organic mixed ionic‐electronic conductor (OMIEC), which exhibits dual ionic and electronic conduction capabilities [9, 10]. Therefore, the development of high‐performance OMIEC materials represents a key determinant for realizing superior OECT performance.

The key parameters for evaluating OECT performance primarily include the transconductance (g _m) and the figure of merit (µC*, the product of charge carrier mobility and volumetric capacitance) [2, 11]. Their relationship is defined by Equation (1),

$$g_{\mathrm{m}}=\frac{Wd}{L}\mu C\ast (V_{\mathrm{T}}-V_{\mathrm{G}})$$ (1)

where W, L, and d correspond to the channel width, length, and deposited film thickness, respectively. The transconductance determines the OECT device's amplification capability for weak ionic signals, while the figure of merit evaluates the device's output and transient characteristics. Based on the type of majority charge carriers, the channel materials can be categorized into p‐type (hole‐transporting) OMIEC materials and n‐type (electron‐transporting) counterparts. Analogous to the development trajectory of organic semiconductors in field‐effect transistors (OFETs), p‐type OMIECs currently exhibit superior performance relative to their n‐type counterparts, with exceptional µC* values exceeding several hundred F cm⁻¹ V⁻¹ s⁻¹. In contrast, the advancement of n‐type OMIECs has been relatively constrained, underscoring a persistent performance disparity in this field [12, 13]. This performance disparity stems from two primary factors. First, the charge carrier mobility of n‐type OMIECs is typically orders of magnitude lower than that of p‐type analogs, a major bottleneck hindering the development of n‐type OECTs [14]. Second, most n‐type OMIECs lack stability under the operational conditions of OECT devices, which necessitate that channel materials maintain structural and functional stability in aqueous, oxygen‐rich environments [15]. Consequently, to overcome the intrinsic limitations of n‐type OECT performance, the rational design and precise synthesis of OMIECs featuring deep‐lying lowest unoccupied molecular orbital (LUMO) energy levels—alongside robust chemical and electrochemical stability in aqueous/oxygen‐rich surroundings—are urgently required.

Based on chemical structural distinctions, OMIECs can be classified into two primary categories. The first category encompasses non‐ionic conjugated polymers (NICPs), which consist of non‐ionic backbones functionalized with oligoethylene glycol (OEG) side chains (Figure 1a) [16]. In 2016, Giovannitti et al. developed one of the benchmark p‐type NICPs, p(g2T‐TT), functionalized with polar OEG side chains; OECT devices fabricated from this polymer exhibited low threshold voltages, minimal hysteresis, and a high µC* of 261 F cm⁻¹ V⁻¹ s⁻¹ [17, 18]. For the design of n‐type OMIECs, the predominant strategy centers on the integration of electron‐withdrawing groups (EWGs), such as cyano (‐CN) groups or fluorine (F) substituents [19, 20]. For instance, Guo et al. adopted a cyano‐substitution strategy to modify bithiophene imide (BTI) derivatives, yielding an n‐type OMIEC material, f‐BSeI2g‐SVSCN (Figure 1a) [21]. This polymer features a downshifted LUMO energy level of −3.91 eV, enabling stable n‐type transport. BTI‐based conjugated polymers exhibit favorable planarity, a feature that facilitates the enhancement of charge carrier transport efficiency [22]. OECT devices based on this polymer demonstrated outstanding performance with a µC* of 190.2 F cm⁻¹ V⁻¹ s⁻¹. However, the limited availability of strong EWGs represents a major bottleneck, impeding the advancement of high‐performance n‐type OMIECs. Most notably, the inherently hydrophobic backbones of NICPs lead to suboptimal ionic transport, a phenomenon attributed to their low volumetric capacitance.

FIGURE 1.

(a) Chemical structures of the representative non‐ionic conjugated polymers (NICPs) and (b) ionic conjugated polymers (ICPs) for high‐performance p‐type and n‐type OMIECs. (c) Schematic of the chemical structure evolution of B ← N functionalized backbone cationic acceptors and related n‐type OMIECs applied in ECG sensors in this work.

The second category comprises ionic conjugated polymers (ICPs), a class of polymers containing charged groups within their repeating units—particularly those with conjugated backbones modified with pendant ionic side chains (e.g., polythiophenes functionalized with alkyl sulfonate ions; Figure 1b) [23, 24, 25]. Ionization can significantly enhance hydrophilicity and ionic transport properties, thereby conferring competitive OECT performance [26, 27, 28]. Nevertheless, the OECT performance of most ICPs remains inferior to that of NICPs, especially for n‐type ICPs. This is because the incorporation of ionic side chains (e.g., sulfonate groups, carboxyl groups) necessitates the replacement of all or part of the hydrophobic alkyl side chains (see PTHS‐TMA⁺‐co‐P3HT in Figure 1b). The steric hindrance and electrostatic repulsion of ionic side chains compromise the continuity of main‐chain π‐π stacking, resulting in disordered polymer packing and the formation of “deflected stack” or S‐bent side chain conformations. This disordered packing substantially increases carrier scattering during electron transport, leading to charge carrier mobilities of ICPs that are typically 1–2 orders of magnitude lower than those of NICPs. While the ionic side chains of ICPs can increase ionic adsorption sites and enhance C* values, this capacitance improvement is often offset by the substantially reduced μ, resulting in an overall µC* value lower than that of NICPs. For example, the ICP PTHS‐TMA⁺‐co‐P3HT exhibited a low OECT hole mobility of 0.017 cm² V⁻¹ s⁻¹ and a µC* value of 1.7 F cm⁻¹ V⁻¹ s⁻¹—values that are two orders of magnitude lower than those of the benchmark p‐type NICP p(g2T‐TT) (∼1.0 cm² V⁻¹ s⁻¹ and 261 F cm⁻¹ V⁻¹ s⁻¹) [18, 29].

