Plasmonic Supraballs for Scalable Broadband Solar Energy Harvesting

Practical solar energy harvesting demands absorbers that combine large-area scalability with strong light-matter interactions spanning the visible to near-infrared (NIR) spectrum. Here, we introduce plasmonic colloidal supraballs─solution-processable assemblies of gold nanospheres (Au NSs)─as a robust and versatile platform for broadband solar energy harvesting. Unlike conventional dielectric supraballs or random Au NS aggregates, our supraballs integrate localized surface plasmon resonances (LSPRs) at the shell with multipolar Mie-type magnetic resonances in the core, achieving synergistic light trapping and near-complete solar absorption. Through confined colloidal self-assembly, we fabricated Au NS supraballs with tunable diameters (460 nm-3 μm) and high lattice fidelity. Despite structural imperfections inherent to spherical crystallization, the supraballs maintained broadband absorption, governed by interparticle capacitive coupling in the visible regime and effective medium behavior in the NIR. Both numerical simulations and Fourier-transform infrared spectroscopy confirmed that supraball films exhibit >90% absorption across the solar spectrum, with an average absorption of ∼88.8% under AM 1.5G illumination, which is nearly twice that of conventional Au NS films. To demonstrate practical utility, supraballs were directly drop-cast onto commercial thermoelectric generator (TEG) modules without any surface modification. The resulting supraball-coated TEG showed stable operation, rapid photoresponse, and a 2.4-fold enhancement in power output compared to Au NS-coated controls. These results establish plasmonic supraballs as a paradigm-shifting, solution-processable class of absorbers that seamlessly bridge nanoscale plasmonics and macroscopic solar energy harvesting, offering a scalable route toward next-generation solar thermoelectric, photothermal, and thermal management technologies.