Luminescence Quantum Efficiency Research Articles

The fluorescence decay times and quantum efficiencies of 1,4,5,8-naphthalisoimides

Presented herein are the luminescence properties of several 1,4,5,8- polinahthalisoimides. The luminescence decay curves after deconvolution exhibit three decay times: τ1, τ2, and τ3.The luminescence lifetimes change along with the growth of excitation energy, as well as the contribution of each luminescence decay mechanism (A1, A2, and A3). The manner of change is distinct for one sample, namely, the pure E-isomer, in comparison to the others. The radiative deexcitation coupled with radiationless vibrational energy transfer to other luminescence centers, mainly other isomer units, is discussed as a possible deexcitation mechanism of fluorescence. The Kasha–Vavilov empirical rule is not fulfilled for the studied samples, as the luminescence quantum efficiency (ΦF) distinctly depends on the excitation wavelength (λexc). The quantum yield of one sample (about 63% of Z-isomer) is quite high, at 1%.

Assessment of luminescent downshifting layers for the improvement of light-harvesting efficiency in dye-sensitized solar cells.

Luminescence downshifting (LDS) of light can be a practical photon management technique to compensate the narrow absorption band of high-extinction-coefficient dyes in dye-sensitized solar cells (DSSCs). Herein, an optical analysis on the loss mechanisms in a reflective LDS (R-LDS)/DSSC configuration is reported. For squaraine dye (550-700 nm absorption band) and CaAlSiN3 :Eu(2+) LDS material (550-700 nm emission band), the major loss channels are found to be non-unity luminescence quantum efficiency (QE) and electrolyte absorption. By using an ideal LDS layer (QE=100 %), a less absorbing electrolyte (Co-based), and antireflection coatings, approximately 20 % better light harvesting is obtained. If the absorption/emission band of dye/LDS is shifted to 800 nm, a maximal short-circuit current density (Jsc ) of 22.1 mA cm(-2) can be achieved. By putting the LDS layer in front of the DSSC (transmissive mode), more significant loss channels are observed, and hence a lower overall efficiency than the R-LDS configuration.

Investigation of sensitizer ions tunable-distribution in fluoride nanoparticles for efficient accretive three-center energy transfer

Cooperative upconversion luminescence of Yb3+-Yb3+ couples and three-center energy transfer mechanisms have been deeply investigated in Yb3+ doped and Yb3+-Tb3+ co-doped β-PbF2 nanoparticles. As sensitizer ions, the distribution of Yb3+ ions, which is a key factor that affects the cooperative upconversion luminescence and three-center energy transfer processes, can be tuned by the structure of nanoparticles. Based on the three-center distributions in tetragonal PbYbxTb1−xF5 nanoparticles, two different energy transfer models, Cooperative Energy Transfer (CET) and Accretive Energy Transfer (AET) mechanisms were established. Especially, AET model is observed and verified in this work for the first time. Experimental results obtained from photoluminescence spectroscopy study are in agreement with the theoretical calculations by applying rate equations in these models, strongly supporting the proposed three-center energy transfer mechanisms. The sensitization between Yb3+ ions only existing in AET process can greatly improve the energy transfer rates, further to enhance the quantum efficiency. The results that the calculated luminescence quantum efficiency in AET quantum cutting process is much higher than that in CET process (134% and 104%, respectively), can benefit for further increasing the conversion efficiency of c-Si solar cells.

