Plasmonic nanoparticles, particularly gold (Au) and silver (Ag), have emerged as powerful tools for enhancing solar energy harvesting due to their unique ability to concentrate light at the nanoscale. This capability arises from localized surface plasmon resonance (LSPR), a phenomenon where conduction electrons oscillate coherently in response to incident light, generating strong electromagnetic fields near the nanoparticle surface. By leveraging LSPR, plasmonic nanoparticles can significantly improve light absorption in solar energy systems, enabling higher efficiencies and novel device architectures.

The primary mechanism by which plasmonic nanoparticles enhance solar energy harvesting is light trapping. When integrated into solar cells or photoelectrochemical systems, these nanoparticles scatter and localize light, increasing the effective optical path length within the active material. For instance, in thin-film solar cells, where absorber layers are often too thin to fully capture sunlight, plasmonic nanoparticles can compensate by concentrating light into the semiconductor. This effect is particularly pronounced at specific wavelengths corresponding to the LSPR peak, which can be tuned by adjusting nanoparticle size, shape, and composition. Spherical Au nanoparticles typically exhibit LSPR in the visible range (520–580 nm), while Ag nanoparticles can achieve resonance at shorter wavelengths (400–450 nm). Anisotropic structures like nanorods or nanostars further extend tunability into the near-infrared, broadening the spectral range for light harvesting.

Hybrid designs combining plasmonic nanoparticles with semiconductors represent a promising strategy for enhancing charge generation and separation. In such systems, the plasmonic near-field enhances absorption in the semiconductor, while the semiconductor itself provides a pathway for charge extraction. For example, Au nanoparticles deposited on titanium dioxide (TiO2) have been shown to boost photocurrent in dye-sensitized solar cells by enhancing light absorption in the dye molecules. Similarly, Ag nanoparticles embedded in silicon solar cells can increase photocurrent by scattering light into the silicon layer. The coupling between plasmonic nanoparticles and semiconductors can also lead to direct electron transfer via hot electron injection. When plasmonic nanoparticles absorb light, they generate high-energy (hot) electrons that can be injected into the conduction band of an adjacent semiconductor, contributing to the photocurrent. This process is particularly efficient in systems with strong interfacial coupling, such as Au-TiO2 or Ag-CdSe heterostructures.

Hot electron generation is another critical aspect of plasmon-enhanced solar energy harvesting. The decay of LSPR can produce hot electrons with energies above the Fermi level, which can be harvested before thermalization. This mechanism bypasses the conventional charge separation process, offering a route to overcome the Shockley-Queisser limit in solar cells. However, hot electron extraction requires careful design of the nanoparticle-semiconductor interface to minimize energy losses. Thin insulating layers or molecular linkers are often employed to facilitate electron tunneling while preventing recombination. For instance, a thin alumina (Al2O3) spacer between Au nanoparticles and TiO2 has been shown to enhance hot electron transfer efficiency by reducing backscattering.

Despite these advantages, plasmonic nanoparticles also introduce parasitic losses that can offset their benefits. Ohmic losses due to electron-phonon scattering dissipate some of the absorbed light as heat, reducing the overall energy conversion efficiency. Additionally, light scattered by nanoparticles may escape the device rather than being absorbed, leading to optical losses. The trade-off between optical enhancement and parasitic losses depends on factors such as nanoparticle density, placement, and the surrounding dielectric environment. Optimized designs often involve core-shell nanostructures, where a plasmonic core is coated with a dielectric or semiconductor shell to balance light trapping and losses. For example, Au-silica core-shell nanoparticles exhibit reduced ohmic losses compared to bare Au nanoparticles while maintaining strong light scattering. Similarly, Ag-TiO2 core-shell structures combine plasmonic enhancement with photocatalytic activity, making them suitable for photoelectrochemical applications.

Applications of plasmonic nanoparticles in solar energy systems span three main categories: solar cells, photoelectrochemical cells, and solar thermal systems. In solar cells, plasmonic nanoparticles are integrated into the active layer or as back reflectors to enhance light absorption. Perovskite solar cells, for instance, have achieved efficiency improvements by incorporating Ag nanoparticles into the hole transport layer. Photoelectrochemical cells benefit from plasmonic enhancement in both light absorption and catalytic activity. Au nanoparticles on bismuth vanadate (BiVO4) photoanodes have demonstrated enhanced water oxidation performance due to improved charge separation and reduced recombination. In solar thermal systems, plasmonic nanoparticles act as nanoheaters, converting sunlight into thermal energy with high efficiency. Ag nanoparticles dispersed in a heat transfer fluid can achieve localized heating for applications such as steam generation or desalination.

Recent advances in core-shell nanostructures have further optimized the performance of plasmonic nanoparticles for solar energy harvesting. For example, Au-core, TiO2-shell nanoparticles combine strong plasmonic effects with the photocatalytic properties of TiO2, enabling dual functionality in photoelectrochemical cells. Similarly, Ag-core, silica-shell nanoparticles minimize ohmic losses while maintaining high scattering efficiency. Multilayered designs, such as Au-silica-Au nanoshells, offer additional tunability of the LSPR peak, allowing precise matching to the solar spectrum. These structures have been employed in thin-film solar cells to achieve broadband light trapping.

The choice between Au and Ag nanoparticles depends on the specific application and operating conditions. Ag nanoparticles generally exhibit stronger plasmonic effects and lower ohmic losses in the visible range, making them ideal for light trapping in solar cells. However, they are prone to oxidation under ambient conditions, which can degrade performance over time. Au nanoparticles, while more stable, suffer from higher ohmic losses but are preferred for applications requiring long-term durability or operation in harsh environments. Alloying Au and Ag to form bimetallic nanoparticles can combine the advantages of both materials, offering tunable LSPR and improved stability.

In summary, plasmonic nanoparticles provide a versatile platform for enhancing solar energy harvesting through LSPR-driven light trapping, hot electron generation, and hybrid designs with semiconductors. While parasitic losses pose challenges, optimized core-shell nanostructures and careful system design can mitigate these effects. Applications in solar cells, photoelectrochemical cells, and solar thermal systems demonstrate the broad potential of plasmonic nanoparticles to advance renewable energy technologies. Future research will likely focus on further reducing losses, improving hot electron extraction efficiency, and developing scalable fabrication methods for large-scale deployment.