Donor centers involved into the quantum cutting in ytterbium‐doped Scheelite‐like crystals

Yb‐doped Scheelite‐like molybdate and tungstate single crystals are promising quantum‐cutting materials. The cooperative downconversion from donor centers to Yb ions occurs in heavily Yb‐doped crystals of this family. This phenomenon can substantially raise the efficiency of crystalline silicon photovoltaic cells. However, the nature of donor centers is still unknown. Herein, several versions of this nature are tested and discussed: Yb${}^{3+}$ ion (its high‐energy charge transfer states); Yb${}^{2+}$ ion; self‐trapped exciton at molybdate/tungstate complex; color centers based on oxygen vacancies; accidental impurities. To test these assumptions, a series of Scheelite‐like molybdate and tungstate single crystals (CaMoO${}_4$, CaWO${}_4$, NaLa(MoO${}_4$)${}_2$, NaGd(MoO${}_4$)${}_2$, NaGd(WO${}_4$)${}_2$, and NaY${}_{1-x}$Gd${}_x$(WO${}_4$)${}_2$) are grown, both undoped and doped with different Yb‐concentrations. The complex spectroscopic investigations of these crystals are performed. As a result of the studies, three versions, self‐trapped excitons at [MoO${}_4$]/[WO${}_4$] complexes, color centers at oxygen vacancies, and accidental impurities, coming into the crystals with MoO${}_3$, are rejected. The applicability of the other versions should be refined. (© 2019 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)