Design principles for (efficient) excited-state absorption-based blue-to-UV upconversion phosphors with Pr³

UV light generation is generally not very efficient, expensive, or may even require toxic elements such as mercury. In contrast, blue light (λ = 450 nm) is cheaply available from semiconductor LEDs and its use in phosphor-converted LEDs is technologically mature and could be envisioned as an intense, sustainable light source in an upconversion scheme. The electronic energy level landscape of the 4f² ion Pr³⁺ does allow such a blue-to-UV upconversion (UC) by resonantly exciting the ³P _J (J = 0, 1, 2) levels with blue light, followed by absorption of a second blue photon, thus populating the 4f¹5d¹ configuration states located in the UV range. While the second absorption step is expected to be efficient based on selection rules, no clear guidelines on how to optimize the expected upconversion efficiency for Pr³⁺ by appropriate choice of a surrounding host are known up to now. Within this work, selected halidoelpasolites, oxyfluorides, garnets, silicates and borates are activated with Pr³⁺ to understand the relation between ESA-based UC efficiency, the energy and configurational offset of the 4f¹5d¹ states as well as the excited-state dynamics. For that purpose, quantum yield measurements, as well as steady-state, time-resolved and temperature-dependent luminescence spectroscopy with different excitation sources and powers are combined. It turns out that several parameters must be carefully mutually matched within a host compound for efficient ESA-based blue-to-UV UC with Pr³⁺. Not only does the decay time of the intermediate ³P₀ level have to be particularly long in an excited-state absorption upconversion scheme, but also the non-radiative crossover from the excited 4f¹5d¹ states needs to be limited. All these conditions are particularly well fulfilled in the Pr³⁺-activated chloridoelpasolite Cs₂NaYCl₆:Pr³⁺, which shows the highest upconversion quantum yield (Φ _UC = 0.11%, P = 0.59 W cm^-2) among all investigated compounds within this work and even surpasses the efficiency of well-known upconverters in this field such as Lu₃Al₅O₁₂:Pr³⁺ (LuAG:Pr³⁺) or β-Y₂Si₂O₇:Pr³⁺ (YPS:Pr³⁺). The relatively high efficiency of this compound compared to the other standards is a consequence of its low cut-off phonon energy and rigid, densely packed structure with large mutual distances between the rare-earth ions.