Polysiloxane-Based Electrolytes

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1		Anil Kumar Bharwal	--	4562	2023-07-13 14:01:27	\|
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3	format	Jason Zhu	-1 word(s)	4561	2023-07-24 03:51:38	\|

This entry is adapted from the peer-reviewed paper 10.3390/coatings13071164

Dye-sensitized solar cells (DSSCs) are a very promising solution as remote sustainable low power sources for portable electronics and Internet of Things (IoT) applications due to their room-temperature and low-cost fabrication, as well as their high efficiency under artificial light. In addition, new achievements in developing semitransparent devices are driving interest in their implementation in the building sector. However, the main obstacle towards the large-scale exploitation of DSSCs mainly concerns their limited long-term stability triggered by the use of liquid electrolytes.

DSSCs polymer electrolyte polysiloxane

1. Introduction

The worldwide power consumption is expected to increase by 47% in the next 30 years due to population and economic growth ^[1]. Hence, there is a growing interest in renewable energy sources and, more precisely, solar energy, which is readily available and abundant. Apart from the abundance of potentially exploitable solar energy, photovoltaic (PV) cells have competitive maintenance costs and can operate silently off-grid, making them ideal for usage on remote sites or mobile applications. According to Greenpeace International, solar PV cells could provide 14% of total electricity generation by 2030 and employ 10.3 million people ^[2]. More than 80% of the PV market for terrestrial applications is currently dominated by mono- and polycrystalline (c-Si) bulk silicon solar cells, which offer the best compromise between cost and performance. To date, this is the most mature PV technology with terrestrial cell efficiencies between 12% and 25% ^[3]. However, although silicon is abundant, a consequent energy budget is consumed for silicon purification ^[4]. Furthermore, recycling is not easy as lower Si grades are obtained but further powder engineering can allow its exploitation in other energy-related domains such as batteries ^[5]^[6].

Beside bulk silicon solar cells, a second generation of thin film PV devices has been developed, which includes amorphous Si, CdS, CdTe, CuInSe₂ (CIS), CuInGaSe₂ (CIGS) and Cu₂ZnSnS₄ (CZTS) ^[7]^[8]. Thin-film technology has always been cheaper but less efficient than conventional c-Si technology. Terrestrial cell efficiencies between 7% and 21% have been established ^[3]^[7], but their large-scale exploitation has been hindered by the high toxicity of constituent materials.

Apart from their lower fabrication cost, third-generation solar cells, including perovskite solar cells, organic solar cells and dye-sensitized solar cells, are appealing nowadays for their tunable color/transparency and the flexible substrate-compatible deposition processes ^[9].

Record efficiencies of up to 25.7% at lab scale were reported for perovskite solar cells ^[10], which still remain environmentally hazardous due to their content in toxic lead and saltlike minerals, and are unstable when exposed to air moisture ^[11].

After more than 20 years of research, a new generation of dye-sensitized solar cells (DSSCs) emerged on the PV market ^[12], striving to level Si-based technology and to target greener processing technologies. Since the first report of O’Regan and Grätzel in 1991 on DSSCs based on liquid electrolytes with the standard

I^{-} / I_{3}^{-}

mediator and a Ru complex as sensitizer reporting modest performances ^[13], the technology has come a long way and performance has doubled. The introduction of new redox couples based on Co-complexes and porphyrin dyes contributed to achieving the benchmark value of 14.7% ^[14]. Indeed, cobalt or copper complex redox mediators have a more positive redox potential and thus enable a higher theoretical maximum photovoltage ^[15]^[16]^[17]^[18]. Recently, that benchmark was overpassed with 15.2% efficiency being reported for Co-cosensitized DSSCs for which the TiO₂ photoanode was preconditioned with hydroxamic acid ^[19]. Although the power conversion efficiency of DSSCs is not as good as that of perovskite or organic solar cells in outdoor sunlight conditions, they outperform them under indoor artificial illumination ^[20], with a 34.5% efficiency record reported ^[21] for cells comprising Cu-redox mediator-based liquid electrolytes. DSSCs can therefore be a solution to the increasing demand for portable and indoor power generation as reflected by the current trends of the indoor photovoltaic market, which was predicted to reach 109 USD in 2021 ^[21].

