KEYWORDS: Tandem solar cells, Perovskite, Optical simulations, Solar cells, Silicon, Oxides, Optics manufacturing, Silicon solar cells, Optimization (mathematics), Multijunction solar cells
Optical simulations of perovskite/silicon tandem solar cells show that nanotexturing both sides of the perovskite top cell yields the strongest antireflective effect. Cells with an intermediate texture in-between the perovskite and silicon sub cells perform comparably to configurations with a fully planar top cell. However, in experiment intermediate-textured solar cells perform slightly better than their planar counterparts. A numerical sensitivity analysis shows that this can be attributed to the thickness of a silicon oxide layer in-between the two sub cells: this thickness affects the optics for a fully planar top cell, but does not affect the performance for intermediate texturing.
Photon upconversion could enable to harvest parts of the solar spectrum that cannot be absorbed in conventional silicon solar cells. However, typical upconversion materials suffer from a low absorption cross-section and a low brightness. Using the sun as energy source, signal enhancement strategies for efficient photon upconversion are required, e.g. by the utilization of metasurfaces providing strongly enhanced electrical near-fields. Here, we placed β-NaYF4:Er3+ nanoparticles on large-area silicon metasurfaces designed to convert near-infrared (1550 nm) excitation light to visible luminescence. We observed a more than 2400-fold enhanced photon upconversion luminescence compared to a planar substrate. Optical simulations allowed attributing this result to the coupling of the excitation source with metasurface resonances at appropriate incident angles. Aiming at broadband applications such as solar energy conversion we also introduced a multi-layer metasurface design: Comparing multi-layer to conventional single-layer structures revealed that the resonances associated with enhanced near-fields are split into multiple different modes and are spectrally broadened. The findings not only permit the significant reduction of the excitation power densities required for photon upconversion of 1550 nm light, but also introduce a new approach for efficient upconversion under broadband excitation conditions. This can open new perspectives for applications of such materials in the third biological excitation window, telecommunication, and photovoltaics.
Optical simulations of perovskite/silicon tandem solar cells show that nanotexturing both sides of the perovskite top cell yields the strongest antireflective effect. Cells with an intermediate texture in-between the perovskite and silicon sub cells perform comparably to configurations with a fully planar top cell. However, in experiment intermediate-textured solar cells perform slightly better than their planar counterparts. A numerical sensitivity analysis shows that this can be attributed to the thickness of a silicon oxide layer in-between the two sub cells: this thickness affects the optics for a fully planar top cell, but does not affect the performance for intermediate texturing.
Currently, perovskites/silicon tandem solar cells and bifacial solar cells are amongst the most-discussed trends within the photovoltaic community.
Accurate numerical simulations of large PV systems consisting of bifacial solar modules are vital their optimization. In this work we present a detailed illumination model for solar modules in a big PV field. This model takes direct and diffuse illumination from the sky and from the ground into account, accounts for shadowing of the modules onto each other and the ground, and allows to calculate the annual energy yield as a function of distance and tilt of the modules.
Using realistic spectral weather data and the spectral reflectivity of the ground, we can perform detailed optimizations for the different tandem solar cell configurations. In general, four-terminal cells yield a higher electricity generation because in contrast to two-terminal (2T) cells no performance losses caused by current mismatch occur, but the balance-of-system costs for 2T cells are lower. By increasing the perovskite thickness and/or decreasing the perovskite bandgap, the top-cell current density can be increased leading to a higher overall current density under bifacial operation. We will compare the optimized energy yield for 2T cells with the energy yield of comparable 4T cells. For these simulations, we use a detailed-balance model with realistic absorption data of perovskite and silicon layers.
Finally, we present minimizations of the LCOE for mono- and bifacial modules as a function of the land cost. We see that increasing the land cost leads to a lower optimal module distance, where the optimal tilt is lower than for larger module distance in order to compensate for more shadowing. With respect to the LCOE corresponding to a module distance determined by a rule-of-thumb, the minimized LCOE can differ significantly especially for higher land cost.
We present optical simulations for a tandem solar cell consisting of a nanostructured thin-film perovskite top cell and a silicon heterojunction (SHJ) wafer bottom cell. The absorption and related current density are calculated using the rigorous simulations in the form of the finite element method for the nanostructured perovskite cell and a semi-empirical method for the SHJ cell. In order to reach the optimal value for the perovskite layer thickness we employ Newton’s method using derivatives obtained directly from the rigorous simulation. Using this we obtain an optimal layer thickness using typically one iteration step and eliminate the need for a parameter scan.
We compare the results for different sinusoidal nanotextures applied to different layers in the multilayer thin-film perovskite top cell. The nanotextures lead to a gain in absorption and power conversion efficiency in comparison to an optimized planar reference. We also present experimental results towards a realisation of the proposed structure. These results give valuable insight for the emerging field of high efficiency perovskite/SHJ tandem solar cells.