Trapping and detrapping inSrAl2O4:Eu,Dypersistent phosphors: Influence of excitation wavelength and temperature

${\mathrm{SrAl}}_{2}{\mathrm{O}}_{4}:\mathrm{Eu},\mathrm{Dy}$ is presumably the best known persistent luminescent phosphor. At room temperature, its green emission remains visible for hours after switching off the excitation. It is known that upon lowering the temperature of the phosphor a second photoluminescence emission band arises in the blue part of the visible spectrum, although its origin is still the subject of discussion. In this paper we thoroughly study the origin of both emission bands in ${\mathrm{SrAl}}_{2}{\mathrm{O}}_{4}:\mathrm{Eu},\mathrm{Dy}$ and we attribute this to europium ions substituting for the two different Sr sites in the phosphor's monoclinic host lattice. The photoluminescence properties, the thermal quenching behavior, and photoluminescence lifetime of both emission bands are investigated. A lanthanide energy level scheme is constructed for both sites. Using an integrated approach, i.e., combining charging, afterglow, and thermoluminescence measurements in the same run, we study the charging or trap filling processes in ${\mathrm{SrAl}}_{2}{\mathrm{O}}_{4}:\mathrm{Eu},\mathrm{Dy}$ upon excitation with site selective excitation wavelengths and at different temperatures. We show that trap filling is a thermally activated process when the green emitting center is excited at 435 nm. Furthermore, we also demonstrate that the distribution of filled traps after charging depends strongly on the excitation wavelength and thus on which ${\mathrm{Eu}}^{2+}$ center has been excited. This suggests trapping of the electron close to the ionized ${\mathrm{Eu}}^{2+}$ ion, without full delocalization to the conduction band during the trapping process. Finally, the quantum efficiency of the persistent luminescence is estimated at 65 (+/\ensuremath{-}10)%.

Direct vapor phase growth process and robust photoluminescence properties of large area MoS2 layers

There has been growing research interest in the use of molybdenum disulfide in the fields of optoelectronics and energy harvesting devices, by virtue of its indirect-to-direct band gap tunability. However, obtaining large area thin films of MoS2 for future device applications still remains a challenge. In the present study, the amounts of the precursors (S and MoO3) were varied systematically in order to optimize the growth of highly crystalline and large area MoS2 layers by the chemical vapor deposition method. Careful control of the amounts of precursors was found to the key factor in the synthesis of large area highly crystalline flakes. The thickness of the layers was confirmed by Raman spectroscopy and atomic force microscopy. The optical properties and chemical composition were studied by photoluminescence (PL) and X-ray photoelectron spectroscopy. The emergence of strong direct excitonic emissions at 1.82 eV (A-exciton, with a normalized PL intensity of ∼55 × 103) and 1.98 eV (B-exciton, with a normalized PL intensity of ∼5 × 103) of the sample at room temperature clearly indicates the high luminescence quantum efficiency. The mobility of the films was found to be 0.09 cm2/(V·s) at room temperature. This study provides a method for the controlled synthesis of high-quality two-dimensional (2D) transition metal dichalcogenide materials, useful for applications in nanodevices, optoelectronics and solar energy conversion.

Coprecipitation synthesis, structure and photoluminescence properties of Eu3+-doped sodium barium borate

Micro-sized phosphors of Eu3+-doped NaBaBO3 (0.5–12mol%) were prepared by the modified co-precipitation synthesis. The samples were characterized by scanning electron micrograph (SEM) images and electron dispersive X-ray (EDX). The polycrystalline samples are well developed to the regular plate-like particles with the uniform morphology. The single crystal-phase NaBaBO3 of the Eu3+-doped sample was confirmed by X-ray powder diffraction (XRD) measurements and the structure refinement. The optical properties are evaluated by photoluminescence (PL) excitation and emission spectra, and decay curves. The absolute luminescence quantum efficiency (QE) and the activation energy (ΔE) are reported. Eu3+-doped NaBaBO3 presents a bright red-emitting color under the excitation of near-UV light around 400nm. The maximum absolute QE of NaBaBO3:Eu3+ is estimated to be 53% at 300K. The phosphor shows an excellent thermal stability on temperature quenching effects. The results were discussed on the relationship between the structure and the luminescence properties.

Quantifying self-absorption losses in luminescent solar concentrators.