2. State-of-the-Art of Polysiloxane-Based Electrolytes in DSSCs

Although they have a slightly lower ionic conductivity than previously presented electrolytes (10⁻⁴–10⁻⁵ S/cm²), polysiloxane electrolytes are appealing for DSSC implementation as they present high chemical and thermal stability, as well as low toxicity ^[22].

As shown in Figure 1, along the years, different polysiloxanes have been prepared for DSSC application owing to their highly flexible backbone, with the barrier energy to bond rotation being only 0.8 kJ mol⁻¹, as well as their very low T_g (−123 °C), high free volumes and polar side chains ^[23].

Figure 1. Structure of polysiloxane-based electrolytes used in DSSCs.

As for poly(ionic liquids), the nonpolar polymer backbone is bulky and determines the electrolyte viscosity and, consequently, the mass transport, whereas the ionic species move more or less freely among the siloxane chains, contributing to the ionic conductivity. The viscosity of polysiloxane increases linearly with the average molecular weight, branch content and branch length ^[24].

In 2001, Ren et al. ^[25], introduced for the first time the use of polysiloxane-based electrolytes for DSSC applications. The novel electrolyte (Si-1 in Figure 1) was crosslinked with PEO chains. Briefly, polymethylhydrosiloxane precursor with Si-H and PEO (macromonomer) were mixed with LiI (20 wt.%), EC/PC (3:1, v:v) and a crosslinker and heated at 80 °C for 30 min. Then, I₂ (5 wt.%) was added. The electrolyte was casted on the photoanode and joining with the Pt counter electrode was realized by in situ crosslinking at 60 °C. The energy conversion efficiency of the DSSC was very low, 2.9%, due to low ionic mobility.

In 2004, a similar efficiency was reached (2.67%) for a polysiloxane electrolyte plasticized with 10% PAN ^[26]. Figure 2 displays the yearly evolution of the photovoltaic parameters for the polysiloxane-based electrolytes in liquid, quasi-solid or solid-state devices.

Figure 2. Yearly evolution of the photovoltaic efficiency for DSSCs based on polysiloxane electrolytes as a function of the electrolyte state (liquid, quasi-solid or gel like and solid).

The lowest reported efficiencies for polysiloxane-based DSSCs were based on solid electrolytes or very viscous ones (Figure 2) with obvious limits in terms of mass and ionic transfer properties. At the opposite end, as expected, much higher efficiencies were reported for polysiloxane-based liquid-state DSSCs (Figure 2).

Much more development was realized for the quasi-solid (gel-like) electrolytes, with record efficiencies of 6.8% and 8.3% reported for iodine containing ^[26] and iodide-free ^[27] electrolytes (see Figure 2). Relying on their high flexibility in terms of side-chain grafting, several ways were explored for controlling viscosity and crosslinking.

As such, low viscosity ionic liquids ^[28]^[29]^[30]^[31]^[32] or polymer crosslinkers ^[25] were blended with the polysiloxane electrolytes to reduce polymer interchain interaction and facilitate ion percolation ^[33].

In 2004, polysiloxane containing quaternary ammonium group (Si-2 in Figure 1), I₂ and 50 wt.% of EC/PC (8:2) (w/w) was used as an electrolyte. The ambient conductivity of this plasticized electrolyte reached up to 1.9 × 10⁻³ S cm⁻¹ and the DSSC showed performances comparable to liquid electrolyte ^[34].

