Upconversion Quantum Efficiency Research Articles

Impact of high sensitizer doping on the transient multiband upconversion luminescence in β-NaYF4:Yb/Ho microcrystals

Under intense near-infrared light excitation, upconversion luminescence (UCL) nanomaterials exhibit multiband emission and high sensitizer quenching concentration, indicating the potential to enhance upconversion (UC) quantum efficiency. In comparison to other common activator ions (e.g. Er3+, Tm3+, Ho3+), Ho3+ possesses abundant energy levels. However, previous studies mostly focused on the red (Ho3+:5F5 → 5I8) and green (Ho3+:5F4,5S2 → 5I8) emissions. In this paper, we have examined the transient UCL characteristics of β-NaYF4:Yb/Ho microcrystals (MCs) excited by a high-power 976 nm ns pulsed laser (∼0.32 GW/cm2). The energy transfer (ET) pathways with varying Yb3+ doping (20–98 mol%) are determined by the time evolution spectra. In high Yb3+ doping system (98 mol%), the electron population of Ho3+ accelerates, and the multiband UC rise is shortened by 6 times (from ∼3 μs to ∼500 ns). We identified a new pathway for electron populating with higher Yb3+ doping ( ≥ 40 mol%), which is Yb3+:2F5/2 + Ho3+:3L9 → Yb3+:2F7/2 + Ho3+:3M9. As Yb3+ concentration increases, the varying lifetime and intensity distribution of UCL leads to variations in the luminescence color evolution. Our result shows that altering the Yb3+ doping concentration provides a way to control the energy flow between energy levels of Ho3+, which can modulate the rise time and color evolution of Yb–Ho co-doped UC MCs. This study is expected to promote the advancement of fast-respond optoelectronic devices, UC displays, and micro-nano lasers. Kewordsmultiband upconversion luminescence; high-power excitation; upconversion luminescence dynamics; high sensitizer concentration; upconversion pathways.

Harnessing Heavy-Atom Effects in Multiple Resonance Thermally Activated Delayed Fluorescence (MR-TADF) Sensitizers: Unlocking High-Performance Visible-to-Ultraviolet (Vis-to-UV) Triplet Fusion Upconversion.

Ultraviolet (UV) light plays a crucial role in various applications, but currently, the efficiency of generating artificial UV light is low. The visible-to-ultraviolet (Vis-to-UV) system based on the triplet-triplet annihilation upconversion (TTA-UC) mechanism can be a viable solution. Metal-free multiple resonance thermally activated delayed fluorescence (MR-TADF) materials are ideal photosensitizers (PSs) apart from the drawback of high photoluminescence quantum yields (PLQYs). Herein, we systematically investigated the impact of the heavy-atom effect (HAE) on the MR-TADF sensitizers. BNCzBr was then synthesized by incorporating a bromine atom into the skeleton of the precursor BNCz. Impressively, the internal HAE (iHAE) leads to a significantly decreased PLQY and a remarkably increased intersystem crossing quantum yield (ΦISC). Consequently, a higher upconversion quantum efficiency of 12.5% was realized. While the external HAE (eHAE) harms the UC performance. This work guides the further development of MR-TADF sensitizers for high-performance Vis-to-UV TTA-UC systems.

Improvement in the upconversion efficiency of an alternating multilayer structure based on 9,10-bis(4-methylphenyl) anthracene and platinum octaethylporphyrin vacuum deposited on an in-plane oriented polythiophene film

Alternating layers of 9,10-bis(4-methylphenyl) anthracene (diMe-DPA) and platinum octaethylporphyrin were fabricated via vacuum deposition on an in-plane-oriented polythiophene (PT) film. The multilayer film showed higher upconversion (UC) quantum efficiency than bilayer films with similar composition and total thickness. The UC quantum efficiency of the multilayer structure deposited on the oriented PT film was three times higher than those deposited on glass substrates, suggesting that the molecular alignment of diMe-DPA increases the UC quantum efficiency.

