Down-conversion Phosphors Research Articles

Visible/near-infrared luminescence and concentration effects of Pr3+-doped Sr2Al2GeO7 downconversion phosphors

The Pr3+ ion has been widely doped into various materials as a red and near-infrared (NIR) emitting center for applications in lighting and solar spectrum downconversion. Herein, the preparation of a new library of Pr3+-doped Sr2Al2GeO7 phosphors was proved by powder x-ray diffraction patterns and Rietveld refinements and characterized by a scanning electron microscope with energy-dispersive x-ray spectrometry. The Sr2Al2GeO7:Pr3+ sample strongly absorbs blue photons over 420–500 nm and yields intense visible emissions with dominant peaks around 490 nm from the Pr3+ 3P0 → 3H4 transition, as well as robust NIR emission bands over 800–1200 nm. In addition to the typical transitions of 1D2 → 3F2 at 880 nm, 1G4 → 3H4 at 1000 nm, and 1D2 → 3F3,4 at 1070 nm, the distinguishable NIR emission at 929 nm was demonstrated from the 3P0 → 1G4 transition via static and dynamic spectroscopic analysis. Most interestingly, for the 3P0 blue-excited state, a considerably elevated concentration of about 10%Pr3+ was optimal for the visible/NIR emissions, in stark contrast to the diluted optimal 1%Pr3+ for the 1D2 state. The relevant cross-relaxation from the 3P0 and 1D2 states between Pr3+ was comprehensively treated by theoretical speculations and experimental results. Such concentrated Pr3+ blue activators would significantly facilitate the blue-to-NIR downconversion through a desired two-step sequential transition from the 3P0 initial state to the 1G4 intermediate level for quantum efficiency exceeding unity. The current results would consolidate the basis of concentrated Pr3+ donors to promote the novel Pr3+/Yb3+ codoping downconversion for greatly increasing Si solar cell efficiency.

(Invited) From Inside Out: How Synthesis-Structure Correlations Conspire to Determine Photoluminescence Traits and Operational Robustness in Non-Blinking Giant Quantum Dots

“Giant” or thick-shell core/shell quantum dots (gQDs) are an important class of solid-state quantum emitter. Without any encapsulation gQDs are characterized by strongly suppressed blinking, or even non-blinking behavior, and resistance to photobleaching at room temperature. In addition, non-radiative Auger processes are significantly reduced in this class of luminescent nanomaterial. Together, these qualities lead to novel functionality as photon sources for a range of ensemble and single-emitter applications, including down-conversion phosphors, direct excitation light-emitting diodes, single-biomolecule tracking, and single-photon generation. For many applications, operation at elevated temperature and under intense photon flux is also desired, and we have shown that gQDs outperform other emitters in this key metric of operational robustness. Interestingly, performance under these conditions is strongly dependent on the synthetic method employed for shell growth. Previously we revealed the mechanistic pathways responsible for thermally-assisted photodegradation, distinguishing effects of hot-carrier trapping and QD charging (ACS Nano 2018, 12, 5, 4206), and subsequently used solid-state spectroscopy to obtain kinetic and thermodynamic parameters of the photothermal degradation process (ACS Appl. Mater. Interfaces 2020, 12, 27, 30695). Here, I will discuss a comprehensive analysis of gQD structural properties from the inside of the nanocrystal to its surface and demonstrate correlations between specific structural features and both causal synthetic parameters and resulting performance metrics. Two synthetic methods—successive-ionic-layer-adsorption-and-reaction and high-temperature continuous-injection—will be compared. Investigated structural features will include interfacial alloying, stacking-fault density and surface-ligand identity, while correlated functionalities will address quantum yield, single-gQD photoluminescence under thermal/photo-stress, charging behavior and quantum-optical properties. I will show several unexpected conclusions, including the surprising finding that while interfacial alloying is the strongest indicator of gQD stability under stress, the parameter is not the determining factor for Auger suppression. Furthermore, quantum yield is strongly influenced by surface chemistry and can approach unity even in the case of a gQD comprising a thick shell with a high defect density with proper choice of excitation wavelength. Interestingly, we find a strong correlation between the existence of zinc-blende stacking faults and whether the gQD exhibits charged-state or purely excitonic emission, and the interplay between charged emission and excitonic emission can be precisely tuned by “mixing” shell-growth methods (Small Sci. 2023,3, 2300092).