To address these limitations, a viable strategy is to ionize the polymer backbone rather than the side chains. This approach endows the polymer with intrinsic hydrophilicity, enabling excellent hydrated ion permeation without relying on additional hydrophilic side chains. Furthermore, Coulombic interactions between the ionic backbone and electrolyte anions direct the formation of ordered ionic transport pathways, lowering ionic transport resistance and facilitating ion‐electron coupled transport. The strategic integration of these ionic moieties into ICP synthesis represents an innovative paradigm for the development of high‐performance OMIECs. However, building blocks with backbone ionization (for n‐type ICPs, this necessitates acceptor‐ or electron‐deficient backbone cationization) pose substantial synthetic challenges, including multistep reaction pathways and stringent purification requirements due to their inherent ionic character. These limitations originate from (1) the thermodynamic instability of cationic intermediates during synthesis and (2) strong electrostatic interactions that hinder purification. Consequently, the successful integration of cationic acceptor backbones into n‐type OMIECs has rarely been demonstrated. To the best of our knowledge, only one report by Lei et al. has documented the introduction of a cationic moiety into a naphthalene donor, yielding a promising µC* value of 62.3 F cm⁻¹ V⁻¹ s⁻¹. To date, no reports have documented the introduction of cationic moieties into the backbones of acceptor units.

Organoboron chemistry has been regarded as a promising molecular engineering platform for rational design of materials with precisely modulated properties. The boron‐nitrogen coordination bond (B ← N), formed between boron and nitrogen moieties in heteroaromatic frameworks, offers distinct advantages, such as high electron affinity, reduced LUMO energy levels, and narrowed band gaps, thereby leading to intriguing organic functional materials [30, 31]. In 2015, the Liu group pioneered the integration of B ← N bonds into conjugated polymers by replacing a C‐C linkage with a B ← N unit in a cyclopentadithiophene derivative [32]. This landmark achievement successfully lowered the LUMO energy level from −3.11 to −3.76 eV, representing the first report on B ← N‐based conjugated polymers [32]. Despite the considerable potential of this promising advancement, the nascent stage of B ← N chemistry has limited accessibility of B ← N‐based polymers for diverse organic electronics applications. Specifically, most B ← N‐based polymers have been explored in light emitting devices, organic photovoltaics, organic field‐effect transistors, and organic thermoelectrics, whereas almost no reports exist regarding applications of such polymers to OECTs [33, 34, 35, 36, 37].

In this study, we present a novel class of backbone cationic acceptor units characterized by a straightforward synthetic route and exceptional electron‐withdrawing capabilities (Figure 1c). Through coordination chemistry involving the lone pair electrons of bidentate ligands (bipyridine and bipyrazine) with boron trifluoride (B ← N functionalization), we have successfully developed two backbone cationic acceptors (BPyBF₃ and BPzBF₃) exhibiting pronounced electron‐withdrawing properties. These building blocks were subsequently employed in the synthesis of two n‐type OMIECs, PBPyBF₃, and PBPzBF₃, which demonstrate remarkably low LUMO energy levels, reaching values as low as −4.0 eV. This significant achievement is attributed to the synergistic interaction between the electron‐withdrawing B ← N‐based cationic moiety and sp²‐hybridized nitrogen atoms. When implemented as channel material in OECTs, PBPzBF₃ demonstrated exceptional performance metrics, achieving a normalized transconductance (g _m,n) of 38.9 S cm⁻¹ and a µC* value of 215.9 F cm⁻¹ V⁻¹ s⁻¹, establishing it as one of the most efficient n‐type OMIECs reported to date. Comprehensive thin‐film morphological characterization reveals that the backbone cationic polymers PBPyBF₃ and PBPzBF₃ can form intermolecular hydrogen bonds with strong hydrogen bond‐donating solvents (hexafluoroisopropanol (HFIP)). This solvent‐mediated hydrogen bonding interaction induces the formation of a porous yet ordered microstructure in the resulting thin films—an architectural feature that synergistically enables high capacitance values while retaining robust charge carrier mobility. Notably, the cationic polymer PBPzBF₃ was successfully integrated into in vitro human electrocardiogram (ECG) sensors based on OECTs, exhibiting significantly higher sensitivity and signal‐to‐noise ratio (SNR) compared to its non‐cationic counterpart PBPz. Beyond demonstrating practical utility in bioelectronics, this work establishes fundamental structure–property–performance relationships for the design and synthesis of high‐performance n‐type OMIECs, thereby laying a critical foundation for the advancement of next‐generation bioelectronic devices with enhanced functionality and reliability.

2. Results and Discussion

2.1. Design and Synthesis of Monomers and Polymers

Organic cations are a class of organic molecules bearing a positive charge. These molecules possess high polarity and pronounced electron deficiency. Additionally, their counteranions readily dissociate, thereby enhancing the electrical conductivity of the resulting materials. However, organic cations typically pose challenges related to intricate synthetic protocols. In the present study, we focus on pyridine derivatives, as the presence of the sp²‐hybridized nitrogen (N) atom strengthens the overall electron‐deficient property. Linking pyridine rings through single bonds facilitates the accumulation of electron deficiency, thereby further enhancing the electron‐accepting capacity of these electron‐withdrawing building blocks [37, 38]. Simultaneously, the lone pair on the nitrogen atom exhibits strong coordinating ability, enabling the formation of stable complexes with a range of Lewis acids [39]. The incorporation of additional sp²‐N atoms permits the design of a more electron‐deficient motif, bipyrazine, while retaining its bidentate ligating ability.

As depicted in Scheme 1, two electron‐deficient building blocks—4,4′‐dibromo‐2,2′‐bipyridine (BPy) and 4,4′‐dibromo‐2,2′‐bipyrazine (BPz)—were selected. Subsequently, two cationic building units, BPyBF₃ and BPzBF₃, were synthesized via B ← N functionalization [40]. The synthetic protocols for both units were analogous: each precursor was dissolved in dichloromethane, followed by the dropwise addition of an excess of boron trifluoride diethyl etherate complex. Precipitation ensued immediately upon the initiation of addition, indicating the rapid progression of the B ← N coordination complexation reaction. Following reaction completion, the precipitate was collected via filtration. This precipitate was soluble solely in highly polar solvents (e.g., N,N‐dimethylformamide (DMF), dimethyl sulfoxide (DMSO)) but exhibited negligible solubility in chloroform. In contrast, BPy and BPz display modest solubility in chloroform.

SCHEME 1.

The synthetic route of the cationic acceptors and their cationic polymers with the non‐cationic counterparts.

