Quantum Efficiency Values Research Articles

Cliff-like conduction band offset at CdS/Cu2ZnSnS4 heterojunction prepared by sputtering CuSn alloy target using different stacking order

The uniformly distributed Cu2ZnSnS4 (CZTS) thin films were grown by a simply sputtered method using two sputtered stacking order: ZnS/CuSn/Mo/SLG (T1) and CuSn/ZnS/Mo/SLG (T2). The highly crystalline tetragonal structure CZTS with Cu-poor, Zn-rich composition was obtained in the two samples. Grain size of the sample T1 was larger than that of sample T2 and some voids appeared in T2 caused by the SnS evaporation during sulfurization. Raman and XPS measurements showed that sample T2 had a small amount secondary phase of ZnS at CZTS/Mo interface, while T1 did not. The CZTS solar cell devices (named D1 and D2) based on the two stacking order were fabricated using CdS buffer layer. The conversion efficiency of 5.22% was obtained from D1, higher than that of 2.53% from D2. The maximum value of external quantum efficiency was 74.6% (D1) and 55.1% (D2) respectively. Accordingly, the device D1 exhibited a higher Voc of 531 mV and Rsh of 698.85 Ωcm2 than that of 481 mV and 286.73 Ωcm2 from the D2, which resulted in the higher fill factor in D1. The effect of the stacking order on conduction band offset (CBO) at CdS/CZTS heterojunction was studied. The typical cliff-like CBO was calculated as 0.22 eV for sample D1 and 0.34 eV for D2. The Cd1-xZnxS were found at the heterojunction in D1 using Zn source layer as the outermost, which made the band alignment smoother. The ZnS/CuSn/Mo/SLG stacking order with smaller CBO can reduce the loss of Voc and greatly improve the performance of solar cells.

An efficient wide range photodetector fabricated using a bilayer Bi2S3/SnS heterojunction thin film

This study presents a detailed optoelectronic investigation on the bilayer Bi2S3/SnS heterojunction photodetector. The Bi2S3 and SnS nanostructures were synthesized by an ultrasonication method and deposited in the form of thin films by a thermal evaporation system to fabricate the photodetectors. The performed optoelectrical measurements showed that the Bi2S3 and Bi2S3/SnS photodetectors are respectively n- and p-type electrical conductivity. This finding was further confirmed by current–voltage (I–V) analysis, Mott–Schottky plots and photoresponse measurements. According to the obtained gain and sensitivity values, the Bi2S3/SnS heterojunction provides a higher photon-to-electrical carrier conversion relative to the Bi2S3 photodetectord due to the enhanced photogeneration of electron–hole pairs and separation of the photoinduced charge carriers under the influence of the electric field build-in at the heterojunction’s interface. The heterojunction exhibited a remarkable photoresponse from 400 to 800 nm by giving the photocurrent gain (G), sensitivity (S), and external quantum efficiency (EQE) values of 8460%, 450% and 45%, respectively. The obtained results represent the Bi2S3/SnS heterojunction photodetector as a potential candidate for devising high-performance optoelectronic devices.

Structural analysis and Judd-Ofelt parameterization of Ca9Gd(PO4)7:Eu3+ nanophosphor for solid-state illumination

Abstract High-brightness, red-emitting Ca9Gd1−x(PO4)7:xEu3+ phosphors were developed via the facile route of solution-combustion. The Rietveld-assisted structural analysis of Ca9Gd(PO4)7 and Ca9Gd0.40Eu0.60(PO4)7 systems unveiled a trigonal crystal system having R3c(161) space-group symmetry. The absence of JCPDS data was compensated by simulating one from refined structural prototype. The band-gap for the host and best luminescent composition were determined to be 4.55 and 4.01 eV, respectively. The ratio of Judd-Ofelt parameters inferred a very high asymmetry around activator ion which can further be enhanced as a function of dopant concentration. The refractive index of Ca9Gd(PO4)7 was determined to be 2.34. The value of stimulated emission cross section, total radiative transition rate and non-radiative transition rate for Ca9Gd0.40Eu0.60(PO4)7 are calculated from detailed photo-luminescent (PL) analysis. Furthermore, very high value of quantum efficiency (98.13%) and radiative lifetime (1.85 ms) suggested that Ca9Gd0.40Eu0.60(PO4)7 nanophosphor have practical applicability in PC-WLEDs (components of solid-state lighting).

