Synthesis and Characterization of Rare-Earth-Doped Nanosilica from Rice Husk for Potential Luminescent Downshifting Applications in Photovoltaics

Abstract

The limited UV–visible photon harvesting capability of silicon solar cells motivates the development of luminescent down-shifting (LDS) materials capable of converting high-energy photons into wavelengths more effectively absorbed by Si. In this study, high-purity nanosilica extracted from five rice cultivars was doped with Dy³⁺, Eu³⁺, and Sm³⁺ to evaluate its suitability as a sustainable LDS material and benchmarked against TEOS-derived silica. The RH-derived silica exhibited high surface areas (>200 m²·g⁻¹) and nanoscale particle sizes (14–70 nm), enabling efficient dopant dispersion and enhanced optical interactions. Bandgap analyses revealed dopant-induced narrowing from ~5.7 eV to ~4.5 eV, confirming the introduction of energy levels that facilitate UV absorption and radiative recombination in the visible region. Photoluminescence measurements further demonstrated strong spectral conversion: Dy³⁺-doped samples produced blue–yellow emissions capable of near-white output (CCT 4,000–6,256 K), Eu³⁺-doped silica displayed intense red emission dominated by the hypersensitive ⁵D₀→⁷F₂ transition (CIE x = 0.627, y = 0.367), and Sm³⁺-doped samples generated tunable orange–red luminescence with concentration-dependent quenching. These emissions align closely with the external quantum efficiency (EQE) peak of Si (~450–650 nm), confirming their usefulness for LDS. Comparative optical studies showed that RH-derived phosphors perform as well as or better than TEOS-derived analogues, driven by defect-assisted sensitization and the highly porous silica network. These synergistic properties validate RE-doped RH nanosilica as a cost-effective, sustainable, and high-performing LDS material capable of enhancing silicon solar-cell spectral utilization, while demonstrating a viable pathway for converting agricultural waste into high-value photonic materials.