Theoretical Quantum Efficiency Research Articles

Broadband sensitized near-infrared emission in Eu2+-Nd3+ co-doped BaAl2O4 as a potential spectral modulator for silicon solar cells.

The near-infrared (NIR) down-conversion process for broadband sensitization has been studied in Eu2+-Nd3+ co-doped BaAl2O4. This material has a broad absorption band of 200-480 nm and can convert photons in the visible region into NIR photons. The NIR emission at 1064 nm, attributed to the Nd3+:4F3/2 → 4I11/2 transition, matches the bandgap of Si, allowing Si solar cells to utilize the solar spectrum better. The energy transfer (ET) process between Eu2+ and Nd3+ was demonstrated using photoluminescence spectra and luminescence decay curves, and Eu2+ may transfer energy to Nd3+ through the cooperative energy transfer (CET) to achieve the down-conversion process. The energy transfer efficiency (ETE) and theoretical quantum efficiency (QE) were 68.61% and 156.34%, respectively, when 4mol% Nd3+ was introduced. The results indicate that BaAl2O4:Eu2+-Nd3+ can serve as a potential modulator of the solar spectrum and is expected to be applied to Si solar cells.

Multimode Luminescent Properties from Tb3+-Yb3+ Codoped CaMoO4 for Solar Cell Spectral Conversion

NIR and visible emitting CaMoO4:Tb3+/Tb3+-Yb3+ phosphors were synthesized to harness the energy losses attribute to wavelength mismatch between response spectrum of incident solar photons and solar cells. The X-ray diffraction (XRD), field emission scanning microscopy (FE-SEM), fourier transform infrared spectroscopy (FTIR), UV–vis diffuse reflectance spectroscopy, photoluminescence (PL), up-conversion and life time measurements were used to characterized the prepared phosphors. XRD study followed by Rietveld refinement confirms the tetragonal crystal structure with space group I41a(88). The phosphors exhibit strong green emission (5D4→7F4) in visible region after an excitation wavelengths 378 nm and 980 nm, respectively through down-shifting (DS) and up-conversion (UC) processes. The UV photons (378 nm) are also split it into two NIR photons (2F7/2→2F5/2) through down-conversion process. Fluorescence quenching was observed by increasing concentration of Tb3+ and Yb3+ ions. Lifetime measurements clearly illustrates the energy transfer from Tb3+ to Yb3+ ions and mechanism involved is co-operative energy transfer. The estimated values of energy transfer efficiency (ETE) and corresponding theoretical internal quantum efficiency are achieved as 69% and 169% respectively.

Intrinsically Stretchable Phosphorescent Light-Emitting Materials for Stretchable Displays.

Intrinsically stretchable organic light-emitting diodes (is-OLEDs) have attracted significant attention for use in next-generation displays. However, most studies conducted to date have focused on how to make fluorescent materials stretchable, utilizing singlet excitons with a theoretical internal quantum efficiency (IQE) of 25%. Although phosphorescent materials have a high theoretical IQE of 100%, no previous work has attempted to develop stretchable phosphorescent light-emitting materials. In this work, we designed a solution-processable and intrinsically stretchable phosphorescent light-emitting layer (isp-EML) by blending various additives together with a mixture of a polymer host, poly(9-vinyl carbazole) (PVK), and a small-molecule emitting dopant, tris(2-phenylpyridine)iridium(III) (Ir(ppy)3). The poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) additive significantly improved the stretchability (∼100% strain), brightness (∼5400 cd/m2), and efficiency (∼25.3 cd/A) of the isp-EML compared with a conventional phosphorescent EML (approximately 3% strain, 3750 cd/m2, and 12.1 cd/A). Furthermore, by changing the emitting dopant in the isp-EML, we could control the red, green, and blue emission colors, with increasing mechanical and electrical properties of the isp-EML. These results highlight the promising potential of the novel blend system using phosphorescent materials and additives for application in highly stretchable and efficient OLEDs.

Efficient and Unidirectional Launching of Surface Plasmons from a Hyperbolic Meta‐Antenna

AbstractTunnel nanojunctions associated with inelastic electron tunneling have demonstrated crucial applications in on‐chip photonic and plasmonic circuitries due to their high photon modulation speed, large‐scale integration capability, and working‐wavelengths range tunability. However, because most electrons tunnel through a junction elastically, the external quantum efficiency of a nanojunction‐based plasmonic source tends to be around 10−4, severely limiting their applications to date. In this work, an integrated high‐efficiency unidirectional plasmonic source composed of an edge‐to‐edge thickness gradient hyperbolic meta‐antenna is proposed. By engineering the extra wavevector dimension, this study demonstrates a theoretical external quantum efficiency of up to 23% for this system. This is attributed to the large local density of optical states from hyperbolic dispersion and wavevector‐match conditions provided by the optical antennas. Furthermore, this study also demonstrates the tunability of this system across a range of wavelengths from 1300 to 1700 nm. The implementations of these metamaterial‐based tunneling structures enable fast and tunable on‐chip high‐efficiency sources for applications in high‐performance plasmonic circuitries.

