<p>In this novel study, the influence of incorporating three types of plasmonic materials—monometallic Cu and Al nanoparticles, and bimetallic Cu–Al nanoparticles—on the performance of Si p–n junction solar cells was investigated. The modified Si p–n junction solar cells were prepared by depositing these plasmonic nanoparticles onto bare Si p–n junctions using a simple room-temperature ion reduction method, in which Cu and Al ions were reduced from a 0.05 M solution. The AlNPs/p–n, CuNPs/p–n, and Cu–AlNPs/p–n solar cells were synthesized by reducing ions from AlCl₃·6H₂O and CuSO₄·5H₂O solutions, as well as from a mixed Cu–Al solution with a 1:1 volumetric ratio. The structural and morphological characteristics of the modified solar cells were systematically analyzed using FE-SEM, AFM, XRD, and EDX measurements. The results revealed that all plasmonic nanoparticle-modified Si p–n junction solar cells exhibited enhanced photovoltaic performance compared to the bare cell. Among them, the Cu–AlNPs/p–n solar cell demonstrated the most significant improvement in photovoltaic parameters relative to both the bare and the monometallic (CuNPs and AlNPs) modified cells. This notable enhancement is mainly attributed to the effective incorporation of plasmonic nanoparticles—particularly the Cu–Al bimetallic system—which promotes broadband plasmonic resonance and consequently increases light absorption within the solar cell.</p>