Highly Effective Near‐Infrared Light‐Activatable Triplet Fusion Upconversion with BODIPY Annihilators: Triplet Modulation via Non‐Conjugated n–π Spatial Electron Coupling

AbstractEffective strategies to manage the triplet state of chromophores are desirable in terms of their crucial role in photonic and optoelectronic technologies. However, triplet excited‐state energy level regulation of organic molecules remains elusive. Here, non‐conjugated n–π spatial electron coupling is demonstrated within boron‐dipyrromethene (BODIPY) and can greatly reduce the low‐lying triplet state (T1) to 1.34 eV without the conventional need for π‐conjugation extension. Moreover, the close alignment between T1 states of this BODIPY derivative and the photosensitizer PtTNP contributes to the reversible triplet energy transfer, namely the intermolecular triplet excited‐state equilibrium. Furthermore, to the best of the knowledge, the first BODIPY‐based near‐infrared light‐activated triplet fusion upconversion (NIR TF‐UC) with an upconversion quantum efficiency of 13.3% is implemented. On this basis, the air‐stable solid‐state NIR TF‐UC material is successfully fabricated, whose upconverted emission can still be seen at an ultralow NIR power density of 6.4 mW cm−2. This new strategy to implement triplet state modulation will accelerate the development of NIR TF‐UC and other important areas involving triplet state, such as photoredox catalysis, photodynamic therapy, and photovoltaic devices.

(Invited) Rational Design of Quantum Dot/Mediator Interfaces for Triplet Energy Transfer and Photon Upconversion

Photon upconversion, where two or more low energy photons are converted into one high energy photon, shows great potential in bioimaging, catalysis and solar energy conversion. Photon upconversion has traditionally been realized with lanthanide-doped nanoparticles, or organic dye sensitized triplet-triplet annihilation (TTA) based upconversion platforms. In recent years, QD sensitized triplet-triplet annihilation based upconversion systems have achieved impressive upconversion quantum efficiency and demonstrated many unique advantages, including high photostability, large extinction coefficient, high spectral coverage and tunability, and low singlet-triplet energy gap. In this talk, we discuss our recent work in developing and understanding QD/mediator interface for efficient QD-sensitized photon upconversion. We summarize the main results of time-resolved spectroscopic studies of various factors affecting the rate of triplet energy transfer (TET) from the QD to the surface attached mediator (TET1) and from the mediator to the emitter in solution (TET2). To identify the key design rules, we compare three PbS sensitized upconversion systems using three mediator molecules with the same tetracene triplet acceptor at different distances from the QD. Our results show that the mediator triplet state is mostly formed by direct TET from quantum dot. With increasing distance between the mediator and PbS QD, the efficiency of the TET1 from the QD to the mediator decreases due to a decrease in the rate of this triplet energy transfer step, while the efficiency of the TET2 from the mediator to emitter increases due to a reduction in the QD induced mediator triplet state decay (via the external heavy atom effect). The rate constant of TET2 is three orders of magnitude slower than the diffusion limited value. We show that the effect of QD/mediator on the total unconversion efficiency measured under CW illumination conditions can be well accounted for by the independently determined efficiencies of TET1 and TET2 steps, providing important insight on the design and rational improvement of efficient photon upconversion systems. Ongoing detailed mechanistic studies of the TET1 process shows that both QD bright and dark exciton states contribute to the TET process and higher lying bridge states play significant role in enhancing TET from core/shell QDs.

