Reduction In Quantum Efficiency Research Articles

Impact of Yb2+ on the anti-Stokes fluorescence cooling performance of Yb-doped silica fibers

To unlock the full potential of laser-cooled silica optical fibers, a better understanding of the internal mechanisms of heat generation is required. This work explores ytterbium-doped aluminosilicate fibers produced via industry-standard modified chemical vapor deposition (MCVD) techniques with varied levels of divalent ytterbium to determine their effect on anti-Stokes fluorescence thermal performance. The inclusion of Yb2+ is shown to have a significant negative impact on cooling potential. Yb2+ ions are shown to correlate with heat generation by two distinct mechanisms, absorption and quenching of active Yb3+ ions. This excess heating represents a reduction in quantum efficiency that is detrimental to Yb-doped fiber lasers and amplifiers beyond the laser-cooling application.

Towards dark current suppression in metallic photocathodes by selected-area oxidation

Oxide-free surfaces of polycrystalline Cu are prepared using acetic acid etching after chemical-mechanical polishing. UV ozone treatment is shown to increase the work function of the cleaned Cu by up to 0.5 eV. There is also a large reduction in quantum efficiency at 265 nm. Cu sheet can be easily masked from ozone exposure by Si or glass, meaning that selected-area oxi-dation is possible. Oxygen plasma treatment has a similar effect to the UV ozone but is more difficult to mask. There is no increase in surface roughness after oxidation, meaning that the larger work function could significantly re-duce dark current in accelerator photocathodes without affecting the desired photoemission region.

Effects of 63 MeV proton irradiation on the performance of MWIR InGaAs/InAsSb nBn photodetectors

An investigation into the effects of 63 MeV proton irradiation on high-sensitivity mid-wave infrared InGaAs/InAsSb nBn devices is performed. Three different structures with various absorber region doping profiles are irradiated and characterized to assess their impact on performance degradation. Minority carrier lifetime is measured using time-resolved photoluminescence and lifetime damage factors are assessed. The majority carrier concentration is determined via capacitance–voltage measurements and dopant introduction rates are calculated. An analysis of dark current density is performed using these material parameters, revealing a reduction in mobility with proton fluence and the emergence of a proton-induced trap energy level. Quantum efficiency is calculated at each proton fluence, and quantum efficiency damage factors show that the graded doping structure exhibits the least reduction of quantum efficiency with dose, attributed to its effective mobility enhancement. Conclusively, detector sensitivity, assessed via shot-noise limited noise-equivalent irradiance, shows that the graded doping structure is the least susceptible to high energy proton irradiation-induced performance degradation.

Room-Temperature 15% Efficient Mid-Infrared HgTe Colloidal Quantum Dot Photodiodes

Mid-infrared HgTe colloidal quantum dot photovoltaic devices previously achieved background-limited infrared photodetection at cryogenic temperatures but also decreased from 20 to 1% efficiency from 150 to 300 K. The reduced quantum efficiency was tentatively attributed to the carrier diffusion length being much shorter than the device thickness of ∼400 nm at room temperature. Here, the carrier diffusion length is measured and is found to peak at 215 nm at 200 K and decrease only to 180 nm at 295 K. It is therefore not the cause of the much larger reduced quantum efficiency. Instead, it is shown that the efficiency drops due to the series resistance. With the device size reduced to 50 by 50 μm, the room-temperature quantum efficiency reaches 10 and 15% for HgTe colloidal quantum dot devices with 2400 cm-1 (4.2 μm) and 2675 cm-1 (3.7 μm) cutoff, respectively. These small-area devices achieve background-limited photodetection at 150 K and a detectivity above 109 Jones at room temperature with a cutoff at 2675 cm-1 (3.7 μm).

Förster Resonance Energy Transfer in Luminescent Solar Concentrators.

