This title appears in the Scientific Report : 2014 
Optical simulation of photonic random textures for thin-film solar cells
Bittkau, Karsten (Corresponding Author)
Hoffmann, Andre
Photovoltaik; IEK-5
2014
2014
SPIE Photonics Europe, Brussels (Belgium), 2014-04-14 - 2014-04-17
Conference Presentation
Thin Film Photovoltaics


Many types of thin-film solar cell demand advanced light-trapping concepts, in order to overcome the limitations from the weak absorptance near the band gap. Mostly, random textures are incorporated that scatters incoming light diffusely prolonging the effective light path in the absorber layer. As an alternative, periodic structures like gratings or photonic crystals incorporated at different interfaces of the device are investigated by several groups. The optical design of optimized textures is often done by rigorous optical simulations.We recently demonstrated that a simple scalar approach sufficiently describes angular resolved scattering in transmission and reflection inside the absorber material. We found that pure random textures scatter light most efficiently in reflection at the back contact, whereas two-dimensional periodic structures show their highest diffraction efficiencies in transmission.We demonstrate that by the combination of both, periodic structure and random texture conformally incorporated at the front and back contact, the high diffraction efficiency in transmission still dominates the light scattering, but the resonance is much broader due to the random structure. The light scattering at the back contact still shows the broad angular distribution around large angles like the random texture without periodic structure. The combined photonic random texture, therefore, benefits from both resulting in optimal transmission and reflection properties.Starting with a randomly textured ZnO:Al layer, that is well-known to provide high-efficiency microcrystalline silicon solar cells, we add a two-dimensional periodic structure with optimized period and height on top of the random texture by applying the scalar approach. The significant improvement of quantum efficiency is verified by Finite-Difference Time-Domain simulations taking into account real layer stack properties. The thus optimized structure outperforms pure periodic and random structures.