Nanopatterned Back-Reflector with Engineered Near-Field/Far-Field Light Scattering for Enhanced Light Trapping in Silicon-Based Multijunction Solar Cells.

Multijunction solar cells provide a path to overcome the efficiency limits of standard silicon solar cells by harvesting a broader range of the solar spectrum more efficiently. However, Si-based multijunction architectures are hindered by incomplete harvesting in the near-infrared (near-IR) spectral range as Si subcells have weak absorption close to the band gap. Here, we introduce an integrated near-field/far-field light trapping scheme to enhance the efficiency of silicon-based multijunction solar cells in the near-IR range. To achieve this, we design a nanopatterned diffractive silver back-reflector featuring a scattering matrix that optimizes trapping of multiply scattered light into a range of diffraction angles. We minimize reflection to the zeroth order and parasitic plasmonic absorption in silver by engineering destructive interference in the patterned back-contact. Numerical and experimental assessment of the optimal design on the performance of single-junction Si TOPCon solar cells highlights an improved external quantum efficiency over a planar back-reflector (+1.52 mA/cm2). Nanopatterned metagrating back-reflectors are fabricated on GaInP/GaInAsP//Si two-terminal triple-junction solar cells via substrate conformal imprint lithography and characterized optically and electronically, demonstrating a power conversion efficiency improvement of +0.9%abs over the planar reference. Overall, our work demonstrates the potential of nanophotonic light trapping for enhancing the efficiency of silicon-based multijunction solar cells, paving the way for more efficient and sustainable solar energy technologies.

Broadband antireflection Mie scatterers revisited-a solar cell and module analysis.

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.

Field stitching approach for the wave optical modeling of black silicon structures.

The interest in black silicon structures as an anti-reflective interface at the front side of silicon solar cells increased strongly with the rise of diamond wire sawing. The application of optical modeling in order to predict optimal structure parameters could be highly valuable. However, due to the random nature of the structure as well as dimensions in the range of the wavelengths of interest, optical modeling is still a challenge. Within this work, the stitching method of rigorously calculated fields is extended and applied to a black silicon structure. A Fourier transform is used to determine the angular intensity distribution in the far field. In combination with the OPTOS formalism, this allows modeling of silicon substrates with black silicon front side and shows a reasonably good agreement with optical measurement results. Implementing the investigated structure into a solar cell configuration reveals not only a low reflectance but also a very good light trapping performance close to that of a Lambertian scatterer.

Optical modeling of structured silicon-based tandem solar cells and module stacks.

Silicon-based tandem solar cells and modules are complex systems that require optical modeling for the optimization towards highest efficiencies. The fact that such devices typically incorporate surface structures of different optical regimes poses high requirements to the involved simulation tools. The OPTOS formalism is ideally suited to deal with such complexity. Within this work OPTOS is extended in order to calculate the layer resolved absorptance in silicon-based tandem solar cells and module stacks. After describing the relevant mathematical details, a good agreement between OPTOS absorptance simulation results and EQE measurements of the current 33.3% record efficiency III-V on silicon two-terminal tandem solar cell is found. Furthermore, a detailed loss analysis is performed for an exemplary perovskite silicon solar cell with and without module encapsulation. The comparison reveals a lower photocurrent density for the module stack due to increased reflectance and absorption in the EVA.

Theoretical study of pyramid sizes and scattering effects in silicon photovoltaic module stacks.

Front side pyramids are the industrial standard for wafer based monocrystalline silicon solar cells. These pyramids fulfill two tasks: They act as anti-reflective structure on the one hand and as a light-trapping structure on the other hand. In recent development smaller pyramids with sizes below 1 µm attract more and more interest. In this paper an optical analysis of periodically arranged front side pyramids is performed. The impact on the reflectance as well as on the useful absorption within the solar cell is investigated depending on the pyramids size, the amount of additional scattering in the system and the quality of the rear side reflector. In contrast to other investigations not only the solar cell, but the full photovoltaic (PV) module stack is considered. This can strongly influence results, as we show in this paper. The results indicate that in a PV module stack with realistic assumptions for the amount of scattering as well as for the rear side reflectance only small differences for pyramids with sizes above 600 nm occur. Preliminary conclusions for random pyramids deduced from these results for periodically arranged pyramids indicate that these differences could become even smaller.

Optical analysis of a III-V-nanowire-array-on-Si dual junction solar cell.

A tandem solar cell consisting of a III-V nanowire subcell on top of a planar Si subcell is a promising candidate for next generation photovoltaics due to the potential for high efficiency. However, for success with such applications, the geometry of the system must be optimized for absorption of sunlight. Here, we consider this absorption through optics modeling. Similarly, as for a bulk dual-junction tandem system on a silicon bottom cell, a bandgap of approximately 1.7 eV is optimum for the nanowire top cell. First, we consider a simplified system of bare, uncoated III-V nanowires on the silicon substrate and optimize the absorption in the nanowires. We find that an optimum absorption in 2000 nm long nanowires is reached for a dense array of approximately 15 nanowires per square micrometer. However, when we coat such an array with a conformal indium tin oxide (ITO) top contact layer, a substantial absorption loss occurs in the ITO. This ITO could absorb 37% of the low energy photons intended for the silicon subcell. By moving to a design with a 50 nm thick, planarized ITO top layer, we can reduce this ITO absorption to 5%. However, such a planarized design introduces additional reflection losses. We show that these reflection losses can be reduced with a 100 nm thick SiO2 anti-reflection coating on top of the ITO layer. When we at the same time include a Si3N4 layer with a thickness of 90 nm on the silicon surface between the nanowires, we can reduce the average reflection loss of the silicon cell from 17% to 4%. Finally, we show that different approximate models for the absorption in the silicon substrate can lead to a 15% variation in the estimated photocurrent density in the silicon subcell.

Optical simulation of photovoltaic modules with multiple textured interfaces using the matrix-based formalism OPTOS.

The OPTOS formalism is a matrix-based approach to determine the optical properties of textured optical sheets. It is extended within this work to enable the modelling of systems with an arbitrary number of textured, plane-parallel interfaces. A matrix-based system description is derived that accounts for the optical reflection and transmission interaction between all textured interfaces. Using OPTOS, we calculate reflectance and absorptance of complete photovoltaic module stacks, which consist of encapsulated silicon solar cells featuring textures that operate in different optical regimes. As exemplary systems, solar cells with and without module encapsulation are shown to exhibit a considerable absorptance gain if the random pyramid front side texture is combined with a diffractive rear side grating. A variation of the sunlight's angle of incidence reveals that the grating gain is almost not affected for incoming polar angles up to 60°. Considering as well the good agreement with alternative simulation techniques, OPTOS is demonstrated to be a versatile and efficient method for the optical analysis of photovoltaic modules.

3D optical simulation formalism OPTOS for textured silicon solar 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/cm2 for a 200 µm thick cell.

Matrix formalism for light propagation and absorption in thick textured optical sheets.

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.