Enhanced Light-trapping in Solar Cells by Directional Selective Optical Filters
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Enhanced Light-trapping in Solar Cells by Directional Selective Optical Filters Carolin Ulbrich1, Thomas Kirchartz2, and Uwe Rau2 1 Institut für Physikalische Elektronik, Universität Stuttgart, Pfaffenwaldring 47, Stuttgart, D70569, Germany 2 IEF5-Photovoltaik, Forschungzentrum Jülich, Jülich, 52425, Germany ABSTRACT Directional selective optical filters increase the photon confinement in solar cells with Lambertian light-trapping schemes. These filters restrict the transmission of incoming sunlight to a cone of limited acceptance angle. This paper models the efficiency of a solar cell with an idealized directional and energy selective filter on top of a Lambertian surface, and compares it to a cell with a Lambertian surface but without filter and to a planar absorber. The enhancement of light-trapping by the directional filter is illustrated by the increase of solar cell quantum efficiency under normally incident light. Simulations of the annual yield demonstrate that the improved light-trapping results in an overall energy density gain of more than 10 % for a tracked system. To achieve this gain at the equator, a filter with small acceptance angle of 5° that is active below the threshold energy ~1.5 eV has to be used. INTRODUCTION Increasing the path length of weakly absorbed light by efficient light-trapping schemes overcomes the drawback of incomplete light absorption in thin solar cells. Next to their lower material consumption, thin solar cells have additional important advantages. In thinner cells, carrier collection is facilitated allowing the use of lower quality absorber materials. The increased minority carrier generation rate per unit volume and the resulting steady state carrier concentration reduce the entropy production per photon, leading to an increased open circuit voltage [1]. The Yablonovitch limit [2] for light-trapping predicts the maximum enhancement of the average path length of weakly absorbed light to be a factor of 4n2, where n is the refractive index of the absorber material. The approach of Yablonovitch, however, is restricted to angle independent absorptances. Exploiting the fact that the sun illuminates the cell only from a very small angular cone has the potential to strongly improve light-trapping. Miñano [3] showed that an idealized step-function-like filter with an opening half angle θth leads to a path length enhancement of 4n²/sin²(θth). Such a filter transmits only the fraction of light that impinges at angles θ < θth. All other rays are blocked and thus cannot leave the cell. However, a filter on a solar cell illuminated by the sun would also block all incident rays originating from diffuse illumination or from solar angles exceeding θ ≥ θth. Obviously, there is a need for a careful investigation of the benefits of light-trapping and losses due to oblique incidence for cells under real-world conditions. This paper focuses on the light-trapping effect of an idealized filter with combined angular and energy selectivity mounted on top of a silicon solar cell. We star
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