<p>Thin films of nano-structure pure ZnO and co-doped with 0.5&#xa0;mol% Er³⁺, and different concentrations (0.5, 1, and 1.5&#xa0;mol%) of Yb³⁺ ions, coded as S, S1, S2, and S3, respectively, have been prepared. Afterwards, the prepared samples were sintered at 500&#xa0;°C for three hours in air. ZnO films have a wide direct band gap of 3.37&#xa0;eV and a single-phase hexagonal wurtzite polycrystalline structure. They were prepared using the seed solution spin coating sol–gel method (SSSCSGM). By XRD analysis, the presence of the nano-structured phase was revealed. Over a broad spectrum, including the visible region, optical transparency is demonstrated by spectrophotometry. The produced samples have been characterized using different spectroscopic techniques, including Raman confocal microscope (RCM) and photoluminescence (PL). The latter was used to detect the PL in both directions (up and down-conversion) utilizing a 635&#xa0;nm excitation laser source. It was observed that the prepared samples exhibited green up-conversion emission at 350 and 493 nm, as well as red down-conversion emission at 1274&#xa0;nm, due to the efficient energy transfer process between Er³⁺ and Yb³⁺ ions. It is proven that (S3) had the highest up-conversion from the edge of the NIR to the middle of the visible region, due to a spectral overlap energy transfer between the&#xa0;²F_5/2&#xa0;band of Yb³⁺&#xa0;and the&#xa0;⁴I_11/2&#xa0;band of Er³⁺ ions. After depositing the S3 sample at the front of silicon solar cell-based devices, the efficiency of collecting solar cell energy increased during the experiment, rising from 10.5% (before S3 deposition) to 13.1% (after S3 deposition), respectively. Consequently, the S3 film offers the advantage of being extremely easy and reasonably priced to manufacture, making it a viable option for Si-solar cells as an energy-conversion layer.</p>