Reabsorption Probability Research Articles

Optical multiscale model for quantification of photon recycling including incoherent light scattering

A comprehensive multiscale modeling framework for photon recycling in textured solar cells and LEDs is introduced, treating light absorption and emission on an equal footing under consideration of the full internal mode spectrum in the device. The framework seamlessly merges a coherent emission model, free from unphysical divergence and ensuring consistency with detailed balance principles, with an incoherent net-radiation model applied to optically thick layers. The scattering of light at nonplanar interfaces is taken into consideration through four characteristic mappings that depict the microscopic scattering process, which are calculated analytically or by using a Monte-Carlo ray-tracing method. The final output of the model consists of energy- and angle-resolved local emission, re-absorption, and energy flux rates for further coupling to electronic transport. To validate the model, it has been compared with analytical solutions for the re-absorption probability in an ideal semiconductor slab with either ideal Lambertian or flat surfaces. The comparison between analytical and numerical calculations shows excellent agreement.

Longitudinal-optical-phonon resonant thermal emission efficiency and spectrum control of metal-GaAs surface micro-stripe structures

Phonon-based tera-Hertz or mid-infrared emission has been investigated because it can be applied to high-efficiency light sources for optical communication and material diagnostics, where thermal energy can be utilized as an energy source of emission. Particularly, nano- and micro-surface structures have been investigated for versatile emission controllability. We present the polarized longitudinal optical (LO) phonon resonant thermal emission of radiation (LORE) peaked at approximately 8.5 THz in a range of 450–650 K from simple surface Au–GaAs–Au micro-stripe structures on undoped (u-) GaAs wafers, where the metal component, which has been avoided for surface phonon polariton (SPhP) emission, is introduced. This LORE is based on electric dipoles formed by coherently oscillating polarization charges on pairs of interfaces and has a feature of structure size or emission direction-independent frequency, which is quite different from the properties of SPhPs. The dependence of emission properties on structural factors and temperature reveals the approximately evenly matched radiative and nonradiative LO-phonon annihilation rates in the local surface field and shows the controllability of emission and absorption balance based on the surface structures. Higher emission efficiency in the higher temperature region is found for structures with narrow emission windows of less than 2 μm. The decrease in the volume occupied by electric dipoles reduces the reabsorption probability of LORE, which shortens the effective radiative lifetime and, resultantly, increases the fraction of the radiative rate in the total annihilation rate. By comparing the emission from a pure graphite wafer, a short LORE mean time interval of approximately 2 ps is found. The background emission subject to Planck’s law is significantly reduced particularly in the region of 250–300 cm−1 using emission window widths narrower than 2 μm because of the sub-diffraction phenomenon.

Photon Recycling in a Solar Cell With Two Lambertian Surfaces

An exact analytical expression for the probability of photon reabsorption and recycling in an idealized solar cell with two Lambertian surfaces is derived. The existing approximations are found to agree with the exact formula to within 5%. The most accurate approximation turned out to be the simplest one that sets the reabsorption probability to the weak-absorption limit of the cell absorptance. The maximal photoconversion efficiency of a silicon solar cell is evaluated to be 29.5% at the base thickness of 98 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m.

Making Eu2+- and Sm2+-Doped Borates Fit for Solar Energy Applications

Despite the development of many luminescent materials for various applications, only a few of these phosphors are applicable for solar energy generation applications. This study used the conventional solid-state reaction method to synthesize different strontium borate compounds codoped with divalent europium and samarium ions. The material was optimized by varying the experimental procedure, the molar ratio of the boron, and the molar ratios of both codopants. Strontium hexaborate doped with a relatively high europium concentration and a low samarium concentration (Sr0.89B6O10:Eu0.1, Sm0.01) gave the optimum optical properties. These properties included a broad excitation range from 220 to 600 nm containing contributions from divalent europium and samarium ions. The material exhibited strong and narrow emission in the region from 650 to 850 nm following radiative transitions within the divalent samarium ions. The internal luminescence quantum efficiency of the optimized material was 79% (λex = 508.5 nm). There was no spectral overlap between the excitation and emission spectra, thereby reducing the reabsorption probability. These marked characteristics make this phosphor material appropriate for use as a solar radiation converter or for use in luminescent solar concentrators.