Enhanced detection of UV fluorescence from food products and tissue using lanthanide-doped polymer composite membranes

We demonstrate polymer composite membranes that can be coupled to conventional spectroscopic tools for enhancing the detection of weak UV fluorescence signals. The membranes incorporate a down-conversion phosphor with lanthanide ions allowing for the detection of UV emission from biomolecules commonly found in food products. We also explore their capability to discern between healthy and wounded tissue through measurements of the UV autofluorescence emission from skin. Our results show that the composite membranes have great potential to improve UV fluorescence detection in applications related to the biomedical, pharmaceutical and food areas.

Unity emission in organic phosphonium antimony halide for high color rendering white LED

Metal halides have attracted increasing attention in luminescent applications because of their fantastic properties in optoelectronics and easy solution processability. However, toxicity of Pb element and low photoluminescence quantum yield (PLQY) hamper their practical application. Herein, we report a lead-free zero-dimensional antimony (Ⅲ) hybrid halide (EPP)2SbCl5 ⋅CH2Cl2 (EPP = Ethyltriphenylphosphonium), in which inorganic luminescent center [SbCl5]2- are surrounded by organic ligands, forming a perfect host-guest system and exhibiting unprecedented photophysical properties. The (EPP)2SbCl5⋅CH2Cl2 exhibits a bright orange-red emission located at 623 nm, a near-unity PLQY of 97.09 %, and a large Stokes shift of 253 nm. The characterization of photophysical properties shows that broadband emission with microsecond decay lifetime (6.98 μs) is derived from self-trapped exciton emission. Together highly efficient and ultra-stable luminescence performance enable (EPP)2SbCl5⋅CH2Cl2 as excellent down-conversion phosphor to successfully fabricate white light-emitting diodes (LEDs) device with a color rendering index of 95. It is proved that the prepared 0D antimony hybrid halide with ultrabroad-band emission and near-unity PLQY is a promising solid-state lighting luminescent material.

A Review on the Applicability of Luminescent Phosphors and Effective Positioning in Dye‐Sensitized Solar Cells for Enhanced Performance and Stability

The narrow range of solar light absorption facilitated by the photoactive organic dyes has been restricting the significant development, and large‐scale applications of dye‐sensitized solar cells (DSSC). The major problem that limits the power conversion efficiency (PCE) of DSSC is the limitations of the organic dyes (light absorbers in DSSC) to absorb beyond the visible region of solar spectrum that resulting in lower PCE (≈13%) as compared with the commercialized technologies. Hence, the focus on developing efficient strategies to harvest the ultraviolet and infrared light regions of the solar spectrum and effective conversion into the absorption limit of dyes and thereby increasing the device performance and stability is the emerging technological development in DSSCs. Rare earth ion‐doped upconversion (UC) and downconversion (DC) phosphors are the most promising solutions for the prevalent issues of the liquid junction solar cells. This review will focus on the emerging strategies for the PCE enhancement of DSSC with UC and DC materials. An exclusive review on the facile approach of simultaneous usage of both the UC and DC phosphors and the effective positioning of these spectral conversion phosphors, to achieve highly stable DSSCs with superior PCE is provided.

Structural and luminescent performance and optical thermometry of Pr3+ doped SrWO4 down-conversion phosphors

