Spectroscopic properties and quantum cutting in Tb³⁺-Yb ³⁺ co-doped ZrO₂ nanocrystals

Ultraviolet-visible to near-infrared quantum cutting (QC) materials are a promising tool to enhance the efficiency of conventional crystalline silicon solar cells. The spectroscopic properties of Tb³⁺-Yb³⁺ co-doped ZrO₂ nanocrystals are presented, and the QC mechanisms in these nanocrystals are investigated. The materials were fabricated using the sol gel method and characterized using X-ray powder diffraction, X-ray absorption near edge structure, and luminescence spectroscopy. The incorporation of Yb ³⁺ ions into the host induced a crystalline phase change of ZrO ₂ from monoclinic to tetragonal to cubic symmetry and influenced the Tb valence state. The Tb³⁺ visible emission, excitation intensity (monitored by the Tb³⁺:⁵D₄ emission), decay time of the Tb³⁺:⁵D₄ emitter level, and down-conversion (DC) emission intensity increased with Yb³⁺ concentration. Furthermore, a sublinear dependence of the DC intensity on the excitation power at the Tb³⁺:⁵D₄ level indicated the coexistence of two different QC mechanisms from Tb³⁺ → Yb³⁺. The first one is a linear process in which one Tb ³⁺ ion transfers its energy simultaneously to two Yb³⁺ ions, known as cooperative energy transfer, and the second one is a non-linear process involving an intermediated virtual level in the Tb³⁺ ion.