Calculation Of Quantum Efficiency Research Articles

Influence of periodic texture profile and parameters for enhanced light absorption in amorphous silicon ultra-thin solar cells.

We have investigated the antireflection and light trapping properties of two-dimensional grating arrays in the hexagonal symmetry with various texture morphologies. Optical simulation based on finite-difference time-domain (FDTD) analysis is carried out to understand the role of the structure profile for different periodicities and heights to achieve enhanced light trapping. The considered active medium of interest is 200-nm-thick hydrogenated amorphous silicon. Although the considered texture profiles possess an incremental change of refractive index from incident medium to active medium, a parabolic-shaped front side texture provides better antireflection effects owing to its high diffraction efficiencies in the higher-order modes as compared to other pattern morphologies. In the back side texture, the parabolic-shaped pattern also dominates with better light trapping efficiencies due to its ability to distribute a major amount of diffracted energy in the higher-order modes. The average reflection calculations in the wavelength range of 300-800 nm confirm that in both side textures, a periodicity of 500 nm with a height of 200 nm can be preferentially recommended for less reflection loss and improved scattering in oblique angles. The quantum efficiency calculation verifies that a device designed with these optimized parameters can offer improved efficiency for ultra-thin solar cells.

Optimizations of GaAs Nanowire Solar Cells

The efficiency of GaAs nanowire (NW) solar cells can be significantly improved with no new processing steps or material requirements. We report coupled optoelectronic simulations of a GaAs NW solar cell with a vertical p-i-n junction and a high-bandgap AlInP passivating shell. Our frequency-dependent model facilitates calculation of quantum efficiency for the first time in NW solar cells. For passivated NWs, we find that short-wavelength photons can be most effectively harnessed by using a thin emitter, while long-wavelength photons are best utilized by extending the intrinsic region to the NW/substrate interface and using the substrate as a base. These two easily implemented changes, coupled with the increase of NW height to 3.5 μm with realistic surface recombination in the presence of a passivation shell, result in an NW solar cell with greater than 19% efficiency.

Theoretical predictions on efficiency of bi-exciton formation and dissociation in chiral carbon nanotubes.

Efficient multiple exciton generation (MEG) in chiral single-wall carbon nanotubes (SWCNTs) is present within the solar spectrum range as shown by the many-body perturbation theory calculations combined with the density functional theory simulations. To describe the impact ionization process, we calculate exciton-to-biexciton decay rates R1→2 and biexciton-to-exciton rates R2→1 in the (6,2) and (10,5) SWCNTs. Within the solar energy range, we predict R1→2 ∼ 1014 s-1, while biexciton-to-exciton recombination is weak with R2→1/R1→2 ≤ 10-2. Also we calculate quantum efficiency (QE), the average number of excitons created by a single absorbed photon, for which we find QE ≃ 1.2-1.6, that is 20%-60%. However, MEG strength in these SWCNTs varies strongly with the excitation energy due to highly non-uniform density of states at the low energy. We hypothesize that MEG efficiency in the chiral SWCNTs can be enhanced by altering the low-energy electronic spectrum via surface functionalization, or by mixing SWCNTs of different chiralities.

Influence of TiO2 optical parameters in a slurry photocatalytic reactor: Kinetic modelling

A kinetic model, based on a proposed reaction photo-mechanism and explicitly dependent on the volumetric rate of photon absorption (VRPA), has been performed and analyzed for the photocatalytic degradation of phenol in aqueous suspensions. UVA–vis simulated solar radiation and three commercial TiO2 powder photocatalysts, with identical chemical structure but with different morphology and hydrodynamic particle size (P25 Aeroxide®, P25/20 VP Aeroperl® and P90 Aeroxide® provided by Evonik) have been used. The effect of TiO2 hydrodynamic particle size on the optical coefficients has revealed that this property is playing an important role on the absorbed radiation evaluation. Radiation profiles inside the photoreactor were determined by solving the radiative transfer equation (RTE), and the corresponding kinetic parameters were obtained.A comparative study of TiO2 loading effect on calculated quantum efficiency (η) revealed P25 was the most photo-efficient catalyst, whereas P25/20 reached similar efficiencies only when higher TiO2 concentrations were used. On the contrary, P90 presented the lowest photo-efficiency with lower photodegradation rates despite absorbing more radiation.The obtained kinetic model has been successfully validated by experimental data, being able to reproduce the evolution of phenol photodegradation at a wide range of catalyst concentrations for the three studied TiO2. In the experimental conditions of this work, low irradiation power conditions can be considered; after simplification, the model adopted a linear dependence on the volumetric rate of photon absorption; showing good agreement between predicted and experimental values, with root-square-mean errors below 5%.