A series of Pr3+ doped SrWO4 phosphors were synthesized using solid-state reaction routes. The phase composition, photoluminescence and temperature sensitivities of as-prepared materials were investigated. Rietveld refinement of measured XRD data was applied to analyze the lattice parameters. The optical transition was elaborated using UV–visible diffuse reflectance spectra and photoluminescence spectra. The photoluminescence measurements confirmed the strong visible light emission from the developed phosphors under both UV excitation at 250 nm and blue excitation at 450 nm. Notably, the maximum emission intensity was observed at a Pr3+ doping concentration of 2 mol%. Furthermore, the fluorescence intensity ratio (FIR) technique was employed, focusing on the emissions originating from the 3P0 and 1D2 state of Pr3+ within the temperature range of 298–598 K. Two FIR groups were adopted in this case, which can verify each other to achieve self-calibration. Typically, SrWO4:0.02Pr3+ exhibited remarkable performance as a temperature sensor, displaying an absolute sensitivity (Sa) of 3.11%K−1 and a relative sensitivity (Sr) of 0.81%K−1. As a result, the significant temperature-dependent PL and remarkable down-conversion luminescence make the SrWO4:Pr3+ material a promising candidate for optical thermometers and UV to visible converters of solar cell applications.

Luminescence and energy transfer of Ce3+/Tb3+ activated fluoroborate BaGaBO3F2 as a color tunable phosphor

Ce3+/Tb3+ co-doped BaGaBO3F2 is a new rare-earth activated luminescent material with efficient green luminescence. The phosphor was synthesized via the conventional solid-state reaction and characterized by X-ray powder diffraction, photoluminescence, and decay lifetimes. The Ce3+/Tb3+ co-doped phosphor exhibits an efficient excitation between 280 and 420 nm, making it suitable for commercial near-UV chips. The 4f–5d transitions of Ce3+ sensitizer allow the phosphor to be excited by the near-UV light, resulting in green and blue luminescence from Tb3+ and Ce3+, respectively. By controlling Ce3+/Tb3+ doping, the luminescence color can be tuned from blue to green. The effective transfer of excitation energy from Ce3+ to Tb3+ of BaGaBO3F2 was confirmed by the spectral characteristics and decay lifetimes of samples with different doping concentrations. The dipole-quadrupole mechanism is responsible for the efficient energy transfer from Ce3+ to Tb3+, with a critical distance of 11.69 Å. These results demonstrate that Ce3+/Tb3+ co-doped BaGaBO3F2 is an efficient near-ultraviolet (NUV) excitation phosphor with tunable luminescence from blue to green. This material has the potential to be used as a down-conversion phosphor for NUV light-emitting diodes.

Zero-dimensional hybrid zinc halides with blue light emissions

To solve the toxicity problem of blue light emissive three-dimensional lead halide perovskite, zero-dimensional lead-free hybrid halides with low cost and high emission efficiency have become one of the most promising solid-state light emitting materials. Herein, by utilizing discrete [ZnBr4]2− tetrahedrons as optical center, a new series of organic–inorganic hybrid zinc halides of AZnBr4 (A = NMPz, MPz, and AMPd) were presented as blue light emitters. Excitingly, these zero dimensional halides exhibit bright blue-light emission with the highest photoluminescence quantum yield of 17.28%, acting as down-conversion phosphors to assembly high-performance white-light emitting diode.

Cubic-phase NaYF4:Pr3+, Yb3+ down-conversion phosphors for optical temperature sensing

The cubic phase NaYF4:Pr3+, Yb3+ single crystals were grown in the resistive furnace by the Bridgman-Stockbarger technique with following milling in an agate mortar. The luminescence intensity ratio (LIR) between 3P1 – 3H5 and 3P0 – 3H5 emissions of Pr3+ was taken a temperature-dependent parameter. The calculated absolute (Sa) temperature sensitivity was 0.0075 at 320 K. The LIR between 3P0 – 1G4 (Pr3+) and 2F5/2 – 2F7/2 (Yb3+) emissions is also temperature-dependent due to phonon-assisted nature of energy transfer between 1G4 of Pr3+ and 2F5/2 of Yb3+. Sr and Sa demonstrated the highest values in the 100–220 K temperature range (Sa (max) = 0.0047 K−1 at 150 K).NaYF4:Pr3+, Yb3+ samples are useful in optical temperature sensing in the broad temperature range from 100 up to 320 K.