Study of energy transfer mechanism in the EuIII and GdIII homobimetallic complexes containing the anti-inflammatory drug naproxen and N,N-donors ligands

Abstract In this work, we present the synthesis, solid state characterization and complete photoluminescence study of new important homobimetallic lanthanide complexes containing the non-steroidal anti-inflammatory drug (NSAID) naproxen. The analytical and spectroscopic techniques reveals the formation of eight compounds of general formula [Ln2(nap)6(H2O)4] (Eu 1 and Gd 2), [Ln2(nap)6(bpy)2] (Eu 3 and Gd 4), [Ln2(nap)6(4,4'-dmbpy)2] (Eu 5 and Gd 6) and [Ln2(nap)6(phen)2] (Eu 7 and Gd 8), where: nap = naproxen ligand, bpy =2,2'-bipyridine, 4,4'-dmbpy = 4,4'-dimethyl-2,2'-bipyridine and phen = 1,10-phenanthroline. Using the RM1 model, the molecular structures of the EuIII complexes were calculated, with your optimized ground state geometries used to obtain all details involved in the energy transfer process. From the respective GdIII complexes were obtained the lowest ligand triplet states, proving that the photoluminescence in the EuIII naproxen complexes is proposed to be a ligand sensitized luminescence process. On the other hand, the position of the triplet states also explains the non-effective energy transfer in the TbIII naproxen complexes. The presence of N,N-donors ligands (bpy, 4,4'-dmbpy and phen) results in an 3–4-fold increase in the quantum efficiency when compared with the EuIII complex without nitrogen ligands. The high values of emission quantum efficiency (η ~ 70 - 98%) show the EuIII complexes can be potential candidates as emitters in biologic assays.

InAs/InGaAs/InAlAs interband quantum well infrared photodetector (IQWIP) with cut-off response wavelength at 1.93 μm

Utilizing the high efficiency of the localized carrier extraction in the low-dimensional semiconductors within a PN junction, an InP-based InAs/InGaAs/InAlAs interband quantum well infrared photodetector has been investigated. Although the thermal energy is much less than the band offset, the detector still realizes photo response up to 1.93 μm. In addition, the detector shows the room-temperature external quantum efficiency of 7% and detectivity of 1.81 × 1010 cm /W without anti-reflection layer. Based on the above quantum efficiency value, an absorption coefficient of 1.1 × 104 cm−1 was also determined. It is clear that our work provides compelling evidences for the possibility of novel and low-cost infrared photodetector.

Photoluminescence properties and thermal stability of RE2-xEuxSn2O7 (RE = Y3+, Gd3+, Lu3+) red nanophosphors: An experimental and theoretical study

Defect-fluorite structured Eu3+-doped Y2Sn2O7 nanoparticles (30–40 nm) were successfully prepared through a facile co-precipitation method without any organic additives or templates followed by calcination in air. Luminescent properties of the as-prepared Y2-xEuxSn2O7 (0.02 ≤ x ≤ 0.6) nanophosphors were fully characterized from an experimental and theoretical point of view (excitation, emission, lifetime, critical concentration, temperature-dependent luminescence and thermal stability, type of interactions and the Judd-Ofelt analysis). The nanophosphors showed relatively sharp excitation bands from 360 to 530 nm and exhibited characteristic emission bands with the most intensive emission centered at 612 nm. In addition, Gd1.98Eu0.02Sn2O7 and Lu1.98Eu0.02Sn2O7 samples were prepared in order to study how substitution of RE3+ (Y3+ with Gd3+ and Lu3+ ions) influences on luminescent properties of RE1.98Eu0.02Sn2O7 and thermal stability of their luminescence. In order to understand better luminescent properties, the Judd–Ofelt analysis was applied to all the synthesized powders. The highest value of quantum efficiency, ~96%, was estimated for Lu1.98Eu0.02Sn2O7. To assess potential application in high-power LEDs, the temperature-dependent emission spectra of Y1.98Eu0.02Sn2O7, Gd1.98Eu0.02Sn2O7 and Lu1.98Eu0.02Sn2O7 nanophosphors were studied.

