Abstract
For advanced optical analysis and optimization of solar cell structures with multi-scale interface textures, we applied a coupled modelling approach (CMA), where we couple the rigorous coupled wave analysis method with ray tracing and transfer matrix method. Coupling of the methods enables accurate optical analysis of solar cells made of thin coherent and thick incoherent layers and includes combinations of nano- and micro-scale textures at various positions in the structure. The approach is experimentally validated on standalone single- and both-side textured crystalline silicon wafers, as well as on complete silicon heterojunction (Si HJ) solar cell structures. Using CMA, fully encapsulated bifacial Si HJ solar cells are optically simulated first by applying single- and both-side illumination, and the effects of introducing nano inverted pyramids and random micro-pyramids at front and/or rear interfaces are analyzed. Secondly, an external light management foil with a three-sided pyramidal micro-texture is applied in simulations to the front and/or rear encapsulation glass, and the related improvements are quantified. For the optimal combination of internal textures in the analyzed structure (random micro-pyramids at the front and nano inverted pyramids at the back) and the use of the light management foil on both sides of the device, a 5.6% gain in the short-circuit current is predicted, compared to the reference case with no light management foil and with random micro-pyramids applied to the front and rear internal interfaces.
Highlights
In solar cells, light management techniques are used to boost the short-circuit current density and the energy conversion efficiency
Within the Coupled Modelling Approach (CMA), the Rigorous Coupled Wave Analysis (RCWA) method is applied for accurate description of the optical situation in nano-textured stacks of thin layers [6], Ray Tracing (RT) method is used to describe reflection and refraction of light at microtextures and light propagation through thick incoherent layers, while Transfer Matrix method (TMM) is applied to optical simulation of flat thin-film stacks
Disagreement between the simulation and the experiment is observed only in the range of 900-1000 nm, which is likely caused by the overly idealized texture considered in simulations as opposed to the significant texture variability in the experimental samples (Fig. 2 and surrounding text)
Summary
Light management techniques are used to boost the short-circuit current density and the energy conversion efficiency. Our approach iteratively couples matrices in full three dimensional space, which offers (i) greater versatility, shown e.g. in simulation of the combined multi-scale (nano+micro) texture at the same interface [6], (ii) greater accuracy, e.g. by fully considering the exact trajectories and lateral positions under which the internally reflected light rays return to the micro texture, which is in some cases crucial for proper calculation of light trapping [1], and (iii) enables a more realistic description of the optical situation in other cases where structures cannot be approximated as a one-dimensional stack An example of such structure is a solar cell containing a grid of contacts on both front and rear sides of the solar cell. We show that application of the cornercube LMF significantly improves performance of the solar cell with flat interfaces, whereas its advantage in the micro or nano textured cell is limited mostly to the lower air-glass reflection losses
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