Analytical equations quantifying self-absorption losses in circular luminescent solar concentrators (LSCs) are presented that can easily be solved numerically by commercial math software packages. With the quantum efficiency, the absorption and emission spectra of a luminescent material, the LSC dimensions, and the refractive index as the only input parameters, the model gives an accurate account of the decrease of LSC efficiency due to self-absorption as a function of LSC radius, thickness, and luminescence quantum efficiency. Results give insight into how many times light is reabsorbed and reemitted, the red shift of the emission spectrum, and on how multiple reabsorptions and reemissions are distributed over the LSC. As an example case the equations were solved for a circular LSC containing a Lumogen F Red 305 dye with 80% luminescence quantum efficiency, and it follows that for an LSC with a 50cm radius the self-absorption reduces the number of photons reaching the LSC edge by a factor of four compared to the case when there would be no self-absorption. The equations can just as well be solved for any material for which the optical properties are known like type I and type II quantum dots.

Oxide Materials with Near Ultra Violet Broadband Absorption for Potential Photovoltaic Applications

For optimal energy conversion in photovoltaic devices based on crystalline silicon (c-Si), the most important requirement is that the total amount of energy of the incident photons is used. The major problem in c-Si photovoltaic cell is the efficiency of the energy conversion which takes place through the cell. In conventional operation, the physics imposes the so-called Shockley–Queisser [1] limit, and only around 29% of the solar spectrum could be converted into electrical energy. However, this limit is estimated to be improved up to 38.4% by modifying the solar spectrum by quantum cutting phosphors that convert one photon of high energy into two photons of lower energy[2]. Quantum cutting conversion from a visible-UV photon, shorter than 500 nm, in two or more infrared photons has been reported in some trivalent lanthanide ions pairs. One of them is always Yb3+. The Yb3+ ion is suitable as an acceptor and emitter because the energy of the only excited level of Yb3+ (~1.2 eV) well-match the bandgap of c-Si (~1.1 eV) and the luminescent quantum efficiency of Yb3+ could reach a value of 100 %. However, since absorption transitions of trivalent lanthanides as a donor are due to the forbidden f-f transitions, the absorption line-widths are sharp and the absorption cross-section is not large. In order to overcome this problem, absorption broadbands of the allowed f-d transitions of Ce3+ and Eu2+ or that of ZnO host has been considered to be used as the sensitizer of Yb3+ luminescence in order to absorb efficiently the high energy region of the solar spectrum. In this work we focused on the host absorption of some perovskite-like compounds: SrTiO3 (STO), Bi4Ti3O12 (BIT) and (Na0.5Bi0.5)TiO3 (NBT). All of them are considered wide-band semiconductors. In addition, trivalent lanthanide can substitute Bi3+ in the last two compounds. As we show, Yb3+ doped STO, BIT and NBT are candidates as wavelength converters for near UV photons to IR photons. We establish the optimal Yb3+ concentration in each compound for near UV to IR photon conversion by luminescence spectroscopy. The compounds were further characterized by scanning electron microscopy, X-ray diffraction, IR spectroscopy and UV-Vis diffuse reflectance spectroscopy. The excitation spectra show broadbands in the near UV region when Yb3+ emission at 980 nm is observed. Because there is no spectral overlap between the absorption band of STO, BIT and NBT and that of Yb3+, the resonant energy transfer by the multipolar interaction and exchange interaction may be discarded. One possibility is the energy transfer by the quantum cutting mechanism as it was suggested for CeO2:Yb3+ [3]. A similar phenomenon ocurrs between Tb3+ and Yb3+. In this case also a non-resonant energy transfer mechanism operates. Acknowledgement. The authors are indebted to the CONACyT (Mexico) for their financial support to carry out this work (Project CB-2010-01 154962, and INFR-2011-1-163250). G. López-Pacheco and G. Santillán-Reyes are grateful to CONACyT for the M. Sc. fellowships granted. Also special thanks to LDRX (T-128) UAM-I for XRD measuremets.