Luminescent solar concentrators (LSCs) are an emerging technology to collect and channel light from a large absorption area into a smaller one. They are a complementary technology for traditional solar photovoltaics (PV), particularly suitable for application in urban or indoor environments where their custom colors and form factors, and performance under diffuse light conditions may be advantageous. Förster resonance energy transfer (FRET) has emerged as a valuable approach to overcome some of the intrinsic limitations of conventional single lumophore LSCs, such as reabsorption or reduced quantum efficiency. This review outlines the potential of FRET to boost LSC performance, using highlights from the literature to illustrate the key criteria that must be considered when designing an FRET‐LSC, including both the photophysical requirements of the FRET lumophores and their interaction with the host material. Based on these criteria, a list of design guidelines intended to aid researchers when they approach the design of a new FRET‐LSC system is presented. By highlighting the unanswered questions in this field, the authors aim to demonstrate the potential of FRET‐LSCs for both conventional solar‐harvesting and emerging LSC‐inspired technologies and hope to encourage participation from a diverse researcher base to address this exciting challenge.

Fluorescence ratio and photochemical reflectance index as a proxy for photosynthetic quantum efficiency of photosystem II along a phosphorus gradient

Sun-induced chlorophyll fluorescence (SIF) is one of the most promising remote-sensing signals to assess spatio-temporal variation in photosynthesis. Yet, it has been shown that the positive linear relationship of SIF and photosynthesis, often reported from satellite and proximal remote sensing, is mainly driven by the amount of absorbed photosynthetic active radiation (APAR). By normalizing SIF by APAR these structural first-order effects can accounted for and SIF is then reflecting physiological regulation of photosynthetic efficiency. However, because of the confounding contribution of non-photochemical energy dissipation, the relationship between SIF and photosynthetic efficiency is non-linear, and therefore additional measurements have to be included to constrain the predictions of photosynthetic efficiency and photosynthetic electron transport.We grew Zea mays at different phosphorus (P) levels to assess if P-induced reduction in quantum efficiency of PSII (ΦPSII), can be estimated by the fluorescence efficiency parameters, APAR normalized fluorescence (Fyield) and the ratio of the two emitted fluorescence peaks (F↑ratio), at leaf level. Results were compared to the photochemical reflectance index (PRI), a well-established index related to the activity of the xanthophyll cycle, a protection mechanism which activates under light-stress conditions. We demonstrate that the relationship between ΦPSII and Fyield is non-monotonic across a P limitation gradient, rendering the prediction of ΦPSII by Fyield alone unfeasible. We show, however, that the pigment corrected PRI (cPRI) and F↑ratio (cF↑ratio) share a strong linear relationship with ΦPSII, thereby enabling the estimation of ΦPSII. We demonstrate that a compensation for reabsorption effects improved the estimation of ΦPSII by Fratio at foliar level. This may allow improved predictions of photosynthetic light use efficiency parameters without the need of measuring green APAR.

Photoluminescence Properties and Quantum Efficiency of Eu<sup>3+</sup>/Mn<sup>2+</sup>- Doped ZnMoO<sub>4</sub> Red Phosphor

In this work, fabrication and characterization of a nanostructured rare-earth-doped ZnMoO4 will be reported. Photoluminescence properties and quantum efficiency of the rare-earth-doped ZnMoO4 with different dopant concentrations have been investigated. These samples were synthesized by sol-gel method. Lattice structure of the fabricated samples was characterized by X-ray powder diffraction (XRD); absorption spectrum was performed on UV-2600 photo spectrometer; PL excitation and emission spectra were recorded by Fluorescence Spectrometers; quantum efficiency was measured by an integrating sphere photoluminescence (PL) system. The results showed that the optimized doping concentration of Eu3+ was around 10 mol% for the highest quantum efficiency at 616 nm emission peak and 465 nm excitation peak. The highest internal quantum efficiency was 91% at low power density excitation (around 50 μW/mm2). Introduction of Mn2+ to Eu3+-doped ZnMoO4 lead to reduced quantum efficiency and electronic lifetime, which can be attributed to defects inside the crystal lattice and energy transfer from Eu3+ to Mn2+ (more non-radiative transition occur).

