The building sector accounts for 36% of energy consumption and 39% of energyrelated greenhousegas emissions. Integrating bifacial photovoltaic solar cells in buildings could significantly reduce energy consumption and related greenhouse gas emissions. Bifacial solar cells should be flexible, bifacially balanced for electricity production, and perform reasonably well under weaklight conditions. Using rigorous optoelectronic simulation software and the differential evolution algorithm, we optimized symmetric/asymmetric bifacial CIGS solar cells with either (i) homogeneous or (ii) gradedbandgap photonabsorbing layers and a flexible central contact layer of aluminumdoped zinc oxide to harvest light outdoors as well as indoors. Indoor light was modeled as a fraction of the standard sunlight. Also, we computed the weaklight responses of the CIGS solar cells using LED illumination of different light intensities. The optimal bifacial CIGS solar cell with gradedbandgap photonabsorbing layers is predicted to perform with 18–29% efficiency under 0.01– 1.0sun illumination; furthermore, efficiencies of 26.08% and 28.30% under weak LED light illumination of 0.0964 mW cm^{2} and 0.22 mW cm^{2} intensities, respectively, are predicted.
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Bifacial flexible CIGS thinfilm solar cells with nonlinearly gradedbandgap photonabsorbing layers
Abstract 
In Part I [
], we used a coupled optoelectronic model to optimize a thinfilm AlGaAs solar cell with a gradedbandgap photonabsorbing layer and a periodically corrugated Ag backreflector combined with localized ohmic Pd–Ge–Au backcontacts, because both strategies help to improve the performance of AlGaAs solar cells. However, the results in Part I were affected by a normalization error, which came to light when we replaced the hybridizable discontinuous Galerkin scheme for electrical computation by the faster finitedifference scheme. Therefore, we reoptimized the solar cells containing an n AlGaAs photonabsorbing layer with either a (i) homogeneous, (ii) linearly graded, or (iii) nonlinearly graded bandgap. Another way to improve the power conversion efficiency is by using a surface antireflection texturing on the wavelength scale, so we also optimized four different types of 1D periodic surface texturing: (i) rectangular, (ii) convex hemielliptical, (iii) triangular, and (iv) concave hemielliptical. Our new results show that the optimal nonlinear bandgap grading enhances the efficiency by as much as 3.31% when then AlGaAs layer is 400 nm thick and 1.14% when that layer is 2000 nm thick. A hundredfold concentration of sunlight can enhance the efficiency by a factor of 11.6%. Periodic texturing of the front surface on the scale of 0.5–2 freespace wavelengths provides a small relative enhancement in efficiency over the AlGaAs solar cells with a planar front surface; however, the enhancement is lower when then AlGaAs layer is thicker. 
Free, publiclyaccessible full text available April 1, 2024

Subramanyam, Guru ; Banerjee, Partha ; Lakhtakia, Akhlesh ; Sun, Nian X. (Ed.)Antireflection coatings are vital for reducing loss due to optical reflection in photovoltaic solar cells. A singlelayer magnesium fluoride (MgF2) antireflection coating is usually used in thin film CIGS solar cells. According to optics, this coating can be effective only for a narrow spec tral regime. Further reduction of reflection loss may require an optimal singlelayer or multilayer coating. Hence, we optimized the refractive indices and thicknesses of single and doublelayer an tireflection coatings for CIGS solar cells containing a CIGS absorber layer with: (i) homogeneous bandgap, (ii) linearly graded bandgap, or (iii) nonlinearly graded bandgap. A relative enhancement of up to 1.83% is predicted with an optimal doublelayer antireflection coating compared to the efficiency with a singlelayer antireflection coating.more » « less

Subramania, Ganapathi S. ; Foteinopoulou, Stavroula (Ed.)We model the e ect of concentrated sunlight on CIGS thin lm gradedbandgap solar cells using an optoelectronic numerical model. For this purpose it is necessary first to solve the timeharmonic Maxwell equations to compute the electric eld in the device due to sunlight and so obtain the electronholepair generation rate. The generation rate is then used as input to a driftdiffusion model governing the flow of electrons and holes in the semiconductor components that predicts the current generated. The optical submodel is linear; however, the electrical submodel is nonlinear. Because the Shockley{Read{Hall contribution to the electronhole recombination rate increases almost linearly at high electron/hole densities, the effciency of the solar cell can improve with sunlight concentration. This is illustrated via a numerical study.more » « less

