Optimal thicknesses determination in a multilayer structure to improve the SPP efficiency for photovoltaic devices by an hybrid FEM — Cascade Neural Network based approach

Abstract —As the global energy needs to grow, there is increasing interest in the electricity generation by photovoltaics (PVs) devices or solar cells. Analytical and numerical methods are used in literature to study the propagation of surface plasmon polaritons (SPP) but the optimal thicknesses in a multilayer structure can’t be established for an optimal propagation by these. In this paper a new method based on cascade Neural Network (NN) is used to predict the propagation characteristics of a multilayer plasmonic structure and coupling FEM analysis of the involved electromagnetic field. The trained NNs are able to provide the required optimal values of the SPP propagation with good accuracy at different value of thicknesses in the multilayer structure. Keywords — Photovoltaics; Surface plasmon polaritons; propagation; cascade neural network; finite element analysis (FEM) I. I NTRODUCTION Photovoltaic (PV) generation is one of the most promising renewable energy utilization methods in the world. Photovol-taic power generation become at high potential, with recent developments in technologies such as photovoltaic panels, energy conversion system, power electronics, energy storage system, and control system [1], [2]. Solar industry has seen an enormous growth during the last decade. The sale of photo-voltaic modules has exceeded 27 GW in 2011, with significant contributions to the market share from all technologies. In-creased production of silicon modules has led to a very fast reduction in their price and remains as benchmark for other technologies. Despite significant improvements in the performance of PV devices in the past half-century, solar cells have not yet to achieve widespread success as an alternative means of pro-ducing electricity. For this reason, the design and optimization of solar cells also aims at maximizing light absorption and improving PV efficiency. Several kinds of solar cells, which differ in the material systems used, design, PV structure, and even the principle of PV conversion, are designed to date. Papers on many aspects of solar cells as the growth and deposition, characterization, and new material development are appeared in literature. A photovoltaic power conversion device has a top surface adapted to receive impinging radiation. The device includes at least two adjacent, serially connected cells. Each cell includes a semi-insulating substrate and a lateral conductivity layer of a first doped electrical conductivity disposed on the substrate. A base layer is disposed on the lateral conductivity layer and has the same electrical charge conductivity there of an emitter layer of a second doped electrical conductivity of opposite electrical charge is disposed on the base layer and forms a p-n junction there between. A PV device based on an interband cascade (IC) structure is proposed for efficiently converting solar and thermal energy to electricity. These IC PV devices employ absorption and transport regions with characteristics that are favorable for achieving high open-circuit voltage and thus possibly improving conversion efficiency over conven-tional PV devices [3]. Among the research on this issue, the emerging field of plasmonics has yielded methods for guiding and localizing light at the nanoscale, well below the scale of the wavelength of light in free space thus a new kind of technological solu-tions can be pursued. Indeed, plasmonic can be used to improve absorption in photovoltaic devices, allowing considerable shrinkage of the photovoltaic layer thickness, while keeping the optical ab-sorption and thus efficiency constant. Also for this reason, in the last year, the field of plasmonics is experiencing a rapid development, due to the interest in studying the behavior of light at the nanometer scale [4]. Computer simulations have proved to be a very useful in-dispensable tool to obtaining a better understanding of solar PV modules performance and to determine optimizing trends for material parameters and solar cell structures. For example light absorption by solar cells patterned with metallic nano-gratings has been investigated [5],[6]. Another possibility is the study of the effects of light incident exciting surface plas-mon-polaritons (SPP) at the metal surface for improving effi-cient capturing of light in solar cells. SPPs are electromagnetic