Self-calibrated multi-mode optical thermometry in up-converting phosphor Cs2NaErCl6: Yb3+

With the development of temperature measurement technology, people put forward higher requirements for the accuracy and precision of temperature measurement. However, most of the current luminescent thermometers use a single optical signal for temperature detection, which has a certain range of temperature insensitivity, resulting in inaccurate temperature measurement. Here, an all-inorganic rare-earth halide double perovskite Cs2NaErCl6: Yb3+ (CNEC: Yb3+) phosphor was synthesized, which not only expressed triple-mode thermometric properties based on thermally coupled levels (TCLs), non-thermally coupled levels (N-TCLs) and lifetime (FL), but also capable of providing self-referencing and high-sensitivity temperature measurements under excitation at 980 nm. Benefiting from the low phonon energy of the matrix, high doping concentration and the excellent internal up-conversion quantum efficiency (UCQY) about 6.52% of optimal composition CNEC: 40%Yb3+, all modes exhibit superior sensitivities, reversibility and thermal resolution under 980 nm irradiation. In addition, a flexible thin film thermometer composed of CNEC: 40%Yb3+ was prepared to realized multi-mode thermal monitoring of local hot spots of electronic components. The excellent three-mode temperature measurement results indicated that CNEC: Yb3+ phosphors have great potential in temperature sensing.

Bodipy Dimer for Enhancing Triplet-Triplet Annihilation Upconversion Performance.

Triplet-triplet annihilation upconversion (TTA-UC) has considerable potential for emerging applications in bioimaging, optogenetics, photoredox catalysis, solar energy harvesting, etc. Fluoroboron dipyrrole (Bodipy) dyes are an essential type of annihilator in TTA-UC. However, conventional Bodipy dyes generally have large molar extinction coefficients and small Stokes shifts (<20 nm), subjecting them to severe internal filtration effects at high concentrations, and resulting in low upconversion quantum efficiency of TTA-UC systems using Bodipy dyes as annihilators. In this study, a Bodipy dimer (B-2) with large Stokes shifts was synthesized using the strategy of dimerization of an already reported Bodipy annihilator (B-1). Photophysical characterization and theoretical chemical analysis showed that both B-1 and B-2 can couple with the red light-activated photosensitizer PdTPBP to fulfill TTA-UC; however, the higher fluorescence quantum yield of B-2 resulted in a higher upconversion efficiency (ηUC) for PdTPBP/B-2 (10.7%) than for PdTPBP/B-1 (4.0%). This study proposes a new strategy to expand Bodipy Stokes shifts and improve TTA-UC performance, which can facilitate the application of TTA-UC in photonics and biophotonics.

Synthesis and Luminescence Properties of Green-to-Red Color-Tunable Upconverting K2Gd(PO4)(WO4):Yb3+,Tb3+,Eu3+ Phosphors

Scientists are increasingly interested in new inorganic luminescence materials that could be excited with near-infrared (NIR) radiation. These materials can be used as luminescent thermometers, bio-imaging agents, anti-counterfeiting pigments, etc. In this manuscript, we report the synthesis and investigation of optical properties of two series of K2Gd(PO4)(WO4):20%Tb3+ (KGPW): the first, KGPW:20%Tb3+ doped with 1–20% Eu3+, and the second, KGPW:10%Yb3+,20%Tb3+ doped with 1–20% Eu3+. The phase-pure specimens were prepared using a solid-state synthesis method. Down-shifting and upconversion luminescence studies have been performed using 340 and 980 nm excitation, respectively. For upconversion emission luminescence, Yb3+ ions were used as sensitizers in the KGPW phosphors. In these phosphors, Yb3+ ions absorb the 980 nm radiation and transfer the energy to Tb3+ ions. At his point, Tb3+ ions either emit themselves or transfer part or all of their energy to Eu3+ ions. It was observed that the emission color of the synthesized phosphors could be successfully tuned from the green to red by varying the Tb/Eu concentration ratio regardless of the 340 or 980 nm excitation. Such color change proves that one luminescent material (KGPW) can provide three colors (i.e., green, orange, and red). Herein, the optical properties, such as reflection, down-shifting excitation and emission spectra, upconversion emission spectra, fluorescence lifetime, thermal quenching, color coordinates, and quantum efficiency, were studied using steady-state and kinetic spectroscopy.

Synchronous upconversion of quantum cascade lasers in AgGaS2.

