Novel rare-earth-free far-red-emitting phosphors, Zn2SnO4:3% Cr3+ (ZTO:Cr3+) and Zn2SnO4:3% Cr3+, 0.6% Al3+ (ZTO:Cr3+, Al3+), were synthesized using a modified high-energy planetary ball milling technique, followed by post-annealing at 1200°C in air. The phosphors presented emission spectra that matched well with plants’ far-red absorption peak of phytochrome pigment. The ZTO:Cr3+, Al3+ phosphor displayed a blueshift in the photoluminescence excitation (PLE) spectra and in the broadband photoluminescence (PL) emission peak compared to ZTO:Cr3+. Under blue light excitation, the ZTO:Cr3+, Al3+ phosphor demonstrated internal quantum efficiency, absorption efficiency, and external quantum efficiency of 83.5%, 40.2%, and 40%, respectively. The PL intensity decreased by only 18% when heated to 473 K, indicating strong thermal stability. A light-emitting diode (LED) device created by coating ZTO:Cr3+, Al3+ onto a blue LED chip exhibited a photoelectric spectrum with two peaks at 450 nm and 730 nm, achieving photoelectric conversion efficiency of 16.3%, with output power of 27.52 mW under current and voltage of 0.1504 A and 3.029 V, respectively. LED bulbs comprising six LEDs, each containing ZTO:Cr3+, Al3+ phosphor mixed with a commercial 660 nm red phosphor with varying mass percentages and a blue LED chip, were successfully produced. The devices showed luminous flux of 55 lm and were used for in situ testing to control the flowering of chrysanthemum plants. Chrysanthemum plants illuminated by LED bulbs demonstrated a better flower bud blossoming rate than the reference plant, highlighting ZTO:Cr3+, Al3+ as a promising phosphor for agricultural LED fabrication.
