A Guide for Cross‐Linking Modulation: The Record Rectifying Ratio of Hydrogel‐Based Ultra Flexible Ionic Diodes

This study focuses on the effect of cross‐linking mechanisms in the development of hydrogel‐based flexible ionic diodes (HBIDs) with desirable features for ionic devices. The research explores one p‐type hydrogel and a series of n‐type hydrogels with varying cross‐linking ratios, investigating their ion‐conducting properties, morphological and electronic structures, as well as the performance of flexible HBIDs constructed using these hydrogels. The results indicate that the cross‐linking ratio significantly influences ionic conductivity, bulk resistance, and capacitance. Additionally, the HBIDs' ability to maintain their electrical performance under repeated mechanical bending is studied concerning the cross‐linking ratio. Notably, the HBID with the highest cross‐linking ratio (HG/12) exhibits relatively higher current densities (up to 8.9 mA cm−2) and considerably higher rectification ratios (up to 1269). Even after 500 cycles of deformation, the HG/12 diode maintained a rectification ratio of 987, demonstrating its excellent performance and durability.


Introduction
[3] Commercial or laboratory-level devices consist of solid and dry components that use electrons as signal carriers.Conversely, there is predominantly ion transport in living tissues in biological systems, and this transport takes place in a liquid medium.For this reason, traditional electronic systems and devices cannot meet the needs of biological systems consisting of living tissues with a soft structure.As a result, the development of more flexible, softer, and wearable ionic devices [4][5][6][7][8][9] has drawn DOI: 10.1002/admi.202300678attention to meet these demands quickly.Because ionic devices, which work with ion transfers instead of electrons, enable features such as softness that are not easily accessible with electronic devices. [10,11]Such ionic devices have been used in pressure and strain sensors [4] and ionic touch panels [5,6] for wearable and bendable applications, as input units, to be integrated into human skin.In addition, a transparent ionic speaker, [7] flexible and wearable triboelectric nanogenerators, [8,9] and ionic power supplies [12] have been tested as output units.Soft gel-based materials, which include elastomers and hydrogels, for creating flexible structures, are utilized to create flexible structures.Among them, hydrogels are distinct as a class of polymer materials such can swell without being dissolved in the liquids.Dissimilar to other polymers, hydrogels with a 3D network structure can retain liquid up to 95% by volume. [13]Due to the many attractive properties of hydrogels, such as high optical transparency, excellent biocompatibility, and high liquid permeability, varied applications of hydrogels have been investigated in the last few years. [14]In addition to their flexibility and natural tissue-like properties, [15][16][17] and also the ion-selective property of the hydrogels is another remarkable feature desired by researchers in ionic devices.This specialty of hydrogels relies on fixed charges in poly-anionic or poly-cationic chains and mobile counterions to achieve charge neutrality.In the 3D crosslinked ionic liquid environment of hydrogels, the movement of the ions can be more efficient and the ion mobility provided is relatively high. [18]Because of all these properties, over the past years, hydrogel-based polymeric materials have been widely used for biosensing, [19,20] in the technology of tissue engineering [21] and medicine. [22]It has also received particular attention in the area of biomedical ionic electronic appliances. [23]Using this unique material, scientists have developed several functional ionic electronic devices, such as transistors [24] and ionic diodes. [17,25]Han et al. [26] In 2009, the first presentation of a prototype combined polyelectrolyte diode and circuit of logic gates was reported.More latterly, Lee et al. [17] suggested a distensible ionic diode using an adjusted poly-electrolyte hydrogel that exhibits corrective behavior under uniaxial strain and maintains its properties under repeated deformity.Lim et al. [27] advanced an HBID that has an open junction for ion-to-ion amplification.Ionic current rectification for this HBID is due to the anti-symmetric electrostatic affect of the hydrogel's charged backbone on the ionic liquid.So, they are exceedingly responsive to the charge density on the hydrogel surface, which is a crucial requisite for charge-based biosensors.Due to these properties, HBIDs are desirable research areas for electrical biosensing. [28]Although diffusion of the small molecules in polyelectrolyte hydrogels has been extensively examined for appliances such as drug delivery, [29] desalination of water, [30] and pressure sensing, [31] the ionic conduction of hydrogels and its role in electronic, there have been relatively few studies on integration in devices. [32,33]] IDs express artificial or biotic ion channels that exhibit the rectification of ionic current.In the meanwhile, according to the charge densities on the surface, which is modulated and reflects a tuned correction [28] representing the transport behavior, the adsorption of charged molecules can be determined.
In this field, ionoelastomers, polyelectrolytes, or HBIDs containing single mobile counterions were first demonstrated. [18,38]uch devices are structurally similar to the semiconductor diode structure based on p-n junctions. [40]The polymeric p-type hydrogel backbone demonstrates a strong negative charge and therefore repulses the co-ions.As a result, the p-type polymeric gel becomes selectively permeable for the transport of counterions that are positively charged.On the contrary, the n-type gel preferably transmits the charge carriers that are negative.When this ionic heterojunction is subjected to the external electric field, the ionic current is corrected relying on polarity and resulting in an anti-symmetric current-voltage (I-V) response. [41]When a forward bias is applied, both anions and cations are repelled toward the junction region. of the hydrogels.Thus, ions accumulate in this junction area.This reduces the resistance in all parts of the system.It should be noted that this forward biasing deposition reduces the local Debye length to allow for continuous ion current flow. [42]Whereas when a reverse bias is applied, anions and cations are withdrawn from the junction area, and the depletion of ions gives rise to an increased resistance within the system.As an outcome of the resulting anti-symmetry and a vary in composition to constitute the typical diode response, the ionic diode produces a current-voltage response such that it exhibits an "on state" against the applied positive potential and an "off state" against the negative potential. [42]ince hydrogels are up-and-coming materials for soft electronics, [28] using them to produce flexible and transparent ionic devices and further investigation is necessary.A common denominator for hydrogel synthesis is the utilization of cross-linkers.Changes in the electrical features of the hydrogels can be awaited due to the change in the cross-linking ratio.The use of hydrogels prepared using different amounts of crosslinkers in diode production should be analyzed in detail because a study on this subject has not been found in the literature within the author's knowledge.In addition, the ion transport properties of the inherently stretchable and porous hydrogels, which have an optimal structure in order to the improvement of flexible, transparent iontronic devices, and the electrical characterization of electronic devices produced with hydrogels have not yet been sufficiently studied experimentally. [41]his study that is focused on highlighting the effect of crosslinking mechanisms in the construction of flexible and transparent HBID is intended to guide the reader about the diode applications consisting of the anionic hydrogel coupled with the cationic hydrogels that have different amounts of cross-linking.In this article, the hydrogels have been classified as n-type and ptype.If the hydrogel has a polyanionic backbone that has mobile cations, it is defined as a p-type hydrogel and if a hydrogel has polycationic chains and mobile anions, it is defined as an n-type hydrogel. [18,27,43]n this article, an exhaustive study of the ion-conducting features, morphological and electronic structure of, one p-type hydrogel and a series of n-type hydrogel with five different crosslinking ratios, as well as the flexible HBID structures produced using these hydrogels is presented.Detailed electrical characterization is also presented.For this purpose, heterojunctions were designed using p-type and n-type hydrogels, and highly stretchable ionic diodes were produced and characterized to experimentally examine the ion transport properties and the produced flexible ionic diodes.In addition, the change in electrical performance of flexible ionic diodes due to mechanical bending is also investigated in terms of cross-linking ratio.The skill and the capacity of the diodes to maintain their stability against repeated mechanical bending were also investigated.

