CuInS2/ZnS (CIS/ZnS) quantum dots (QDs) are extensively employed as fluorophores in luminescent solar concentrators (LSCs), due to their excellent optical properties and low toxicity. However, the power conversion efficiency (PCE) of LSCs employing CIS/ZnS QDs is significantly constrained by the energy losses from the overlap between the absorption and emission spectra. Herein, we propose a novel and facile approach to enlarge the Stokes shift of CIS/ZnS QDs using the off-stoichiometric thiol–ene (OSTE) polymers. Significantly, the photoluminescence (PL) spectrum of CIS/ZnS QDs in OSTE experienced a substantial redshift of 65 nm without compromising the PLQY. This resulted in a remarkably enlarged Stokes shift of 585 meV and a minimal spectral overlap integral of 0.126, marking the first instance of a nanocomposite featuring CIS/ZnS QDs with such an enlarged Stokes shift. Further investigation demonstrates that thiol monomers, characterized by a strong electron-donating property, altered the Cu+ defect energy states in CIS QDs, resulting in a significant redshift in the PL spectrum. Consequently, a certified record PCE of 1.36% was achieved for a large-area (29 × 29 cm2) LSC device with an average visible transmittance (AVT) of 51%, representing the highest PCE value for such devices. Our findings not only present a viable strategy for augmenting the efficiency of CIS/ZnS QD-based LSCs for building-integrated photovoltaics (BIPV) but also establish a new avenue for controlling the optical properties of QD/polymer nanocomposites for other optoelectronic devices.