Universal measure of photon collection efficiency of dye luminescent solar concentrators

Luminescent solar concentrators (LSC) are recently getting attention as promising components for use in building integrated photovoltaics (BIPV). Ongoing efforts to reduce their optical losses result in active search for new luminophores with large Stokes shift in order to reduce the reabsorption probability. Here we present a novel figure of merit for a large group of Gaussian dye luminophores that provides an accurate assessment of optical losses. Our approach originates in simplified analytical solution for spectral overlapping integral of absorption/emission bands and results in assessment of average reabsorption probability. The procedure is then generalized for any Gaussian dye LSC, by considering its basic parameters (Stokes shift, absorbance and size), thus resulting in a simple useful tool for designing practical LSC devices. Calculations are correlated with the experimental results of reabsorption probability obtained by measuring edge fluorescence spectra of various LSCs. • Width of gaussian spectral bands of molecules is related to Stokes shift. • Their relation enables prediction of reabsorption losses based on Stokes shift. • We numerically derive universally valid expression for losses of Gaussian dye LSCs. • Experiments on selected dye LSCs confirms the validity of our theoretical approach.

Rigorous wave-optical treatment of photon recycling in thermodynamics of photovoltaics: Perovskite thin-film solar cells

The establishment of a rigorous theory on thermodynamics of light management in photovoltaics that accommodates various loss mechanisms as well as wave-optical effects in the absorption and reemission of light is at stake in this contribution. To this end, we propose a theoretical framework to calculate the open-circuit voltage enhancement resulting from photon recycling ($\Delta V^{\mathrm{PR}}_{\mathrm{oc}}$) with rigorous wave-optical treatment. It can be applied to both planar thin-film and nanostructured single-junction solar cells. We derive an explicit expression for $\Delta V^{\mathrm{PR}}_{\mathrm{oc}}$, which reveals its dependence on internal quantum luminescence efficiency, parasitic reabsorption, and on photon escape probabilities of reemmited photons. While the internal quantum luminescence efficiency is an intrinsic material property, both latter quantities can be determined rigorously for an arbitrary solar cell architecture by three-dimensional electrodynamic dipole emission calculations. We demonstrate the strengths and validity of our framework by determining the impact of photon recycling on the $V_{\mathrm{oc}}$ of a conventional planar organo-metal halide perovskite thin-film solar cell and compare it to established reference cases with perfect antireflection and Lambertian light scattering. Our calculations reveal $\Delta V^{\mathrm{PR}}_{\mathrm{oc}}$ values of up to 80 mV for the considered device stack in the absence of angular restriction and up to 240 mV when the escape cone above the cell is restricted to $\theta_{\mathrm{out}}=2.5^\circ$ around the cell normal. These improvements impose severe constraints on the parasitic absorption as a parasitic reabsorption probability of only 2\% reduces the $\Delta V^{\mathrm{PR}}_{\mathrm{oc}}$ to 100 mV for the same angular restriction. Our work here can be used to provide design guidelines.

Steady-State Fluorescence of Highly Absorbing Samples in Transmission Geometry: A Simplified Quantitative Approach Considering Reabsorption Events.

A simplified methodology to acquire steady-state emission spectra and quantum yields of highly absorbing samples is presented. The experimental setup consists of a commercial spectrofluorometer adapted to transmission geometry, allowing the detection of the emitted light at 180° with respect to the excitation beam. The procedure includes two different mathematical approaches to describe and reproduce the distortions caused by reabsorption on emission spectra and quantum yields. Toluene solutions of 9,10-diphenylanthracence, DPA, with concentrations ranging between 1.12 × 10-5 and 1.30 × 10-2 M, were used to validate the proposed methodology. This dye has significant probability of reabsorption and re-emission in concentrated solutions without showing self-quenching or aggregation phenomena. The results indicate that the reabsorption corrections, applied on molecular emission spectra and quantum yields of the samples, accurately reproduce experimental data. A further discussion is performed concerning why the re-emitted radiation is not detected in the experiments, even at the highest DPA concentrations.

Multiwavelength time-resolved detection of fluorescence during the inflow of indocyanine green into the adult’s brain

Optical technique based on diffuse reflectance measurement combined with indocyanine green (ICG) bolus tracking is extensively tested as a method for clinical assessment of brain perfusion in adults at the bedside. Methodology of multiwavelength and time-resolved detection of fluorescence light excited in the ICG is presented and advantages of measurements at multiple wavelengths are discussed. Measurements were carried out: 1. on a physical homogeneous phantom to study the concentration dependence of the fluorescence signal, 2. on the phantom to simulate the dynamic inflow of ICG at different depths, and 3. in vivo on surface of the human head. Pattern of inflow and washout of ICG in the head of healthy volunteers after intravenous injection of the dye was observed for the first time with time-resolved instrumentation at multiple emission wavelengths. The multiwavelength detection of fluorescence signal confirms that at longer emission wavelengths, probability of reabsorption of the fluorescence light by the dye itself is reduced. Considering different light penetration depths at different wavelengths, and the pronounced reabsorption at longer wavelengths, the time-resolved multiwavelength technique may be useful in signal decomposition, leading to evaluation of extra- and intracerebral components of the measured signals.

