Insights into the energy transfer mechanism in ${\mathrm{Ce}}^{3+}-{\mathrm{Yb}}^{3+}$ codoped YAG phosphors

Two distinct energy transfer (ET) mechanisms have been proposed for the conversion of blue to near-infrared (NIR) photons in YAG:${\mathrm{Ce}}^{3+},{\mathrm{Yb}}^{3+}$. The first mechanism involves downconversion by cooperative energy transfer, which would yield two NIR photons for each blue photon excitation. The second mechanism of single-step energy transfer yields only a single NIR photon for each blue photon excitation and has been argued to proceed via a ${\mathrm{Ce}}^{4+}-{\mathrm{Yb}}^{2+}$ charge transfer state (CTS). If the first mechanism were operative in YAG:${\mathrm{Ce}}^{3+},{\mathrm{Yb}}^{3+}$, this material would have the potential to greatly increase the response of crystalline Si solar cells to the blue/UV part of the solar spectrum. In this work, however, we demonstrate that blue-to-NIR conversion in YAG:${\mathrm{Ce}}^{3+},{\mathrm{Yb}}^{3+}$ goes via the single-step mechanism of ET via a ${\mathrm{Ce}}^{4+}-{\mathrm{Yb}}^{2+}$ CTS. The photoluminescence decay dynamics of the ${\mathrm{Ce}}^{3+}$ excited state are inconsistent with Monte Carlo simulations of the cooperative (one-to-two photon) energy transfer, while they are well reproduced by simulations of single-step (one-to-one photon) energy transfer via a charge transfer state. Based on temperature dependent measurements of energy transfer and luminescence quenching we construct a configuration coordinate model for the ${\mathrm{Ce}}^{3+}$-to-${\mathrm{Yb}}^{3+}$ energy transfer, which includes the ${\mathrm{Ce}}^{4+}-{\mathrm{Yb}}^{2+}$ charge transfer state.

Figure 1

The AM1.5D solar spectrum (yellow curve) illustrating the fraction that can directly be converted by crystalline Si solar cells (green area), and the additional solar power that can be utilized through 1-to-2 downconversion (blue area) and 2-to-1 upconversion (red area).

Figure 2

(a) Excitation spectra by monitoring the ${\mathrm{Ce}}^{3+}$: ${5d}^1$ → $4f$ emission at 570 nm and the ${\mathrm{Yb}}^{3+}$: ${}^2{F}_{5/2}$ → ${}^2{F}_{7/2}$ emission at 1029 nm in YAG:${\mathrm{Ce}}^{3+},{\mathrm{Yb}}^{3+}$. (b) Visible-to-NIR emission spectra of YAG:${\mathrm{Ce}}^{3+}$ codoped with various ${\mathrm{Yb}}^{3+}$ concentrations under excitation of ${\mathrm{Ce}}^{3+}$ at 455 nm.

Figure 3

Photoluminescence decay curves of the ${\mathrm{Ce}}^{3+}$: ${5d}^1$ → $4f$ emission at 530 nm in YAG:1% ${\mathrm{Ce}}^{3+},x$% ${\mathrm{Yb}}^{3+}$ ($x$ = 1, 2, 5, 10, 20). The experimental data (dots) are measured upon pulsed laser excitation at 445 nm. Solid lines in (a) show the results of a fit to the model of a single-step energy transfer mechanism via a CTS [Eq. (5)], (b) shows the results of a fit to single-step energy transfer via dipole-dipole coupling [Eq. (4)], while (c) shows the results of a fit to a cooperative energy transfer mechanism [Eq. (6)]. Insets show the quality of the fits.

Figure 4

Average decay rate of the ${\mathrm{Ce}}^{3+}$ $5d^1$ excited state as a function of ${\mathrm{Yb}}^{3+}$ concentration in YAG. The solid line shows the linear trend as expected for a single-step energy transfer mechanism, while the dashed line represents a quadratic trend as expected for a cooperative energy transfer mechanism.

Figure 5

Temperature dependent visible-to-NIR emission spectra (a) and decay curves of ${\mathrm{Ce}}^{3+}$: ${5d}^1$ → $4f$ at 530 nm (b) in YAG:1% ${\mathrm{Ce}}^{3+}$,5% ${\mathrm{Yb}}^{3+}$. The inset of (a) shows the temperature dependence of the integrated luminescence intensities of the ${\mathrm{Ce}}^{3+}$: $5d^1$ → $4f$ and the ${\mathrm{Yb}}^{3+}$: ${}^2{F}_{5/2}$ → ${}^2{F}_{7/2}$ emission.

Figure 6

${\mathrm{Yb}}^{3+}$-concentration dependent luminescence decay curves of ${\mathrm{Yb}}^{3+}$: ${}^2{F}_{5/2}$ → ${}^2{F}_{7/2}$ at 1029 nm under pulsed laser excitation of ${\mathrm{Ce}}^{3+}$ at 441.4 nm.

Figure 7

Configuration coordinate model showing the $4f^1$ and $5d$ states of ${\mathrm{Ce}}^{3+}$, the $4f^{13}$ state of ${\mathrm{Yb}}^{3+}$ (solid parabolas) and the ${\mathrm{Ce}}^{4+}-{\mathrm{Yb}}^{2+}$ CTS (dashed parabolas), illustrating the single-step energy transfer via a ${\mathrm{Ce}}^{4+}-{\mathrm{Yb}}^{2+}$ CTS and the possible pathways of temperature quenching. Entirely nonradiative return to the ground state after excitation of ${\mathrm{Ce}}^{3+}$ to its $5d$ state is possible via the CTS, followed by intersystem crossing over barrier $\Delta E_3$. As it is hard to depict the ${\mathrm{Ce}}^{4+}-{\mathrm{Yb}}^{2+}$ CTS for both ions in a single diagram, the configuration coordinate diagrams of ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Yb}}^{3+}$ are depicted separately.