Sphagnum outcompetes feathermosses in their photosynthetic adaptation to postharvest black spruce forests

Mosses dominate the understory of black spruce forest, and changes in moss community composition and functional traits may influence many important ecosystem processes, particularly paludification due to accumulation of peat (mainly Sphagnum). To understand performance and fitness of ground-cover mosses in early forest succession following clearcutting, we investigated photosynthetic traits in Sphagnum and three feathermoss species (i.e., Hylocomium, Ptilium, and Pleurozium) coexisting in mature stands and decade-old black spruce harvest blocks. The results showed that all of these mosses have a significantly higher light saturation point at the harvested plots than the forested plots. Feathermosses at the harvested plots all underwent a reduction in quantum efficiency, chlorophyll b content, and chlorophylls:carotenoids ratio related to photoinhibition, particularly Pleurozium, which experienced a significant decrease in maximum net photosynthesis. In contrast, Sphagnum showed a significant increase in maximum gross photosynthesis, dark respiration, and the ratio of chlorophyll a:b at the harvested plots compared with the forested plots. The distinctive responses of moss photosynthetic traits indicate the potential proliferation of Sphagnum and decrease of feathermosses, and thus consequent peat accumulation. Our results emphasize the importance of the photosynthetic traits of moss as indicators of postharvest conditions for enhancing understory vegetation management to maintain and improve productivity of black spruce.

The potential benefits of using higher X-ray energies for macromolecular crystallography

Using X-ray energies higher than those normally used (5-15 keV) for macromolecular X-ray crystallography (MX) at synchrotron sources can theoretically increase the achievable signal as a function of dose and reduce the rate of radiation damage. In practice, a major stumbling block to the use of higher X-ray energy has been the reduced quantum efficiency of silicon detectors as the X-ray energy increases, but hybrid photon-counting CdTe detectors are optimized for higher X-ray energies, and their performance has been steadily improving. Here the potential advantages of using higher incident beam energy together with a CdTe detector for MX are explored, with a particular focus on the advantages that higher beam energies may have for MX experiments with microbeams or microcrystals. Monte Carlo simulations are presented here which for the first time include the efficiency responses of some available X-ray detectors, as well as the possible escape of photoelectrons from the sample and their entry from surrounding material. The results reveal a `sweet spot' at an incident X-ray energy of 26 keV, and show a greater than factor of two improvement in diffraction efficiency at this energy when using microbeams and microcrystals of 5 µm or less.

The potential benefits of using higher X-ray energies for macromolecular crystallography

Using X-ray energies higher than those normally used (5-15 keV) for macromolecular X-ray crystallography (MX) at synchrotron sources can theoretically increase the achievable signal as a function of dose and reduce the rate of radiation damage. In practice, a major stumbling block to the use of higher X-ray energy has been the reduced quantum efficiency of silicon detectors as the X-ray energy increases, but hybrid photon-counting CdTe detectors are optimized for higher X-ray energies, and their performance has been steadily improving. Here the potential advantages of using higher incident beam energy together with a CdTe detector for MX are explored, with a particular focus on the advantages that higher beam energies may have for MX experiments with microbeams or microcrystals. Monte Carlo simulations are presented here which for the first time include the efficiency responses of some available X-ray detectors, as well as the possible escape of photoelectrons from the sample and their entry from surrounding material. The results reveal a `sweet spot' at an incident X-ray energy of 26 keV, and show a greater than factor of two improvement in diffraction efficiency at this energy when using microbeams and microcrystals of 5 µm or less.

Does Spraying of Atrazine on Triazine-Resistant Canola Hybrid Impair Photosynthetic Processes?