Preparation of the Hydrogels and Hydrogel-Based Flexible Ionic Diodes
Both types of hydrogels were synthesized in aqueous media using free radical polymerization techniques.The p-type (anionic) polymer-based swelling hydrogel was entitled p-HG and synthesized as mentioned before in detail. [44]To obtained the p-type hydrogel, Sodium 4-vinyl benzenesulfonate (VBS), which was the anionic monomer, the cross-linker N,N′ methylene bisacrylamide (MBAAm), and Acrylic acid (AA) that was partially neutralized with 70% NaOH solution, was purchased from Sigma-Aldrich.0.01 mol of VBS was used to set the aqueous solution in 18 MΩ deionized water at 1 m and the molar ratio of AA was retained at 0.5.MBAAm, and initiator, ammonium persulphate (APS) that was procurement from Merck was used in 1% mol.Combined ingredients in 15 mL volumes were placed into the falcon tubes and vortexed for 3 min.
During the synthesis of the n-type (cationic) hydrogels as in, [17] the 2-methacryloyloxyethyl trimethylammonium chloride (Q-DMC) was utilized as the monomer.The ratios of Q-DMC and not neutralized AA were retained at 0.5 and the cross-linker MBAAm ratios were increased from 0.4 to 1.2 by 0.2 for each of the hydrogels.The synthesized hydrogels were entitled n-HG/4, n-HG/6, n-HG/8, n-HG/10, and n-HG/12 about the cross-linking ratio.Weighed out the indicated amounts of monomer and crosslinker, dissolved in deionized water, and added to Falcon tubes.
Both polymerization reactions of p-type and n-type hydrogels were realized for 16 h at 55 °C.Then the hydrogels were removed from the Falcon tubes and sliced into as many as possible small pieces for effectively washing with deionized water to eliminate unreacted monomer residues.Next, they were oven-dried at 55 °C until they reached a certain weight and milled with a ceramic grinder.FTIR analysis was performed by Thermo Scientific Nicolet IS 5 FT-IR Spectrometer to determine the structural modification of hydrogels depending on the cross-linking ratio.
Used throughout the HBID production, 175 μm thick polyethylene terephthalate coated with indium tin oxide (PET-ITO), transparent and flexible substrates that had 18 Ω sq −1 surface resistivity, were obtained from Solaronix. 1 mg of p-type hydrogel powder was placed on the PET-ITO substrate homogeneously and pressed under a pressure of ≈3 tons.The same procedure was operated on another PET-ITO substrate with 1 mg of n-type hydrogel powder.Subsequently, the layer of ionic gels that were covered on the two PET-ITO electrodes was moistened with 0.02 mL of the ionic liquid that contains iodide/triiodide redox couple.Then the electrodes were put on top of each other and were sandwiched by using a 60 μm thick spacer.The hydrogel-based heterojunction that had a 2 × 2 mm 2 active area was pressed again under a pressure of ≈6 tons.The ionic liquid that was used to moisten the hydrogel layers was occurred by using 0.7 m 4-tert-butylpyridine, 0.08 m I 2 , 0.7 m N Butyl-Methylimidazoliumiodide and 0.02 m LiI in 50:50, v/v valeronitrile and acetonitrile.All chemicals that were utilized in ionic liquid were retained from Sigma-Aldrich.
This procedure was repeated by using five different n-type hydrogels.At every turn, the p-type p-HG hydrogel was combined with another n-type hydrogel that had a different amount of crosslinking.The devices were kept for 12 h in dark, room conditions and the combined hydrogel-based heterojunctions swelled with the ionic liquid.Hereby the flexible HBID structure was completed, and the schematic configuration of the HBID structure is represented in Figure 1.
All of the hydrogel layers and the cross-sectional scanning electron microscope (SEM) images of the heterojunction were obtained by FEI Inspect SEM.Samples were prepared by swelling and the fully swollen porous hydrogels were then freeze-dried, mounted onto metal stubs, and coated with 3 nm gold for SEM observation.Before imaging, the imaged portion of the horizontally cut samples was dried and then coated with 5 nm gold.

