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In this study, numerical optimization and analysis utilizing AMPS-1D software package of a simple model for tunnel junction (AlGaAs) between the top cell (AlGaAs) and bottom cell (SiGe) of cascade solar cells. The electrical properties and the photovoltaic performance parameters of AlGaAs/SiGe multijunction solar cells. The possible effects of base of the top cell layer thickness and doping level on solar cell performance parameters are addressed. The conversion efficiency of the solar cell has been found to increase significantly with the doping concentration in the range from 1015 to 1017 cm-3 of 19.52% to 31.19% under the AM1.5G spectrum and one sun. These results are very promising for future potential applications in multijunction and high performance AlGaAs/SiGe solar cells technology. [ABSTRACT FROM AUTHOR]

Carbon black (CB) filled polymers have become the platform to study a number of interesting properties including percolation mechanisms, localization effects, and multiscale modeling of interface and interphase regions surrounding filler particles. A systematic microwave study of the effective complex permittivity of CB filled diglycidylic ether of bisphenol A based epoxy samples, determined by the impedance bridge (35 GHz) and the cavity perturbation (2.4 and 9.5 GHz) techniques, is reported. A series of composite materials was fabricated by mechanical mixing with three types of CB (Monarch and Sterling) particles differing with their average particle size and surface area. On the basis of the limited experimental evidence presented here, our distinctive features are seen in the data: (i) We found no enhancement of the effective permittivity near the dc percolation threshold; (ii) the calculation of the effective permittivity based on Lichteneker and Rother's mixing law with constant k close to zero reproduces the measured CB volume fraction dependence of the effective permittivity very well for the series of samples containing Monarch particles, (iii) the data are not well fit with the Bruggeman equation for supercolative samples. This is an indication that a mean field model is not strictly applicable because this simple model assumes a given microstructure for the composite material; and (iv) the two exponent phenomenological percolation equation (TEPPE) can yield good predictive values of the imaginary part of the effective complex permittivity over the range of frequencies and CB volume fractions with non-universal values of the percolation exponents. It is concluded that fitting the experimental data with mixing law and effective medium equation predictions has limited applicability because these models assume a given microstructure for the composite material. [ABSTRACT FROM AUTHOR]

In this work, the relaxation dynamics (RD) in carbon black (CB)-polymer composites under stress is investigated by two complementary approaches: effective permittivity and electric modulus formally defined as the reciprocal of the complex relative permittivity. Experimental measurements of the microwave data over the of CB volume fractions below the percolation threshold and microwave frequencies studied agree well with the predictions of the Cole-Cole (CC) phenomenological model of RD. Similar conclusions about the RD of subpercolative samples could be reached by viewing the data in terms of their &eh; and M representations. For subpercolative systems, the CB aggregates show nearly no influence on the dielectric relaxation through the matrix. However, we show that the relaxation time scale is increased by two orders of magnitude when CB volume fraction is increased from below to above the percolation threshold at about 8.5 vol%. Extraction of the RD information from CC modeling shows that when the system is subpercolative the dielectric relaxation kinetics is proportional to λ-1/λ2, which is consistent with the Gaussian molecular network (affine network) model in the low-strain case. [ABSTRACT FROM AUTHOR]

A thorough investigation of the ac electrical properties of carbon black (CB) mixed into ethylene butylacrylate copolymer has been conducted using ac impedance spectroscopy in the frequency range 10 Hz–100 kHz and over the temperature range of 150 (Tg-48 K)–319 (Tg+121 K) K. For this investigation, a series of eight samples were prepared with various filler contents above the percolation threshold. Using the electric modulus formalism it has been found that the Cole–Cole equation of dielectric relaxation expressed in the electric modulus form is capable of quantitatively describing the experimental data from which we extract the relaxation time and a parameter α which gauges the broadening of the loss spectrum. The small values of α ranging from 0.06 to 0.10 suggest a behavior close to the state of a single relaxation time. Furthermore, the relaxation time as a function of temperature is characterized by an Arrhenius behavior. While the effective activation energy is on the order of 60 meV and is insensitive to CB volume fraction, the Arrhenius prefactor is found to strongly depend on this volume fraction. The observed relaxation mechanism does not result simply from thermally activated dipolar interactions but is a collective cooperative effect of the CB mesostructure. [ABSTRACT FROM AUTHOR]