In 2011, Yang et al., grafted imidazolium iodide moieties to the polysiloxane by simply mixing TESPIm⁺I⁻ ionic liquid with tetraethoxysilane (TEOS) in ethanol in the presence of HCl ^[30]. It is the first report of polysiloxane electrolyte modification with ionic liquids for DSSC application. To obtain a membrane, the previously prepared composition was mixed with PVDF in a 2/1 weight proportion and an antisolvent. Membrane crosslinking was achieved by heating at 80 °C. The membrane swelled with an iodide-based liquid electrolyte sandwiched between the photoanode and the counter electrode without a separator, but modest PCE values were reported (3.61%). The same year, Jung et al. ^[35] reached higher efficiencies, 5.2%, for DSSC comprising a new ionic siloxane hybrid electrolyte. The iodide-oligosiloxane monomer was synthesized by a simple sol gel condensation of 3-iodopropyltrimethoxysilane and diphenylsilanediol and the membrane crosslinking was achieved through thermal excitation in the presence of 2-benzimidazolone (Si-3 in Figure 1). They concluded that both the composition and concentration of the oligosiloxane used in the electrolyte affect the performance of the DSSCs.

In 2012, Bae et al. ^[36] fabricated an oligosiloxane gel electrolyte by introducing a novel in situ gelation of the liquid electrolyte (Si-4 in Figure 1). The alkoxysilane monomers are capable of gelling the liquid electrolyte through a sol–gel reaction, resulting in an effective infiltration and contact. The DSSC showed reduced charge recombination and an improved PCE of 5.8% with long-term stability (1000 h at 50 °C).

The same year, a PSEO gel was synthesized by Wand et al. ^[31] through the hydroxilation of poly(methylhydrosiloxane) (PMHS) and poly(ethyl glycol) methyl ether methacrylate (PEGMEMA), followed by the addition of 0.5 M NH₄I, 0.1 M TBAI, 0.5 M DMPII, 0.1 M LiI, 0.2 M I₂ and solvent evaporation. However, poor PCE performances were obtained, most certainly due to the high viscosity of the gel electrolyte, which was not able to properly penetrate into the TiO₂ photoanode network.

In 2013, Lee et al. ^[37] used oligosiloxanediimidazolium iodides (SiDII1, SiDII2, and SiDII3) having different viscosities (Si-5 in figure 1). The electrolytes based on SiDII1 and SiDII2 showed a maximum efficiency of 6.2% and 6.0%, respectively, owing to their superior ionic conductivity. Later the same year, Lee’s group developed electrolytes based on functionalized oligosiloxane by replacing the oligosiloxanediimidazolium iodides with pyridinium iodides (Si-6 in figure 1 ^[29]. The SiDPI2 electrolyte showed a maximum efficiency of 6.8% due to its superior diffusion coefficient and 60 days of device stability at RT.

In 2014, Manca et al. ^[28] reported the implementation of poly[(3-N-methylimidazolium-propyl)methylsiloxane-co-dimethylsiloxane]iodides (Si-7 in figure 1) as suitable polymeric hosts for a novel class of in situ crosslinkable iodine/iodide-based gel electrolytes for DSSCs. The polymer gel electrolytes were prepared by dissolving the poly(3-iodopropylmethylsiloxane-co-dimethylsiloxane) polymer (40 wt.%) in the liquid electrolyte consisting of I₂, LiI, 1,2-dimethyl-3-propylimidazolium iodide (DMPII) in MPN. The overall value of the iodide species in electrolyte was around 1M. A stoichiometric amount of bis(3-aminopropyl)-terminated poly(dimethylsiloxane) was then added to the polymeric solutions, acting as a crosslinking agent. The electrolyte was injected between the two electrodes and cured in situ at low temperature (75 °C), showing a maximum efficiency of 5.84% for an electrolyte, which is rather viscous. Interestingly, this high viscosity and its Newtonian behavior (lower viscosity for higher shear rates) indicate compatibility with blade-coating processes and hot lamination is possible as the electrolyte thermal stability exceeds 250 °C. Recently, the same group reported the synthesis of an ion conductive polysiloxane, named poly[(3-N-methylimidazoliumpropyl)methylsiloxane-co-dimethylsiloxane]iodide (IP-PDMS) (Si-8 in figure 1) ^[38]. The electrolyte was prepared by dissolving about 40 wt.% of synthesized IP-PDMS in liquid electrolytes consisting of 0.9 M DMPII ionic liquid and 0.15 M I₂ in MPN. The final amount of solvent in the prepared electrolyte was rather high, about 60 wt.%, which allowed facile cell filling. The electrolyte was cured in situ at 60 °C and conversion efficiencies of about 6.45% and stable operation over 1000 h under light soaking at 40 °C at 0.44 sun could be achieved.