Exploring the Electroluminescent Applications of Imidazole Derivatives.

Recently, the fields of organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs) have improved tremendously with regard to tunable emission, efficiency, brightness, and thermal stability. Imidazole derivatives are excellent deep blue-green light-emitting layers in the OLED or LEC devices. This Review summarizes the major breakthroughs of various electroluminescence (EL) layers with imidazole-containing organic or organometallic derivatives, the molecular design principles, and their light-emitting performances as effective EL materials. The highly tunable chemical structures and flexible molecular design strategies of imidazole-based compounds are advantages that provide great opportunities for researchers. They can provide a good basis for the design and development of new EL materials with narrower emission and higher efficiency. Moreover, imidazole compounds have demonstrated breakthrough performances in thermally activated delayed fluorescence (TADF) properties where triplet excitons are utilized to inhibit anti-intersystem quenching, showing great promise in breaking the theoretical external quantum efficiencies (EQE) limits in traditional fluorescent devices.

Photoemission from GaAs-based photocathodes with multilayer complex structures

To enhance the near-infrared response of photocathodes in the application of image detection, two kinds of multilayer complex structures of GaAs-based photocathodes are developed by changing the structure of the buffer layer underneath the emission layer, wherein one is the graded bandgap structure, and the other is the distributed Bragg reflector (DBR) structure. The theoretical quantum efficiency suitable for the reflection-mode (r-mode) GaAs-based photocathode with these two kinds of buffer structures are deduced based on one-dimensional continuity equations, among which the reflectivity changing with the wavelength of incident light is considered in particular. By comparison of spectral characteristics of photocathodes with the two different structures, and analysis of cathode structure parameters, it is found that the photoelectric performance of the photocathodes with the two structures are quite different, and the structure parameters especially the thickness of GaAs emission layer have a great impact on the spectral characteristic. The quantum efficiency of photocathode with graded bandgap structure is improved due to the introduction of the built-in electric field and the decrement of interface recombination, while for cathode with DBR structure, the quantum efficiency is improved by the multiple reflection of light between two parallel mirrors in the stack of alternating quarter-wave layers of high and low refractive. The theoretical quantum efficiency calculation and analysis would provide theoretical guidance for the better design of GaAs-based photocathodes.

Enhancement of near-infrared response for GaAs-based photocathode with laminated graded-bandgap structure: theory and experiment

To enhance the quantum efficiency of GaAs-based photocathode in the near-infrared range, a laminated graded-bandgap photocathode structure consisting of a distributed Bragg reflection layer and a graded-bandgap emission layer is proposed. The theoretical optical properties and quantum efficiency of this proposed photocathode are simulated based on the finite-different time-domain method and the one-dimensional continuity equations, respectively. The absorptivity at 1064 nm can be significantly increased for the new-type photocathode because of the secondary absorption caused by the distributed Bragg reflection layer. With the improvement of absorptivity at 1064 nm, the quantum efficiency at specific wavelength is enhanced significantly. The contribution of the DBR structure and built-in electric field to the quantum efficiency is theoretical investigated respectively. According to theoretical structure design, the cathode samples grown by epitaxial technique are prepared to verify the theoretical prediction of quantum efficiency enhancement. After the treatment of surface cleaning and Cs/NF3 activation, the experimental results show that the minimum reflectivity at 1064 nm is realized, and the quantum efficiency of the sample with proposed structure is ten times than that of the sample without distributed Bragg reflection layer at 1064 nm. This research provides a guidance for the design of novel photocathode structures with enhanced response at special wavelength.