Fabrication of 12 µm pixel-pitch 1280 × 1024 extended short wavelength infrared focal plane array using heterojunction type-II superlattice-based photodetectors

We present an intial demonstration of a 1280 × 1024 extended short-wavelength infrared focal plane array (FPA) imager with 12 μm pixel-pitch based on type–II InAs/AlSb/GaSb superlattice heterojunction photodetectors, with a novel bandstructure-engineered photo-generated carrier extractor as the window layer in the hetero structure to efficiently extract the photo-generated carriers. This heterostructure with a larger bandgap top window/contact layer leads to the device having lower dark current density compared to conventional pn junction devices. The large format FPA was fabricated with 12 μm pixel-pitch using a developed fabrication process. Test pixels fabricated separately exhibit 100% cut–off wavelengths of ∼2.22, ∼2.34 μm, and ∼2.45 μm at 150, 200 K, and 300 K. The test devices achieve saturated quantum efficiency values under zero bias of 54.3% and 68.4% at 150 and 300 K, under back-side illumination and without any anti-reflection coating. At 150 K, these photodetectors exhibit dark current density of 1.63 × 10−7 A cm−2 under −20 mV applied bias providing a specific detectivity of 1.01 × 1011 cm · Hz1/2/W at 1.9 μm.

Spectroscopic Investigation of Yb -Doped Silica Glass for Solid-State Optical Refrigeration

We have argued that a high-purity Yb-doped silica glass can potentially be cooled via anti-Stokes fluorescence optical refrigeration. This conclusion is reached by showing, using reasonable assumptions for the host material properties, that the non-radiative decay rate of Yb ions can be made substantially smaller than the radiative decay rate. Therefore, an internal quantum efficiency of near unity can be obtained. Using spectral measurements of the fluorescence emission from a Yb-doped silica optical fiber at different temperatures, we estimate the minimum achievable temperature in Yb-doped silica glass for different values of internal quantum efficiency.

Broad-band emission and color tuning of Eu3+-doped LiCa2SrMgV3O12 phosphors for warm white light-emitting diodes

In this study, series of Eu3+-doped LiCa2SrMgV3O12 (LCSMV) phosphors with broad-band emission and color tunable feature were prepared via solid phase reaction. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) results presented a pure cubic phase product with micron-sized and homogeneous distribution of element. Their spectroscopic properties were investigated systematically by photoluminescence excitation (PLE) and emission (PL) spectra, temperature-dependent PL spectra and luminescence decay curves. The LCSMV phosphors displayed a strong absorption to ultraviolet light and a broad cyan emission. Moreover, in Eu3+-doped LCSMV phosphors, Eu3+ ion characteristic emissions at 589, 610, 651 and 705 nm, attributing to the 5D0→7F1, 7F2, 7F3 and 7F4 transitions, were observed. Along with Eu3+ ion concentrations increasing, the emission colors could be readily tuned from cyan to orange and the decay lifetimes of (VO4)3- became shorter. Meanwhile, electric dipole-dipole interaction was responsible to energy migration from (VO4)3- groups to Eu3+ ions. Further, the quantum efficiency (QE) values were estimated to be 32.5% for LCSMV host and 39.3% for LCSMV: 0.01Eu3+ sample. Finally, a LED lamp was prepared by integrating the blend of the LCSMV: 0.01Eu3+ phosphors and commercial blue-emitting BaMgAl10O17:Eu2+ phosphors with NUV chip (365 nm) and exhibited warm white light (CCT = 3655 K, Ra = 90), which may be applied in lighting and display field.

