External Quantum Yield Research Articles

Temperature and time-dependent luminescence of single crystals of KTb3F10

This work deals with the spectroscopic properties of single crystalline samples of KTb3F10. Green luminescing KTb3F10 is an efficient UV radiation converter with a quantum yield of about 40% upon UV-A excitation, while it is also an efficient scintillator upon excitation by 20–50 kV X-ray photons. The temperature dependence of the integral photoluminescence as well as the decay time of KTb3F10 monitored for the Tb3+ emission show that the internal quantum yield between 100 and 800 K is almost constant, while the external quantum yield is modulated, likely due to a change in reabsorption at elevated temperatures. In contrast, the low temperature range between 3 and 63 K reveals a change of the local Tb3+ environment in KTb3F10, which is indicated by an anomaly in the heat capacity and by a strong change of the decay time as well as by a distinct shift of the photoluminescence spectra at low temperatures. In addition, temperature dependent magnetic susceptibility measurements and a structure refinement based on single crystal X-ray diffraction data were performed.

Luminescent properties of Cu+ doped aluminosilicate glasses: Effect of optical basicity and doping content

Various Cu+ doped transparent aluminosilicate glasses were elaborated by melt quenching method. X-ray diffraction (XRD), transmission spectra, luminescent spectra and fluorescence decay curves were used to study their structural and luminescent properties systematically. By increasing optical basicity of glasses, excitation and emission peaks of Cu+ shift from 300 to 321 nm and from 445 to 536 nm, respectively. Furthermore, longer excitation (330 nm) and emission wavelengths (545 nm) are realized via growing Cu+ concentration. It suggests that the absorption and emission band of Cu+ in aluminosilicate glasses can be modified by altering the optical basicity of glass composition and Cu+ content. More importantly, the as-prepared Cu+ doped glasses show highly external quantum yields (35.8–57.4%) and excellently thermal stability. At 428 K, Cu+ doped glass excited by 330 nm can keep 75% emission intensity of that at 308 K. And chromaticity shows only a tiny shift with increasing temperature. Our research may provide a strategy for extending the practical application of Cu+ doped transparent luminescence glasses.

Relation between Fluorescence Quantum Yield and Open‐Circuit Voltage in Complete Perovskite Solar Cells

Bringing the Voc of a perovskite solar cell toward its radiative value, corresponding to a 100% external fluorescence quantum yield (QY) of the cell, has been pursued to reach the highest performance photovoltaic devices. Therefore, much research has been focused on maximizing the QY of the active layer isolated from the rest of the cell layers. However, such quantity does not often correlate with the Voc following the ideal diode relation. Herein, the QYs of complete FA0.8MA0.2PbI3−yBry solar cells are reported, ranging from 0.1% to 3%, and compared with their Vocs, ranging from 1 to 1.13 V. By combining these measurements with electromagnetic simulations based on a full‐wavevector detailed balance and a fluorescence power‐loss model, it is demonstrated that a nonoptimal Voc in mixed‐cation lead halide perovskite cells is not only due to nonradiative photocarrier recombination at traps. In addition to the expected parasitic absorption of the emitted photons in the electrode layers, discrepancies appear between Voc and QY. These discrepancies are attributed to the rise of energy barriers, a side effect of trap removal. Indeed, although surface passivation may enhance the QY, its beneficial effect may be counterbalanced by the emergence of such barriers between active and charge‐transporting layers.

Observation of plasmon boosted photoelectrochemical activities on single Au/Cu2O nanoelectrode

Plasmon related photoelectrochemical (PEC) reactions have been widely reported on metallic nanostructures in a large scale. However, the detailed information on single nanostructure has been rarely reported. Herein, we construct nanoelectrode based on single Au/Cu2O nanowire to unveil how surface plasma in Au nanowire impact the PEC behavior of Au/Cu2O nanoelectrode with diameter as small as 100 nm. Even in such low dimension, we could still observe obvious photocurrent in the scale of nanoamperes. The contribution of plasma in Au nanowire has been confirmed by the external quantum yield spectra and the exponential curve of photocurrent versus light intensity. Importantly, H2 bubble has also been observed in the water reduction process. Thus, insights on PEC reactions in nanoscale have been provided.

