Stylianos P. Savaidis

Optical Modeling of Hybrid Polymer Solar Cells Using a Transmission-Line Model and Comparison With Experimental Results

Organic photovoltaic (OPV) cells provide an efficient alternative solution compared to traditional PV devices. Their conversion efficiency is directly dependent to the light intensity in the photoactive layer. The present work proposes a fast, accurate, and easy-to-implement transmission-line model, which evaluates the light intensity in any layer inside a multilayer OPV cell. In particular, a plane optical wave impinging normally on a planar multilayer OPV configuration is considered and next the optical field distribution is determined. Numerical results and optimization guidelines for a hybrid PV configuration are presented and discussed.

Scattering by an Infinite Elliptic Metallic Cylinder

The scattering of a plane electromagnetic wave by an infinite elliptic metallic cylinder is considered. Two different methods are used for the evaluation. In the first the electromagnetic field is expressed in terms of elliptical-cylindrical wave functions. In the second, a shape perturbation method, the field is expressed in terms of circular-cylindrical wave functions only, while the equation of the elliptical boundary is given in polar coordinates. Analytical expressions are obtained for the scattered electromagnetic field and the various scattering cross-sections, when the solution is specialized to small values of the eccentricity h = c/2a, (h ≪ 1), with c the interfocal distance of the elliptic cylinder and 2a the length of its major axis. In this case exact, closed form expressions are obtained for the expansion coefficients g(2) and g(4) in the relation S(h) = S(0)[1 + g(2)h2 + g(4)h4 + O(h6)] expressing the scattered field and the scattering cross-sections. Both polarizations are considered for normal incidence. Numerical results are given for various values of the parameters.

Simulation of Light Emission From Planar Multilayer OLEDs, Using a Transmission-Line Model

In this paper, we propose a fast, accurate, and easy to implement transmission-line model, which evaluates light emission from planar multilayer OLED devices. Total radiated power and radiation patterns of OLED devices are calculated adopting the classic consideration, which relates photon emission in the active layer with the power that is radiated by a randomly oriented dipole antenna. The simulation model considers the under study configurations as equivalent electric and magnetic transmission lines in Fourier space, which are excited by voltage or current sources properly calculated. The proposed model demonstrates also an inherent ability to calculate the waveguided modes and can be also easily transformed to tackle problems of material inhomogeneity and nonlinearity. The accuracy of the transmission-line model has been verified against alternative but also exact calculation methods. Finally, numerical results are provided and discussed for OLED devices, which are constructed by materials with a complex refractive index and emit light within a zone inside the active layer.

Conformal Patch Antenna Arrays Design for Onboard Ship Deployment Using Genetic Algorithms

Conformal antennas and antenna arrays (arrays) have become necessary for vehicular communications where a high degree of aerodynamic drag reduction is needed, like in avionics and ships. However, the necessity to conform to a predefined shape (e.g., of an aircraft’s nose) directly affects antenna performance since it imposes strict constraints to the antenna array’s shape, element spacing, relative signal phase, and so forth. Thereupon, it is necessary to investigate counterintuitive and arbitrary antenna shapes in order to compensate for these constraints. Since there does not exist any available theoretical frame for designing and developing arbitrary-shape antennas in a straightforward manner, we have developed a platform combining a genetic algorithm-based design, optimization suite, and an electromagnetic simulator for designing patch antennas with a shape that is not a priori known (the genetic algorithm optimizes the shape of the patch antenna). The proposed platform is further enhanced by the ability to design and optimize antenna arrays and is intended to be used for the design of a series of antennas including conformal antennas for shipping applications. The flexibility and performance of the proposed platform are demonstrated herein via the design of a high-performance GPS patch antenna.

Reflection and transmission calculations in a multilayer structure with coherent, incoherent, and partially coherent interference, using the transmission line method.

A generalized transmission line method (TLM) that provides reflection and transmission calculations for a multilayer dielectric structure with coherent, partial coherent, and incoherent layers is presented. The method is deployed on two different application fields. The first application of the method concerns the thickness measurement of the individual layers of an organic light-emitting diode. By using a fitting approach between experimental spectral reflectance measurements and the corresponding TLM calculations, it is shown that the thickness of the films can be estimated. The second application of the TLM concerns the calculation of the external quantum efficiency of an organic photovoltaic with partially coherent rough interfaces between the layers. Numerical results regarding the short circuit photocurrent for different layer thicknesses and rough interfaces are provided and the performance impact of the rough interface is discussed in detail.

Outcoupling efficiency optimization of phosphorescent and fluorescent based hybrid red, green and blue emitting OLED devices

The outcoupling efficiency of a hybrid OLED device with a hole-injection layer formed by either an organic semiconducting material, namely PEDOT:PSS, or an under-stoichiometric molybdenum oxide is examined. Because the device is considered for the three primary colour emissions, the active layer is composed of a wide band-gap blue emitting host polymer doped with either phosphorescent or fluorescent guest emitters. Critical parameters, such as the emission zone profile for the doped active layer with either the organic hole-injection layer or the inorganic metal oxide, are studied. Contour charts have been derived providing the areas of maximum outcoupling efficiency with the corresponding thickness pairs of the active layer and the hole-injection layer. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Application of the transmission line method for the study of highly nonlinear multilayer optical structures

Abstract. In this work, we numerically investigate and analyze the properties of an optical structure composed of successive thin film layers that can possess high values of nonlinear susceptibility, affecting the refractive index and/or the absorption coefficient. By applying the transmission line method properly modified to resolve the inclusion of third-order nonlinearity, the spectral reflectivity and transmission of such a device are presented. Specifically, the method is applied to a conceptual design of a distributed Bragg reflector. Optical bistability can be observed, which translates not only to a change in the value of reflectivity as the input power increases, but also to a shift of the Bragg wavelength.

Cutoff wavelengths of metallic waveguides with elliptical cross section

The cutoff wavelengths λcmn of metallic waveguides with elliptical cross section are determined analytically. Two alternative methods, namely, a field and a shape perturbation method are used in the proposed analysis. Exact, closed form expressions, free of Mathieu as well as of Bessel functions, are obtained for the cutoff wavelengths, when the solution is specialized to small values of the eccentricity of the elliptical cross section. Numerical results for all types of modes are also included.