The structures of these cationic units were characterized via nuclear magnetic resonance (NMR) spectroscopy. ¹H, ¹³C, ¹¹B, and ¹⁹F NMR spectra of the intermediates and target products were acquired (Figures S1–S15). In the ¹H NMR spectra of BPyBF₃ and BPzBF₃, two new resonances emerged at chemical shifts of 5.76 and 2.08 ppm. Comparison with the ¹H NMR spectrum acquired after treating the compounds with potassium fluoride (KF) in DMSO‐d₆ (Figure 2a) revealed that these new resonances arise from the hydration of fluoride ions by residual water in the deuterated solvent DMSO‐d₆. This finding further validates the successful synthesis of the cationic units. In the ¹¹B NMR spectra, a resonance was observed in the high‐field region, indicative of tetrahedral (four‐coordinate) boron centers (Figure 2b). This observation corroborates the successful synthesis of the cationic building blocks via B ← N coordination. The molecular weights of the cationic units, determined via matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF), were consistent with the theoretical values (Figures 2c,d and S16).

FIGURE 2.

(a) The ¹H NMR spectra of the cationic acceptor units BPzBF₃, BPyBF₃ and compound KF in dimethyl sulfoxide‐d₆. (b) ¹¹B NMR spectra of the cationic acceptor units BPzBF₃ and BPyBF₃. MALDI‐TOF mass spectra of (c) BPzBF₃ and (d) BPyBF₃.

Next, thiophene‐flanked diketopyrrolopyrrole (DPP) was selected as the copolymer unit due to its excellent planarity and ease of side‐chain modification [41, 42]. Branched OEG chains were chosen as solubilizing side chains to provide sufficient solubility for the polymer while enabling ample doping capacity for ions. The synthetic route followed our previously reported reference [43]. The organotin reagent was synthesized according to Scheme S1. This tin compound should be used immediately after purification for polymerization due to its relatively instability. The target polymers were synthesized via Stille polycondensation. The detailed synthetic route is provided in the Supporting Information. Adding a small amount of the co‐catalyst CuI aided in obtaining higher molecular weight polymers [19]. Due to the very low solubility of BPyBF₃ and BPzBF₃ in chlorobenzene (CB), a mixed solvent of CB/DMF (1:1, v/v) was used for the polymerization. The conjugated polymers based on bipyridine derivatives, PBPy and PBPyBF₃, displayed slower reaction kinetics, requiring 48 h for complete reaction. In contrast, the conjugated polymers incorporating bipyrazine units, PBPz and PBPzBF₃, underwent polymerization very rapidly, necessitating reaction termination after only 10 min. Prolonged extension of the reaction time resulted in the polymers becoming insoluble in chloroform and hexafluoroisopropanol (HFIP), thereby impeding the solution‐based processing of thin‐film devices. Subsequent to polymerization, the polymers were precipitated into hexane and subsequently subjected to Soxhlet extraction using acetone and hexane sequentially to remove residual monomers and catalysts. Owing to the presence of cationic moieties within their chemical structures, the cationic polymers PBPyBF₃ and PBPzBF₃ were insoluble in aprotic solvents such as chloroform, with a solubility of < 1 mg mL⁻¹, and exhibited solubility exclusively in HFIP. In contrast, the neutral polymers PBPy and PBPz demonstrated favorable solubility in chloroform, CB, and HFIP. Since the polymers modified with OEG side chains could not be characterized for molecular weight using trichlorobenzene (TCB)‐based high‐temperature gel permeation chromatography (GPC), we evaluated the number‐average molecular weights (M _n) and polydispersity indices (Đ) of all four polymers using room‐temperature GPC with HFIP as the eluent [44], the results are summarized in Table 1 and Figure S17. The M _n of all polymers exceeded 10 kDa. The molecular weights of PBPy and PBPyBF₃ were half those of PBPz and PBPzBF₃, consistent with the observed differences in reaction rates. The exceptionally high reactivity of BPz and BPzBF₃ is likely attributable to the activation of the ortho C‐Br bonds by the para‐position nitrogen atoms in the pyrazine ring. Due to the presence of ionizable groups within the BPyBF₃ and BPzBF₃ molecules, PBPyBF₃ and PBPzBF₃ belong to a category of polyelectrolytes. The thermal stability of the polymer series was measured (Figure S18a). The decomposition temperatures (T _d), defined as the temperature for 5% weight loss, are listed in Table 1. PBPy and PBPz exhibited higher T _d values (> 300°C), while those of PBPyBF₃ and PBPzBF₃ were significantly lower, possibly due to the volatility of the counterions [45]. Differential scanning calorimetry (DSC) analysis showed no discernible thermal transitions within the 0°C–100°C range (Figure S18b).

TABLE 1.

The molecular weight, optical, electrochemical properties of PBPy, PBPyBF₃, PBPz, and PBPzBF₃.

Polymer	M n a (kDa)	Đ a	${\lambda }_{\max }^{\mathrm{sol}.}($ nm)	${\lambda }_{\max }^{\mathrm{film}}$ (nm)	E LUMO b (eV)	E HOMO b (eV)	E g c (eV)	T d (°C)
PBPy	17.2	2.01	594, 361	709, 405	−3.58	−5.15	1.57	341
PBPyBF3	16.5	2.10	573, 361	682, 388	−3.71	−5.17	1.46	273
PBPz	35.5	2.31	630, 393	743, 428	−3.90	−5.31	1.41	348
PBPzBF3	35.9	2.24	622, 396	736, 423	−3.96	−5.32	1.36	277

^a Measured by HFIP as the eluent at room temperature.

^b Measured by CV method.

^c E _g = E _LUMO − E _HOMO.