Recently, we studied the effect of hexagonal sinusoidal textures on the reflective properties of perovskite-silicon tandem solar cells using the finite element method (FEM). We saw that such nanotextures, applied to the perovskite top cell, can strongly increase the current density utilization from 91% for the optimized planar reference to 98% for the best nanotextured device (period 500 nm and peak-to-valley height 500 nm), where 100% refers to the Tiedje-Yablonovitch limit.* In this manuscript we elaborate on some numerical details of that work: we validate an assumption based on the Tiedje-Yablonovitch limit, we present a convergence study for simulations with the finite-element method, and we compare different configurations for sinusoidal nanotextures.
KEYWORDS: Perovskite, Tandem solar cells, Solar cells, Silicon, Absorption, Nanophotonics, Finite element methods, Computer architecture, Fourier transforms, Crystals
Currently, perovskite–silicon tandem solar cells are one of the most investigated concepts for overcoming the theoretical limit for the power conversion efficiency of silicon solar cells. For monolithic tandem solar cells, the available light must be distributed equally between the two subcells, which is known as current matching. For a planar device design, a global optimization of the layer thicknesses in the perovskite top cell allows current matching to be reached and reflective losses of the solar cell to be minimized at the same time. However, even after this optimization, the reflection and parasitic absorption losses add up to 7 mA / cm2. In this contribution, we use numerical simulations to study how well hexagonal sinusoidal nanotextures in the perovskite top-cell can reduce the reflective losses of the combined tandem device. We investigate three configurations. The current density utilization can be increased from 91% for the optimized planar reference to 98% for the best nanotextured device (period 500 nm and peak-to-valley height 500 nm), where 100% refers to the Tiedje–Yablonovitch limit. In a first attempt to experimentally realize such nanophotonically structured perovskite solar cells for monolithic tandems, we investigate the morphology of perovskite layers deposited onto sinusoidally structured substrates.
Light management is a key issue for highly efficient liquid-phase crystallized silicon (LPC-Si) thin-film solar cells and can be achieved with periodic nanotextures. They are fabricated with nanoimprint lithography and situated between the glass superstrate and the silicon absorber. To combine excellent optical performance and LPC-Si material quality leading to open circuit voltages exceeding 640 mV, the nanotextures must be smooth. Optical simulations of these solar cells can be performed with the finite element method (FEM). Accurately simulating the optics of such layer stacks requires not only to consider the nanotextured glass-silicon interface, but also to adequately account for the air-glass interface on top of this stack. When using rigorous Maxwell solvers like the finite element method (FEM), the air-glass interface has to be taken into account a posteriori, because the solar cells are prepared on thick glass superstrates, in which light is to be treated incoherently. In this contribution we discuss two different incoherent a posteriori corrections, which we test for nanotextures between glass and silicon. A comparison with experimental data reveals that a first-order correction can predict the measured reflectivity of the samples much better than an often-applied zeroth-order correction.
KEYWORDS: Silicon, Thin film solar cells, Solar cells, Thin films, Antireflective coatings, Diffraction gratings, Numerical simulations, Finite element methods, Thin film solar cells, Refractive index, Interfaces, Polarization, Maxwell's equations, Absorption
Hexagonal sinusoidal nanotextures are well suited to couple light into silicon on glass at normal incidence, as we have shown in an earlier publication [K. Jäger et al., Opt. Express 24, A569 (2016)]. In this manuscript we discuss how these nanotextures perform under oblique incidence illumination. For this numerical study we use a rigorous solver for the Maxwell equations. We discuss nanotextures with periods between 350 nm and 730 nm and an aspect ratio of 0.5.
We present a numerical method to characterize the symmetry properties of photonic crystal (PhC) modes based on field distributions, which themselves can be obtained numerically. These properties can be used to forecast specific features of the optical response of such systems, e.g. which modes are allowed to couple to external radiation fields. We use 2D PhCs with a hexagonal lattice of holes in dielectric as an example and apply our technique to reproduce results from analytical considerations. Further, the method is extended to fully vectorial problems in view of 3D PhCs and PhC slabs, its functionality is demonstrated using test cases and, finally, we provide an efficient implementation. The technique can thus readily be applied to output data of all band structure computation methods or even be embedded – gaining additional information about the mode symmetry.
Maxwell solvers based on the hp-adaptive finite element method allow for accurate geometrical modeling and high numerical accuracy. These features are indispensable for the optimization of optical properties or reconstruction of parameters through inverse processes. High computational complexity prohibits the evaluation of the solution for many parameters. We present a reduced basis method (RBM) for the time-harmonic electromagnetic scattering problem allowing to compute solutions for a parameter configuration orders of magnitude faster. The RBM allows to evaluate linear and nonlinear outputs of interest like Fourier transform or the enhancement of the electromagnetic field in milliseconds. We apply the RBM to compute light-scattering off two dimensional photonic crystal structures made of silicon and reconstruct geometrical parameters.