High-Power GaN-Based Vertical-Cavity Surface-Emitting Lasers with AlInN/GaN Distributed Bragg Reflectors

High-efficiency and high-power operation have been demonstrated for blue GaN-based vertical-cavity surface-emitting lasers (VCSELs) with AlInN/GaN distributed Bragg reflectors. The high-efficiency performance was achieved by introducing a novel SiO2-buried lateral index guide and adjusting the front mirror reflectivity. Lateral optical confinement has been shown to greatly lower the otherwise significant loss of transverse radiation exhibited by typical VCSELs based on GaN. Employing a long (10λ) cavity can also enhance the output power, by lowering the thermal resistance of the VCSEL and increasing the operating current associated with thermal rollover. This modification, in conjunction with optimized front mirror reflectivity and a buried SiO2 lateral index guide, results in a blue VCSEL (in the continuous wave mode with an 8 μm aperture at 20 °C) having a superior differential quantum efficiency value of 31% and an enhanced 15.7 mW output power. This unit also exhibits a relatively high output power of 2.7 mW at temperatures as high as 110 °C. Finally, a 5.5 μm aperture VCSEL was found to generate a narrow divergence (5.1°) single-lobe far field pattern when operating at an output power of approximately 5 mW.

Study on Optical Properties and Internal Quantum Efficiency Measurement of GaN-based Green LEDs

In this paper, the optical properties of GaN-based green light emitting diode (LED) are investigated and the internal quantum efficiency (IQE) values are measured by temperature dependent photoluminescence (TDPL) and power dependent photoluminescence (PDPL) methods. The "S-shaped” shift of peak wavelength measured at different temperature disappears gradually and the spectra broadening can be observed with increasing excitation power. The IQE calculation results of TDPL, which use the integrated intensity measured at low temperature as unity, can be modified by PDPL in order to acquire more accurate IQE values.

Efficient blue fluorescent emitters based on imidazole-pyridine derivatives for organic light-emitting diodes

Imidazole-pyridine derivatives such as 2-(1-phenyl-5-(pyridin-2-yl)-1H-imidazol-4-yl)pyridine were synthesized for the efficient blue light-emitting materials. OLED devices using these materials were fabricated in the following sequence; Indium tin oxide (ITO) (150 nm)/N,N'-diphenyl-N,N'-(2-napthyl)-(1,1'-phenyl)-4,4'-diamine (NPB) (50 nm)/Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) (10 nm)/Blue materials (20 nm)/2,2′,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) (30 nm)/Lithium quinolate (Liq)/Aluminum (Al) (2/100 nm). Particularly, a device using 9,10-dihydro-9,9-dimethyl-10-(4-(1-phenyl-4,5-di(pyridin-2-yl)-1H-imidazol-2-yl)phenyl)acridine as a blue emitter showed maximum values of luminous efficiency, power efficiency, and external quantum efficiency of 0.21 cd/A, 0.10 lm/W, 0.26% at 20 mA/cm2, respectively with the Commission Internationale d’Énclairage (CIE) (x,y) coordinates of (0.19, 0.11) at 6.0 V.

Influence of TIO2-rGO optical properties on the photocatalytic activity and efficiency to photodegrade an emerging pollutant

TiO2-reduced graphene oxide (TiO2-rGO) photocatalysts with different mass ratios (GO:TiO2 0.1-0.5-1%) were synthesized via a hydrothermal method and compared through their physico-chemical properties and photocatalytic activity in the degradation of an emerging contaminant, clofibric acid. The optical properties of the TiO2-rGO nanocomposites were first estimated in order to calculate the local volumetric rate of photon absorption inside a photocatalytic reactor. Radiation models were solved using the Monte Carlo method. The effect of rGO as well as the photocatalyst loadings on the radiation absorption was evaluated. The lowest photodegradation rate found in P25-rGO 1% was ascribed to an excess of rGO that could well favor charge carriers recombination leading to detrimental photoactivity. A GO/TiO2 mass ratio of 0.5% provided the fastest initial photodegradation rate under the operating conditions studied here. Finally, the photo-efficiency of all these photocatalysts was also analyzed by calculating the quantum efficiency parameter. The highest value of quantum efficiency was achieved with P25-rGO 0.5% at 100 mg L−1, with an increase of 11% compared to the value obtained for P25-rGO 0%.