Realizing high energy density supercapacitors assisted by light-induced charging

The increasing demand for energy storage devices has initiated research on alternative sustainable energy storage mechanisms, such as supercapacitors. Here, we report a skillful design strategy that harvests visible light energy and has immense potential applications in boosting the storage capacity of supercapacitors – one of the energy storage devices. A down-conversion phosphor layer is introduced over a reduced graphene oxide-based supercapacitor electrode having an optical window to harvest visible light. The fabricated cell exhibits an excellent areal energy density of 233 μWh/cm2 (at an areal current density of 4 mA/cm2 for the voltage ranging from 2 to 4.25 V) under optical illumination, which is 2.5 times higher than that achieved without illumination. Under light illumination, the cell operates across a high voltage range (0–4 V) and current density (40 mA/cm2) and shows a light-induced charging voltage of 354 mV, without any external bias. Thus, such exceptional supercapacitor performance generates significant possibilities for developing novel energy storage devices with high energy and power densities, through which portable devices can be charged using visible light.

Upconversion phosphor thermometry for use in thermal barrier coatings

Measuring the temperature below the surface of a thermal barrier coating (TBC) using a thin phosphor layer is challenging primarily due to the absorption and scattering of laser excitation light and phosphor luminescence as they propagate through the coating. One way to increase phosphor luminescence could be to use upconversion phosphor thermometry, which is investigated in the current study. It is attractive because using longer excitation wavelengths reduces the absorption and scattering in TBCs as 8% wt. yttria-stabilize zirconia (8YSZ) generally has lower scattering and absorption coefficients around 1000 nm than at 532 and 355 nm. Therefore, the viability of upconversion to measure the temperature at the bottom of a TBC was evaluated for the first time and was compared with the more conventional downconversion phosphor thermometry. The current work involved an experimental study of several phosphors with lanthanides doped in the 8YSZ host, which were excited through downconversion by pulsed 355 nm and 532 nm laser light and through upconversion with 965 nm laser light. The YSZ:Er,Yb and YSZ:Ho,Yb phosphors show promise for upconversion phosphor thermometry. The experimentally acquired optical phosphor characteristics were used to simulate laser light and phosphor luminescence propagation in TBCs using Kubelka–Munk theory. This was to evaluate the signal strength with upconversion excitation compared to downconversion excitation. Upconversion excitation resulted greater signal strength from an embedded phosphor layer than 532 nm excitation and much higher than 355 nm excitation. Upconversion lifetime phosphor thermometry also resulted in improved phosphor lifetime temperature sensitivity. Coupled with reduced interference from background luminescence from impurities in TBCs with upconversion, it is a promising method for temperature measurements with the thermographic phosphor embedded in or underneath a TBC.

Multi-dynamics and emission tailored fluoroperovskite-based down-conversion phosphors for enhancing the current density and stability of the perovskite solar cells

In this study, a successful attempt to drive the limit of photocurrent density (Jsc) beyond the typical values for conventional PSCs is carried out. This was achieved by using a nonlinear optical phenomenon called down-conversion (DC).

Enhancement of photoluminescence and tunable properties for Ce3+, Eu2+ activated Na2CaSiO4 downconversion phosphor: A novel approach towards spectral conversion

In the present work, we report a potential blue-emitting Na2CaSiO4:Eu2+, Ce3+ phosphor with tunable emission for ultraviolet/near ultraviolet (UV/NUV) based white light emitting diodes (WLEDs) and solar applications. In this investigation, blue-emitting Na2CaSiO4:Eu2+, Ce3+ phosphors were successfully synthesized by the sol-gel method. Their phase purity, structure, particle size, morphological behaviour, luminescence and I–V characteristics have been studied. Under 346 nm UV excitation, Na2CaSiO4:Eu2+, Ce3+ phosphor exhibits broad and strong blue emission around 440 nm. The energy transfer from Ce3+ to Eu2+ has been detected in Ce3+ and Eu2+ codoped Na2CaSiO4. The CIE chromaticity coordinates were calculated through the OSRAM SYLVANIA color calculator, which was observed in the blue region for the optimum concentration of Na2CaSiO4:Eu2+, Ce3+ phosphor. The chromaticity coordinate was found in the blue region for synthesized phosphors. In addition, the synthesized material was coated with a commercial silicon solar cell by doctor blade method and it was found that silicon-based solar cell efficiency increased around 16.48% under the solar simulator and 20.89% under direct sunlight. The entire study and results suggested that synthesized materials are potential candidates for WLEDs and solar cell efficiency enhancement.

