Microporous Polymer–Conducting Polymer Gel Electrolytes: Enhancing Stability and Performance in Quasi-Solid-State Dye-Sensitized Solar Cells

Microporous Polymer–Conducting Polymer Gel Electrolytes: Dye-sensitized solar cells (DSSCs) have attracted sustained interest as a low-cost photovoltaic technology, yet their practical application is limited by the instability of conventional liquid electrolytes. Recent advances in quasi-solid-state gel electrolytes provide an effective strategy to improve device durability while maintaining efficient charge transport. This article reviews the design and performance of a microporous poly(glycidyl acrylate)–polypyrrole (PGA–PPy) hybrid gel electrolyte developed for DSSC applications. The hybrid material combines the high absorbency and solvent-retention capability of poly(glycidyl acrylate) with the electrical conductivity and electrocatalytic activity of polypyrrole. Structural and electrochemical analyses demonstrate that the microporous polymer network enables high electrolyte uptake and ionic conductivity, while the conducting polymer component enhances charge-transfer kinetics of the iodide/triiodide redox couple. When applied in quasi-solid-state DSSCs, the PGA–PPy gel electrolyte delivers improved interfacial stability and a power conversion efficiency of 5.03% under simulated solar illumination. These results highlight the potential of multifunctional polymer gel electrolytes as a promising pathway toward more stable and practical dye-sensitized solar cell technologies.

Background and Motivation: Advancing Electrolytes for Dye-Sensitized Solar Cells

Since their first demonstration by O’Regan and Grätzel in 1991, dye-sensitized solar cells (DSSCs) have been widely investigated as a promising photovoltaic technology due to their low fabrication cost, relatively simple manufacturing process, and good performance under diffuse light conditions.

Early high-efficiency DSSCs predominantly employed liquid electrolytes based on the iodide/triiodide (I⁻/I₃⁻) redox couple, achieving power conversion efficiencies exceeding 11%. Leakage, solvent volatilization, corrosion of cell components, and poor long-term stability are widely recognized challenges that compromise device durability and operational safety. These issues have driven intensive research efforts toward alternative electrolyte systems.

In response, several electrolyte concepts have emerged, including all-solid-state electrolytes, ionic liquid electrolytes, and quasi-solid-state gel electrolytes. Among them, gel electrolytes have attracted particular attention because they combine the high ionic conductivity of liquid systems with the enhanced mechanical stability of solids. Typically, gel electrolytes consist of a polymer host that immobilizes a liquid electrolyte within a three-dimensional network, significantly reducing leakage while maintaining efficient ion transport.

Conventional polymer hosts often suffer from insufficient absorbency for organic solvents or limited electrochemical activity. Therefore, developing advanced polymer architectures that integrate high absorbency with enhanced electrochemical functionality is a key research focus.

In this context, hybrid polymer systems that combine superabsorbent polymers with conducting polymers offer a compelling strategy. Superabsorbent polymers provide a microporous network capable of retaining large amounts of electrolyte, while conducting polymers can improve charge-transfer kinetics and catalytic activity toward redox reactions. The development of such hybrid gel electrolytes represents an important step toward more stable, efficient, and commercially viable dye-sensitized solar cells.

Design Strategy of the PGA–PPy Hybrid Gel Electrolyte

The design of an effective gel electrolyte for dye-sensitized solar cells (DSSCs) requires a careful balance between mechanical stability, electrolyte retention, ionic conductivity, and electrochemical activity. Conventional polymer gel electrolytes often rely on inert polymer hosts that primarily serve as physical matrices to immobilize liquid electrolytes. While this approach reduces leakage, it does not actively contribute to charge transport or redox kinetics. The poly(glycidyl acrylate)–polypyrrole (PGA–PPy) hybrid gel electrolyte represents a more advanced design strategy, integrating complementary functionalities within a single polymer system.

