Ca3La2Te2O12:Mn4+,Nd3+,Yb3+: an efficient thermally-stable UV/visible–far red/NIR broadband spectral converter for c-Si solar cells and plant-growth LEDs

Abstract

A series of novel Mn⁴⁺,Nd³⁺,Yb³⁺-doped Ca₃La₂Te₂O₁₂ (CLTO) phosphors were prepared by substituting Te⁶⁺ for W⁶⁺ in the Ca₃La₂W₂O₁₂ compound using a high-temperature solid-state reaction method. A Mn⁴⁺ singly-doped CLTO phosphor (CLTO:0.004Mn⁴⁺) showed a bright deep red-light emission corresponding to a narrow band around 707 nm (14 144 cm⁻¹) due to a Mn^{4+ 2}E_g → ⁴A_2g spin-forbidden transition upon 365 nm UV excitation, which largely overlapped the absorption spectrum of plant phytochrome P_fr. The excitation spectrum (λ_em = 707 nm) presented a broad band ranging from 250 nm (40 000 cm⁻¹) to 600 nm (16 667 cm⁻¹), which can be decomposed into four Gaussian bands peaking at 318 nm (31 431 cm⁻¹), 355 nm (28 148 cm⁻¹), 410 nm (24 385 cm⁻¹), and 476 nm (20 992 cm⁻¹), corresponding to a Mn⁴⁺–O²⁻ charge transfer (CT) transition, and Mn⁴⁺ transitions ⁴T_1g ← ⁴A_2g, ²T_2g ← ⁴A_2g and ⁴T_2g ← ⁴A_2g, respectively. Moreover, three kinds of Mn⁴⁺ emission sites were analyzed with the assistance of time-resolved spectra. With the single incorporation of Nd³⁺/Yb³⁺ into CLTO:Mn⁴⁺, energy transfer phenomena from Mn⁴⁺ to Nd³⁺/Yb³⁺ ions can be observed, which proceeded via non-radiative resonant and phonon-assisted mechanisms, respectively, resulting in the broadband spectral conversion of the UV/blue to NIR light. When Nd³⁺ and Yb³⁺ were co-doped into CLTO:Mn⁴⁺ to form the tri-doped CLTO phosphors, a successive energy transfer process, Mn⁴⁺ → Nd³⁺ → Yb³⁺, was determined based on the simultaneous energy transfer processes Mn⁴⁺ → Nd³⁺ and Nd³⁺ → Yb³⁺ in the co-doped samples, further enhancing the broadband spectral conversion process of the UV/blue to NIR region, which can be absorbed by photosynthetic bacteria and show high response when applied to c-Si solar cells. More attractively, the luminescence thermal stabilities of both CLTO:0.004Mn⁴⁺ and CLTO:0.004Mn⁴⁺,0.04Nd³⁺,0.20Yb³⁺ showed excellent performance, and the temperature-dependent luminescence properties of the Mn⁴⁺,Nd³⁺,Yb³⁺ tri-doped materials have been investigated for the first time. These results indicate that this kind of phosphor can be potentially applied to improving spectral conversion efficiency for c-Si solar cells and plant-growth far-red/NIR LEDs. In addition, this report provides a strategy wherein hosts for Mn⁴⁺ doping can be well enriched by substituting Te⁶⁺ for W⁶⁺ in certain tungstate compounds, which is highly desired in searching for novel red-emitting phosphors.