Diffractive gratings for crystalline silicon solar cells—optimum parameters and loss mechanisms

Authors


Marius Peters, Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstraße 2, D-79110 Freiburg, Germany.

E-mail: marius.peters@ise.fraunhofer.de

ABSTRACT

In this paper, we present guidelines for the design of backside gratings for crystalline silicon solar cells. We use a specially developed method based on a combination of rigorous 3D wave optical simulations and detailed semiconductor device modeling. We also present experimental results of fabricated structures. Simulation-based optimizations of grating period Λ and depth d of a binary grating and calculations of the optical and electrical characteristics of solar cells with optimized gratings are shown. The investigated solar cell setup features a thickness of dbulk = 40 µm and a flat front surface. For this setup, we show a maximum increase in short-circuit current density of ΔjSC = 1.8 mA/cm² corresponding to an efficiency enhancement of 1% absolute. Furthermore, we investigate different loss mechanisms: (i) an increased rear surface recombination velocity S0,b because of an altered surface caused by the introduction of the grating and (ii) absorption in the aluminum backside reflector. We analyze the trade-off point between gain due to improved optical properties and loss due to corrupted electrical properties. We find that, increasing the efficiency by 1% absolute due to improved light trapping, the maximum tolerable recombination velocity is S0,b(max) = 5.2 × 103 cm/s. From simulations and measurements, we conclude that structuring of the aluminum backside reflector should be avoided because of parasitic absorption. Adding a dielectric buffer layer between silicon and the structured aluminum, absorption losses can be tuned. We find that for a planar reflector, the thickness of a SiO2 buffer layer should exceed math formula = 120 nm. Copyright © 2011 John Wiley & Sons, Ltd.

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