Photoluminescence of rare-earth ion (Eu³⁺, Tm³⁺, and Er³⁺)-doped and co-doped ZnNb₂O₆ for solar cells

Visible converted emissions produced at an excitation of 286 nm in ZnNb₂O₆ ceramics doped with rare-earth ions (RE = Eu³⁺, Tm³⁺, Er³⁺ or a combination of these ions) were investigated with the aim of increasing the photovoltaic efficiency of solar cells. The structure of RE:ZnNb₂O₆ ceramics was confirmed by x-ray diffraction patterns. The undoped ZnNb₂O₆ could emit a blue emission under 286-nm excitation, which is attributed to the self-trapped excitons' recombination of the efficient luminescence centers of edge-shared NbO₆ groups. Upon 286-nm excitation, Eu:ZnNb₂O₆, Tm:ZnNb₂O₆, and Er:ZnNb₂O₆ ceramics showed blue, green, and red emissions, which correspond to the transitions of ⁵D₀→⁷F_J (J = 1-4) (Eu³⁺), ¹G₄ → ³H₆ (Tm³⁺), and ²H_11/2/⁴S_3/2 → ⁴I_15/2 (Er³⁺), respectively. The calculated CIE chromaticity coordinates of Eu:ZnNb₂O₆, Tm:ZnNb₂O₆, and Er:ZnNb₂O₆ are (0.50, 0.31), (0.14, 0.19), and (0.29, 0.56), respectively. RE ionco- doped ZnNb₂O₆ showed a combination of characteristic emissions. The chromaticity coordinates of Eu/Tm:ZnNb₂O₆, Eu/Er:ZnNb₂O₆, and Tm/Er:ZnNb₂O₆ were calculated to be (0.29, 0.24), (0.45, 0.37), and (0.17, 0.25).