2.2. Theoretical Calculations of Monomers and Polymers

Density functional theory (DFT) calculations were performed to elucidate the fundamental properties of the new backbone cationic polymers. The cationic acceptor units (BPyBF₃ and BPzBF₃) and their non‐cationic counterparts (BPy and BPz) were initially investigated via DFT calculations at the B3LYP‐D3(BJ)/6‐31G(d) level of theory [46]. For BPyBF₃ and BPzBF₃, surface electrostatic potential (ESP) maps were simulated to identify the regions of highest ESP, which were used to determine the positions of counterions. As illustrated in Figure 3a, owing to the conformational locking effect induced by B ← N coordination, BPyBF₃ and BPzBF₃ adopt a conformation opposite to that of BPy and BPz. The regions of maximum ESP for both BPyBF₃ and BPzBF₃ are localized on the side opposite to the boron atom and coplanar with the aromatic ring system. Additionally, BPyBF₃ and BPzBF₃ exhibit an extended blue region along the entire backbone of the acceptor moieties. These observations indicate that prior to the formation of the B ← N coordination‐based cationic structure, the electron‐deficient nature of the BPy and BPz units is primarily conferred by the sp²‐hybridized nitrogen atoms. In contrast, following the formation of the B ← N coordination‐based cationic structure, the B ← N functionality in BPyBF₃ and BPzBF₃ becomes dominant and serves as the primary source of electron deficiency. Furthermore, this electron deficiency is markedly enhanced by acceptor backbone cationization via B ← N functionalization. The LUMO and highest occupied molecular orbital (HOMO) energy levels of BPy, BPyBF₃, BPz, and BPzBF₃ were also computed to compare the energy level trends among these four monomers. It should be noted that the counterions are localized at these regions of maximum ESP. The optimized LUMO/HOMO configurations and energy levels are presented in Figures 3b and S19. Upon formation of B ← N coordination, cationic molecules are generated, which results in a substantial downshift of the LUMO energy level, with the LUMO electron density uniformly delocalized across the entire molecular framework. In contrast, the HOMO energy level undergoes an upward shift, and the HOMO electron density becomes strongly localized on the counterions. This finding demonstrates that B ← N coordination‐driven cationization constitutes an effective strategy for constructing strongly electron‐deficient comonomer units with lower‐lying LUMO energy levels and reduced band gaps, thereby facilitating the synthesis of n‐type polymers.

FIGURE 3.

(a) The ESP distribution of the cationic acceptors BPyBF₃ and BPzBF₃ and their non‐cationic counterparts. (b) Frontier molecular orbital distribution diagrams of the cationic acceptor BPzBF₃ and its non‐cationic counterpart. (c) Optimal geometries and frontier molecular orbital distribution diagrams of the backbone cationic polymer PBPyBF₃ and its non‐cationic counterpart (methyl‐substituted trimers) from DFT calculations. (d) The color‐filled map of IRI on system plane of PBPy and PBPzBF₃. (e) Dihedral rotation potential barrier curves of PBPy, PBPyBF₃, PBPz, and PBPzBF₃.

Subsequently, the LUMO and HOMO energy levels of the polymers were computed (Figures 3c and S20). It is found that increasing the number of nitrogen atoms in the comonomer unit (PBPy vs. PBPz) or introducing B ← N coordination cationization (PBPy vs. PBPyBF₃ or PBPz vs. PBPzBF₃) effectively downshifts both the LUMO and HOMO energy levels. With respect to the frontier molecular orbital (FMO) distribution, the results demonstrate that the LUMO electron density of all four polymers is delocalized over the central dimer segments; for PBPyBF₃ in particular, the LUMO electron density is distributed across nearly the entire conjugated backbone. Regarding the HOMO electron density, all four polymers exhibit localization on the DPP units. This indicates that the polymers may exhibit prominent n‐type charge transport properties. Among the four polymers, PBPyBF₃ chains adopt a planar, pseudo‐linear conformation, whereas the other polymers display a wavy conformation. For instance, from the side view of the PBPy polymer manifold, it exhibits a dihedral angle of 20°, compared to 0° for PBPzBF₃ (Figure 3c). In addition, the color‐filled map of interaction region indicator (IRI) on system plane of both polymers are shown in Figure 3d. In PBPy, the isosurfaces between DPP and BPy are very weak while it shows larger area in the case of PBPzBF₃. The pseudo‐linear conformation with better comonomer interaction is considered to facilitate electron transport along the polymer chains [47]. It should be noted that in PBPyBF₃ and PBPzBF₃, the fluorine (F) atoms reside out‐of‐plane relative to the conjugated backbone. This structural feature may promote the formation of a less compact packing arrangement, potentially facilitating ion doping during operation in OECTs. Collectively, introducing backbone cationization via B ← N functionalization effectively downshifts both the LUMO and HOMO energy levels and favors the realization of n‐type ionic‐electronic transport behavior.

We also performed torsional barrier scanning for the thiophene unit within the DPP and the electron‐withdrawing comonomer unit. The results are shown in Figure 3e. PBPy exhibits the lowest torsional barrier. Upon boron‐nitrogen coordination cationization, the torsional energy barrier increases. The two polymers based on the pyrazine unit exhibit very high torsional energy barriers, exceeding 30 kcal mol⁻¹. A larger torsional energy barrier facilitates the adoption of a planar conformation, which can enhance the effective conjugation length, leading to superior charge carrier transport behavior [48, 49].

2.3. Photophysical and Electrochemical Properties

To investigate the photophysical properties of the polymers in both solution and thin‐film states, ultraviolet‐visible‐near‐infrared (UV‐vis‐NIR) absorption spectra were recorded, as shown in Figure 4a,b. The corresponding absorption maxima are summarized in Table 1. The UV‐vis‐NIR absorption spectra exhibit characteristic dual‐band absorption behavior: the absorption band between 500 and 700 nm corresponds to the intramolecular charge‐transfer (ICT) process, while the band between 300 and 450 nm corresponds to the π‐π* transition. All polymers show similar spectral profiles. The absence of discernible 0‐0 and 0–1 vibronic peaks indicates a loss of vibronic fine structure, suggesting that in dilute solution (using HFIP as the solvent), the polymer chains exhibit a very low degree of pre‐aggregation. Following B ← N coordination cationization, a slight blue shift in the absorption maximum is observed, attributed to the enhanced electron‐deficient character of the B ← N functional group. Upon transitioning from solution to the thin‐film state, a significant red shift (> 110 nm) in the absorption maximum occurs, accompanied by the emergence of vibronic fine structure. This indicates that during film formation, the polymer chains undergo a certain degree of pre‐aggregation, which facilitates the development of a suitably ordered microstructure. Under OECT operating conditions, the polymer films are in a heavily doped state. To investigate the ion doping behavior of the polymer films under an applied electric field in detail, spectroelectrochemistry measurements were performed (Figures 4c and S21), utilizing three‐electrode setup with an Ag/AgCl reference electrode. PBPy exhibits p‐type doping behavior, whereas PBPyBF₃, PBPz, and PBPzBF₃ exhibit n‐type transport behavior. Upon application of voltage, all polymers display characteristic electrochemical doping behavior: the intensity of the ICT absorption band gradually decreases (electrochemical bleaching), concurrently with the emergence of a polaron absorption band in the 900–1200 nm region. To compare the differences in doping behavior among the NICPs and ICPs in detail, the change in absorption peak intensity as a function of applied voltage is plotted in Figure 4d,e. For PBPy, the ICT absorption peak does not show a distinctly visible change until 0.6 V. However, the polaron absorption band can be observed even at 0 V and becomes progressively enhanced with increasing applied voltage. The doping behavior of PBPyBF₃ exhibits a high onset doping potential and a low doping level. PBPz and PBPzBF₃ show similar electrochemical doping behavior; however, PBPzBF₃ has a lower onset doping potential (−0.6 V vs. −0.4 V), exhibits stronger electrochemical bleaching, and displays a distinct polaron absorption peak at high voltages. These results demonstrate backbone cationization using B ← N coordination enhances the doping capability of the polymer films.

