Cooperative down-conversion of UV light in disordered scheelitelike Yb-doped NaGd(MoO4)2 and NaLa(MoO4)2 crystals

Concentration series of disordered scheelitelike Yb:NaGd(MoO₄)₂ and Yb:NaLa(MoO₄)₂ single crystals are grown by the Czochralski method. The actual concentrations of Yb³⁺ ions in the crystals are determined by optical-absorption spectroscopy. The luminescence of Yb³⁺ ions in these crystals in the region of 1 μm is studied under UV and IR excitation. In the case of UV excitation, this luminescence appears as a result of nonradiative excited state energy transfer from donor centers of unknown nature to ytterbium. The character of the concentration dependence of Yb³⁺ luminescence indicates that the energy transfer at high Yb concentrations occurs with active participation of a cooperative mechanism, according to which the excitation energy of one donor center is transferred simultaneously to two Yb³⁺ ions. In other words, the quantum yield of this transfer exceeds unity, which can be used to increase the efficiency of crystalline silicon (c-Si) solar cells.