In 2017, new poly(1-N-methylimidazolium-pentylpolydimethylsiloxane)iodide electrolytes were prepared by Bharwal et al. ^[39] with different degrees of ionic functionalization (low—ImIPDMS1, medium—ImIPDMS2 and high—ImIPDMS3, Si-9 in figure 1). This proved to be an effective way of controlling both viscosity and T_g, i.e., with increasing functionalization, the T_g decreased and the viscosity increased. Unfortunately, low photovoltaic performances were obtained due to the poor electrolyte penetration into the photoanode and the low ionic mobility. In order to decrease the PILs viscosity and improve the ionic transport, MPITFSI and MPII ionic liquids were added ^[32]. The functional properties of these different blends depend on both the IL nature and its concentration ^[39]. A large improvement of the ionic conductivity was obtained for ImI-PDMS2:MPITFSI (1:1) and ImI-PDMS2: MPII (1:1) mixes with efficiencies reaching 5.6%–5.9%.

In addition to their good solubility in ionic liquids, polysiloxanes are also highly soluble in high-boiling solvents such as PC (propylene carbonate) and EC (ethylene carbonate). Even more, adding EC as plasticizer to ImI-PDMS3 increased the ionic diffusion by facilitating the ions dissociation ^[32] following a Grotthuss mechanism with conduction taking place through the iodide ions.

Due to the high nonpolar and hydrophobic character of the siloxanes backbone and high ionicity of side chains in ImI-PDMS3, hydrophobic/hydrophilic phase-separated domains are formed with ionic imidazolium groups solvated and favorably dissociated by the highly polar EC solvent. A record PCE value and outstanding long-term stability were obtained for the optimized ImI-PDMS3:EC 3:1 ratio, 6.3% and 250 days aging in ambient conditions, respectively.

In 2019 ^[40], comparable efficiencies (5.8%) were reported for the ImI-PDMS2/MPII containing a higher proportion of MPII (1:3 weight ratio) in combination with a double porosity (8–10 nm mesopores + 60–70 nm macropores) home-designed TiO₂, as opposed to the commercial mesoporous (20 nm pore size) TiO₂ ^[32]. It was the first time that the influence of porosity was studied for DSSCs based on polysiloxane polymer electrolytes, thus illustrating the importance of effective electrolyte penetration and interfacial contact for device performance.

These polysiloxanes are very attractive for future printing processes as they are thermally and (electro)chemically stable, have good ionic conductivity, can act both as electrolyte and redox mediator ^[27], are good solvents for other polymers ^[25]^[30] and ionic liquids ^[31]^[32]^[36]^[38] and are compatible with high-boiling solvents like MPN.

3. Iodine-Free Polysiloxane Electrolytes

The presence of elemental iodine (I₂) in standard liquid electrolytes generates visible-light-absorbing

I_{3}^{-}

species, which competes with the photosensitizer (dye) adsorbed on the TiO₂ surface, reducing the maximum J_sc that can be achieved by DSSC ^[41]. Because of the proximity effect, the oxidized dye and the surplus

I_{3}^{-}

may form an ion pair, which speeds up the electron recombination process. In order to reduce the resistance of DSSCs and increase the overall conversion efficiency, metal counter electrodes are commonly used for large-scale commercial DSSCs modules. However, scaling up DSSCs is complicated due to the corrosion of the

I_{3}^{-} / I^{-}

redox couple with the metallic counter electrodes, which could also affect the stability of DSSC modules. It is crucial to create I₂ free electrolytes to counteract these drawbacks.