Interface Effects in Sunlight-Driven Ag/g-C3N4 Composite Catalysts: Study of the Toluene Photodegradation Quantum Efficiency.

Metallic silver (ranging from 1 to 10 wt %) was deposited onto a graphite-like carbon nitride photocatalyst through a microemultion method. Surface, morphological, and structural properties of the resulting materials were characterized using BET and porosity measurements, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and UV-vis and photoluminescence spectroscopy. The activity of the composite samples under sunlight-type and visible illumination was measured for toluene photodegradation and was analyzed by means of the reaction rate and the quantum efficiency parameter. To obtain the latter observable, the lamp emission properties as well as the radiation field interaction with the catalyst inside the reactor were modeled and numerically calculated. The stability of the samples under both illumination conditions was also studied. The results evidence that the composite samples containing 1-10 silver wt % outperform carbon nitride for sunlight-type and visible illumination, but the optimal use of the charge generated after light absorption is obtained for the sample with 1 wt % of silver acording to the quantum efficiency calculation. The study shows that the optimum silver-g-C3N4 contact is able to outperform TiO2 reference systems (nano-TiO2 and P25) under sunlight illumination and points out that this occurs as a direct consequence of the charge handling through the interface between catalyst components. This indicates that composite systems based on g-C3N4 can be competitive in sunlight-triggered photodegradation processes to eliminate tough polluctants such as toluene, rendering active and stable systems.

Effect of annealing conditions on structural and luminescencent properties of Eu3+-doped Gd2Ti2O7 thin films

Here we report on preparation of Eu3+-doped Gd2Ti2O7 pyrochlore luminescent thin films by pulsed laser deposition technique and their structural, morphological and optical characterization. The influence of annealing temperature and background gas (air vs. argon) on film photoluminescence is examined for the optimization of post-deposition annealing conditions. As-deposited amorphous films become pure pyrochlore crystalline after calcination at temperatures higher than 1000°C. Atomic force microscopy showed increase in the grain size from ∼20nm in the as-deposited to ∼60nm in the crystalline sample annealed at 1100°C. Scanning electron microscopy showed dense films with the uniform thickness of about 700nm. Luminescence spectra of crystalline films were complex and composed of better resolved emission lines than in the amorphous sample. Emission spectra showed that symmetry of Eu3+ sites become disturbed in annealed films due to the extrinsic thermal stress. Films treated in argon displayed similar emission and excitation spectral features like air-treated ones, but with better resolved emission lines. Calculated quantum efficiency of emission showed that optimization of annealing conditions led to an enhancement of films luminescence. The highest quantum efficiency of emission and the longest lifetime is found for the sample annealed at 1100°C in presence of argon.

The effect of band offsets in quantum dots

The insertion of quantum dots in a host material produces band offsets which are greatly dependent on the field of strains brought about by this insertion. Based on the Empiric KP Hamiltonian model, the energy spectrum of the quantum dot/host system is easily calculated and a relationship between the conduction and valence band offsets is determined by the energy at which the lowest peak of the sub-bandgap quantum efficiency of an intermediate band solar cell is situated; therefore knowledge of the valence band offset leads to knowledge of both offsets. The calculated sub-bandgap quantum efficiency due to the quantum dot is rather insensitive to the value of the valence band offset. However, the calculated quantum efficiency of the wetting layer, modeled as a quantum well, is sensitive to the valence band offset and a fitting with the measured value is possible resulting in a determination of both offsets in the finished solar cell with its final field of strains. The method is applied to an intermediate-band solar cell prototype made with InAs quantum dots in GaAs.