Electrical Characterization
Absorption coefficient calculation was used in addition to conventional electrochemical impedance spectroscopy (EIS) to examine the optical, electrical/dielectric properties, ionic conductivity, and capacitive behavior of hydrogels with different ratios of cross-linking.The hydrogels' absorption spectra were acquired by using the Hach Lange DR 5000 UV-vis Spectrometer.Structural features that significantly affect the conductivity mechanism were also characterized by SEM analysis for all hydrogels.
200 mg of powdered solid hydrogels were pelletized under a pressure of 6 tons and they were in a situation of 12.7 mm diameter, ≈1.1 mm thick was acquired and 5 mL of liquid electrolyte was appended to each one.Then these samples were used for the EIS and dielectric measurements.The prepared hydrogel pellets were stored in ionic liquid for 3 days to ensure adequate liquid uptake.At the end of this period, the unabsorbed ionic liquid was filtered out.Electrical and dielectric experiments, which procure the most elaborated knowledge on conduction mechanisms, were performed to describe samples, using the parallel plate capacitor technique, at room temperature by a 16451-B text fixture attached Agilent 4284A LCR meter.For all samples, the frequency-dependent measurements of the dielectric properties were performed according to capacitance and dissipation factor values, and the parameters that were associated with conductivity and dielectric were computed by derived from these values.From 20 Hz to 1 MHz frequency range was adjusted and a 10 mV AC oscillator signal was implemented during the experiment.
The applied potential versus current (I-V) characterization of the HBID in the range of ±1.5 V was performed under dark conditions using a Keithley 2612 computer-controlled source meter and rectification ratios were calculated.The I-V characteristics of every HBID were obtained by swept bias voltage against −1.5 to +1.5 V at a constant ramp rate of 1 mV s −1 .
The most important feature of flexible HBIDs was that both positive and negative ions was movable.These ions diffuse independently across the interface outwardly being withdrawn or repulsed by the polymer or surface that was charged.In ionic diodes due to different structural properties, high or inferior diffusion/transfer proportions of ions were used to generate unidirectional ion flux at every diode terminal by performing ion rectification.Contrary to ordinary ion correction devices with settled charges, flexible HBIDs acquiesce free ion transportation of negative and positive ions, as in biological systems. [45]Since the dissemination of non-reactive counterions in the hydrogel determines the actual rectification behavior, [13] if the implemented external electric field changes too quickly, the ions could not had enough time to reposition.Hence, a steady −1.5 V reverse bias voltage was enforced on the diode for 10 min previously the start of several scan cycles to guarantee a steady-state distribution of counterions in each cycle.
On the other hand, to comprehend charge and ion transport mechanisms inside devices in diode applications, a detailed EIS analysis was performed.The Nyquist plots of the flexible HBIDs were obtained in a Faraday cage, at room temperature, and in the dark by using VersaSTAT 3 Potentiostat/Galvanostat controlled by VersaStudio 2.60.6 computer program.A kept constant at 10 mV AC oscillator signal was applied to the samples at the high, medium, and low-frequency regions.The range of frequency was set between 0.01 Hz and 1 MHz.
To evaluate the electrochemical performance and the capacitive effects of the hydrogels; two electrode cyclic voltammetry (CV) technique by using VersaSTAT 3 Potentiostat/Galvanostat, was examined for p-type hydrogel from −0.7 to +1 V, 10-100 mV s −1 scanning speeds, and was examined for ntype hydrogels in the same voltage range at scanning rates of 10-500 mV s −1 , and the results were obtained.
The switching times of all flexible HBIDs were also measured using the AA TECH AWG-1020 function/arbitrary Waveform Generator and the computer-controlled digital AGILENT MSO9064A Mixed Signal Oscilloscope.The switching times of the diodes were obtained by applying a square wave alternating current signal with an amplitude of ± 1.5 V and a frequency of 100 mHz.
Additionally, after being subjected to angular bending, the electrical measurements of the hydrogel ionic diodes were repeated.The rectification ratio was recalculated for each diode from the I-V characteristics obtained at the end of the cycle repeated 500 cycles by bending 0°, 30°, 60°and again to 0°.