Visible to near infrared energy transfer in Ce3+, Yb3+ co-doped YAG crystal prepared by the co-precipitation method

Nanoscaled Ce3+, Yb3+ co-doped yttrium aluminum garnets (YAG: Ce3+, Yb3+) with Yb3+ concentration of 0–5 mol% were synthesized by the co-precipitation method. The microstructure and surface morphology of the samples were analyzed using X-ray diffraction and scanning electron microscope. The energy transfer (ET) from Ce3+ to Yb3+ in YAG: Ce3+, Yb3+ was verified by excitation spectra, emission spectra, and the luminescence decay time. The maximum ET efficiency (ηET) and the theoretical total quantum efficiency (ηQE) reach 91.2% and 191.2%, respectively. The near infrared cathodoluminescence (CL) of Yb3+ was also investigated. Compared to the single-doped YAG: Ce3+, co-doped YAG: Ce3+, Yb3+ shows a 2.3 times higher emission intensity, demonstrating the possibility of the ET process in CL. The YAG: Ce3+, Yb3+ prepared by the co-precipitation method can be used as a potential scintillator for high energy radiation applications.

White Fluorescent Organic Light-Emitting Diodes with 100% Power Conversion

Energy-efficient lighting sources are desired to provide another solution of carbon emission reduction. White organic light-emitting diodes are promising, because of theoretical internal quantum efficiencies for 100% electric-to-light conversion. However, pure organic fluorescent materials still face a challenge in harvesting triplet excitons for radiation. Herein, we report a white fluorescent organic light-emitting diode having an external quantum efficiency of 30.7% and a power efficiency of 120.2 lm W −1 . In the single emissive layers, we use blue thermally activated delayed fluorescent emitters to sensitize a yellow fluorescent emitter. Transient photoluminescence and electroluminescence analyses suggest that a blue thermally activated delayed fluorescent molecule with ~100% reverse intersystem crossing efficiency and negligible triplet nonradiative rate constant completely converts triplet to singlet, suppressing triplet quenching by a yellow fluorescent emitter and ensuring 100% power conversion.

Spectroscopic properties and dynamic analysis of three-photon quantum-cutting luminescence in Er doped NaYF4 nanoparticles

Spectroscopic properties and dynamic analysis play a significant role during the design and development of novel luminescent materials. In this work, Er3+ doped NaYF4 nanoparticles were successfully synthesized by coprecipitation method. The absorption spectrum, fluorescence spectra, and decay curves were measured at room temperature. The spectroscopic properties, including the radiative and nonradiative transition probability, fluorescent branch ratio, cross-relaxation rate and the fluorescence lifetime, were calculated based on the Judd-Ofelt theory. Furthermore, the rate equation models were established to analyze the dynamic processes of three-photon near-infrared quantum cutting luminescence. The time-dependent population densities of excited levels of Er3+ ions were simulated and the theoretical quantum efficiency was calculated to be 252.40%, which is consistent with our experimental results.

Effect of silver co-doping on luminescence of the Pr3+-doped lithium tetraborate glass

Spectroscopic and luminescent properties of the Pr-doped and Pr–Ag co-doped borate glasses with Li2B4O7 (Li2O–2B2O3) composition are studied and discussed. Experimental and theoretical oscillator strengths (fexp and ftheor), phenomenological parameters (Ω2, Ω4, Ω6), radiative properties (Arad, β, τrad), and quantum efficiencies (η) of the Pr3+ luminescence were calculated basing on the obtained experimental results and Judd–Ofelt theory. Most intense band of the Pr3+ luminescence is peaked at 601 nm (1D2 → 3H4 transition). Presence of the isolated Ag+ (4d10, 1S0) ions, small non-plasmonic Ag aggregates (Agmn+ nanoclusters), and plasmonic Ag metallic nanoparticles in the Li2B4O7:Pr,Ag glass was proposed basing on analysis of the photoluminescence and optical absorption spectra. Observed enhancement of luminescence intensity and increasing of stimulated emission cross-section and quantum efficiency of luminescence in the Li2B4O7:Pr,Ag glass is explained by energy transfer from Ag species to the Pr3+ ions as wells local-field effect induced by Ag nanoparticles.

UV–Vis–NIR broadband response of GaAs-based photocathode with multilayer graded-band cascade structure

