Front side antireflection concepts for silicon solar cells with diffractive rear side structures
For thin silicon solar cells, standard pyramidal textures cannot be used as antireflection structures due to their thicknesses of several micrometres. In this paper, we investigate two alternative front side antireflection surfaces, which in combination with diffractive rear side structures could lead to low surface reflectance and advanced light trapping: firstly, planar multi-layer antireflection coatings, optimized using experimentally determined refractive indices, have been produced. The resulting hemispheric reflectance weighted with the AM1.5g spectrum (280 nm to 1000 nm) for two and three optimized planar layers were 3.67 % and 3.52 %, respectively, slightly higher than for a sample with inverted pyramids with 3.09 %. Secondly, black silicon was investigated. This acicular nanostructured silicon surface has a very low reflectance over a wide range of the spectrum (weighted reflectance 1.21 %). Reflection and transmission measurements show that a black silicon surface leads to scattering and light trapping. As we see significant absorption for energies below the silicon bandgap for all samples with structured surfaces, possible mechanisms leading to these measurement results were analysed.