Enhanced Vis-to-UVC Upconversion in Pr3+-Doped A3(BO3)2 (A = Sr, Ca) under 444 nm Excitation: Suppression of Cluster Formation via Alkali Ion Co-Doping

Upconverting materials represent a diverse and significant class of luminophores, with those capable of converting visible (vis) light into ultraviolet C (UVC) playing a particularly important role. This is due to the germicidal properties of UVC radiation, which is widely used for disinfection, sterilization, and purification applications. In this study, we investigate the vis-to-UVC upconversion process in A3(BO3)2:Pr3+ (A = Sr, Ca) compounds, with a particular focus on the effects of Li+ and Na+ co-doping as charge compensation. Our findings reveal that Na+ co-doping significantly enhances upconversion luminescence in both Sr3(BO3)2 (SBO) and Ca3(BO3)2 (CBO). Moreover, the co-doped samples exhibited improved crystallinity as a result of the flux effect. Interestingly, we observed no concentration quenching of the 1D2 level in singly doped samples, while co-doping with Na+ led to a reduction in 1D2 decay time with increasing Pr3+ content. Numerical simulations based on a multilevel model revealed that when cross-relaxation (CR) dominates over radiative decay, as in clustered Pr3+ ions, concentration quenching effects on decay time are minimal. Co-doping with Na+ or Li+ suppresses cluster formation, reducing nonradiative losses and improving upconversion efficiency. A more pronounced effect was observed for Na+ ions, attributed to their closer ionic radius match with the substituted Sr2+ and Ca2+ sites in the host lattice. Overall, this study provides a detailed analysis on the influence of CR processes on the decay kinetics of the 1D2 level in compounds with high energy of phonons. Furthermore, the obtained results bring valuable insight into the roles of charge compensation and cluster formation in Vis-to-UVC upconversion materials, an area that remains largely underexplored.