In Part I [
], we used a coupled optoelectronic model to optimize a thinfilm ${\mathrm{C}\mathrm{u}\mathrm{I}\mathrm{n}}_{1<\#comment/>\mathrm{\xi <\#comment/>}}{\mathrm{G}\mathrm{a}}_{\mathrm{\xi <\#comment/>}}{\mathrm{S}\mathrm{e}}_{2}$ (CIGS) solar cell with a gradedbandgap photonabsorbing layer and a periodically corrugated backreflector. The increase in efficiency due to the periodic corrugation was found to be tiny and that, too, only for very thin CIGS layers. Also, it was predicted that linear bandgapgrading enhances the efficiency of the CIGS solar cells. However, a significant improvement in solar cell efficiency was found using a nonlinearly (sinusoidally) gradedbandgap CIGS photonabsorbing layer. The optoelectronic model comprised two submodels: optical and electrical. The electrical submodel applied the hybridizable discontinuous Galerkin (HDG) scheme directly to equations for the drift and diffusion of charge carriers. As our HDG scheme sometimes fails due to negative carrier densities arising during the solution process, we devised a new, to the best of our knowledge, computational scheme using the finitedifference method, which also reduces the overall computational cost of optimization. An unfortunate normalization error in the electrical submodel in Part I came to light. This normalization error did not change the overall conclusions reported in Part I; however, some specifics did change. The new algorithm for the electrical submodel is reported here along with updated numerical results. We reoptimized the solar cells containing a CIGS photonabsorbing layer with either (i) a homogeneous bandgap, (ii) a linearly graded bandgap, or (iii) a nonlinearly graded bandgap. Considering the meager increase in efficiency with the periodic corrugation and additional complexity in the fabrication process, we opted for a flat backreflector. The new algorithm is significantly faster than the previous algorithm. Our new results confirm efficiency enhancement of 84% (resp. 63%) when the thickness of the CIGS layer is 600 nm (resp. 2200 nm), similarly to Part I. A hundredfold concentration of sunlight can increase the efficiency by an additional 27%. Finally, the currently used 110nmthick layer of${\mathrm{M}\mathrm{g}\mathrm{F}}_{2}$ performs almost as well as optimal single and doublelayer antireflection coatings. 
Abstract We investigate an inverse scattering problem for a thin inhomogeneous scatterer in R m , m = 2, 3, which we model as an m − 1 dimensional open surface. The scatterer is referred to as a screen. The goal is to design target signatures that are computable from scattering data in order to detect changes in the material properties of the screen. This target signature is characterized by a mixed Steklov eigenvalue problem for a domain whose boundary contains the screen. We show that the corresponding eigenvalues can be determined from appropriately modified scattering data by using the generalized linear sampling method. A weaker justification is provided for the classical linear sampling method. Numerical experiments are presented to support our theoretical results.more » « less

In this paper we consider the inverse problem of determining structural properties of a thin anisotropic and dissipative inhomogeneity in
,\begin{document}$ {\mathbb R}^m $\end{document} from scattering data. In the asymptotic limit as the thickness goes to zero, the thin inhomogeneity is modeled by an open\begin{document}$ m = 2, 3 $\end{document} dimensional manifold (here referred to as screen), and the field inside is replaced by jump conditions on the total field involving a second order surface differential operator. We show that all the surface coefficients (possibly matrix valued and complex) are uniquely determined from far field patterns of the scattered fields due to infinitely many incident plane waves at a fixed frequency. Then we introduce a target signature characterized by a novel eigenvalue problem such that the eigenvalues can be determined from measured scattering data, adapting the approach in [\begin{document}$ m1 $\end{document} 20 ]. Changes in the measured eigenvalues are used to identified changes in the coefficients without making use of the governing equations that model the healthy screen. In our investigation the shape of the screen is known, since it represents the object being evaluated. We present some preliminary numerical results indicating the validity of our inversion approach