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Reflectance, reduction, and light trapping enhancement are essential to maximize the absorption of silicon solar cells. The industrial state of the art method to improve the solar cell optics is wet chemical texturization of the front surface in combination with the deposition of antireflection coatings. This work analyzes an alternative route, namely a TiO2 pillar structure on the front side of a planar silicon solar cell encapsulated in ethylene vinyl acetate (EVA) and glass. It focuses on parameter variations of the structured TiO2 layer while taking the module encapsulation into account. It is shown that internal reflections at the front interface of the module play a crucial role for the structure design. This leads to optimized structures working in a different optical regime. While state of the art structures optimized for a half infinite encapsulation act as effective media, structures optimized for the full module show an improved performance by making use of diffractive effects. It could be shown that weighted reflectance of 4.7% can be reached for a solar module with TiO2 pillar structure on top of the silicon surface compared to 5.5% for a two-layer ARC with a TiO2 bottom layer and 2.3% for an isotexture, which is the state of the art structure for multicrystalline silicon cells.

In this paper we introduce the three-dimensional formulation of the OPTOS formalism, a matrix-based method that allows for the efficient simulation of non-coherent light propagation and absorption in thick textured sheets. As application examples, we calculate the absorptance of solar cells featuring textures on front and rear side with different feature sizes operating in different optical regimes. A discretization of polar and azimuth angle enables a three-dimensional description of systems with arbitrary surface textures. We present redistribution matrices for 3D surface textures, including pyramidal textures, binary crossed gratings and a Lambertian scatterer. The results of the OPTOS simulations for silicon sheets with different combinations of these surfaces are in accordance with both optical measurements and results based on established simulation methods like ray tracing. Using OPTOS, we show that the integration of a diffractive grating at the rear side of a silicon solar cell featuring a pyramidal front side results in absorption close to the Yablonovitch Limit enhancing the photocurrent density by 0.6 mA/cmsup2/supfor a 200 µm thick cell.

In this paper, we introduce a simulation formalism for determining the Optical Properties of Textured Optical Sheets (OPTOS). Our matrix-based method allows for the computationally-efficient calculation of non-coherent light propagation and absorption in thick textured sheets, especially solar cells, featuring different textures on front and rear side that may operate in different optical regimes. Within the simulated system, the angular power distribution is represented by a vector. This light distribution is modified by interaction with the surfaces of the textured sheets, which are described by redistribution matrices. These matrices can be calculated for each individual surface texture with the most appropriate technique. Depending on the feature size of the texture, for example, either ray- or wave-optical methods can be used. The comparison of the simulated absorption in a sheet of silicon for a variety of surface textures, both with the results from other simulation techniques and experimentally measured data, shows very good agreement. To demonstrate the versatility of this newly-developed approach, the absorption in silicon sheets with a large-scale structure (V-grooves) at the front side and a small-scale structure (diffraction grating) at the rear side is calculated. Moreover, with minimal computational effort, a thickness parameter variation is performed.

Light trapping is becoming of increasing importance in crystalline silicon solar cells as thinner wafers are used to reduce costs. In this work, we report on light trapping by rear-side diffraction gratings produced by nano-imprint lithography using interference lithography as the mastering technology. Gratings fabricated on crystalline silicon wafers are shown to provide significant absorption enhancements. Through a combination of optical measurement and simulation, it is shown that the crossed grating provides better absorption enhancement than the linear grating, and that the parasitic reflector absorption is reduced by planarizing the rear reflector, leading to an increase in the useful absorption in the silicon. Finally, electro-optical simulations are performed of solar cells employing the fabricated grating structures to estimate efficiency enhancement potential.