To improve the lattice matching, quantum efficiency and response range of the transmission-mode GaAs-based photocathode , a novel photocathode with multilayer graded-band cascade emission layer is proposed. The one-dimensional steady-state continuity equations and finite-difference time-domain methods are utilized to obtain the quantum efficiency and optical absorption characteristics of the proposed photocathode, respectively. The results show that, the built-in electric fields generated by the varying-composition and varying-doping structure can assist the photoelectrons generated in the sublayer itself and the front sublayers to transport towards the emitting surface and escape to the vacuum. Compared with the conventional AlGaAs/InGaAs structure, the proposed structure possesses better lattice matching and enhanced quantum efficiency in the UV and NIR wavelength range. A three-fold increase of quantum efficiency can be realized at 400 nm and the theoretical quantum efficiency at 1064 nm can reach 2%. Besides, the integral absorptivity is increased by 6.8% in the visible wavelength range, and 12.9% in the NIR wavelength range. It is found that the increase of the thickness of AlGaAs sublayer and GaAs sublayer mainly reduces the quantum efficiency in the UV–Vis wavelength range. When the InGaAs sublayer is thin enough, the increase of thickness would enhance the quantum efficiency in the NIR wavelength range and deteriorate that in the UV–Vis wavelength range. This work can contribute to the performance improvement of UV–Vis–NIR broadband GaAs-based photocathode. • A transmission-mode photocathode with multilayer graded-band cascade structure is proposed. • The quantum efficiency model of the proposed multilayer structure is deduced. • The quantum efficiency and optical absorption characteristics are investigated. • The lattice matching, quantum efficiency and response range are improved.

Quantum cutting properties in KYF4:Tb3+, Yb3+ phosphors: Judd-Ofelt analysis, rate equation models and dynamic processes

Spectroscopic properties play a significant role during the design and development of novel luminescent materials. In this work, quantum cutting (QC) luminescent materials KYF4:Tb3+,Yb3+ phosphors were successfully synthesized by sol–gel method. The crystal structure, morphology, fluorescence spectra, and decay curves were investigated by X-ray diffractometer (XRD), scanning electron microscope (SEM) and spectrofluorometer. The Judd-Ofelt (J-O) parameters, Ωλ (λ = 2, 4, 6), were directly derived from the emission spectra and decay curves, attributed to the transition intensities of Tb3+ ions were dependent on the J-O parameters. Then, the radiative and nonradiative transition rates, fluorescent branching ratios, cross-relaxation rates and lifetimes of QC luminescence were calculated based on the obtained J-O parameters. Moreover, the rate equation models of QC luminescence were established and the time-dependent population densities in excited levels of Tb3+ ions and Yb3+ ions were simulated. Finally, the dynamic processes of QC luminescence were discussed carefully and the corresponding theoretical quantum efficiency was estimated. Our studies would provide an insight into the QC luminescence and give theoretical guidance for developing novel luminescent materials.

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.

Down-Shifting in the YAM: Ce3+ + Yb3+ System for Solar Cells.

In this work, we investigate Ce3+ to Yb3+ energy transfer in Y4Al2O9 (YAM) for potential application in solar spectrum down-converting layers for photovoltaic devices. Photoluminescence properties set, of 10 samples, of the YAM host activated with Ce3+ and Yb3+ with varying concentrations are presented, and the Ce3+ to Yb3+ energy transfer is proven. Measurement of highly non-exponential luminescence decays of Ce3+ 5d band allowed for the calculation of maximal theoretical quantum efficiency, of the expected down-conversion process, equal to 123%. Measurements of Yb3+ emission intensity, in the function of excitation power, confirmed the predominantly single-photon downshifting character of Ce3+ to Yb3+ energy transfer. Favorable location of the Ce3+ 5d bands in YAM makes this system a great candidate for down-converting, and down-shifting, luminescent layers for photovoltaics.

Down-shifting and down-conversion emission properties of novel CdO–P2O5 invert glasses activated with Pr3+ and Pr3+/Yb3+ for photonic applications

The down-shifting and down-conversion emission properties of novel CdO–P2O5 invert glasses activated with Pr3+ and Pr3+/Yb3+ were respectively studied. The down-shifting emission spectra of Pr6O11 singly doped glasses upon 443 nm excitation (Pr3+: 3H4 → 3P2), displayed the feature Pr3+ transitions in the visible and near-infrared (NIR) regions, reaching the optimum intensity at 0.7 and 0.3 mol% of Pr6O11, respectively. The emission tonality can be adjusted from the reddish-orange to orange-pink region, depending on the Pr6O11 content. Such fact coupled with the superposition of the blue light excitation might be attractive for white light-emitting diodes (W-LEDs) applications. The cross-relaxation processes involved between Pr3+-Pr3+ pairs seem to be mediated by an electric quadrupole-quadrupole interaction, as revealed by the Inokuti-Hirayama model. The emission spectra of the Pr6O11 and Yb2O3 doped glasses upon 443 nm excitation showed, in addition to the Pr3+ related transitions, a band at 977 nm associated with the Yb3+: 2F5/2 → 2F7/2 transition, which gradually grows at expenses of non-radiative energy transfer from Pr3+. This process might lead to a down-conversion (quantum cutting) emission with theoretical quantum efficiencies up to 144%. Analysis complemented by the Inokuti-Hirayama or Dexter model revealed that the non-radiative Pr3+ → Yb3+ energy transfer process arisen from Pr3+: 3P0 and 1D2 levels are most likely dominated by electric dipole-dipole and quadrupole-quadrupole interactions, respectively. The global emission properties suggest that the Pr3+ and Pr3+/Yb3+ activated CdO–P2O5 invert glasses might be interesting for WLEDs and crystalline silicon (c-Si) solar cell applications.