Photon reabsorption in fluorescent solar collectors

Understanding photon transport losses in fluorescence solar collectors is very important for increasing optical efficiencies. We present an analytical expression to characterize photon reabsorption in fluorescent solar collectors, which represent a major source of photon loss. A particularly useful universal form of this expression is found in the limit of high reabsorption, which gives the photon reabsorption probability in a simple form as a function of the absorption coefficient and the optical étendue of the emitted photon beam. Our mathematical model predicts fluorescence spectra emitted from the collector edge, which are in excellent agreement with experiment and provide an effective characterization tool for photon transport in light absorbing media.

Monte-Carlo simulations of light propagation in luminescent solar concentrators based on semiconductor nanoparticles

Semiconductor nanoparticles have a wide absorption band and small reabsorption probability, which makes them great candidates for luminescent solar concentrators (LSCs). We use Monte-Carlo simulations of photon transport to predict the performance of LSCs based on “type-II” CdSe-CdTe quantum dots. These computations suggest that semiconductor-based LSCs can be highly efficient. The optimum performance is reached with a fairly long LSC with a photovoltaic cell covering only one edge. In addition, when the LSC has CdSe-CdTe nanorods that are aligned perpendicular to the top surface, the escape of photons from the top surface is significantly reduced. These results are encouraging for cost-effective LSC designs based on semiconductor nanoparticles.

Patterned dye structures limit reabsorption in luminescent solar concentrators

This work describes a method for limiting internal losses of a luminescent solar concentrator (LSC) due to reabsorption through patterning the fluorescent dye doped coating of the LSC. By engineering the dye coating into regular line patterns with fill factors ranging from 20 - 80%, the surface coverage of the dye molecules were reduced, thereby decreasing the probability of the re-emitted light encountering another dye molecule and the probability of reabsorption. Two types of fluorescent dyes with different quantum yields were used to examine the effects of patterning on LSC performance. The effect of various dimension and geometry of the patterns on the efficiency and edge emission of LSC are presented and analyzed.

AlGaInP/GaP Heterostructures Bonded with Si Substrate to Serve as Solar Cells and Light Emitting Diodes

In this study, an AlGaInP/GaP-based heterostructure featuring a silicon substrate and a /indium tin oxide/Ag omnidirectional reflector, using a metal-to-metal bonding technique, serves as a dual-function device operating in light emitting and photovoltaic modes. To enhance the light extraction efficiency and conversion efficiency, AlGaInP/Si heterojunction devices with a periodic texture applied to the surface layer using photolithography and a wet etching process are also presented. Using the light emitting mode and a 350 mA current injection, the external quantum efficiencies of AlGaInP/Si light emitting diodes (LEDs) with (LED-I) and without (LED-II) a textured surface are measured at approximately 17.3 and 11.8%, respectively. The enhancement of the output power in LED-I can be attributed to a multitude of bowl-shaped notches on the surface, resulting in a reduction in the reabsorption probability of photons inside the device because the photon path length of LED-I is shorter than LED-II before photons escape into free space. When devices are operated in a photovoltaic mode, measured under an air mass 1.5 condition, the typical efficiency and fill factor are around 4.67 and 83%, respectively, for devices with a textured surface.

Improved Light Extraction Efficiency in AlGaInP Light-Emitting Diodes by Applying a Periodic Texture on the Surface

In this letter, AlGaInP-GaP-based light-emitting diodes (LEDs) were fabricated with an Si substrate and an SiO2-ITO-Ag omni-directional reflector using a metal-to-metal bonding technique. To enhance light extraction efficiency, a periodic texture was applied to the (Al0.5Ga0.5)0.5In0.5P surface layer of AlGaInP-Si LEDs by photolithography and a wet etching process. The exterior of the etched texture consists of a series of bowl-shaped recesses. With a 350-mA current injection, the typical output power of the AlGaInP-Si LEDs with and without the textured surface (LED-I and LED-II, respectively) were measured at approximately 118 and 81 mW, respectively, when the LED chips were bonded on the TO 46 without resin encapsulation. The enhancement of output power in LED-I can be attributed to a multitude of bowl-shaped notches on the surface, which resulted in a reduction of the reabsorption probability of the photons due to the fact that the photon path length in LED-I is shorter than in LED-II before the photons escape into the free space.