Highly efficient luminescence in bulk transparent Sr2GdF7:Tb3+ glass ceramic for potential X-ray detection

The urgent demand for radiation detection in diverse applications boosts extensive investigations on scintillators. However, the current scintillating materials are mostly limited to mono-crystals with high cost, restricted volume and time-consuming fabrication techniques. Here, we report an efficient and transparent Tb3+-doped Sr2GdF7 glass ceramic (GC) prepared via simple melt-quenching method, and its optical response to X-ray. By incorporating Tb3+ into crystallized Sr2GdF7 inside a typical aluminosilicate glasses, highly enhanced green emission was observed under UV and X-ray excitation. After thermal treatment, the internal and external photo-luminescence quantum yields can be as high as 59.1 and 26.4%, respectively, with the high transparency (65%@550 nm) preserved. More interestingly, this composite material, combining the advantages of oxide glasses and fluoride crystals, presents strong X-ray excited luminescence (XEL). Its XEL intensity is 194% of that of commercial BGO scintillator. Evidenced by detailed optical and structural properties, it can be concluded that rare earth ions activated bulk oxyfluoride glass ceramics with unique merits, such as low-cost, high efficiency and intense radio-luminescence, could be alternative scintillating materials for high energy ray detection.

Enhancing upconversion photoluminescence by plasmonic-photonic hybrid mode.

Upconversion photoluminescence (UCPL) of rare-earth ions has attracted much attention due to its potential application in cell labeling, anti-fake printing, display, solar cell and so forth. In spite of high internal quantum yield, they suffer from very low external quantum yield due to poor absorption cross-section of rare-earth ions. In the present work, to increase the absorption by rare earth ions, we place the emitter layer on a diffractive array of Al nanocylinders. The array is designed to trap the near infrared light in the emitter layer via excitation of the plasmonic-photonic hybrid mode, a collective resonance of localized surface plasmons in nanocylinders via diffractive coupling. The trapped near-infrared light is absorbed by the emitter, and consequently the intensity of UCPL increases. In sharp contrast to the pure localized surface plasmons which are bound to the surface, the hybridization with diffraction allows the mode to extend into the layer, and the enhancement up to 9 times is achieved for the layer with 5.7 µm thick. This result explicitly demonstrates that coupling the excitation light to plasmonic-photonic hybrid modes is a sensible strategy to enhance UCPL from a thick layer.

Polymer Films with Silver Nanoparticles Improve the Spectral Characteristics of Photovoltaic Converters

A method for the synthesis of polyvinyl butyral (PVB) films containing silver nanoparticles has been developed. The properties of films applied as antireflection coatings have been studied on silicon photovoltaic converters with and without Si3N4 layers. It was established that the PVB coating with the maximum concentration of silver nanoparticles (7 mmol/L) provided a more than 20% increase in the external quantum yield in the 500–1000 nm spectral range for solar cells without a Si3N4 layer. In the case of these functional coatings on photovoltaic converters with a Si3N4 layer, the quantum yield increased on average by 10% in the 400–1000 nm wavelength interval and by 15–20% in the 320–400 nm interval.

Квантовый выход кремниевого лавинного фотодиода в диапазоне длин волн 120-170 nm

The external quantum yield of silicon avalanche photodiode in the wavelength range of 120-170 nm was performed. It was shown that the engineered avalanche photodiode has the external quantum yield of 24-150 electron/proton under reverse bias voltage of 230-345 V, respectively. The testing of worked out avalanche photodiode by means of pulse flash of 280 and 340 nm wavelength demonstrates the speed, corresponding to the bandwidth not less than 25 MHz.

MICRODISPLAY STRUCTURES BASED ON ORGANIC GREEN LIGHT EMITTING DIODES USING THERMALLY ACTIVATED DELAYED FLUORESCENCE MATERIALS