Efficient charge carrier injection into sub-250 nm AlGaN multiple quantum well light emitting diodes

The design and Mg-doping profile of AlN/Al0.7Ga0.3N electron blocking heterostructures (EBH) for AlGaN multiple quantum well (MQW) light emitting diodes (LEDs) emitting below 250 nm was investigated. By inserting an AlN electron blocking layer (EBL) into the EBH, we were able to increase the quantum well emission power and significantly reduce long wavelength parasitic luminescence. Furthermore, electron leakage was suppressed by optimizing the thickness of the AlN EBL while still maintaining sufficient hole injection. Ultraviolet (UV)-C LEDs with very low parasitic luminescence (7% of total emission power) and external quantum efficiencies of 0.19% at 246 nm have been realized. This concept was applied to AlGaN MQW LEDs emitting between 235 nm and 263 nm with external quantum efficiencies ranging from 0.002% to 0.93%. After processing, we were able to demonstrate an UV-C LED emitting at 234 nm with 14.5 μW integrated optical output power and an external quantum efficiency of 0.012% at 18.2 A/cm2.

Eu2+‐Doped Yellow‐Emitting CsBaB3O6 Glass‐Ceramic Prepared by Melt Quenching and Re‐Crystallization Method

Eu3+‐doped cesium barium borate glass with the composition of Cs2O·2BaO·3B2O3 was prepared by the conventional melt quenching method. The glass‐ceramic sample was obtained from the re‐crystallization of the as‐made glass to change the amorphous glass into a crystalline host. This reduces the Eu3+ in glass to Eu2+ ions resulting in a yellow‐emitting phosphor of Eu2+‐activated CsBaB3O6. The samples were investigated by the XRD patterns and SEM micrograph, the optical absorption, the photoluminescence spectra, and decay curves. The as‐made glass has only Eu3+ centers. Under the excitation of blue or near‐UV light, Eu2+‐doped CsBaB3O6 presents yellow‐emitting color from the allowed inter‐configurational 4f–5d transition in the Eu2+ ions. The maximum absolute luminescence quantum efficiencies of Eu2+‐doped CsBaB3O6 phosphor was measured to be 47% excited at 430 nm light at 300 K. By taking into account the efficient excitation in blue wavelength region, this new phosphor could be a potential yellow‐emitting phosphor for an application in white light‐emitting diodes fabricated with blue chips.

Synthesis, Aggregation‐Induced Emission, and Electroluminescence of Dibenzothiophene‐ and Dibenzofuran‐Containing Tetraarylethenes

AbstractDibenzothiophene‐ and dibenzofuran‐functionalized ethanes were synthesized by the McMurry coupling reaction. The luminogens are faintly emissive when molecularly dissolved in good solvents, but emit intensively when aggregated as nanoparticles in poor solvents or fabricated as solid thin films, demonstrating the phenomenon of aggregation‐induced emission (AIE). Their organic light‐emitting diode (OLED) applications were explored, utilizing the AIE effect. Electroluminescence devices with the configuration of indium tin oxide (ITO)/N,N′‐di(1‐naphthyl)‐N,N′‐diphenyl‐(1,1′‐biphenyl)‐4,4′‐diamine (NPB; 60 nm)/dye (20 nm)/1,3,5‐tri(1‐phenyl‐1H‐benzo[d]imidazol‐2‐yl)phenyl (TPBi; 10 nm)/tris(8‐hydroxyquinoline)aluminum (Alq3; 30 nm)/LiF (1 nm)/Al (100 nm) were fabricated. The OLED device emits at 510 nm with a maximum luminescence and external quantum efficiency of 104 cd/m2 and 2.1 %, respectively. The OLED behavior of the E/Z isomers was also studied.

Aggregation effects on the optical emission of 1,1,2,3,4,5-hexaphenylsilole (HPS): a QM/MM study.