For an optimized light harvesting while using diverse periodic photonic light-trapping architectures in low cost thin film crystalline silicon (c-Si) solar cells, it is also of prime importance to tune the features of their lattice point basis structure. In view of this, tapered nanoholes would be of importance for envisaged better light in-coupling due to graded index effect and also from the point of fabrication feasibility. Using a 3D finite element method based computational simulator, we investigate the basis structural influence of triangular as well as honeycomb lattice-structured experimentally feasible tapered air nanoholes in ~400 nm thick c-Si absorber on a glass substrate. We present a detailed convergence analysis of volume absorption in Si absorber with cylindrical as well as tapered nanoholes. For a wavelength rage of 300 nm to 1100 nm, we present the computed results on light absorption of the engineered Si nanoholes for a lattice periodicity of 600nm. In particular, we study the influence of tapering angle of engineered nano air holes in Si thin film for the absorption enhancement in photonic triangular and honeycomb lattice structured tapered nanoholes. Further we make a comparative analysis of cylindrical and tapered nanoholes for a range of light incident angles from 0° to 60°. For the presented triangular as well as honeycomb lattice structured nanoholes, we observe that in comparison to the cylindrical nanoholes, the tapered nanoholes perform better in terms of light trapping for enhanced light absorption in textured Si thin films even when the effective volume fraction of Si is lower in the absorber layer with tapered nanoholes in comparison to that of cylindrical ones. From the maximum achievable short circuit current density estimation in the present study, the performance of c-Si absorbing layer engineered with triangular lattice structured tapered air holes harvests light efficiently owing to its higher lattice symmetry among periodic structures as well as graded index effect of the tapered nanoholes.
Jolly Xavier, Jürgen Probst, Philippe Wyss, David Eisenhauer, Franziska Back, Eveline Rudigier-Voigt, Christoph Hülsen, Bernd Löchel, Christiane Becker
We present our results on optical absorption enhancement in crystalline silicon (c-Si) absorber structured with transversely quasicrystalline lattice geometry for thin-film photovoltaics. c-Si nanoarchitectures are prepared on the nanoimprinted ten-fold symmetry quasicrystalline textured substrate. The structural features of the fabricated Si nanostructures are analyzed to confirm the defining characteristics of the quasicrystalline texturing of the absorber film. We present the optical absorption plots for a spectrum of incident light for varying angle of light incidence in these fabricated higher symmetry crystalline Si architectures. Neither any back reflector nor antireflection coating is considered in the present study, where use of such layers could further improve the light absorption. The realized quasicrystalline textured silicon nanoarchitectures with higher rotational symmetry lattice geometry are observed to improve the isotropic and broad band absorption properties of the thin film c-Si absorber and envisaged to have efficiency enhanced thin film photovoltaics effective in terms of cost and performance.
A smart light trapping scheme is essential to tap the full potential of polycrystalline silicon (poly-Si) thin-film solar cells. Periodic nanophotonic structures are of particular interest as they allow to substantially surpass the Lambertian limit from ray optics in selected spectral ranges. We use nanoimprint-lithography for the periodic patterning of sol-gel coated glass substrates, ensuring a cost-effective, large-area production of thin-film solar cell devices. Periodic crystalline silicon nanoarchitectures are prepared on these textured substrates by high-rate silicon film evaporation, solid phase crystallization and chemical etching. Poly-Si microhole arrays in square lattice geometry with an effective thickness of about 2μm and with comparatively large pitch (2 μm) exhibit a large absorption enhancement (A900nm = 52%) compared to a planar film (A900nm ~ 7%). For the optimization of light trapping in the desired spectral region, the geometry of the nanophotonic structures with varying pitch from 600 nm to 800 nm is tailored and investigated for the cases of poly-Si nanopillar arrays of hexagonal lattice geometry, exhibiting an increase in absorption in comparison to planar film attributed to nanophotonic wave optic effects. These structures inspire the design of prospective applications such as highly-efficient nanostructured poly-Si thin-film solar cells and large-area photonic crystals.
Large grained polycrystalline silicon (poly-Si) absorbers were realized by electron beam induced liquid phase crystallization on 2 μm periodically patterned glass substrates and processed into a-Si:H/poly-Si heterojunction thin-film solar cells. The substrates were structured by nanoimprint lithography using a UV curable hybrid polymer sol-gel resist, resulting in a glassy high-temperature stable micro-structured surface. Structural analysis yielded high quality poly-Si material with grain sizes up to several hundred micrometers. An increase of absorption and an enhancement of the external quantum efficiency in the NIR as a consequence of light trapping due to the micro-structured poly-Si/substrate interface were observed. Up to now, only moderate solar cell parameters, a maximum open-circuit voltage of 413 mV and a short-circuit current density of 8 mA cm-2, were measured being significantly lower to what can be achieved with liquid phase crystallized poly-Si thin-film solar cells on planar glass substrates indicating that the substrate texture has impact on the electrical material quality. By reduction of the SiC interlayer thickness at the micro-structured poly- Si/substrate interface defect-related parasitic absorption was considerably minimized. This encourages the implementation of nanoimprinted tailored substrate textures for light trapping in liquid phase crystallized poly-Si thinfilm solar cells.
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