Results and Discussion
This study that is focused on highlighting the effect of crosslinking mechanisms in the construction of HBIDs is intended to guide the reader about the diode applications consisting of the ptype hydrogel coupled with the n-type hydrogels that have different amounts of cross-linking.Hydrogel structures were prepared in a creamy semi-solid form that could make satisfactory contact with each other using ionic liquid to serve electron transfer, and hydrogels were joined together to form micrometer-sized heterojunctions.The relationship between the rectification behavior and the response to bending of the flexible HBIDs, in terms of the cross-linking ratios of hydrogels has not been experimentally or numerically studied yet.The article also examines the electrical performance variation of the ionic diode due to mechanical bending, thanks to its high flexibility.
Two different types of hydrogels with acceptable mechanical properties and high ion selectivity were used in the fabrication and experimental investigation of flexible HBIDs.The n-type hydrogels were modified in five different configurations to have different cross-linking ratios.The highly flexible ionic diodes were fabricated with one p-type, and n-type hydrogels that have modified cross-linking ratios.The rectifications of the diodes were studied as the function of cross-linking ratios.
The hydrogels prepared in this manuscript were synthesized by copolymerizing monomer molecules containing either anionic or cationic groups along with acrylic acid.Copolymerizing monomer molecules containing anionic or cationic groups with acrylic acid instead of using single polymeric materials directly when synthesizing hydrogels has several advantages.Copolymer hydrogels have excellent mechanical properties, such as high tensile strength and flexibility, which are superior to those of pure acrylic acid hydrogels.Thereby they can be used to create flexible and stretchable devices that can withstand mechanical deformation. [46,47]On the other hand, copolymer hydrogels have different electrical and optical properties depending on their charge type and cross-linking ratio, which can be exploited for different functions.According to the literature, [40] the rectification ratio determines the performance of ionic diodes, and it can be modulated by the charge density and mobility of the ions in the hydrogel, which are influenced by the copolymer composition and cross-linking ratio.Additionally, copolymer hydrogels can achieve a higher rectification ratio than single-polymer hydrogels.By using copolymers of acrylic acid and either sodium acrylate or acrylamide, researchers were able to tune the rectification ratio by changing the cross-linking ratio. [45]o modify the gels, NaOH-treated AA that has functional carboxyl groups was adjoined as a condenser.Acrylic acid, also known as vinyl formic acid is an elementary carboxylic acid that is unsaturated and consisted of vinyl groups that are directly attached to carboxylic ends.Q-DMC, with the steady positive charges because of the ammonium groups that are existed in the polymeric backbones, which were used as the monomer together with movable Cl − ions for the n-type hydrogels.These mobile chloride ions in hydrogel structures exhibit a behavior similar to the behavior of an electron in an n-type semiconductor.In the p-type hydrogel, on the other hand, VBS, which supplies stable negative charges because of the sulfonate groups in the polymer chains, together with movable sodium ions, was chosen as the monomer.These mobile Na + ions in the structure of the hydrogel have a similar behavior to a hole in a p-type semiconductor. [48]tructural features that significantly affect the conductivity mechanism of the hydrogels, which are strongly dependent on the cross-linking ratio, were characterized by FT-IR and SEM measurements.FTIR spectra of the p-type and n-type hydrogels obtained for further clarification of the structures and determination of the cross-linking ratio's effect on the structural modification of hydrogels are shown in Figure 2.For the FTIR spectrum of the p-type hydrogel that given in Figure 2a, the characteristic bands observed at 620, 750, 950, 1040, and 1160 cm −1 due to the sulfonate groups' symmetrical and asymmetrical vibrations, belong to the VBS-AA. [43,44]In the spectra, the band that is observed at 1675 cm −1 is the asymmetric vibrational band of C═O in the group of the COO and the peak observed at 1745 cm −1 is the band belonging to the vibration of the acid form C═O band.The peak emerging at 3045 cm −1 was ascribed to the asymmetric C═C─H stretches that are exhibited to the aromatic groups of VBS-AA.The broadband between 3000 and 3700 cm −1 was followed for the O─H group of the acrylic acid. [44]he data in the FTIR spectra of the n-type hydrogels that were given in Figure 2b, shows the availability of severe broadband between 3760 and 3390 cm −1 emerging from stretches of the O─H groups of AA.The bands at 2642 cm −1 and at 1720 cm −1 are the C─H tension band in the methylene groups, and the asymmetric vibrational band of C═O in the COO group respectively.[51] Of note is the potency of the signal of % transmittance, that is originated from hydroxyl stretching and the other functional groups in n-HG/12 hydrogel; and as compared to the others, n-HG/12 hydrogel may specify higher concentration, polarity or availability of the functional groups in Q-DMC.And this situation may indicate the higher polarity in n-HG/12, as compared to the others. [52]On the other hand, when focusing on n-type hydrogels, an overall increase in peaks is observed with an increasing cross-linking ratio, indicating a successful crosslinking.
Hydrogels are porous and flexible, which are sought after for the improvement of iontronic devices that are flexible.In terms of examining the morphological structure of hydrogels, the porosity, and tortuosity of different ionic species were investigated and were correlated with ionic conductivity.The attained outcomes will procure elementary design guidelines for modulation the structure and ionic conductivity of hydrogels for bioelectronic device applications.Hydrogels with different types and different ratios of cross-linking were characterized by SEM images for morphological evaluation.As shown in Figure 3a-e, the porous morphology was clearly observed in the hydrogels.
While low cross-linked hydrogels show a relatively smoother surface, the increase in porosity with the enhancement of the cross-linking ratio in n-type hydrogels with different crosslinking ratios is also noteworthy.Especially in the n-HG/12 hydrogel, the porous structure is much more obvious than the others.This porous morphology in the hydrogel raises electrolyte fluid retention by the osmotic pressure while favoring the movement of ions.Also since the increased porosity also allows ions to be transported faster and more ions can move freely within the gel was leads to greater ionic conductivity. [53]Because the electrical conductivity in polymer structures is essentially achieved through the motion of charge carriers throughout the chains.When they can hold a highly ionic liquid, the probability of charge carrier motion among chains will also increase the electrical conductivity. [17]f studied at the molecular level, in hydrogels, the polar solvent will either bind to hydrophilic groups that are polar or fill in between the network chains or the interstitial spaces in the porous structure.In the polymer chain, the hydrophilic groups immobilize the molecules of the polar solvent through van der Waals affinity and bonding of hydrogen. [54]However, additional crosslinking not only increases porosity but also creates other avenues for available charge transport.Therefore, as the cross-linking increases, the electrical conductivity will also rise. [17]To prove this situation, frequency-dependent electrochemical impedance and dielectric mechanisms affecting the conductivity characterization of all samples were studied in detail in low, medium, and high regions.
The bulk resistance (R b ) of the hydrogel can be acquired by using the Nyquist plot that is given in Figure 4a, and used in Equation ( 1) for the calculation of the conductivities () of the hydrogels, in which, R b is the bulk resistance as Ω, A is the area as m 2 and t is the thickness of sample as m. [55]The point where the Nyquist curve intersects with the x-axis gives the bulk resistance value of the hydrogel, accordingly, it was observed that the bulk resistance value decreased with the increase of the cross-linking ratio The calculation results are represented in Figure 4b for all hydrogels.However, it has been proved by calculating the R b bulk resistances [56] and conductivity from the Nyquist plots of the hydrogels given in Figure 4a that the conductivity of the hydrogels changes in direct proportion with the cross-linking ratio and is given in Figure 4b.
EIS is a preferred technique for certain the mobility and charge density of hydrogels.For this purpose, EIS analyzes the electri-cal attitude of the hydrogels that are enclosed among two electrodes by implementing an alternating electrical signal.During that electrical signal is enforced to the sample, negative and positive ions periodically gather at the corresponding electrodes, constituting a dual layer.Impedance spectroscopy can be used to study the electrical and capacitive behavior of hydrogels.As it is known, increasing the charge densities of hydrogels and maximizing their ion-selective properties is one of the most important points.The preferred amount of monomer during hydrogel production is effective in increasing the counterion concentration and swelling of the hydrogels with the effect of high osmotic pressure.The increase in the amount of cross-linking during the synthesis of hydrogels with high concentrations can increase the degree of swelling [8] ; while increasing the porosity and therefore the fragility of the hydrogel.Although this may seem like a disadvantage for various studies, it turns into an advantage for this study as it increases the contact surfaces of hydrogels because of ionic liquid with each other and with the electrodes while forming p-n junctions.