Spatial effects of radiation trapping in an optically thick atomic vapor excited by a laser beam

We describe the influence of radiation trapping on the spatial distribution of the excited-state population in sodium vapor, interacting with a linearly polarized Gaussian laser beam. Counterintuitively the excitation volume experiences a strong spatial shrinking at high optical depth. The interpretation of this nonlocal effect is based on the numerical solution of the stationary Holstein equation, which takes into account the full frequency dependence of the reabsorption probability within the process of diffusion of radiation. The calculations performed are in good qualitative agreement with the spatial effects observed experimentally. \textcopyright{} 1996 The American Physical Society.

Subthreshold antiproton andK ? production in heavy ion collisions

Subthreshold ¯p andK− and energeticπ− production was studied in Ne + NaF, Cu, Sn and Bi, and in Ni + Ni collisions with incident energies between 1.6 and 2 GeV/u. The measured cross sections indicate a dominant contribution of baryonic resonances. This is also consistent with a generalized scaling behaviour of the cross sections with the energy available in the collision and the energy necessary to produce particles as observed with Ne induced reactions. Deviations from scaling especially pronounced in the Ni-Ni system will be discussed in terms of absorption effects. The flat slope of the excitation function for ¯p production can be related to a reduced production threshold caused by a reduction of the antiproton mass in the dense and heated medium by about 100—150 MeV/c2. A similar in-medium mass reduction is also indicated forK− mesons. An increased ¯p reabsorption probability for the heavier systems is concluded from the comparison of the ¯p yields in Ne + NaF, Ne + Sn and Ni + Ni collisions.

Some remarks on the Σ− hyperon conversion rate

The reabsorption probability for Σ − hyperons produced by K − meson interactions at rest in freon CF 3Br has been found to be 0.50 ± 0.05, in agreement with previous results obtained in various light or heavy elements. The implications of this result on some aspects of the K − meson capture process are discussed.

The Ionization Structure of Planetary Nebulae--VIII: Models of the Nebulae NGC 7662 and IC 418

Using computer programs which are based on the. theory developed in the previous papers in this series a number of models of the planetary nebulae NGC 7662 and IC 418 have been computed. An essential feature of the models is the inclusion of stellar atmosphere fluxes calculated by Hummer & Mihalas (private communication) which fit the observational data for the central stars of these two nebulae. A detailed comparison is made between observed and calculated forbidden line intensities and values derived for the abundances of nitrogen, oxygen and neon relative to hydrogen in the nebulae. The value of particularly the neon abundance is found to be substantially less than the mean values for B-stars and for planetary nebulae given by Aller (1961 and 1964 respectively). It is shown that the intensity of the collisionally excited C IV λ 1549 resonance doublet relative to the He II λ4686 recombination line is insensitive to the physical parameters of the model but sensitive to the probability of re-absorption of He II Ly α photons produced in the nebula. Measurement of the intensity of the λ1549 1ine (from above the Earth's atmosphere) for a few high-excitation planetary nebulae would, therefore, be important to the understanding of the He II Ly α transfer problem.

Antiproton annihilations in complex nuclei

An investigation of 356 antiproton annihilations in nuclear emulsion is reported. The results are generally analysed to yield information on primary processes, assumed to be annihilations with single protons or neutrons into a number of pions with or without a K-meson pair. The average pion multiplicity is 4.68±0.12 for annihilations at rest, and 5.11±0.12 for annihilations in flight (the antiproton energy being on the average 166 MeV). This increase of 0.4±0.2 pions is to be compared with an expected average increase of 0.1 pion. The pion energy distributions are given with mean values of 391±10 MeV total energy (at rest) and 390±9 MeV (in flight). Emission of K-meson pairs is found to take place in 3±2% of the annihilation reactions. The average number of pions emitted together with K-mesons is 2.2±0.5. The correction for K-meson pairs unobservable in usual emulsion experiments has been computed. The result that about 40% of the K-meson pairs are undetectable, means that the correction for detection efficiency is larger than previously assumed. A recalculation of all published data using emulsion detectors results in a world average of 5.0±1.1% of the annihilations with K-meson pairs for annihilations at rest and in flight (antiproton energy below 250 MeV). The annihilation probability at rest with neutrons is found to be less than that with protons on the basis of a determination of the number of stars with an odd number of charged pions relative to that with an even number. The ratio odd/even is 0.73±0.09. The reabsorption probability of the pions in complex emulsion nuclei is determined. It is found that 18% of the pions are absorbed on the average (at rest) and 32% (in flight). The difference is understood as a result of the movement of the centre-of-momentum and the penetration of the antiprotons into the nuclei for the cases produced in flight. Comparison of our experimental results with the predictions of various theories is carried out. For the pion production, however, no decision in favour of a particular theoretical model can be made. As regards the production of K-mesons, all theories fail to predict the experimentally found production frequency. The world data on antiproton annihilations are collected, reanalysed and discussed.