FIGURE 4.

The normalized UV‐vis‐NIR absorption spectra of PBPy, PBPyBF₃ (a) and PBPz, PBPzBF₃ (b) in HFIP solution and thin‐film. (b) Potential‐dependent UV‐vis‐NIR absorption spectra of PBPz and PBPzBF₃. Changes in the absorbance at the ICT peak (d) and polaron peak (e). (f) The diagram of frontier orbital energy levels of PBPy, PBPyBF₃, PBPz and PBPzBF₃, which is drawn based on the CV results.

Subsequently, cyclic voltammetry (CV) was employed to assess the FMO energy levels of the polymer series. The CV curves and the LUMO/HOMO energy level diagram are presented in Figures 4f and S22, respectively. The results demonstrate that substituting pyridine with pyrazine (i.e., increasing the number of sp²‐hybridized nitrogen atoms) can simultaneously and effectively downshift both the LUMO and HOMO energy levels. The downshift in the LUMO energy level exceeds 0.30 eV, while that in the HOMO energy level exceeds 0.15 eV. For instance, the LUMO and HOMO energy levels of PBPy are −3.58 and −5.15 eV, respectively, whereas those of PBPz are −3.71 and −5.17 eV, respectively. This indicates that PBPy may exhibit predominantly p‐type charge transport properties, while PBPz can display enhanced n‐type charge transport properties. These observations are consistent with previously reported findings in the literature [22, 50, 51]. Upon formation of the B ← N‐coordinated cation, the LUMO energy level further undergoes a large downshift, with a magnitude ranging from 0.06 to 0.13 eV. However, the HOMO energy level exhibits almost no change. This indicates that the backbone cationization using B ← N functionalization exerts a more pronounced effect on the LUMO energy level, whereas the HOMO energy level remains largely unchanged, as it is influenced by the counterions. Notably, the LUMO energy level of PBPzBF₃ is close to −4.0 eV, which is conducive to the realization of decent n‐type charge transport behavior. Collectively, these CV experimental results are consistent with the DFT calculation results presented above.

2.4. OECT Performance

Next, parallel‐structured OECTs were fabricated to assess the OECT performance of the backbone cationic polymers and their non‐cationic counterparts. The semiconductor layers were prepared by spin‐coating onto substrates with gold electrodes pretreated via UV‐ozone to enhance the wettability of the polymer solutions. HFIP was initially adopted as the processing solvent owing to its excellent solvating power for the polymers in this study. The detailed fabrication procedure is provided in the Supporting Information. The transfer and output characteristics of the four polymers are presented in Figures 5a–d, S23, and S24, with the corresponding results summarized in Table 2. OECT devices based on PBPy exhibited p‐type transport behavior, consistent with their designation as p‐type polymers. This is attributed to their relatively higher LUMO energy levels, which facilitate hole induction and transport via ion doping, which is consistent with the findings from spectroelectrochemical and CV measurements. Its cationized counterpart via B ← N functionalization, PBPyBF₃, exhibited downshifted LUMO and HOMO energy levels and demonstrated n‐type transport behavior, albeit with inferior performance (Table 2). OECT devices based on PBPz and PBPzBF₃ displayed ideal n‐type transport behavior (Figure 5a–d). PBPz achieved a geometrically normalized transconductance (g _m,n) of 5.4 S cm⁻¹ and a µC* value of 57.7 F cm⁻¹ V⁻¹ s⁻¹, whereas its cationized counterpart (PBPzBF₃) exhibited a fourfold improvement in g _m,n (reaching 23.9 S cm⁻¹) and an 88% increase in µC* (reaching 109.0 F cm⁻¹ V⁻¹ s⁻¹; Table 2). Electrochemical impedance spectroscopy (EIS) was employed to measure the capacitance (C*) of the four materials, and μ _OECT was calculated using C* and µC* (Table 2 and Figures S25–S28). Upon formation of the cationized backbone via B ← N coordination, a substantial increase in C* was observed relative to the non‐cationized counterparts, indicating that the B ← N‐based cationic units possess superior ion affinity. The LUMO energy level of PBPz was sufficiently low to support n‐type charge transport, and backbone cationization via B ← N coordination further enhanced electron injection and transport. Consequently, PBPzBF_3—with its higher capacitance and higher μ _OECT—achieves an excellent µC*.

FIGURE 5.

Transfer and corresponding transconductance curves of OECTs based on the polymer (a) PBPz and (b) PBPzBF₃ processed by HFIP. The corresponding output curves of (c) PBPz and (d) PBPzBF₃ processed by HFIP. Transfer and corresponding transconductance curves of (e) PBPy, (f) PBPyBF₃, (g) PBPz, and (h) PBPzBF₃ processed by the strategy of MS. The corresponding output curves of (i) PBPz and (j) PBPzBF₃ processed by the strategy of MS. Response time (k) and long‐term on‐off switching curves (l) of PBPzBF₃.

TABLE 2.

Summary of OECT parameters for of PBPy, PBPyBF₃, PBPz, and PBPzBF₃.