We investigate synchronous upconversion of a pulsed, tunable quantum cascade laser (QCL) in the important 5.4-10.2 µm range, with a 30 kHz, Q-switched, 1064 nm laser. The possibility to accurately control the repetition rate and pulse duration of the QCL results in a good temporal overlap with the Q-switched laser, leading to an upconversion quantum efficiency of 16% in a 10 mm-long AgGaS2 crystal. We investigate the noise properties of the upconversion process in terms of pulse-to-pulse energy stability and timing jitter. For QCL pulses in the 30-70 ns range the upconverted pulse-to-pulse stability is approximately 1.75%. The demonstrated combination of broad tunability and high signal to noise in the system is well-suited for mid-IR spectral analysis of highly absorbing samples.

(Keynote) Rational Design of Quantum Dot/Mediator Interfaces for Triplet Energy Transfer and Photon Upconversion

Photon upconversion, where two or more low energy photons are converted into one high energy photon, shows great potential in bioimaging, catalysis and solar energy conversion. Photon upconversion has traditionally been realized with lanthanide-doped nanoparticles, or organic dye sensitized triplet-triplet annihilation (TTA) based upconversion platforms. In recent years, QD sensitized triplet-triplet annihilation based upconversion systems have achieved impressive upconversion quantum efficiency and demonstrated many unique advantages, including high photostability, large extinction coefficient, high spectral coverage and tunability, and low singlet-triplet energy gap. In this talk, we discuss our recent work in developing and understanding QD/mediator interface for efficient QD-sensitized photon upconversion. We summarize the main results of time-resolved spectroscopic studies of various factors affecting the rate of triplet energy transfer (TET) from the QD to the surface attached mediator (TET1) and from the mediator to the emitter in solution (TET2). To identify the key design rules, we compare three PbS sensitized upconversion systems using three mediator molecules with the same tetracene triplet acceptor at different distances from the QD. Our results show that the mediator triplet state is mostly formed by direct TET from quantum dot. With increasing distance between the mediator and PbS QD, the efficiency of the TET1 from the QD to the mediator decreases due to a decrease in the rate of this triplet energy transfer step, while the efficiency of the TET2 from the mediator to emitter increases due to a reduction in the QD induced mediator triplet state decay (via the external heavy atom effect). The rate constant of TET2 is three orders of magnitude slower than the diffusion limited value. We show that the effect of QD/mediator on the total unconversion efficiency measured under CW illumination conditions can be well accounted for by the independently determined efficiencies of TET1 and TET2 steps, providing important insight on the design and rational improvement of efficient photon upconversion systems. Ongoing detailed mechanistic studies of the TET1 process shows that both QD bright and dark exciton states contribute to the TET process and higher lying bridge states play significant role in enhancing TET from core/shell QDs.

Lanthanide-doped nanocrystals in high-Q microtoroids for stable on-chip white-light lasers

The plentiful energy states of lanthanide ( Ln 3 + )-doped nanomaterials make them very promising for on-chip integrated white-light lasers. Despite the rapid progresses, the Ln 3 + -based white upconversion emissions are strongly restricted by their low upconversion quantum efficiency and the color stability. Herein, we combine the CaF 2 : Yb 35 Tm 1.5 Er 0.5 nanocrystals and the high- Q microtoroids, and experimentally demonstrate the chip-integrated stable white-light laser. By optimizing the sizes, density, and distributions of Ln 3 + -doped nanocrystals, the Q factors of Ln 3 + -doped microtoroids are maintained as high as 5 × 10 5 . The strong light matter interaction in high- Q microtoroids greatly enhances the upconversion emission and dramatically reduces the laser thresholds at 652 nm, 545 nm, and 475 nm to similarly low values ( 1.89 – 2.10 mJ cm − 2 ). Consequently, robust white-light microlaser has been experimentally achieved from a single microtoroid. This research has paved a solid step toward the chip-scale integrated broadband microlasers.

Modulating TTA efficiency through control of high energy triplet states.

