Optics in Thin-film Silicon Solar Cells with Integrated Lamellar Gratings
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1153-A07-20
Optics in Thin-film Silicon Solar Cells with Integrated Lamellar Gratings Rahul Dewan, Darin Madzharov, Andrey Raykov and Dietmar Knipp School of Engineering and Science, Jacobs University Bremen, 28759 Bremen, Germany.
ABSTRACT Light trapping in microcrystalline silicon thin-film solar cells with integrated lamellar gratings was investigated. The influence of the grating dimensions on the short circuit current and quantum efficiency was investigated by numerical simulation of Maxwell’s equations by a Finite Difference Time Domain approach. For the red and infrared part of the optical spectrum, the grating structure leads to scattering and higher order diffraction resulting in an increased absorption of the incident light in the silicon thin-film solar cell. By studying the diffracted waves arising from lamellar gratings, simple design rules for optimal grating dimensions were derived. INTRODUCTION Efficient light management concepts are needed to increase the short circuit current and quantum efficiency of thin film solar cells. High efficiencies have been achieved by introducing randomly textured interfaces in the solar cell [1-3]. Introducing nano textured interfaces leads to reduced reflection losses and enhanced scattering and diffraction of light in the device. The optical path length is increased, which leads to a distinctly enhanced short circuit current and quantum efficiency in the red and infrared part (wavelength 600 – 1100 nm) of the optical spectrum [4-5]. For shorter wavelengths (300 – 600 nm) the short circuit current and quantum efficiency remain almost constant, since the absorption length is significantly smaller than the thickness of the solar cell. Subsequently the blue and green light will be absorbed even before reaching the back reflector. In order to understand the wave propagation within the nano textured solar cell and to optimize the nano texturing process, near field optics has to be considered when modeling the devices. Therefore, to describe the wave propagation in such devices simple geometric or wave optics is not sufficient, rather Maxwell’s equations have to be solved rigorously [6]. Different approaches of using Maxwell’s solvers have been used to analyze the wave propagation in solar cells [6-8]. In this work, a Finite Difference Time Domain (FDTD) simulation tool (OptiFDTD®) was used to investigate the wave propagation for nano textured microcrystalline silicon solar cells. So far FDTD has only been used to investigate the optical wave propagation within amorphous solar cells [7]. The analysis of the wave propagation within a randomly textured solar cell is complex; hence a simple model system was selected which approximates the randomly textured solar cell. The texturing was modeled by lamellar gratings. The results on smooth substrates are used as a reference to investigate the influence of the grating parameters on the solar cell parameters. Based on grating designs that were derived to maximize the short circuit current and the efficiency of the microcrystallin
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