In this paper, we study the limits of light trapping for amorphous silicon thin film solar cells using surface metallic gratings. Adopting a method used recently by Sheng et al. [31], arbitrarily shaped periodic surface textures described by Fourier series with limited terms are considered, and global inverse optimization techniques such as Simulated Annealing are used to adjust the structural variations of the unknown texture to yield maximum light trapping. The optimization is done with respect to two objective functions: enhancement in the number of absorbed photons and, maximal spectral absorptivity enhancement. We show that compared with the rectangular structures previously studied, curved structures result in additional waveguide modes and more broadband enhancement in absorptivity of silicon. An overall improvement of over 60% is achievable in the number of absorbed photons for polarized incident sunlight using the shape functions we will describe. We compare the results with conventional Lambertian limit of light trapping [1] and with the more recent theoretical limits of Yu et al. [30] and Sheng et al. [31] for thin films. We show that at near-infrared ranges, absorptivity enhancements remarkably higher than those results can be achieved using the proposed structures and inverse optimization.

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