Abstract
Optimizing upconversion (UC) composites is challenging as numerous effects influence their unique emission mechanism. Low scattering mediums increase the number of dopants excited, however, high scattering mediums increase the UC efficiency due to its non-linear power dependency. Scattering also leads to greater thermal effects and emission saturation at lower excitation power density (PD). In this work, a photoluminescence quantum yield (PLQY) increase of 270% was observed when hexagonal NaYF4:(18%)Yb3+,(2%)Er3+ phosphor is in air compared to a refractive index-matched medium. Furthermore, the primary inner-filter effect causes a 94% PLQY decrease when the excitation focal point is moved from the front of the phosphor to 8.4 mm deep. Increasing this effect limits the maximum excitation PD, reduces thermal effects, and leads to emission saturation at higher excitation PDs. Additionally, self-absorption decreases the PLQY as the phosphor’s thickness increases from 1 to 9 mm. Finally, in comparison to a cuboid cuvette, a 27% PLQY increase occurs when characterizing the phosphor in a cylindrical cuvette due to a lensing effect of the curved glass, as supported by simulations. Overall, addressing the effects presented in this work is necessary to both maximize UC composite performance as well as report their PLQY more reliably.
Highlights
Optimizing upconversion (UC) composites is challenging as numerous effects influence their unique emission mechanism
At the lowest investigated power density (PD), the internal PLQY (iPLQY) was increased by 270% compared to the refractive index (RI)-matched medium
This disparity decreases as the PD increases; the PD experienced by the sample influences both the rate of emission saturation and the magnitude of excitation induced thermal effects15,16,28. iPLQY saturation occurs at lower PDs in a high scattering medium because it experiences a greater local PD because of scattering
Summary
Optimizing upconversion (UC) composites is challenging as numerous effects influence their unique emission mechanism. The primary inner-filter effect causes a 94% PLQY decrease when the excitation focal point is moved from the front of the phosphor to 8.4 mm deep Increasing this effect limits the maximum excitation PD, reduces thermal effects, and leads to emission saturation at higher excitation PDs. self-absorption decreases the PLQY as the phosphor’s thickness increases from 1 to 9 mm. UC materials possess a non-linear dependence on excitation power due to the multi-photon absorption process of UC mechanisms This adds an additional layer of complexity to PLQY characterization p rocedures[15]. Our results yield a deeper understanding of how these effects can be addressed to optimize UC materials and increase reliability in their PLQY measurements The latter is crucial due to the comparability challenges associated with characterizing UC materials
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