Absorption Enhancement in Plasmonic Solar Cells by Incorporation of Periodic Nanopatterns
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Absorption Enhancement in Plasmonic Solar Cells by Incorporation of Periodic Nanopatterns W. Wang1, S. Wu1, Y.L. Lu2, Kitt Reinhardt3, S.C. Chen4 Materials Science and Engineering, The University of Texas at Austin, Austin, TX 78712, USA 2 Laser Optics Research Center, Physics Department, United States Air Force Academy, CO 80840, USA 3 United States Air Force Office of Scientific Research, AFOSR/NE, 875 North Randolph Street, Suite 326, Arlington, VA 22203, USA 4 NanoEngineering Department, University of California, San Diego, CA 92093, USA
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ABSTRACT Currently, the performances of thin film solar cells are limited by poor light absorption and carrier collection. In this research, large, broadband, and polarization-insensitive light absorption enhancement was realized via incorporation of different periodic nanopetterns. By studying the enhancement effect brought by different materials, dimensions, coverage, and dielectric environments of the metal nanopatterns, we analyzed the absorption enhancement mechanisms as well as optimization criteria for our designs. A test for totaling the absorption over the solar spectrum shows an up to ~30% broadband absorption enhancement when comparing to conventional thin film cells. INTRODUCTION Active materials used in thin film solar cells are usually polycrystalline or amorphous silicon (p-Si or a-Si) because of their low cost, nontoxicity, abundance and mature processing technology. Yet, such great benefits are balanced by a short carrier diffusion length in silicon, resulting in a much lower conversion than that in crystalline solar cells. To achieve a nearly complete absorption the absorbing layer’s thickness should be at least a few micrometers. Unfortunately, this is unrealistic because of high and defect-related carrier recombination [1]. As a result, improving the absorption in thin film Si solar cells has become crucial. In the past years, many light-trapping techniques have been investigated, among which a typical example is the use of scattering surface textures [2]. However, they are balanced by the induced surface roughness that is almost of the same order as the film thickness and by the resulting large surface area which causes an increased surface recombination. Recently, notable progresses in the field of surface plasmon polaritons (SPPs) [3] have provided a promising way of light-trapping, and have consistently drawn an increasing amount of attention [4-9]. Upon excitation, SPPs cause strong near-field amplitude of the incident electromagnetic (EM) field, and a resonantly enhanced scattering cross section (SCS) as well. With regard to light absorption enhancement in thin film solar cells, SPPs are thought to be useful because the dissipation energy is proportional to the electric field (E-field) squared, a larger E-field causes a larger absorption, and because a stronger SCS redirects more incident photons into the absorbing layer, substantially increasing the absorbing length. Early work in this area carried out in late 1990s involved incorporation of small
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