Mutual energy transfer luminescent properties in novel CsGd(MoO4)2:Yb3+,Er3+/Ho3+ phosphors for solid-state lighting and solar cells

Abstract

In this work, we prepared a novel kind of Yb³⁺,Er³⁺/Ho³⁺ co-doped CsGd(MoO₄)₂ phosphors with a different structure from the reported ALn(MoO₄)₂ (A = Li, Na or K; Ln = La, Gd or Y) compounds using a high-temperature solid-state reaction method. X-ray diffraction showed that the as-prepared samples had a pure phase. Based on the efficient energy transfer from Yb³⁺ to Er³⁺/Ho³⁺, the up-conversion (UC) luminescence of the optimal CsGd(MoO₄)₂:0.30Yb³⁺,0.02Er³⁺ sample showed intensely green light with dominant emission peaks at 528 and 550 nm corresponding to Er³⁺ transitions ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2, respectively, as well as a weak emission peak originating from ⁴F_9/2 → ⁴I_15/2 at 671 nm, under 975 nm laser excitation. The CsGd(MoO₄)₂:Yb³⁺,Ho³⁺ samples mainly displayed two emission bands around 540 and 660 nm together with a negligible one at 755 nm, which corresponded to Ho³⁺ transitions ⁴F₄,⁵F₂ → ⁵I₈, ⁵F₅ → ⁵I₈ and ⁴F₄,⁵F₂ → ⁵I₇, respectively, under 975 nm laser excitation. With increasing Yb³⁺ concentration in CsGd(MoO₄)₂:Yb³⁺,Ho³⁺ phosphors, the emission color could be tuned from orange red to light yellow due to the large energy gap between levels ⁴F₄,⁵F₂ and ⁵F₅. In addition, the CsGd(MoO₄)₂:Yb³⁺,Er³⁺ showed green light under 376 nm UV irradiation similar to that upon 975 nm laser excitation. However, the emissions for CsGd(MoO₄)₂:Yb³⁺,Ho³⁺ samples under 358 nm UV or 449 nm blue excitation showed dominant emission peaks at 540 nm and weak 660 nm and 752 nm peaks, which were a bit different from those under 975 nm excitation. Interestingly, we observed efficient energy transfer phenomena (possible quantum cutting) from Er³⁺/Ho³⁺ to Yb³⁺ and a Yb³⁺–O²⁻ charge transfer (CT) transition in the molybdates, which was deduced from the visible and near-infrared emission spectra and the decrease of the Er³⁺/Ho³⁺ luminescent lifetimes with increasing Yb³⁺ concentration in the CsGd(MoO₄)₂:Yb³⁺,Er³⁺/Ho³⁺ samples. The luminescence properties of these phosphors suggest their potential possibility for applications in solid-state lighting and displays as well as in c-Si solar energy conversion systems.