Regulating the Singlet and Triplet Emission of Sb3+ Ions to Achieve Single-Component White-Light Emitter with Record High Color-Rendering Index and Stability.

The rapid development of solid-state lighting technology has attracted much attention for searching efficient and stable luminescent materials, especially the single-component white-light emitter. Here, we adopt a facile ion-doping technology to synthesize vacancy-ordered double perovskite Cs2ZrCl6:Sb. The introduction of Sb3+ ions with a 5s2 active lone pair into Cs2ZrCl6 host stimulates the singlet (blue) and triplet (orange) states emission of Sb3+ ions, and their relative emission intensity can be tuned through the energy transfer from singlet to triplet states. Benefiting from the dual-band emission as a pair of perfect complementary colors, the optimum Cs2ZrCl6:1.5%Sb exhibits a high-quality white emission with a color-rendering index of 96. By employing Cs2ZrCl6:1.5%Sb as the down-conversion phosphor, stable single-component white light-emitting diodes with a record half-lifetime of 2003 h were further fabricated. This study puts forward an effective ion-doping strategy to design single-component white-light emitter, making practical applications of them in lighting technologies a real possibility.

Remote Optogenetics Using Up/Down-Conversion Phosphors.

Microbial rhodopsins widely used for optogenetics are sensitive to light in the visible spectrum. As visible light is heavily scattered and absorbed by tissue, stimulating light for optogenetic control does not reach deep in the tissue irradiated from outside the subject body. Conventional optogenetics employs fiber optics inserted close to the target, which is highly invasive and poses various problems for researchers. Recent advances in material science integrated with neuroscience have enabled remote optogenetic control of neuronal activities in living animals using up- or down-conversion phosphors. The development of these methodologies has stimulated researchers to test novel strategies for less invasive, wireless control of cellular functions in the brain and other tissues. Here, we review recent reports related to these new technologies and discuss the current limitations and future perspectives toward the establishment of non-invasive optogenetics for clinical applications.

Energy transfer mechanism of KAlF4:Dy3+, Eu3+ co-activated down-conversion phosphor as spectral converters: An approach towards improving photovoltaic efficiency by downshifting layer

Rare earth activated fluoride phosphors have attention in recent years in the field of solid state lighting and solar cell efficiency enhancement. In the present study, RE (RE = Dy3+, Eu3+) activated/co-activated KAlF4 phosphor has been synthesized by low temperature wet chemical method. The structural feature of the samples was analyzed by X-ray Diffraction (XRD), which showed that the samples were crystallized in a well known structure. Photoluminescence (PL) measurement of KAlF4:Dy3+ phosphors we find that they can be efficiently excited at 351 nm and possess dominant emission bands centered at 482 nm (blue) and 572 nm (yellow). From Photoluminescence (PL) measurement of KAlF4:Eu3+ phosphors a strong orange emission at the peak wavelength of 594 nm and 616 nm were observed which could be attributed to the 5D0 → 7F1 and 5D0 → 7F2 transition in Eu3+ ions. On co-doping of rare earths in the host lattice, a strong blue and yellow emissions were observed together with the red emission. This co-doped phosphor is the coated on Si-solar cell by doctor blade method. Solar cell efficiency has been checked by I-V characteristics of the coated and blank cell. Theoretical quantum efficiency has been determined by Judd–Ofelt parameters for the proposed phosphor.

Synthesis of Highly Luminescent InP/ZnS Quantum Dots with Suppressed Thermal Quenching

InP quantum dots (QDs) are promising down-conversion phosphors for white light LEDs. However, the mainstream InP QDs synthesis uses expensive phosphorus source. Here, economic, in situ-generated PH3 is used to synthesize InP QDs and a two-step coating of ZnS shells is developed to prepare highly luminescent InP/ZnS/ZnS QDs. The QDs show a photoluminescence quantum yield as high as 78.5%. The emission can be tuned by adjusting the halide precursor and yellow emissive InP/ZnS/ZnS QDs are prepared by judiciously controlling the synthetic conditions. The yellow QDs show suppressed thermal quenching and retain >90% room temperature PL intensity at 150 °C for the growth solution. Additionally, the PL spectrum matches with the eye sensitivity function, resulting in efficient InP QD white light LEDs.