We investigate the photophysical property for 1,1,2,3,4,5-hexaphenylsilole (HPS) through combined quantum mechanical and molecular mechanical (QM/MM) simulations. Under the displaced harmonic oscillator approximation with consideration of the Duschinsky rotation effect (DRE), the radiative and nonradiative rates of the excited-state decay processes for HPS are calculated by using the analytical vibration correlation function approach coupled with first-principles calculations. The intermolecular packing effect is incorporated through electrostatic interaction modeled by a force field. We find that from the gas phase to the solid state (i) the side phenyl ring at the 5-position becomes coplanar with the central silacycle, which increases the degree of conjugation, thus accelerating the radiative decay process, and (ii) the rotation of the side phenyl ring at the 2-position is restricted, which blocks the excited-state nonradiative decay channels. Such a synergetic effect largely enhances the solid-state luminescence quantum efficiency through reducing the nonradiative decay rate by about 4 orders of magnitude, leading to the radiative decay overwhelming the nonradiatvie decay. In addition, the calculated solid-phase absorption and emission optical spectra of HPS are found to be in agreement with the experiment.

Dramatically Enhanced Luminescence of Layered Terbium Hydroxides as Induced by the Synergistic Effect of Gd3+ and Organic Sensitizers

Host-guest chemistry allows the engineering of new functional materials with tunable properties. This study focuses on layered terbium hydroxides (NO3-LTbH) codoped with Gd3+ (NO3-LTbH:Gd), which were prepared using the hydrothermal method and successfully modified using sensitizers (L). Luminescence studies showed that compared with the NO3-LTbH precursor the sensitizer-modified NO3-LTbH:Gd exhibited high luminescence intensity and high luminescence quantum efficiency (Phi = 33%). This performance results from the synergistic effect of codoped Gd3+ and intercalated sensitizers in the organic inorganic hybrid materials, which led to stronger luminescence properties, and synergistic effect on the enhancement of Tb3+ luminescence was investigated by the spectroscopic characteristics, UV-vis absorption spectrum, low-temperature phosphorescence, and crystal structures of the layered rare-earth hydroxides. Studies demonstrate that the mechanism for synergistic effect of significant enhancement of Tb3+ luminescence is mainly caused by Gd3+ inducing a cascaded energy transfer from the host to Tb3+ via organic sensitizers. The very interesting thing is that LTbH:Gd has multiple roles, including energy-transfer bridges that connect the sensitizers and Tb3+ (in host) to enhance the characteristic emission of Tb3+, significantly enhancing phosphorescence of sensitizers, and acting as host matrices for sensitizers. Second, the effects of intercalation conditions on luminescence were also investigated. Furthermore, a novel transparency luminescent composite film device [PMMA/L-LTbH:Gd] that exhibits strong luminescence property was also fabricated using the solvent-casting method.

Synthesis and spectroscopic behavior of highly luminescent trinuclear europium complexes with tris-β-diketone ligand

A new tris-β-diketone ligand, 2-[4,6-bis-(1-benzoyl-2-oxo-2-phenyl-ethyl)-[1,3,5]triazin-2-yl]-1,3-diphenyl-propane-1,3-dione (H3L), and its trinuclear europium complexes, Eu3(DBM)6L (C1), Eu3(DBM)6(Bipy)3L (C2) and Eu3(DBM)6(Phen)3L (C3) were synthesized and their spectroscopic behaviors were studied by FT-IR, 1H NMR, UV–vis and photoluminescence spectroscopic techniques. These europium complexes exhibited the characteristic emission bands that arise from the 5D0→7FJ (J=0–4) transitions of the europium ion in solid state. The Ω2 and Ω4 intensity parameters, lifetime (τ) and luminescence quantum yield (η) were calculated according to the emission spectra and luminescence decay curves in solid state. The results indicated that these trinuclear europium complexes displayed a longer lifetime (τ) and higher luminescence quantum efficiency (η), especially complexes C2 (τ=0.820ms, η=46.5%) and C3 (τ=0.804ms, η=47.4%), which due to the effect of two additional europium ion lumophors and the introduction of the third ligands, Bipy or Phen in trinuclear complexes. Their Ω2 values demonstrated that the europium ion in these complexes is in a highly polarizable chemical environment.

