Single-phase far-red-emitting ZnAl2O4:Cr3+ phosphor for application in plant growth LEDs

Single-phase far-red-emitting ZnAl2O4:Cr3+ phosphor has been successfully synthesized by a sol-gel method. The phosphor can be well excited by NUV, violet, and green lights, and it strongly emits multi-peak broadband emissions in the far-red region peaking at 687 and 698 nm, matching well with the absorption band of phytochrome. The maximum PL intensity is achieved for the ZnAl2O4:0.8%Cr3+ phosphor annealed at 1400 °C. The lifetime and activation energy of the optimal ZnAl2O4:0.8%Cr3+ phosphor annealed at 1400 °C are 25.3 ms and 0.302 eV, respectively. Three types of far-red-emitting LEDs have been successfully fabricated by coating the optimal ZnAl2O4:0.8%Cr3+ phosphor on the surface of NUV (395 nm), violet (410 nm), and green (510 nm) chips. The chromatic coordinates of the corresponding LEDs are (0.2990; 0.2199), (0.2725; 0.1594), (0.2703; 0.5743), respectively. The quantum efficiency of the ZnAl2O4:Cr3+ phosphor excited by different LEDs is calculated and reported for the first time. The obtained results indicate that the ZnAl2O4:Cr3+ phosphor has a high potential for pc-converted plant growth LED application.