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

Abstract

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 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 3PJ (J = 0, 1, 2) levels with blue light, followed by absorption of a second blue photon thus populating the 4f15d1 conguration 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, oxyuorides, garnets, silicates and borates are activated with Pr3+ to understand the relation between ESA-based UC eciency, the energy and congurational onset of the 4f15d1 states as well as the excited-state dynamics. For that purpose, quantum yield measurements, 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 those conditions are particularly well fullled in the Pr3+-activated chloridoelpasolite Cs2NaYCl6:Pr3+, which shows the highest up-conversion quantum yield (ΦUC = 0.11%, P = 0.59Wcm-2) among all investigated compounds within this work and even surpasses the efficiency of well-known upconverters in that 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.