An ideal annihilator in triplet–triplet annihilation photon upconversion (TTA-UC) can achieve a maximum of 50% quantum efficiency. This spin statistical limit depends on the energies of the triplet states of the annihilator molecule, with only 20% quantum efficiencies possible in less-optimal energy configurations (ET2 ≤ 2ET1). Our work utilises three perylene analogues substituted with phenyl in sequential positions. When substituted in the bay position the isomer displays drastically lowered upconversion yields, which can be explained by the system going from an ideal to less-ideal energy configuration. We further concluded position 2 is the best site when functionalising perylene without a wish to affect its photophysics, thus demonstrating how molecular design can influence upconversion quantum efficiencies by controlling the energetics of triplet states through substitution. This will in turn help in the design of molecules that maximise upconversion efficiencies for materials applications.

Crystal phase modified blue upconversion on Tm3+/Yb3+:BCZT ceramic phosphor benefits multifunctionality in white-light applications.

Rare earth (RE) doped perovskite oxide hosts especially titanates, are promising phosphor materials in terms of white-light emission owing to their extraordinary properties such as an exceptional hosting environment for RE-ions and a switchable crystal phase near the phase boundary. Here, we report a new strategy of crystal phase modification to enhance the blue upconversion (UC) efficiency to such an extent that the combinational mixing of blue and green/red-emitting phosphor gives intense white emission. The Lead free (Ba0.85Ca0.15)(Zr,Ti)O3 ceramics were synthesised at different sintering temperatures by incorporation of Tm3+/Yb3+ ions as dopants. The UC quantum efficiency of the Tm3+/Yb3+:BCZT sample sintered at 1300 °C was recorded at different excitation power densities. It was observed that the crystal phase transformation from tetragonal to rhombohedral symmetry in the sample near the phase boundary plays a cruicial role in improving the quantum efficiency. White-light emission applications were demonstrated by preparing biphasic samples with powder mixing of a BCZT:Tm3+/Yb3+ (blue-emitting) + BCZT:Er3+/Yb3+ (green/red-emitting) phosphor, and their composition were optimised at a mixed ratio. Thereafter, photometric characterization (CIE chromaticity, colour purity and corelated colour temperatures) was performed, and it indicated the suitability of the current biphasic samples in direct white-light (cooler) applications on an industrial scale. Crystal phase modified blue emission efficiency enhancement is a key feature of this work, which helps to generate approximately pure white-light with ideal chromacity (∼0.333, 0.343) emission when Tm3+/Yb3+:BCZT is mixed with a green emitting BCZT:Er3+/Yb3+ phosphor.

(Invited) Rational Design of Quantum Dot/Mediator Interfaces for Efficient Triplet- Annihilation-Based Photon Upconversion Systems

Photon upconversion, where two or more low energy photons are converted into one high energy photon, shows great potential in bioimaging, catalysis and solar energy conversion. Photon upconversion has traditionally been realized with lanthanide-doped nanoparticles, or organic dye sensitized triplet-triplet annihilation (TTA) based upconversion platforms. In recent years, QD sensitized triplet-triplet annihilation based upconversion systems have achieved impressive upconversion quantum efficiency and demonstrated many unique advantages, including high photostability, large extinction coefficient, high spectral coverage and tunability, and low singlet-triplet energy gap. In this talk, we discuss our recent work in developing and understanding QD/mediator interface for efficient QD-sensitized photon upconversion. We summarize the main results of time-resolved spectroscopic studies of various factors affecting the rate of triplet energy transfer (TET) from the QD to the surface attached mediator (TET1) and from the mediator to the emitter in solution (TET2). To identify the key design rules, we compare three PbS sensitized upconversion systems using three mediator molecules with the same tetracene triplet acceptor at different distances from the QD. Our results show that the mediator triplet state is mostly formed by direct TET from quantum dot. With increasing distance between the mediator and PbS QD, the efficiency of the TET1 from the QD to the mediator decreases due to a decrease in the rate of this triplet energy transfer step, while the efficiency of the TET2 from the mediator to emitter increases due to a reduction in the QD induced mediator triplet state decay (via the external heavy atom effect). The rate constant of TET2 is three orders of magnitude slower than the diffusion limited value. We show that the effect of QD/mediator on the total unconversion efficiency measured under CW illumination conditions can be well accounted for by the independently determined efficiencies of TET1 and TET2 steps, providing important insight on the design and rational improvement of efficient photon upconversion systems.