ABSTRACT: Canola is an important rotation crop for the winter season and the use of atrazine-resistant hybrids can lead to an increase in yield. This work was aimed at evaluating the effect of atrazine on photochemical and biochemical processes of photosynthesis in triazine-resistant canola. The experiment was conducted in a greenhouse, with triazine-resistant hybrid Hyola® 555TT, in a randomized block design with three replications. The treatments consisted of application or no application of atrazine on canola plants. The plants were assessed at one, three, five, and eight days after application (DAA) for chlorophyll indexes, modulated chlorophyll a fluorescence and gas exchange. Chlorophyll indexes were higher in canola plants treated with atrazine. Application of atrazine caused an increase in fluorescence at steady state and a reduction in quantum efficiency of photosystem II and electron transport rate, at 1 DAA, and a reduction in photochemical quenching, at 1 and 3 DAA. Lower stomatal conductance, at 1 DAA, and higher net carbon assimilation rate, at 8 DAA, were found in plants treated with atrazine. The application of atrazine temporarily reduces electron transport between photosystems and increases chlorophyll indexes in resistant canola plants, raising the net carbon assimilation rate at eight days after application.

Brightness of femtosecond nonequilibrium photoemission in metallic photocathodes at wavelengths near the photoemission threshold

The operation of photoemission electron sources with wavelengths near the photoemission threshold has been shown to dramatically decrease the minimum achievable photocathode emittance, but at the cost of significantly reduced quantum efficiency (QE). In this work, we show that for femtosecond laser and electron pulses, the increase in required laser intensities due to the low QE drives the photocathode electronic distribution far from static equilibrium. We adapt an existing dynamic model of the electron occupation under high intensity laser illumination to predict the time-dependent effects of the nonequilibrium electron distribution on the QE, mean transverse energy (MTE), and emission brightness of metal photocathodes. We find that multiphoton photoemission dramatically alters the MTE as compared to thermal equilibrium models, causing the MTE to no longer be a monotonic function of photon excess energy.

Design and characterization of InGaP-based InP quantum dot solar cells

In an ideal intermediate-band (IB) concept, the host semiconductor generates current by absorbing short-wavelength light and the IB is used to absorb long-wavelength light. Here, we investigate the impact of the host absorber thickness at the front side of the cells on the properties of InGaP-based InP quantum dot (QD) solar cells. We prepared the InGaP-based InP QD cells with the front i-InGaP layer and compared them with the cells without the front i-InGaP layer. The insertion of the front i-InGaP layer results in an increase in quantum efficiency in the visible region, indicating that the short-wavelength light absorbed at the InGaP host increases the short-circuit current density in the cell. In addition, a thick host layer leads to reduced quantum efficiency below the host bandgap energy, which indicates that the thermal escape from QDs is suppressed. These results indicate that the optimization of the host semiconductor thickness is critical for achieving the ideal operation of the IB concept in the QD solar cells.

Phytoremediation of chlorpyrifos in aqueous system by riverine macrophyte, Acorus calamus: toxicity and removal rate.

The potential of Acorus calamus to remove chlorpyrifos from water was assessed under laboratory conditions. Toxic effects of the insecticide in A. calamus were evaluated using pulse-amplitude modulated chlorophyll fluorescence techniques as well. At exposure concentrations above 8mgL(-1), A. calamus showed obvious phytotoxic symptom with significant reduction in quantum efficiency of PSII (ΦPSII) and photochemical quenching coefficient (qP) in 20-day test; the inhibition of maximal quantum efficiency of PSII (Fv/Fm) was accompanied by a significant rise in initial chlorophyll fluorescence (Fo) within 15-day exposures. Fv/Fm and Fo recover to the normal level after 20-day exposure. The reduced removal rate to chlorpyrifos was observed with increase of initial chlorpyrifos concentrations. At application levels of 1, 2, and 4mgL(-1), the disappearance rate of chlorpyrifos in the hydroponic system with plants was significantly greater than that without plants during the 20-day test periods. Chlorpyrifos was taken up from medium and transferred to above ground tissues by the plant and significant amounts of chlorpyrifos accumulated in plant tissues. The result indicated that A. calamus can promote the disappearance of chlorpyrifos from water and may be used for phytoremediation of water contaminated with a relatively low concentration of chlorpyrifos insecticide (<4mgL(-1)).