Photoemission and thermoluminescence characteristics of Dy3+-doped zinc sodium bismuth borate glasses

The present study reports preparation and characterization of Dy3+-doped PbO free alkali-heavy-metal-borate glasses for luminescent applications. The amorphous nature of the glasses and the presence of various structural groups like BiO6, BO3 and BO4 were confirmed through XRD and FTIR measurements. The Oscillator strength of Dy3+ absorption transitions as well as the bond formed with the O2− ion in the glass network were determined using the absorption spectra. Radiative parameters extracted from Judd-Ofelt theory proved the degree of suitability of prepared glasses for lasing applications. The photoluminescence spectra exhibited the emission characteristics of Dy3+ ions and used to calculate the branching ratio, stimulated emission cross-section and radiative lifetime. Metastable lifetimes of Dy3+ ions were obtained through exponential fitting of respective decay profiles. The luminescence quenching phenomena was reasoned using Inokuti-Hirayama (IH) model. The optical gain and quantum efficiency values clearly suggested Dy3+ doping at 0.3 mol% was optimum for 587 nm lasing action. Further, CIE chromaticity plot indicated the possible applications in solid state lighting devices. The thermoluminescence glow curves of 0.3 mol% Dy3+-doped glasses were considered for gamma irradiation and kinetic parameters of the same were evaluated using Computerized Glow Curve Deconvolution (CGCD) technique.

Effect of ZnO‐based TCO on the performance of a‐Si H(n)/a‐Si H(i)/c‐Si H(p)/Al BSF(p+)/Al heterojunction solar cells

Inclusion of ZnO‐based transparent conducting oxide (TCO) film layer into amorphous silicon/crystalline silicon (a‐Si/c‐Si) solar cell enhances its photovoltaic conversion efficiency. These findings have been confirmed here, using Afors HIT software. The simulation study is performed onto a solar cell that was originally lab prepared so as to use its measured parameters in the simulation. The ZnO‐based TCO films were electrodeposited on n‐type (100) silicon wafer and were simulated. X‐ray diffraction (XRD) pattern confirms the zinc blende nature of the TCO layer, and show that the preferred orientation of ZnO films is (002). Scanning electron microscopic (SEM) imaging confirms nano‐size nature of the ZnO based TCO film. For comparison purposes, cells with and without TCO layers are investigated by simulating photo‐current versus applied potential (J–V) plots. Values of fill factor (FF), short circuit current density (Jsc) open circuit potential (VCO) and conversion efficiency (η) are extracted. The energy band diagram, current density and charge carrier generation/recombination phenomena are in‐depth analyzed to understand the mechanism of enhancement in the hetero‐junction cell performance. Values of quantum efficiency (QE) are also simulated. The results show that the solar cell heterojunction is hypersensitive to the ZnO layer. The added value of using the ZnO‐based transparent conductive oxide (TCO) layer, in enhancing intrinsic thin layer (HIT) solar cell conversion efficiency, is assessed by critically comparing it with a control cell having no ZnO layer. © 2018 American Institute of Chemical Engineers Environ Prog, 2018 © 2018 American Institute of Chemical Engineers Environ Prog, 38:e13114, 2019

Synthesis and luminescence properties of LiGd3(MoO4)5:Eu3+ phosphors for white LED applications

Eu3+ activated LiGd3(MoO4)5 phosphors were synthesized proficiently by simple sol gel method. Structural, morphological and luminescence characteristics of the phosphors were investigated in detail. The photo physical parameters and quantum efficiency values are studied by using Judd–Ofelt concepts. All the outcomes imply that LiGd3(MoO4)5:0.07Eu3+ phosphor may be a meritful candidate as red emitting component for white light emitting diodes applications.