Development of a high-efficiency DF-STEM detector

We experimentally evaluated the signal intensity produced by an electron on three types of scintillators. A powder scintillator showed high efficiency at a low accelerating voltage, whereas a single crystal scintillator showed high efficiency at higher accelerating voltages. On the basis of these characteristics, for a dark-field (DF) scanning transmission electron microscope (STEM), we have developed a new hybrid type scintillator, which consisted of a powder deposited on a single crystal substrate. The luminescent quantum efficiency of the hybrid scintillator was measured to be twice as large as that of the single crystal type detector at 60 kV and was about 8 times higher than that of the powder type detector at 300 kV. The developed detector has advantages of powder and single crystal type detectors, and covers the observation at the accelerating voltages from low to high voltages. Especially, it is useful for low voltage observations of carbon-based materials consisted of few atomic layers that produces weak scattering of electron.

Photofunctional hybrids of lanthanide (Eu3+, Tb3+)/beta-diketonate functionalized MCM-41/SBA-15 mesoporous host prepared with 1,4,7,10-tetraazacyclododecane modified siloxane as covalent linkage

Several kinds of novel lanthanide organic–inorganic mesoporous hybrid luminescent materials have been assembled by introducing the ternary lanthanide ions (Eu3+, Tb3+) and beta-diketone (TTA, TAA) into the functionalized mesoporous hosts through the coordination bond and covalent bond by using 3-chloropropyltriethoxysilane modified 1,4,7,10-tetraazacyclododecane (Cyclen-Si) as an organic bridge molecule. These multicomponent hybrids are characterized by FTIR, TEM, XRD, thermogravimetric analysis, diffuse reflectance UV–Vis spectra and nitrogen adsorption–desorption isotherms, especially the photoluminescence properties of them. The luminescent spectra of the hybrids reveal that the MCM-41 based materials show strong emission intensities, high luminescent quantum efficiencies, long luminescent lifetimes and exhibit characteristic red and green luminescence originated from lanthanide ions (Eu3+ or Tb3+), which suggesting the Cyclen-Si bridge molecule is favorable for the assembly of lanthanide functionalized luminescent hybrids.

Stretch-stowage-growth strategy to fabricate tunable triply-amplified electrochemiluminescence immunosensor for ultrasensitive detection of pseudorabies virus antibody.

Triply amplified electrochemical biosensors have attracted particular attention in the detection of low-abundance biomarkers. The universal construction routes for nonenzymatic triply amplified and even multiply amplified biosensors are extremely desirable but remain challenging. Here, we proposed a "stretch-stowage-growth" strategy to tunably fabricate a nonenzymatic triply amplified or multiply amplified electrochemiluminescence (ECL) immunosensor for ultrasensitive determining pseudorabies virus (PrV) antibody. Based on the matrix role of gold nanoparticle-graphene nanosheet (Au-GN) hybrids, carrier role of silicon nanoparticles (SNPs) and bridge role of "biotin-streptavidin-biotin" (B-SA-B) structure, the establishment processes were defined as "stretch", "stowage", and "growth", respectively. Relying on the interaction of antigen-antibody and of B-SA, the "Au-GN/PrV (Ag)/PrV antibody (Ab1)/biotinylated IgG (B-Ab2)/SA/biotinylated Ru(bpy)3(2+)-encapsulated SNPs (B-Ru@SNPs)" triply amplified biosensor could be fabricated and exhibited better analytical performance not only toward monoclonal PrV antibody with a linear detection range from 50 ng mL(-1) to 1 pg mL(-1) and a detection limit of 0.40 pg mL(-1), but also toward actual serum samples when compared with enzyme-linked immunosorbent assay and fluorometry. Furthermore, multiply amplified biosensors could be conveniently fabricated by controllably repeating the combination of B-Ru@SNPs and SA to form the B-SA-B structure. After it was repeated three times, the multiply amplified biosensor stretched to the maximum of signal amplification and achieved a luminescence quantum efficiency about 23.1-fold higher than the triply amplified biosensor. The tunable biosensor exhibits good stability, acceptable reproducibility and accuracy, suggesting its potential applications in clinical diagnostics.