Highly efficient triplet-triplet annihilation upconversion in high viscosity phthalate ester media

Movement of molecules are always affected by the surrounding solvent molecules, so solvents have an essential effect in triplet-triplet annihilation (TTA) upconversion. In this article, the TTA upconversion system with Diiodo-BODIPY (MBDP-2I) as sensitizer and perylene as acceptor in six solvents of differing viscosity were investigated (toluene, 1,4-dioxane and four high viscosity phthalate ester solvents DMP, DBP, DNOP and DOP). The upconversion efficiency was positively correlated with the solvent viscosity. Among the six solvents, the highest upconversion quantum efficiency of 9.59% was observed in high viscosity DOP (η = 72.4 mPa s), almost 1.5 times higher than that in 1,4-dioxane (η = 3.2 mPa s, ΦUC = 6.36%). The efficient upconversion was attributed to the long-lived triplet excited states of MBDP-2I, efficient fluorescence quantum yield of perylene and efficient TTA in high viscosity phthalate esters solvents. The mechanism underlying the highly efficient TTA upconversion in the high viscosity phthalate esters solvents was investigated by solvent cage effect. In addition, significant room temperature phosphorescence of MBDP-2I at 765 nm was observed in the viscous phthalate esters solvents.

Evolution from Tunneling to Hopping Mediated Triplet Energy Transfer from Quantum Dots to Molecules.

Efficient energy transfer is particularly important for multiexcitonic processes like singlet fission and photon upconversion. Observation of the transition from short-range tunneling to long-range hopping during triplet exciton transfer from CdSe nanocrystals to anthracene is reported here. This is firmly supported by steady-state photon upconversion measurements, a direct proxy for the efficiency of triplet energy transfer (TET), as well as transient absorption measurements. When phenylene bridges are initially inserted between a CdSe nanocrystal donor and anthracene acceptor, the rate of TET decreases exponentially, commensurate with a decrease in the photon upconversion quantum efficiency from 11.6% to 4.51% to 0.284%, as expected from a tunneling mechanism. However, as the rigid bridge is increased in length to 4 and 5 phenylene units, photon upconversion quantum efficiencies increase again to 0.468% and 0.413%, 1.5-1.6 fold higher than that with 3 phenylene units (using the convention where the maximum upconversion quantum efficiency is 100%). This suggests a transition from exciton tunneling to hopping, resulting in relatively efficient and distance-independent TET beyond the traditional 1 nm Dexter distance. Transient absorption spectroscopy is used to confirm triplet energy transfer from CdSe to transmitter, and the formation of a bridge triplet state as an intermediate for the hopping mechanism. This first observation of the tunneling-to-hopping transition for long-range triplet energy transfer between nanocrystal light absorbers and molecular acceptors suggests that these hybrid materials should further be explored in the context of artificial photosynthesis.