An important breakthrough for the development of iodine-free DSSCs was reported in 2021 by Bharwal et al. ^[42], which reported high efficiencies (3.55%) for the ImI-PDMS3:EC iodine-free electrolyte in back-illuminated flexible devices with TiO₂ nanotubes on Ti foil as photoelectrodes. Even more, outstanding long-term stability was reported under accelerated aging, 500 h under 1 sun and 50 °C, owing to the reduced charge recombination and extended electron lifetime. In 2022, the same authors published the highest ever reported efficiency for iodine-free polysiloxane, 8.3% ^[27], for the same type of EC modified ImI-PDMS3 polysiloxane electrolyte used with traditional mesoporous TiO₂ photelectrodes. The devices retained 84% of their initial efficiency after ambient aging for 26 months.

Following, the thermophysical, rheological and electrochemical properties, as well as the mass and ionic transport properties, of polysiloxane-based electrolytes are described in view of their future exploitation using up-scalable deposition techniques.

4. Properties of Polysiloxane-Based Electrolytes

4.1. Thermophysical Properties

The polysiloxanes are thermally stable well above 200 °C, when dehydration and then decomposition occur ^[25]^[34]^[37]. However, the upper limit of the thermal stability range of DSSCs is determined by the dye sensitizer, whose stability is generally below 100 °C ^[43] but well above the normal DSSC-operating temperature.

Thermogravimetric analysis of the pure polysiloxane-based electrolytes have shown that the glass transition temperature (T_g) is lower than −100 °C ^[39], meaning poor chain mobility with negative consequences on mass and ion transport but also on the final electrolyte film flexibility. The T_g of polysiloxanes could be increased by simply functionalizing ^[44], modifying the siloxane chain length ^[37] or mixing the polysiloxanes with ionic liquids ^[32]. These additives act as spacers between the polymer chains, reducing the interactions between them and leading to reduced viscosity and improved electrolyte film processability along with improved ionic conductivity. Thus, they represent viable strategies for controlling the viscosity in view of printing. Similarly, adding plasticizers ^[45]^[46] or inorganic fillers ^[47]^[48]^[49] increase the T_g with a positive effect on ionic conductivity.

Polysiloxane-based gel electrolytes with good thermal stability were prepared by Cipolla et al. ^[38] by mixing various amounts of IP-PDMS (50%–80%) with 0.9 M of DMPII, 0.15 M of I₂ in 3-methoxypropionitrile. Thermal stability up to 270 °C and only a slight drop in viscosity with heating from RT to 50 °C followed by a steady variation up to 80 °C reported for the gel electrolytes containing at least 50% of PGE polymer.

4.2. Ionic Conductivity

The polysiloxane-based electrolytes are amorphous at room temperature ^[50], and the Vogel–Tammann–Fulcher (VTF) model was used to explain the ionic conductivity variation with temperature ^[32]^[34]^[44]. According to the VTF model (Equation (1)) the charge transport through the free volume is favored by polymer segments movement.