Processed solvent	Polymer	d a (nm)	V T b (V)	g m,n c (S cm−1)	µC* d (F cm−1 V−1 s−1)	C* e /F cm−3	μ OECT f (cm2 V−1 s−1)	Log[I on/I off]
HFIP	PBPy	29.9 ± 3.6	−0.57 ± 0.02	39.2 (36.5 ± 2.5)	122.1 (113.9 ± 7.1)	199	1.02 (0.96 ± 0.06)	5–6
	PBPyBF3	31.9 ± 3.1	0.68 ± 0.02	1.1 (0.97 ± 0.06)	6.4 (5.7 ± 0.4)	208	0.03 (0.027 ± 0.002)	3–4
	PBPz	31.8 ± 2.2	0.69 ± 0.03	5.4 (4.8 ± 0.4)	57.7 (51.8 ± 6.0)	272	0.21 (0.19 ± 0.02)	4–5
	PBPzBF3	28.1 ± 1.5	0.58 ± 0.02	23.9 (20.6 ± 2.5)	109.0 (103.9 ± 4.2)	388	0.28 (0.27 ± 0.02)	4–5
MS	PBPy	55.9 ± 2.8	−0.67 ± 0.02	85.6 (81.6 ± 4.5)	305.7 (291.2 ± 12.3)	151	2.02 (1.92 ± 0.07)	5–6
	PBPyBF3	46.6 ± 2.7	0.79 ± 0.02	3.6 (3.2 ± 0.3)	33.3 (29.1 ± 2.7)	249	0.13 (0.12 ± 0.01)	4–5
	PBPz	48.1 ± 1.1	0.68 ± 0.02	13.7 (12.6 ± 0.7)	91.6 (84.0 ± 5.7)	364	0.25 (0.23 ± 0.02)	5–6
	PBPzBF3	51.4 ± 1.7	0.64 ± 0.02	38.9 (36.7 ± 1.7)	215.9 (203.7 ± 8.7)	581	0.37 (0.35 ± 0.02)	5–6

^a Film thickness.

^b Threshold voltage.

^c Transconductance normalized by the channel geometry factors.

^d Extracted by Equation (1).

^e Obtained by measuring and fitting the capacitance and volume of the materials.

^f Calculated from the µC* and the volumetric capacitance C*.

To further enhance the OECT figure of merit (µC*), a mixed‐solvent (MS) processing strategy was employed for fabricating OECT devices. In our study, HFIP was a strong hydrogen‐bond donor that interacts with the OEG side chains, thereby facilitating complete dissolution of the polymers in this solvent. The incorporation of a small quantity of a poor solvent could induce effective pre‐aggregation of polymer chains in the solution phase, thereby enhancing device performance. Dimethoxyethane (DME)—which had a slightly higher boiling point than HFIP (85°C vs. 60°C) and was miscible with HFIP—served as a poor solvent for the four polymers. The effect of incorporating different proportions of DME on the OECT performance of PBPz was investigated, with the results presented in Table S1 and Figure S33. A small incorporation of DME resulted in a modest improvement in µC*; when the proportion was increased to 10%, the enhancement became more pronounced. However, increasing the proportion to 20% resulted in a drastic decline in OECT performance. Excessive DME might reduce carrier mobility by inducing unfavorable packing structure, thereby deteriorating overall device performance. Consequently, 10% DME was selected as the optimal additive ratio for OECT device processing. The corresponding results are presented in Figures 5e–j and S34 and Table 2. MS processing resulted in substantial improvements in μ _OECT and capacitance, which resulted in a notable improvement in µC* (Table 2 and Figures S29–S32). On one hand, the incorporation of an appropriate quantity of poor solvent promoted polymer chain pre‐aggregation, and the resulting polymer films inherited this pre‐aggregated state from the solution phase, leading to a more ordered film structure with elevated crystallinity (thin‐film GIWAXS study, vide infra). This leads to increased grain size with a concomitant reduction in grain boundary defects and enhanced carrier mobility. On the other hand, the pre‐aggregation can form a more continuous polymer film network, which can drastically shorten the ion diffusion length (thin‐film morphology study, vide infra). It affords a substantially larger ion penetration depth and contact area, and enables a higher level of electrochemical doping, thereby enhancing the volumetric capacitance C* (the number of electrochemically active sites per unit volume). Furthermore, Furthermore, we employed heated‐substrate processing to fabricate PBPyBF₃ and PBPzBF₃ films with dense, homogeneous morphologies closely resembling those of their neutral counterparts (see polymer morphologies in Figure S35). The corresponding OECT devices were then prepared, and their performance metrics are summarized in Figure S36 and Table S2. Although the porous network leads to a geometric inflation of the apparent C*, the heated‐substrate control experiments show that, compared with the native neutral polymer PBPz, which likewise exhibits a dense film morphology, dense PBPzBF₃ still delivers a pronounced performance enhancement. This observation firmly demonstrates that acceptor‐backbone cationization does not improve OECT performance solely through an increase in volumetric capacitance. Instead, acceptor‐backbone cationization synergistically lowers the LUMO energy level, modulates film morphology, and optimizes microstructural packing. Collectively, the essence of the MS strategy lies in the direct translation of solution‐state molecular engineering and nanostructural design into solid‐state films, thereby maximizing the balance among molecular packing, charge transport, and ion penetration while preserving the defining characteristic of OMIECs, namely, mixed ion‐electron transport.

The response times of the devices were also recorded (Figures 5k and S37), with PBPz and PBPzBF3 exhibiting significantly shorter response times compared to PBPy and PBPyBF₃. Furthermore, the on‐time (τ _on) and off‐time (τ _off) were comparable, indicating that electrolyte ions could rapidly ingress and egress the semiconductor layer upon application or removal of the gate voltage. This was critical for the development of highly sensitive, low‐power sensors. The operational stability of the devices was also evaluated (Figures 5l and S38). Devices based on the p‐type polymer PBPy exhibited good operational stability. For the three n‐type polymers, devices based on PBPz and PBPzBF₃ exhibited superior operational stability compared to those based on PBPyBF₃, attributed to the deeper LUMO energy levels of PBPz and PBPzBF₃. It is worth noting that, because PBPyBF₃ and PBPzBF₃ are backbone‐cationic polyelectrolytes, their initial counterions (F⁻) are associated with the polymer matrix only through electrostatic interactions. During OECT operation in an aqueous environment, these counterions can readily leach into the electrolyte and undergo exchange with electrolyte anions during the electrochemical doping/dedoping processes. Notably, the unique porous film morphology provides efficient ion‐transport pathways, thereby reducing transport resistance and facilitating this ion‐exchange process (thin‐film morphology study, vide infra). X‐ray photoelectron spectroscopy (XPS) further substantiates this behavior, providing clear evidence of counterion exchange between the initial polymer‐bound anions and electrolyte anions before and after device operation (Figure S42). These results suggest that the combination of backbone cationization via B ← N coordination and sp²‐hybridized N substitution constitutes an effective strategy for the fabrication of high‐performance n‐type OEIMC materials.

