Enriching the Deep-Red Emission in (Mg, Ba)MGeO: Mn (M = Al, Ga) Compositions for Light-Emitting Diodes.

Red emission from Mn-containing oxides inspired the development of high color rendering and cost-effective white-light-emitting diodes (WLEDs). Aiming at this fact, a series of new crystallographic site modified (Mg, Ba)MGeO: Mn (M = Al, Ga) compositions were developed with strong deep-red emission in the reaction to UV and blue lights. The MgAlGeO host is composed of three phases: orthorhombic-MgGaGeO, orthorhombic-MgGeO, and cubic-MgAlO. However, MgGaGeO secured an orthorhombic crystal structure. Interestingly, MgAlGeO: Mn showed a 13-fold more intense emission than MgGaGeO: Mn since Mn occupancy was preferable to [AlO] sites compared to [GaO]. The coexisting phases of MgAlO and MgGeO in MgAlGeO: Mn contributed to Mn luminescence by providing additional [AlO] and [MgO] octahedrons for Mn occupancy. Further, these sites reduced the natural reduction probability of Mn to Mn in [AlO] tetrahedrons, which was confirmed using cathodoluminescence analysis for the first time. A cationic substitution strategy was employed on MgMGeO: Mn to improve the luminescence, and MgBaMGeO: Mn (M = Al, Ga) phosphors were synthesized. Partial substitution of larger Ba ions in Mg sites caused structural distortions and generated a new Ba impurity phase, which improved the photoluminescence. Compositionally tuned MgBaAlGeO: 0.005Mn exhibited a 35-fold higher emission than that of MgGaGeO: 0.005Mn. Additionally, this could retain 70% of its ambient emission intensity at 453 K. A warm WLED with a correlated color temperature (CCT) of 3730 K and a CRI of 89 was fabricated by combining the optimized red component with YAlO: Ce and 410 nm blue LED. By tuning the ratio of blue (BaMgAlO: Eu), green (CeTbMgAlO), and red (MgBaAlGeO: 0.005Mn) phosphors, another WLED was developed using a 280 nm UV-LED chip. This showed natural white emission with a CRI of 79 and a CCT of 5306 K. Meanwhile, three red LEDs were also fabricated using the MgBaAlGeO: 0.005Mn phosphor with commercial sources. These could be potential pc-LEDs for plant growth applications.