Novel Intense Emission-Tunable Li1.5La1.5WO6:Mn4+,Nd3+,Yb3+ Material with Good Luminescence Thermal Stability for Potential Applications in c-Si Solar Cells and Plant-Cultivation Far-Red-NIR LEDs

Novel pure Li1.5La1.5WO6:Mn4+,Nd3+,Yb3+ (LLWO:Mn4+,Nd3+,Yb3+) materials with intense emission-tunable far-red-near-infrared (far-red-NIR), potentially applied in crystalline silicon (c-Si) solar cells and plant-cultivation applications, were prepared using a high-temperature solid-state reaction approach. In optimal LLWO:0.03Mn(4+), a far-red emission band peaking at 714 nm, corresponding to Mn4+ E-2(g) -> (4)A(2g) transition of the 3d(3) electron in a [MnO6] octahedron, is observed upon the 345 nm ultraviolet (UV) excitation. The broad excitation spectrum (lambda(ex) = 714 nm) can be decomposed into four Gaussian bands centered at 323 nm (30 960 cm-1), 350 nm (28 571 cm(-1)), 399 nm (25 063 cm(-1)), and 470 nm (21 277 cm(-1)), respectively, which are assigned to Mn4+ <- O2- charge transfer, Mn4+ T-4(1g) <- (4)A(2g), T-2(2g) <- (4)A(2g), and T-4(2g) <- (4)A(2g) transitions. When Nd3+/Yb3+ is introduced into the LLWO:0.03Mn(4+) system, an energy-transfer process from Mn4+ to Nd3+/Yb3+ is demonstrated, which realizes the spectral conversion of broadband UV-visible to far-red emission and NIR However, the energytransfer mechanisms for them seem to be different, in which the former one intends to be a resonant process of electronic dipole dipole interaction, whereas the latter one is proposed as a multiphonon-assisted mechanism. In the case of codoping dipole dipole interaction, whereas the latter one is proposed as a multiphonon-assisted mechanism. In the case of codoping Nd3+ and Yb3+ into LLWO:0.03Mn(4+), the Yb3+ emission is enhanced based on a elucidated successive energy-transfer process of Mn4+ -> Nd3+ -> Yb3+. Consequently, the UV-visible light can be effectively converted into tunable NIR emission in this system based on the synergistic effect of dual Mn4+ -> Nd3+ -> Yb3+ and Mn4+ -> Yb3+ energy-transfer processes, matching well with the high spectral response region for c-Si solar cells. Moreover, the acquired far-red-NIR emission also couples well with the absorption spectrum of biologically active states P-fr and photosynthetic bacteria, which suggests its potential application in plant -cultivation LEDs. Additionally, the good luminescence thermal stability is found in LLWO:0.03Mn(4+),0.02/3Nd(3+),0.05/3Yb(3+), further supporting its feasibility.