Short-Wave Infrared Emissions from Te4+–Ln3+ (Ln: Er, Yb)-Codoped Cs2NaInCl6 Double Perovskites

A Cs2NaInCl6 double perovskite is environmentally benign, and its wide band gap (∼5.1 eV) makes it photoinactive and photostable in the ultraviolet, visible, and short-wave infrared (SWIR) regions. Interestingly, the octahedrally coordinated In3+ lattice site is suitable for doping lanthanide ions like Er3+ and Yb3+, which can emit SWIR radiation at 1540 nm (0.805 eV) and 994 nm (1.247 eV), respectively. But the optical excitation of lanthanides is Laporte forbidden, and the host requires excitation energy >5.1 eV. The large Stokes shift for the excitation and SWIR emission reduces the power conversion efficiency. Here, we codoped Te4+ with Er3+ or Yb3+ into Cs2NaInCl6. Te4+ absorbs at the sub-band-gap level at around 3.1 eV (400 nm) because of 5s2 → 5s15p1 electronic transitions. Then, the excited Te4+ transfers its energy nonradiatively to an Er3+ or Yb3+ codopant. The de-excitation of Er3+ or Yb3+ through f–f electronic transitions emits SWIR radiation at 1540 and 994 nm, respectively, along with weak visible-light emissions. Temperature (8–300 K) dependent photoluminescence excitation, emission, and lifetime measurements reveal the mechanism of these energy transfer processes. Finally, we fabricated a simple phosphor-converted light-emitting diode (pc-LED) emitting SWIR radiation.