Commercially available solar cells (e.g. CdS/CdTe, CIGS, c-Si) have a spectrally narrow absorption band when compared to the solar emission spectrum. The UV-wavelength response of a solar cell (SC) can be improved by the application of a luminescent down-shifting layer (LDSL) on the SC front side, permitting the conversion of short-wavelength photons to the longer wavelength photons better matching the SC’s absorption spectrum. The ideal down-shifting material must possess a large Stokes shift and have a high luminescence quantum yield. We propose the use of the LDSL containing ZnO nanoparticles of less than 5 nm in diameter able to absorb UV light (λ < 400 nm), where the solar cell spectral response (SR) is low, and re-emitting at longer wavelengths (λ > 425 nm), where the typical SC’s SR increases. ZnO nanoparticles were synthesized by a low energy cluster beam deposition (LECBD) technique and their luminescent properties were studied as a function of the oxygen partial pressure (OPP) applied during the deposition process. The stoichiometry and crystallinity of ZnO nanoparticles can be controlled via the adjustment of the OPP. It was also observed that there exists an optimal value of the oxygen pressure introduced during the LECBD process, which permits to obtain the highest visible photoluminescence emission, necessary for an efficient down-shifting. The yield of the down-shifting in ZnO nanoparticle layer was determined varying the excitation wavelength using the photoluminescence excitation technique.