Enhanced Quantum Cutting via Li⁺ Doping from a Bi³⁺/Yb³⁺-Codoped Gadolinium Tungstate Phosphor

The Bi³⁺/Yb³⁺-codoped gadolinium tungstate phosphor has been synthesized through a solid-state reaction method. The structural characterization reveals the crystalline nature of the phosphor. The Bi³⁺-doped phosphor emits visible radiation from the blue to red regions upon excitation with 330 and 355 nm. The addition of Yb³⁺ to the Bi³⁺-doped phosphor reduces the emission intensity in the visible region and emits an intense near-infrared (NIR) photon centered at 976 nm through a quantum-cutting (QC) phenomenon. This is due to cooperative energy transfer (CET) from the ³P₁ level of Bi³⁺ to the ²F_5/2 level of Yb³⁺. The presence of Li⁺ ions in the Bi³⁺/Yb³⁺-codoped phosphor enhances the emission intensity in the NIR region up to by 3 times, whereas the emission intensity in the visible region is significantly reduced. The energy transfer (ET) from the Bi³⁺ ions to the Yb³⁺ ions is confirmed by lifetime measurements, and the lifetime for the ³P₁ level of Bi³⁺ decreases continuously with increasing Yb³⁺ concentration. The ET efficiency (η_ETE) and corresponding QC efficiency (η_QE) are calculated and found to be 29% and 129%, respectively. The presence of Li⁺ enhances the QC efficiency of the phosphor up to 43%. Thus, the Bi³⁺/Yb³⁺/Li⁺-codoped phosphor is a promising candidate to enhance the efficiency of a crystalline-silicon-based solar cell through spectral conversion.