In this study we present measurements and data analysis of the dielectric relaxation in carbon black (CB) loaded epoxy composites over a wide range of CB concentration (below and above the percolation threshold) and a broad range of frequency (180–1.5×107 Hz). We show that our data are in conflict with the expectation that the fit parameters of the much-discussed two exponent phenomenological percolation equation (TEPPE) represent the critical exponents of the standard percolation theory. This result therefore gives new insight to the problem of why the TEPPE has a limited applicability in representing the effective permittivity versus CB volume fraction functional relationship. We believe that the reason for this discrepancy is that the true dependence of the internal length scales of the mesostructure and the elasticity network is too complicated to fulfill a simple unified functional equation. [ABSTRACT FROM AUTHOR]

Technologically, an understanding of the temperature influence on the transport properties is essential to the study of many random conductor-insulator composites, while fundamentally it is related to a variety of questions in statistical physics, dielectrics, and materials science, to name a few. Variable-temperature measurements of the frequency dependent complex effective permittivity were performed on amine-cured epoxy resins loaded with carbon black (CB). Two series of prepercolative samples differing from the kind of CB particles (Raven 2000 and Raven 5000) mixed in an amine-cured epoxy matrix (diglycidylic ether of bisphenol F) were studied. In this effort to contribute to our understanding of the role of frequency (100 Hz–15 MHz) and temperature (from ambient temperature up to 90 °C) on the complex effective permittivity which describes the linear response of the system to an electromagnetic wave, we investigate these composites with CB loadings below the percolation threshold. Two features are observed. First, our observations cannot be understood in the typical framework of a simple Debye-like dipolar process. In this analysis, we argue that the appearance of the broad temperature and frequency dependent maximum loss can be understood within the heuristic framework proposed by Jonscher which applies to disordered heterogeneous systems. This theoretical framework is consistent with several aspects of the experiments, notably the power-law decays of the real and imaginary parts of the effective permittivity characterized by two fractional exponents m and n. These exponents are both positive and smaller than unity. We further quantified their different temperature variations: while m is strongly decreasing with increasing temperature, n takes a value close to 1. Second, the observed maximum loss frequency found for each CB volume fraction shifts to higher frequencies with increasing temperature and exhibits a non-Arrhenius temperature dependence well represented by a Vogel–Tammam–Fulcher (VTF) fit. Well below the percolation threshold, the associated activation energy and ordering temperature of the VTF fit are not significantly sensitive upon the CB concentration. Such results are compared to previous related work. [ABSTRACT FROM AUTHOR]

Filled polymers have been a fertile area for modeling studies of microwave dynamics, percolation, and elasticity network formation to mention but a few. To better understand the basic concepts relevant to wave transport in filled polymers, an experimental validation is paramount for theories that predict the dielectric response and, hence, the polarization mechanisms of these heterostructures. Reproducible measurements of the effective complex permittivity, at ambient temperature, are reported in the frequency range of 200 Hz–15 MHz for series of carbon black (CB)-epoxy composites that provide a detailed and quantitative characterization of the dielectric relaxation behavior. We provide observational evidence showing that dipolar relaxation and anomalous low-frequency dispersion are likely the transport mechanisms in these samples below and above percolation, respectively. The various fractional exponents of the power-law decay of these spectral models are analyzed, and Hill plots can be used to rationalize the effective complex permittivity observations for these series of samples. Our results have important implications for designing and optimizing engineered CB/polymer materials for a broad spectrum of engineering applications, e.g., solid broadband tissue simulant materials and electromagnetic absorbers. [ABSTRACT FROM AUTHOR]