Pd-Pt bimetallic Nb-doped TiO2 for H2 photo-production: Gas and liquid phase processes

The performance of binary PtPd co-catalysts supported on Nb-TiO2 was tested in the gas and liquid phase photo-reforming of methanol. Co-catalyst formulations having Pt:Pd ratios from 2:1 to 1:4 were tested and the activity measured through the calculation of the quantum efficiency. Characterization of the initial and post-reaction materials showed that optimum activity is achieved in both processes by samples containing binary co-catalysts where Pd suffers an oxidation process and a PtPd alloying metallic phase is formed under reaction conditions. The analysis of catalytic output through an experimental design considering tree factors (methanol:water ratio, irradiance level, and catalyst volumetric concentration at liquid phase or surface concentration at gas phase) shows that the liquid phase process presents a rather high quantum efficiency value compared with the gas phase but the latter process displays higher atom efficiency as well as renders products with lower toxicity.

Systematic studies on Sr4La6(SiO4)6M2:Eu3+ (M = F/Cl) phosphors: effects of the halogen anions on photoluminescence

We successfully prepared Sr4La6(SiO4)6F2:Eu3+ (SLSOF:Eu3+) and Sr4La6(SiO4)6Cl2:Eu3+ (SLSOC:Eu3+) oxyapatite phosphors via the traditional solid-state method. The phosphors’ structure and photoluminescence properties were investigated by scanning electron microscope (SEM), X-ray powder diffraction (XRD), photoluminescence (PL) spectroscopy, ultraviolet–visible spectra and temperature-dependent emission spectra. Among them, the irregular morphology of SLSOF and SLSOC particles was displayed by SEM images, and the phase formation of both sets of phosphors was illustrated by XRD analysis. Additionally, PL spectra indicated that the phosphor could be efficiently excited by the near ultraviolet (NUV) light, and then emitted shining red light, agreeing well with that the calculated color coordinates are within the red region. Comparatively, the luminescence intensity of SLSOF:Eu3+ is higher than that of SLSOC:Eu3+ on the basis of the greater electronegativity of F−, which directly leads to the enhancement of 5D0→7F2 transition. The temperature-dependent emission spectra indicated that samples have excellent thermal stability. Internal quantum efficiency value of phosphors were calculated. These results indicated that the halogen anions of these Sr4La6(SiO4)6M2:Eu3+ play important roles in their PL properties and both phosphors have great potential to serve as red emitting phosphor for NUV white light emitting diodes.

M-ary pulse position modulation performance in strong atmospheric turbulence.

The performance of an M-ary pulse position modulated (PPM) optical wireless communication system operating in strong atmospheric turbulence is investigated. Bit error rate (BER) is employed as the measure for the performance. In our overall performance formulation, average received power as measured by a finite-sized avalanche photodiode (APD) detector is used by the help of the extended Huygens-Fresnel principle. For the aperture averaged scintillation evaluations, the asymptotic Rytov theory with the gamma-gamma intensity statistics is utilized. Gamma-gamma statistics together with the large-scale and the small-scale log-intensity variances yield the scintillation index valid both in weak and strong atmospheric turbulence regimes. BER variations versus the plane wave scintillation index are examined at different values of receiver aperture diameters, data bit rates, M values of M-ary PPM, quantum efficiency, and average APD gain.

Radiation effects, photoluminescence and radioluminescence of Eu-doped (Y0.7Gd0.3)2O3 nanoparticles with various sizes

(Y1-xLnx)2O3 oxides (Ln = Y, Gd, Lu) doped with Eu3+ ions are widely used as commercialized scintillators in medical and industrial scanning applications, with high performance and attractive properties. In this work, effects of high-energy radiation on luminescence properties of (Y1-xGdx)2O3: Eu3+ powders with different particle sizes in the nanometric range are analyzed. Powders were prepared by polymer complex solution route, followed by annealing. Nanoparticles were obtained with an average size between 20 and 120 nm according to annealing conditions (time and temperature). Samples were exposed to gamma-irradiation (doses up to 4 MGy) on a commercial gamma-irradiation plant. The effect of irradiation on different nanoparticle size is followed by XRD, TEM and monitoring their luminescence properties. No change was observed in structure, morphology and steady-state emission. On the opposite after irradiation, excited-state lifetimes and quantum efficiency values are particle size dependent.