2.5. Thin‐Film Microstructure Study

To gain deeper insights into the origins of the performance differences in the OECTs based on the backbone cationic polymers and their non‐cationic counterparts, we systematically analyzed the thin‐film microstructure. By employing 2D grazing‐incidence wide‐angle x‐ray scattering (GIWAXS), we rigorously characterized the microstructure of the polymer films (Figures 6, S39, and S40, with crystallographic parameters summarized in Table S3). When HFIP was utilized as the processing solvent, the backbone cationic polymers and their non‐cationic counterparts, both solubilized by identical OEG side chains, exhibited comparable lamellar stacking distances (d ₁₀₀) in the out‐of‐plane direction. The d ₁₀₀ of the cationic polymers is marginally larger than that of the non‐cationic polymers, and their coherence length (L _C) is notably smaller. This phenomenon is ascribed to the additional steric hindrance introduced by B ← N coordination cationization and its associated counter ions, which disrupts the ordering along the lamellar stacking direction. It is crucial to note that the stacking distance of 4.2–4.4 Å, calculated from q _z of 1.5 Å⁻¹, is attributed to sidechain stacking interactions, rather than the (010) backbone π–π stacking (d₀₁₀). Following B–N coordination cationization, this stacking distance also increased by 0.2–0.3 Å due to steric hindrance (Figure 6e,f). Collectively, the cationic polymers exhibit lower ordering compared to their non‐cationic counterparts, which is detrimental to efficient electron/hole transport between polymer chains. Conversely, the loose packing is advantageous for ion penetration, thereby enhancing the C*. Notably, the implementation of the MS processing strategy significantly improves the polymer packing ordering (Figure 6b,d). For the (h00) diffraction peaks, the lamellar stacking distances of all polymers decrease, indicating that the addition of a poor solvent promotes ordered packing in this direction. For the two non‐cationic polymers, PBPy and PBPz, distinct higher‐order (h00) diffraction peaks up to (300) and (400) are observable (Figure 6b,g). Although clear higher‐order diffraction peaks remain challenging to discern for the two cationic polymers (Figures 6d,h and S40) the π–π interactions were markedly enhanced. The d ₀₁₀ of PBPzBF₃ is only 3.55 Å (Table S2), which is substantially shorter than that of its non‐cationic counterpart PBPz (3.67 Å). This reduction effectively mitigates the adverse effects of steric hindrance resulting from B ← N coordination cationization. Overall, for the cationic polymers, MS processing significantly enhances packing order—particularly with respect to π–π stacking order—which facilitates interchain electron hopping transport (resulting in a higher μ _OECT). Meanwhile, the loose lamellar packing facilitates ion penetration, thereby conferring a higher C*. As a result, PBPzBF₃ achieves an excellent figure of merit (µC*).

FIGURE 6.

GIWAXS diffraction pattern of PBPz processed by (a) HFIP and (b) the strategy of MS. GIWAXS diffraction pattern of PBPzBF₃ processed by (c) HFIP and (d) the strategy of MS. One‐dimensional XRD line profiles of (e) out‐of‐plane and (f) in‐plane directions, extracted from the corresponding two‐dimensional GIWAXS patterns processed by HFIP. One‐dimensional XRD line profiles of (g) out‐of‐plane and (h) in‐plane directions, extracted from the corresponding two‐dimensional GIWAXS patterns processed by the strategy of MS.

2.6. Thin‐Film Morphology Analysis

Subsequently, atomic force microscopy (AFM) was employed to investigate the morphology of the polymer thin films, with the results presented in Figures 7a–d and S41. When HFIP was used as the processing solvent, both non‐cationic polymers (NICPs), PBPy, and PBPz, exhibited featureless, flat, and smooth morphologies with relatively low root mean square roughness (R _q) values. In contrast, both cationic polymers showed an increased R _q and displayed a porous morphology with sparse, isolated pores. Particularly, PBPzBF₃ exhibited a porous morphology with denser, interconnected pores forming a network‐like structure. This porous structure was also observed in the scanning electron microscopy (SEM) images (Figure 7e,f). When the processing strategy of MS was employed, the thin films of both NICPs developed a morphology with small, fragmented fibrous features, accompanied by an increase in R _q. The same trend was observed for the cationic polymers, which displayed a denser and more uniform porous morphology (Figure 7c,d). Typically, the construction of porous morphologies requires complex procedures, such as template‐assisted methods, phase separation methods, and etching methods. However, our cationic polymers can in situ form such porous morphologies via a simple spin‐coating process. We discussed the potential mechanism for the formation of the porous morphology, as illustrated schematically in Figure 7i. HFIP is a strong hydrogen‐bond donor. The presence of abundant fluorine atoms and fluoride ions in our cationic polymers may facilitate hydrogen‐bonding interactions with HFIP. This could result in the retention of some condensed HFIP within the film during spin‐coating. During subsequent annealing, these hydrogen bonds dissociate, allowing the condensed HFIP to evaporate and leaving behind pores. In contrast, the NICPs, lacking sufficient hydrogen‐bonding sites, cannot retain HFIP and thus do not form pores upon annealing. To verify this hypothesis, Fourier‐transform infrared (FTIR) absorption spectra of a PBPzBF₃ thin film spin‐coated from HFIP were characterized before and after annealing, as shown in Figure 7g. The as‐cast film exhibited a strong vibrational absorption band around 3300 cm⁻¹, attributed to the hydroxyl group of HFIP (the condensed HFIP). After annealing, this hydroxyl band was almost undetectable, indicating the absence of HFIP in the film (the dissociation of hydrogen bonds). Furthermore, 2D ¹H‐¹⁹F heteronuclear Overhauser effect spectroscopy (HOESY) NMR was conducted, as presented in Figure 7h. The results revealed clear interactions between the hydroxyl protons of HFIP and the fluorine atoms within the polymer repeat unit, supporting the existence of hydrogen bonding. The porous morphology observed in the cationic polymers is favorable for ion penetration and doping, which is expected to enhance the C* of the film and consequently improve OECT performance.

