Review on deep red-emitting rare-earth free germanates and their efficiency as well as adaptability for various applications

Deep red-emitting phosphors are becoming the hot area of research during these times owing to their wide variety of applications in various fields, including lighting, display, imaging, and many others. The transition metal ions, especially Mn4+ and Cr3+ activated oxide phosphors possess admirable merits such as high chemical and thermal stability, eco-friendly preparation, and emission in the deep-red region. It is important to mention that Mn4+ doped germanates usually show emission spectra peaking above 650 nm. Thanks to this emissive property, germanates can be the best choice for display applications for providing a wide colour gamut in ultra-high-definition TV. Moreover, the emission of these phosphors can be extended to the near-infrared region with long persistence. Most of the reviews published on Mn4+ activated phosphors concentrate on the white light-emitting diode applications. Whereas the reviews on Cr3+ activated phosphors are focused on their persistent near-infrared emission. In this article, we mainly focused on the Mn4+ and Cr3+-activated deep red-emitting germanate phosphors since octahedral Ge4+ sites in germanates are ideal for accommodating these transition metal ions. We have thoroughly discussed the crystal structure-dependant luminescence behaviour of the reported germanates. In addition to that, we have also discussed various methods for improving the luminescence properties of these transition metal ions. Finally, we have presented possible potential applications of the reviewed phosphors. We believe that this article will give the insight to develop new deep red-emitting phosphors and it will be helping the researchers to improve the luminescence properties of different existing phosphors. After all, it will be good to make a better understanding of the reported work in this area in a short time.