When the Nyquist plots in Figure 4a and Table 1 are examined, the resulting Nyquist plots are semicircular.The point where the Nyquist curve intersects with the x-axis gives the resistance value of the hydrogel, while the diameter of the Nyquist curve semicircle corresponds to the resistance of the charge transfer (R CT ) in the interface between the hydrogel and electrode with the capacitive behavior (C PE ). [57]he semicircles obtained from n-type hydrogels appear to belong from smallest to largest, n-HG/12, n-HG/10, n-HG/8, n-HG/6, and n-HG/4 samples respectively.Not only does the frequency-dependent development of hydrogels' behaviors, close to each other, but also they are compatible within themselves in terms of cross-linking ratio.The n-HG/12 hydrogel was determined that it has the smallest semicircle among the n-type hydrogels after p-HG due to the lowest absorption coefficient.The ionic conductivity of all hydrogels was measured and it was found that the ionic conductivity increased from 0.210 to 0.524 S m −1 by increasing the cross-linking ratio from 0.4 to 1.2.The result was established to be important in that it confirms the previous practical experiment data and shows that the highest ionic conductivity of 0.524 S m −1 belongs to n-HG/12 coded hydrogel due to its high mobility in the hydrogel channels. [58]As a result, it was reported that the highest ionic conductivity belonged to the n-HG/12 coded hydrogel and the ionic conductivity tends to increase in hydrogels due to increasing ionized groups and charge carrier density in direct proportion to the cross-linking ratio.While the highest value of the absorption coefficient was recorded for the n-HG/12 coded hydrogel, the lowest value was recorded for the n-HG/4 coded hydrogel, which supports the resistance and conductivity values obtained for all samples.This was found to be important in terms of showing that the lowest resistance value of 40.6 Ω and accordingly the n-HG/12 sample has the greatest electrical conductivity, supporting the results of both SEM analysis and bulk resistance calculations.On the other hand, charge transfer resistance R CT also shows the lowest value of 6.4 Ω cm −2 for n-HG/12 and the highest value of 10.1 Ω cm −2 for n-HG/4.The C PE values of n-HG/12 hydrogel is 2.72 μF also much higher than 0.28 μF of n-HG/4, suggest that n-HG/12 hydrogel offers a much larger capacitance among the other n-type hydrogels.The effectiveness of these values has been reinforced by the data obtained using the Cyclic Voltammetry (CV) technique.
The UV spectra of the hydrogels were obtained at room temperature and between 200 and 800 nm wavelength.The peak values of the samples were observed between 450 and 600 nm as given in Figure 4c.The absorbance values of the hydrogels with various cross-linking ratios increased exponentially with the increase of wavelength was determined, among which the n-HG/4 hydrogel has the lowest and the n-HG/12 hydrogel has the highest absorbance values.For all hydrogels, the high values of the absorbance in the stated region are thought that can be attributed to the innervation of dipolar oscillations.On the other hand, it is guessed that the reason why the absorbance values of the hydrogels show a fluctuating behavior throughout the entire wavelength depending on the cross-linking ratio is the molecular-level interactions between the ionic electrolyte and hydrogel bonds. [59]t has been determined that the largest and smallest values of the absorption coefficient are inversely proportional to the resistance and impedance values for all samples as given in Figure 4a,b.][61][62][63] It is clear that the different absorption values observed in hydrogels are related to their different particle sizes and shapes, liquid holding capacities, and dielectric environments due to the different cross-linking ratios. [60,62]For this reason, using the values of absorbance that were attained by UV spectrum results of all samples, the energy bandgaps were computed by using the slope among the (h) 2 versus h graphs with the Tauc law. [64]As shown in Figure 4d, while the lowest energy bandgap among to the hydrogels belonged to the n-HG/12 with 2.67 eV, and the highest one belonged to the n-HG/4 with 2.91 eV was determined.n-HG/12 encoded hydrogel is thought to have the lowest energy band because of the effects such as the quantum confinement and the Fermi-Dirac distribution.It was also determined that the energy bandgap of the hydrogels tends to reduce with the increase of the cross-linking ratio.It has been determined that the oscillation time changes regularly due to the increased interface interactions due to the rising in the crosslinking ratio, thus the energy bandgap changes in inverse proportion to the cross-linking ratio.The n-HG/4 coded hydrogel's energy bandgap value is higher compared to other hydrogels, so it can be qualified as an inspiring material for use in electronic devices that can be operated at high temperatures and withstand high voltage variations. [58]The increase in the cross-linking ratio accelerates the load-carrying dynamics on their surfaces and decreases the required energy level.Thus, the charge carriers in the hydrogel can easily acquire enough energy to overcome the energy bandgap and jump into the empty spaces provided by the hydrogel. [65]As a result of this analysis, it has been proven once again that the conductivity of the n-HG/12 coded hydrogel will be better than the others since the band energy is less.In addition, as given in Table 1, it has been proven that the p-type hydrogel's bandgap value is 1.67 eV and shows p-type conductivity at low temperatures. [66]The capacitance values and the behavior of the charge carriers of hydrogels can be determined by dielectric measurement studies.The graphs of the dielectric loss and the dielectric permittivity according to the frequency of hydrogels were analyzed by the EIS technique and the results are given in Figure 5a,b, respectively.Additionally, the frequency-dependent variation of the dielectric/electrical features such as tangent factor, permittivity, conductivity, surface resistivity, and capacitance of the hydrogels that have different cross-linking ratios, was analyzed using impedance spectroscopy, and the capacitance values were calculated by using Equation (2).
The capacitance values and the behavior of the charge carriers of hydrogels can be determined by dielectric measurement studies.The graphs of the dielectric loss and the dielectric permittivity according to the frequency of hydrogels were analyzed by the EIS technique and the results are given in Figure 5a,b, respectively.In the low-frequency region, due to the interfacial polarization caused by frequency, and effects such as space charge, dipolar, ionic, atomic, and electronic polarization, ion movement in the direction of the applied electric field is prevented and a carrier charge accumulation occurs. [59]As a result, dielectric permittivity and dielectric loss variables took high values in the low-frequency region due to the interface polarization.The movable charge carriers partially induce negative charges in the polymer chains of the hydrogel, resulting with the reorientating of the dipoles.For all hydrogels, the polarization mechanisms lose their effectiveness with increasing frequency.While the values of the dielectric permittivity of all samples decrease linearly, then decrease calmly respectively with the increment of frequency, in the low and highfrequency regions.The dielectric permittivity and dielectric loss values of all hydrogels are relatively high in low-frequency regions.These values gradually decrease and reach a steady state at high-frequency values.Because in the high-frequency range, the free ions in the hydrogels do not have enough time to orient toward the implemented external electric field.Calmly decreasing and the low values of dielectric permittivity and dielectric loss in the high-frequency region of all hydrogels, can be explained by related to the unified effects of; dipole relaxation, conducting of the cations, and Maxwell dynamics approach. [58]These parameters reduce with an increment of the ionic conductivity in the system of Maxwellian at high frequency.In light of the data obtained, it has been observed that the dielectric permittivity and dielectric loss values of the n-HG/12 have the lowest values throughout the entire frequency region and the mobile charge carrier behaviors in hydrogels are not dependent.
This situation also supports the obtained conductivity results and shows that the values of the ionic conductivity increase as the cross-linking ratio increases in accordance with the implemented bias voltage.In addition, when conductive materials with different work functions come into contact to form a junction, bound up with the work function of conductors an energy barrier occurs. [67]The possible energy barrier describes the charge movement across the contact.The variation of the cross-linking ratio and thus the polar architecture in the hydrogel also causes a variation in the work function. [68]The measurement of the dielectric constant also indicated a variation in polar architecture and charge accumulation observed that occurred at the interfaces due to the changing energy barrier height. [17]ince both a high dielectric dissipation and a low leakage current are requested in organic field effect transistors (OFETs) that work on low-voltage, the polarization and mobility at the electrode-hydrogel interface is the reason for the preference for low-voltage OFETs for use in biological systems.[71] On the other hand, it can be said that it has the potential to be used in biological systems for the applications of cell membranes with its high dielectric constant values in the low-frequency region. [72]o evaluate the electrochemical performance and thus the capacitive effect, the results of the CV technique were obtained and for p-type hydrogel was given in Figure 6a, and for n-type hydrogel structures were given in Figure 6b-f.Asymmetry in the distribution of charge carriers leads to asymmetric I-V properties. [48] shown in Figure 6, n-HG/12 exhibits larger CV curves than all other n-type hydrogels, indicating that n-HG/12 can store higher energy than all other n-type hydrogels. [73]The shape of the CV curves, that varing from a rectangular shaped to a leaflike appearance, is related to the restriction of the interaction between the ions in the hydrogel and the electrode surface. [74]Compared to the CV curves of all other hydrogels, the CV shapes of p-HG and n-HG/12 show a more semi-rectangular shape, which shows the electrical bilayer charge storage behavior characteristic (Figure 6a,f). [75]Also, the shape of the CV curve was preserved at almost all scan rates as the scan rate increased from 10 to 500 mV s −1 .The obtained results agree well with the obtained results of the capacitance values obtained by electrochemical impedance spectroscopy and dielectric measurements and indicate the best ion migration performance of n-HG/12.The results are also convincing that n-HG/12 hydrogel has a much better electrochemical performance than all other hydrogels.
When two p-type and n-type hydrogels are brought together to form a junction, it has been noted that the the attitude of contact among the two types of hydrogel is similar to a p-n junction of the semiconductors, based on the spreading action of the mobile Na + and Cl − ions in the structure of the hydrogels. [48]This heterojunction exhibits the behavior of an ionic diode, and when an external electric field is applied to this ionic heterojunction, it corrects the ionic current depending on the polarity, causing an asymmetric current-voltage (I-V) response.When the applied electric field disappears, the entrainment flow of the moving ions returns to its original equilibrium state, and the established potential along the electric field and the depletion region also returns to their initial levels, leading to a gradual decrease in the electron flow in the external circuit. [48]The obtained I-V curve varies depending on the cross-linking ratio of the n-type hydrogel used in the het-erojunction.In Figure 7 forward and reverse biases are defined as positive (in left) and negative (in right) voltages applied to flexible ionic diodes, respectively.