Poly(glycidyl acrylate) (PGA) is selected as the primary polymer host due to its superabsorbent nature and three-dimensional microporous network. The presence of polar functional groups in PGA enables strong interactions with organic solvents commonly used in DSSC electrolytes, allowing the polymer to absorb and retain large amounts of liquid electrolyte. This high absorbency is crucial for maintaining continuous ionic pathways while minimizing solvent leakage and volatilization. Moreover, the crosslinked porous structure of PGA provides mechanical robustness, ensuring dimensional stability during device operation.

However, while PGA excels in electrolyte uptake, it is electrically insulating and offers limited contribution to interfacial charge-transfer processes. To address this limitation, polypyrrole (PPy), a well-known conducting polymer, is introduced into the PGA network through an in situ polymerization process. PPy is widely recognized for its high electrical conductivity, redox activity, and electrocatalytic properties, particularly toward the iodide/triiodide (I⁻/I₃⁻) redox couple used in DSSCs. By polymerizing pyrrole monomers within the pre-formed PGA matrix, a homogeneous hybrid structure is achieved, ensuring intimate contact between the absorbent framework and the conducting component.

This two-step solution polymerization strategy offers several advantages. First, it preserves the microporous architecture of PGA while uniformly distributing PPy throughout the network. Second, the incorporation of PPy reduces charge-transfer resistance at the electrolyte–electrode interface, enhancing redox reaction kinetics. Third, the hybrid structure improves ionic salt tolerance, enabling higher electrolyte loading without phase separation or conductivity loss.

Overall, the PGA–PPy hybrid gel electrolyte exemplifies a rational materials design approach in which structural and electrochemical functions are synergistically combined. By integrating a superabsorbent polymer host with a conducting polymer, this strategy directly addresses the key limitations of traditional gel electrolytes and provides a versatile platform for improving the efficiency and stability of quasi-solid-state DSSCs.

Structural and Electrochemical Characteristics of PGA–PPy Gel Electrolytes

Understanding the structural and electrochemical properties of polymer gel electrolytes is essential for correlating material design with device performance in dye-sensitized solar cells (DSSCs). In the case of the poly(glycidyl acrylate)–polypyrrole (PGA–PPy) hybrid electrolyte, a combination of morphological, spectroscopic, and electrochemical analyses reveals how the hybrid architecture contributes to enhanced ionic transport and redox activity.

Scanning electron microscopy (SEM) provides direct evidence of the microporous network structure of the PGA-based polymer host. The interconnected pores form a three-dimensional framework capable of absorbing and retaining a large volume of liquid electrolyte. This morphology facilitates efficient ion diffusion by creating continuous pathways for iodide and triiodide species while simultaneously suppressing electrolyte leakage. After in situ polymerization of pyrrole, the SEM images indicate that polypyrrole is uniformly distributed within the PGA network without collapsing the porous structure, confirming the structural integrity of the hybrid material.

Fourier transform infrared (FTIR) spectroscopy further validates the successful formation of the PGA–PPy hybrid. Characteristic absorption bands associated with the ester groups and epoxy-related functionalities of PGA are clearly observed, while additional peaks corresponding to the conjugated backbone of PPy confirm its incorporation into the polymer matrix. The coexistence of these functional groups suggests strong interactions between the two polymer components, which contribute to improved electrolyte affinity and mechanical stability.

Electrochemical characterization highlights the functional advantages of introducing PPy into the gel electrolyte. Cyclic voltammetry (CV) measurements demonstrate enhanced electrocatalytic activity toward the I₃⁻/I⁻ redox couple, a critical process in DSSC operation. Compared to gel electrolytes without PPy, the PGA–PPy system exhibits higher peak currents and reduced peak-to-peak separation, indicating faster redox kinetics and lower charge-transfer resistance. These improvements are attributed to the conductive and redox-active nature of PPy, which facilitates electron transfer at the electrolyte–electrode interface.

