We assessed the influence of Yb³⁺ and Er³⁺ dopant concentration on the relative spectral distribution, quantum yield (Φ_UC), and decay kinetics of the upconversion luminescence (UCL) and particle brightness (B_UC) for similarly sized (33 nm) oleate-capped β-NaYF₄:Yb³⁺,Er³⁺ upconversion (UC) nanoparticles (UCNPs) in toluene at broadly varied excitation power densities (P). This included an Yb³⁺ series where the Yb³⁺ concentration was varied between 11%-21% for a constant Er³⁺ concentration of 3%, and an Er³⁺ series, where the Er³⁺ concentration was varied between 1%-4% for a constant Yb³⁺
concentration of 14%. The results were fitted with a coupled rate
equation model utilizing the UCL data and decay kinetics of the green
and red Er³⁺ emission and the Yb³⁺ luminescence at 980 nm. An increasing Yb³⁺ concentration favors a pronounced triphotonic population of ⁴F_9/2 at high P by an enhanced back energy transfer (BET) from the ⁴G_11/2 level. Simultaneously, the Yb³⁺-controlled UCNPs absorption cross section overcompensates for the reduction in Φ_UC with increasing Yb³⁺ concentration at high P, resulting in an increase in B_UC. Additionally, our results show that an increase in Yb³⁺ and a decrease in Er³⁺ concentration enhance the color tuning range by P.
These findings will pave the road to a deeper understanding of the
energy transfer processes and their contribution to efficient UCL, as
well as still debated trends in green-to-red intensity ratios of UCNPs
at different P.