The aim of the work is to develop a new highly efficient light-emitting structure of microdisplays based on organic light-emitting diodes (OLED) for the modernization of the microdisplayes MDO 02 of the mass production. It is also intended to use the new OLED structure in subsequent developments of new series of microdisplays, including a green glow. Сomplete microdisplay element consists of а active matrix and OLED structure, which is a set of layers of low molecular weight organic materials. The active matrix of the microdisplay MDO 02 contains 800×3(RGB)×600 pixels for the full-color version and 800×600 pixels for the monochrome version. Microdisplay MDO 02 has the following characteristics: the nominal brightness of the full-color glow is 140 cd/m2, the monochrome glow is – 560 cd/m2, the unevenness of the brightness is not more than 15 %, the contrast in relative units is not less than 100:1, the power consumption is not more than 450 mW, the operating time on refusal not less than 5000 hours. To improve these characteristics, it is proposed to use OLED structure, including materials with thermally activated delayed fluorescence (TADF). Materials with TADF have a much simpler synthesis scheme, an expanded selection of starting components and do not need expensive rare and rare-earth metals, which are used for the synthesis of phosphorescent materials. A structure with high light (external quantum yield up to 26.2 %) and electrical parameters with the described dopant synthesis process was selected from a number of OLED structure. This structure consists of four organic layers: hole-injection, hole-transport, emission and electron-transport. As a dopant for the emission layer, material aICTRZs based on indocarbosol derivatives was used. The dopant aICTRZs was synthesized by us according to the proposed synthesis method. The characteristics of this structure were evaluated using an ITO / TAPC (30 nm) / TCTA (10 nm) / CBP (25 nm) / Bphen (30 nm) / LiF (0.5 nm) / Al (150 nm). Although the optical characteristics of such an LED did not reach the declared values, they showed quite good results. As a result, such an OLED structure can be used as an initial one and, with its further development, one can count on stable and high results of the optical and electrical characteristics of MDs.

Stabilization of cationic zinc phthalocyanine monomers by adsorption on clay nanosheet colloid and photogeneration of singlet oxygen

To determine the optimal conditions for the stabilization of cationic zinc phthalocyanine (ZnPc4+) monomers within an aqueous medium, the adsorption behavior of ZnPc4+ on the surface of an anionic clay nanosheet was investigated. It was determined that ZnPc4+ could be adsorbed as a monomer on the surface of an anionic clay nanosheet dispersed in an aqueous medium, containing up to 0.9 ZnPc4+ per cationic exchange site on the clay nanosheet. It was observed that the concentration of ZnPc4+ monomers in the aqueous solution was negligible when the concentration of ZnPc4+ was low. When ZnPc4+ monomers – after having been adsorbed on the surface of the anionic clay nanosheet – were photoexcited by visible light (690 nm) in the presence of 9,10-antracenediyl-bis(methylene) dimolonoic acid (ADMA), the typical absorbance of ADMA at 398 nm decreased with an increase in irradiation time. This is indicative of the photogeneration of a singlet oxygen in aqueous ZnPc4+, due to a highly stabilized ZnPc4+ monomer on anionic clay nanosheet surface. The external quantum yield of decomposed ADMA in the presence of the anionic clay nanosheet colloid in aqueous medium was measured to be 2.62 × 10−5.

Radiative Efficiency and Charge‐Carrier Lifetimes and Diffusion Length in Polycrystalline CdSeTe Heterostructures

CdTe‐based photovoltaics is a rapidly developing energy technology with one of the lowest levelized costs of electricity. But nonradiative Shockley–Read–Hall (SRH) recombination limits the efficiency of CdTe solar cells. Partial mitigation of bulk and grain‐boundary SRH recombination was achieved by alloying CdTe with Se, and CdSexTe1−x absorbers are used in high‐efficiency solar cells. Recently, interface recombination was significantly reduced with alumina passivation, but other properties of Al2O3/CdSexTe1−x/Al2O3 heterostructures have not yet been investigated. Herein, further progress in understanding and developing polycrystalline heterostructures with x = 0.2 is reported. It is shown that external photoluminescence quantum yield is increased to 0.2%, quasi‐Fermi‐level splitting to 950 mV, minority carrier lifetimes to 750 ns, and mobility to 100 cm2 Vs−1. Such polycrystalline CdSexTe1−x electronic characteristics match or exceed CdTe single‐crystal properties. The resulting charge‐carrier diffusion length of ≈14 μm is several times greater than the absorber thickness in test structures and in typical CdTe solar cells. Herein, with passivation and absorbers, polycrystalline CdSeTe solar cell open‐circuit voltage is increased from the current 76% of the Shockley–Queisser limit to 82%, or close to 1 V is reported.

Spectral analysis of the electroluminescence and photoresponse of heterostructures with InGaAs quantum objects

In this work, physical and optical properties of In0.8Ga0.2As quantum dots (QDs) embedded in the structure of a single-junction GaAs solar cell (SC) grown by MOVPE technique were investigated using spectral characteristics of external quantum yield (EQE) and electroluminescence (EL). It has been found that, in characterizing QD physical parameters, simplified model of a thin stressed quantum well can be applied to a wetting layer (WL). It has been demonstrated that the EL spectra allows determining the absorption energy of photons in WL and QDs more accurately compared to the EQE spectra. Energies of “Ee-Ehh” and “Ee-Elh” transitions in WL have been determined and were 1.325 eV and 1.388 eV respectively. The calculated values of the WL thickness (5.63 Å) and In composition of QDs (xIn = 80%) coincide with the technological parameters used in the epitaxial growth of the device.