Enhanced multiple exciton generation in amorphous silicon nanowires and films

ABSTRACTMultiple exciton generation (MEG) in nanometer-sized hydrogen-passivated silicon nanowires (NWs), and quasi two-dimensional nanofilms depends strongly on the degree of the core structural disorder as shown by the perturbative many-body quantum mechanics calculations based on the density functional theory simulations. Working to the second order in the electron–photon coupling and in the screened Coulomb interaction, we calculate quantum efficiency (QE), the average number of excitons created by a single absorbed photon, in the Si29H36 quantum dots (QDs) with crystalline and amorphous core structures, simple cubic three-dimensional arrays constructed from these QDs, crystalline and amorphous NWs, and quasi two-dimensional silicon nanofilms, also both crystalline and amorphous. Efficient MEG with QE ranging from 1.3 up to 1.8 at the photon energy of about 3Eg, where Eg is the electronic gap, is predicted in these nanoparticles except for the crystalline NW and crystalline film where QE ≃ 1. MEG in the amorphous nanoparticles is enhanced by the electron localisation due to structural disorder. Combined with the lower gaps, the nanometer-sized amorphous silicon NWs and films are predicted to have effective carrier multiplication within the solar spectrum range.

Heterogeneous photocatalysis: Light-matter interaction and chemical effects in quantum efficiency calculations

In this contribution we analyze the calculation of the quantum efficiency observable in the gas-phase heterogeneous photocatalytic oxidation of toluene. To this end, we carried out porosimetry, UV–visible optical, and TEM tomography measurements together with the mathematical modeling of the most significant physico-chemical factors influencing the quantum efficiency observable. Critical factors concern physical properties related to the light-matter interaction as well as the chemical properties defining the outcome of the reaction. Among the firsts, we analyze the effective catalyst surface area illuminated and provide a detailed formulation of the radiation model, including main absorbance, transmittance and reflectance events. For chemical properties not only the activity but also the selectivity was considered. The work was carried out with a dual aim. First, by scanning a basis set containing most common reactor geometries and light sources, we illustrate quantitatively (numerically) and qualitatively (relative among factors) the importance of each one of the above mentioned factors in quantum efficiency calculations. Second, using a series of samples with “continuous” variation of chemical, textural, morphological and optical properties we calibrate the goodness of common simplifications utilized to represent the above mentioned physical/chemical factors and measure their numerical effects in the quantum efficiency observable. Combination of these two approaches provides a general scheme to bundle and interpret all physico-chemical factor effects in quantum efficiency outputs and can serve as a guide to calibrate current quantum efficiency formulations accuracy.

Enhanced 1.53 μm band fluorescence in Er3+/Ce3+ codoped tellurite glasses containing Ag NPs

The silver nanoparticles (Ag NPs) was introduced into Er3+/Ce3+ codoped tellurite glasses with composition of TeO2–ZnO–La2O3 and the effects of Ag NPs on the 1.53μm band spectroscopic properties of Er3+ ions, structural nature and thermal stability of glass hosts were investigated. The absorption spectra, upconversion emission spectra, 1.53μm band fluorescence spectra together with the X-ray diffraction (XRD) patterns, differential scanning calorimeter (DSC) curves and transmission electron microscopy (TEM) image were measured. It was found that the near-spherical shape Ag NPs with average diameter of about 19nm dispersed uniformly in the glass matrix, and the Er3+/Ce3+ codoped tellurite glasses containing Ag NPs exhibited an obvious enhancement in the 1.53μm band fluorescence, which is attributed to the intensified local electric field induced by Ag NPs and the possible energy transfer from Ag NPs to Er3+ ions. The enhanced effect of Ag NPs on the 1.53μm band fluorescence was elucidated by the obtained large Judd–Ofelt intensity parameter Ω6 and the calculated quantum efficiency of Er3+:4I13/2 level. In addition, the thermal stability of prepared glass samples increased with the introduction of Ag NPs and the amorphous structural nature was demonstrated by the measured XRD patterns with no sharp diffraction peak. The present results indicate that the prepared Er3+/Ce3+ codoped tellurite glass with Ag NPs has good prospect as a gain medium applied for 1.53μm band Er3+-doped fiber amplifiers (EDFAs).

Modeling and simulation of acid generation in anion-bound chemically amplified resists used for extreme ultraviolet lithography

Extreme ultraviolet (EUV) lithography is the most promising candidate technique for the high-volume production of semiconductor devices with half-pitches of sub-10 nm. An anion-bound polymer, in which the anion part of onium salts is polymerized, has attracted much attention from the viewpoint of the control of acid diffusion. In this study, we modeled the acid generation processes in the anion-bound chemically amplified resists upon exposure to EUV radiation and developed a Monte Carlo simulation code. Using the developed simulation code, the dependence of the quantum efficiency of acid generation on the concentration of acid generator units was calculated. The calculated quantum efficiencies well agreed with the experimental values with a fitting error of less than 10%. The thermalization distance was considered to be approximately 3 nm. The blur of proton distribution intrinsic to the reaction mechanisms of anion-bound chemically amplified resists was roughly estimated to be 4.5–6.5 nm.

