Strong self-sensitized green and NIR emission in NaYS2 doped with Pr3+ and Yb3+ by inducing Laporte allowed and charge transfer transitions
Down conversion of solar UV and blue light is one of the few ways of surpassing the Shockley-Queisser limit of solar cells. One of the most efficient down conversion systems is the Pr3+ – Yb3+ lanthanide pair. However, these ions do not provide any strong absorption for solar UV and blue light, thus require sensitizing. In this work, we report self-sensitization by inducing strong Laporte allowed f → d and charge transfer transitions on Pr3+ and Yb3+, respectively. By replacing oxygen or fluorine anions with the more polarizable and reducible S2−, the chemical bonds become more covalent, the d-shell of Pr3+ and the anion-to-Yb3+ charge transfer states are lowered into the near-UV and blue range. This avoids the need for a third strongly absorbing specie that may be difficult to include into the matrix or might provide other quenching routes by, for example, introducing new defects. The down conversion system can thus be kept simple, without more dopants types than the two lanthanides. The straightforward two-step synthesis of NaYS2 is realized without toxic H2S, and both precursor chemicals and produced material can be handled in air. The optical and chemical properties of NaYS2, no need for an additional sensitizer and easy synthesis are strong arguments that the little explored NaYS2-type materials deserves more attention.