Two distinct energy transfer (ET) mechanisms have been proposed for the conversion of blue to near-infrared (NIR) photons in YAG:Ce3+,Yb3+. 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 Ce4+-Yb2+ charge transfer state (CTS). If the first mechanism were operative in YAG:Ce3+,Yb3+, 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:Ce3+,Yb3+ goes via the single-step mechanism of ET via a Ce4+-Yb2+ CTS. The photoluminescence decay dynamics of the Ce3+ 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 Ce3+-to-Yb3+ energy transfer, which includes the Ce4+-Yb2+ charge transfer state
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