Development of High-Bandgap AlGaInP Solar Cells Grown by Organometallic Vapor-Phase Epitaxy

AlGaInP solar cells with bandgaps between 1.9 and 2.2 eV are investigated for use in next-generation multijunction photovoltaic devices. This quaternary alloy is of great importance to the development of III–V solar cells with five or more junctions and for cells optimized for operation at elevated temperatures because of the high bandgaps required in these designs. In this work, we explore the conditions for the organometallic vapor-phase epitaxy growth of AlGaInP and study their effects on cell performance. Initial efforts focused on developing ∼2.0-eV AlGaInP solar cells with a nominal aluminum composition of 12%. Under the direct spectrum at 1000 W/m2 (AM1.5D), the best of these samples had an open-circuit voltage of 1.59 V, a bandgap-voltage offset of 440 mV, a fill factor of 88.0%, and an efficiency of 14.8%. We then varied the aluminum composition of the alloy from 0% to 24% and were able to tune the bandgap of the AlGaInP layers from ∼1.9 to ∼2.2 eV. While the samples with a higher aluminum composition exhibited a reduced quantum efficiency and increased bandgap-voltage offset, the bandgap-voltage offset remained at 500 mV or less, up to a bandgap of ∼2.1 eV.

CMOS-sensors for energy-resolved X-ray imaging

Due to their low noise, CMOS Monolithic Active Pixel Sensors are suited to sense X-rays with a few keV quantum energy, which is of interest for high resolution X-ray imaging. Moreover, the good energy resolution of the silicon sensors might be used to measure this quantum energy. Combining both features with the good spatial resolution of CMOS sensors opens the potential to build ``color sensitive" X-ray cameras. Taking such colored images is hampered by the need to operate the CMOS sensors in a single photon counting mode, which restricts the photon flux capability of the sensors. More importantly, the charge sharing between the pixels smears the potentially good energy resolution of the sensors. Based on our experience with CMOS sensors for charged particle tracking, we studied techniques to overcome the latter by means of an offline processing of the data obtained from a CMOS sensor prototype. We found that the energy resolution of the pixels can be recovered at the expense of reduced quantum efficiency. We will introduce the results of our study and discuss the feasibility of taking colored X-ray pictures with CMOS sensors.

Quantification and Modeling of Spectral and Angular Losses of Naturally Soiled PV Modules

Spectral and angle of incidence (AOI) losses on naturally soiled crystalline silicon photovoltaic (PV) modules have been investigated in this study. The test modules designated as “moderately soiled (3 g/m2 )” and “heavily soiled (74.6 g/m2)” showed short-circuit current ( $I_{{\rm sc}}$ ) losses of about 10% and 41%, respectively. The spectral reflectance and quantum efficiency (QE) losses were also quantitatively determined. In the wavelength range of 350–1100 nm, the average reflectance of moderately and heavily soiled modules increased (as compared with the clean surface) by 58.4% and 87.2%, respectively. In the moderately soiled module, the 26.3% (average) reduction in QE is mainly because of 23% of absorption and 5.5% of reflection in the dust. In the highly soiled module, the 75.3% (average) reduction in QE is mainly because of 62% of absorption and 31% of reflection in the dust particles. It is also seen that the typical critical AOI of 57° for cleaned PV modules decreased to 38° for the moderately soiled module and 20° for the heavily soiled module. This influence is crucial for fixed tilt modules as they experience a wide range of AOI during daily operation, and a significant fraction of energy is generated at higher AOIs.

