The blend ratio and viscosity must be balanced, not maximized. In a 9 wt.% PEO/PMMA polymer gel electrolyte, a 7/3 PEO-to-PMMA ratio produces an approximately 16.9 Pa·s viscosity, which is reported to support both reliable paste printing and penetration into mesoporous TiO₂ photoelectrodes. This balance improves electrolyte–electrode contact, lowers interfacial charge-transfer resistance, and can contribute to higher Voc and power-conversion efficiency, including the reported 8.48% result.
Core takeaway: A polymer gel that is too fluid may spread or drain without maintaining a uniform printed layer, while one that is too viscous may not infiltrate the TiO₂ pores. The optimum blend ratio creates sufficient flow during printing while retaining enough structure for controlled deposition and deep penetration.
Why Blend Ratio Controls Electrolyte Behavior
PEO and PMMA perform different rheological roles
Changing the PEO/PMMA ratio changes the polymer network, solvent retention, and resistance to flow. The resulting composition determines whether the gel behaves more like a printable paste or a highly viscous film-forming material.
The ratio also affects how readily the electrolyte can move through the small interconnected pores of the TiO₂ photoelectrode. Therefore, composition is a processing variable as well as an electrochemical variable.
Total polymer concentration sets the viscosity range
At a fixed PEO/PMMA ratio, increasing total polymer concentration generally produces a more viscous and mechanically stable gel. Lower concentrations flow more easily but may provide poorer shape retention and thinner, less uniform coverage.
The reported 9 wt.% total polymer concentration provides a useful reference point, but the optimum is not universal. Solvent content, salt concentration, molecular weight, temperature, and printing conditions can shift the viscosity required for successful infiltration.
How Viscosity Affects Printability
Moderate viscosity supports controlled deposition
A viscosity of approximately 16.9 Pa·s for the 9 wt.%, 7/3 PEO/PMMA formulation is reported to enable effective paste printing onto the photoanode. It is sufficiently fluid under printing shear to pass through the process while remaining stable after deposition.
This balance helps maintain a consistent wet-film thickness and reduces uncontrolled spreading across the substrate.
Excessively low viscosity causes poor layer control
If the gel is too fluid, it may spread beyond the intended printed area, drain through the electrode, or produce thickness variations. These effects can create incomplete coverage and inconsistent electrolyte loading from cell to cell.
A low-viscosity formulation may also fail to remain in the desired position during handling or before gelation.
Excessively high viscosity restricts printing
A gel with excessive viscosity may require higher printing pressure or force and can produce incomplete transfer from the screen or coating tool. It may also show poor leveling, leaving voids, ridges, or nonuniform regions.
High viscosity can additionally slow ion transport and make infiltration into narrow mesopores more difficult.
How Viscosity Affects Electrode Penetration
Penetration requires flow through connected pores
The electrolyte must enter the mesoporous TiO₂ network rather than remain as a layer on top of the photoelectrode. Appropriate viscosity allows the printed gel to flow into the pore structure under the pressure and shear generated during coating, printing, or assembly.
Deep infiltration increases the physical contact area between the electrolyte and the semiconductor surface.
Improved contact reduces interfacial losses
Better pore filling can improve ionic access and contact at the photoanode/electrolyte interface. This can reduce interfacial charge-transfer resistance, commonly represented as Rct, and support more effective charge transfer.
The same principle applies at the counter-electrode interface, where poor contact can increase interfacial resistance and limit cell performance.
Printing conditions are part of the viscosity equation
The reported viscosity cannot be separated from the application method. Screen geometry, shear rate, coating speed, wet-film thickness, substrate temperature, and drying or gelation conditions all influence the apparent flow and final penetration.
Consequently, the same formulation may print and infiltrate differently if the process parameters change.