Lead halide perovskites and other metal halide complexes as inorganic capping ligands for colloidal nanocrystals.

Lead halide perovskites (CH3NH3PbX3, where X = I, Br) and other metal halide complexes (MXn, where M = Pb, Cd, In, Zn, Fe, Bi, Sb) have been studied as inorganic capping ligands for colloidal nanocrystals. We present the methodology for the surface functionalization via ligand-exchange reactions and the effect on the optical properties of IV–VI, II–VI, and III–V semiconductor nanocrystals. In particular, we show that the Lewis acid–base properties of the solvents, in addition to the solvent dielectric constant, must be properly adjusted for successful ligand exchange and colloidal stability. High luminescence quantum efficiencies of 20–30% for near-infrared emitting CH3NH3PbI3-functionalized PbS nanocrystals and 50–65% for red-emitting CH3NH3CdBr3- and (NH4)2ZnCl4-capped CdSe/CdS nanocrystals point to highly efficient electronic passivation of the nanocrystal surface.

Eu3+-site occupations and peculiar reddish orange luminescence in aluminates Sr3RAl2O7.5:Eu3+ (R=Y, Lu)

A novel reddish orange-emitting phosphor of Eu3+-doped aluminate Sr3RAl2O7.5 (R=Lu, Y) was successfully synthesized by wet-chemistry sol–gel method. The samples were characterized by X-ray powder diffraction (XRD) and scanning electron microscope (SEM). The luminescence properties such as photoluminescence excitation and emission spectra, decay curves, CIE color coordinates and luminescence quantum efficiency were investigated. This phosphor can be efficiently excited by UV and near-UV light and exhibits a bright reddish orange luminescence with the abnormally strongest 5D0→7F0 transition. Two distinct Eu3+ sites can be observed on the emission spectra under the excitation of UV light (excitation at charge transfer band of Eu3+ ions) or dye laser in the 5D0→7F0 wavelength region (selective excitation into each site). The site-selective luminescence decay curves have been investigated in the 5D0→7F0 region of each Eu3+ site by using a pulsed, tunable, and narrowband dye laser. Each Eu3+ site presents the distinct luminescence lifetime with different rise time. The Stark energy levels of two Eu3+ sites were assigned from the site-selective emission spectra. The energy transfer can happen between the closely neighbored Eu3+ sites in Sr3RAl2O7.5 (R=Lu, Y). The luminescence mechanism responsible for the abnormally strong 5D0→7F0 transition was discussed.

Er3+–Yb3+ co-doped TeO2–PbF2 oxyhalide tellurite glasses for amorphous silicon solar cells

In this study, we successfully prepared Er3+–Yb3+ co-doped TeO2–PbF2 oxyfluoride tellurite glasses with different Yb3+ concentrations and characterized their upconversion properties. Intense emission bands at 527, 544, and 657nm corresponded to the Er3+ transitions, and the maximum was obtained at an Yb3+-to-Er3+ molar ratio of 3. When this glass was applied at the back of amorphous silicon solar cells in combination with a rear reflector, a 0.45% improvement in efficiency was obtained under co-excitation of AM1.5 and 400mW 980nm laser radiation. Maximum external quantum efficiency and luminescence quantum efficiency of 0.27% and 1.35%, respectively, were achieved at 300mW excitation.