σ = \frac{A}{\sqrt{T}} \exp (\frac{- E_{a}}{R (T - T_{0})})

(1)

In this formula, A is a pre-exponential factor related to the number of charge carriers (S cm⁻¹ K^1/2), E_a is pseudo activation energy (J mol⁻¹) corresponding to ion-carrier diffusion, and T₀ is the ideal glass transition temperature at which ion mobility goes to zero (K). Above T₀, thermal motion of the polymer chains initiates the transportation of ions. The value of T₀ is usually 50 K or 25 K below T_g. The nonlinearity seems to be more noticeable for highly functionalized polysiloxane electrolytes ^[51]. The trend does not change after mixing the same polysiloxanes with ionic liquids ^[51]. Nevertheless, the type of solvent or plasticizing agent seems to have a different effect on the ionic conductivity variation with temperature ^[27]. It is interesting to notice that the highly functionalized ImI-PDMS3 polymer has a higher ionic conductivity despite its higher viscosity. In highly viscous systems, carrier transportation through an electron exchange mechanism (Grotthus-like mechanism) was proposed ^[27]. This mechanism relies on the hole hopping and bond exchange between the polyiodide species grouped in the hydrophilic domains, owing to the high ionicity of the side chains. The hydrophilic domains are thus enclosed between the hydrophobic domains constituted by the polysiloxane backbone. Polyiodide dissociation can be encouraged by using highly polar solvents such as EC ^[32].

According to the Einstein–Stokes formula (Equation (2)) the ionic conductivity and diffusion coefficient of the electrolytes largely depend on the viscosity ^[52]^[53]. Here, k is the Boltzmann constant, T is the temperature, μ is the viscosity of the solvent and Rion is the spherical radius of diffusion species. Thus, a large solute ion radius and high fluidity (1/μ) are expected to cause high ion mobility.

D_{g e n} \propto \frac{k T}{µ R_{i o n}}

(2)

To understand how structural parameters affect ionic conductivity, the relationship between the conductivity and viscosity (the ionicity concept) of the polymer blends can be examined according to Walden’s rule, i.e., mobility (μ) and molar conductivity (Λ) are proportional to fluidity according to μ∼1/_η and Λɳα = constant (α is the slope of the Walden line) ^[53]^[54]. Ideally, for α > 1, the polymer segmental relaxation phenomena do not influence the ionic conductivity of the electrolyte. When the slope equals to unity (α = 1), the polymer electrolyte is fully dissociated with no ion–ion interaction, as for classical dilute KCl solution.

Bharwal et al. studied the influence of polysiloxane degree of functionalization and ionic liquid proportion on the viscosity–ionic conductivity decoupling ^[39]. The lower functionalized ImIPDMS1 and ImIPDMS2 electrolyte points are below the ideal Walden line, indicating that the ionic conductivities are somewhat decreased as a result of ion-pair interaction, meaning the polymer chain movement due to shear stress or temperature factors are involved in the ion percolation mechanism.

By increasing the polysiloxane functionalization, a decoupling of the viscosity and ionic conductivity could be achieved (point above the KCl line) owing to a nanoscale phase separation between the hydrophobic polymer backbone and hydrophilic ionic groups, following the previously mentioned Grotthuss mechanism. The same authors showed that, with increasing ionic liquid amount, the ionicity of the electrolyte increases with points remaining below the ideal KCl line.

Ideal Walden behavior was reported for polysiloxane electrolytes based on oligo/poly(methyl(2-(tris(2-H methoxyethoxy)silyl)ethyl)siloxane mixed with the LiTFSI, LiFSI and LiPF6 ionic liquids ^[55]. Indeed, for these systems, the rate of ionic diffusion is much faster than the rate of structural relaxation of polymer molecules. A better decoupling was observed with increasing polymer chain length.

4.3. Rheological Properties

As a matter of fact, all the polysiloxane-based electrolytes are characterized by non-Newtonian behavior ^[38]^[39] i.e., the viscosity decreases with the increasing shear rate, with a shift in the yield point as a function of composition. Wang et al. ^[43] have demonstrated that the presence of Newtonian or non-Newtonian behavior is highly dependent on the degree of functionalization of polysiloxane-based liquid electrolytes used in DSSCs. They reported a rather Newtonian behavior up to 90 °C for solvent-free low functionalized polysiloxane electrolytes and temperature-dependent non-Newtonian (shear-thinning) behavior for highly functionalized counterparts.