Visible-to-ultraviolet (

In this article, visible-to-ultraviolet photon upconversion (UV-UC) by triplet-triplet annihilation in the emission range shorter than 340 nm, which has not been explored well, is presented and the relevant physicochemical characteristics are elucidated. Investigations were carried out in several deaerated solvents using acridone and naphthalene derivatives as a sensitizer and emitter, respectively. Both upconversion quantum efficiency and sample photostability under continuous photoirradiation strongly depended on the solvent. The former dependence is governed by the solvent polarity, which affects the triplet energy level matching between the sensitizer and emitter because of the solvatochromism of the sensitizer. To elucidate the latter, first we investigated the photodegradation of samples without the emitter, which revealed that the sensitizer degradation rate is correlated with the difference between the frontier orbital energy levels of the sensitizer and solvent. Inclusion of the emitter effectively suppressed the degradation of the sensitizer, which is ascribed to fast quenching of the triplet sensitizer by the emitter and justifies the use of ketonic sensitizers for UV-UC in solvents. A theoretical model was developed to acquire insight into the observed temporal decays of the upconverted emission intensity under continuous photoirradiation. The theoretical curves generated by this model fitted the experimental decay curves well, which allowed the reaction rate between the emitter and solvent to be obtained. This rate was also correlated with the difference between the frontier orbital energy levels of the emitter and solvent. Finally, based on the acquired findings, general design guidelines for developing UV-UC samples were proposed.

Lasing Frequency Up-Conversion by Using Thermal Population

We theoretically demonstrate a model which can be used to analyze frequency up-conversion of a laser wavelength by using thermal population. The proposed model uses a rate equation model of ytterbium-doped fiber with thermal population effect. The rate and power propagation equations are set up and numerically analyzed to elucidate the dependence of frequency up-conversion efficiency and thermal-optical conversion efficiency on ambient thermal power. The analytical techniques and numerical results show that using pump laser at 1 000 nm, the wavelength can be converted into 900 nm with an up-conversion quantum efficiency of about 99.97% and a cooling efficiency of about 11.1%. This theoretical model is a promising candidate for vast applications in energy-efficient laser and energy-utilizing field.

Sensitivity analysis of the theoretical performance of semiconductor upconversion nanostructures

Detailed balance models of the performance of upconverter-backed single-junction solar cells show significantly improved solar cell efficiency of over 47% under 1-sun and 63% under concentration. Realizing these predicted gains, however, requires finding or engineering materials that can realize the upconversion performance needed. Semiconductor upconversion heterostructures show the greatest potential in this respect, with over 39% solar cell efficiency predicted using a kinetic rate model to describe the upconverter photophysics. Although the kinetic rate model used was based on realistic material parameters, material design and engineering requires trade-offs that should be informed by a sensitivity analysis of the upconverter kinetic rate model assumptions. Here, we analyze the robustness of the kinetic rate model by considering how the internal upconversion quantum efficiency is affected by variations in the solar spectrum splitting (i.e., photon absorption energy ranges), upconverter absorption cross section, carrier relaxation and recombination rates, and solar concentration. We further analyze the upconverter-backed solar cell performance as a function of these variations using detailed balance methods. The results show that the theoretical performance of this upconversion paradigm under concentrated sunlight agrees with previous models and exceeds 60% solar energy conversion efficiency. More importantly, the results show that the predicted performance is relatively insensitive to the assumptions made in the model, suggesting that practical realization of such a semiconductor upconverter heterostructure paradigm is possible.

Efficient upconversion fluorescence in NaYF4:Yb3+, Er3+/mNaYF4 multilayer core-shell nanoparticles

Here, we show a core-shell nanostructure which effectively improves NIR-to-visible upconversion quantum efficiency and luminescence intensity. The NaYF4:Yb3+, Er3+/mNaYF4 (m = 0, 1, 2, 3, 4, 5) core-shell nanoparticles are synthesized using a thermal decomposition method. The structure, morphology and fluorescence spectrum of the nanoparticles are confirmed by XRD, TEM and fluorometer. The average size of the as-prepared NaYF4:Yb3+, Er3+/mNaYF4 cores is about 8 nm while the thickness of the shells varies from 0.5 nm to 8 nm for different m values. With 980 nm excitation, the nanoparticles exhibit distinct emission peaks at 522 nm, 544 nm and 650 nm. It is noticed that an increasing shell thickness enhances their fluorescence intensity. The fluorescence lifetime and quantum yield of obtained multilayer NaYF4:Yb3+, Er3+/mNaYF4 nanoparticles are also discussed.