Changes in leaf gas exchange and biomass of Eucalyptus camaldulensis in response to increasing drought stress induced by polyethylene glycol

Under increasing short-term drought stress, E. camaldulensis plantlets showed reduced stomatal conductance, then decreased carboxylation efficiency, and finally, reduced quantum efficiency of photosynthesis and chlorophyll content. We evaluated the physiological responses of a drought-tolerant (DT) clone and a drought-sensitive (DS) clone of Eucalyptus camaldulensis to drought stress. We evaluated leaf gas exchange, chlorophyll fluorescence, chlorophyll content, and biomass of 4-month-old eucalyptus plantlets during a graduated series of drought stress treatments over 24 days. The plantlets were grown in half-strength Hoagland’s solution, and drought stress was imposed by adding polyethylene glycol (PEG 6000) to the nutrient solution. The plantlets were subjected to three stepwise levels of drought stress, each 8 days long. The osmotic potential of the nutrient solution was −70, −140, and −280 kPa under mild, moderate, and severe drought stress, respectively. Compared with unstressed plantlets, the drought-stressed plantlets of both clones showed significant decreases in leaf gas exchange, chlorophyll fluorescence parameters, and biomass accumulation. The limitation of photosynthesis under mild drought stress was mainly because of reduced stomatal conductance. The reduction in the photosynthetic rate (P n ) under moderate to severe drought stress was because of both stomatal and non-stomatal limitation; that is stomatal closure, reduced carboxylation efficiency, reduced quantum efficiency, and decreased chlorophyll content. Both clones used stomatal control as an avoidance mechanism under drought stress. The differences between the two clones were already shown under mild drought stress. Compared to its non-stressed condition, the DS clone showed larger decreases in P n , stomatal conductance, and transpiration rate than the DT clone. The stepwise scheme in imposing drought stress can differentiate the drought response traits in E. camaldulensis.

Synthesis of dibenzothiophene-based host materials and their blue phosphorescent device performances

Two structural isomeric host materials, 9-(4-(dibenzo[b,d]thiophen-4-yl)phenyl)-9H-pyrido-[2,3-b]indole (pDBTCb) and 9-(3-(dibenzo[b,d]thiophen-4-yl)phenyl)-9H-pyrido-[2,3-b]indole (mDBTCb), were designed and synthesized, incorporating dibenzothiophene (DBT) and α-carboline moieties via phenyl linkages and their device performances of phosphorescent organic light-emitting diodes (PHOLEDs) were also investigated. The different linkages between DBT and α-carboline on central phenyl spacer play an important role in the structure–property correlations. Although their photophysical properties were similar regardless of different linkage positions, the bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III)-based blue device with mDBTCb, which adopted a meta-linkage showed a significantly higher maximum quantum efficiency of 19.8% as compared to its para-linkage analog, pDBTCb (16.2%). A high quantum efficiency of 19.8% and only ca. 10% reduction of quantum efficiency at 1000cd/m2 were demonstrated from the blue PHOLEDs with the mDBTCb host material.

Formation of nanotextured surfaces on microtextured Si solar cells by metal-assisted chemical etching process.

We investigated a nanotexturization process on the microtextured surface of monocrystalline Si solar cells which utilized a metal assisted chemical etching process. P-type Si solar cell wafers were used for nanotexturing followed by saw damage removal and a microtexturing process. As the nanotexturing time was increased, green and red-orange photoluminescence spectra at wavelengths of 506, 507, and 637 nm were observed from the nanotextured cells, indicating that the quantum size effect caused the confinement of charge carriers in nanocrystalline silicon. The nanotextured cells showed a low photoreflectance of 4.5% in the visible spectral region of 400-600 nm. However, the reduced quantum efficiency from the nanotextured samples suggests that a shallow nanopore depth and density are required to prevent surface-related phenomena of charge recombination and surface current leakage.