Polymer gel electrolytes were prepared by Cipolla et al. ^[38] by dissolving IP-PDMS into an ionic liquid consisting of 0.9 M of DMPII, 0.15 M of I₂ in 3-methoxypropionitrile. The yield point (the lowest shear stress above which the electrolyte will behave like a liquid) increased with increasing polymer content. With the increasing polymer amount, the chain-packing density increases with less free volume available for the alignment of polymer chains, thus pushing the flowing point toward higher shear rates.

The polymer amount affects the viscoelastic properties during printing as well as the green properties, such as the strength, density and topological structure after drying. Rheological measurements using either rotational and oscillation tests are used to determine the flow behavior when stress is applied onto the sample or to study the viscoelastic behavior, respectively. The plot between storage modulus (G′) and loss modulus (GG″) versus the shear strain provides the linear viscoelastic region in the amplitude sweep test and gives information about the polymer gel or membrane stability as well as on the elastic properties domination over the viscous properties. The storage G′ and loss G″moduli are the real and imaginary parts of the complex modulus. The storage modulus G′ represents the elastic portion of the viscoelastic behavior, which partly describes the solid-state behavior of the sample. The loss modulus G″ characterizes the viscous portion of the viscoelastic behavior, which can be attributed to the liquid-state behavior of the sample. Otherwise, the complex shear modulus G* (in Pa) is defined by Equation (3), where ζ is the shear-stress amplitude (ζ = F/A) in Pa and γA is the strain amplitude which is dimensionless or expressed in % (γ = s/h, where s is the liquid plates deflection path and h is the distance between the plates ^[56].

G* = ζ/γ (3)

The shear-thinning effect is less important for screen or blade-coating deposition techniques, for which rather low deposition speeds are used and thick wet layers are deposited. However, the same phenomenon becomes important in spray or inkjet printing processes, as it prevents spraying/printing head clogging, enables obtaining a continuous jet while maintaining a high throughput and controls the printing resolution. The influence of polymer ink rheological properties on the quality of the inkjet-printed pattern is clearly highlighted. For the polymer ink without the S-hBN (sulphonated–hexabornnitride) filler addition, the loss modulus (G″) dominates in the range of shear stress from 0.1 to 100 Pa, revealing a liquidlike behavior, which leads to the collapse of the printed pattern due to the low storage modulus ^[57]. Once the S-hBN filler is added, different viscoelastic properties are observed: the storage modulus (G′) of the inks is higher than the loss modulus (G″) in the region of 10⁻¹ to 10 Pa, indicating a solid-like behavior with an increase of 3–4 orders of their values. Beyond the yield point (point where the G′ decreases), the viscous characteristic becomes dominant in the high shear stress region. With increasing filler concentration, chain alignment is hampered due to the strong filler–polymer interaction at low shear stress, thus explaining the increase in the static viscosity. However, at high shear stress, this interaction is weaker, allowing the orientation of the polymer chains along the flow.

In conclusion, the polymer inks with S-hBN filler could be extruded smoothly through the nozzle at high shear stress with structure recovery after stress release. The polymer ink containing 2% of filler showed the highest storage modulus (>5 × 10³ Pa) with a high difference between G′ and G″, and, thus, a stiff structure after printing enabling the maintenance of the printed architecture as well as satisfying ionic conductivity (0.47 mS cm⁻¹).

Solvent-free polysiloxane-based gel electrolytes were prepared by the sol–gel reaction of PEG-functionalized polymethylsiloxane, followed by dissolution of LiTFSI and radical polymerization of terminal vinyl moieties for Li-based battery applications ^[58]. The rheological analysis highlights that the size of the methoxy-terminated chains and the addition of an inorganic filler can influence the structural integrity. The density of crosslinking is higher in the electrolyte HP5 containing shorter PEG side chains compared to HP3, leading to a higher plateau storage modulus. This is constant over the studied temperature domain, proving a solid elastic behavior.