Due to a forward bias applied to the diode (left part in Figure 7) and the repulsive force provided by the internal electric field (culombic interactions), the anions (Cl − ) in the hydrogel medium to the p-type hydrogel, and the cations (Na + ) to the n-type hydrogel, ie the junction migrates to the region.As a result, the diodes have a very low current (<0.03 mA cm −1 ), and the threshold voltage for the HG/12 diode with the best performance was measured below 0.227 V (Figure 9a).Redox reactions begin at the heterojunction when the Threshold voltage is high enough to cross the barriers for electrode reactions.In this case, the measured ion current increased significantly as the forward bias increased, turning the diode "ON".In particular, it should be noted that the theoretical threshold voltage for the electrode reaction in our research is consistent with the presented experimental results.Conversely, when a reverse bias is applied (right in Figure 7), anions (Cl − ) in the hydrogel medium move towards the n-type hydrogel, and cations (Na + ) move towards the p-type hydrogel; however, migration of anions/cations is inhibited due to the presence of high potential on the electrode surface, resulting in the failed formation of the circuit and hence the "OFF" state of the diode.
As shown in the cross-sectional SEM image in Figure 8a, p and n-type hydrogels create a heterojunction interface.Moreover, as demonstrated in Figure 8b, the HBID has great flexibility, which can be suitable for application to wearable and flexible devices thanks to the intrinsically flexible components used in its manufacture, as well as its transparency that is quite clear in Figure 8c.
In the absence of an asymmetrical heterojunction, hydrogels have high conductivity, which is found to increase with the crosslinking ratio.However, when two different types of hydrogels are contacted to form an asymmetrical heterojunction, significant rectification is observed. [45]The distribution of ions in the hydrogel has a strong effect on rectification.Within the scope of the study, the factors thought to affect the rectification behavior of ionic diodes were investigated and evaluated for the first time in the literature in terms of the dependence of flexible HBID behavior on the cross-linking ratios of hydrogels.
The I-V curves of the flexible HBIDs depending on the crosslinking ratio of the n-type hydrogel are shown in Figure 9a.It clearly shows that the interfacial current between p-type and ntype hydrogels flows in only one direction.
As shown in the inset of Figure 9a, a very low current of 0.03 mA −1 in the HG/12 flexible HBID was measured with a bias voltage below 0.227 V.When the bias voltage is high enough to exceed this value, the measured ion current turns the diode "ON" and increases significantly as the forward bias increases.It is noteworthy that the threshold voltage of flexible ionic diodes changes depending on the cross-linking ratio of the n-type hydrogel, and the threshold voltage decreases while the cross-linking ratio increases.The forward current increases in proportion to the conductivity of the electrolyte, while the reverse current is not affected as much as the forward current.The reverse current is limited by the electrode structure rather than the conductivity of the hydrogel medium.When the factors affecting this performance change observed in flexible HBIDs were investigated, it was determined that the threshold voltage of the diode changed due to the superior ionic conductivity of the hydrogels, which was determined to vary depending on the cross-linking ratio, and even the superior ionic conductivity of the hydrogel supported the rectification ability of the diode.This phenomenon is given in Figure 9b.Due to the dominant dependence of the forward current on the hydrogel conductivity, the rectification ratio of the device also increases with the increase in cross-linking. [76]Rectification ratios (ratio of current at +1.5 V to current at −1.5 V) and current densities of flexible HBIDs were observed.By applying a constant negative bias of −1.5 V to the flexible ionic diodes for 5 min just before each measurement, the migration of ions in the hydrogel layers was directed towards the nearest electrode so that each measurement starts from the same state of ion distribution.While the p-type hydrogel was fixed in all ionic diodes, the forward current of the diode increased due to the increased conductivity of the n-type hydrogel due to the cross-linking ratio.
As a result, we observed relatively higher current densities up to 8.9 mA cm −2 and considerably higher rectification ratios up to 1269 for HG/12 and 769 for HG/10 diodes (Figure 9b).The rectification ratio achieved in this study is unusually high, with values of up to 1269.High rectification ratios are ideal for ionic diodes as they express efficient control of ions.This control is necessary to realize the efficient distribution of ions in integrated flexible electronic circuits. [75]On the other hand, the observed current densities are normally within the range observed for ionic diodes, or in some cases better than that.The rectification ratio is the main factor because of their expression of the efficient control of ions and they are wanted to be relatively high for ionic diodes.This effective control is necessary to achieve the efficient distribution of ions in integrated flexible electronic circuits. [77]As stated in previous studies, by increasing the ion concentration, the nonreactive counterions in the system increase, and thus larger potential drops occur at the electrodes.This results in higher current densities in the forward bias, while no significant current change occurs in the reverse bias.This has a very important effect on rectification and the higher rectification ratio is in good agreement with the conductivity results, proving that the increase in cross-linking rate yields more charged species. [77]owever, it should be noted that a higher surface area provides higher charge densities, which supports the evaluations determined from SEM images (Figure 3) when examining the morphological features of the hydrogels.All experimental findings support and agree with each other.][80][81] It is claimed that these data obtained from the literature are the values of the maximum correction ratio and it should not be forgotten that they are obtained at different voltage values.
The switching times of the flexible were obtained by applying a square wave alternating current signal with an amplitude of ± 1.5 V-100 mHz and the current responses were recorded in real time.As shown in Figure 10, for both reverse and forward bias values, during the cycles, the current decreased rapidly to a constant value from the inceptive relatively high values depending on time.This is proof that ion mobility is supported, and as expected, the increase in response time with decreasing cross-linking ratio reinforces the other experimental data obtained.The resulting responses of "ON" and "OFF" times were ≈7 and 150 ms respectively for HG/12 coded diode.As shown in Figure 10a, over many cycles, the current density and the rectification ratio values of the flexible ionic diode were reasonably stable.
The HBIDs produced in the present study are highly flexible and transparent.To determine the suitability of HBIDs for flexible electronic applications, also their performance under various bending states was validated.The electrical measurements of the flexible HBIDs were repeated when bending the devices.
The rectification ratios that were given in Figure 11a were recalculated for each diode from the I-V characteristics that were obtained at the end of the cycle repeated 500 cycles by bending 0°, 30°, 60°, and again to 0°, as representatively shown in Figure 11b,c.Additionally, for the bending tests of the flexible devices, the bending radiuses (r) for each bending angle, which is another critical parameter, were given in Figure 11c.It is clear that the bending radius of 0.62 cm at 60°bending is noticeably reduced compared to 1.36 cm at 30°bending.Flexible HBIDs exhibited continuous correction over hundreds of bending cycles, proving that the devices have very high performance even after deformation and they are highly suitable for biosensing platforms.
The bending angle of 60°is an unusually advanced bending angle in the literature, and it is very important data that stability can be maintained at this angle value.In addition, the high transparency of the diodes is maintained during angular bending.As shown in Figure 11 and Table 2, a slight change in the rectification ability of the ionic diode is observed when the bending angle changes from 0°to 60°and then returns to 0°.When evaluated in terms of rectification ratios, the fact that the HG/12 ionic diode, which has the highest performance in the absence of bending, performs the best at the end of 500 cycles, among others, is very important and remarkable in terms of the efficiency of the diode.
When the rectification ratios are examined, both the rectification ratio and current/current density values of flexible HBIDs are comparable to or even higher than the values reported in previous studies. [13,79,82]In addition, the improvement of the rectification performance of the diodes, which are returned to their original state (0 again) after angular bending, is remarkable and is an expression of stability in terms of performance, which can be considered quite good.The rectification ratio, which was 1269 at the beginning, remained at 987, which is considered an acceptable value in the literature, at the end of 500 cycles, which indicates that the HG/12 diode has a very high performance even after deformation.Likewise, the performance of other devices is remarkable in terms of maintaining their stability after deformation.The hydrogels used in the study ensured stability during angular bending due to their effective fluid release and reentrapment capability.In the light of this data, which was determined as a result of the morphological analysis, the fact that it has the most porous structure has brought the HG/12 ionic diode, in which the n-HG/12 hydrogel is used, to the fore in this respect.This shows that diodes are very stable in bending states and ionic diodes are promising in future flexible electronics applications.The main reason for the deterioration of diode performance during angular bending is the elongation of the free path due to an applied external mechanical input and shape bending, the local separation of the contact surfaces disrupting the original equilibrium state and limiting the movement of ions due to an enlarged space charge region. [48]