Ga(In)AsP Lateral Nanostructures as the Optical Component of GaAs-Based Photovoltaic Converters

The possibility of using lateral Ga(In)AsP nanostructures grown by the catalytic method in a quasi-closed volume from phosphorus and indium vapors on the GaAs (100) surface as an antireflection coating for photovoltaic devices is considered for the first time. It is shown that, at a fixed growth temperature, it is possible to control the surface morphology by changing the growth duration. The surface morphology is examined by scanning electron and atomic force microscopies. It is shown that the antireflection properties of the surface in the range 400–800 nm are related to its structure. The use of such a coating in GaAs-based photocells demonstrated a significant increase in the external quantum yield of photovoltaic converters.

Aggregation-Induced Dual-Phosphorescence from Organic Molecules for Nondoped Light-Emitting Diodes.

Aggregation-induced emission (AIE) is a beneficial strategy for generating highly effective solid-state molecular luminescence without suffering losses in quantum yield. However, the majority of reported AIE-active molecules exhibit only strong fluorescence, which is not ideal for electrical excitation in organic light-emitting diodes (OLEDs). By introducing various substituent groups onto the biscarbazole compound, a series of molecular materials with aggregation-induced phosphorescence (AIP) is designed, which exhibits two distinctly different phosphorescence bands and an absolute solid-state room-temperature phosphorescence quantum yield up to 64%. Taking advantage of the AIE feature, the AIP molecules are fabricated into OLEDs as a homogeneous light-emitting layer, which allows for relatively small efficiency roll-off and shows an external electroluminescence quantum yield of up to 5.8%, more than the theoretical limit for purely fluorescent OLED devices. The design showcases a promising strategy for the production of cost-effective and highly efficient OLED technology.

Synthesis and optical properties of efficient orange emitting GdB5O9:Sm3+ phosphors

In the present study the single phase polycrystalline GdB5O9:Sm3+ target materials were prepared by aqueous sol–gel method. The powder X-ray diffraction was used to monitor the crystal phase formation. Samples doped with 1 and 2.5% Sm3+ showed bright orange–red emission upon excitation with near-UV radiation. The emission possessed excellent colour saturation and good colour coordinate stability. Emission spectra recorded at different temperatures showed that the thermal quenching activation energy is around 35 meV. Moreover, the temperature dependent photoluminescence (PL) decay measurements revealed that PL lifetime values barely change in the temperature range of 77–500 K showing that the external quantum yield decreases much faster than the internal one. The highest quantum yield of ca. 60% was obtained for the sample doped with 1% Sm3+ ions. Further increase of Sm3+ content resulted in concentration quenching as a consequence of increased probability of cross-relaxation. Rather high quantum yield and well resolved sharp emission lines makes these phosphors suitable for application as an orange–red component in near-UV LEDs, on the one hand, and the luminescent security pigments, on the other hand.

Comparative Study of p-type CuBi2O4 Films and CuBi2O4 Nanopillars Photocathode for High Performance Photoelectrochemical Water Splitting

Traditional thin films and nanostructure are the most reasonable candidates to build the next generation of photoelectrochemical water splitting system with outstanding optical and electrical properties. Especially the use of nanostructure arrays as photoelectrodes might complement the traditional semiconductor photoelectrodes in providing close transfer distance of photoinduced carriers and the increase in the surface reaction sites than thin films. Both of the reasons reduce the probability of carriers recombination and thus enhancing the photoelectrochemical performances. In this work, we demonstrated highly efficient water splitting performance of CuBi2O4 nanopillars compared to thin film CuBi2O4 photocathode. The CuBi2O4 nanopillars were fabricated by electrodeposition on anodized aluminum oxide (AAO) template. The CuBi2O4 nanopillars photocathode gave a notable improvement in photocurrent, from −0.50 to −1.50 mA cm−2 at −0.45 V vs. Ag/AgCl by the external quantum yield more than 3 times at wavelength 420 nm. Finally, the result of the study appealed that the photoelectrode based on CuBi2O4 nanostructure arrays is an encouraging system for showing efficient water splitting system under visible light.

