Modeling light trapping and electronic transport of waffle-shaped crystalline thin-film Si solar cells |
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Authors: | R Brendel D Scholten |
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Institution: | Bavarian Center for Applied Energy Research (ZAE Bayern), Am Weichselgarten 7, D-91058 Erlangen, Germany (E-mail: brendel@zae.uni-erlangen.de), DE Institut für Angewandte Physik, Universit?t Erlangen-Nürnberg, Staudtstrasse 7, D-91058 Erlangen, Germany, DE
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Abstract: | Monocrystalline Si films from the novel perforated-Si process are candidates for the fabrication of thin-film solar cells
because their waffle shape enhances the optical absorption and hence permits the use of films with a thickness of only a few
microns. We study the optics of waffle cells by three-dimensional Monte Carlo ray-tracing. A high photogeneration of 38 mA/cm2 from a film of thickness Wf=4 μm is possible due to a detached Al-back surface reflector that has an effective reflectance of 99.7% at 1250 nm. Our analytical
model for light trapping in thin films explains this high reflectance. Two-dimensional numerical transport modeling reveals
the existence of an optimum texture period p≈2Wf that originates from a carrier collection efficiency that increases with texture period while the photogeneration decreases
with period. For well-passivated cells the optimum thickness Wf is at least one fifth of the diffusion length L. Efficiencies of 17% to 18% are feasible with waffle films of 1 to 3 μm in
thickness. We introduce an analytic model for the minority carrier transport that agrees with two-dimensional numerical modeling
to within 10% and reduces the computation time by orders of magnitude. This analytic model is also applicable to conformal
thin-film geometries differing from the waffle geometry.
Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999 |
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Keywords: | PACS: 84 60 J 42 15 D 72 20 Jv |
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