Conclusion
In this study that focused on the crucial role of cross-linking ratios of the hydrogels, in the specification of hydrogel-based ionic diodes (HBIDs), firstly the n-type and p-type hydrogels' morphological, optical, dielectric, and electrical properties have been explored.By the formation of p-n heterojunctions by using these hydrogels that have been electrically characterized, it was proved that the interfacial current between p-type and n-type hydrogels flows in only one direction and creates ionic diodes with exceptional rectification performance that has been demonstrated.Therewithal their remarkable potential in advanced electronic and sensing applications has been shedded light.
It's evident that the cross-linking ratio strongly influences hydrogel properties especially the n-HG/12 hydrogel that stands out with its high conductivity and absorption coefficient, with increased ratios leading to higher porosity, and improved ionic conductivity, besides high rectification ratios, and stable response times.Moreover, these flexible and transparent HBIDs by maintaining performance even after repeated bending have showcased their potential for integration into various systems, particularly in bio-interfaced applications.
As a result, it has been proven by trying many different test methods that the current-voltage characteristics of the flexible and transparent ionic diodes to be produced by forming hydrogelbased p-n heterojunctions can be adjusted by modifying the cross-linking ratios of the hydrogels to be used.The adaptability abilities of HBIDs to bending conditions, dependent on the crosslinking ratio underscores their potential for next-generation applications in bio-interfaced systems.The collective body of experimental evidence has supported and agreed with each other these findings, paving the way for innovative and multifaceted appli-cations of hydrogel-based ionic diodes in the realm of advanced electronics and beyond.

