Host-Sensitized NIR Quantum Cutting Emission in Nd 3+ Doped GdNbO 4 Phosphors and Effect of Bi 3+ Ion Codoping Praveen Kumar Shahi,* ,† Priyam Singh, † Shyam Bahadur Rai,* and Amresh Bahadur Department of Physics, Banaras Hindu University, Varanasi-221005, India ABSTRACT: Host-sensitized near-infrared quantum cutting (QC) emission has been demonstrated in Nd 3+ doped Gd 1-x Nd x NbO 4 phosphors for various x values. Further, the effect of Bi 3+ ion addition as a sensitizer on near-infrared QC is studied in detail. X-ray diffraction confirms a monoclinic structure for pure and Nd 3+ doped phosphors. Pulsed laser excitation at 266 nm of Gd 1-x Nd x NbO 4 and Gd (0.99-x) Nd x Bi 0.01 NbO 4 causes efficient room-temper- ature energy transfer from the NbO 4 3- to the Nd 3+ ions and the NbO 4 3- and Bi 3+ ions to the Nd 3+ ions, respectively, which emits more than one near- infrared photon for single impinging ultraviolet photon. The emission band of Nd 3+ shows unusual character where the intensity of the 4 F 3/2 - 4 I 9/2 transition at 888 nm is higher than the intensity of the transition 4 F 3/2 - 4 I 11/2 at 1064 nm, due to energy transfer from GdNbO 4 host to Nd 3+ ion. Using photoluminescence lifetime studies, the quantum cutting efficiencies are found to be the maximum 166% and 172% for Gd (0.95) Nd 0.05 NbO 4 and Gd (0.94) Nd 0.05 Bi 0.01 NbO 4 , respectively. The present study could establish Nd 3+ ion as an alternative of Yb 3+ ion for near-infrared quantum cutting. This work facilitates the probing of Nd 3+ ions doped phosphor materials for next generation Si-solar cells. 1. INTRODUCTION In recent years, rare-earth doped downconverting (DC)/ downshifting (DS) materials have become a very dynamic field of research due to their wide range of practical applications in the field of plasma display panel, mercury free fluorescent tubes, and photovoltaic cell, etc. 1-6 Some of the down-converting materials have a specific potential to split a high-energy photon into two or more low energy photons with an appreciable efficiency tending to 200%. 1 The process of conversion of a high energy (VUV and UV) photon into two or more low energy (VIS and NIR) photons is known as quantum cutting (QC), and these materials may be advantageous in enhancing the e fficiency of silicon (Si) solar cells by spectral modifications. 7 The major issue with Si-solar cells is the spectral mismatching of incident photons [i.e., low energy photons (λ > 1100 nm) as well as high energy photons (λ < 400 nm)], which are not absorbed efficiently and cause thermalization losses. 7 In this context, the use of QC material to adapt the solar spectrum to the solar cell is proposed and explored. Trupke et al. were the first who had experimentally shown the effect of using QC material on the front surface of solar cells and obtained an efficiency up to 40% under unconcentrated sunlight. 8 Many other ideas were proposed to enhance the efficiency of Si-solar cell, including organic dyes and quantum dots. 9,10 Organic dyes have very broad absorption band and efficient luminescence property. Unfortunately, these materials show poor chemical stability, toxicity, and reabsorption problem. 9 Coropceanu et al. reported that use of quantum dots solved the reabsorption issue; however, the problems of chemical stability and toxicity are still unresolved. 10 It is time to be conscious about searching for a material with good chemical stability, broad absorption, and efficient NIR emission (in the solar cell response region ∼1000 nm). In this context, much attention has been focused on lanthanides as most of these give intense NIR emission, which can offer many applications without any constraints associated with organic dyes and quantum dots. 1-7 The process of QC in rare-earth ions was first predicted by Dexter in 1957. 11 Research on QC phosphors was started with singly activated ions like Pr 3+ , Tm 3+ , and Gd 3+ through cascade emission. The attention later shifted to explore the QC process in the pair of lanthanide ions through the energy-transfer (ET) mechanism. 7,12-14 There are a large number of reports on Ln 3+ -Yb 3+ (Ln 3+ = Tm 3+ , Pr 3+ , Tb 3+ , Ho 3+ ) codoped NIR quantum cutting phosphors. 7,15 However, in most of the cases, Ln 3+ and Yb 3+ ions show weak NIR emission due to weak absorption as 4f-4f transition is parity forbidden. 16 As it is well-known that the emission intensity of lanthanides is dependent on host matrix and symmetry, so the limitation of weak emission can be sorted out by choosing a self-activated host matrix that transfers its excitation energy to activators, that is, Ln 3+ ions. 17 Because Nd 3+ ions are well- known for their strong NIR emission, the advantage of using Nd 3+ ion over Yb 3+ ion is that it does not show reducing tendency (charge transfer state) and has rich energy levels, which results in an intense NIR emission in comparison to Yb 3+ . 18,19 Nd 3+ ions can be directly sensitized by UV activated host, which is not easily feasible with Yb 3+ ions. Although Nd 3+ has several required features, as far as we know, it has not been tried for NIR QC. 20 Received: October 14, 2015 Article pubs.acs.org/IC © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.5b02370 Inorg. Chem. XXXX, XXX, XXX-XXX