Ionic conductivity measurements further confirm the superior performance of the hybrid electrolyte. The PGA–PPy gel achieves an ionic conductivity of 12.83 mS cm⁻¹ at room temperature, comparable to or exceeding many conventional gel electrolytes. Temperature-dependent conductivity studies reveal thermally activated ion transport behavior, suggesting efficient ion mobility within the polymer network. Importantly, the electrolyte also demonstrates good ionic salt tolerance, maintaining high conductivity even at elevated salt concentrations.

Collectively, these structural and electrochemical characteristics underscore the effectiveness of the PGA–PPy hybrid design. The synergy between the microporous PGA framework and the conductive PPy component enables high electrolyte uptake, efficient ion transport, and enhanced redox kinetics, making this material highly suitable for quasi-solid-state DSSC applications.

Performance of Quasi-Solid-State DSSCs Using PGA–PPy Gel Electrolytes

The ultimate evaluation of a newly developed gel electrolyte lies in its performance within a fully assembled dye-sensitized solar cell (DSSC). When the poly(glycidyl acrylate)–polypyrrole (PGA–PPy) gel electrolyte is incorporated into a quasi-solid-state DSSC (QS-DSSC), notable improvements in photovoltaic performance and device stability are observed, underscoring the practical advantages of this hybrid electrolyte system.

The QS-DSSCs are typically constructed using a nanocrystalline TiO₂ photoanode sensitized with a ruthenium-based dye, a platinum-coated counter electrode, and the PGA–PPy gel electrolyte containing the iodide/triiodide (I⁻/I₃⁻) redox couple. Owing to the strong electrolyte absorbency of the PGA framework, the gel effectively infiltrates the mesoporous TiO₂ network, ensuring intimate interfacial contact between the electrolyte and the photoanode. This efficient pore filling is critical for maintaining continuous ionic pathways and minimizing internal resistance within the device.

Electrochemical measurements indicate that the incorporation of polypyrrole significantly reduces the charge-transfer resistance at the counter electrode–electrolyte interface. Polypyrrole’s intrinsic conductivity and electrocatalytic activity toward the I₃⁻/I⁻ redox reaction facilitate faster electron exchange, which directly contributes to improved photocurrent generation. As a result, DSSCs employing the PGA–PPy gel electrolyte exhibit enhanced short-circuit current density (J_sc) and fill factor compared to devices based on non-conductive polymer gel electrolytes.

Under standard simulated solar illumination (AM 1.5, 100 mW cm⁻²), the QS-DSSC incorporating the PGA–PPy electrolyte achieves a power conversion efficiency of 5.03%. While this efficiency remains lower than that of optimized liquid-electrolyte DSSCs, it represents a competitive value for quasi-solid-state systems and demonstrates a favorable balance between efficiency and operational stability. Importantly, the gel-based device shows improved resistance to electrolyte leakage and solvent evaporation, which are key factors affecting long-term performance.

Furthermore, the mechanical robustness of the gel electrolyte contributes to enhanced device reliability during thermal cycling and prolonged illumination. The immobilization of the liquid electrolyte within the polymer matrix suppresses mass transport losses and mitigates degradation at the electrode interfaces. These attributes highlight the potential of the PGA–PPy gel electrolyte for applications where stability and durability are prioritized alongside reasonable photovoltaic efficiency.

Overall, the performance of QS-DSSCs based on PGA–PPy gel electrolytes illustrates how rational electrolyte design can bridge the gap between high-efficiency liquid systems and stable solid-state configurations, advancing the practical viability of dye-sensitized solar cell technology.

Implications and Future Outlook for Polymer Gel Electrolytes in Solar Energy

The development of the poly(glycidyl acrylate)–polypyrrole (PGA–PPy) hybrid gel electrolyte highlights the growing importance of advanced polymer design in overcoming long-standing limitations of dye-sensitized solar cells (DSSCs). By integrating a superabsorbent polymer framework with a conducting polymer component, this system demonstrates how multifunctional materials can simultaneously address electrolyte leakage, ionic conductivity, and interfacial charge-transfer efficiency. These results have broader implications not only for DSSCs but also for next-generation solar energy technologies that demand both performance and durability.