Both the storage (G′) and loss (G″) moduli increase with temperature, with G′ always higher than G″ in the 0.1 to 100 Pa shear stress domain (as expected for a crosslinked polymer), and with no visible flow/melting point up to 100 °C. However, when TiO₂ is added as a filler in the same electrolyte, the polymer electrolyte loses rigidity with the plateau shear modulus decreasing and the appearance of an inflexion point at temperatures higher than 60 °C. The electrolyte application domain is slightly limited, as abrupt falls in G′ and G″ are noticed for shear rates above 40 s⁻¹ at RT, indicating gel structure corruption.

Chen et al. ^[59] studied the linear viscoelastic properties of polysiloxane electrolytes with phosphonium and oligo(ethylene oxide) side chains containing ionic monomers. They observed that, by increasing the ionic content, the polymer relaxation is delayed (λ = 1/ωc, in s), thus extending the solidlike elastic behavior (G′ > G″), with no flow point limit visible at temperatures around T_g and high deformation frequencies.

Iodopropyl-branched polysiloxane gel electrolytes with low temperature thermal crosslinking were reported by De Gregorio et al. ^[28]. At the initial stage, these electrolytes appear as viscous liquids but after a few minutes curing at 75 °C, an abrupt increase of both moduli is observed with G′ crossing G″, which then reaches a plateau marking the end of the crosslinking process. The elastic modulus (G′) reaches a maximum value of around 1.1 kPa after 240 min of curing and the electrolyte ionic conductivity stabilizes at 6.65 × 10⁻² mS cm ^{− 1} for lower quaternization rates (GL11_Q55°). The amount of unquaternized iodopropyl influenced both the viscoelastic properties and the ionic conductivity of these systems. The polysiloxane electrolyte gelation time depends on the chain length and crosslinker ^[60]. Shi et al. ^[61] showed that the polysiloxane electrolyte flexibility can be improved by increasing the poly(ethylene glycol) diacrylate—PEGDA reticulation precursor chain length owing to T_g decreasing, as highlighted by the blue shift of the loss tangent (tan δ = G′/G″) peak maximum.

Polymer inks’ gelation temperature- and time-dependent behavior significantly affect their printability and shape retention performance, whereas the mechanical strength of the ink is important for the structural stability of the entire DSSC. There are not enough reports on the viscoelastic properties of the polysiloxane-based nor other types of polymer electrolytes used in DSSCs. Thus, much research is needed to optimize the viscoelastic properties of the electrolytes to fit the different spraying or printing techniques and to fulfill the temperature, ionic-conductivity, chemical and electrochemical operating conditions.

4.4. Electrochemical Stability and Redox Potential

The electrochemical stability of the electrolyte impacts the stability of DSSCs. A number of factors affect the electrochemical stability of redox active ions and molecules in an electrolyte solution. They are connected with (i) the intrinsic electronic properties of both oxidized and reduced forms of a given redox couple, and (ii) their interactions with the environment. The latter are largely determined by the structural changes of the surrounding electrolyte solution accompanying the electron transfer reaction.

All the polysiloxane electrolytes prepared so far for DSSCs rely on the iodine/iodide redox couple as redox mediator. The usable electrochemical window is determined by the potential at which the oxidation/reduction reactions of the electrolyte occur and can be determined by cyclic voltammetry.

The polysiloxane electrolytes redox potential gives an indication of the expected open-circuit potential and the projected cell performance. The lower the redox potential, the higher the open-circuit potential. In iodine-containing polysiloxane, the redox potential depends on the chemical environment. As such, shifting of the

I^{-} / I_{3}^{-}

redox potential with the degree of polysiloxane substitution or the type of ionic liquid could be observed and related to a more or less facile redox species dissociation [x]. In their study, Assary et al. ^[62] have shown that various electron-donating and withdrawing substituents influence the oxidation potential of polysiloxanes.

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