Mn–Yb co-doped CaGdAlO4 nanoparticles as efficient blue-to-NIR converters for photovoltaic enhancement

Nano-sized Mn–Yb doped CaGdAlO4 powder samples were synthesized using an auto-combustion sol-gel method at a relatively low temperature of 900 °C, which is significantly lower than the temperatures typically reported for such processes, offering a cost-effective and energy-saving route. Nanoparticles with sizes ranging from 14 to 20 nm were successfully formed. Such nanoscale dimensions not only induce a blue shift in the optical absorption edge but also reduce light scattering. The samples were prepared with a fixed Mn concentration of 0.04 mol. % and varying Yb concentrations (0%, 0.1%, 0.3%, and 0.5 mol. %). Under 445 nm of excitation, a strong photoluminescence (PL) emission at 675 nm was observed in the Mn-doped sample, attributed to the excitation of Mn4+ ions from the 4A2 to 4T2 states, followed by a non-radiative relaxation to the 2Eg level and a radiative transition to the 4A2 ground state. In Yb-doped samples, an additional intense near-infrared (NIR) PL signal appeared at 930 nm, arising from energy transfer from the 2Eg state of Mn4+ to the 2F5/2 level of Yb3+, followed by the 2F5/2 → 2F7/2 transition. The rate of energy transfer (RET) between Mn and Yb ions increased with Yb content, indicating a dipole–dipole interaction mechanism. The sample with 0.3 mol. % Yb content exhibited optimal downconversion performance, effectively converting blue light to NIR emission. These findings highlight the potential of Mn–Yb doped CaGdAlO4 nanoparticles as efficient spectral converters for photovoltaic applications, particularly as optical windows for enhancing the spectral response of Si-based solar cells.