Role of electron transfer in Ce³⁺ sensitized Yb³⁺ luminescence in borate glass

In a Ce³⁺-Yb³⁺ system, two mechanisms are proposed so far namely, the quantum cutting mechanism and the electron transfer mechanism explaining Yb³⁺ infrared luminescence under Ce³⁺ excitation. Among them, the quantum cutting mechanism, where one Ce³⁺ photon (ultraviolet/blue) gives rise to two Yb³⁺ photons (near infrared) is widely sought for because of its huge potential in enhancing the solar cell efficiency. In present study on Ce³⁺-Yb³⁺ codoped borate glasses, Ce³⁺ sensitized Yb³⁺ luminescence at ∼1 μm have been observed on Ce³⁺ 5d state excitation. However, the intensity of sensitized Yb³⁺ luminescence is found to be very weak compared to the strong quenching occurred in Ce³⁺ luminescence in Yb³⁺ codoped glasses. Moreover, the absolute luminescence quantum yield also showed a decreasing trend with Yb³⁺ codoping in the glasses. The overall behavior of the luminescence properties and the quantum yield is strongly contradicting with the quantum cutting phenomenon. The results are attributed to the energetically favorable electron transfer interactions followed by Ce³⁺-Yb³⁺ ⇌ Ce⁴⁺-Yb²⁺ inter-valence charge transfer and successfully explained using the absolute electron binding energies of dopant ions in the studied borate glass. Finally, an attempt has been presented to generalize the electron transfer mechanism among opposite oxidation/reduction property dopant ions using the vacuum referred electron binding energy (VRBE) scheme for lanthanide series.