Figure 1 .
Figure 1.Schematically representation of HBID (side view in left, top view in right).

Figure 2 .
Figure 2. a) FTIR spectra of the p-type hydrogel and b) the n-type hydrogels with different cross-linking ratios.

Figure 4 .
Figure 4. a) Nyquist plots with the equivalent circuit, b) graphs of the electrical conductivity and the R b bulk resistances, c) spectra of the UV-vis absorption, and d) (h) 2 versus h graphs of the hydrogels.

Figure 5 .
Figure 5.The real part graph of a) the dielectric permittivity and b) dielectric loss variation versus frequency plots of the n-type hydrogels.

Figure 7 .
Figure 7.A close-up impression of the hydrogel p-n junction reveals the anionic character of p-HG and the cationic character of n-HG and the working mechanism of the device under a) forward bias and b) reverse bias (reds represent positively charged ions, and blues represent negatively charged ions).Asymmetry in the distribution of charge carriers leads to asymmetric I-V properties.[48]

Figure 8 .
Figure 8. a) Cross-section of the SEM image of the p-n junction.b) Photographs that show the transparency and the flexibility (c) of the HBID.

Figure 9 .
Figure 9. a) Graphs of current (I-V) as a function of applied voltage under an AC bias of ± 1.5 V amplitude, obtained for flexible HBIDs, based on the cross-linking ratios of n-type hydrogels.b) Calculated at ±1.5 V rectification ratios of flexible HBIDs based on crosslinking ratios of n-type hydrogels.

Figure 10 .
Figure 10.a) Current-time response of HG/12 coded flexible HBID under a square wave bias of ±1.5 V at 0.1 Hz. b) "ON" and "OFF" response time graphs of the flexible HBIDs.

Figure 11 .
Figure 11.a) I-V curves of the flexible HBIDs in terms of the bending angles.b) Representative drawings and c) the photos of the bending angles and radiuses of the flexible HBIDs at 30°(left) and 60°(right).d) Recalculated rectification ratio graphs of the diodes for bending angles.

Table 1 .
The measured and calculated characteristic parameters of the hydrogels.

Table 2 .
Recalculated rectification ratios of the flexible HBIDs for bending angles.