Long-term Stability Improvement in Perovskite Solar Cell through Low Temperature Thin Film Encapsulation with Nanoparticles Embedded PMMA-PU Interpenetrating Polymer Composite

The commercialization of perovskite solar cells (PSCs) has faced challenges due to their poor long-term stability, especially in humid and high-temperature environments, which accelerate their degradation and limit reliability. Polymer-nanoparticle composites offer a versatile platform for achieving tunable properties that are difficult to obtain from individual components. In this study, we developed an advanced encapsulation material using an interpenetrating network (IPN) composite of poly(methyl methacrylate) (PMMA) and polyurethane (PU). To further improve its moisture barrier capabilities, hydrophobic CeO₂ nanoparticles were incorporated into the IPN polymer.
Unlike the polymer blend which the polymers remain as individual, separate phases that may or may not mix well, the PMMA/PU IPN composite leverages the physical entanglement of PMMA, a mechanically stiff polymer, and PU, an elastic and flexible polymer, to create a robust, interconnected structure. This entanglement reduces the free volume within the polymer, restricting molecular motion and effectively limiting the diffusion of water molecules. By adjusting the PMMA/PU composition, we were able to fine-tune both the mechanical strength and water resistance of the composite. Increased PMMA content in the PU phase resulted in a more rigid structure, further decreasing water permeability by reducing the mobility of water molecules through the network.
Incorporating CeO₂ nanoparticles into the IPN matrix provided additional improvements in barrier performance. Due to their unique electronic structure, specifically the inaccessibility of Ce ions' 4f electrons, CeO₂ nanoparticles exhibit strong hydrophobicity, which prevents interaction with water. These nanoparticles are dispersed well in polymeric solution and fill the remaining free spaces in the polymer matrix and interact with the hydrophobic CH_x groups of the polymer, further reducing free volume and enhancing resistance to water diffusion, while also increasing the structural rigidity of the composite.
To test the effectiveness of the PMMA/PU IPN-CeO₂ composite as an encapsulation layer for PSC devices, we coated the composite onto PSC and PSC-Si tandem solar cells. The long-term stability of the encapsulated devices was evaluated under accelerated aging conditions of 85°C and 85% relative humidity (85-85 conditions). The power conversion efficiency (PCE) of the encapsulated PSCs showed only a 17% decrease after 816 hours of continuous testing, demonstrating the remarkable moisture-blocking and stability-enhancing capabilities of the IPN-CeO₂ composite. This performance is significantly enhanced compared to the rapid degradation typically observed in unencapsulated or conventionally encapsulated PSC devices under similar conditions.
These findings demonstrate the promise of PMMA/PU IPN composites with CeO₂ nanoparticles as highly effective encapsulation materials for PSCs. By harnessing the benefits of interpenetrating polymer networks and hydrophobic nanomaterials, this approach presents a viable solution for enhancing the long-term stability and commercialization potential of PSC devices.