Near-infrared quantum cutting in Tb 3+ , Yb 3+ co-doped calcium tungstate via second-order downconversion
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Near-infrared quantum cutting involving the conversion of one visible photon into two nearinfrared photons was demonstrated in Ca0.99xYbxWO4: Tb0.01 phosphors. From the analysis of the refinement of x-ray diffraction patterns, the suitable concentration range of Yb3+ in Ca0.99WO4: 0.01Tb3+ was determined to be 0–20%. By investigating their luminescent spectra and decay lifetimes, second-order downconversion from Tb3+ to Yb3+ were proved and the possible quantum cutting mechanism was proposed. Quantum efficiency related to Yb3+ concentration was calculated and the maximum efficiency was reached at 140.4%. Because the energy of Yb3 + 2F7/2 / 2F5/2 transition matches well with the band gap of the crystalline Si, the Ca0.99xYbxWO4: Tb0.01 phosphors could be potentially applied in silicon-based solar cells.
I. INTRODUCTION
Recently, quantum cutting (QC) of rare-earth-doped phosphors has attracted increasing attention as a significant development in the search for luminescent materials with high quantum efficiency (QE).1–8 QC is a phenomenon that occurs in materials, which could generate two or more low-energy photons by absorbing one high-energy photon. Primitively, intense research activities were focused on the vacuum ultraviolet-excited QC phosphors because it was indispensable for applications in plasma display panels and mercury-free fluorescent lamps.1–5 Afterwards, the scope of investigation on QC phosphors has been extended to the near-infrared (NIR) region. If conversion of one visible (VIS) photon into two NIR photons is realized and the energy of the NIR photon is just above the band gap of Si (Eg 5 1.12 eV), then the NIR QC materials could be applied in silicon-based solar cells.6–9 Till date, the main system of NIR QC materials is Tb3+ -Yb3+ co-doped phosphors.9–16 In this system, the phosphor could emit two NIR photons (Yb3+: 2F7/2 / 2F5/2) through the absorption of one VIS photon (Tb3+: 5D4 / 7F6). Moreover, the Yb3 + 2F7/2 / 2F5/2 emission is situated around 1000 nm, which matches well with the band gap energy of Si.9,10 Experimental evidences have been presented for QC in Tb3+ and Yb3+ co-doped fluorides (YF3 and TbF3),10–12 borates (GdBO3 and LaBaB9O16),15,16 and phosphates (YPO4).9 However, QC in alkaline metal tungstates has not been investigated. According to Su et al.,17 rare-earth (Tb3+, Eu3+, Dy3+, and Sm3+)-doped calcium tungstates a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.6 J. Mater. Res., Vol. 26, No. 5, Mar 14, 2011
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have been extensively studied because of their excellent luminescent properties. In this study, we synthesized the phosphors Ca0.99xYbxWO4: Tb0.01 and investigated their QC phenomenon in VIS–NIR region. II. EXPERIMENTAL
Calcium carbonate (CaCO3, 99%), tungsten oxide (WO3, 99%), terbium oxide (Tb4O7, 99.99%), and ytterbium oxide (Yb2O3, 99.99%) were used as raw materials. All of them were sufficiently commixed in agate mortar with appropriate amount of ethanol. Then the powders
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