Well-defined Gd(OH)CO3, Gd2O(CO3)2·H2O, and Gd2O3 compounds with multiform morphologies and adjustable particle sizes: Synthesis, formation process, and luminescence properties

A variety of monodisperse size-tunable Gd(OH)CO3 nanospheres and Gd2O(CO3)2·H2O microplates have been synthesized via a facile homogeneous precipitation route with a mixing solvent of isopropanol (IPA) and H2O. The morphology and particle size of the gadolinium compounds can be tuned in a controlled manner by simply adjusting the volume ratio of IPA/H2O in solvent or reaction temperature and time. Both the amorphous Gd(OH)CO3 nanospheres and crystalline Gd2O(CO3)2·H2O microplates have been transformed into cubic phase of Gd2O3 after a calcination process in air. Interestingly, the Gd2O3 products perfectly inherit the well-defined morphology, good unifomity and dispersity of the precursor samples except for a shrinkage in particle size due to crystallization and coalescence at high temperature. The Gd2O3:Eu3+ and Gd2O(CO3)2·H2O:Tb3+ samples show red and green emission colors under ultraviolet excitation, and Gd2O3:Yb,Ln3+ (Ln = Er and Ho) samples exhibit characteristic orange-red and green emissions when excited at near-infrared (NIR) excitation. The morphology- and particle size-dependent luminescence performances of phosphors are studied in detail. Moreover, the light emitting diode (LED) devices fabricated with the corresponding down-conversion or up-conversion phosphors can display dazzling characteristic multicolor emissions, revealing that the as-synthesized luminescent materials have potential application prospect in the fields of LEDs, light display systems, and optoelectronic devices.

Photoluminescence and electron-vibrational interaction in 5d state of Eu2+ ion in Ca3Al2O6 down-conversion phosphor

In the present work, a series of Eu2+ activated Ca3Al2O6 down-conversion phosphors have been successfully prepared via solid-state reaction method at 800 °C. Under 334 nm excitation, PL emission spectra shows broad band emission with the peak centred at 445 nm. This broad band emission is assigned to the allowed 4f65d1 → 4f7 electronic transition of Eu2+ ions. The highest emission intensity was observed for Ca3Al2O6: 0.5 mol%Eu2+ phosphor. The CIE chromaticity diagram shows high colour purity and excellent chromaticity coordinates. This CIE diagram reveals (0.2127, 0.1102) chromaticity coordinates in the blue region with 84.21% colour purity. Further, the electron-vibrational interaction parameters such as Stokes shift, Huang-Rhys factor, effective phonon energy, zero phonon line position, and red shift have been calculated using a spectrum fitting method. These results indicate that the Eu2+ activated Ca3Al2O6 down-conversion phosphor has great potential in the field of white LEDs.

Unraveling the Luminescence Quenching of Phosphors under High-Power-Density Excitation

Laser-driven phosphor-converted white light sources have received great attention for next-generation high-brightness lighting and display technologies. However, luminescence quenching of down-conversion phosphors under high-power-density excitation severely restricts the luminous efficacy and light output of laser-driven lighting devices. Till now, the quenching mechanism, once assigned to thermal quenching, is still argued, making it difficult to select appropriate laser phosphors. In this work, we identify the optical excitation quenching (a non-thermal quenching effect) rather than thermal quenching that dominates the luminescence loss of Ce3+- and Eu2+-doped phosphors under high-power-density excitation. The optical excitation quenching is mainly caused by the energy-transfer upconversion (ETU), and the quenching rate is greatly dependent on the decay time of phosphors, evidenced by combining the experimental data with theoretical models. A critical parameter, named as optical excitation quenching behavior, is finally proposed to screen laser phosphors.