David Eisenhauer

5 × 5 cm² silicon photonic crystal slabs on glass and plastic foil exhibiting broadband absorption and high-intensity near-fields.

Crystalline silicon photonic crystal slabs are widely used in various photonics applications. So far, the commercial success of such structures is still limited owing to the lack of cost-effective fabrication processes enabling large nanopatterned areas (≫ 1 cm2). We present a simple method for producing crystalline silicon nanohole arrays of up to 5 × 5 cm2 size with lattice pitches between 600 and 1000 nm on glass and flexible plastic substrates. Exclusively up-scalable, fast fabrication processes are applied such as nanoimprint-lithography and silicon evaporation. The broadband light trapping efficiency of the arrays is among the best values reported for large-area experimental crystalline silicon nanostructures. Further, measured photonic crystal resonance modes are in good accordance with light scattering simulations predicting strong near-field intensity enhancements greater than 500. Hence, the large-area silicon nanohole arrays might become a promising platform for ultrathin solar cells on lightweight substrates, high-sensitive optical biosensors, and nonlinear optics.

Quasicrystalline-structured light harvesting nanophotonic silicon films on nanoimprinted glass for ultra-thin photovoltaics

We present nanophotonic light harvesting crystalline silicon (c-Si) thin films on glass exhibiting ten-fold transversely quasicrystalline lattice geometry on 6 x 8 mm2 area. The c-Si architectures with a nearest neighbor distance of 650 nm are fabricated by nanoimprinting the desired quasicrystalline geometry into sol-gel coated glass substrates followed by Si deposition of 240 nm to 270 nm thickness, self-organized solid phase crystallization and selective chemical etching. Broadband absorption measurements on these quasicrystalline-structured c-Si architectures yield a very significant improvement in light trapping in the near infrared regime and an enhanced light coupling due to a graded index effect in comparison to the unstructured sample. The average value of maximum achievable short circuit current density jsc, max of solar cells with such quasicrystalline-structured c-Si absorber geometry (19.3 mA/cm2) is more than double in comparison to the jsc, max of unstructured planar films of the same thickness (9.3 mA/cm2) and remains stable for light incident angles up to 60°. In comparison to a 320 nm thick c-Si film on textured ZnO:Al substrate as widely used for light trapping in amorphous-microcrystalline Si thin-film photovoltaics, still a 65% increased jsc, max is observable for the presented quasicrystalline c-Si structures. The nanophotonic light trapping efficiency of these transversely quasicrystalline c-Si nanoarchitectures is among the highest values for experimentally realized structures, revealing their promising influence for broadband and isotropic light trapping for economically viable and efficient ultra-thin solar cells.

Smooth anti-reflective three-dimensional textures for liquid phase crystallized silicon thin-film solar cells on glass

Recently, liquid phase crystallization of thin silicon films has emerged as a candidate for thin-film photovoltaics. On 10 μm thin absorbers, wafer-equivalent morphologies and open-circuit voltages were reached, leading to 13.2% record efficiency. However, short-circuit current densities are still limited, mainly due to optical losses at the glass-silicon interface. While nano-structures at this interface have been shown to efficiently reduce reflection, up to now these textures caused a deterioration of electronic silicon material quality. Therefore, optical gains were mitigated due to recombination losses. Here, the SMooth Anti-Reflective Three-dimensional (SMART) texture is introduced to overcome this trade-off. By smoothing nanoimprinted SiOx nano-pillar arrays with spin-coated TiOx layers, light in-coupling into laser-crystallized silicon solar cells is significantly improved as successfully demonstrated in three-dimensional simulations and in experiment. At the same time, electronic silicon material quality is equivalent to that of planar references, allowing to reach Voc values above 630 mV. Furthermore, the short-circuit current density could be increased from 21.0 mA cm−2 for planar reference cells to 24.5 mA cm−2 on SMART textures, a relative increase of 18%. External quantum efficiency measurements yield an increase for wavelengths up to 700 nm compared to a state-of-the-art solar cell with 11.9% efficiency, corresponding to a jsc, EQE gain of 2.8 mA cm−2.

Combining tailor-made textures for light in-coupling and light trapping in liquid phase crystallized silicon thin-film solar cells

We present tailor-made imprinted nanostructures for light management in liquid phase crystallized silicon thin-film solar cells providing both, increased jsc by enhanced absorption and excellent electronic material-quality with Voc-values >640mV. All superstrate textures successfully enhance light in-coupling in 10-20µm thick liquid phase crystallized silicon thin-films. Moreover, the effect of combining imprinted textures at the front side with individually optimized light trapping schemes at the rear side of the absorber layers on the optical properties is analyzed. With a silicon absorber layer thickness of 17µm maximum achievable short-circuit current density of 37.0mA/cm2 is obtained, an increase by + 1.8mA/cm2 (or 5.1%) compared to the optimized planar reference.

Nanophotonic light management for perovskite–silicon tandem solar cells

Abstract. 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 harvesting quasicrystalline nanophotonic structures for crystalline silicon thin-film solar cells

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.

Antireflective nanotextures for monolithic perovskite-silicon 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.

CsxFA1–xPb(I1–yBry)3 Perovskite Compositions: the Appearance of Wrinkled Morphology and its Impact on Solar Cell Performance

1 Helmholtz-Zentrum Berlin für Materialien und Energi e GmbH; a Young Investigator Group Perovskite Tandem Solar Cells, b Institute of Silicon Photovoltaics, c Young Investigator Group Hybrid Materials Formation and Scaling, d Young Investigator Group Nano-SIPPE, Kekuléstraße 5, 12489 Berlin, Germany. 2 Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, Germany. 3 University of Potsdam, Institute of Physics and As tronomy, Karl-Liebknecht-Str. 24-25, 14476 Potsdam-Golm, Germany. 4 Departamento de Ciencias, Sección Física, Pontifici a Universidad Católica del Perú, Av. Universitaria 1801, Lima 32, Peru.

Improved Light Management in Crystalline Silicon Thin-Film Solar Cells by Advanced Nano-Texture Fabrication

We present a texturing method for liquid phase crystallized silicon thin-film solar cells enabling a maximum achievable short-circuit current density of 36.5mA cm−2 due to optimized light management compared to current textured devices.

Optical simulations of advanced light management for liquid-phase crystallized silicon thin-film solar cells

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.