Optical spectroscopy of Eu3+ ions doped in YAl3 (BO3)4 crystal

The electronic structure of Eu3+ ions doped in YAl3 (BO3)4 crystal are investigated by means of room temperature absorption and emission spectroscopy in the 15,000–30,000cm−1 range. The observed transitions are assigned and analyzed on the basis of group theory assuming that Eu3+ ion occupies a D3 symmetry site. Eu3+ Free-ion energy diagram is presented and the corresponding free ion parameters are calculated. Calorimetric parameters of the Eu3+ ions red emission in YAB are calculated (x=0.67, y=0.32). The Judd–Ofelt parameterization is applied to the analysis of the room temperature absorption and emission spectra. The intensities parameters are determined as: Ω2=2.26×10–20cm2, Ω4=5.11×10–20cm2 and Ω6=0.78×10–20cm2. These parameters are used to calculate, the line strength, radiative transition probability, fluorescence, branching ratio and radiative lifetime. The calculated 5D0 radiative life time around 3.6ms is compared with the experimental value. The calculated quantum efficiency is about 35%.

Spectral characteristics of intense red luminescence in Pr:Y2O3 nanophosphor on UV excitation

Detailed structural and spectroscopic characterizations of Pr:Y2O3 nanophosphor have been carried out. On ultraviolet light (at 266 nm) excitation, intense red emission along with weak emissions in blue, green and infrared regions have been observed. The phenomena for the observed emission have been thoroughly explained in the present work and it was finally concluded that the direct excitation through low-lying 4f15d band of Pr3+ ions and through the conduction band of Y2O3 nanophosphor are the main channels for the observed emissions. The calculated quantum efficiency (QE) for the red emission is found to be ~80 % while the CIE coordinates are (0.65, 0.34) quite close to the standard values. These studies clearly indicate that Pr:Y2O3 nanophosphors are promising and futuristic red luminescent material for the development of various display devices and UV detectors. Time resolved spectroscopy has been further used to investigate luminescence dynamics.

Design Rules for High-Efficiency Quantum-Dot-Sensitized Solar Cells: A Multilayer Approach.

The effect of multilayer sensitization in quantum-dot (QD)-sensitized solar cells is reported. A series of electrodes, consisting of multilayer CdSe QDs were assembled on a compact TiO2 layer. Photocurrent measurements along with internal quantum efficiency calculation reveal similar electron collection efficiency up to a 100 nm thickness of the QD layers. Moreover, the optical density and the internal quantum efficiency measurements reveal that the desired surface area of the TiO2 electrode should be increased only by a factor of 17 compared with a compact electrode. We show that the sensitization of low-surface-area TiO2 electrode with QD layers increases the performance of the solar cell, resulting in 3.86% efficiency. These results demonstrate a conceptual difference between the QD-sensitized solar cell and the dye-based system in which dye multilayer decreases the cell performance. The utilization of multilayer QDs opens new opportunities for a significant improvement of quantum-dot-sensitized solar cells via innovative cell design.

Spectral Response and Noise Temperature of a 2.5 THz Spiral Antenna Coupled NbN HEB Mixer

Abstract We report on a 2.5 THz spiral antenna coupled NbN hot electron bolometer (HEB) mixers, fabricated with in-situ process. The receiver noise temperature with lowest value of 1180 K is in good agreement with calculated quantum efficiency factor as a function of bias voltage. In addition, the measured spectral response of the spiral antenna coupled NbN HEB mixer shows broad frequency coverage of 0.8-3 THz, and corrected response for optical losses, FTS, and coupling efficiency between antenna and bolometer falls with frequency due to diffraction-limited beam of lens/antenna combination.

Efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes with nonuniform current spreading

We demonstrate that the efficiency droop phenomenon in multiple quantum well InGaN/GaN light-emitting diodes (LEDs) may be connected to the current crowding effect. A numerical model of internal quantum efficiency calculation is presented that takes into account nonuniform lateral carrier injection in the active region. Based on this model, we examine the effect of current crowding on the efficiency droop using comparison of simulated internal quantum efficiency of InGaN LEDs with low and high uniformity of current spreading. The results of simulations and measurements show that the devices with low uniformity of current spreading exhibit higher efficiency droop and lower roll-off current value.

Influence of anomalous dispersion mirror properties on quantum efficiency of InGaAs/GaAs resonant cavity photodetector

AbstractWe present a theoretical analysis of the quantum efficiency of a resonant cavity enhanced InGaAs/GaAs p-i-n photodetector (PD) for the ultrashort optical connections. The numerical method of calculation of quantum efficiency combining a transfer matrix method and an energy conservation law is offered. Using anomalous dispersion (AD) mirror, flat-topped QE spectrum has been obtained. Conditions for ideal flat-topped spectral response have been received. A design with a maximum QE of 93.5% and 3-nm bandwidth at 0.02 dB below the peak is presented.

Modelling the quantum efficiency of cadmium telluride solar cells

A simple analytical model is presented describing the quantum efficiency of cadmium telluride (CdTe) solar cells. This model is based on a consistent set of parameters that were extracted from electrical and optical measurements. These measurements also reveal the CdTe solar cells to mainly rely on carrier generation as well as carrier collection within the space-charge region. Recombination in this part of the cell is hence taken into account. As a result, quantum efficiency spectra can be closely fitted by an expression that includes the lifetime of the minority carriers and the width of the space-charge region as free variables. The comparison of the calculated quantum efficiency curves with the experimental ones gives fundamental insight into the specific operation of CdTe solar cells.

Monte Carlo Modeling of VLWIR HgCdTe Interdigitated Pixel Response

Increasing very long-wave infrared (VLWIR, λ c ≈ 15 μm) pixel operability was approached by subdividing each pixel into four interdigitated subpixels. High response is maintained across the pixel, even if one or two interdigitated subpixels are deselected (turned off), because interdigitation provides that the preponderance of minority carriers photogenerated in the pixel are collected by the selected subpixels. Monte Carlo modeling of the photoresponse of the interdigitated subpixel simulates minority-carrier diffusion from carrier creation to recombination. Each carrier generated at an appropriately weighted random location is assigned an exponentially distributed random lifetime τ i , where 〈τ i 〉 is the bulk minority-carrier lifetime. The minority carrier is allowed to diffuse for a short time dτ, and the fate of the carrier is decided from its present position and the boundary conditions, i.e., whether the carrier is absorbed in a junction, recombined at a surface, reflected from a surface, or recombined in the bulk because it lived for its designated lifetime. If nothing happens, the process is then repeated until one of the boundary conditions is attained. The next step is to go on to the next carrier and repeat the procedure for all the launches of minority carriers. For each minority carrier launched, the original location and boundary condition at fatality are recorded. An example of the results from Monte Carlo modeling is that, for a 20-μm diffusion length, the calculated quantum efficiency (QE) changed from 85% with no subpixels deselected, to 78% with one subpixel deselected, 67% with two subpixels deselected, and 48% with three subpixels deselected. Demonstration of the interdigitated pixel concept and verification of the Monte Carlo modeling utilized λ c (60 K) ≈ 15 μm HgCdTe pixels in a 96 x 96 array format. The measured collection efficiency for one, two, and three subelements selected, divided by the collection efficiency for all four subelements selected, matched that calculated using Monte Carlo modeling.

An efficient compact 300 mW narrow-linewidth single frequency fiber laser at 15 μm

An efficient single frequency fiber laser by using a newly-developed Er(3+)/Yb(3+) co-doped single mode phosphate glass fiber with the net gain coefficient of 5.2 dB/cm and propagation loss coefficient of 0.04 dB/cm has been demonstrated. Over 300 mW stable continuous -wave single transverse and longitudinal mode seed lasering at 1.5 microm has been achieved from a 2.0 cm-long active fiber. The measured slope efficiency and the calculated quantum efficiency of laser emission are found to be 30.9% and 0.938 +/- 0.081, respectively. It is found that the linewidth of the fiber laser is less than 2 kHz, and the measured relative intensity noise (RIN) is around -120 dB/Hz in the frequency range of 50 to 500 kHz.