Toward an\nIdeal Light Source for Indoor Photosynthesis:\nBroadband Red Emission in Zero-Dimensional Hafnium-Based Metal Halide\n(TPP)<sub>2</sub>HfCl<sub>6</sub>·4C<sub>2</sub>H<sub>3</sub>N:Sb<sup&

With\nsuitable electron–phonon coupling strength, a near-unity\nbroadband photoluminescence quantum yield (PLQY) can be achieved in\norganic–inorganic hybrid metal halides (OIHMHs) via self-trapped\nexciton (STE) emission. However, it is still challenging to obtain\nhigh-quality red emission from OIHMHs with a desirable emission wavelength\nand high chemical stability, which hinders their practical application\nin high-performance displays, plant-growth lighting, and biomedical\nimaging. Herein, a series of hafnium-based zero-dimensional (TPP)<sub>2</sub>HfCl<sub>6</sub>·4C<sub>2</sub>H<sub>3</sub>N (TPP: tetraphenylphosphonium)\nsingle crystals with different Sb<sup>3+</sup> doping levels are synthesized.\nThe Sb<sup>3+</sup>-doped (TPP)<sub>2</sub>HfCl<sub>6</sub>·4C<sub>2</sub>H<sub>3</sub>N shows dual-band red emission with a full width\nat half-maximum of 178 nm and a high PLQY of 91.09%. This broad dual-band\nemission originates from dopant-induced extrinsic free excitons and\nSTEs. Furthermore, (TPP)<sub>2</sub>HfCl<sub>6</sub>·4C<sub>2</sub>H<sub>3</sub>N:Sb<sup>3+</sup> was employed as a luminescence converter\nin a light-emitting diode (LED) for plant growth regulation. A correlated\ncolor temperature of 4055 K and a color rendering index of 82.13 were\nachieved upon excitation of the LED at 365 nm. These results provide\nfundamental perspectives on the emission behavior of Sb<sup>3+</sup>-doped OIHMHs and illustrate their promise for use in plant-growth\nlighting.