Connecting Penetration to Solar-Cell Performance
Better infiltration can increase Voc
A well-infiltrated electrolyte provides more uniform interfacial contact and can reduce regions where charge recombination or inefficient transfer occurs. This may contribute to a higher open-circuit voltage, although Voc also depends on the redox chemistry, electrode properties, illumination, and recombination pathways.
The 8.48% efficiency associated with the cited formulation should therefore be treated as a formulation-and-process result, not as a universal performance guarantee.
Uniformity improves reproducibility
Even when average efficiency is acceptable, nonuniform gel loading can cause substantial variation between devices. Consistent viscosity and controlled printing help produce repeatable electrolyte thickness, pore filling, and interfacial contact.
For laboratory development, reproducibility is often as important as the best single-cell efficiency.
Understanding the Trade-offs
Printability versus pore infiltration
Lower viscosity generally favors flow and penetration, but too little viscosity can compromise film integrity. Higher viscosity improves shape retention and may reduce leakage, but can impede pore filling and ion transport.
The practical target is a shear-responsive, moderately viscous paste that flows during application and stabilizes after deposition.
Ionic conductivity versus mechanical stability
Polymer gel electrolytes retain liquid electrolyte within a polymer matrix, improving thermal and mechanical stability relative to a free liquid electrolyte. However, increasing polymer content or reducing solvent content can restrict ion mobility.
Excess solvent has the opposite problem: it can improve ionic transport but reduce mechanical strength and increase the risk of dimensional instability.
Performance versus process window
A formulation may have excellent electrochemical properties but a narrow printing window. Small changes in temperature, coating speed, or drying time can then produce different film thicknesses and penetration depths.
A slightly less conductive formulation with robust, repeatable processing may be more useful for device fabrication than a nominally superior formulation that is difficult to deposit consistently.
Avoid treating one viscosity as universal
The 16.9 Pa·s value is a useful benchmark for the stated PEO/PMMA system and concentration. It should not be transferred directly to other polymer families, solvents, salt concentrations, temperatures, or printing geometries without rheological and infiltration testing.
How to Apply This to Your Fabrication Process
Start by treating the blend ratio, polymer concentration, and printing parameters as one integrated process, rather than optimizing viscosity in isolation.
- If your primary focus is print definition: Use a formulation near the reported 9 wt.% PEO/PMMA, 7/3 composition as a starting point, then adjust viscosity and shear conditions to preserve edge definition and uniform wet-film thickness.
- If your primary focus is mesoporous TiO₂ penetration: Avoid excessive polymer concentration and viscosity, and verify that the gel infiltrates the pores under the actual printing or coating conditions.
- If your primary focus is device efficiency: Optimize pore filling and interfacial contact together with ionic conductivity, because improved penetration alone cannot compensate for poor electrolyte chemistry or electrode interfaces.
- If your primary focus is reproducibility: Precisely control coating speed, wet-film thickness, temperature, drying or gelation, and assembly pressure while measuring viscosity under relevant shear conditions.
A successful quasi-solid-state solar-cell electrolyte is not simply the most viscous or most conductive formulation—it is the one that provides controlled printing, deep pore infiltration, stable interfaces, and repeatable electrochemical performance.
Summary Table:
| Factor | Impact on Printability | Impact on Electrode Penetration | Optimal Balance |
|---|---|---|---|
| Blend Ratio (PEO/PMMA) | 7/3 ratio provides moderate viscosity suitable for controlled deposition | 7/3 ratio allows deep infiltration into TiO₂ pores | Moderate ratio balances printing and penetration |
| Viscosity (Pa·s) | ~16.9 Pa·s supports reliable printing without spreading | Moderate viscosity enables flow into mesopores | Viscosity around 16.9 Pa·s for balanced performance |
| Polymer Concentration | Higher concentration increases viscosity, aiding shape retention | Higher concentration may restrict pore penetration | 9 wt.% concentration is a good starting point |
| Shear Conditions | Adjustable to improve printing precision | Adjustable to enhance infiltration | Optimize shear rate and coating speed |
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