Effect of ancillary ligands on visible and NIR luminescence of Sm3+β-diketonate complexes.

Two new Sm3+ complexes with pyrazolic β-diketones bearing a CF3 group acting as main ligands and with 2,2'-bipyridine or 1,10-phenanthroline being the ancillary ligand were studied, and their energy level structure was established. Stark splitting observed in the photoluminescence spectra of the complexes points to their non-cubic symmetry, confirmed by the calculated Judd-Ofelt intensity parameters. Internal quantum yields obtained for the compounds by the Judd-Ofelt calculations were of the order of 5.5%, whereas the measured external quantum yields were 0.75% and 1.5% for Sm3+ complexes involving 2,2'-bipyridine and 1,10-phenanthroline ancillary ligands, respectively, with the corresponding sensitization efficiencies calculated as 0.16 and 0.26. It was demonstrated that replacing the 1,10-phenanthroline ancillary ligand with 2,2'-bipyridine provides an increase in the intensity of 650 nm emission of the Sm3+ complexes, with the branching ratio reaching 55%. Intensive emission of the studied complexes at 650 nm offers hope for their use as spectrally pure red emitters.

Triazine-Acceptor-Based Green Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes.

High-efficiency thermally activated delayed fluorescence (TADF) is leading the third-generation technology of organic light-emitting diodes (OLEDs). TADF emitters are designed and synthesized using inexpensive organic donor and acceptor derivatives. TADF emitters are a potential candidate for next-generation display technology when compared with metal-complex-based phosphorescent dopants. Many studies are being conducted to enhance the external quantum efficiencies (EQEs) and photoluminescent quantum yield of green TADF devices. Blue TADF reached an EQE of over 35% with the support of suitable donor and acceptor moieties based on a suitable molecular design. The efficiencies of green TADF emitters can be improved when an appropriate molecular design is applied with an efficient device structure. The triazine acceptor has been identified as a worthy building block for green TADF emitters. Hence, we present here a review of triazine with various donor molecules and their device performances. This will help to design more suitable and efficient green TADF emitters for OLEDs.

Highly Efficient and Selective Oxidation of Ethanol to Acetaldehyde by a Hybrid Photocatalyst Consisting of SnO2 Nanorod and Rutile TiO2 with Heteroepitaxial Junction.

Single-crystal SnO2 nanorods were grown on rutile TiO2 with a heteroepitaxial relation of SnO2 {001}/TiO2 {001} (SnO2 -NR#TiO2 ) by a hydrothermal reaction. Resulting compressive lattice strain in the SnO2 -NR near the interface induces a continuous increase in the a-axis length extending over 60 nm to relax towards the [001] direction from the root to the tip. UV-light irradiation of the robust SnO2 -NR#TiO2 stably progresses the selective oxidation of ethanol to acetaldehyde with an external quantum yield of 25.6 % at excitation wavelength (λex )=365 nm under ambient temperature and pressure. Spectroscopic analyses and density functional theory simulation results suggested that the extremely high photocatalytic activity stems from the smooth interfacial electron transfer from TiO2 to SnO2 -NR through the high-quality junction and subsequent efficient charge separation due to the lattice strain-induced unidirectional potential gradient of the conduction band minimum in the SnO2 -NR.

Nearly perfect near-infrared luminescence efficiency of Si nanocrystals: A comprehensive quantum yield study employing the Purcell effect

Thin layers of silicon nanocrystals (SiNC) in oxide matrix with optimized parameters are fabricated by the plasma-enhanced chemical vapor deposition. These materials with SiNC sizes of about 4.5 nm and the SiO2 barrier thickness of 3 nm reveal external quantum yield (QY) close to 50% which is near to the best chemically synthetized colloidal SiNC. Internal QY is determined using the Purcell effect, i.e. modifying radiative decay rate by the proximity of a high index medium in a special wedge-shape sample. For the first time we performed these experiments at variable temperatures. The complete optical characterization and knowledge of both internal and external QY allow to estimate the spectral distribution of the dark and bright NC populations within the SiNC ensemble. We show that SiNCs emitting at around 1.2–1.3 eV are mostly bright with internal QY reaching 80% at room temperature and being reduced by thermally activated non-radiative processes (below 100 K internal QY approaches 100%). The mechanisms of non-radiative decay are discussed based on their temperature dependence.