A nanocomposite of metal nanoclusters/OSTE is fabricated through off-stoichiometric thiol-ene polymerization, incorporating adamantanethiol-protected electrum nanoclusters Au_23-xAg_x(SAdm)₁₅ (where x = 7.44) along with the OSTE monomer. During the photopolymerization, there is a transforfation of the precursor nanoclusters and the nanocomposite achieves a maximum photoluminescence quantum yield of ≈73% at 740 nm and 60% at the 850 nm emission peak. The photophysical characteristics of nanocomposite AuAgNCs@OSTE are examined at both ambient and low temperatures, revealing an improved radiative recombination mechanism through the interactions with polymer radicals. This high photoluminescence quantum yield near-infrared-emitting AuAgNCs@OSTE material, distinguished by a larger Stokes shift, is utilized to fabricate luminescent solar concentrators measuring 5 × 5 × 0.13 cm³. Experimental measurements are conducted to determine the absorption coefficient, reabsorption coefficient, absorption cross-section, and volume concentration of the device. Additionally, theoretical evaluations of waveguiding efficiency and power conversion efficiency are performed and compared with quantum dot-based alternatives. The findings indicate that the metal NCs@OSTE nanocomposite has the potential to function as a highly efficient, heavy-metal-free nanophosphor, demonstrating superior overall performance for semi-transparent luminescent solar concentrator devices and being suitable for a broad range of light conversion applications in the NIR spectrum.