FIGURE 7.

AFM height images of PBPz processed by (a) HFIP and (b) the strategy of MS. AFM height images of PBPzBF₃ processed by (c) HFIP and (d) the strategy of MS. SEM images of (e) PBPz and (f) PBPzBF₃‐based thin films. (g) FTIR spectra of PBPzBF₃ thin films processed with HFIP as the solvent before and after annealing. (h) ¹H‐¹⁹F HOESY spectra of the solvent HFIP and the cationic acceptor BPzBF₃. (i) Schematic illustration of thin‐film fabrication and formation for the cationic polymer PBPzBF₃ and its non‐cationic counterpart PBPz.

The performance of OECT devices is governed by two critical factors: (1) charge carrier transport efficiency and (2) ion doping capability, as evidenced by previous studies. These properties are quantitatively characterized by the corresponding physical parameters μ _OECT and C*. The usual strategy of OEG sidechain substitution can enhance C* by facilitating ion accessibility and increasing ion storage capacity. However, this enhancement in C* inherently compromises charge carrier transport efficiency (μ _OECT) due to increased scattering centers and reduced interchain connectivity. Minimizing this adverse effect while preserving the advantages is crucial for enhancing the performance of OECT devices. Through backbone cationizaiton via B ← N functionalization and using MS processing in this study, PBPzBF₃ achieves optimal device performance: (1) the combination of electron‐withdrawing B ← N‐based cationic moiety and sp²‐hybridized nitrogen atoms leads to the lowest LUMO energy level, enhancing electron injection and transport efficiency; (2) the highest backbone torsion barrier maximizes the effective conjugation length and improves microstructure ordering, thereby promoting efficient intra‐ and interchain charge transport; (3) HFIP‐DME MS processing induces the formation of a porous yet ordered microstructure in the resulting thin films via solvent‐mediated hydrogen bonding interaction. This architectural feature synergistically enables high capacitance values while retaining robust charge carrier mobility, resulting in both high μ _OECT and C*. Therefore, it positions PBPzBF₃ as a high‐performance n‐type OMIEC for advanced OECT applications.

2.7. In Vitro Human ECG

OECTs have garnered considerable attention for wearable electrophysiological sensors owing to their intrinsic high transconductance and low operating voltage. Each heartbeat generates electrical signals that can be acquired by electrodes and transduced into electrochemical signals. Upon placing OECT electrodes in contact with the skin, cardiac electrical signals are transduced into measurable changes in source‐drain current, thereby enabling the acquisition of ECG signals. Two n‐type OMIEC materials, PBPz and PBPzBF₃, were selected for the fabrication of OECT‐based ECG sensors in this study. As illustrated in Figure 8a, commercial electrodes were employed to collect cardiac electrical signals. The subject was fitted with a commercial electrode (connected to the OECT gate) proximal to the cardiac region and a grounded commercial electrode at a distal site. The acquired cardiac signals modulate the conductivity of the OECT semiconductor layer, and the temporal variation of the source‐drain current was recorded, as shown in Figure 8b. ECG sensors based on the two n‐type OMIEC materials were successfully fabricated, and the electrical signals generated by cardiac activity were reliably recorded with the expected waveform features. To systematically compare the sensitivity and SNR between the two OMIEC materials, the key signal parameters (I _on/I _off and I _{peak‑to‑peak}) were extracted and summarized in Figure 8c. The PBPzBF₃‐based ECG sensor exhibited a higher on/off ratio and larger I _{peak‑to‑peak}, indicating superior sensitivity and improved SNR, which yielded clearer ECG signals. OECT‐based ECG sensing technology holds substantial potential in terms of portability, sensitivity, and real‐time responsiveness, and is anticipated to serve as a critical tool for personal health monitoring and telemedicine applications in the future.

FIGURE 8.

(a) Schematic illustration of ECG recording using OECTs. (b) ECG signal tracings recorded from devices based on PBPz and PBPzBF₃. (c) Comparison of the on/off ratio and I _{peak‑to‑peak} of ECG signals obtained from OECTs fabricated with different OMIEC materials.

3. Conclusion

In summary, we have developed the B ← N functionalization strategy as a straightforward and adaptable platform for designing electron‐withdrawing cationic acceptor units (BPyBF₃ and BPzBF₃). Leveraging these units, we have synthesized two backbone cationic polymers (PBPyBF₃ and PBPzBF₃), which function as high‐performance n‐type OMIECs. PBPyBF₃ and PBPzBF₃ not only feature low‐lying LUMO energy levels, but also in situ form a porous microstructure via a simple spin‐coating process. This architectural trait synergistically achieves high capacitance while preserving robust charge carrier mobility, thereby yielding both high μ _OECT and C*. The lead material, PBPzBF₃, exhibits outstanding n‐type OECT performance, with a g _m,n of 38.9 S cm⁻¹ and a high µC* of 215.9 F cm⁻¹ V⁻¹ s⁻¹, outperforming its non‐cationic analog (PBPz) by a considerable margin. This superior performance arises from two synergistic key factors: (1) the strong electron deficiency (E _LUMO ≈ −4.0 eV) and elevated backbone torsional barrier imparted by the B ← N‐functionalized bipyrazine scaffold; and (2) the porous yet ordered thin‐film microstructure facilitated by solvent‐mediated hydrogen‐bonding interactions. Furthermore, we have successfully integrated PBPzBF₃ into ECG sensors, which delivers a high SNR. This work offers a transformative molecular design concept that utilizes B ← N coordination to build highly electron‐deficient cationic acceptor units, enabling high‐performance n‐type OMIECs and pushing forward the field of n‐type organic bioelectronics.

Author Contributions

Dongsheng Yan: data curation, validation, investigation, formal analysis, writing – original draft, writing – review and editing. Wei Song: data curation, investigation, writing – review and editing. Zhiwei Zhao: investigation, writing – review and editing. Yuqing Ding: investigation, writing – review and editing. Zhen Jiang: investigation, writing – review and editing. Qingxiang Qi: investigation, writing – review and editing. Ziyi Ge: supervision, resources, writing – review and editing. Yang Wang: conceptualization, methodology, investigation, validation, supervision, funding acquisition, visualization, project administration, resources, writing – original draft, writing – review and editing, formal analysis. Yunqi Liu: writing – review and editing, supervision.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie73464‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding authors on reasonable request.