A Plastic Scintillator Based on an Efficient Thermally Activated Delayed Fluorescence Emitter 9‐(4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)‐2‐methylphenyl)‐3,6‐dioctyl‐9H‐carbazole for Pulse Shape Discrimination Measurement

AbstractPulse shape discrimination (PSD)‐capable plastic scintillator is in demand for the detection of high‐energy neutrons in the presence of gamma radiation background. Conventional PSD plastics are based on delayed fluorescent dye via triplet–triplet annihilation of a fluorescent dye additive, which is inefficient to harvest the energy of triplet excitons. In recent years, thermally activated delayed fluorescence (TADF) emitters have gained substantial success in organic light‐emitting diodes due to their efficient utilization of the energy of triplet excitons and thereby achieving theoretical unity internal quantum efficiencies. Herein, a highly efficient TADF dye 9‐(4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)‐2‐methylphenyl)‐3,6‐dioctyl‐9H‐carbazole is reported for the PSD application. This TADF dye may be loaded up to 30 wt% in polyvinyltoluene matrix and still retains high optical transparency. High scintillation light yield and PSD figures of merit are obtained from the TADF plastic. With the introduction of a secondary dye to further increase the utilization efficiency of the excitation energy, the light yield is increased to 6948 photons MeV−1. The measured alpha/gamma PSD figure of merit is 1.12 at the energy threshold of 100 keVee and neutron/gamma PSD figure of merit is 1.32 at the threshold of 1000 keVee.

Efficient rare earth cerium(III) complex with nanosecond d-f emission for blue organic light-emitting diodes.

In the field of RGB diodes, development of a blue organic light-emitting diode (OLED) is a challenge because of the lack of an emitter which simultaneously has a short excited state lifetime and a high theoretical external quantum efficiency (EQE). We demonstrate herein a blue emissive rare earth cerium(III) complex Ce-2 showing a high photoluminescence quantum yield of 95% and a short excited state lifetime of 52.0 ns in doped film, which is considerably faster than that achieved in typical efficient phosphorescence or thermally activated delayed fluorescence emitters (typical lifetimes >1 μs). The corresponding OLED shows a maximum EQE up to 20.8% and a still high EQE of 18.2% at 1000 cd m−2, as well as an operation lifetime 70 times longer than that of a classic phosphorescence OLED. The excellent performance indicates that cerium(III) complex could be a candidate for efficient and stable blue OLEDs because of its spin- and parity-allowed d−f transition from the Ce3+ ion.

Enhanced Surface Charge Separation Induced by Ag Nanoparticles on WO3 Photoanode for Photoelectrochemical Water Splitting

Monoclinic WO3 is considered to be a promising photoanode due to high theoretical photocurrent density and quantum efficiency. However, the practical photoelectrochemical (PEC) performance is consi...

55 W kilohertz-linewidth core- and in-band-pumped linearly polarized single-frequency fiber laser at 1950 nm.

Based on core- and in-band-pumped polarization-maintaining ${{\rm Tm}^{3 + }}$Tm3+-doped single-cladding fiber (PM-TSF, the core diameter is 9 µm) by a 1610 nm fiber laser and a distributed Bragg reflector seed laser, a linearly polarized single-frequency fiber laser (LP-SFFL) at 1950 nm with an output power of 55.3 W and a laser linewidth of 6.95 kHz is demonstrated. The output beam qualities of ${M}_x^2$Mx2 and ${ M}_y^2$My2 are measured to be 1.01 and 1.03, respectively. The slope efficiency with respect to the launched pump power is 71.0%, in comparison with a theoretical quantum efficiency of 82.6%. A polarization-extinction ratio of 19 dB and an optical signal-to-noise ratio of 58 dB are obtained from the 1950 nm LP-SFFL. To the best of our knowledge, to date, this is the highest power of 2.0 µm SFFL output directly from a strict single-mode active fiber. Our experiment offers a promising solution to the current limitations of the high-performance fiber lasers at 2.0 µm, which is particularly essential for coherent detection.